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Martha Moon Larson, DVM, MS, DACVR Blacksburg, VA and Jeri C. Jones, DVM Department of Small Animal Clinical Sciences, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic and State University, Blacksburg, Virginia, USA Imaging of the Urinary System
Martha Moon Larson, DVM, MS, Dipl. ACVR Kidneys The kidneys are fairly easily visualized on survey abdominal radiographs as long as there is sufficient retroperitoneal fat for contrast. Renal size, location, and occasionally shape can often be determined without other imaging procedures. Renal size in the cat (measured on the VD radiograph) varies from 2.4-3.0 times the length of L-2 (second lumbar vertebral body). However, it is fairly common for older cats (over 10 years of age) to have kidneys that range from 2.0-2.4 times the length of L-2, without any clinical signs, or hematological changes that might indicate renal disease. In the dog, renal size should be 2.5-3.5 times the length of L2. Mineralized foci in the renal pelvis usually indicates the presence of calculi. Occasionally linear calcifications in the area of the diverticuli are seen, especially in older animals. Nephrocalcinosis ( diffuse mineralization of the renal parenchyma) can be seen with calcium and phosphorus disorders such as hyperparathyroidism, or dietary abnormalities. Additional imaging techniques such as excretory urography or ultrasound are usually needed to achieve a more definitive diagnosis, and in many cases, a biopsy eventually is necessary. Excretory urography Several intravenous organic iodinated contrast agents are available. They are composed of an anion (usually iothalamate or diatrizoate) and a cation, either sodium, meglumine, or a mixture of the two. The contrast agents differ also in their concentration of iodine. Despite the large number of products on the market, there is little practical difference once the iodine concentration has been decided on. All of these contrast agents are hypertonic, and while reactions are rare and usually mild, they may occasionally be fatal. The most common reaction is vomiting, usually seen during a rapid bolus injection of contrast material. More severe reactions, including urticaria (hives) and bronchospasm may occur. Hypotension, cardiac, and respiratory arrest are rare, but have been reported. The kidneys can also have adverse reactions to the contrast agent, both directly, and indirectly as a result of hypotension. Acute oliguria/anuria may occur following intravenous contrast administration. This is seen more commonly with dehydration, therefore all patients should be well hydrated prior to the study. Azotemia is not a contraindication for intravenous urography (IVU) if the patient is well hydrated. In fact, there is no correlation between the BUN and the quality of the study, unless urine specific gravity is low. In people, IVU is contraindicated in cases of diabetes mellitus, multiple myeloma, and combined renal and hepatic failure. Although not substantiated in the veterinary literature, it may be wise to avoid intravenous contrast agents in patients with these conditions. Nonionic intravenous contrast agents (such as those used for myelography) are available, and may cause fewer reactions. However, they are not used routinely for intravenous studies in veterinary medicine because of the greater cost. Technique
Normal appearance The nephrogram phase occurs when contrast is present in the renal tubules and vasculature, in the first 3 minutes post-injection. This phase allows good visualization of renal size, shape, and location, and is an indication of renal perfusion. The kidneys should have a dense, uniform opacification, which starts to fade as contrast filters into the collecting system of the kidney. This initiates the pyelogram phase, where contrast is visible in the renal pelvis, diverticuli, and ureters. The pyelogram phase starts within 3 minutes, and may persist for more than 2 hours in the normal dog and cat. Normally, the renal pelvis is visualized as a thin (less than 2mm diameter) curvilinear structure with well defined, paired diverticuli branching away into the parenchyma. The diverticuli may not be visualized in some normal dogs without abdominal compression; they are usually better visualized in the cat. The ureters are seen as narrow (less than 2mm diameter) linear structures extending through the retroperitoneal space to enter the trigone of the bladder. They may curve cranially as they enter the bladder, but are otherwise fairly straight. The entire ureter is rarely visualized on a single film as peristaltic contractions will decrease visualization of some portions. Abnormal appearance Abnormalities can occur in all phases of the IVU. Abnormalities in the nephrogram phase include increased or decreased opacity, as well as persistent opacity of the kidney. A faint nephrogram may indicate an inadequate contrast dose, or chronic renal disease (inability to concentrate). A persistently dense nephrogram without a subsequent pyelogram is seen with acute renal failure, hypotension, or acute obstruction. An irregular or mottled pattern of opacification may indicate avascular structures such as cysts, or some tumors. The nephrogram is also useful for visualization of changes in renal size and shape. Infarcts, tumors and congenital anomalies may be visualized during this phase. Abnormalities in the renal pelvis, diverticuli, and ureters are best visualized during the pyelogram phase. Dilation of the pelvis and diverticuli (hydronephrosis) can occur with obstruction in the ureter or at the bladder trigone. Rupture of the ureter can cause a functional obstruction, with subsequent proximal dilatation. Chronic pyelonephritis usually results in a slightly dilated pelvis, with blunted, distorted diverticuli. The proximal ureter is often dilated, and the kidney may appear small and irregular. Filling defects within the contrast in the renal pelvis can be caused by calculi, tumors, and cellular debris. Air bubbles within the pelvis are seen occasionally if they have passed up the ureters from an air filled bladder. Ultrasound examination of the kidneys Numerous indications exist for renal ultrasound. Abdominal radiographs allow visualization only of renal size and shape. If abdominal contrast is lacking, the kidneys may not be seen at all. Contrast studies may be done to enhance renal visualization, but are deficient if renal function is poor. Ultrasound allows visualization of not only renal size and shape, but also internal architecture. Decreased abdominal contrast due to effusion or lack of fat inhibits visualization of disease on abdominal radiographs, but may enhance ultrasound imaging. Indications for renal ultrasound include abnormal renal size or shape based on palpation or abdominal radiographs, azotemia, hematuria, abnormal urinalysis, or inability to visualize or palpate kidneys. Technique The right kidney is often more difficult to scan in the dog as it lies partially under the rib cage and may be obscured by bowel gas. It is especially difficult to image in deep chested or large canine breeds, but is usually easily accessible in cats. The cranial pole of the right kidney is associated with the caudate liver lobe. Right lateral intercostal (between 11th and 12th intercostal spaces) approaches are often needed to more completely visualize the right kidney. The left kidney is easier to image consistently as it lies just caudal to the last rib. The spleen may be used as an acoustic window. Longitudinal and transverse scans of the entire kidney should be performed. In cats and small dogs, a 7.5 MHz transducer works well for both kidneys. In larger dogs, a 5.0 or 3.5 MHz probe may be needed for the right kidney. The 7.5 MHz probe may be adequate for the left kidney even in large dogs because of the its more superficial location. Normal renal ultrasound anatomy The renal capsule is a thin, linear hyperechoic density, but is usually not seen in its entirety around the kidney. The renal cortex is uniformly echoic, usually hypo- or isoechoic compared to the liver (the right kidney is easily compared to the caudate liver lobe). However, in some normal cats, especially at higher transducer frequencies, the renal cortex may actually be hyperechoic to the liver. Many cats seem to have increased renal cortical echogenicity due to normal fat deposition. The renal cortex should be hypoechoic to the spleen (the left kidney is used for comparison), and there should be a clear demarcation between cortex and medulla. The renal medulla is anechoic to hypoechoic, and is separated into compartments by linear hyperechoic interlobar vessels and diverticula which radiate from the renal sinus. The renal sinus is hyperechoic and centrally located at the renal hilus (medial). The hyperechogenicity is due to peri-pelvic fat. High intensity echoes can be seen at the renal periphery, corticomedullary interface, and in the renal sinus, and are produced by reflective fibrous or fat interfaces in the capsule, pelvic diverticula, and vasculature. Renal size is sometimes difficult to measure accurately, even with on-screen calipers. There may be poor definition of renal borders, difficulty in getting standard planes to measure, and the cranial and caudal poles may extend outside the sector image, forcing extrapolation of renal measurements. However, in cats, 3.0--4.4cm is considered the normal range, with the cortex measuring 2--5mm in thickness. In dogs, because of breed variation, renal measurements range from 3--10cm, with cortical thickness 3--8mm. Diuresis may cause slight dilation of the renal pelvis. Ureters are not normally visualized. Abnormal renal ultrasound Diffuse renal parenchymal disease may have a variable appearance, showing no changes in many cases. Increased cortical echogenicity, with enhanced cortico-medullary definition may be seen, or there may be increased cortical and medullary echogenicity, with loss of the distinct cortico-medullary junction. As renal echogenicity approaches that of the surrounding perirenal fat, the kidney becomes difficult to discern. With chronic renal disease, there is often a decrease in size. Diseases that may cause such changes include nephrocalcinosis, glomerulonephritis, chronic pyelonephritis, congenital renal dysplasia, end stage kidney disease, and chronic interstitial nephritis. Other diseases showing the same ultrasound changes, but also showing renal enlargement include FIP, amyloidosis, acute tubular necrosis ( as seen with ethylene glycol toxicity) , and lymphosarcoma. Occasionally a hyperechoic line at the cortico-medullary junction is visible. This has been reported with hypercalcemic nephropathy, lymphosarcoma, and multiple myeloma. However, it has also been seen as an incidental finding in normal animals, so the significance is unknown. Focal disease in the kidney has many differentials. Cysts, abscesses, hematomas, primary and metastatic neoplasia, and infarcts should be considered. Focal cysts are occasionally seen incidentally in the renal cortex. The size is variable, with large cysts protruding from and distorting the cortex. True cysts are anechoic, with acoustic enhancement, are round and smooth, and may be single or multiple. Polycystic kidney disease usually results in renal enlargement, with multiple cysts of 1.0 cm or larger. Hepatic cysts may also be present. Neoplasia can often be readily diagnosed on ultrasound examination. Renal lymphosarcoma may be seen as hypoechoic mass lesions, or as a diffuse hypoechogenicity. In cats, lymphosarcoma has also been reported to cause a hyperechoic renal cortex. Other renal neoplasias such as carcinoma and nephroblastoma tend to be more complex, and are focal, multifocal, or diffuse. Hyperechoic areas due to fibrosis are mixed with anechoic areas (due to cystic cavities, old hemorrhage, and necrosis). Tumor type typically cannot be differentiated on ultrasound exam alone, and other differentials such as abscess, hematoma, and necrosis should be included. Infarcts Acute renal infarcts are typically hypoechoic, although they increase in echogenicity with age. They may have a typical wedge shape, with the base towards the capsule, and apex pointing towards the medulla. Other conditions Hydronephrosis may be caused by renal calculi, ureteral stricture or calculi, ureteral obstsruction by an extrinsic mass, or obstructive mass at the bladder trigone. With hydronephrosis, there is a separation of the normally hyperechoic central sinus echoes by an anechoic space. With mild or moderate hydronephrosis an anechoic crescent shape located within the normally hyperechoic area of the renal sinus is present. With severe hydronephrosis, the renal parenchyma becomes compressed into a thin layer around an anechoic, dilated renal pelvis. A dilated, anechoic tubular structure representing the ureter may be seen if ureteral involvement is present. The dilated ureter should be followed as distally as possible to try to determine the cause of obstruction. The bladder should also be evaluated for a possible trigone mass. Diuresis may cause a mild bilateral dilation of the renal pelvis. If hydronephrosis is questionable on ultrasound, it may be confirmed or ruled out by an intravenous urogram. Pyelonephritis causes a mild-to-moderate pelvic dilation (pyelectasia), usually not as marked as in hydronephrosis. There may also be increased cortical echogenicity , and loss of the cortico-medullary definition if the disease is chronic. Acute pyelonephritis may cause no changes Renal calculi can usually be easily visualized, even if the calculi are radiolucent on radiographs. The calculi are highly echogenic, often with acoustic shadowing. Shadowing is best visualized when high frequency transducers are used, and the calculus is within the focal zone of the transducer. Occasionally, pelvic dilation will be associated with the calculus if obstruction is present. The highly echogenic fat in the renal sinus can be mistaken for renal calculi in some cases. Perinephric pseudocysts are diagnosed when large amounts of fluid accumulate around one or both kidneys. The entire kidney is surrounded by fluid, and thus may appear falsely hyperechoic. There frequently are abnormalities associated with the kidneys themselves with this condition. Imaging techniques of the bladder The urinary bladder is usually well visualized on survey abdominal radiographs unless the bladder is empty, or the patient lacks abdominal contrast. Because of its distensible nature, bladder size is variable. The bladder is soft tissue opacity in the normal animal. Radiopaque calculi can be visualized in the dependent portions (middle) of the bladder. Dystrophic calcification of the bladder wall may be secondary to chronic inflammation or neoplastic disease. Air is not normally present within the bladder, although air bubbles occasionally are seen after bladder catheterization or cystocentesis. Emphysematous cystitis results in gas within the bladder wall. This is reported most commonly with diabetic patients. Cystography
Normal appearance The bladder wall should be thin and uniform, approximately 1-2mm thick. The mucosal surface should also be smooth and uniform. On a double contrast study, the positive contrast puddle should be well defined, with uniform margins and density. Occasionally, reflux of contrast, either positive or negative can be seen extending up the ureters and into the renal pelvis. This can occur in young patients, or with overdistension, and is not considered pathologic. However, in patients with cystitis, this may predispose to pyelonephritis. It should be remembered that bladder wall changes have to be fairly advanced before changes will be visible on a cystogram. Therefore, a normal cystogram does not rule out bladder disease. Abnormal appearance Contrast material is visualized free in the peritoneum after bladder rupture. If only a small bladder leak is present, the bladder must be fully distended with contrast material before the site of the rupture is visible. If a rupture is suspected, but not visible on a positive contrast study, a ureteral or urethral rupture should be considered. Occasionally, contrast can be seen within the bladder wall and subserosa, appearing to extend into the peritoneum adjacent to the bladder. This is has been reported after maximum bladder distension, but will also occur in cats with minimal bladder distension, and does not usually result in a clinical problem. Cystitis, especially chronic or severe, causes thickening of the bladder wall, primarily the cranio-ventral aspect. In some cases the entire bladder wall is affected. Neoplasia can have a similar appearance, and may require a biopsy to differentiate from severe cystitis. In most cases however, bladder tumors are seen as focal (single or multiple) mass lesions involving the bladder wall. If the mass involves the trigone area, an intravenous urogram is helpful for detection of ureteral involvement and obstruction. For intraluminal filling defects, double contrast cystography is best. Calculi are visible as focal, slightly irregular filling defects in the middle of the positive contrast puddle. Air bubbles have smooth margins and tend to accumulate at the periphery of the contrast puddle. Blood clots are seen as irregular filling defects of variable size, usually mobile, and falling into the middle of the contrast puddle. Ultrasound examination of the urinary bladder The bladder is usually easily visualized in the caudal abdomen as an anechoic fluid-filled structure. The size is variable depending on the volume of urine at the time of the scan, but scanning is best when the bladder is at least partially distended. It is usually fairly superficial, so that 7.5 MHz transducers are often adequate for complete examination. Longitudinal and transverse views along the entire bladder length should be performed. The bladder serves as an acoustic window for the caudal aorta, caudal vena cava, sublumbar lymph nodes (not seen unless enlarged), and the enlarged uterus. Normal bladder ultrasound anatomy The bladder lumen is usually anechoic with acoustic enhancement, but occasionally may contain urine sediment which creates intraluminal echoes. Slice thickness artifacts can create the impression of luminal echoes. The wall should be thin and uniform in thickness in the distended bladder. Normal bladder wall thickness is approximately 1-2mm when distended, but may appear thickened with less complete bladder distension. The colon may impinge on the bladder dorsally, causing distortion, and can mimic wall masses. The entrance of the ureters into the bladder may be seen as a small focal thickening. Occasionally, ureteral "jets" are seen as ureters empty into the bladder. The urethra is inconsistently evaluated. Abnormal bladder ultrasound Calculi are highly echogenic, usually with acoustic shadowing, even if radiolucent. They tend to be mobile, and will lie on the dependent side. Occasionally urine sediment will cause shadowing, but this sediment is usually easily resuspended by bladder agitation. Blood clots are usually irregular, hyperechoic masses which may be mobile or adhered to the bladder wall. When adhered, blood clots are difficult to differentiate from bladder tumors on ultrasound exam alone. Cystitis may cause thickening or irregularity of the mucosa, mainly involving the cranioventral bladder wall. Blood clots may be associated with cystitis, as are calculi and echogenic urine sediment. Neoplasia can have a variable appearance. Tumor masses may project into the lumen, or cause diffuse irregular thickening of the bladder wall. Tumor masses are usually a complex (mixed) echogenicity. Invasive tumors may have little demarcation between normal and abnormal areas. Neoplasia must be differentiated from severe cystitis and blood clots. Biopsies are usually required. It is important in cases of suspectedbladder neoplasia to check the sublumbar area for enlarged lymph nodes. Bladder rupture is not reliably diagnosed on ultrasound exam alone, although the presence of free abdominal fluid is easily seen. If saline or sterile water is injected into the bladder while scanning, flow may be seen through a defect in the bladder wall. Imaging of the Gastrointestinal Tract
Martha Moon Larson, DVM, MS, Dipl.ACVR The stomach and intestines can be imaged using several different imaging modalities. Each has advantages and disadvantages, and more than one imaging technique may be needed to make a final diagnosis. Survey radiology of the stomach The stomach varies in appearance depending on contents and patient position. On lateral radiographs, the axis of the stomach, from the dorsal fundus down to the ventral pylorus, is perpendicular to the spine, parallel to the ribs, or somewhere in between. The pylorus is located in the cranioventral abdomen, caudal to the liver, and may occasionally be superimposed over the gastric body . In right lateral recumbency, the pylorus may be fluid filled, mimicking a mass or foreign body. On ventrodorsal (VD) views, in the canine, the cardia, fundus, and body of the stomach are typically located to the left of midline, with the pylorus to the right; the stomach may be perpendicular to the spine, or "U" shaped. The feline has more of a "J" shaped stomach, with the pylorus located on the midline. Positioning affects the appearance of the stomach due to different distributions of fluid and gas. In right lateral recumbency, gas is present in the fundus and body, while fluid accumulates in the pylorus. In left lateral recumbency, the opposite occurs, with gas located in the pylorus and fluid in the body and fundus. In dorsal recumbency (VD view), gas accumulates in the pylorus, while fluid fills the fundus and body. Finally, on DV views, gas will be located in the fundus and body, and fluid in the pylorus. Gastric wall thickness is not accurately determined on most survey abdominal radiographs. Some gastric abnormalities may be diagnosed on plain abdominal radiographs, without the need of contrast agents. These include gastric dilatation and torsion, some radiopaque foreign bodies, and rugal mineralization secondary to uremia or hyperadrenocorticism. Pyloric outflow obstructions may be suspected on the basis of visualization of a distended, fluid-filled, or gas and fluid-filled stomach. If plain films are not definitive, contrast studies, ultrasound, or endoscopy may be needed to confirm the diagnosis. Radiopaque foreign bodies such as bones or metallic objects usually are well visualized on plain films. Radiolucent foreign objects may be more difficult to visualize, especially if surrounded by gastric fluid. The use of both right and left lateral radiographs results in different distributions of air and fluid, and may better demonstrate intraluminal structures. Gastric foreign bodies tend to lodge in the pylorus. In these cases, foreign bodies may be better visualized on left lateral views, where the object is surrounded by pyloric gas. If plain films are non-diagnostic, a gastrogram may be helpful. Negative or double contrast studies cause the foreign body to be surrounded by radiolucent air, or a thin coating of positive contrast. Some foreign objects may cause a filling defect within a positive contrast filled stomach, but in some cases, may be obscured by a large amount of contrast. Repeating films after the stomach is partially empty may allow visualization of the foreign body, especially if it has an absorbent surface, such as cloth, and barium is retained within it. Survey radiology of the normal intestines The small intestines typically are distributed throughout the peritoneal space filling in the areas not occupied by other viscera. They are sometimes distributed more to the right side of the abdomen, and in obese animals, may appear centered within the mid-abdomen. The small intestines may be fluid filled, gas filled, or both. Fluid within a bowel loop can mimic bowel thickening; therefore bowel wall thickness is not reliably assessed on plain radiographs. The diameter of the small bowel is important to note on survey films, as intestinal obstruction is often suspected in vomiting patients. In the canine, normal small intestinal diameter should not exceed the height of the midbody of L2, or be greater than twice the width of a rib. The cat has a more uniform body size, so a diameter of 12 mm is fairly consistent for upper limits of bowel diameter. Another rule of thumb is a feline small intestine diameter of less than two times the height of the mid body of L4. The duodenum in both species may be slightly larger in diameter than the rest of the small bowel. Dilation of the small intestines can result from either a paralytic or an obstructive ileus. Because the etiology and treatment differ markedly, it is important to try to differentiate between the two. Ileus Ileus is a failure of intestinal contents to pass through the bowel lumen, and may be due to lack of peristalsis (paralytic, adynamic, or functional ileus), or due to obstruction (obstructive, mechanical, or dynamic ileus). Paralytic ileus results in normal or mildly dilated gas filled bowel loops, and is usually generalized. Peritonitis, post-op abdomen, pain, enteritis, systemic infection, metabolic change, anticholinergic drugs, hypokalemia, and spinal trauma have all been reported to result in paralytic ileus. Occasionally, focal paralytic ileus can occur with pancreatitis (usually affecting the descending duodenum) or thrombosis of segmental mesenteric artery. Obstructive ileus results in more dramatically dilated bowel loops, with only those segments orad (proximal) to the obstruction being affected. A low obstruction will cause more diffuse bowel dilation however, and must be differentiated from a paralytic ileus. A high obstruction, resulting from a duodenal lesion, may show no bowel dilation, as duodenal contents may be passed back into the stomach, and relieved by vomiting. Obstructive ileus may be due to foreign body, neoplasia, stricture, granuloma, hernia, adhesions, or intussusception. The actual cause of the obstruction, such as foreign body, may not be visualized radiographically, but surgery is indicated if an obstructive pattern is present. In some cases, parvoviral enteritis (a paralytic ileus) can mimic obstructive ileus, with generalized, more dramatic bowel dilation. Mesenteric volvulus may also result in generalized gaseous distension of bowel, resembling obstructive ileus. Chronic obstructions (partial) usually result in fluid distended bowel loops. A linear foreign body will cause an intestinal obstruction, but may have no visible bowel dilation. Typical survey radiographic signs of linear foreign body include plication of the small bowel, centralization of bowel, and eccentric, comma-shaped intraluminal gas bubbles. If perforation has occurred, free fluid and/or free gas may be visible radiographically. Infiltrative disease of the small intestines is difficult to visualize on survey abdominal radiographs unless a large mass is present, or there is severe spasticity/corrugation of the bowel wall. In these cases, or in any disease process not adequately diagnosed on survey films of the abdomen, a contrast series may provide valuable additional diagnostic information. GI contrast techniques An upper GI series is occasionally necessary for further diagnosis or evaluation of stomach or small intestine. Because of the time involvement and expense, the GI series should be done properly to get as much information as possible from the study. The choice of oral contrast material depends on the patient and suspected diagnosis. For routine studies where GI perforation is not suspected, barium is the agent of choice. Commercial preparations are preferred over the powdered agent which must be mixed with water. Most barium suspensions come as 60% w/v solutions. The choice of full strength or diluted with water to make a 30%w/v solution is up to the individual. Oral iodine products are available for use in patients in which a bowel perforation is suspected. These iodinated agents are hypertonic, and will draw fluid into the stomach and bowel after oral administration, resulting in dilution of contrast, and possible dehydration and electrolyte imbalance in the fragile patient. If aspirated, pulmonary edema may result. However, the iodinated agents are safe if free in the peritoneal space. They also traverse the bowel more rapidly than barium. Nonionic contrast agents (iopamidol, iohexol), typically used for myelograms, are very safe for upper GI series, as they are less hypertonic than the regular ionic iodinated contrast agents. They cause no reaction if leaked into the peritoneal space, and minimal reaction if aspirated. They also give a much better mucosal coating than the regular oral iodinated contrast agents. The disadvantage to the non-ionic contrast agents is their expense, which mostly limits their use to cats and small dogs.
Transit time Transit time in the dog varies, but barium contrast should begin to enter the duodenum within 15 to 30 minutes. Normal small bowel transit time is three to five hours. The stomach should be empty, or have only a minimal coating of contrast by the time that most of the contrast is in the colon. Transit time is faster in the cat, and contrast material may reach the colon in 30 minutes. Average transit time is one hour, although in some cats it takes three to four hours to reach the colon. When organic iodine contrast agents are used, transit times are more rapid. Contrast may reach the colon within an hour in dogs, and 30 minutes in the cat. These agents can be useful in quickly determining the patency of the bowel lumen. Abnormally shortened transit times may indicate an inflammatory condition of the bowel and have been associated with acute enteritis. Delayed transit times associated with normal bowel diameters are consistent with previous administration of anticholinergic drugs or insufficient contrast dose (insufficient volumes to stimulate peristaltic activity). If transit times are delayed in the face of dilated bowel lumen, an obstruction should be suspected. Use of contrast for gastric disease If plain films are non-diagnostic for a suspected foreign body, a gastrogram may be helpful. Negative or double contrast studies cause the foreign body to be surrounded by radiolucent air, or a thin coating of positive contrast. Some foreign objects may cause a filling defect within a positive contrast filled stomach, but in some cases, may be obscured by a large amount of contrast. Repeating films after the stomach is partially empty may allow visualization of the foreign body, especially if it has an absorbent surface, such as cloth, and barium is retained within it. Pyloric outflow obstruction may be caused by foreign bodies, antral hypertrophy, and infiltrative disease. Acute obstruction, typically due to foreign bodies, results in gastric distension primarily with air. With chronic obstruction, a persistently distended, fluid-filled stomach is present on plain abdominal radiographs. A positive contrast gastrogram can also be used to determine gastric emptying times. A significant delay in passage of contrast into the duodenum (greater than 30 minutes) indicates mechanical obstruction, or a functional problem (decreased gastric motility). Gastric neoplasia can occur in any portion of the stomach. Typically a mass lesion is present, protruding into the gastric lumen. These are rarely visualized on plain abdominal radiographs, but if large enough, may be seen as filling defects on positive contrast gastrograms. Gastric neoplasia may also be present as a more diffuse, infiltrative process. Circumferential narrowing of the stomach may be visualized resulting in a narrowed, irregular lumen on positive contrast studies. Non-distensible areas of the stomach may be seen when serial radiographs are made. In some cases, negative contrast gastrograms are sufficient to see gastric wall thickening of mass lesions protruding into the air filled stomach. Use of contrast for intestinal disease Each contrast-filled bowel segment should be evaluated for its mucosal appearance, wall thickness, diameter, and location within the abdomen. To be considered significant, a lesion should be visible on serial radiographs, as changes associated with normal peristalsis can mimic strictures or filling defects. Transient peristaltic contractions are normal and are especially prominent in the feline duodenum, giving rise to the "string of pearls" appearance. The contrast-mucosal interface should be smooth to slightly fimbriated, depending on the degree of small bowel distention. Smudging of this interface, along with flocculation of barium may indicate mucosal inflammation, edema, and increased mucus production and can be seen in patients with enteritis. This might be accompanied by a very rapid transit time, and increased segmentation of the bowel. Variations in lumen size and mucosal surface are seen with infiltrataive diseases such as neoplasia and severe enteritis. Positive contrast allows evaluation of the location and extent of mass lesions. Intraluminal masses are visible as lucent filling defects within the contrast column and can be pedunculated or sessile. Intramural masses vary from intraluminal protrusions to diffuse thickening and rigidity of the bowel wall. They can cause eccentric or annular narrowing of the bowel lumen, along with partial obstruction. Smooth masses tend to be benign in nature, while those with irregular mucosal surfaces are more likely to be malignant. The "apple core" sign is an irregular, annular thickening indicative of intramural neoplasia. The most common intestinal tumors reported are adenocarcinoma, leiomyosarcoma, leiomyoma, and lymphosarcoma. Lymphosarcoma tends to be multicentric, while other neoplastic processes are more localized. Other differentials for intramural lesions include granuloma, scar tissue and adhesions, abscess, and hematoma. While neoplastic masses can cause partial or complete intestinal obstruction, a more common cause is an obstructing foreign body. A radiopaque foreign body might be seen on survey radiographs, along with accompanying signs of obstruction such as intestinal dilation. However, in some cases, administration of contrast material is necessary to confirm the obstruction or to differentiate obstructive ileus from paralytic ileus. In animals with incomplete obstruction, the barium should outline the foreign body as it passes around the site. With complete obstruction, the contrast column stops abruptly at the point of obstruction. It is important to note again that if no foreign body is visible radiographically but a classic obstruction pattern is present, a contrast series is not necessary to locate or identify the foreign body. Surgery would be the logical next step. Ultrasound of the GI tract Ultrasound of the gastrointestinal tract has become an increasingly popular and useful diagnostic procedure for evaluation of GI obstructions, mass lesions, and infiltrative and inflammatory bowel diseases. Although the stomach and bowel may be incompletely evaluated because of intraluminal gas and resultant shadowing artifact, GI wall thickness and integrity of layers, peristaltic activity, and bowel contents can often be imaged. Normal gastric ultrasound anatomy When the stomach is gas filled, only the near wall is well visualized as reverberation artifact and shadowing obscure the lumen and far wall. However, wall layers and wall thickness in the near wall can be examined. In the non-distended stomach rugal folds may be prominent, and it is important to measure between the rugal folds for accurate assessment of gastric wall thickness. In the empty stomach, rugal folds produce a linear, striated appearance. In cross section, rugal folds may appear as spokes on a wheel. To evaluate the stomach using ultrasound, both longitudinal and cross sectional imaging planes should be used. The canine stomach lies horizontal to the long axis of the body so longitudinal imaging planes allow cross sectional images of the stomach. The transducer is placed at the xiphoid, angled caudal to the liver, and the stomach followed, from left sided, dorsal fundus region, to the right, more ventrally located pylorus. The pylorus will extend cranially to its junction with the duodenum in most dogs, and will need to be followed up beneath the rib cage. This is more difficult in deep chested dogs. After completing the cross sectional imaging of the stomach, the transducer is turned 90 degrees to evaluate in the longitudinal plane. This will be perpendicular to the spine. Similar techniques should be used in the cat, remembering that the stomach lies more parallel to the spine, and the pylorus lies closer to midline. Five individual layers are present in both the gastric and intestinal wall. From outside to inside these layers include: serosa: thin hyperechoic layer muscularis: thin hypoechoic layer submucosa: thin hyperechoic layer mucosa: prominent hypoechoic layer (typically the thickest layer) mucosal surface/lumen interface: hyperechoic layer in the center of the bowel These individual layers are best visualized with higher frequency transducers. Normal wall thicknesses have been established in the dog and cat for the stomach and various segments of intestine:
Four to five contractions/minute should be seen with normal stomach peristaltic activity, with 1-3 noted in the small intestine. Ultrasound appearance of the small intestines The small intestines can be seen throughout the abdomen, both end-on and longitudinally oriented. The duodenum has a slightly larger diameter than the rest of the small intestinal loops, and is the most lateral and ventral bowel loop in the right cranial abdomen. It can be located usually just ventral and lateral to the right kidney, and followed cranially into the pylorus. In the cat, the ileum has a distinctive cross-sectional appearance (resembling spokes on a wheel) and can be visualized as it enters the colon, just medial to the right kidney. The colon typically is gas filled, with poor visualization of the lumen. Abnormal gastric ultrasound Ultrasound may be used in place of contrast studies to locate or diagnose foreign objects, especially if the stomach is fluid filled. A curvilinear echogenic interface is typical of ball foreign bodies. Depending on the composition of the ball, shadowing or through transmission may be present deep to the rounded interface. Pyloric foreign bodies may also result in fluid distension of the stomach, enhancing their visualization. Ultrasound is particularly useful in the diagnosis of pyloric abnormalities. Frequently, the stomach is distended with fluid in these conditions, enhancing ultrasound visualization. Circumferential thickening of the pylorus is present with antral hypertrophy. Both lymphosarcoma and adenocarcinoma can cause diffuse or focal pyloric thickening with loss of the normal wall layers. Fine needle aspiration or tru-cut biopsy may be performed with ultrasound guidance to help differentiate benign from neoplastic lesions of the pylorus. Ultrasound is also helpful in determining gastric motility, as peristaltic contractions can be visualized and counted, helping in the differentiation between pyloric obstruction and motility dysfunction. Gastric masses may be well visualized on ultrasound examination, depending on the amount of air or fluid within the lumen. Lymphosarcoma has been reported to result in uniform hypoechoic thickening of the gastric wall, with either focal or diffuse involvement. Loss of the normal wall layers becomes apparent. There may also be focal decreased gastric motility. Regional lymph node enlargement often accompanies gastric lymphoma. Gastric carcinoma may show the same ultrasonographic features, along with a potential "pseudolayered" appearance. Pseudolayering refers to the appearance of a moderately echogenic zone surrounded by outer and inner less echogenic lines within the thickened gastric wall. Gastric leiomyomas may appear as small, focal, mass lesions that are often incidental findings. Gastric inflammatory disease may show no changes both radiographially and on ultrasound examination. If severe, it can be difficult to differentiate inflammatory disease from neoplasia. Inflammation results in varying degrees of gastric wall thickening, and may be diffuse or focal. Gastritis can result in increased width of rugae with a narrow interrugal space. On ultrasound examination in these cases, wall thickening with loss of normal wall layers may be present. Ultrasound guided fine needle aspiration of tru-cut biopsy of the gastric lesion or enlarged regional lymph nodes may result in a more definitive diagnosis. Abnormal intestinal ultrasound Intestinal obstruction may be suspected when bowel loops show significant dilation, either by gas or fluid. Peristaltic activity is usually increased in the acute obstruction, but may be decreased with chronic obstruction or functional (paralytic) ileus. Foreign bodies occasionally can be visualized, depending on their physical properties. A curvilinear shadowing in the intestine, as in the stomach, is suggestive of a ball foreign body, but could also represent gas or ingesta. Rocks, needles, and some balls attenuate the sound beam, resulting in a reflective near surface and acoustic shadowing. Linear foreign bodies may result in visible bowel plication, with the presence of an echogenic linear structure extending through the affected bowel segment. The presence of intraluminal fluid enhances foreign body visualization. Intussusceptions can usually be reliably diagnosed on ultrasound examination by their characteristic "target", or "ring" appearance. The presence of a bowel within a bowel results in multilayered concentric bowel wall layers with a central echogenicity. (in cross section). Again, the bowel proximal to the intussusception is often dilated. Increased thickness of gastric or intestinal wall, and/or loss of the normal layered appearance is the most common ultrasound sign of infiltrative disease, and should be carefully evaluated. Neoplasia may result in either a localized mass, or a diffuse wall thickening, as noted earlier for the stomach. Moderate to severe wall thickening, with loss of layers, is the most common appearance. Regional lymphadenopathy may also be present. Inflammatory diseases may cause a less severe wall thickening, typicallly with preservation of wall layers. As mentioned earlier, fine needle aspirate or tru-cut biopsy of the affected area is usually necessary for a definitive diagnosis. Imaging Techniques for the Pancreas
Martha Moon Larson, DVM, MS, Dipl.ACVR Introduction Pancreatitis is a common consideration in dogs and in an increasing number of cats presented for vomiting, anorexia, lethargy, or abdominal pain. The disease however, is difficult to diagnose definitively. Clinicopathologic data, including amylase and lipase values are used routinely when pancreatitis is suspected. However, they may be normal, or elevated from other disease processes. Newer tests, including canine and feline pancreatic lipase immunoreactivity may be more sensitive, but do not appear to be the gold standard for the diagnosis of pancreatitis, especially in the cat. Imaging techniques have become an essential part in the workup of patients suspected of having pancreatitis. Radiology of the pancreas Both acute and chronic pancreatitis can result in changes visible on survey abdominal radiographs. Potential radiographic signs include: any of the following:
Computed tomography is a sensitive means of making a diagnosis of pancreatis or pancreatic neoplasia in people, but has more limited availability, is expensive, and usually requires general anesthesia in veterinary medicine. Ultrasound, therefore, has become the imaging modality of choice in the diagnosis of pancreatic disease. Ultrasound examination of the pancreas can be difficult, however. The pancreas, unlike the liver or spleen, is often not seen as a discrete organ, and therefore the pancreatic region and its anatomic landmarks must be examined. The pancreas is surrounded by gas filled structures, including the duodenum, stomach, and colon, which may obscure visualiztion. Finally, abdominal pain in patients with pancreatitis may prevent the firm pressure on the transducer needed to visualize the pancreatic area. Ultrasound of the pancreas Since the canine pancreas is frequently not visualized as a discrete organ, the sonographer must be very familiar with the anatomy and anatomic landmarks used to identify the pancreatic region. The use of a 7 MHz transducer will aid in visualization of the pancreatic tissue. The pancreas is divided into left and right lobes which are joined together at the pancreatic body. The right pancreatic lobe lies dorsomedial to the duodenum, ventral to the right kidney, lateral to the portal vein, and dorsomedial to the cecum. The cranial and caudal pancreaticoduodenal veins course through the right lobe, parallel to the descending duodenum. The right lobe may be imaged by scanning lngitudinally along the right lateral abdomen, using the descending duodenum and right kidney as landmarks. The descending duodenum can be visualized lateral to the right kidney, as the largest diameter small intestinal loop, and one which follows the straightest course longitudinally down the body wall. The right lobe of the pancreas lies dorsomedial to the descending duodenum. The pancreaticoduodenal vein is frequently visualized extending parallel to the descending duodenum. The descending duodenum should be followed caudally until it turns medially and becomes the ascending duodenum. Alternatively, the duodenum can be followed cranially in cross section, visualizing the pancreatic area medially and dorsally. The pancreaticoduodenal vein is often well visualized in these cross sectional images. In large or deep chested dogs, a lateral intercostal approach may be needed. Using a right lateral transverse view through the 10-12th intercostal space, the right lobe will be located ventral to the right kidney, ventrolateral to the portal vein, and dorsomedial or dorsal to the descending duodenum. The pancreatic body joins the two lobes and is located immediately ventral to the portal vein and cranioventral to the right kidney. The body lies caudal to the pyloric region. The left pancreatic lobe may be more difficult to visualize due to surrounding gas-filled structures (stomach and colon). It may be located in a triangular region cranial to the left kidney, caudal to the stomach, and medial to the spleen in the left cranial abdomen. It lies caudo-dorsal to the stomach and craniodorsal to the transverse colon. In the cat, the body and left limb of the pancreas may be routinely visualized, especially if using higher frequency transducers (8-10 MHz). The transducer is placed slightly obliquely in the right cranial abdomen, until the portal vein (longitudinal view) is located entering into the liver hilus. The portal vein can be followed slightly caudally, and the feline pancreas may then be located just dorsal to the pylorus and body of the stomach, and ventral to the portal vein. Normal appearance The normal pancreas may on occasion be visualized, especially when using high frequency (7-10 Mhz) transducers, or in patients with hepatomegaly. The enlarged liver pushes abdominal viscera caudally, away from the rib cage, making organs more easily visible. In the dog, the normal pancreas appears slightly more echogenic than the liver, and slightly less than the spleen. The pancreaticoduodenal vein can be seen as a undulating vessel running parallel to the duodenum, in the right limb. In the cat, the pancreatic duct is prominent, and is located centrally within the body of the pancreas. The feline pancreas is typically hypoechoic to surrounding fat (similar to hepatic echogenicity), and well marginated. The right limb of the feline pancreas is small and difficult to routinely visualize. Abnormal appearance Pancreatitis In the dog the inflammation, edema, necrosis, and hemorrhage associated with acute pancreatitis results in an enlarged, irregular, hypoechoic pancreas. The hypoechoic areas may be diffuse, or patchy. They are typically surrounded with hyperechoic saponified fat. Other associated changes occur in the descending duodenum, which can appear thickened, dilated and fluid-filled (ileus), or corrugated secondary to duodenitis. Pancreatitis is the most common cause of extrahepatic biliary obstruction, and can result in dilation of the gall bladder and common bile duct. Eventually, after 5-7 days of complete obstruction, the intrahepatic bile ducts will also dilate. Focal peritoneal effusion often accompanies acute pancreatitis. The right lobe seems to be the most commonly affected portion of the pancreas with acute disease, but this may simply be because it is more easily imaged than the left lobe. Chronic, low grade, or resolving pancreatitis appears as a slightly more echogenic pancreas with better defined, smooth borders. Other associated changes such as hyperechoic peripancreatic fat, duodenitis, and abdominal effusion usually are absent. In the cat, ultrasound changes of pancreatitis are less well defined. Acute pancreatitis has been reported to cause an irregular pancreatic outline, hypoechoic changes to the parenchyma, and hyperechoic peripancreatic mesentery. However, it appears that ultrasound changes of pancreatitis, both acute and chronic, are non-specific in the cat, and may be insensitive. Pancreatic pseudocyst Pancreatic pseudocysts are focal collections of pancreatic enzymes, blood, and products of tissue digestion within the pancreas. Eventually, a thick fibrous or granulation tissue capsule forms around the fluid collection. Pseudocysts are associated with acute pancreatitis, but require several weeks to develop. If small (less than 4 cm), they may resolve on their own. Larger or persistent (more than 6 weeks) pseudocysts require surgical drainage or removal. On ultrasound examination, they appear generally hypoechoic or anechoic, may have acoustic enhancement, and usually contain echogenic debris. Often, they are surrounded by a thick wall. It is difficult to differentiate a pancreatic pseudocyst from a pancreatic abscess on ultrasound exam alone. A needle aspirate may be required for a definitive diagnosis. One report of percutaneous ultrasound guided drainage of a pancreatic pseudocyst resulted in gradual resolution of the cyst. Complications of a persistent untreated pseudocyst include secondary infections, rupture, and hemorrhage. Pancreatic abscess Abscesses are collections of purulent material and necrotic tissue within the pancreas or extending into adjacent tissues. Pancreatic abscesses develop from secondary infection of necrotic pancreatic tissue, and are a serious complication of acute pancreatitis. They are often fatal unless there is surgical intervention. It is difficult to differentiate between pancreatic abscesses, pseudocysts, and even neoplastic disease. Abscesses may appear as ill defined mass lesions within the pancreas, containing both hypoechoic and hyperechoic areas. A thick or poorly defined wall may surround the abscess. These changes are similar to pseudocysts and tumors, so a needle aspirate or surgical biopsy is needed for a definitive diagnosis. Pancreatic neoplasia Insulinomas (islet cell tumors) are suspected in some animals with hypoglycemia. These tumors generally are small (about 1 cm), focal, hypoechoic nodules which may be located in any part of the pancreas. Because they are easily missed, a negative ultrasound exam should not rule out insulinomas. Pancreatic carcinomas are a differential for any mass lesion within the pancreas. However, ultrasound cannot reliably differentiate neplasia from pancreatitis, pseudocyst, or abscess. There may be a localized change in echogenicity, with an irregular border. Secondary changes may include metastasis to the liver or regional lymph nodes, and possibly biliary obstruction. Imaging Of The Central Nervous System:
Radiology Versus Alternate Imaging J. C. Jones Introduction For most animals with suspected CNS disease, conventional diagnostic radiography is of limited value. The main reasons for low diagnostic sensitivity include superimposition of overlying structures, insufficient contrast resolution, and silhouetting by adjacent tissue or fluid of similar density. Advanced imaging techniques such as computed tomography and magnetic resonance imaging are much more sensitive for detecting intracranial disease. For this reason, they are the preferred initial examination techniques for suspected CNS disease in humans. 1 As advanced imaging techniques become increasingly available and less expensive, they are rapidly becoming standard diagnostic tools for small animals as well.2-16 Conventional Diagnostic Radiography Basic principles and techniques The standard radiographic views for the calvarium include lateral and dorsoventral projections.17,18 19 Brachycephalic breeds may require more penetration than mesatocephalic or dolichocephalic breeds. The dorsal margin of the calvarium and the foramen magnum may be demonstrated in a 24-40 degree, closed mouth, rostrocaudal oblique projection. Diagnostic sensitivity of skull radiographs is maximized by the use of good radiography equipment, film/screen combinations, technique charts, film processing techniques, and patient positioning. 20 Chemical restraint (heavy sedation or general anesthesia) is highly recommended to insure accurate positioning. 18,21,22 Radiolucent positioning sponges, tape, or gauze also help to achieve symmetrical positioning and minimize personnel exposure. The use of a small focal spot and large object-film distance (air gap) may be used to help magnify smaller structures of interest. 23,24 The areas of clinical concern should be centered in the x-ray beam in order to minimize geometric distortion and maximize spatial resolution. Normal findings The brain is normally not visible in plain radiographs, due to superimposition of overlying bone. The calvarium is smoothly marginated, with swirling convolutions on the dorsal and lateral surfaces. The cribriform plate is visible as a rostrally convex, curvilinear, bone opacity between the calvarium and the caudal nasal cavity. Open fontanelles may be visible in the dorsal calvarium in normal immature animals. Small breed dogs and cats have a more domed shape to the dorsal calvarium, and frontal sinuses may be poorly developed. The walls of the calvarium also appear thinner than those seen in large breed dogs. Clinical applications In plain radiographs, diseases of the brain may sometimes be inferred from secondary changes in the skull. Calvarial enlargement, increased doming, or thinning of the bony wall are radiographic signs of congenital or acquired hydrocephalus. 25 There may also be decreased visualization of the normal convolutions. The severity of distortion depends on the rate of fluid accumulation, severity of ventricular enlargement, and the stage of ossification at the onset of disease. Enlargement of the foramen magnum is a characteristic of occipital dysplasia.26-28 Some authors theorize this malformation may cause an increased risk for intermittent tentorial herniation. Signs may include occipito-cervical pain, personality changes, scratching of one ear, protrusion of the tongue, dysphagia, ataxia, or convulsions. Other authors consider the malformation to be an incidental finding, especially in the Pekingese. Neoplasms of the calvarium may cause invasion or compression of adjacent brain tissue. Lesions may be either osteolytic or osteoblastic. Most osteosarcomas of the cranial vault are osteoblastic, with regular well defined borders and are characterized by evenly distributed granular calcific densities.29 Multilobular tumors of bone appear as lobulated, mixed soft tissue and bone opacity masses, that may or may not be locally invasive.30 Synonyms include osteochondroma, osteochondrosarcoma, or chondroma rodens. In some cats, intracranial meningioma may be visible as a focal calcification or thickening of the calvarium.31 Skull fractures may be associated with brain compression due to displaced fragments, or hematomas. Fractures are visible as radiolucent lines within the calvarium, that may or may not be associated with normal suture lines. 32 A step defect is visible when there is malalignment of the fracture fragments. Intracranial gas may be seen if there is a communication with the skin surface, nasal cavity or paranasal sinuses. Computed Tomography Basic principles and techniques Computed tomography (CT) is a digital imaging technique that uses xray energy and computer processing to make cross-sectional (transverse) images of structures. 33-35The xray tube is housed within a ring-shaped structure called the gantry. A motorized table advances the patient through the gantry for each slice. Slices are made when the xray tube rotates in a circle around the patient. The energy of transmitted xrays are recorded opposite the patient by detectors. Detectors convert the xray energy to an electrical signal. Each slice is divided into a matrix of cubes (voxels). A computer converts the electrical signal associated with each cube of tissue into numerical (digital) data. These data are referred to as CT numbers. They are units of density relative to water and are expressed in Hounsfield units (HU). Mean CT numbers are calculated for each cube of tissue and displayed as grey-scale picture elements (pixels) on the viewing monitor. White is assigned to pixels with higher CT numbers (ex) bone. Varying shades of grey are assigned to pixels with intermediate CT numbers (ex) soft tissues, fat and fluid. Black is assigned to pixels with lower CT numbers (ex) lung, air-filled organs. The higher the number of pixels per unit volume of tissue, the higher the spatial resolution. The main advantages of CT over radiography are the ability to detect more subtle tissue density differences than radiography, elimination of superimposition, the ability to adjust image data as needed to improve visualization of structures. Operators can adjust the contrast (window width) and brightness (window level) of images as needed to better see tissues of interest. Bony structures are usually viewed at window widths greater than +500. Soft tissue structures are usually viewed at window widths less than +500. CT scanners are classified in generations, with the numbers based primarily on technologic advancements in x-ray tube movement and detector design. 3rd generation scanners are configured such that the x-ray tube and arc of detectors rotate together around the patient for each slice. 4th generation scanners have an x-ray tube that rotates around a stationary ring of detectors. Spiral (helical) CT scanners are the newest technology. With spiral scanning, the table moves continuously while x-ray tube is rotating around patient. This allows acquisition of all the volume data at one time, so that slice thickness can be altered retrospectively as needed. In spiral scanning, the table speed can be adjusted (pitch). The slower the table speed, the more samples are obtained per unit of tissue and the higher the image resolution. Extremely fast examination times are also possible (ex) 30 seconds for a brain scan, but yield slightly lower image resolution . A motorized patient table supports the patient in the center of the ring-shaped gantry opening. The maximum weight limit for most tables is 300-400 pounds. The table is incrementally advanced by the CT computer. This controls the slice thickness and intervals. The gantry houses the x-ray tube and detectors. The gantry can be tilted as needed to adjust angulation of the slices through the anatomic region of interest. A collimator, positioned between the x-ray tube and the patient, adjusts the thickness of beam. The detectors are positioned opposite the x-ray tube, so that they can record the amount of x-ray energy passing through the patient. The operator console of the CT computer controls the technique settings (kVp, mAs), slice thickness/interval, size of the area to be scanned (field size), size of the area to be displayed (image size), and the number of scans per slice. The CT computer processor creates the image from the numerical data. It also may be used to reformat the data in a set of CT images in order to view structures in the sagittal, dorsal, or oblique planes. Advanced computer processing techniques also allow three-dimensional reconstructions and selective color displays. Images are most commonly stored on x-ray film, using either a multiformat or laser camera. Digital image data are temporarily stored on the computer hard drive, then archived on tape cartridges or optical discs. The most common CT artifacts are streak and partial volume artifacts. Streak artifacts appear as white or black lines that go across the CT image. Most are caused by errors in computer interpretation. Common kinds of streak artifacts include patient motion, density change, beam-hardening, and field of view. Patient motion causes parallel, blurred, white streaks in images. The streaks are oriented parallel to the direction of motion. Density change artifacts appear as bright white, sharp lines that radiate outward from a high density object (ex) EKG lead, gunshot, bone plates. Beam hardening artifacts appear as black, blurred streaks across soft tissues adjacent to dense bone. They are caused when dense bone differentially absorbs the lower energy portion of x-ray beam (ex) cerebellum/brainstem. Field-of-view artifacts appear as parallel, sharply marginated, white lines across whole image. They are usually caused by a body part or wire being postioned outside of the scanner's field of view. Partial volume artifacts appear as a false area of increased or decreased opacity in the image. They are caused by a voxel/pixel translation problem. The displayed greyscale is determined from an average density of tissues within a given slice. If high density and low density tissues are adjacent to each other and included in the same slice, the computer averages their density and displays the greyscale accordingly. Partial volume artifacts can be differentiated by looking at adjacent slices or re-scanning the area of concern using thinner slices. Other image quality factors include patient positioning, targeting, slice thickness, and scan speed.36 It is important to make sure anatomic region of interest is oriented perpendicular to the slice plane. Oblique positioning may cause a false positive diagnosis of anatomic assymmetry. Targeting is performed by choosing an image size that is limited to the region of interest (ex) spine and paraspinal region. This allows the computer to enlarge the image and assign smaller pixels per unit area. Patients are placed under routine general anesthesia so that accurate positioning can be maximized and motion artifacts minimized. For head imaging, we prefer sternal recumbency. The head is positioned within an extension cradle and adjusted as needed with foam sponges. The nose is slightly elevated such that the hard palate is parallel to the table surface. The endotracheal tube is taped to the extension cradle to avoid changes in head position as the table moves. Saphenous vein catheterization is preferred, but access to a cephalic vein catheter may be facilitated by positioning the forelimbs caudally. Lateral and ventrodorsal digital radiographs (pilot, scout image) of the region of interest is obtained with the CT scanner. Positioning is adjusted as needed and radiographs repeated. Transverse slices are posted on the final radiographs, with the cribriform plate as the first slice and the foramen magnum as the last slice. We use slice thicknesses of 2 mm for cats and small dogs, 4 mm for medium dogs, and 5-8 mm for large dogs. Image sizes range from 120 to 240 mm. Survey scans are first examined, then the scan is repeated immediately following a rapid intravenous injection of iodinated contrast medium at a dose of 400 mg I/kg. Normal findings For a detailed identification of individual anatomic structures, the reader is referred to several brain CT atlases. 4,7,37-39 In general, all normal paired structures should be symmetrical. Bony structures should be smoothly marginated and well-defined. Cortical bone appears bright white and medullary bone exhibits varying shades of grey. The tentorium cerebelli may be calcified in some normal dogs. 40 Soft tissue structures are usually homogenous, with some variation in shades of grey caused by slight differences in tissue density. To a limited extent, white matter can be distinguished from grey matter by a slightly lower density. The ventricles of the brain appear slightly darker grey than brain parenchyma (hypodense), because the cerebrospinal fluid is approximately 2% less dense than brain tissue.37 The fourth ventricle and its communication with the cerebellomedullary cistern heps distinguish the cerebellum from the medulla. The position of the thalamus and interthalamic adhesion can be inferred from the relationship between the lateral and third ventricles. The intercrural cistern helps delineate the region of the pituitary gland. Bony landmarks such as the dorsum sellae, hypophyseal fossa, and rostral clinoid process help in identifying the anatomic structures not distinguishable by their tissue density alone. After administration of intravenous contrast medium, there should be no focal enhancement within the normal brain parenchyma. The exception to this rule is the pituitary gland. This structure may enhance in normal animals, because there is no blood-brain barrier. Venous structures of the normal brain (intracranial venous sinuses, parenchymal veins, choroid plexus, falx cerebri) may enhance after administration of contrast material. Clinical applications The general CT characteristics of brain disease include a visible mass; change in ventricular size, shape or position; deviation of the falx cerebri (falx shift), and focal change in brain opacity.5,8,9,16,36,41,42 11,15 CT is more sensitive than MRI for acute hemorrhage, soft tissue calcification, and intracranial gas. CT is less sensitive than MRI for edema, infarcts, low grade masses, and caudal fossa masses. Administration of iodinated contrast medium intravenously helps improve visibility of many brain lesions. Contrast is administered using a rapid bolus injection of 400 mgI/lb. Focal accumulation of contrast medium in the brain parenchyma is a sensitive but not specific indicator of brain disease. Enhancement occurs in locations where there are venous sinuses, disruption of the blood brain barrier, damaged blood vessels, or malformed vessels (neovascularization). Because CT characteristics of brain lesions are not specific, cerebrospinal fluid analysis and brain biopsy are needed for a definitive diagnosis. New devices for minimally invasive, stereotactic, CT-guided biopsy of canine brain lesions have recently been developed. 43-46 New software features also allow CT dose planning for radiation therapy of intracranial masses.8,47 Common characteristics of intracranial neoplasms in dogs and cats have been documented, but some overlap exists.8,9,11 Meningiomas are usually peripherally located (extra-axial), broad-based at the edge of the brain or on the midline, markedly enhancing, large at the onset of clinical signs, and exhibit a "dural tail". A dural tail is a region of linear enhancement that is associated with thickening of the dura mater adjacent to the mass. Meningiomas may also contain focal calcifications or be associated with bone remodelling. Gliomas tend to be centrally located (intra-axial), peripherally enhancing (ring enhancement), and surrounded by a zone of edema. Choroid plexus papillomas are often located either within or adjacent to a ventricle, appear hyperdense relative to surrounding brain tissue, exhibit marked enhancement, and are associated with hydrocephalus. Pituitary macroadenomas and adenocarcinomas are most commonly located in the mid-ventral fossa of the cranial vault, displace the 3rd ventricle dorsally, enhance uniformly, and may exhibit a "mushroom cloud" shape. New spiral CT techniques show promise for differentiating pituitary microadenomas based on changes in pituitary perfusion.48 Metastatic neoplasia more commonly appears as multifocal regions of contrast enhancement, that may or may not be associated with ventricular displacement. Hydrocephalus is evident as generalized or localized ventricular enlargement. Localized enlargement is more likely to be obstructive. Generalized enlargement is more likely to be nonobstructive. Assymmetry of the lateral ventricles may be indicative of obstructive hydrocephalus, but this finding has also been reported as a normal anatomic variant in some breeds. Edema may be visible as patchy areas of decreased opacity that are non-enhancing. Hemorrhage varies in opacity, depending on the duration.14 Acute hemorrhage (24-72 hrs) appears as a region of increased opacity. Chronic (>72 hrs)hemorrhage usually exhibits a decreased opacity. Patchy regions of edema and increased meningeal enhancement may also be seen with inflammatory brain disease. Abscesses and chronic hematomas may mimic gliomas, in that they are often centrally located and ring-enhancing.49 Inflammatory brain disease may mimic neoplasia in appearance.15 There may be solitary or multifocal regions of contrast enhancement. Central vestibular disease may be underdiagnosed in CT images, due to beam hardening artifacts in the caudal fossa. Magnetic Resonance Imaging Basic principles and techniques Magnetic resonance imaging (MRI) is an imaging technique that uses a strong magnetic field and pulses of radiofrequency energy to cause tissues to emit characteristic energy signals. 33 A motorized table centers the patient in a tube-shaped or open gantry in which there is a constant strong magnetic field. While inside the gantry, hydrogen atoms within the patient's tissues align themselves with the magnetic field. Tissues are intermittently exposed to brief pulses of radiofrequency energy to temporarily knock the hydrogen atoms out of alignment. A weak energy signal (resonance) is released from the tissues as the hydrogen atoms realign themselves with the magnetic field. A receiver coil is placed near the anatomic region of interest to record the signal coming from the tissues. The strength of the returning signal varies based on multiple factors: inherent tissue factors, concentration of hydrogen atoms, interactions of the atoms with each other, strength of the magnetic field, technique settings assigned by the computer operator, duration of each radio-pulse, frequency of radio pulses (repetition time or TR), and how long the signal is recorded by the receiver coil after the pulse occurs (echo time or TE). The MRI computer converts the signal intensity to varying shades of grey in the image. Tissues with higher signal intensity are assigned whiter colors. Those with lower signal intensities are assigned darker grey colors. Tissues having no signal appear black. MRI system components include a magnet, receiver coil, computer station, and gradient coils. The magnet maintains a strong external magnetic field around the patient. The magnetic strength is measured in Tesla units (1 Tesla = 10,000 X earth's magnetic field). Three ranges of magnetic field strength are available for medical MRI scanners: 1) low field = <0.5 Tesla, 2) mid field = 0.5-1.0 Tesla, and 3) high field = > 1.0 Tesla. The two most common types of magnet construction are superconducting or permanent. Superconducting magnets are made using coils of electrical wires that are cooled with liquid helium or nitrogen. Permanent magnets consist of magnetic discs, usually made of iron. The receiver coil detects the electromagnetic signals being emitted by the tissues. Receiver coils are available in different sizes and shapes, so they can be as close to the area of interest as possible. This helps maximize the signal to noise ratio and improve image quality. The computer station controls the technical parameters and radiofrequency pulse sequences. The plane of scanning can be altered without moving the patient by the use of gradient coils. These gradient coils cause slight changes in the main magnetic field, that are used as localization tools by the MRI computer. Numerous radiofrequency pulse sequences have been designed in order to improve visualization of specific tissues of interest. The possibilities are nearly endless. However, the most commonly used pulse sequence is the spin-echo technique. This involves the use of a 90 degree radiofrequency pulse followed by a 180 degree radiofrequency pulse. This technique uses characteristics called T1 and T2 relaxation times to help distinguish individual tissues. T1-weighted images are created when short TE and short TR intervals are used in a spin echo pulse sequence (ex) 20-35 ms, 300-500 ms respectively. Tissues that have high signal intensity (appear bright white) in T1-weighted images are those that have short T1 relaxation times. Such tissues include fat, gadolinium contrast medium, and proteinaceous fluid. Tissues that appear dark on T1-weighted images are those that have long T1 relaxation times. These include all other fluids, edema, air, bone, and fast-flowing blood. T2-weighted images are created using long TE and long TR intervals (ex) 75-150 ms, 1500-2500 ms respectively. Tissues with high signal intensity in T2-weighted images are those that have long T2 relaxation times. These include fluid and edema. Tissues that appear dark on T2-weighted images are those that have short T2 relaxation times. These include soft tissue, air, bone, and fast-flowing blood. Other pulse sequences used for small animal MRI may include fluid-attenuated inversion recovery (FLAIR), diffusion-weighted, magnetization transfer, and fat-saturation techniques. Future developments of contrast material for MR imaging include non-gadolinium compounds, intrathecal contrast media, cerebral blood flow and volume evaluation, and, possibly, antibody-labeled contrast agents. 50 Common MRI artifacts include motion, ferromagnetic, signal void, and signal drop-off. Motion appears as blurred streaks that run perpendicular to the direction of motion. They are present in all images obtained during a given pulse sequence. Ferromagnetic artifacts are caused by such objects as gunshot fragments or pellets, vascular clamps, skin staples, intravenous catheter needles, or orthopedic fixation devices. These artifacts appear as a large black void that surrounds the metallic object. The void may obscure all adjacent structures or distort their shape. High field strength magnets may also cause metallic objects to move or heat up during scanning. A signal void artifact is caused by fast-moving blood within a vessel. The protons that are knocked out of alignment by the radiofrequency pulse move out of the scan field before they can release their resonating signal. Signal drop-off artifact occurs at the edges of the receiver coil. As the signal to noise ratio drops, the image becomes increasingly dark and grainy in appearance. The main advantages of MRI versus CT include 1) no beam hardening artifacts, 2) higher sensitivity for subtle changes in soft tissue chemical properties, 3) the ability to acquire images in any plane desired, and 4) absence of ionizing radiation. For example, MRI is much more sensitive than CT for early infarcts and edema. The main disadvantages of MRI vs. CT include 1) higher cost, 2) shifting or heating up of metallic implants, 3) severe distortion artifacts caused by metallic objects in the scan field, 4) more sensitive to motion artifacts and 5) less sensitive for soft tissue calcification or bone proliferation. High field magnets cannot be used for patients or personnel in early pregnancy or with cardiac pacemakers. The intravenous contrast agent most commonly used is gadolinium. This is a paramagnetic substance that causes adjacent hydrogen nuclei to relax more quickly. Tissues accumulating gadolinium have shorter T1 times and therefore appear bright in T1-weighted images. Causes of focal enhancement are similar to those previously described for CT. Recent advances in MR scanner technology now allow evaluation of intracranial vessels without the use of intravenous contrast material. The procedure is termed magnetic resonance angiography, because it resembles a conventional angiogram in appearance.51 With this technique, signals from soft tissues are dampened by rapidly pulsing them with radiofrequency energy. The blood entering the slice of interest therefore emits the strongest signal and appears bright white. Normal findings For a detailed identification of anatomic structures, the reader is referred to several brain MRI atlases.4,52 53 In general, all normal paired structures should be symmetrical. There should be no focal contrast enhancement (with the exception of the pituitary gland, veins, and sometimes the choroid plexus). T1-weighted images yield the best spatial resolution and morphologic detail for soft tissues overall. 52 Cerebrospinal fluid within the ventricles and subarachnoid spaces exhibits very low signal intensity and appears dark grey or black. Ventricles are normally well-visualized in dogs, but may be more difficult to appreciate in cats. 53 Ventricular assymmetry may be present as a normal anatomic variant, especially in beagles and labrador retrievers. 54,55 Fat in the bone marrow of the skull, subcutaneous tissue, and fascial planes has high signal intensity and appears bright white. Proton-density weighted images yield an image very similar to T1-weighted images, with improved contrast resolution between grey matter and white matter. White matter has slightly lower signal intensity than grey matter. T2-weighted images are of overall lower signal intensity compared to other pulse sequences and yield darker images. The spatial resolution is also decreased, with a more grainy appearance. This technique provides the best contrast resolution between grey and white matter of the brain. Also, cerebrospinal fluid within the ventricles and subarachnoid spaces appears bright white. In all pulse sequences, arteries and veins with fast-moving blood exhibit low signal intensity because of signal void artifacts. Cortical bone also appears dark black in all pulse sequences because the protons are so rigidly bound they cannot move out of alignment when pulsed. Air within the tympanic bullae, nasal cavities, frontal sinuses, and nasopharynx also appears black in all pulse sequences, due to the low concentration of hydrogen protons. Clinical applications Common veterinary applications for head MRI are similar to those for head CT: suspected intracranial neoplasia, non-neoplastic brain lesions, or central vestibular disease. 9,14,15,56 Also similar to CT, typical MRI characteristics of common brain neoplasms have been established and there are some exceptions to the rules. A definitive diagnosis still requires a biopsy. Meningiomas usually have an extra-axial location, are broad-based and appear sharply marginated. They are isointense in pre-contrast T1 weighted images, hyperintense in T2-weighted images, and uniformly enhancing. Choroid plexus adenomas are most commonly found in periventricular or cerebellopontine locations. They are solitary, and often associated with hydrocephalus. They appear isointense in pre-contrast T1 weighted images, hyperdense in T2-weighted images and are uniformly enhancing. Gliomas typically have an intra-axial location. Ependymomas and oligodendrogliomas are often periventricular. Medulloblastomas are often in the cerebellum. Gliomas tend to be hypointense in pre-contrast T1 weighted images, and hyperintense in T2 weighted images. They exhibit variable enhancement. Low grade gliomas may not enhance at all. High grade gliomas may have cavitary areas and marked peritumoral edema. Pituitary adenomas are in the suprasellar region and are usually fairly sharply marginated. They appear isointense in pre-contrast T1 weighted images and are often uniformly enhancing. Characteristics of nonobstructive and obstructive hydrocephalus are similar to those described for CT. With MRI, periventricular edema may be easier to see as an indicator of possible acute hydrocephalus or inflammation. Periventricular edema appears as a zone of increased T2 signal intensity that surrounds the ventricles. Obstructive hydrocephalus secondary to a Chiari malformation may also be more readily identified with MRI. The most common form is the Chiari I malformation, which is characterized by caudal displacement of a portion of the cerebellum through the foramen magnum. Syringohydromyelia of the cervical spine may also be present. 57 Intraparenchymal hemorrhage varies in signal intensity, based on the stage of hemoglobin breakdown.14,58 Within the first few hours, a hematoma will usually be T1-hypointense and T2-hyperintense. In the first few days, the T1 signal may vary from hypointense to hyperintense while the T2 intensity remains hypointense. As hemolysis occurs over the next few weeks, the T1 intensity ranges from hyperintense to hypointense, while the T2 signal becomes more consistently hyperintense. Non-hemorrhagic Infarcts are usually visible with MRI earlier than with CT. Initially, the infarct may appear hyperintense in T2 and proton-weighted images. Parenchymal enhancement is uncommon within the first few days. After several weeks, the infarct decreases in size. There is often focal atrophy of the adjacent brain tissues, with dilation of nearby sulci and ventricles. In experimental canine studies, MRI was found to be more sensitive than CT for identification of early meningitis. 59 Post-gadolinium T1-weighted images demonstrated increased leptomeningeal enhancement earlier than enhancement was seen with CT. Early intraparenchymal edema from encephalitis may be seen as ill-defined regions of increased T2 signal intensity, primarily within the white matter. Encephalitis may also be associated with ventricular assymmetry. Ring-enhancing abscesses may mimic gliomas. Multifocal, contrast-enhancing granulomas may mimic metastatic neoplasia. Vascular disorders may also mimic a solitary brain neoplasm. Magnetic resonance angiography can be used to depict aneurysms, malformations, occlusive disease and fistulas noninvasively. 51 MRI is much more sensitive than CT for identifying central vestibular disease, primarily due to the absence of beam hardening artifacts in the brainstem.56,60 References:
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