March 2004

Radiology

Anne Bahr, DVM, MS, DACVR

College of Veterinary Medicine, Texas A&M University



Diagnostic Imaging of the Thorax in Small Animals

Basic Interpretation of Thoracic Radiographs

A brief overview of normally visualized structures on thoracic radiographs is necessary to provide a foundation for future discussions. The main reason that structures are visualized on the radiograph is due to differentiating opacities (i.e. air Vs soft tissue).

Lateral view
On either the left lateral or right lateral view, it is expected to see the heart, and at least one set of the cranial lobar vessels (order is Artery, Bronchus, Vein from dorsal to ventral). In some animals, the vessels supplying the lung overlying the heart (right middle or caudal subsegment of the left caudal) can be easily seen. The caudal pulmonary vessels generally are overlapping and often are hard to distinguish as separate, discrete structures. However, the tertiary vessels extending into the caudal lobar lung fields should be easily seen, particularly in cats. The trachea should be easily seen up to the level of the carina. Occasionally, a small amount of air may be seen in the esophagus (particularly if the animal is panting or dyspneic) which is of no clinical consequence.

DV/VD view

Again, the heart should be well visualized. The trachea often deviates slightly to the right. The caudal pulmonary vessels are visualized best on the DV view. The right caudal pulmonary vein often silhouettes with the caudal vena cava and can't be seen. The cranial lobar vessels may not be distinct, as there is often superimposition of the scapulas. The cranial mediastinum is usually distinct and should be approximately the width of the vertebral bodies. However, in very obese animals or chondrodystrophic breeds, fat may accumulate in this area and create a widened mediastinum. This can be difficult to differentiate from a cranial mediastinal mass.

Radiographic Pulmonary Patterns

There are 3 basic radiographic pulmonary patterns:

Pulmonary PatternHallmark Radiographic Sign
AlveolarAir Bronchogram
InterstitialLoss of visualization of vessels
BronchialDonuts/Train Tracks


Alveolar Lung Disease

The hallmark sign of alveolar disease is air bronchograms. However, in many cases, the air bronchograms may be difficult to visualize. Other signs to look for include
1) Lobar borders-disease is confined to one lung lobe but adjacent lobe is airfilled creating an air/soft tissue interface; 2) Silhouette sign with other organs particularly the heart-on the VD/DV view the cardiac silhouette should be distinct, if a portion of the border is not well visualized, this is often due to silhouette sign with lung infiltrate.

Interstitial Lung Disease

Interstitial infiltrates may not be evident clinically-i.e. lung sounds will be normal as the airways may not be affected. Interstitial disease is usually a component of all lung diseases but in some instances may be the predominant type. Interstitial infiltrates can take three forms:

Miliary<5 mm sharply marginated
ST nodules
Nodular5mm-4cm sharp or irregular
>4cm=mass
UnstructuredIndistinct opacities which cause indistinctness of vasculature; haziness to lung


Important to differentiate 'end-on' vessels from miliary interstitial nodules. Remember, they are more numerous in the peri-hilar region. They also get smaller towards the periphery and are associated with linear vascular shadows of the same or greater diameter. End-on vessels are usually of greater opacity than the linear vessel.


Bronchial Disease

Considerable changes must be present before changes can be seen radiographically. That is why a cat may present clinically like an asthmatic but have no radiographic changes. Remember that normal bronchi may be seen radiographically, particularly in the perihilar region. If there is no wall thickening, than it may be a normal structure. Bronchial changes are most commonly associated with inflammatory etiologies (allergy, infection, etc). Occasionally, a bronchus will become completely filled with secretions and look like a vessel.

Differential Diagnosis of Lung Disease


The type of pulmonary pattern(s) and their distribution can often help create a short list of differential diagnoses.

Localized Alveolar Disease
  • Pneumonia- ventral distribution is commonly associated with aspiration
  • Pulmonary Edema - usually symmetrically located in caudal dorsal lung fields; can be seen with cardiogenic and non-cardiogenic edemas
  • Atelectasis- Commonly seen in Rt. Middle lung lobe in cats with Feline Asthma (bronchial occlusion due to secretions causes atelectasis)
  • Lung lobe torsion - usually associated with pleural effusion
  • Hemorrhage - usually associated with contusions from trauma
Generalized Alveolar Disease
  • Can be associated with severe versions of the above
Interstitial Disease - Miliary
  • Granulomatous nodules from mycotic pneumonia (blasto, histo)
  • Metastatic Neoplasia - if well defined- most types of sarcomas (except hemangio); if ill-defined- thyroid, mammary, transitional cell carcinoma or hemangiosarcoma
  • Pulmonary Osteomas (Osseous Metaplasia)- mineral opacity; usually incidental
Interstitial Disease- Nodular
  • Solitary mass - malignant primary tumor, abscess, metastasis, granuloma
  • Solitary nodule - malignant primary tumor, metastasis, granuloma, hematoma, artifact (superimposed nipple, engorged tick, skin nodule)
  • Multiple nodules - metastasis, granulomas
  • Cavitary nodules - congenital cyst/bulla, traumatic bulla, abscess, neoplasia
Unstructured Interstitial Disease
  • Fibrosis - usually associated with a previous disease insult (No such thing as "Old Dog Lungs" but often-used terminology
  • Pneumonia - can be early in the disease process or as the disease is resolving
  • Edema - cardiogenic or non-cardiogenic
  • Neoplasia - usually associated with lymphoma
Bronchial Disease
  • 'donuts' and 'tramlines' are usually seen due to inflammatory/allergic airway disease
  • in cats, occasionally metastatic lesions will be cavitary and look like bronchial changes
  • differentiate by taking films 3-4 weeks later and see progression of disease.

Radiographic Imaging of the Pleural Space

Knowledge of the pertinent anatomy of the thoracic structures facilitates understanding the abnormal. This is especially true in radiographic evaluation of thoracic lesions.
The pleura is a mesothelial lined connective tissue membrane covering the internal thoracic wall, the mediastinum, heart, and lungs. The pleural space is a potential space containing a small amount of lymph for lubrication. The pleural lined lung lobes have a fairly constant anatomical arrangement. Knowledge of this arrangement or the lung fissures is important in recognition of pleural effusion or thickened pleura.


Notice the natural fissures that are formed where the individual lung lobes meet. Radiographic visualization of these fissures (air or soft tissue opacity) is usually due to disease of the pleural space. Pleural fissure lines may occasionally be seen in older dogs or dogs recovered from previous thoracic disease. These lines represent thickened pleural membranes that have been penetrated tangentially (end-on) by the x-ray beam.

Pleural effusion is the presence of any type of fluid within the pleural space (transudate, modified transudate, and exudate). The radiological differentiation of thickened pleural membranes and slight pleural effusion may be difficult or impossible. Occasionally the divergence of the fissure as it progresses toward the periphery of the lung field is an indication of pleural effusion.

The amount of pleural fluid affects the ease with which it is recognized. Close to 100 ml of free fluid maybe unrecognized in a medium sized dog. Theoretically, the earliest area to detect pleural fluid is at the costodiaphragmatic angle. Fluid may collect here and cause blunting of the caudal lung lobe tips. Also, minimal amounts of fluid may be seen collecting in the lung fissure between the accessory and left caudal lung lobe. These changes are best seen on the recumbent VD view. It is important to remember that there is normally fat accumulation in the mediastinum between the accessory and left caudal lung lobe.

In the recumbent DV view, fluid collects along the sternum, obscuring the cardiac apex and widening the cranial mediastinum. With massive pleural fluid, lobes retract toward the hilus. The fluid obscures pulmonary detail. The lungs may undergo partial or complete atelectasis. In partial atelectasis the lung tissue retains its normal shape due to its natural "form elasticity". Its volume diminishes but it still can be recognized as lung by its shape and air opacity. When complete atelectasis occurs the cranial and middle lung lobes are most frequently involved. As the lungs retract, the lung/fluid border takes on a scalloped appearance. The fluid may be so extensive as to silhouette (absence of radiographic contrast between two objects) with the caudal heart and the cranial diaphragm. This may present confusion in distinguishing between a diaphragmatic hernia and pleural effusion alone.



Extrapleural Lesions and the Extra Pleural Sign

The extrapleural space is a potential space between the parietal pleura and the body wall or diaphragm. Enlargement of this space can be caused by: subpleural hemorrhage, subpleural infection, neoplasia, or organ displacement (hernia). Extrapleural lesions may be confused with pulmonary parenchymal nodules or masses that touch the thoracic wall and conform to the wall and appear as part of the wall. Generally, extrapleural lesions take on a characteristic shape. Due to the dense fibrous attachment of the pleura, masses originating within its covering are prevented from spreading significantly under this attachment. The path of least resistance is outward. Subpleural masses then, tend to form a uniform mass with its convex border facing toward the lung and having equal longitudinal and transverse diameters. The widest part of the mass is opposite its attachment to the thoracic wall. The edges of the mass taper on reflection to the thoracic wall. The tapered reflection of the edges of a mass that silhouettes with the pleura is identified as the extrapleural mass.
Pulmonary masses located near the periphery of a lung lobe may become deformed as it presses against the thoracic wall. It may resemble an extrapleural mass but close inspection will show that its margins do not blend with the parietal surface.

Horizontal Beam Radiography for Pleural Effusion

Occasionally there are situations where fluid may be mixed with tumor masses or where fluid obscures adequate visualization of the lung parenchyma. Erect VD positioning will help by redistributing the fluid to the dependent portion of the animal. If there is an effusion, the fluid will gravitate to the costophrenic angle and the left accessory lung lobe/caudal lung lobe fissure. Rounding of the angles and widening of the fissure indicates fluid. Remember, free fluid will be influenced by gravity, but encapsulated fluid will not. When performing an erect VD, make sure to include the area of interest-Cranially, if interested in the cranial mediastinum, etc and ventrally if evaluating for the presence of fluid.

Radiographic Imaging of the Mediastinum

Normal Anatomy

The mediastinum is the central portion of the pleural space and is composed of the left right pleural sacs. Mediastinal disease usually involves the space between the mediastinal pleural layers. It extends from the thoracic inlet to the diaphragm and is primarily in the median plane of the thorax. In some animals the mediastinum is fenestrated. It communicates with the fascial planes of the thoracic inlet as well as the retroperitoneal space. The main mediastinal organs that are identified radiographically are the heart, aorta, trachea and caudal vena cava. There are many other structures located in the mediastinum but are not normally identified. The ventral aspect of the medistinum is narrow and oblique to the median plane. The mid portion of the medistinum is wider as it contains a variety of structures and then it is narrow dorsally again. The dorsal mediastinum is parallel to the median plane.
The cranial mediastinum is visualized on the ventrodorsal view and normally should not be wider than the width of the vertebrae except for the brachycephalic breeds which may accumulate fat in this location. In young animals, the thymus may be visualized extending to the left and is called the "sail" sign. On the lateral view, the cranial waist of the heart is composed of structures within the mediastinum.

The parallel lines cranially represent the dorsal mediastinum. The oblique line cranially is the ventral mediastinum. The arrow is pointing to the caudal mediastinal reflection. The thickness of these structures are variable depending upon the amount of fat present.

Mediastinal Abnormalities

There are basically 4 conditions that commonly affect the mediastinum. Mediastinal shift, masses, fluid and pneumomediastinum. A mediastinal shift occurs when there is a loss of lung volume allowing the medistinum to shift towards the loss of volume. This is typical of atelectasis. A shift can also occur in the presence of tension pneumothorax. This shift is usually identified on the VD view.
Masses often affect the mediastinum, most commonly seen cranially. Common radiographic signs include dorsal displacement of the trachea, caudal displacement of the heart and a loss of visualization of the cranial waist of the heart. Fluid can also accumulate in the mediastinum due to such etiologies as infection, inflammation, trauma or neoplasia.

Pneumomediastinum is often detected due to increased visualization of mediastinal structures. Often, the brachycephalic trunk is noted. Pneumomediastinum can lead to pneumretroperitoneum or subcutaneous emphysema. Common causes include trauma to the lung, subcutaneous gas in the neck extending into the thoracic inlet, tracheal rupture, esophageal perforation, retroperitoneal gas or gas producing organisms.


Non-Cardiac Thoracic Ultrasound

Scanning Technique

Radiographic Examination of the thorax is helpful when determining a sonographic approach to the thorax. This will help to localize masses, fluid, etc. Normally, aerated lung prevents visualization of internal structures except in areas of acoustic windows (cranial ventral - heart region). If fluid is present, it can be used as an acoustic window and placing the animal in a recumbent position and scanning from beneath may also improve visualization of structures. Usually a sector/vector type probe is best as an intercostal approach is most common. A subcostal approach is also possible particularly in smaller animals. If interested in the cranial mediastinum, a thoracic inlet approach may also be possible. As always, the highest frequency transducer that permits adequate depth is recommended for use.

Normal Anatomy - Mediastinum


In most areas of the thorax, the structures of the chest wall are visualized as hypoechoic layers. Just deep to this is the aerated lung which is seen as a thin, highly echogenic linear line which moves with respiration. Reverberation artifact is seen as the ultrasound beam interacts with the air in the lung. Only the surface of aerated lung can be seen as the ultrasound beam can not penetrate through it.

Imaging of the cranial mediastinum can be performed using a cardiac window or the thoracic inlet window. The mediastinum is divided into cranial, middle, and caudal portions. The cranial medistinum in the normal animal often is difficult to image due to the fat that typically accumulates there. Usually, it is uniform in echotexture except the vascular structures, trachea and esophagus that can be seen coursing through it. The middle mediastinum contains the heart and the caudal mediastinum the major caudal vessels.

Mediastinal Abnormalities


Ultrasound is particularly useful in evaluation of the mediastinum because many diseases will also cause pleural effusion along with mediastinal abnormalities. The effusion makes it difficult to evaluate the mediastinum due to silhouetting on radiographs. The most common abnormality seen in the mediastinum is masses. Etiologies include enlarged lymph nodes from inflammation or neoplasia (lymphoma), heart base masses (neoplasia) and neoplasia of other structures (thymoma) or abscesses or granulomas. It is not possible to differentiate the etiologies solely on sonographic findings. Abscesses often are cavitary with thick walls, while enlarged lymph nodes are solid and uniform in echotexture with a thin, echogenic periphery. Thymomas often have small cavitations. Ultimately, a sample of the mass (aspirate or biopsy) is necessary for diagnosis. In cats, branchial cysts may be seen as thin walled cystic masses.


Examples of Cranial Mediastinal Masses

Pleural Abnormalities

Pleural effusion enhances the sonographic imaging of intrathoracic structures as it creates an acoustic window. The sonographic characteristics of effusion depend upon its type and can range from anechoic to echogenic. Sampling of the effusion may add in diagnosis as well. The presence of fluid may also help identify thickening or masses of the pleural surfaces. History and time course may help in building a differential list if this is identified as long standing effusion can create thickening and fibrin may accumulate and form mass-like lesions as well.


Pleural Effusion

Pulmonary Abnormalities

In the normal animal, only the visceral surface of the lung can be seen as the air reflects almost all of the ultrasound beam back to the transducer. Infiltrates in the lung, which replace the air, can be visualized if they are on the periphery. Usually, infiltrates are seen as hypoechoic areas within the lung. Differentials include inflammation (pneumonic infiltrates), neoplasia, and vascular anomaly (lung lobe torsion). Differentiating the etiology is not possible based on ultrasound alone but sampling of the area can be directed using ultrasound. Atelectasis will also result in loss of air from the lung lobe, but it will also cause a loss of volume (which does not occur in the other problems). Therefore, atelectic lung lobes will be small and usually have sharp margins.

Diaphragmatic Abnormalities


Evaluation of the diaphragm can be useful. Complete assessment often relies on the presence of free pleural and abdominal fluid to accurately visualize the diaphragm. In the normal animal, a subcostal approach (when evaluating the liver) allows examination of the major portion of the diaphragm. However, without free fluid, care must be taken not to mistake mirror image artifact for a hernia. The abnormality seen in diaphragmatic hernias is disruption of the diaphragmatic line with or without concurrent visualization of structures into the thorax. Congenital peritoneal pericardial diaphragmatic hernias are easier to diagnose sonographically as abdominal organs are visualized within the pericardial sac. Visualization of the hernia is not necessary. This is a condition, which commonly may be an incidental finding in cats.


Diagnostic Imaging of the Abdomen in Small Animals

Visceral Surface Detail

Visualization of structures in the abdominal cavity is due to the differential radiopacity of the soft tissue structures which are primarily surrounded by fat. This is the key factor as there is very little free peritoneal fluid present in the normal dog or cat. Enhanced visualization of visceral surfaces in the abdomen can be due to pneumoperitoneum. Free air in the peritoneal space allows visualization of structures which are not normally seen: examples include the cranial pole of the right kidney, the visceral surface of the stomach and the diaphragm. Possible causes for pneumoperitoneum include iatrogenic causes (surgery, peritoneocentesis), penetrating wounds, ruptured viscus or gas forming bacteria.

Diagnosis of pneumoperitoneum is dependent upon the amount of free air present. Sometimes routine radiography may be sufficient. However, the use of positional radiographs may be necessary to detect smaller amounts of air. This utilizes a horizontally directed x-ray beam. Often, left lateral recumbency will allow air to accumulate in the right cranial quadrant away from the fundus of the stomach.

Loss of visceral detail can be due to a number of causes such as lack of abdominal fat in young or emaciated animals. This is usually not clinically significant. Other causes include accumulation of soft tissue opacity which silhouettes with the abdominal structures such as fluid or cellular infiltrates. Fluid such as transudates, modified transudates or exudates can cause loss of detail. Cellular infiltrates such as carcinomatosis can also cause loss of detail.

Compression of abdominal contents can also cause a loss of visceral detail. This often is seen with large mass effects in the abdominal such as occurs with neoplasia or a large and distended urinary bladder. Often, ultrasound can be used to help determine the loss of visceral detail - to differentiate fluid accumulation vs mass effect. It can also be used to obtain a sample of fluid, particularly if there is a small amount present.

Normal Abdominal Structures - Stomach

The stomach is located just caudal to the liver with the visualization dependent upon the amount of surrounding fat as well as the contents of the stomach. Often, it is recognized because of the food within the lumen. If the stomach is empty of food, gas often helps to determine its location. In the dog, the fundus is located in the left cranial quadrant with the pylorus on the right. On the VD view, the axis should be perpendicular to the spine. In the cat, the pylorus is located on midline and is C-shaped on the VD view. In both species, the axis of the stomach should be parallel to the ribs on the lateral view. In addition, if gas is present it can be manipulated using gravity to view the different portions of the stomach. It is necessary to realize that the fundus is slightly more dorsal than the body of the stomach as is the pylorus. Thus, if the animal is in left lateral recumbency, gas can be recognized in the pylorus. In right lateral recumbency gas rises to the fundus. On the VD view, gas will be in the body and on the DV view, gas will be in the pylorus and fundus.

Diseases of the Stomach


Foreign Bodies are usually observed by their different opacity from normal contents. This is dependent upon many factors such as the make up of the foreign body (metal vs fabric, etc). Often, one can use the mobility of gas on different views to help highlight a foreign body. Sometimes, a contrast study may be necessary to confirm this finding. The main radiographic finding in this case is to visualize a filling defect or contrast residue adhering to the foreign body (this normally occurs when the FB is fabric).

Gastric dilatation/volvulus (GDV) is a common problem involving the stomach. The first radiographic view to obtain is the right lateral recumbent view. A "double bubble" or compartmentalization of the stomach is seen on this view because the pylorus typically has moved dorsally and to the left. The right lateral recumbent view promotes accumulation of gas in the pylorus and thus the "double bubble" appearance. If the right lateral view is inconclusive, then a DV view may be helpful. Again, in this position, air should fill the pylorus and help determine its location (should be on the right).

Normal Abdominal Structure - Small Intestine

The duodenum is fixed in position by its mesenteric attachments. It is normally the most lateral intestine on the right side and the descending duodenum is parallel to the body wall. The jejunum is relatively mobile. The serosal margins are visualized due to the surrounding fat. Mucosal margins can NOT be definitively visualized without positive contrast to outline the surface. In general, the overall diameter of the small intestine should not exceed 1-2 (sometimes up to 3) times the width of a rib (or 12 mm in the cat).

Diseases of the Small Intestine

The most common problem detected on survey radiography is ileus. By definition this strictly means failure of passage of the intestinal contents. There are two basic categories of ileus 1)Mechanical (or obstructive or dynamic) 2)Functional (or paralytic or adynamic). Mechanical ileus is usually associated with a physical obstruction of the intestines due to an intraluminal or extraluminal blockage such as a foreign body or tumor. The radiographic signs of mechanical ileus is focal dilation of the bowel with distension proximal to the obstruction. Often, a stacking of loops with hair pin turns may be seen. Linear foreign bodies can cause plication.

Functional ileus is due to a generalized neurologic/muscular dysfunction of the intestines such that progressive motility is lost. This is commonly seen in enteritis, dysautonomia, etc. The radiographic signs include generalized dilation of the bowel.

Contrast Studies of the GI Tract

Upper GI- the purpose is to distend the gi tract with an alternative radiopacity which interfaces with the mucosal surface.

Technique - Use 37% W/V commercial Barium Sulfate suspension. Avoid the old USP powder as it does not contain emulsifiers and will precipitate out. If possible, prepare animal by withholding food for 12-24 hours and enemas to cleanse the gi tract. Obtain survey radiographs prior to beginning the study to verify proper preparation and for comparison to contrast films. Administer 6 ml/lb via stomach tube (dog). May need slightly more in small dogs and cats and less in large dogs. The objective is to distend the stomach with barium. After administration, obtain RL, LL, VD, and DV views. Then obtain 2 orthogonal views every 15 minutes for 1 hour then every hour until the contrast is in the colon. In cats, after the first hour, obtain views every 30 minutes.

Evaluation - In the stomach, the wall should be thin and uniform. The emptying time in the dog is 1-4 hours and the transit time should be a maximum of 15 minutes. The small intestines have a normal fimbriated border on the mucosal surface. In the duodenum, pseudoulcers may be seen (lymphoid aggregates) on the antimesenteric border. In cats, a "string of pearls" may be seen in the duodenum due to strong circular muscle contractions. The transit time in the dog is up to 5 hours in the SI and 3 hours in the cat.

Remember that if a perforation is suspected, use an iodine based contrast agent. Leakage of barium into the peritoneal space can incite a severe granulomatous reaction.

Normal Abdominal Structure - Liver

The liver is located just caudal to the diaphragm. Normally in most breeds, it is located within the costal arch and has sharp margination. In cats, it is common to see a large falciform fat pad ventral to the liver giving it the appearance of "floating". On the lateral view, the axis of the stomach should be essentially parallel to the ribs.

Diseases of the Liver

Enlargement of the liver can be generalized or focal. Generalized enlargement may be due to any infiltrative process. This can be detected best on the lateral view. Usually caudal displacement of the stomach(pylorus) is noted as well as extension of the ventral liver margin beyond the costal arch. Remember that in some breeds (deep chested dogs) slight extension beyond the arch is acceptable. Focal enlargement is usually due to a mass. Look for specific displacements of the abdominal structures (i.e. displacement of the pylorus to the left, etc).

A small liver (microhepatica) is due to loss of hepatic mass such as seen with cirrhosis or portosystemic shunting. This is usually visualized by cranial displacement of the stomach axis.

Normal Abdominal Structure - Spleen

The head of the spleen is held in position by the gastrosplenic ligament. It can be seen on the lateral view just dorsal to the kidney in the cranial dorsal abdomen. On the VD view, it is seen just caudo-lateral to the stomach. The tail is mobile and can be found anywhere in the mid to caudal abdomen. The margination should be smooth and sharp.

Diseases of the Spleen

Generalized enlargement of the spleen is difficult to diagnose as it can have a variable size. It is also affected by sedatives and general anesthesia. The most common abnormality diagnosed radiographically is a mass involving the spleen. A mass in the tail is most commonly seen in the mid ventral abdomen just caudal to the stomach. Neoplasia is the most common cause but hematomas and nodular hyperplasia should also be considered. Splenic torsion is relatively uncommon but radiographically can be diagnosed by the classic reverse "C" appearance.

Normal Abdominal Structures - Kidneys

The kidneys are located in the retroperitoneal space. They are best seen on the VD view and are measured compared to the length of L2. In the dog, normal kidneys may be 2.5-3.5 times the length of L2 and in the cat 2.4-3 times the length of L2. The kidneys should be evaluated for disparity of size and margination as well as changes in opacity. The kidneys are normally at the level of T13-L4. The right kidney is usually more cranially located in the dog. In the cat, the kidney may be pendulous.

Diseases of the Kidneys

Uniform increases in kidney size may be due to

Compensatory Hypertrophy
Neoplasia
Hydronephrosis
FIP
Perirenal pseudocysts
Acute pyelonephritis

Focal increases in kidney size may be due to

Neoplasia
Subcapsular hemorrhage
Hematoma
Abscess
Renal Cysts

Decrease in kidney size may be due to

Chronic pyelonephritis
Chronic infarcts
Cortical Hypoplasia
Chronic Progressive Renal Disease

Normally the kidneys should be soft tissue opacity and surrounded by the retroperitoneal fat. An increase in opacity is usually associated with nephroliths or nephrocalcinosis. Loss of visualization of the kidneys may be due to fluid accumulation within the retroperitoneal space.

Normal Abdominal Structures - Ureters

The ureters not normally seen radiographically. Anatomically, the proximal ureter is retroperitoneal while the most distal portion is peritoneal in location. The only way to evaluate the ureters is with contrast radiography

Contrast Radiography of the Kidneys/Ureters


Excretory Urogram - Indications - with the advent of ultrasound most uses of the EU to evaluate the kidneys has been replaced. However, it is still useful to evaluate the size and location of the kidney if ultrasound is not available or if there is difficulty evaluating the kidneys on ultrasound. The most common indication for an EU today is to diagnose the location of an ectopic ureter.

Technique - Clinical dehydration is the main contraindication. Prepare animal as for other contrast studies (NPO, enemas). Survey radiographs should be obtained immediately prior to beginning study. Adminster 400mg/lb of iodinated contrast media as a bolus intravenously. Immediate radiographs (2 views) and then views at 5, 10, 20 and 40 minutes should be obtained. These times can be adjusted depending on the goal. Visualization of contrast within the kidneys requires: renal blood flow, functional glomeruli, tubular reabsorption, and a patent collecting system.

Interpretation - The kidneys can be evaluated for size, shape, opacity, etc during the nephrogram phase. The ureters will be visualized during the pyelogram phase. The are usually 1-2 mm in diameter and should have peristalsis present. Therefore, the entire length of the ureter will not be visualized at any one time. Pneumocystography may be helpful in determining the location of the ureteral termination.

Normal Abdominal Structure - Urinary Bladder

Survey radiographs are useful only for evaluating bladder position, opacity, shape and size. However, it may be necessary to perform a contrast study to complete the evaluation. The bladder is divided into 3 areas - vertex, body, neck. The trigone is the dorsal aspect of the neck.

Diseases of the Urinary Bladder

Changes in bladder position may be due to herniation or displacement due to enlargement of adjacent viscera. Changes in opacity is normally due to the presence of calculi. Radiopaque calculi include those that contain calcium or phosphorus. A decrease in opacity can be seen with air in the lumen from iatrogenic causes or air in the wall from emphysematous cystitis. Emphysematous cystitis is often associated with diabetes mellitus or hyperadrenocorticism. Changes in size is subjective but can be due to obstruction, an inability to eliminate. A complete lack of visualization of the urinary bladder can be due to rupture or just an empty bladder.

Contrast Studies of the Urinary Bladder

There are 3 types of studies - Negative, positive and double contrast cystograms. A negative contrast cystogram is usually only to determine the position of the urinary bladder. The positive contrast cystogram is used to determine the integrity of the urinary bladder. The double contrast cystogram is used to evaluate the mucosal and luminal areas of the urinary bladder. The latter has recently been replaced with the use of ultrasound.

Technique - Negative contrast cystogram. This is fast and inexpensive. Position a catheter within the urinary bladder and distend with either room air or CO2. CO2 is considered ideal as it is less likely to cause embolism. Distend the urinary bladder until it palpates turgid (take care not to over distend) and then obtain radiographs.

Positive Contrast Cystogram - Similar to the Negative Contrast Cystogram except use iodinated contrast media. Often can dilute iodinated contrast by 50-75%. Distend urinary bladder until it is turgid. This is critical if interested in leakage as small holes may not leak until the bladder is distended.

Double Contrast Cystogram - Position a catheter in the urinary bladder. Remove as much urine as possible. Instill5-15 mls of iodinated contrast and then instill negative contrast (air or CO2) until the bladder is distended. Terminate the injection if back pressure if felt. Obtain radiograph in RL, LL, VD, DV in order to distribute the contrast puddle to all areas of the bladder.

Evaluation - Double Contrast Cystogram
The mucosal surface and wall can be evaluated. The wall should be thin and uniform. Thickening of the cranial ventral wall is often associated with cystitis. Neoplasia is typically found in the trigone. However, inflammation or neoplasia can cause changes in any part of the bladder. The lumen can be evaluated and the following table details these findings.

Filling Defect Shape Borders Location
Air Bubbles Round Smooth Periphery of contrast puddle
Calculi Round to Irregular Indistinct Center of contrast puddle
Blood Clot Irregular Irregular to indistinct In the contrast puddle or near the wall




Computed Tomography: What is it and When is it Useful?

Introduction

Survey radiography has been the mainstay of diagnostic imaging for years. Its advantages include its widespread availability, ease of acquisition and relative economy. However, one of its main disadvantages if the superimposition of structures in the image that is generated. Remember that a radiograph is basically of summation of the 'shadows' that are created due to attenuation of the x-ray beam as they penetrate through the area of interest. With the advent of computers, cross sectional imaging was made possible.
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are the current modalities in use to perform cross sectional imaging. The main advantage of both of these imaging modalities is the lack of superimposition of structures. The object of this presentation is to provide a basic understanding of the methods by which CT provides the images as well as an overview of when this modality may be useful.

Computed Tomography

CT utilizes x-rays and their interaction with tissue to create the image. The basic premise is that each type of tissue attenuates the x-ray beam in a unique amount (which is represented by the attenuation coefficient of that tissue). The second premise is that the internal structure of an object can be reconstructed from multiple projections of that object. In reality, these projections are the amount of transmitted radiation detected passing through the object, which are obtained, from a variety of angles. All CT scanners are made of some form of x-ray generating device and x-ray detectors. In general, an x-ray beam (of some finite thickness) is passed through the patient. The amount of transmitted radiation is detected on the side opposite the x-ray generator. The x-ray beam is passed through the patient in the same location from a variety of angles. A computer is then used to calculate the attenuation coefficient of the different projections and ultimately can calculate the location of these different tissues. This action produces a single 'slice' of the patient. The CT scanner is linked to the couch upon which the patient is resting and it is calibrated to determine the location of each slice. To obtain another slice in a different location, the couch is moved (with the patient on it) to the next area to be imaged via computer control.

To create the image, which is viewed, the CT computer displays the attenuation coefficients as a variation of a shade of gray rather than a number. Thus, tissues that are blacker in the image have a lower attenuation coefficient than areas that are more light, or whiter. Most machines are calibrated to display the attenuation coefficient relative to that of water; this is called the Hounsfield unit or CT number. The CT number can range from -1000 to + 1000. One problem is that the human eye can only resolve about a limited number of shades of gray at a time. Thus a technique called window leveling and width is employed. The window level or center is centered at the CT number of the tissue of most interest. The window width then is the number of CT numbers to be displayed around the center. This creates an image with the area of interest represented as gray while CT numbers outside the window width will be displayed as all black or all white. An example of when these settings are critical is seen when evaluating the tympanic bullae. If a soft tissue window is used, it is not possible to evaluate the bone of the bullae as they are all displayed as white. A bone window is necessary to make this evaluation.

Contrast

In CT, contrast is often used to help facilitate evaluation of abnormal structures. CT utilizes intravenously administered iodinated contrast. Areas that are abnormal may accumulate the iodinated contrast (which has a high attenuation coefficient) and show as an area of hyperattenuation when compared to a noncontrast study. In addition, with the advent of very fast imaging techniques using helical CT with acquisition of many slices simultaneously, CT angiography has become possible. Using 3-D reconstruction, a virtual model of any vascular structure can be obtained.

Uses of Cross sectional Imaging

In general, most people would agree that CT is far superior to survey radiography for evaluation of structures without superimposition. CT is particularly useful in imaging of bone and air filled structures due to the high detail that can be achieved. In some practices, it is becoming common for CT to be used for pulmonary metastasis checks (A study can be performed in less than a minute using the new helical, multislice CT systems that are available). CT is also commonly used to evaluate the neurological system (brain and spinal cord). Many brain tumors, spinal cord tumors and intervertebral disc disease are commonly imaged using CT. In some instances, old invasive techniques such as myelograms have been replaced with cross sectional imaging. The detail is improved and the morbidity is greatly reduced. Other uses include orthopedic and nasal evaluations. CT is the current optimum, noninvasive diagnostic test for evaluation of syndromes such as fragmented medial coronoid disease. It permits visualization of abnormalities that even arthroscopic evaluation may not elucidate. An example is when an animal has an in situ fragment with an intact cartilage surface. Arthroscopy may miss this lesion but it easily seen on CT evaluation.

With a new fast, multislice CT scanners, images can be acquired so rapidly, that motion may be evaluated. For example, cardiac motion and therefore cardiac function, may be evaluated using these newer scanners.

Other Considerations

There are relative few problems associated with the use of CT. It may require general anesthesia in order to properly position the patient and obtain the images. This may be true of brain, spinal and orthopedic evaluations. However, studies such as pulmonary metastatic checks are often performed with sedation and minimal restraint. The primary pitfall in CT is that it utilizes radiation for formation of the image. However, with appropriate technique and setup, CT personnel should not be exposed to any additional risk than the average radiology technician. Exposure to the patient is not considered a problem as the evaluation is considered medically necessary. Also, metallic materials such as bone plates, screws, etc. may cause artifacts, which make the image difficult to evaluate in some instances.

Conclusion

Overall, the invention of computed tomography has greatly advanced diagnostic imaging and has provided additional information that other modalities could not. Cross sectional imaging will likely replace standard radiography and become the standard of care at some point in the future and will advance the quality of veterinary medical care on a large scale.

Overview of Ultrasound Physics

Sound waves with a frequency up to 20,000 Hz are in the audible range. Diagnostic ultrasound uses frequencies in the 1-15 MHz most commonly. It is particularly useful for evaluating internal soft tissue structures and it allows visualization of dynamic events in real time.

The properties of the sound wave include: velocity, frequency and wavelength. They are interrelated by the formula
Velocity = frequency x wavelength.
Velocity is measured in m/s; frequency is the number of wavelengths passing a point per second (measured in Hz) and the wavelength is measured in meters. In most soft tissues, the velocity is approximately 1540 m/s.

In general, the transducer is both the producer and receiver of the ultrasound waves and echoes. These devices convert electrical energy into sound waves and then back to electrical energy. The two main types 1)linear (image is same size in near and far fields) 2) sector (small near field, large far field). The transducer emits ultrasound waves about 1% of the time and receives returning echoes about 99% of the time.

The ultrasound image is formed based upon the time it takes for an echo to return to the transducer. Since the velocity is 1540 m/s, one half the time it takes for the echo to return multiplied by the velocity will give the distance the reflector is away from the transducer. The intensity of the echo is displayed in the image as brightness: 1) low intensity echo is displayed as a dark, or hypoechoic, spot 2) high intensity echo is displayed as a white, or hyperechoic, spot.

The frequency of the transducer is directly related to the resolution in the image and inversely related to the depth of penetration. Therefore, one should choose the highest frequency transducer for use that allows the depth of imaging required.

Basic Sonography of the Small Animal Abdomen

Patient preparation is essential to obtaining quality images. In general, the hair should be removed as air is often trapped in the hair and this will cause degradation of the images. Imaging can be performed in either ventral dorsal or lateral recumbency. A coupler (usually acoustic gel) is needed to eliminate any air between the skin and transducer. Rubbing alcohol can also be used. Do not use mineral oil as this can destroy the transducer housing.

Initial scanning is usually performed using the highest frequency transducer possible. A sector scanner is useful as it allows a broad view of the organs being imaged. Also, sector scanners usually have a relatively small footprint which makes it is easier to maintain good contact. The following can be used as a general guideline
7-10 MHz - cats/small dogs
5 MHz - medium dogs (30-50 pounds)
3 MHz- large/giant dogs

Often, particularly in larger animals, I perform a general scan using a sector transducer of relatively low frequency and then switch to a higher frequency (often linear) for imaging of specific portions of the organs.

The general convention for all scanning (except cardiac) is for the cranial or right side of the animal is presented on the left side of the image. Thus, in a parasagittal plane, the cranial is on the left side of the screen and caudal is on the right side.

In general, sonographic interrogation can be used to evaluate the echotexture of a structure as well as the margination and internal derangement of normal architecture. In the abdomen, the relative echogenicity of organs is a mainstay in evaluation. The common terminology would include 1)hyperechoic - bright strong echos 2) hypoechoic - weak, dark echos 3) isoechoic - structures of the same echogenicity ( doesn't necessarily convey the brightness or darkness) 4) heterochoic - a combination of hyper and hypoechogenicities. The spleen is the most echogenic organ while the liver and kidney cortex are of approximately the same echogenicity and are hypoechoic compared to the spleen.

LIVER
The liver is bounded by the diaphragm cranially and the stomach caudally. The right kidney is adjacent to the caudate liver lobe. The falciform fat is typically found ventrally. Sonographically, the liver has a coarse uniform echotexture. The prominent vascular structures that are visualized are the portal veins (denoted by hyperechoic walls) and the hepatic veins (isoechoic walls). In the normal dog and cat, hepatic arteries and bile ducts are not visualized. The gall bladder is typically "tear drop" shaped and thin walled. Echogenicity can be seen the bile, but is often not considered significant unless it is immobile and forming a mass effect (gall bladder mucocele).

SPLEEN
The dorsal (or head) of the spleen is held in place by the gastric-splenic ligament by the greater curvature of the stomach. The ventral portion (or tail) of the spleen can be found in a variable position. Sonographically, the spleen has a uniform echotexture and is the most echogenic organ in the abdomen. Splenic veins can be easily seen entering the hilus. The capsular margin is normally smooth with gradation of thickness from the extremities to the center.

KIDNEYS
The cortex and medulla can be seen distinctly. The cortex has a fine, homogenous echotexture, which is approximately isoechoic to the liver. The medulla is anechoic to hypoechoic. The pelvis is usually filled with fat with no dilation. There is no reliable method to evaluate kidney size sonographically. In cats, a general rule is that the sagital length should be approximately 4 cm.

URINARY BLADDER
The urinary bladder is found ventral to the colon (and uterus in females). The wall is normally thin (less than 2-3 mm) and should have anechoic contents. In cats, the urine may contain some echoes, which may be clinically insignificant (possible crystals??).

GI TRACT
Visualization of the structures depends upon the luminal contents and the amount of gas present. The duodenum is typically the most lateral loop of small intestine in the right cranial quadrant. The small intestines typically show a laminar arrangement of the layers. The luminal or mucosal surface is hyperechoic. The next layer is the mucosa, which is hypoechoic. The next layer is the submucosa which is hyper echoic. The muscularis is hypoechoic and the outer layer is the serosa, which is hyperechoic. Typical, normal thickness of the stomach is 3-5 mm. The intestines are 2-3 mm thick (except the duodenum which can be 1-2 mm thicker).

PROSTATE
The normal prostate is bilobed in shape with a relatively uniform echotexture with the exception of the muscular fibers around the urethra as it traverses the prostate. Size is variable depending upon the reproductive status of the animal. Castrated dogs typically have a very small prostate, which may not visible abdominally.

PANCREAS
In the dog, the pancreas is found medial to the duodenum (right limb) with the left limb found caudal to the greater curvature of the stomach ( but does not cross midline). In the cat, the right limb is medial to the duodenum but the left limb crosses midline and extends to the level of the left kidney. In the normal animal, the echogenicity of the pancreas may be similar to the surrounding fat. Visualization of the pancreaticoduodenal vessels can help in identification.

ADRENAL GLANDS
The left adrenal gland is found medial to the cranial pole of the left kidney adjacent to the aorta. The left renal artery is a landmark as the left adrenal gland can be found just cranial to it. The right adrenal gland is found medial to the cranial pole of the right kidney adjacent to the caudal vena cava. The left gland is "peanut" shaped and has a cortex and a medulla. The right gland is "arrow" shaped. There are no known normal measurements, but a useful guide can be that the width should not be more than 30% of the length.

Ultrasound Guided Biopsy/Aspiration Techniques

Ultrasound guided biopsy/aspiration techniques are useful because there is increased safety in needle placement with less morbidity due to inadvertent damage. Also, the diagnostic yield of these samples is increased due to more accurate needle placement.

Preparation of the Patient

The problem associated with biopsy/aspiration of structures is hemorrhage. Thus, the patient should be assessed for coagulopathies prior to the procedure. This assessment is made from history, physical examination, signalment and laboratory tests. If a coagulopathy exists, it should be corrected or treated prior to the procedure. The skin over the biopsy/aspiration site should be aseptically prepared. The demeanor of the patient determines the need for anesthesia or sedation. Note: pure opioids often induce panting and should be avoided.

Techniques

There are basically three types of methods that can be used to biopsy or aspirate the structure of interest. The first is called the indirect guidance method. Basically, it entails using ultrasound to find the structure of interest and then the ultrasound probe is removed. The needle is then passed blindly in the area of interest. This is best for extremely large targets that don't require direct visualization of the needle as it is passed into the structure to be biopsied/aspirated. In general, I don't use this technique.

The other two techniques allow direct visualization of the needle as it is passed into the organ of interest. It is critical that the needle pass at an oblique angle to the ultrasound beam in order to see it on the image. A needle that passes parallel to the beam will not create any useful echoes. The freehand technique is performed by handling the ultrasound probe in one hand (usually the non-dominant hand) and the needle in the opposite hand. The probe is used to visualize the structure of interest and then the needle is passed concurrently. Care must be taken to make sure the needle is at an oblique angle to the beam. Also, make sure to not stick the ultrasound probe! This method requires good hand-eye coordination and practice to master it. Many sonographers prefer this technique because it allows more choices as to how to approach the structure.

The last technique is the needle guide technique. Many ultrasound machines have a manufactured needle guide which fits onto the transducer. This equipment will guide the needle in a preset angle and maintains the orientation to the transducer. Software on the machine will project the needle path onto the screen, which allows very accurate placement of the needle. One problem with this approach is that the needle guide is often bulky and thus the approach choices may be limited, particularly in small animals. Because the needle is passed through the needle guide, sterile technique is essential. The guide must be sterilized prior to use. In addition, a sterile covering is usually applied to the transducer as well (a sterile surgical glove works well). This is accomplished by placing coupling gel into the glove and then fitting it over the transducer. The guide is then attached.

Any mass, which can be viewed using ultrasound, can be sampled. However, the risk of complications should be weighed against the usefulness of obtaining the sample. It is especially risky to sample the gall bladder, urinary bladder, structures near major vessels and intestines, which have a thin wall. In general, the spleen should never be biopsied (aspirate only). It is often difficult to sample structures in the presence of a large amount of effusion.

Biopsy Phantom

To practice aspiration/biopsy techniques, a biopsy phantom can be created. The simplest one utilizes a heavy plastic bag (an empty intravenous saline bag works well). Culinary gelatin is mixed at double the recommend concentration and poured into the bag. After 15-30 minutes of refrigeration, as the gelatin begins to set, targets are inserted into the middle of the gelatin (grapes, cherries, chick peas work well). Care should be taken to not create any air bubbles. The gelatin is allowed to completely set and then the bag is sealed. The outer surface can then be ultrasounded and a needle passed easily through the bag to the target.

ADRENAL SONOGRAPHY

Radiographic examination of the adrenal glands is not particularly useful. In addition, with the improvements in the newer ultrasound machines, visualization of even normal adrenal glands is routine.

TECHNIQUE

Imaging of the adrenal glands should be performed using the highest frequency transducer that will provide the depth needed. The left adrenal can be found medial to the cranial pole of the left kidney, immediately adjacent to the aorta and cranial to the renal artery. When imaging the animal in ventrodorsal recumbency, image the left kidney and position the transducer so that the cranial pole (in a parasagittal plane) is in the middle of the screen. Slowly point (with out moving the footprint) the transducer in a medial direction until the aorta is visualized. The left renal artery should be seen at this point. The adrenal will be just cranial to the renal artery. Often, the transducer must be rotated slightly clockwise in order to image the maximal width of the left adrenal gland. The right adrenal gland is found medial to the cranial pole of the right kidney immediately adjacent to the caudal vena cava. The right adrenal is often more difficult to image due to interposed intestinal gas, especially in deep chested dogs. The right kidney should be imaged in a parasagittal plane and then the transducer is pointed medially until the caudal vena cava is seen. The transducer should then be pointed slightly laterally until the right adrenal gland is seen.

The left adrenal gland is commonly described as "peanut" shaped while the right adrenal is somewhat arrow shaped. This is dependent upon the breed and size of animal. Often, particularly in smaller animals, the cortex and medulla of the glands can be visualized. The medulla is hypoechoic compared to the cortex. In cats, the glands are oval shaped. Furthermore, mineralization of the gland in cats is considered an incidental finding while in dogs, it is indicative of neoplasia (50% are malignant).

There is no completely reliable method to evaluate adrenal gland size. There has been some suggestion that the upper limit of normal diameter is 7.4 mm in the dog and 4.3 mm in the cat. Another method suggests that the maximal width should not be more than 30% of the length. As definitive measurements are not reliable, clinical experience in evaluation of the shape of the adrenal gland should also be included in evaluation of the glands. Irregularity or rounding may suggest the gland is abnormal.

Adrenal Gland Enlargement

Symmetrical enlargement of both adrenal glands is suggestive of pituitary dependent hyperadrenocorticism. Assessment of adrenal size and shape as well as signalment and history should be combined to determine the significance of these findings. Pituitary dependent hyperadrenocorticism is most common in smaller breed dogs. Other differentials would include adrenal gland tumor with metastasis to the opposite gland. This is a possibility because the adrenal glands share lymphatics and this creates a passageway for metastasis to occur. Adrenal gland tumors include adenomas, adenocarcinomas and pheochromocytomas. There are no differentiating sonographic findings to separate these types of tumors. In some instances, adrenal tumors are found as incidental findings during an exam. It may be difficult to differentiate adrenal gland enlargement due to pituitary dependent disease versus adrenal tumor. It has been suggested however, that tumor is more likely if the width of the gland is more than 2 cm. Adrenal tumors can invade the surrounding vasculature including the caudal vena cava, renal artery/vein, and aorta. If a tumor is suspected, the surrounding vasculature should be interrogated for invasion and/or thrombosis. As a note, a left adrenal gland tumor has been seen to cross midline and invade the caudal vena cava. In cats, hyperadrenocorticism, if present, is often seen in conjunction with diabetes mellitus.

Percutaneous biopsy of adrenal masses can be performed, however, it is not recommended because if the mass is a pheochromocytoma, a hypertensive crisis may result. Also, due to the proximity of the major vasculature, critical hemorrhage may also be a problem.

There is no known lower limit for adrenal gland size. However, in my experience, if I can't visualize at least the left adrenal gland, small size should be suggested. It is common not to see the glands in hypoadrenocorticism or in animals being treated for hyperadrenocorticism.

Reproductive Sonography

Uterus - Examination Technique

The highest frequency transducer available should be used for examination of the uterus. The normal uterus is composed of three, distinct layers - mucosa, muscularis and serosa. The body of the uterus is located dorsal to the urinary bladder and ventral to the colon. The size depends upon the reproductive status of the animal. A normal, non-gravid uterus may be visualized but is usually small, and does not contain fluid in its lumen. In these situations, the layers are often not discernible.

Pyometra

The most common uterine abnormality is pyometra. It usually occurs secondary to an abnormal response to progesterone, which allows endometrial hyperplasia to occur. This environment is then prone to infection. Distension of the uterus with mucin fluid accumulation may occur from the endometrial hyperplasia. This is termed hydrometra or mucometra. Ultrasound is the modality of choice for diagnosis of these problems as it is more sensitive and specific than radiography. The main sonographic finding in pyometra is distension of the uterus with fluid, which is typically echogenic. Hydrometra usually involves the presence of anechoic fluid while mucometra may have echogenic fluid present. This makes it difficult to differentiate it from pyometra solely on sonographic findings. Clinical findings (such as fever, elevated WBC, vulvar discharge, etc) may be evaluated in conjunction with the sonographic findings to determine a likely differential diagnosis. Aspiration of the uterine contents percutaneously is not recommended as the potential for leakage/rupture is high, especially is the uterus is very distended. If medical management is pursued for pyometra (in a breeding animal) it can take upto 4 weeks for the uterus to return to normal sonographically.

Pregnancy

Early pregnancy diagnosis is a common request. In the past, palpation of the gestational sacs from 20-35 days of pregnancy was the method of choice. Ultrasound can provide the diagnosis as early as 10-11 days after breeding. It is imperative however that the animal be cooperative and fasted prior to examination to try to decrease the presence of ingesta/gas which may obscure the gestational sacs. In general, ultrasound has been a poor predictor of fetal number, primarily because gas can hide a small fetus during early pregnancy and there is considerable overlap in late term pregnancy. It is recommended that radiography still be the method of choice after 45 days of pregnancy (preferably > 55 days) to accurately determine fetal number.

In addition, prediction of gestational age, particularly in the dog, is often inaccurate as well. This is primarily because canine sperm may remain fertile for about 7 days. Therefore, breeding may take place on day 3 of estrus, but actual fertilization may not occur for upto a week. Early pregnancy diagnosis based is based primarily upon visualization of the gestational sacs. These are thin-walled structures which contain anechoic fluid. The canine embryo is usually seen about 23 days post luteinizing hormone peak. It is an echogenic structure seen within the anechoic gestational sac. Besides detection of pregnancy, assessment of viability is also a possible request. This is usually assessed by the detection of cardiac activity. In the dog, this may be seen 23 days post LH peak. It is often seen as just a flutter of movement initially within the embryo. As the embryo grows, distinct chambers of the heart may be seen by day 40. Fetal movement may be seen around day 30-33. As the fetus reaches late term, assessment of fetal heart rate can help determine fetal stress and viability. The fetal heart rate should be at least twice the maternal heart rate and in general, a heart rate of less than 200 bpm is of concern. The best way to determine the fetal heart rate is to place an M mode cursor across the chambers and obtain an M mode tracing. Most machines have a calculation package available that will automatically calculate the heart rate based upon the marking the systolic excursions.

Estimation of Fetal Age

Estimation of fetal age can be accomplished via ultrasound; however, one must realize that it is only an estimation primarily because it is hard to definitely determine the exact date of conception. The following are useful formulas derived from a variety of sources. The days before parturition can be calculated by subtracting the GA from 65 for the dog and 61 for the cat.

Gestational Age (GA) in the Dog ( 3 days)

Less than 40 days (GSD = gestational sac diameter; CRL = crown-rump length)
GA = (6 x GSD) + 20
GA = (3 x CRL) + 27

More than 40 days (HD = head diameter, BD = body diameter)
GA = (15 x HD) + 20
GA = (7 x BD) + 29
GA = (6 x HD) +(3 x BD) + 30

Gestational Age in the Cat ( 2 days)

Greater than 40 days
GA = 25 x HD + 3
GA = 11 x BD + 21

Prostate and Testes

Examination of the Prostate

As always, the highest frequency transducer that allows complete visualization should be used. Remember that the prostate may have small cystic changes, which may not be visible except with a 7.5 MHz transducer or higher. Usually a sector/vector type transducer is required to visualize the prostate as it may be somewhat intrapelvic (especially in the castrated male) and the small footprint allows manipulation around the pubic shadowing. A rectal transducer may also be necessary but use of this type of probe usually requires general anesthesia or very heavy sedation.

The easiest method to find the prostate is to visualize the urinary bladder and then scan caudally along the trigone. The prostate is found just caudal to the neck of the urinary bladder. It is a bilobed structure and completely encircles the prostatic urethra. In the normal prostate it is uniform in echogenicity with the exception of the region around the urethra, which is, hypoechoic compared to the rest of the parenchyma. The size of the prostate depends upon the reproductive status, age and pathologic status. Animals, which have been castrated for a long period of time often, have a small prostate, which is intrapelvic and difficult to visualize. Intact animals can have a prostate that is abdominal in location. However, it is recommended to use radiography to assess prostatic size for pathologic determinations.

Benign Prostatic Hyperplasia

This is a common condition seen in intact males and is due to glandular hyperplasia from hormone stimulation. The prostate is usually enlarged but may range from homogenous echogenicity to a heterogenous echotexture. Also, commonly seen are cysts within the parenchyma, which may range in size (up to about 2 cm in diameter). Usually the capsule is not disrupted and mineralization is not seen within the parenchyma.

Prostatitis

Prostatitis is usually due to an acute or chronic bacterial infection (although fungal prostatitis has been seen). Usually the gland is enlarged and heterogenous. Large cystic cavitations (often greater than 2 cm) may be seen and contain purulent material. It may be difficult to differentiate this problem from benign prostatic hyperplasia or neoplasia. Aspiration of cystic areas and correlation with clinical findings (fever, elevated WBC, etc) are necessary for diagnosis. Chronic prostatitis may induce dystrophic mineralization as well.

Neoplasia

Usually some form of carcinoma is the most common type of neoplasia seen in the prostate. Usually the gland will be enlarged with heterogenous echogenicity. Cavitations with secondary infection may be seen. Mineralization is also commonly seen in neoplasia. Disruption of the glandular capsule is also indicative of neoplasia. Diagnosis of neoplasia may be difficult as benign hyperplasia and infection can also be seen concurrently.

Testes

The testes are found in the scrotum and are uniform in echotexture. There is a distinct mediastinum testis which is seen as a bright linear echo. The epididymus is composed of a head, body and tail. The head and tail may be seen at the cranial and caudal aspects of the testis.

The most common reason to ultrasound the testes is to encourage castration. If a lesion is found, it is often easier to convince an owner that castration is necessary. Neoplasia is a common finding. Interstitial cell tumors, Sertoli cell tumors and seminomas are the most common. Unfortunately, the sonographic findings of testicular tumors are not specific for type. Lesions can range from hypoechoic masses to hyperechoic masses and may be well defined or ill-defined.




© 2004 - Anne Bahr, DVM, MS, DACVR - All rights reserved