Introduction

There has been a rapid development in the use of ultrasonography in private practice in the last ten years. This has resulted in a tendency for practitioners to reach first for the ultrasound transducer, rather than the xray machine when looking for diagnostic information from the abdominal cavity. In many instances such as pregnancy, pyometra, ascites etc. this is a correct decision. However radiography should not be overlooked as much valuable information can be gleaned from good quality images.

In order to ensure consistent quality in the production of abdominal radiographs a standard radiographic technique should be instigated.

Manual restraint is not an option in many countries under radiation protection regulations. Sedation is usually necessary except with cooperative or semicomatose animals. The choice of sedation should be made carefully as drugs such as some alpha 2 agonists may cause gastric dilatation. General anaesthesia is contraindicated as it has profound effects on the gastrointestinal tract. Limb ties, sponges and sandbags are required in order to maintain animals in standard positions.

Technique

For lateral abdominal radiographs the hind limbs should be positioned parallel to each other and at right angles to the spine so that there is minimal axial rotation of the abdomen. They should be drawn caudally and clear of the abdomen but not overextended as this tightens the skin and the skin folds may cause artefacts. The lateral view should be centered halfway between the top of the last rib and the iliac crest. The xray beam should be collimated but should include the diaphragm to the pelvic inlet. Commonly the xray beam is centered too far caudally. In giant breeds two separate views will be required. The largest cassette possible should be used rather than obtaining several small images.

The exposure factors are critical and the use of an exposure chart can aid consistent results. A High Mas technique helps to avoid movement blur.

Medium speed screens and the use of a latitude film will also aid consistent results and keep repeat exposures to a minimum. Computed and digital imaging systems generally deliver good quality images provided the algorithms are set up correctly.

At least two orthogonal views should be obtained. These usually are the right lateral recumbent and ventrodorsal views. Exceptions might be animals which are in pain or respiratory distress and in which case the VD view is not advisable.

General Abdomen

The presence of fluid (ascites) in the abdomen generally precludes accurate evaluation of the abdominal organs. If the intestines are displaced peripherally from their usual central location then the presence of an intra-abdominal mass should be suspected. However a full bladder displacing intestines dorsally or cranially may sometimes be mistaken for a mass.

Focal lack of serosal detail in the abdomen is indicative of a regional peritonitis. Pancreatitis may cause a focal lack of serosal detail in the right cranial quadrant of the abdomen. Intraperitoneal haemorrhage tends to be more focal in distribution than transudates. Adrenal tumors sometimes invade the great vessels and cause intraperitoneal haemorrhage. Post trauma cases may also contain some intra-abdominal fluid. Differentiation between fluids such as blood and urine is not possible with plain images and either abdominocentesis, contrast radiography and or ultrasonography ± fine needle aspiration of the fluid would be required. Uroperitoneum tends to cause a more generalised peritonitis. Bile peritonitis may also be seen as a result of abdominal trauma. Carcinomatosis may cause a regional or generalised peritonitis.

Gastrointestinal Tract

Often opposing lateral radiographs--right lateral and left lateral recumbent views--are most informative. Such positional studies moves fluid and gas through the GIT and sometimes radiolucent foreign bodies may be outlined by the gas in the tract. Air moving from one side of the stomach to the other is also advantageous in outlining foreign bodies or delineating the gastric wall thickness. In cases of suspected gastric torsion the right lateral recumbent view is often diagnostic. If the torsion is greater than 180 degrees the so called 'compartmentalisation line' may be identified. Sometimes all 4 views (RLR, LRL, VD, DV) are required in order to move gas around the stomach and allow the identification of the pylorus which is usually markedly displaced.

The distal colon usually lies at the junction of the dorsal and middle thirds of the caudal abdominal cavity. Dorsal displacement of the colon may be seen with prostatomegaly, uterine enlargement and uterine stump abscessation. Perforation of the GIT may cause a focal peritonitis and clumping of the small intestine. Lymphadenopathy of the medial iliac lymph nodes may enlarge and displace the colon ventrally.

Urinary Tract

Identification of the renal shadows is possible in about 50% of cases. They can be difficult to identify in thin animals due to the small amount of retroperitoneal fat. The bladder when full is usually seen and recognised by its standard location and shape. It lies well within the abdominal cavity in female dogs and in cats. If the bladder is full and the bladder neck is located at or within the pelvic cavity in the female or adult entire male it is in an abnormal location. Occasionally other soft tissue opacities in the caudal abdomen will require supplementary diagnostic techniques such as contrast radiography or ultrasonography. Radio-opaque urethral calculi may be identified in the prostatic and penile urethra.

Prostate

The prostate is usually located at the pelvic inlet. It is larger in some breeds such as the Scottish terrier but this breed also is predisposed to prostatic carcinoma. In adult male dogs the prostate may extend slightly further cranially and consequently displace the bladder further cranially into the abdominal cavity. Sometimes the two lobes may be discerned particularly in fat animals or in animals with prostatomegaly. Any change to the normal round smooth border should be investigated further with ultrasonography. The presence of two or more bladder shaped soft tissue opacities in the caudal abdomen presents a challenge as differentiation of the prostate and bladder will be required. Prostatomegaly may be due to prostatic abscess, prostatic neoplasia, paraprostatic cysts and ultrasonography if available would be more advantageous. Otherwise positive contrast urethrography is necessary. Paraprostatic cysts occasionally have a mineralised rim. Focal mineralisation within the prostate is often associated with neoplasia and the retroperitoneal space and the lumbar spine should be carefully evaluated for signs of metastases. Long standing chronic prostatitis may also cause focal areas of mineralisation.

Liver

The liver is usually located within the rib cage with the caudoventral margin extending caudally just beyond the costal arch. It is important to assess whether the film has been obtained in expiration before a diagnosis of hepatomegaly is made. Inspiratory radiographs may cause the liver to project beyond the costal margin. The shape and contour of the liver edge is also important as an irregular or rounded border is suspicious of liver pathology. Often the long axis of the stomach is a useful indicator of the liver size. Mineralisation within the liver often has a dramatic appearance but may or may not be of current clinical significance. It can be caused by bile duct calcification, dystrophic calcification of the hepatic parenchyma or bile duct carcinoma. Occasionally focal mineralisation may be seen in the vicinity of the gall bladder suggesting the presence of choleliths. An enlarged gall bladder may sometimes be seen in the caudoventral abdomen as a rounded margin projecting caudally particularly in cats. Displacement of the gastric axis cranially may be seen with diaphragmatic ruptures--acquired or congenital. A small liver may be seen with portosystemic shunts or cirrhosis.

Spleen

In dogs the spleen is usually identified on right lateral recumbent views. The tail is located ventrally just caudal to the pylorus. In the German shepherd dog the spleen often extends along the ventral abdominal floor to the cranial margin of the bladder. The head of the spleen is occasionally seen in the dorsocranial abdominal quadrant. On the ventrodorsal view the head of the spleen is located lateral to the fundus of the stomach. The body may extend along the left abdominal wall before it crosses towards the right. In cats the spleen is rarely seen on radiographs and if seen may warrant further investigation. Splenomegaly may result in displacement of adjacent organs. Splenic tumours result in splenomegaly and changes to the normal triangular shape on the lateral view. Rupture of a splenic neoplasm can cause local metastatic seeding resembling peritonitis and intra-abdominal haemorrhage may also be present. Mineralisation of the spleen is occasionally seen and may be caused by dystrophic calcification of haematomas, abscesses or neoplasia.

References

1.  Kealy JK, McAllister H. Fourth edition Elsevier USA 2005

2.  Dennis R, Kirberger RM, Wrigley RH, Barr F. Handbook of Small Animal Radiological Differential Diagnosis. WB Saunders UK 2001.

3.  Barr FJ, Kirberger RM. Manual of Canine and Feline Imaging of the Dog and Cat. BSAVA pubs UK 2006

4.  Thrall DE. Textbook of Veterinary Diagnostic Radiology Fifth edition. Elsevier USA 2007