BASIC RADIOGRAPHY OF THE ABDOMEN

BASIC RADIOGRAPHY OF THE ABDOMEN

Abdominal radiography is indicated for primary abdominal diseases and other disorders which may involve, or extend into the abdomen. It is complementary with ultrasonography. When using both (which is recommended), it is better to perform radiography first, to help direct the ultrasonographic examination and avoid radiographic artefacts, created by the ultrasound gel.

The interpretation of abdominal radiographs requires a logical, consistent approach and a good knowledge of normal anatomy and variations due to age, breed, body condition, the effect of sedation and diet. Carefully observing changes and deviations from the normal appearance on the image can direct us toward the pathological process.

The abdomen has greater superimposition of structures than most other body areas and relatively poorer radiographic contrast, caused by the similar opacity of the tissues and a greater amount of scatter created in the abdomen. The main source of contrast is fat between the viscera, which is less radiopaque than soft tissue and enables visual separation of the organs (this feature is termed serosal detail). An inherently poor contrast must be taken into account when choosing the radiographic technique. Exposure factors should be selected in a way to maximize radiographic contrast – use low kilovoltage (kV) and high milliampere second (mAs). To reduce the scatter (which is proportional to the volume of tissue exposed), a grid can be used. Grids are recommended with the depth of tissue > 10 cm and even in obese animals at depth < 10 cm. The use of grid requires higher exposure factors and increases the need to reduce movement of the animal (consequently chemical restraint might be necessary). In very obese animals, or the ones with a large volume of free abdominal fluid, higher exposure factors may be required.

Abdominal radiographs should be taken at the end of expiration, when the respiratory pause minimizes movement blur and the diaphragm is positioned most cranially, resulting in increased abdominal volume and greater separation of the organs.

Sedation of the patient facilitates the procedure, although in sick or compliant animals radiography can be done without it. For gastrointestinal contrast studies, sedation or general anaesthesia is contraindicated as it reduces gastrointestinal motility and poses a risk of barium or gastrointestinal content aspiration. If it is not possible to perform the study in a conscious animal, light sedation can be used with neuroleptanalgesia, which has little effect on oesophageal or gastrointestinal motility.

In order to reduce some disturbing factors and obtain the maximum amount of information from the image, it is wise to prepare the patient and perform radiography as an elective procedure when possible. The superimposition of ingesta and faecal material on the organs can make interpretation difficult, causing artefacts and obscuring pathology. Distended gastrointestinal tract and bladder occupy a significant amount of space and increase superimposition of the other structures. It is recommended to withhold the food for 12 hours prior to the elective procedure to have a relatively empty stomach and small intestine and give the patient the opportunity to defecate and urinate. In case of urinary tract studies, a non-irritant enema (e.g. warm water) 2 hours prior to examination ensures empty large intestine. An enema administration is contraindicated when bowel obstruction is suspected because it will alter the native pattern of bowel gas and fluid. The animal’s coat should be dry and free of dirt. Wet hair can cause streaky appearance and dirt on the coat can be mistaken for mineralisation and calculi.

The minimum recommendation for radiographic evaluation of the abdomen is obtaining two orthogonal views (lateral and ventrodorsal) for accurate localization of the structures. When a gastrointestinal obstruction is suspected, it is advisable to take both lateral views in addition to the VD, as this allows the redistribution of gas and fluid within the intestine and stomach. Different locations of gas and fluid may make the cause of obstruction visible on only one view.

Laterolateral views

Either left or right lateral view can be obtained, but the selection should be consistent to familiarize with the normal appearance and aid the interpretation. The appearance differs slightly between both recumbencies due to the movement of gas and fluid within the gastrointestinal tract and the effect of gravity on the mobile viscera.

  • On the right lateral view, the gas (if present) is visible within the gastric fundus. Fluid gravitates to the pylorus, where it may mimic a round soft tissue opacity and can appear as a mass or foreign object. Renal silhouettes are more separated, the left kidney is more ventral. There is greater visibility of the tail of the spleen.
  • On the left lateral view, the gas is within pylorus and the fluid gravitates to the gastric fundus. Duodenum is seen more consistently, and gas is often seen in proximal duodenum (especially if the dog is laid in left lateral recumbency first). Kidneys are more superimposed. Liver appears larger than on right lateral view, the spleen is usually not visible. This view is best for detecting free abdominal gas.

The animal is positioned in chosen lateral recumbency with hindlimbs extended slightly caudally, avoiding hyperextension which would tighten the abdominal wall. Foam wedges can be placed under the sternum and between the femurs to prevent axial rotation. In accurate, non-rotated position, rib heads, transverse processes and hip joints should be superimposed. The X-ray beam is centred slightly caudal to the caudal-most part of the last rib. The picture should include the diaphragm and greater trochanter. In large dogs, it may be necessary to take two radiographs to image the entire abdomen. For lower urinary tract studies, the collimation should extend caudally to include the perineum and penis or vulva to ensure that the entire urethra is included in the image. A lateral view with the hindlimbs drawn cranially is used to evaluate the lower urinary tract in male dogs with the assessment of the full length of the urethra without superimposition of the femurs.

Ventrodorsal view (VD)

The VD view is preferred to DV. The abdominal contents “spread out” in dorsal recumbency resulting in less superimposition of the abdominal structures and a reduction in soft tissue depth. The patient is placed in dorsal recumbency and supported with foam wedges. It is helpful to place thorax in the positioning trough and put pads under thighs to reduce tilting and axial rotation. Thoracic and lumbar vertebrae should be in a straight line without lateral curvature. The hindlimbs are preferably in “frog-legged” position, which minimizes superimposition of the skin folds on the caudal abdomen. The X-ray beam is centred on the umbilicus and collimated to include diaphragm and greater trochanter.

DV radiographs of the abdomen are rarely made and usually don’t form the basis of the orthogonal view. They may have to suffice when a patient cannot be positioned in dorsal recumbency, but the abdomen will be more crowded and organ visibility will be reduced.

LITERATURE:

Holloway A., McConnell F.: BSAVA Manual of Canine and Feline Radiography and Radiology. A Foundation Manual. BSAVA, 2016

O’Brien R., Barr F.: BSAVA Manual of Canine and Feline Abdominal Imaging. BSAVA, 2009

Dennis R. et al.: Handbook of Small Animal Radiology and Ultrasound. Techniques and Differential Diagnoses, 2nd Ed. Elsevier, 2010

Thrall D. E.: Textbook of Veterinary Diagnostic Radiology, 7th Ed. Elsevier, 2018