Daniel A. Feeney, DVM, MS, DACVR
Professor of Radiology, University of Minnesota College of Veterinary Medicine, St. Paul, MN, USA
A. Abdominal preparation & positioning
1. When & How Should Abdominal Preparation Be Performed?
Ingesta & stool in the gastrointestinal tract can obscure abdominal organ detail and limit identification of abnormal densities in and around organs regardless of whether these organs are intraperitoneal or retroperitoneal. Therefore, withhold food for 24 hours and give at least one cleansing enema 1–2 hours before routine abdominal radiography & special radiographic procedures. For examinations of a suspected "acute abdomen", however, this is not advised because of delays and deleterious mechanical effects on compromised viscera. Be aware that gas can be introduced into the small bowel and that the small bowel can appear "fluid-filled" after a vigorous enema so allow time for these effects to clear before filming. If you are suspecting alimentary causes for the acute abdomen, administer nothing and get the views suggested below.
2. What About Positioning for Abdominal Radiography?
Routine positioning for small animal abdominal radiography should be in right lateral and ventrodorsal (VD) recumbency and exposures made using a vertically (downward) directed X-ray beam and a grid to control scatter radiation reaching the screens. The right lateral recumbency is preferred to left lateral recumbency because the kidneys are separated more in a craniocaudal direction limiting superimposition. In addition, the diaphragmatic crura are parallel in right lateral recumbency (vs. crossed in the left lateral). Be aware that the fluid in the pyloric region of the stomach can resemble a "ball" in the stomach. If this causes confusion in a patient in which you suspect a gastric foreign body, displaced stomach or impeded gastric emptying, repeat the lateral view in left lateral recumbency wherein the gastric pylorus will fill with gas (there is usually some gas in the stomach in most small animals). For the lateral views, the hind legs should be pulled caudally to allow more clear visualization of the pelvic inlet. For the VD view, the hind legs should be "splayed" laterally and pulled somewhat caudally to limit superimposition on the pelvic organs. However, it is not necessary to position the legs in the extended position like that used for hip dysplasia! The goal should be to include the region from the diaphragm to the anus on all routine views (use multiple films on giant breeds as necessary). Expose the film during end expiration to limit organ crowding and motion.
B. Basics of Abdominal Interpretation
Use the Geometric Roentgen Signs (size, shape, margination/contour, number, & location), the Density Roentgen Signs (air, fat, soft-tissue/water, bone/mineral, & metal) and the Silhouette Sign concept (two structures of equal radiographic density cannot be distinguished at the point of contact). These will allow you to determine whether each of the organs and spaces listed below are morphologically normal (within limits of radiographic sensitivity) and to infer something about the Functional Roentgen Signs (continuity, intact borders or walls, propulsion, & excretion). Use special radiographic procedures (e.g., upper GI study, air or barium enema, ...), if necessary.
C. Overview of the Abdominal Radiograph
1. Identify the following: (Note: "region of..." indicates the organ can't be seen per se)
a. Diaphragm - all parts
b. Portal vein-associated organs:
i. Liver (use character of margins & fundic/pyloric angle to determine size)
ii. Region of gall bladder
iv. Region of pancreas
c. Alimentary organs
i. Stomach and gastroesophageal junction
ii. Small intestine
d. Urinary organs
i. Kidneys (retroperitoneal)
ii. Region of ureters (mostly retroperitoneal)
iii. Urinary bladder
iv. Region of urethra (some retroperitoneal)
e. Genital organs
a) Prostate gland
a) Region of ovary
b) Region of uterus
c) Region of vagina
f. Peritoneal and retroperitoneal spaces (look for fluid, nodules, etc. & their consider their relationship to the regional organs)
g. Abdominal wall (including spine, pelvis & subcutaneous tissues)
2. Are there any masses or abnormal densities found that are not associated with specific organs or "regions" in C-1 not found during the overview (e.g., mesenteric mass?)
3. Formulate rule-outs & determine of more complex imaging procedures are indicated/necessary?
D. Radiographic Evaluation of the Acute Abdomen
Evaluation of an abdominal radiograph:
1. Is there evidence for bowel obstruction?
a. Fluid/air accumulation (? S.I. > lumbar vertebral body length)
b. Differentiate dynamic ileus (obstruction, 2 or more populations of small bowel diameter) from adynamic ileus (atony on population of mildly fluid or gas-distended small bowel)
2. Are there intra/retroperitoneal effusions?
c. Leakage (urine, alimentary tract contents, other)
d. Beware of young and emaciated animals! (judge abdominal girth and compare peritoneal to retroperitoneal contrast)
3. Is there free air?
a. Alimentary tract rupture
b. External wound
c. Confirm using left decubitus view (left side down, horizontal beam technique)
d. Differentiate from intraparenchymal air (emphysematous cystitis/cholecystitis, pneumatosis coli necrosis, abscess, foreign body...)
4. Is there an abdominal mass?
a. Try to determine organ of origin
b. Displacement of other organs = key
5. Is there organ malposition?
a. Torsion/volvulus - stomach
b. Torsion - spleen
c. Volvulus - mesentery (often looks like distal high-grade small bowel obstruction)
d. Hernia - diaphragmatic, inguinal, nonspecific abdominal, scrotal
e. Horizontal/opposite recumbent views will help identify affected organ
6. Are there abnormal densities?
a. Metal (rocks, bullets)
7. Is there periabdominal evidence of trauma?
a. Axial/appendicular skeletal fractures
b. Subcutaneous emphysema
c. Swelling of soft tissues (?hernia, hematoma, etc.)
8. Compare and further analyze patient clinical demeanor, history, physical exam combined with 1–7 to determine patient status:
a. Patient is critical -> Immediate intervention (e.g., surgery)
b. Patient is stable -> Can tolerate detailed evaluation (alternative recumbency view radiographs, horizontal-beam radiographs or either abdominal ultrasound or an organ or system specific special radiographic procedure)
E. Causes of Acute Abdomen in Small Animals
1) Obstruction or overdistention, 2) Inflammation or sepsis, 3) Organ displacement or malposition, 4) Perforation or rupture, 5) Vascular compromise (ischemia), or 6) Other (e.g., gunshot, calculi, adverse drug reactions or specific organ effects).