David S. Biller, DVM, DACVR
Professor and Head of Radiology, Kansas State University, College of Veterinary Medicine, Department of Clinical Sciences
Manhattan, KS, USA
Abdominal Compression Radiography
Abdominal compression radiography is the use of a radiolucent device to compress an area of interest, thus decreasing subject thickness and allowing anatomic isolation of a structure. This technique provides for evaluation of the size, shape, location, and opacity of a specific organ, without the degree of superimposition of structures seen on survey radiographs. Indications include a suspected abnormality on survey abdominal radiographs, which is inconclusive due to superimposition of structures. Compression radiography of the abdomen provides additional information in cases, which helped with diagnosis, prognosis, and therapeutic options.
The equipment necessary is minimal and consists of varying sizes of wooden or plastic (Lucite or Plexiglas) stirring spoons or paddles. Any rigid radiolucent material may be used. The size of the compression surface should be based on the area of interest. Ideally the compression surface should be 1.5 times the area of the target organ. The field size should be collimated to include only the region of interest. This will improve image quality and decrease personnel exposure by decreasing scatter radiation. It is important to recognize that compression of the animal will decrease the subject thickness, therefore exposure factors should be adjusted accordingly. A decrease in kVp by approximately 10% to 15% is usually adequate. The animal can be measured while compressed if exposure factors are in question. Failure to decrease technique will result in overexposure. Tranquilization may be helpful to aid in positioning, but is not required.
This technique should not be used when there is a large increase in uterine size with suspected pyometra because there is the potential of uterine rupture. The same holds true for very large masses which are cystic in nature, such as hemangiosarcoma. Diaphragmatic hernia is a contraindication because of the potential to further displace abdominal contents into the thoracic cavity. Compression radiography can also be used in combination with contrast procedures of the gastrointestinal or upper and lower urinary tract.
Compression radiography is helpful in delineation of the origin of a mass, size and shape of an organ, and abnormal opacities (gas or mineralization) such as renal, ureteral, or cystic calculi. This technique has also been used when assessing for fetal death. This allows for improved detail of fetal skeletal structures when checking for normal alignment.
Cases for compression should be selected based on the area of interest and the size of the animal. The region of interest must be a compressible area. For example, this technique could not be used on structures underlying the rib cage. Cats and small to medium size dogs are more readily examined because abdominal organs are easier to separate. This technique has been used on giant breeds with success. Compression can be used with any position, although lateral recumbency is most common.
Contraindications include enlarged organs in which rupture of the structure may occur. Examples would include a severely enlarged uterus or severe splenic enlargement due to a cystic mass. Diaphragmatic hernia is also a contraindication because compression may displace abdominal viscera into the thoracic cavity.
Radiography of the gastrointestinal tract is indicated in cases of foreign body ingestion, vomiting, regurgitation, abdominal pain, abdominal distention, weight loss, anorexia, and abnormal abdominal palpation. Standard survey abdominal radiographs include a ventrodorsal and either a left or right lateral recumbent view. The determination of which lateral radiograph is obtained is often personal preference. There are differences in the appearance of various organs, such as the kidneys, spleen, and gastrointestinal tract, between the right and left lateral recumbent radiograph. This varied appearance is particularly noticeable in the gastrointestinal tract, which is dependent on gas to provide contrast for visualization of the mucosal surface.
Fluid and gastric contents are extremely mobile and tend to move to the dependent portion of the stomach during postural changes. Gas will rise to the non-dependent portion of the stomach. For example, in right lateral recumbency gas accumulates in the fundus, and in left lateral recumbency redistribution to the pyloric region occurs.
Gas within the gastrointestinal tract serves as a negative contrast media. Specifically, the change in positioning of the animal for the opposite lateral abdominal radiographs will allow for the redistribution of gas already present in the stomach, small, and large intestines. The position of gas in the stomach changes in the following manner. If the animal is in left lateral recumbency gas will be present in the pylorus if the stomach is in its normal location. Conversely, if the animal is in right lateral recumbency gas will be present in the fundus. It is important to realize that the amount of gas will have and effect on which portions of the stomach will contain gas. In a severely gas distended stomach, gas may be in all portions of the stomach on both lateral abdominal radiographs. Even in these situations the location of the pylorus can be determined. The right lateral recumbent radiograph is recommended in determining gastric dilatation from gastric dilatation with volvulus.
Even in cases in which ileus is detected it is often helpful to gain additional information to help with surgical planning and prognosis.
The fluid filled pylorus is an area, which can be misdiagnosed as a cranial abdominal mass when the right lateral recumbent radiograph is taken. When the left lateral abdominal radiograph is obtained the pylorus will be filled with gas.
This technique may not always be useful if there is minimal air within the gastrointestinal tract. Air can be introduced via and orogastric tube and is especially useful in disorders of the stomach.
Indications for the use of contrast in evaluation of the stomach include:
1. Suspicion of luminal or mural gastric masses
2. Radiolucent gastric foreign bodies
4. Recurrent or non-responsive vomiting
5. Gastric localization, identification, size, shape, and margination
6. To evaluate motility
Preparation of the patient include survey radiographs which should always precede contrast studies. Survey radiographs allow evaluation for subsequent adjustment of technical exposure settings for the contrast study. The animal should be fasted 12-24 hours before radiography. Cleansing enemas should be done the night prior and 2-3 hours before the procedure. Contrast procedures should always be individualized. If a patient has acute abdominal pain or there is a potential for time delay which may make a difference the enema and fasting should be overlooked. Many drugs affect motility and these drugs should be discontinued for an appropriate interval before any contrast study is done. Contrast agents include negative (room air) and positive (barium sulfate suspension which is micropulvarized). If perforation is suspected an organic iodinated solution like Iohexol should be used. Other equipment includes mouth gag and an orogastric tube. Gastrograms are most often used as part of an otherwise standard upper GI series (the small bowel evaluation follows the introduction of a positive contrast media). Technique includes dosage of barium of approximately 5 mls/lb that is administered via a gastric tube. Radiographs are routinely taken in right lateral and ventrodorsal but for complete and accurate evaluation of the stomach a DV and left lateral films may be taken. Films are taken immediately to evaluate the complete stomach, before it starts to empty.
Partial Barium Enema / Pneumocolon
A helpful technique in differentiating distended small intestine from large intestine is a partial barium enema. This technique also helps localize the colon in the abdomen. Technique includes introduction of either barium or gas (room air) at a dosage of 6 ml/Kg. Introduce it with a flexible catheter as far into the colon as possible. Then take a ventrodorsal and well as right and left lateral abdominal radiographs.