Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

312 Small Animal Radiolo g y and Ultrasono graphy
Portal Vein Thrombosis. Thrombosis of the portal vein has been reported infrequently.
In most cases an underlying problem such as pancreatitis, hypercoagulopathy, or
neoplasia was present. 322 Congenital lesions are rare. 323 Imaging techniques have included
contrast radiography of the portal system, which revealed acquired portosystemic shunts,
and ultrasonography, which revealed intravascular masses or abnormal patterns of blood
flow or both. 324,325
Cirrhosis. Doppler ultrasonography has been used in the evaluation of experimentally
induced cirrhosis. 326 Extensive extrahepatic portosystemic shunts were identified.
Portal blood flow velocity was markedly reduced from a normal level of 18.1 mm/sec to 9.2
mm/sec. Mean portal blood flow was also reduced from 31 ml/min per kg to 17.2 ml/min
per kg. Large incident angles can introduce significant velocity errors, and in some dogs it
is impossible to attain a proper incident angle. In these dogs, accurate measurement of portal
blood flow velocity cannot be obtained. Qualitative as well as quantitative information
may be obtained from portal Doppler ultrasonography. This includes determination of
patency of the portal vein and direction of flow, evaluation of portosystemic shunts,
recognition of arteriovenous malformations, and discriminating between dilated bile ducts
and hepatic vessels.
ALIMENTARY OVERVIEW
One approach to evaluating the alimentary tract is to view it as a tube that begins at the
mouth and ends at the anus. Functions that are performed along the tract include secretion
and lubrication, propulsion, storage, digestion, absorption, flow control, and fluid
equilibration. Any imbalance in these can create a difference in the size of the organ, or
segment of the organ if only part of it is affected, or the relative amounts of fluid and gas
in the organ(s). Even displaced organs have changes in their fluid or gas content, which
may be the first clue to organ malpositioning.
The initial interpretive pass over the alimentary tract should include (1) a determination
of the relative position of these organs and (2) a determination of the relative amount
of fluid and gas present in the various segments. Any evidence of disproportionate gas or
fluid accumulation should be scrutinized further. Some generalities about fluid-gas balance
may be helpful. Although not unequivocally specific, a large buildup of gas in the small
intestine usually is associated with high-grade, complete obstruction (e.g., foreign body or
intussusception), fulminant inflammation (e.g., no peristalsis due to severe viral enteritis),
or severe circulatory disorders (e.g., mesenteric volvulus). By comparison, for the stomach,
a large buildup of gas may indicate aerophagia related to nausea, pain, or dyspnea, with or
without outflow obstruction. A buildup of fluid relative to gas in the stomach, small bowel,
or large bowel is usually indicative of partial obstruction (the gas moves on but the fluid
does not), moderate inflammation or irritation (including exudative, transudative, or hemorrhagic
disease), and the presence of osmotically active substances (either ingested or produced
in the bowel by chemical or bacterial reactions). From a survey radiographic
perspective, any disruption of an even distribution of fluid and gas among the stomach,
small bowel, and large bowel, as well as within the organs, particularly the small bowel,
should be viewed as suspicious.
The second interpretive pass over the alimentary tract should include (1) a consideration
of whether additional views, including repeating the survey radiographs, are indicated
to determine if any suspected fluid-gas imbalance is reproducible, (2) an assessment based
on species, age, breed, possibly gender, history, and clinical signs about whether what is
seen on the survey radiography fits with the clinical picture, and (3) a determination of
whether conservative management and recheck versus immediate more aggressive diagnostic
(e.g., contrast radiography, ultrasonography) or therapeutic procedures (e.g., organ
decompression, laparotomy) is indicated. Detailed interpretation of survey radiographs
can lead to the appropriate choice between endoscopic and contrast radiographic procedures
should additional diagnostics be deemed necessary. For instance, fluid-distended
segments and normal segments of small intestine will not be investigated appropriately
using endoscopy. A knowledge of possible diseases, the species, breed, and age predispositions
for certain diseases, the effects of drugs (systemic sedative or specific alimentary