Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Three The Abd omen 275
Abdominal Vasculature. Within the abdomen certain vascular structures may be seen in
some normal animals. In animals with large amounts of fat, the aorta and, in some cases,
the caudal vena cava may be seen on the lateral view. They can be recognized because of
their linear shape and position. The aorta is slightly ventral to the spine and branches at the
level of L5, the deep circumflex iliac arteries, and at the level of L6-L7, the external iliac
arteries. The caudal vena cava runs obliquely from the caudal dorsal abdomen in a cranioventral
direction toward the liver.
Vascular anatomy can be examined using ultrasonography. 59-62 The caudal vena cava and
the aorta and their branches, the hepatic veins, and the portal vein can be detected and examined.
In most vessels, flow can be observed when high-resolution transducers are used with
relatively high gain settings. Color-flow Doppler techniques facilitate flow detection and
qualitative characterization of flow direction, velocity, and uniformity. Flow can be quantitated
using Doppler ultrasonography. The aorta and caudal vena cava can be examined in
both longitudinal and transverse planes. The aorta is dorsal and slightly to the left of the caudal
venal cava. The aortic pulsations can be observed. The wall of the aorta is slightly thicker
than that of the caudal vena cava, but in smaller dogs the wall usually is not identified. 59
Aortic pulsations may not be readily apparent and may be transmitted to the caudal vena
cava, making discrimination between these vessels difficult. Compression of the vessels using
the ultrasonographic transducer helps discriminate between the aorta and the vena cava,
because the cava is more easily compressed. 59 The celiac and cranial mesenteric arteries can
be detected if the stomach and intestines are relatively empty. They can be followed a short
distance from their origin before being obscured by intestines. The phrenicoabdominal artery
can be identified just cranial to the left kidney and is used as a landmark to locate the left
adrenal gland. The renal arteries may be seen at their origin; however, they are rarely seen in
the renal hilus. Renal arteries can be detected using Doppler imaging even though they cannot
be identified specifically. The terminal branches of the aorta can be seen most readily
when the aorta is followed in the transverse plane. The splitting of the aorta into several
round vessels can be identified. These branches are more difficult to follow in the longitudinal
plane. It is difficult to follow the vessels beyond the inguinal ring. The caudal vena cava is
wider than the aorta and lacks the pulsations observed in the aorta. The wall of the vena cava
can be identified in some larger dogs. The renal veins sometimes can be identified. Splenic
veins can be identified but usually cannot be traced to the portal vein. The vena cava may be
followed in the transverse or longitudinal planes to the liver, where the hepatic veins can be
observed to join the vena cava. The portal vein can be traced from the confluence of the
mesenteric veins to the liver, where it branches in the porta hepatis. The presence of hyperechoic
walls helps to identify the portal vein. The portal vein, hepatic vein, and extrahepatic
bile ducts come together at the porta hepatis. Because of the small size of the extrahepatic bile
ducts compared with that of the portal veins, it is usually easy to recognize the difference
between these structures. The echogenicity of the walls of the portal vein is helpful in recognizing
the intrahepatic portal branches. The hepatic veins course toward the caval hiatus of
the diaphragm, facilitating differentiation of these from dilated intrahepatic bile ducts.
Doppler ultrasonography can facilitate characterization of these three structures. Portal
blood flow can be measured using pulsed Doppler ultrasonography. An overestimation of
portal flow usually results. A scanning plane through the right eleventh or twelfth intercostal
space is recommended. 62 Doppler techniques can be used to examine intraabdominal arteries
and veins. The examination is technically difficult in an awake, unsedated animal. The
value and predictability of this technique are unproved in veterinary medicine; however, it is
helpful in ruling out thrombi, hepatofugal portal venous flow, and dirofilariasis. 63-66
GASTROINTESTINAL SYSTEM
SPECIAL PROCEDURES: GENERAL CONSIDERATIONS
The abdominal organs are affected by many lesions that exert their pathologic effect at a
physiologic or anatomical level not discernible by survey radiography. These lesions may
be identified by one or more special (contrast) procedures. Many different special
procedures may be performed to evaluate the abdominal organs. Recommended methods