PRACTICAL EXAMPLES WITH COLOR, SPECTRAL AND POWER DOPPLER
Renal blood flow
Hemodynamic alterations of the microcirculation of the vascular bed may well precede the clinical presence of renal disease.
Ultrasonography has become an increasingly important modality in the diagnosis of renal disease. Color and power Doppler now
allow demonstration of the entire renovascular tree from the main renal arteries to the arcuate arteries as well as their
terminal branches. For the detection of discrete losses of blood flow in the peripheral cortex, power Doppler has the advantage
over color Doppler. Color Doppler and PD have also been used to detect blood flow in renal allograft cortices in humans and
to compare findings of flow alterations in the cortices with histopathologic diagnoses. It is generally considered that abnormal
flow patterns are highly predictive for both rejection and acute tubular necrosis despite the non-specific nature of the findings.
Normal mesenteric vein flow is monophasic with slight undulations in maximal velocity. Mean portal flow velocity has been
reported from 14.7 (+/- 2.5) to 18.1 (+/- 7.6) cm/s and maximal velocity at 32 cm/s. Portal hypertension occurs as a result
of thromboembolism, stenosis, or compression by enlarged hepatic lymph nodes or other masses or chronic liver disease. Portosystemic
shunts are another cause. Posthepatic causes include compression of the hepatic veins, vena cava or right heart disease. Sonographic
findings include reduced blood flow velocity, multiple tortuous varices (portal collaterals), chronic hepatic changes (small
liver), and free abdominal fluid.
Splenic, portal and mesenteric vein thromboembolism
The absence of flow in a vein can be due to thromboembolism, external compression or neoplastic invasion. To detect subtle
changes in echogenicity of the thrombus with gray-scale imaging, machine settings must be optimized and high-frequency transducers
are required. A recently formed thrombus is hypoechoic and can be difficult to detect. PRF and filter settings should be set
as low as possible. These parameters increase the sensitivity for detecting low velocity flow. Sensitivity to low flow is
also increased with higher Doppler frequencies. Recanalization may occur and be recognized as small color or power Doppler
signals in and around the thrombus. Pulsed Doppler cranial to the thrombus will show absent or reduced flow. Color Doppler
shows absence of flow within the venous lumen. Any absence of flow in color Doppler should be confirmed with power and spectral
Doppler which have better sensitivity.
Veins within the abdomen have minimal resistance to flow and phasic changes occur in response to cardiac activity and changes
in intra-abdominal and intrathoracic pressures. Masses adjacent to the caudal vena cava, hepatic or renal veins can cause
venous compression. Budd-Chiari syndrome is due to obstruction (compression or thrombosis) of the hepatic veins or caudal
vena cava. Spectral and color Doppler will show reduced flow velocities and signals. Dilation of the veins can be seen with
right heart failure, pulmonary hypertension, pericardial effusion, constrictive pericarditis, atrial tumor and tricuspid valve
disease. The liver may be enlarged in all of these instances and ascites may be present. The typical Doppler wave form will
be altered with reduced velocities towards the heart and possible increase in velocities away from the heart (atrial contraction).
by Johnny D. Hoskins
DVM, PhD, Dipl. ACVIM
Dr. Hoskins is owner of Docu-Tech Services. He is a diplomate of the American College of Veterinary Internal Medicine with
specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail: