Value of examining low velocity abdominal blood flow in the dog

Q: Please review use of examining low-velocity blood flow in the dog's abdomen.

A: Dr. Lorrie Gaschen gave an excellent lecture, "Optimizing low-velocity Doppler sonography in the abdomen," at the 2008 American College of Veterinary Internal Medicine Forum. Here are some relevant points from the lecture:

There are many instances when examination of low-velocity blood flow in the abdomen can be of diagnostic importance. Detection of arterial flow in major arteries is not as technically difficult as that of small arteries such as in the renal cortex. Venous blood flow having a much lower velocity can prove difficult. Detection of portal blood flow is one example.

Assessing organ parenchymal perfusion is a new area under investigation in veterinary medicine. Low velocity blood flow occurs mainly in small veins, venules and capillaries. The minimum detectable flow velocity is inversely dependent on the ultrasound wave frequency; when using a 2.5 MHz probe, the minimum detectable flow velocity is 1.5 cm/s. In addition, commercial Doppler devices are typically limited to the frequency below 15 MHz; so the minimum detectable flow velocity for most commercial Doppler devices is around 0.35 cm/s.

Optimizing low velocity blood-flow imaging

To accurately identify slow flow and not mistakenly consider flow to be absent, the Doppler parameters must be optimized for detection of slow flow. First, one must be sure that the B-mode parameters are optimized, including depth of field, location and number of focal zones, output power, and time-compensated gain. Spectral or power Doppler ultrasound is more sensitive in the detection of slow flow than is standard color Doppler flow ultrasound. The wall filter settings should be as low as possible so that low-frequency signals arising are not eliminated. The Doppler angle should be optimized, not only by correcting the angle with the computer, but by positioning the animal so as to allow the transducer to be placed in an appropriate superficial location. The strength of the color flow image decreases with the Doppler angle; if the angle is too small, the transducer should be moved to a more suitable position. In addition, the color box should be as small, narrow and superficial as possible, whereas the gate used for pulsed Doppler sampling should be wide. A larger or wider box (i.e., greater depth) requires a longer round-trip time for pulses, reducing the Pulse Repetition Frequency (PRF), increasing the signal processing time, degrading the image, and diminishing the ability to depict slow portal venous flow. The velocity range should be adjusted to a level appropriate for flow within the portal vein.

The velocity scale must be adjusted differently for high versus low velocity flow. For examining low velocity flow and smaller vessels, a low PRF is required. Too high a PRF setting will not allow sampling of the lower velocity flows of veins and small vessels. If the PRF is not reduced, veins will appear to be void of flow, possibly leading to a false diagnosis of ischemia due to torsions, embolism or trauma. Neighboring veins and small arteries in other organs should be sampled to ensure that the velocity scale settings are correct. For vessels with very slow flow (portal venous flow, splenic and mesenteric veins), a high filter setting may cause them to be inadvertently obscured or missed. Filters operate at variable frequencies to eliminate signals from low velocity blood flow. To avoid the loss of signal that characterize slow flow, filter settings should be kept at the lowest possible setting (typically in the 50-100-MHz range).

Under-correction of the Doppler angle will result in a falsely low flow estimate. This may be of importance when examining an animal for portal hypertension where flow velocity within the portal veins is an important criterion. Increasing color Doppler gain to improve visualization of low flow vessels often can lead to noise artifacts, as these will also be enhanced by higher gain. These degrade the image making small vessels impossible to examine. For this reason, power Doppler allows improved visualization of small, low flow vessels that cannot be detected with color Doppler. Power Doppler (PD) is a technique in which a color map displays the integrated amplitude of the reflected Doppler signal from red blood cells. This technique is important since it allows detection of much lower flow velocities in small vessels, such as those in the renal cortex, when compared to color Doppler. Since PD records only amplitude, gain can be increased more than with color Doppler so that small vessels are seen without being degraded by noise.

The operator-adjusted gate defines the size and location of the area from which Doppler information is obtained. The gate should be as small as possible to exclude erroneous signals arising from adjacent vessels or marginal flow. Too large a gate may admit erroneous signals from adjacent vessels or may lead to acquisition of data from extraneous parenchyma. Too small a gate may give the false impression of reduced or even absent flow. For veins and small vessels, the gate should cover the lumen of the vessel but should not be larger than the vessel.

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.

Portal hypertension

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.

Hepatic congestion

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: jdhoskins@mindspring.com.