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