Page 57 - Feline Cardiology

52 Section B: Diagnostic Testing

reflected sound wave depends on the direction and the baseline (Figure 7.1) and on color-flow Doppler,
velocity of the reflector and the transmitted frequency. aliasing is represented by color reversal (Figure 7.2); in
Since the ultrasound frequency emitted from the both situations, aliasing means that an accurate mea-
machine and the velocity of sound in soft tissue and surement of blood velocity is not possible. The Nyquist
blood are known, the velocity of red blood cells can be limit is controlled by ultrasound beam depth and trans-
Diagnostic Testing graph to plot curves, called envelopes, that represent the interest, the lower the transducer frequency that will be
calculated and the information used by the echocardio-
ducer frequency. Therefore, the deeper the structure of
movement of blood in a chosen location. Color-flow
necessary. In small animals such as cats, a 7.5 mHz probe
Doppler superimposes color labeling (of red blood cells)
is usually adequate for a complete cardiac examination,
on the black-and-white 2D image, typically with blood
but in a particularly large or obese cat a lower frequency
moving toward the transducer labeled as red and blood
moving away from the transducer labeled as blue. probe (e.g., 5 mHz) may be necessary.
With tissue Doppler imaging (TDI), the velocity of
Spectral Doppler (either pulsed-wave, continuous-wave, myocardial wall motion (rather than blood) in a region
or high-pulse repetition) calculates the blood flow veloc- of interest is measured. Spectral Doppler and TDI results
ity in a region of interest, selected by the operator by are displayed in graph form (velocity against time with
moving a cursor on the 2D image, and displays the a simultaneous ECG). Blood flow and tissue velocities
results graphically. in the direction of the transducer are displayed as posi-
Pulsed-wave and continuous-wave Doppler modali- tive signals and those away from the transducer are dis-
ties are complementary. Pulsed-wave Doppler allows played as negative signals. Tissue Doppler techniques are
one to examine the velocity and direction of blood flow used for identifying otherwise undetectable myocardial
in a specific region of interest because the sample volume wall motion abnormalities, and hold great promise for
is chosen by the operator. However, pulsed-wave Doppler early detection of subtle lesions such as the earliest
cannot record the maximal velocity or direction of flow stages of hypertrophic cardiomyopathy (Nagueh et al.
when blood flow velocity is high, as occurs in various 2003).
pathologic states. With high-velocity blood flow, pulsed-
wave Doppler signals often are aliased, meaning that ECHOCARDIOGRAPHIC EQUIPMENT
they are of too high a velocity to be accurately inter-
preted with the ultrasound probe and Doppler modality Optimal 2D images are obtained when structures
being used. Thus, once the site of abnormal flow is are perpendicular to the ultrasound beam, whereas
located with pulsed-wave Doppler (spectral or color- results are most accurate with Doppler echocardiogra-
flow), the direction and velocity of flow can be measured phy when the Doppler sound waves are directed parallel
using continuous-wave Doppler. Continuous-wave to the target. As the angle between the ultrasound
Doppler samples along the entire cursor line, so although beam and moving red blood cells increases, the velocity
it is not encumbered by velocity limits, the precise ana- measured with Doppler will become less accurate.
tomic localization of the measured flow is not possible. Angles greater than 25° yield unacceptable estimates
Therefore, both modalities are necessary to fully define of velocity. Many ultrasound machines have a feature
the characteristics of abnormal, high velocity flow (as that corrects for poor alignment (typically called angle
occurs with mitral regurgitation, ventricular septal correction). Unfortunately, this feature is prone to error
defects, etc.). and should not be used, because correction for poor
Pulsed-wave and color-flow Doppler signals alias angulation in one plane does not account for poor
when flow velocities are above the Nyquist limit angulation in either of the other two dimensions.
(half the pulse repetition frequency; the maximum Incorrect velocity calculations are common with angle
velocity of flow that can be measured accurately). correction software.
Therefore, measurements of blood velocity by frequency Echocardiographic machines with these basic capa-
shifts—the Doppler principle that underlies all of bilities are becoming more and more affordable. Image
echocardiography—are only accurate when the pulse quality remains the most critical capability, which is also
repetition frequency is at least twice the detected largely affected by the probe (transducer). For cardiac
maximal velocity. The limited maximum pulse repeti- applications, sector probes are superior to linear probes
tion frequency of ultrasound probes explains the fre- because of a smaller surface area that allows improved
quent occurrence of aliasing with pulsed-wave Doppler, imaging through small feline intercostal spaces (smaller
and the limited range of velocities that can be measured “footprint”), and they come in several types. Mechanical
by this technique. Aliasing is depicted on the spectral probes are most common with basic machines and
display as a band signal (envelope) that “wraps around” contain an oscillating piezoelectric crystal. These probes