o_1977r8vv9vk1ts2ms0kd8pksa.pdf

76 D. Maulik
Fig. 6.6. Color M-mode sonogram. Top: Two-dimensional
color Doppler echocardiogram of the fetal heart (fourchamber
view). M-mode cursor is indicated by the dotted
line. Bottom: Color M-mode tracing of flow from the right
atrium (RA) to the right ventricle (RV) across the tricuspid
orifice. The movement of the tricuspid valve (TV) is visible
in the middle of the panel. As the flow is toward the transducer,
it is coded red. Note the aliasing at the tricuspid orifice
level (blue and yellow)
with the events of the cardiac cycle. For this reason,
color M-mode sonography is a useful tool for fetal
echocardiographic examination.
Operational Considerations
Transducer Frequency
The operating frequency of the transducer is an important
contributor to the functional efficacy of the system.
A high carrier frequency results in better spatial
resolution of the image but reduces the depth of penetration.
The Doppler mode is more vulnerable to depth
limitation than gray-scale tissue imaging. For most obstetric
applications, a frequency of 2±4 MHz provides
adequate penetration and resolution and is usually preferred.
For transvaginal applications, a higher frequency
(5±12 MHz) is used, as penetration is not a
problem. Currently, piezoelectric elements capable of
resonating at more than one frequency are available,
so a single transducer can operate at multiple frequencies.
This capability offers a unique advantage for color
flow mapping, as a low frequency may be used for the
combined tissue imaging and color Doppler mode, and
one may then default to a high frequency for tissue
imaging to ensure a higher gray-scale resolution.
Pulse Repetition Frequency
Doppler color flow mapping is based on pulsed Doppler
insonation. The magnitude of the frequency shift
in color flow Doppler sonography therefore depends
on the PRF of the transmitted ultrasound. The PRF is
Fig. 6.7a±c. Effect of pulse repetition frequency (PRF) on
the color flow map. The images show atrioventricular flow
on the oblique apical four-chamber view of the heart.
a Low PRF setting resulting in severe aliasing with mosaic
patterns and spatial overrepresentation of flow. b Effect of
a modest increase in the PRF is partially improved image
quality. Overrepresentation of flow and mosaic appearance
are still present. c More appropriate color flow depiction
with another substantial increase in the PRF. Note the presence
of aliasing across the tricuspid orifice. RA right atrium,
RV right ventricle, LA left atrium, LV left ventricle