Diagnostic ultrasound ( PDFDrive )

CHAPTER 47 Doppler Sonography of the Brain in Children 1595
A
B
FIG. 47.3 Occipital View Through Foramen Magnum. (A) Normal landmark in the foramen magnum view shows rounded medulla just
anterior to the very echogenic clivus (arrow). (B) With color low Doppler imaging, the V-shaped vertebral arteries (blue) join to form the basilar
artery at the medulla-pons junction. Cursor is on the left vertebral artery. Spectral Doppler ultrasound waveforms in the basilar artery show low
away from the transducer.
assessing stroke risk in children with sickle cell anemia, guidelines
regarding normal versus abnormal/conditional velocities were
validated in large clinical trials using nonduplex equipment that
was not angle-corrected. 20-23 Angle correction can signiicantly
increase velocities in vessels that are not coursing directly toward
the transducer. 24 Although angle correction has been suggested
as a way to correct for variations between the imaging and
nonimaging examinations, the lack of published data for anglecorrected
velocities currently limits this approach. 12,13,25 Audible
optimization for curser placement is key in obtaining similar
values with TCD and TCDI without angle correcting. Placing a
cursor on a two-dimensional color image may not result in the
most optimized tracing because the third dimension of the vessel
may be out of the image plane. For this reason, curser placement
based solely on a color image may result in a lower velocity than
curser placement utilizing the highest achievable audible Doppler
frequency. Because the MCA and OA usually course almost
directly toward or away from the transducer using the technique
described here, angle correction is less of an issue in these vessels.
ACA and PCA velocities are more variable because of their
tortuous course.
An index of pulsatility, either the pulsatility index (PI) (PSV
− DV [diastolic velocity]/mean velocity) or resistive index (RI)
(PSV − DV/PSV), can also be evaluated. Both of these indices
are ratios and therefore minimize the efect of vessel
angulation.
Physiologic variables including age, gender, hematocrit, viscosity,
carbon dioxide, temperature, blood pressure, and mental or
motor activity can impact low velocity. 17 Age-dependent reference
values are available for velocities of the various intracranial
vessels. 26,27 Early in life, velocities increase with age, demonstrating
a rapid increase ater birth and a slower increase until 6 to 8
years of age. Aterward and through adolescence, a slow decrease
is seen until 70% of the maximal velocity is reached around 18
years of age. Regarding gender, slightly higher velocities are
observed in pubertal girls when compared to boys. 28 An inverse
relationship is present between hematocrit and viscosity and
low velocity; a decrease in 30% to 40% of the hematocrit will
result in an increase in velocities around 20%. 17 If anemia is the
cause of elevated velocities, the changes should be detected in
all of the intracranial arteries. A focal velocity increase suggests
a focal cause.
Changes in arterial carbon dioxide (CO 2 ) afect cerebral blood
low and arterial velocities. Hyperventilation causes a decrease
in the MCA mean velocity and an increase in the PI. Hypoventilation
causes an increase in MCA mean velocity and decrease in
PI. If a child is sleeping or in pain and is hyperventilating, TCD
results may be afected.
Heart rate also inluences intracranial arterial velocities. he
sonographer may need to adjust Doppler display sweep time in
extreme cases of bradycardia or tachycardia. If autoregulation
is intact, cardiac output should not have an afect on TCD
velocities.
Velocities can demonstrate daily and interobserver variability.
Studies evaluating these variations suggest that normal daily
changes should be less than 10 cm/sec and interobserver variability
can be 7.5% (measured at the same time) to 13% (daily variation).
hey suggest that a variation of more than 14% be considered
abnormal. 29
Normal mean velocity in the MCA in adults range from 50
to 80 cm/sec; in the ACA, 35 to 60 cm/sec; in the PCA, 30 to
50 cm/sec; and in the basilar artery, 25 to 50 cm/sec. PSVs up
to 150 cm/sec have been described in patients with SCD secondary
to anemia. 30,31 he velocity in the OA is normally about one-fourth