Diagnostic ultrasound ( PDFDrive )

450 PART II Abdominal and Pelvic Sonography
oten used; it has a higher probability of seeing the entire renal
artery in obese patients. Also, achieving an acceptable Doppler
angle of less than 60 degrees is easier from the lank. Breath
holding is oten used, although it does not work well in many
patients, either because the patient is dyspneic, or because of a
slow but steady cephalad drit of the kidneys, which occurs in
some patients despite breath holding. Even without breath holding,
the examination oten is successful.
When possible, we avoid examining patients who are in heart
failure, who have acute dyspnea, who have acutely decreased
mobility, or who are on ventilators. All these patients have a
reduced ability to accomplish the breath holding. Renal duplex
Doppler sonograms generally are not emergent. he examination
will likely become much easier, more successful, and more highly
accurate once the patient’s condition improves.
Performance of the renal artery duplex Doppler study mainly
relies on obtaining accurate velocities throughout the main renal
artery. However, measurement of the resistive index (RI) is also
important. Treatment of renal artery stenosis is less likely to be
efective in reducing blood pressure when the segmental artery
RI is high (≥0.80), probably because of irreversible damage to
the small blood vessels in kidneys with a high RI. 92
Attention to intrarenal waveforms is also of some importance.
A highly abnormal waveform can be a valuable indicator of
stenosis. A delayed systolic peak (tardus, i.e., “tardy”) and velocities
that are greatly decreased (parvus, i.e., “puny”) can be a
strong sign of a more proximal stenosis. he intrarenal waveform
can be analyzed quantitatively by calculating the systolic rise
time and the acceleration (Fig. 12.21). Although we calculate
these parameters, a qualitative assessment of the appearance
of the waveform usually serves just as well. We rely on a
tardus-parvus waveform to make the diagnosis only when
the inding is pronounced (compare Fig. 12.19B and E; see also
Fig. 12.20A and B).
Finally, because of the regular occurrence of renal lesions or
abnormalities that afect patient care, we believe that ultrasound
of the kidneys should be considered when renal artery duplex
Doppler ultrasound is performed, unless the patient has had
recent (<1 or 2 years) cross-sectional imaging. Incidental indings
in our ultrasound department have included xanthogranulomatous
pyelonephritis, adrenal tumors, hydronephrosis, and several renal
cell carcinomas. 93
Doppler Interpretation. here are many proposed guidelines
for Doppler interpretation. Proposed parameters to assess for
stenosis include the peak systolic velocity (PSV), renal aortic
ratio (RAR; deined as highest systolic velocity in renal artery
divided by aortic systolic velocity, with aortic velocity measured
at or above SMA origin), 94 acceleration time, acceleration index,
renal interlobar ratio, 95,96 and renal-renal ratio. 95,97,98 An
inluential article suggested a combination of RAR of 3.5 or
greater or PSV of 200 or greater as the criterion for renal artery
stenosis of more than 60%. 75 We use a variation of these criteria,
using the same RAR as the study but a higher PSV of 350.
False-Positive/False-Negative Results. To obtain the
highest accuracy, it is important to avoid relying solely on the
numerical data obtained. When a high velocity is seen or when
the renal aortic ratio is high, the interpreting radiologist must
also actively look for secondary signs of stenosis, such as a
characteristic harsh audible signal at the site of stenosis, increased
diastolic low, color bruit, color aliasing, and poststenotic turbulence
(Fig. 12.22, Video 12.10). Without ancillary indings,
the interpreter must consider the possibility that the high velocity
or high RAR represents a false-positive result.
False-negative indings are a risk primarily when visualization
is marginal and the entire artery has not been adequately evaluated.
In perhaps 5% to 10% of patients, accurate diagnosis cannot
be made because of inadequate visualization of one or both
arteries.
he examination is challenging to the uninitiated operator,
but establishment of a renal artery duplex Doppler program can
be rewarding. Because of the lower cost versus other diagnostic
tests, Doppler ultrasound lowers the threshold for the diagnosis
of renovascular hypertension. Hurdles mainly relate to the learning
curve and the initial investment of time. Starting a program is
more feasible in a large center where demand will likely be higher
than in a smaller facility. Once the program is mature, the study
is inancially viable and can result in improved patient care.
Acceleration = Velocity / systolic rise time
Velocity
Time
FIG. 12.21 Renal Artery Systolic Waveform. Normal waveform.
In early systole there is a rapid acceleration of blood in the renal artery.
The systolic rise time (purple arrow) is the time expended during this
rapid acceleration. The acceleration is the change in velocity during the
systolic rise (blue arrow) divided by the systolic rise time.
FIG. 12.22 Secondary Signs of Stenosis. Increased diastolic low,
as seen here, can help conirm the presence of renal artery stenosis.
Other secondary signs include color bruit, poststenotic turbulence, and
a tardus-parvus waveform. See also Video 12.10.