Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 27
FIG. 1.40 Impedance. High-resistance waveform in brachial artery (A), produced by inlating forearm blood pressure cuff to a pressure
above the systolic blood pressure. As a result of high peripheral resistance, there is low systolic amplitude and reversed diastolic low. Low-resistance
waveform in peripheral vascular bed (B), caused by vasodilation stimulated by the prior ischemia. Immediately after release of 3 minutes of
occluding pressure, the Doppler waveform showed increased amplitude and rapid antegrade low throughout diastole.
Fig. 1.40 provides a graphic example of the changes in the
Doppler spectral waveform resulting from physiologic changes
in the resistance of the vascular bed supplied by a normal brachial
artery. A blood pressure cuf has been inlated to above systolic
pressure to occlude the distal branches supplied by the brachial
artery. his occlusion causes reduced systolic amplitude and
cessation of diastolic low, resulting in a waveform diferent than
that found in the normal resting state. During the period of
ischemia induced by pressure cuf occlusion of the forearm vessels,
vasodilation has occurred. Immediately ater release of the
occluding pressure the Doppler waveform relects a low-resistance
peripheral vascular bed with increased systolic amplitude and
rapid low throughout diastole, typical for vasodilation.
Doppler indices include the systolic-to-diastolic ratio (S/D
ratio), resistive index (RI), and pulsatility index (PI) (Fig. 1.41).
hese compare blood low in systole and diastole, show resistance
to low in the peripheral vascular bed, and help evaluate the
perfusion of tumors, renal transplants, the placenta, and other
organs. With Doppler ultrasound, it is therefore possible to
identify vessels, determine the direction of blood low, evaluate
narrowing or occlusion, and characterize blood low to organs
and tumors. Analysis of the Doppler shit frequency with time
can be used to infer both proximal stenosis and changes in distal
vascular impedance. Most work using pulsed wave Doppler
imaging has emphasized the detection of stenosis, thrombosis,
and low disturbances in major peripheral arteries and veins. In
these applications, measurement of peak systolic and end diastolic
FIG. 1.41 Doppler Indices. Doppler low indices used to characterize
peripheral resistance are based on the peak systolic frequency or
velocity (A), the minimum or end diastolic frequency or velocity (B),
and the mean frequency or velocity (M). The most frequently used
indices are the systolic-to-diastolic ratio (A/B); resistive index [(A-B)/A];
and pulsatility index [(A-B)/M]. In calculation of the pulsatility index,
the minimum diastolic velocity or frequency is used; calculation of the
systolic-to-diastolic ratio and resistive index use the end diastolic value.
frequency or velocity, analysis of the Doppler spectrum, and
calculation of certain frequency or velocity ratios have been the
basis of analysis. Changes in the spectral waveform measured
by indices comparing low in systole and diastole indicate the
resistance of the vascular bed supplied by the vessel and the
changes resulting from a variety of pathologic conditions.