Diagnostic ultrasound ( PDFDrive )

32 PART I Physics
A
B
C
D
FIG. 1.46 Aliasing. Pulse repetition frequency (PRF) determines the sampling rate of a given Doppler frequency. (A) If PRF (arrows) is suficient,
the sampled waveform (orange curve) will accurately estimate the frequency being sampled (yellow curve). (B) If PRF is less than half the frequency
being measured, undersampling will result in a lower frequency shift being displayed (orange curve). (C) In a clinical setting, aliasing appears in
the spectral display as a “wraparound” of the higher frequencies to display below the baseline. (D) In color Doppler display, aliasing results in a
wraparound of the frequency color map from one low direction to the opposite direction, passing through a transition of unsaturated color. The
velocity throughout the vessel is constant, but aliasing appears only in portions of the vessel because of the effect of the Doppler angle on the
Doppler frequency shift. As the angle increases, the Doppler frequency shift decreases, and aliasing is no longer seen.
zone. his has led to an interest in HIFU as a means of destroying
noninvasive tumor and controlling bleeding and cardiac conduction
anomalies.
HIFU exploits thermal (heating of tissues) and mechanical
(cavitation) bioefect mechanisms. As ultrasound passes through
tissue, attenuation occurs through scattering and absorption.
Scattering of ultrasound results in the return of some of the
transmitted energy to the transducer, where it is detected and
used to produce an image, or Doppler display. he remaining
energy is transmitted to the molecules in the acoustic ield and
produces heating. At the spatial peak temporal average (SPTA),
intensities of 50 to 500 mW/cm 2 used for imaging and Doppler,
heating is minimal, and no observable bioefects related to tissue
heating in humans have yet been documented with clinical devices.
With higher intensities, however, tissue heating suicient to
destroy tissue may be achieved. Using HIFU at 1 to 3 MHz, focal
peak intensities of 5000 to 20,000 W/cm 2 may be achieved. his
energy can be delivered to a small point several millimeters in
size, producing rapid temperature elevation and resulting in tissue
coagulation, with little damage to adjacent tissues (Fig. 1.48).
he destruction of tissue is a function of the temperature reached
and the duration of the temperature elevation. In general, elevation
of tissue to a temperature of 60°C for 1 second is suicient to
produce coagulation necrosis.
Because of its ability to produce highly localized tissue destruction,
HIFU has been investigated as a tool for noninvasive or
minimally invasive treatment of bleeding sites, uterine ibroids,
and tumors in the prostate, liver, and breast. 19,20 As with diagnostic
ultrasound, HIFU is limited by the presence of gas or bone
interposed between the transducer and the target tissue. he
relection of high-energy ultrasound from strong interfaces
produced by bowel gas, aerated lung, or bone may result in tissue
heating along the relected path of the sound, producing unintended
tissue damage.
Major challenges with HIFU include image guidance and
accurate monitoring of therapy as it is being delivered. Magnetic