Diagnostic ultrasound ( PDFDrive )

42 PART I Physics
FIG. 2.6 Acoustic Cavitation Bubbles. This cavitation activity is
being generated in water using a common therapeutic ultrasound device.
(Courtesy of National Center for Physical Acoustics, University of
Mississippi.)
FIG. 2.8 Collapsing Bubble Near a Boundary. When cavitation is
produced near boundaries, a liquid jet may form through the center of
a bubble and strike the boundary surface. (Courtesy of Lawrence A.
Crum.)
a therapeutic ultrasound device; the setup is backlighted (in
red) to show the bubbles and experimental apparatus. he
chemiluminescence emissions are the blue bands seen through
the middle of the liquid sample holder. he light emitted is
suicient to be seen by simply adapting one’s eyes to darkness.
Electron spin resonance can also be used with molecules that trap
free radicals to detect cavitation activity capable of free radical
production. 38 A number of other chemical detection schemes are
presently employed to detect cavitation from diagnostic devices
in vitro.
FIG. 2.7 Chemical Reaction Induced by Cavitation Producing
Visible Light. The reaction is the result of free radical production.
(Courtesy of National Center for Physical Acoustics, University of
Mississippi.)
of stabilized gas bubbles should provide a source of cavitation
nuclei, as discussed later in the section on ultrasound
contrast agents.
Sonochemistry
Free radical generation and detection provide a means to observe
cavitation and to gauge its strength and potential for damage.
he sonochemistry of free radicals is the result of very high
temperatures and pressures within the rapidly collapsing bubble.
hese conditions can even generate light, or sonoluminescence. 36
With the addition of the correct compounds, chemical luminescence
can also be used for free radical detection and can be
generated with short pulses similar to those used in diagnostic
ultrasound. 37 Fig. 2.7 shows chemiluminescence generated by
Evidence of Cavitation From Lithotripters
It is possible to generate bubbles in vivo using short pulses with
high amplitudes of extracorporeal shockwave lithotripsy (ESWL).
he peak positive pressure for lithotripsy pulses can be as high
as 50 MPa, with a rarefactional pressure of about 20 MPa. Finite
amplitude distortion causes high frequencies to appear in
high-amplitude ultrasound ields. Although ESWL pulses have
signiicant energy at high frequencies because of inite amplitude
distortion, a large portion of the energy is actually in the 100-kHz
range, much lower than frequencies in diagnostic scanners. he
lower frequency makes cavitation more likely. Aymé and
Carstensen 39 showed that the higher frequency components in
nonlinearly distorted pulses contributed little to the killing of
Drosophila larvae.
Interestingly, evidence indicates that collapsing bubbles
play a role in stone disruption. 40-42 A bubble collapsing near
a surface may form a liquid jet through its center, which
strikes the surface (Fig. 2.8). Placing a sheet of aluminum
foil at the focus of a lithotripter generates small pinholes. 40
he impact is even suicient to pit solid brass and aluminum
plates.
At very high acoustic amplitudes, tissue can be disrupted
and even emulsiied using ultrasound in a process termed
“histotripsy.” 43-45 Peak rarefactional pressures used in this
form of therapeutic ultrasound can be as high as 25 to