Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 31
With current devices operating in imaging modes, concerns
about bioefects are minimal because intensities suicient to
produce measurable heating are seldom used. With Doppler
ultrasound, the potential for thermal efects is greater. Preliminary
measurements on commercially available instruments suggest
that at least some of these instruments are capable of producing
temperature rises of greater than 1°C at sot tissue/bone interfaces,
if the focal zone of the transducer is held stationary. Care is therefore
warranted when Doppler measurements are obtained at or near
sot tissue/bone interfaces, as in the second and third trimester
of pregnancy. hese applications require thoughtful application
of the principle of ALARA (as low as reasonably achievable).
Under ALARA the user should use the lowest possible acoustic
exposure to obtain the necessary diagnostic information.
FIG. 1.45 Spectral Broadening. The range of velocities detected at
a given time in the pulse cycle is relected in the Doppler spectrum as
spectral broadening. (A) Normal spectrum. Spectral broadening may
arise from turbulent low in association with vessel stenosis. (B) and (C)
Artifactual spectral broadening. This may be produced by improper
positioning of the sample volume near the vessel wall, use of (B) an
excessively large sample volume, or (C) an excessive system gain.
modes. In addition, to avoid aliasing and other artifacts with
Doppler imaging, it is oten necessary to use higher PRFs than
with other imaging applications. Longer pulse duration and higher
PRF result in higher duty factors for Doppler modes of operation
and increase the amount of energy introduced in scanning. Color
Doppler, although a scanned mode, produces exposure conditions
between those of real-time and Doppler imaging because color
Doppler devices tend to send more pulses down each scan line
and may use longer pulse durations than imaging devices. Clearly,
every user needs to be aware that switching from an imaging to
a Doppler mode changes the exposure conditions and the potential
for biologic efects (bioefects).
Bioeffects and User Concerns
Although users of ultrasound need to be aware of bioefects
concerns, another key factor to consider in the safe use of
ultrasound is the user. he knowledge and skill of the user are
major determinants of the risk-to-beneit implications of the
use of ultrasound in a speciic clinical situation. For example,
an unrealistic emphasis on risks may discourage an appropriate
use of ultrasound, resulting in harm to the patient by preventing
the acquisition of useful information or by subjecting the patient
to another, more hazardous examination. he skill and experience
of the individual performing and interpreting the examination
are likely to have a major impact on the overall beneit of the
examination. In view of the rapid growth of ultrasound and its
proliferation into the hands of minimally trained clinicians, many
more patients are likely to be harmed by misdiagnosis resulting
from improper indications, poor examination technique, and
errors in interpretation than from bioefects. Misdiagnosis (e.g.,
of ectopic pregnancy) and failure to diagnose a clinically important
anomaly are real dangers, and poorly trained users may be the
greatest current hazard of diagnostic ultrasound.
Understanding bioefects is essential for the prudent use of
diagnostic ultrasound and is important in ensuring that the
excellent risk-to-beneit performance of diagnostic ultrasound is
preserved. All users of ultrasound should be prudent, understanding
as fully as possible the potential risks and obvious beneits of
ultrasound examinations, as well as those of alternate diagnostic
methods. With this information, operators can monitor exposure
conditions and implement the principle of ALARA to keep patient
(and in obstetric imaging, the fetal) exposure as low as possible
while fulilling diagnostic objectives.
THERAPEUTIC APPLICATIONS: HIGH-
INTENSITY FOCUSED ULTRASOUND
Although the primary medical application of ultrasound has
been for diagnosis, therapeutic applications are developing rapidly,
particularly the use of high-intensity focused ultrasound (HIFU).
HIFU is based on three important capabilities of ultrasound: (1)
focusing the ultrasound beam to produce highly localized energy
deposition, (2) controlling the location and size of the focal zone,
and (3) using intensities suicient to destroy tissue at the focal