Diagnostic ultrasound ( PDFDrive )

CHAPTER 2 Biologic Effects and Safety 35
to the operator regarding the relative potential for bioefects.
herefore informed decision making is important concerning
the possible adverse efects of ultrasound in relation to the desired
diagnostic information. Current FDA regulations that limit the
maximum output are still in place, but in the future, systems
might allow sonographers and physicians the discretion to increase
acoustic output beyond a level that might induce a biologic
response.
Although the choices made during sonographic examinations
may not be equivalent to the risk-versus-beneit decisions associated
with imaging modalities using ionizing radiation, the operator
will be increasingly responsible for determining the diagnostically
required amount of ultrasound exposure. hus the operator
should know the potential bioefects associated with ultrasound
exposure. Patients also need to be reassured about the safety of
a diagnostic ultrasound scan. he scientiic community has
identiied some potential bioefects from sonography, and although
no causal relation has been established, it does not mean that
no efects exist. herefore it is important to understand the
interaction of ultrasound with biologic systems.
PHYSICAL EFFECTS OF SOUND
he physical efects of sound can be divided into two principal
groups: thermal and nonthermal. Most medical professionals
recognize the thermal efects of elevated temperature on tissue,
and the efects caused by ultrasound are similar to those of any
localized heat source. With ultrasound, the heating mainly results
from the absorption of the sound ield as it propagates through
tissue. However, “nonthermal” sources can generate heat as well.
Many nonthermal mechanisms for bioefects exist. Acoustic
ields can apply radiation forces (not ionizing radiation) on the
structures within the body at both the macroscopic and the
microscopic levels, resulting in exerted pressure and torque. he
temporal average pressure in an acoustic ield is diferent from
the hydrostatic pressure of the luid, and any object in the ield
is subject to this change in pressure. he efect is typically
considered smaller than other efects because it relies on less
signiicant factors in the formulation of the acoustic ield. Acoustic
ields can also cause motion of luids. Such acoustically induced
low is called streaming.
Acoustic cavitation is the action of acoustic ields within a
luid to generate bubbles and cause volume pulsation or even
collapse in response to the acoustic ield. he result can be heat
generation and associated free radical formation, microstreaming
of luid around the bubble, radiation forces generated by the
scattered acoustic ield from the bubble, and mechanical actions
from bubble collapse. he interaction of acoustic ields with
bubbles or “gas bodies” (as they are generally called) has been
a signiicant area of bioefects research for many years.
THERMAL EFFECTS
Ultrasound Produces Heat
As ultrasound propagates through the body, energy is lost through
attenuation. Attenuation causes loss of penetration and the
inability to image deeper tissues. Attenuation is the result of two
processes, scattering and absorption. Scattering of the ultrasound
results from the redirection of the acoustic energy by tissue
encountered during propagation. With diagnostic ultrasound,
some of the acoustic energy transmitted into the tissue is scattered
back in the direction of the transducer, termed backscatter, which
allows a signal to be detected and images to be made. Energy
also is lost along the propagation path of the ultrasound by
absorption. Absorption loss occurs substantially through the
conversion of the ultrasound energy into heat. his heating
provides a mechanism for ultrasound-induced bioefects.
Factors Controlling Tissue Heating
he rate of temperature increase in tissues exposed to ultrasound
depends on several factors, including spatial focusing, ultrasound
frequency, exposure duration, and tissue type.
Spatial Focusing
Ultrasound systems use multiple techniques to concentrate or
focus ultrasound energy and improve the quality of measured
signals. he analogy for light is that of a magnifying glass. he
glass collects all the light striking its surface and concentrates it
into a small region. In sonography and acoustics in general, the
term intensity is used to describe the spatial distribution of
ultrasonic power (energy per unit time), where intensity = power/
area and the area refers to the cross-sectional area of the ultrasound
beam. Another common beam dimension is the beam
width at a speciied location of the ield. If the same ultrasonic
power is concentrated into a smaller area, the intensity will
increase. Focusing occurs on both transmission of the ultrasound
and when receiving the backscattered signals used to form the
image. he transmit focusing is of importance in terms of potential
biologic efects because this phenomenon controls the applied
energy to the tissue. here are ultrasound imaging systems that
use plane wave transmission or limited transmit focusing, which
may reduce the local intensity, but all ultrasound systems must
still operate under the FDA limits.
Focusing in an ultrasound system can be used to improve
the spatial resolution of the images. he side efect is an increased
potential for bioefects caused by heating and cavitation. In
general, the greatest heating potential is between the scanhead
and the focus, but the exact position depends on the focal distance,
tissue properties, and heat generated within the scanhead itself.
Returning to the magnifying glass analogy, most children
learn that the secret to incineration is a steady hand. Movement
distributes the power of the light beam over a larger area, thereby
reducing its intensity. he same is true in ultrasound imaging.
hus imaging systems that scan a beam through tissue reduce
the spatial average intensity. Spectral Doppler and M-mode
ultrasound imaging maintain the ultrasound beam in a stationary
position (both considered unscanned modes) and therefore
provide no opportunity to distribute the ultrasonic power
spatially, whereas color low Doppler, power mode Doppler,
and B-mode (also called gray-scale) ultrasound imaging require
that the beam be moved to new locations (scanned modes)
at a rate suicient to produce the real-time nature of these
imaging modes.