Diagnostic ultrasound ( PDFDrive )

38 PART I Physics
Ultrasound imaging systems include speciic modalities that
rely on nonlinear efects. In tissue harmonic imaging, or
native harmonic imaging, the image is created using the backscatter
of harmonic components induced by nonlinear propagation
of the ultrasound ield. his has distinct advantages in
terms of reducing image artifacts and improving lateral resolution
in particular. In these nonlinear imaging modes the
acoustic output must be suiciently high to produce the efect.
he acoustic power currently used is still within the FDA limits,
but improvements in image quality using such modes may
create the need to modify or relax the regulatory restrictions.
Similarly, in elasticity or shear wave imaging, the acoustic pressure
may need to be increased to provide suicient acoustic
radiation force for imaging tissue stifness. his detail will be
discussed in the section on considerations for increasing acoustic
output.
Transvaginal ultrasound is important to note because of the
proximity of the transducer to sensitive tissues such as the ovaries.
As discussed later, temperature increases near the transducer
may provide a heat source at sites other than the focus of the
transducer. In addition, the transducer face itself may be a
signiicant heat source because of ineiciencies in its conversion
of electric to acoustic energy. Such factors must be considered
in the estimation of potential thermal efects in transvaginal
ultrasound and other endocavitary applications.
Hyperthermia and Ultrasound Safety
Knowledge of the bioefects of ultrasound heating is in part
based on the experience available from other, more common
forms of hyperthermia, which serve as a basis for safety criteria.
Extensive data exist on the efects of short-term and extended
temperature increases, or hyperthermia. Teratogenic efects from
hyperthermia have been demonstrated in birds, all the common
laboratory animals, farm animals, and nonhuman primates. 5 he
wide range of observed bioefects, from subcellular chemical
alterations to gross congenital abnormalities and fetal death, is
an indication of the efectiveness or universality of hyperthermic
conditions for perturbing living systems. 6
he National Council on Radiation Protection and Measurements
(NCRP) Scientiic Committee on Biological Efects of
Ultrasound compiled a comprehensive list of the lowest reported
thermal exposures producing teratogenic efects. 7,8 Examination
of these data indicated a lower boundary for observed thermally
induced bioefects. Questions remain, however, about the relevance
of this analysis of hyperthermia to the application of diagnostic
ultrasound. 9 Ater a careful literature review, O’Brien and colleagues
10 suggested a more detailed consideration of thermal
efects with regard to short-duration exposures. Fig. 2.5 shows
the recommended approach to addressing the combination of
temperature and duration of exposure. Note that the tolerance
of shorter durations and higher temperatures suggests a substantial
safety margin for diagnostic ultrasound. Regardless, it is beneicial
to provide feedback to the ultrasound operator as to the relative
potential for a temperature rise in a given acoustic ield under
conditions associated with a particular examination. his will
allow an informed decision as to the exposure needed to obtain
diagnostically relevant information.
Temperature (°C)
58
55
52
49
46
43
40
37
0.1 1 10
Time (s)
100 1000
FIG. 2.5 Conservative Boundary Curve for Nonfetal Bioeffects
Caused by a Thermal Mechanism. Note the increase in temperature
tolerance associated with shorter durations of exposures, a modiication
to the earlier American Institute of Ultrasound in Medicine (AIUM)
Conclusions Regarding Heat statement (March 26, 1997). The AIUM
approved a revised thermal statement on April 6, 2009. For a complete
description of the origins of this curve, see O’Brien and colleagues. 10
(With permission from O’Brien Jr WD, Deng CX, Harris GR, et al. The
risk of exposure to diagnostic ultrasound in postnatal subjects: thermal
effects. J Ultrasound Med. 2008;27[4]:517-535. 10 )
Thermal Index
Based on analysis of hyperthermia data, the NCRP proposed a
general statement concerning the safety of ultrasound examinations
in which no temperature rise greater than 1°C is expected.
In an afebrile patient within this limit, the NCRP concluded that
there was no basis for expecting an adverse efect. In cases where
the temperature rise might be greater, the operator should weigh
the beneit against the potential risk. To assist in this decision,
given the range of diferent imaging conditions seen in practice,
a thermal index (TI) was approved as part of the Standard for
Real-Time Display of hermal and Mechanical Acoustical Output
Indices on Diagnostic Ultrasound Equipment of the American
Institute of Ultrasound in Medicine (AIUM). 11 his standard
provides the operator with an indication of the relative potential
risk of heating tissue, with calculations based on the imaging
conditions and an on-screen display showing the TI. his standard
was subsequently adopted as an international standard through
the International Electrotechnical Commission (IEC). 12
The Thermal Index
To more easily inform the physician of the operating conditions
that could, in some cases, lead to a temperature elevation
of 1°C, a thermal index is deined as
W
TI =
W
where W deg is the ultrasonic source power (in watts) calculated
as capable of producing a 1°C temperature elevation under
speciic conditions. W 0 is the ultrasonic source power (in
watts) being used during the current exam.
Reproduced with permission of American Institute of Ultrasound in
Medicine (AIUM).
0
deg