Diagnostic ultrasound ( PDFDrive )

CHAPTER 2 Biologic Effects and Safety 39
he NCRP ultrasound committee introduced the TI concept. 8
he purpose of the TI is to provide an indication of the relative
potential for increasing tissue temperature, but it is not meant
to provide the actual temperature rise. he NCRP recommended
two tissue models to aid in the calculation of the ultrasound
power that could raise the temperature in tissue by 1°C: (1) a
homogeneous model in which the attenuation coeicient is
uniform throughout the region of interest, and (2) a ixedattenuation
model in which the minimum attenuation along the
path from transducer to a distant anatomic structure is independent
of the distance because of a low-attenuation luid path
(e.g., amniotic luid). 8,13,14 Because of concern for the patient, it
was recommended that “reasonable worst case” assumptions be
made with respect to estimation of temperature elevations in
vivo. he FDA, AIUM, and National Electrical Manufacturers
Association (NEMA) adopted the TI as part of the output display
standard. hey advocate estimating the efect of attenuation in
the body by reducing the acoustic power/output of the scanner
(W 0 ) by a derating factor equal to 0.3 dB/cm-MHz for the sot
tissue model. 11
he AIUM hermal Index Working Group considered three
tissue models: (1) the homogeneous tissue or sot tissue model,
(2) a tissue model with bone at the focus, and (3) a tissue model
with bone at the surface, or transcranial model. 11 he TI takes
on three diferent forms for these tissue models.
Homogeneous Tissue Model (Soft Tissue)
he assumption of homogeneity helps simplify the determination
of the efects of acoustic propagation and attenuation, as well as
the heat transfer characteristics of the tissue. Providing one of
the most common applications for ultrasound imaging, this model
applies to situations where bone is not present and can generally
be used for fetal examinations during the irst trimester (low
calciication in bone). In the estimation of potential heating,
many assumptions and compromises had to be made to calculate
a single quantity that would guide the operator. Calculations of
the temperature rise along the axis of a focused beam for a
simple, spherically curved, single-element transducer result in
two thermal peaks. he irst is in the near ield (between the
transducer and the focus), and the second appears close to the
focal region. 15,16 he irst thermal peak occurs in a region with
low ultrasound intensity and wide beam width. When the beam
width is large, cooling will occur mainly because of perfusion.
In the near ield the magnitude of the local intensity is the chief
determinant of the degree of heating. he second thermal peak
occurs at the location of high intensity and narrow beam width
at or near the focal plane. Here the cooling is dominated by
conduction, and the total acoustic power is the chief determinant
of the degree of heating.
Given the thermal “twin peaks” dilemma, the AIUM hermal
Index Working Group compromised in creating a TI that included
contributions from both heating domains. 11 heir rationale was
based on the need to minimize the acoustic measurement load
for manufacturers of ultrasound systems. In addition, adjustments
had to be made to compensate for efects of the large range of
potential apertures. he result is a complicated series of calculations
and measurements that must be performed, and to the
credit of the many manufacturers, there has been considerable
efort in implementing a display standard to provide user feedback.
Diferent approaches to these calculations were considered, 10 but
changes will require that the currently accepted implementation
be reexamined and approved for use by the FDA and considered
by the IEC.
Tissue Model With Bone at the Focus
(Fetal Applications)
Applications of ultrasound in which the acoustic beam travels
through sot tissue for a ixed distance and impinges on bone
occur most oten in obstetric scanning during the second and
third trimesters. Carson and colleagues 13 recorded sonographic
measurements of the maternal abdominal wall thickness in various
stages of pregnancy. Based on their results, the NCRP recommended
that the attenuation coeicients for the irst, second,
and third trimesters be 1.0, 0.75, and 0.5 dB/MHz, respectively. 7
hese values represent “worst case” estimates. In addition, Siddiqi
and colleagues 17 determined the average tissue attenuation coeficient
for transabdominal insoniication (exposure to ultrasound
waves) in a patient population of nonpregnant, healthy volunteers
was 2.98 dB/MHz. his value represents an average measured
value and is much diferent from the worst-case estimates previously
listed. his leads to considerable debate on how such
parameters should be included in an index.
In addition, bone is a complex, hard connective tissue with
a calciied collagenous intercellular structure. Its absorption
coeicient for longitudinal waves is a factor of 10 greater than
that for most sot tissues (see Fig. 2.2). Shear waves are also
created in bone as sound waves strike bone at oblique incidence.
he absorption coeicients for shear waves are even greater than
those for longitudinal waves. 18-20
Based on the data of Carstensen and colleagues 2 described
earlier, the NCRP proposed a thermal model for bone heating.
Using this model, the Bone hermal Index (TIB) is estimated
for conditions in which the focus of the beam is at or near bone.
Again, assumptions and compromises had to be made to develop
a functional TI for the case of bone exposure, as follows:
• For unscanned mode transducers (operating in a ixed position)
with bone in the focal region, the location of the
maximum temperature increase is at the surface of the bone.
herefore the TIB is calculated at the distance along the beam
from the transducer where it is maximized, a worst-case
assumption.
• For scanned modes, the Sot Tissue hermal Index (TIS)
is used because the temperature increase at the surface is
either greater than or approximately equal to the temperature
increase with bone in the focus.
Tissue Model With Bone at the Surface
(Transcranial Applications)
For adult cranial applications, the same model as that with bone
at the focus is used to estimate the temperature distribution in
situ. However, because the bone is located at the surface, immediately
ater the acoustic beam enters the body, attenuation of
the acoustic power output is not included. 11 In this situation the
equivalent beam diameter at the surface is used to calculate the