o_1977r8vv9vk1ts2ms0kd8pksa.pdf

96 D. Maulik
Acoustic Output of Diagnostic
Ultrasound Devices
and Its Regulation
A central issue in the safe use of diagnostic ultrasound
is the power output of the instruments. By enacting
the Medical Devices Amendment to the US
Food, Drug and Cosmetic Act on May 28, 1976, the
United States Congress empowered the FDA to control
the output limits to the devices through the mandatory
premarketing approval process. The approval
was based on the manufacturers' ability to demonstrate
substantial equivalence of each new device in
safety and efficacy to diagnostic ultrasound devices
on the market prior to the enactment date. The principle
of substantial equivalence in safety was based
on the assumption that the pre-enactment devices
were safe and was supported by the available scientific
data which provided no evidence of independently
confirmed adverse significant biological effects in
mammalian tissue exposed in vivo to I SPTA below 100
mW/cm 2 [11]. In 1985, the FDA introduced the application
specific output standards of substantial equivalence
covering four areas of use: cardiac, peripheral
vascular, fetal imaging and other, and ophthalmic. In
the early 1990s, based on an NCRP technical report
called the Output Display Standard (ODS) [12], the
AIUM and the National Electrical Manufacturers Association
(NEMA) led an initiative to increase the intensity
in exchange for on-screen labeling which resulted
in a modification of the existing regulation by
the FDA. The track 3 option allows devices to increase
the overall maximum output limit to 720 mW/
cm 2 provided they incorporate the ODS [13]. Equipments
used in ophthalmology were exempted. The
track 3 devices can have a substantial increase in the
power output and it is the responsibility of the sonographer
to use the display to limit the intensity of fetal
exposure. Given the potential for adverse effects,
the need for user education and training in this area
can not be overstressed. This provides a compelling
rationale for this chapter.
The ODS consists of two risk indicators, a thermal
index (TI) for thermal bioeffects, and a mechanical
index (MI) for nonthermal bioeffects [12]. The TI
was further refined in 1998 adding three tissue-specific
TI models [14]. These are further described below:
n The TI is the ratio of total acoustic power to the
acoustic power that would be required to raise
temperature by 18C for a specific tissue model. As
a ratio it is dimensionless and provides an estimate
of maximum temperature rise rather than the
actual increase. Thus a TI of 2 indicates a higher
temperature elevation than a TI of 1, but does not
imply an actual rise of 2 8C in the insonated tissue.
There are three tissue-specific thermal indices:
± TIS=Thermal index for soft tissue is concerned
with temperature rise within homogeneous soft
tissue.
± TIB=Thermal index bone is related to temperature
elevation in bone at or near the focus of
the beam.
± TIC=Thermal index cranial bone indicates temperature
increase of bone at or near the surface,
such as during a cranial examination.
n The MI is also a dimensionless quantity and is derived
from the peak rarefactional pressure at the
point of the maximal intensity divided by the
square root of the center frequency of the pulse
bandwidth. It is an indicator of potential nonthermal
bioeffects, especially those that are cavitationrelated.
According to the FDA, the MI may range
up to 1.9 except for ophthalmic usage. The higher
the value of MI, the higher the risk of a mechanical
effect.
The relevance and practical utilization of the indices
for safe use of diagnostic sonography are further discussed
later.
Mechanisms of Bioeffects
Any review of bioeffects of ultrasound must include
considerations of the known mechanisms by which
propagating ultrasound reacts with biological systems.
The following effects are currently recognized:
1. Thermal effects which are mediated by insonationinduced
tissue heating
2. Nonthermal or mechanical effects which include
those not related to heat generation.
The thermal and the mechanical effects of diagnostic
acoustic exposure and their relevance for the safety
of Doppler ultrasound usage are further discussed below.
Thermal Effects
As a beam of ultrasound propagates through a tissue
medium, a portion of its energy is absorbed and converted
to heat because the frictional forces in the medium
oppose the ultrasound-related molecular oscillations.
The rate of temperature elevation in the insonated
tissue is determined by the balance between
the rates of heat production and of heat dissipation.
The rate of heat generation depends on the characteristics
of the transmitted ultrasound and the tissue
medium [15].