142 Chapter 6: Safety
Safety
Fixed-path tissue models, in which soft tissue thickness is held constant, sometimes are used to
estimate In Situ acoustic exposures when the beam path is longer than 3 cm and consists largely of
fluid. When this model is used to estimate maximum exposure to the fetus during transabdominal
scans, a value of 1 dB/cm MHz may be used during all trimesters.
Existing tissue models that are based on linear propagation may underestimate acoustic exposures
when significant saturation due to non-linear distortion of beams in water is present during the
output measurement.
The maximum acoustic output levels of diagnostic ultrasound devices extend over a broad range of
values:
• A survey of 1990-equipment models yielded MI values between 0.1 and 1.0 at their highest
output settings. Maximum MI values of approximately 2.0 are known to occur for currently
available equipment. Maximum MI values are similar for real-time 2D and M Mode imaging.
• Computed estimates of upper limits to temperature elevations during transabdominal scans
were obtained in a survey of 1988 and 1990 pulsed Doppler equipment. The vast majority of
models yielded upper limits less than 1° and 4°C (1.8° and 7.2°F) for exposures of first-trimester
fetal tissue and second-trimester fetal bone, respectively. The largest values obtained were
approximately 1.5°C (2.7°F) for first-trimester fetal tissue and 7°C (12.6°F) for second-trimester
fetal bone. Estimated maximum temperature elevations given here are for a “fixed path” tissue
model and are for devices having I
SPTA
values greater than 500 mW/cm
2
. The temperature
elevations for fetal bone and tissue were computed based on calculation procedures given in
Sections 4.3.2.1-4.3.2.6 in “Bioeffects and Safety of Diagnostic Ultrasound” (AIUM, 1993).
Intended Uses
The intended uses for each exam type are contained here. See the intended transducer for exam type
in
Table 2, “Transducer, Exam Type, and Imaging Mode” on page 44.
Abdominal Imaging Applications
This system transmits ultrasound energy into the abdomen of patients using 2D, M Mode, color
Doppler (Color), color power Doppler (CPD), Tissue Harmonic Imaging (THI), and pulsed wave
(PW) Doppler to obtain ultrasound images. The liver, kidneys, pancreas, spleen, gallbladder, bile
ducts, transplanted organs, abdominal vessels, and surrounding anatomical structures can be
assessed for the presence or absence of pathology transabdominally.
Cardiac Imaging Applications
This system transmits ultrasound energy into the thorax of patients using 2D, M Mode, color
Doppler (Color), Tissue Harmonic Imaging (THI), pulsed wave (PW) Doppler, and continuous
wave (CW) Doppler to obtain ultrasound images. The heart, cardiac valves, great vessels,
surrounding anatomical structures, overall cardiac performance, and heart size can be assessed for
the presence or absence of pathology. The heart can be imaged transthoracic.
The patient’s electrocardiogram (ECG) may be obtained and is used for accurate timing of diastolic
and systolic function.
Warning: The ECG is not used to diagnose cardiac arrhythmias and is not designed for long
term cardiac rhythm monitoring.
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