I’=I*R
F
Where I’ is the intensity in soft tissue, I is the time-averaged intensity measured in
water.
Tissue Model:
Tissue temperature elevation depends on power, tissue type, beam width, and scanning mode. Six models are
developed to mimic possible clinical situations.
Thermal Models Composition Mode Specification Application
1
TIS
Soft tissue
Unscanned
Large aperture (>1cm
2
)
Liver PW
2 TIS Soft tissue Unscanned
Small aperture (<1cm
2
)
Pencil Probe
3 TIS Soft tissue Scanned Evaluated at surface Breast color
4 TIB Soft tissue and bone Scanned Soft tissue at surface Muscle color
5 TIB Soft tissue and bone Unscanned Bone at focus Fetus head PW
6 TIC Soft tissue and bone Unscanned/scanned Bone at surface Transcranial
Soft tissue:
Describes low fat content tissue that does not contain calcifications or large gas-filled spaces.
Scanned: (auto-scan)
Refers to the steering of successive burst through the field of view, e.g. B and color mode.
Unscanned:
Emission of ultrasonic pulses occurs along a single line of sight and is unchanged until the transducer is moved to a
new position. For instance, the PW, and M mode.
TI:
TI is defined as the ratio of the In Situ acoustic power (W.3) to the acoustic power required to raise tissue temperature
by 1°C (W
deg
), TI = W.3/W
deg
.
Three TIs corresponding to soft tissue (TIS) for abdominal; bone (TIB) for fetal and neonatal cephalic; and cranial
bone (TIC) for pediatric and adult cephalic, have been developed for applications in different exams.
An estimate of the acoustic power in milli-watts necessary to produce a 1°C temperature elevation in soft tissue is:
W
deg
= 210/fc, for model 1 to 4, where fc is the center frequency in MHz.
W
deg
= 40 K D for model 5 and 6, where K (beam shape factor) is 1.0, D is the aperture diameter in
cm at the depth of interest.
MI:
Cavitation is more likely to occur at high pressures and low frequencies in pulse ultrasound wave in the tissue, which
contains the bubble or air pocket (for instance, the lung, intestine, or scan with gas contrast agents). The threshold
under optimum conditions of pulsed ultrasound is predicted by the ration of the peak pressure to the square root of the
frequency.
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