Acoustic Output
23-3
C
MI
= 1 (MPa /
MHz
)
For the frequency 1 MHz and the peak-rarefactional acoustic pressure 1 MPa, MI
becomes 1. It is possible to think MI to be one threshold of the cavitations generation.
Especially, it is important to keep MI value to be low when both gases and the soft tissues
exist together, for such as lung exposure in cardiac scanning and bowel gas in abdominal
scanning.
TI (Thermal Index):
TI is determined by the ratio of the total acoustic power to the acoustic power required to
raise
the tissue temperature by 1 degree C. In addition, because the temperature rises is
greatly different according to tissue structures, TI is divided three kinds: TIS (Soft-tissue
Thermal Index), TIB (Bone Thermal Index) and TIC (Cranial-bone Thermal Index).
îš„ TIS: Thermal index related to soft tissues, such as abdominal and cardiac
applications.
îš„ TIB: Thermal index for applications, such as fetal (second and third trimester) or
neonatal cephalic (through the fontanel), in which the ultrasound beam passes
through soft tissue and a focal region is in the immediate vicinity of bone.
îš„ TIC: Thermal index for applications, such as pediatric and adult cranial applications,
in which the ultrasound beam passes through bone near the beam entrance into the
body.
Although the output power is automatically controlled for the selected applications, high TI
values should be kept to a minimum or avoided in obstetric applications. WFUMB (World
Federation for Ultrasound in Medicine and Biology) guidelines: state that temperature
increase of 4 degree C for 5 min or more should be considered as potentially hazardous to
embryonic and fetal tissue.
23.4.2 MI/TI Display
TI and MI values are displayed in the upper part of the screen in real-time. The operator
should monitor these index values during examinations and ensure that exposure time
and output values are maintained at the minimum amounts needed for effective
diagnosis.
NOTE:
If there is a value of MI or TI exceeds 1.0, you must be careful to practice the ALARA
principle.
The display precision is 0.1.
23.5 Acoustic Power Setting
îš„ Acoustic power adjustment
Use the soft menu control to click [A. Power] in the menu to adjust the acoustic power
percentage, and its value is displayed on the soft menu item as well as at the top of the
screen. The greater the acoustic power percentage, the greater the current acoustic
output. When the image is frozen, the system stops transmitting acoustic power.
îš„ Default setting of acoustic power
Selection of diagnostic applications is the most important factor for controlling ultrasound
output. The permissible level of intensity of ultrasound differs depending on the region of
interest. For fetal examinations, in particular, much care must be exercised.
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