14
Wdeg = 210/fc, for models 1 to 4, where fc is the mean frequency in MHz.
Wdeg = 40 KD for models 5 and 6, where K (beam shape factor) is 1.0 and D is the diameter of
the aperture in cm at the depth of the area being examined.
MI:
Cavitation is more likely to occur at high pressures and low frequencies of the ultrasonic wave pulse in tissue that con-
tains bubbles or air pockets (for instance, the lungs, intestines, or when scanning with contrast agents that contain
gas). The threshold under optimum conditions of pulsed ultrasound is calculated by the ratio of the peak pressure to
the square root of the frequency.
MI = Pr '/ sqrt (fc)
Pr’ is the de-rated (0.3) peak rarefactional pressure in Mpa at the point where PII is the maximum, and fc is the mean
frequency in MHz. PII is the pulse intensity integral that is the total energy per unit area of the wave carried for the
duration of the pulse. The peak rarefactional pressure is measured in a hydrophonic, negative maximum voltage,
normalised by the hydrophonic calibration parameter.
Display guideline:
Depending on the operating mode, there are different types of display. However, only one index needs to be shown at a
time.
Display is not required if the maximum MI for any operator setting is less than 1.0, or if the
maximum TI for any setting of the operator mode is less than 1.0. For TI, if the TIS and TIB are both greater than 1.0, the
scanners need not be capable of displaying both indices simultaneously. If the index falls below 0.4, no display is re-
quired. The display increments are no greater than 0.2 for index values less than one and no greater than one for index
values greater than one. (e.g. 0.4, 0.6, 0.8, 1, 2, 3).
Display and report
Located in the upper middle section of the system display monitor, the acoustic output display provides the operator
with a real-time indication of the acoustic levels being generated by the system.
For the scan
Only display and report MI, and start from 0.4 if the maximum MI > 1.0, display in increments of 0.2.
Below is a simple guideline for the user when the TI exceeds the exposure time limit by 4(6-TI) minutes, based on the
recommendations of the 'National Council on Radiation Protection and Measurements. Exposure criteria for medical
ultrasound diagnostics: I. Criteria based on thermal mechanisms. Report no. 113 1992'.
Operating characteristics:
The user should be aware that certain operator controls may affect the acoustic output. It is recommended to use the
default (or lowest) output power setting and to compensate using the gain control to acquire an image. Unlike with the
output power setting in the soft menu, which has the most direct impact on the power, the PRF, image sector size,
frame rate, depth, and focus position also have a slight impact on the output power. The default setting is normally
around 70% of the allowable power depending on the examination's application mode.