BACKGROUND:
The subject paragraph of the Ultrasonic Therapy Performance Standard requires that
units having an amplitude-modulated (pulsed) output waveform provide an indication of the
temporal-maximum ultrasonic power and intensity. The requirement also states that the sum
of the errors in the indications of temporal-maximum ultrasonic power and ratio of the
temporal-maximum to temporal-average intensities may not exceed + 20% for all emissions
greater than 10 percent of the maximum emission.
This accuracy requirement was specified in this way, i.e., as a sum of errors, in order
to relate the error in the indication to the uncertainty in measurement of the
temporal-average power. In the current state-of-the-art, the measurement of
temporal-average power can be made with greater accuracy (e.g., by radiation force
techniques) than the measurement of temporal-maximum power, which currently must rely on
hydrophone measurements. Furthermore, many ultrasonic therapy devices are capable of
operating in either the pulsed or continuous mode; since the production testing of such
devices would include measurement of the temporal-average power (to calibrate the
continuous mode), it is logical to relate the calibration of the pulsed mode to the same
measurement.
The actual wording of this section of the standard has caused some confusion among
users and manufacturers. The *Center for Devices and Radiological Health* intends to
formally amend the standard so that the requirement will be stated more clearly. The
amendment process requires a certain amount of time; for the interim period, the Agency
policy regarding the calibration of the pulsed mode will be as specified below.
POLICY:
The intent regarding the calibration of the pulsed mode can be stated as follows: The
temporal-maximum power and intensity must be indicated, and the ratio of temporal-peak to
temporal-average intensities must be specified. For a given setting of the output control,
the indicated temporal-maximum power divided by the ratio of intensities will yield a
value for temporal-average power. This calculated value must agree with the measured value
of the temporal-average power, for the same setting of the output control, to within + 20
percent. This policy will guide the *CDRH's* compliance testing, and should guide
manufacturers in production testing. Manufacturers and interested parties will be notified
when the proposed amendment is published for comment and when the formal amendment process
has been completed.
*Material between asterisks is new or revised*
Issued: 10/1/80
Revised: 3/95