13
WATER, TANK SIZE, TRANSDUCER PLACEMENT &
TEMPERATURE CONSIDERATIONS
Water as a Measurement Medium: The measurements
are to be performed in de-gassed water because ultra-
sound propagation in water closely approximates that in
tissues (see UL-1-1981, AIUM/NEMA Standard Publica-
tion). The ultrasonic attenuation in water can be taken
as a lower limit on the attenuation which will be encoun-
tered in the body. Large areas in the body can consist of
low attenuating material such as urine and amniotic fluid.
The use of water prevents measurements in a more
highly attenuating material such as liver equivalent gels
from representing the highest possible intensities which
might be encountered in the body. A measurement tem-
perature of 24°C ± 3°C (75°F ± 5°) is chosen for conven-
ience.
De-Gassed Water: Ultrasound power measurement ac-
curacy is affected (by lowering the power reading) if the
water contains more than five parts per million of air. To
de-gas, boil distilled water for one half-hour, then pour
into a suitable container, seal tightly and place in refrig-
erator. This process will give the required quality. The
container should be heat resistant glass; or thick plastic
may be used after the water has been cooled. Before
testing, pour water into tilted test tank to minimize turbu-
lence. The test tank water surface will absorb oxygen
and a change of de-gassed water is recommended be-
fore each test. An alternative method of de-gassing wa-
ter is to heat the water to the boiling point, then put a
vacuum to it for five minutes.
Water temperature affects accuracy; use a testing tem-
perature of 21 to 27°C ambient. Sonic energy agitates
the water surface through heating and scattering. Test-
ing time should be limited to a few minutes; prolonged
testing, particularly at higher power levels, will drive out
dissolved air and air bubbles will be visible on surfaces in
the tank. These bubbles can be gently brushed off.
Transducer Wetting and Placement: After tilting the
transducer into the water at a 45° angle, verify that the
surface is uniformly wetted; if not, wipe the surface clean.
The transducer should be positioned above the cone
target. Small variations will occur due to placement. Try
various positions above target to obtain a maximum
power reading.
ULTRASOUND RADIATION LEVELS
There are no maximum limits in the U.S. for therapy
power, only the verification of the displayed setting accu-
racy to ± 20% of actual output is required. Exposure lev-
els for physical therapy applicator heads range from
100mW/cm
2
to 3W/cm
2
. Total power requires multiplica-
tion by the radiated cross sectional area in cm
2
.
The power limits shown in the following table for diagnos-
tic ultrasound have been extracted from FDA Section
510(k) guidance to manufacturers on submissions and
clearance as of February 1993. Refer to the AIUM and
FDA publications for complete and up to date testing
standards and interpretations. Measurements are done
in all standard modes of operation. Power intensity is
rated as Spatial Peak Temporal Average (I
SPTA
) and Spa-
tial Peak Pulse Average (I
SPPA
). The values in mW/cm
2
are derated for tissue and in parenthesis for the water
medium (use the chart below):
PRE-AMENDMENT ACOUSTIC OUTPUT LIMITS
Use I
SPTA
Tissue
(mW/cm
2
)
Water
I
SPPA
Tissue
(mW/cm
2
)
Water
Peripheral Vessel 720 1500 190 350
Fetal Imaging & Other * 430 730 190 350
Cardiac 94 180 190 350
Opthalmic 17 68 28 110
* Abdominal, intra-operative, pediatric, small organ (breast, thyroid, testes, etc.), neonatal cephalic, adult cephalic
To Next Page
Loading...
Need help?
General
