Section 6
Specifications
6
- 3
Probe Material: Lead Metanioboate
Nominal Center Frequency:
A-scan: 10 MHz
B-scan: 10 MHz
Pulse Repetition Frequency:
A-scan: 19.2 Hz
B-scan:
3800
Hz
Type: , A-scan and B-scan.
Typical Ultrasonic Intensities in tissue listed
below:*
B-SCAN
B-scan
(autoscanning)
I
SPTA
, .881 mW/cm
2
I
SPPA
11.45 W/cm
2
MI 0.151
Ultrasonic Power 0.133 mW
A-SCAN
I
SPTA
, 0.0068 mW/cm
2
I
SPPA
3.31 W/cm
2
MI 0. 085
Ultrasonic Power .00134 mW
*Energy emitted in bursts for A-scan. Measurement must
be repeated for new burst. At measured transducer focus
(1.7 cm from probe tip for A-scan, 1.5 cm for B-scan).
Focal point adjusted for attenuation of tissue. The output
power is not adjustable. Thus, these values are also the
global maxima.
The energy will always be attenuated by the
tissue between the transducer and the focus
when used as recommended. The values
presented here are the values at the focal point,
the point of maximum intensity.
It is not possible to vary the output energy of the
transducer. However, to minimize exposure,
measurements should be kept as short as
possible during use.
If more accuracy is desired, the intensity in the
body at any transducer point may be calculated
according to the formula recommended by the
FDA:
I
t
= I
w
exp(-0.069fz),
where I
t
= is the estimated in situ intensity, I
w
is
the measured intensity in water at the focus of
the transducer (indicated in the above chart), f is
the ultrasonic frequency in megahertz, and z is
the distance from the face of the probe to the
transducer focus in centimeters, which is the
point of measurement. For this device, f = xx
and z = yy. This formula was also used to
calculate the derated values shown above.
Transducer parameters show considerable
variation from transducer to transducer. The
measured and calculated values shown above
were those for an actual transducer, whose
values deviated slightly from the values in the
specification above, and whose values are likely
to be different from the transducer with your
system. However, the values in the specification
should give results that are accurate enough for
any practical purpose, since the intensities are
very low.
There is no control for adjusting the output
energy. Thus, the transducer should be applied
to the eye for as short a period as possible.
Remove the transducer from the eye when not
making measurements. One should always
minimize exposure by limiting the ultrasonic
transmission to as short periods as possible.
Velocity of sound used by system (values for
different eyes) are tabulated in Appendix A of
this manual.
Global Maxima
The values given above are the global maxima.
Accuracy
The A-scan system measurements have an
accuracy of 0.1 mm. The B-scan accuracy is 0.1
mm. These are the values that have been found
in clinical practice. The corresponding
theoretical values are 0.484 mm (B-scan) and
0.023 mm (A-scan). These theoretical values do
not include errors caused by operator technique
or uncertainty in the velocity of sound. The A-
scan accuracy can be expected over a range of
18 to 40 mm of axial length. The B-scan
accuracy at an acquisition of 256 rays is valid to
ALARA SECTION AND EMISSIONS
(As Low As Reasonably Possible)
