
2.1 A-Scan Function
One of the major applications of A-Scan is
to measure the axial length of the eye in
order to calculate the necessary power of an
intraocular lens replacement (such as
required in the treatment of cataracts).
In A-Scan, a beam of ultrasound is
transmitted along a fixed line through the
eye and the reflected echoes are displayed.
An echo is produced whenever the
ultrasound beam encounters a boundary
between two media having different values
of acoustic impedance. The acoustic
impedance is a function of density and
elasticity and the amount of energy reflected
depends upon the difference in the acoustic
impedance of the two media. A-Scan is
equipped with software that calculates the
distance between the echoes.
The probe acts as the interface between the
instrument and the patient. The probe
contains a piezoelectric crystal, which
converts electrical energy into ultrasound
when transmitting and then acts as a detector
to convert the received ultrasound echoes
into electrical signals for display and
measurement. The A-Scan probe contains a
red LED fixation light in its center. The
fixation light provides a target, which assists
the operator to align the visual axis of the
eye being examined. To minimize
indentation of the eye, which is caused by
the operator applying excessive pressure
while holding the probe on the eye, A-scan
probes feature "Soft-Touch". This feature
minimizes the compression effect by
allowing the probe handle to float on
bushings located inside the probe handle.
2.2 B-Scan Function
Theory of Operation The operation of the B-Scan is similar to a
radar operation. A short ultrasound wave is
produced by a transducer and sent to the
eye. Part of the energy of that wave is
reflected back to the transducer from various
structures of the eye (lens, retina, etc.). The
reflected energy is converted into electrical
signals that are amplified and displayed on
the LCD as an intensity. Since the reflected
energy is proportional to the reflected
properties of the different eye structures, one
can examine the eye by observing the
resulting grey or color scale image.
The ultrasound transducer is located in the
B-Scan probe and is driven by two (2)
solenoids to produce a 60°sector scan. The
sector scan is divided into 256 rays. The
data acquired along each ray is digitized at
512 discrete time intervals (pixels) and each
sample voltage is encoded into 7 bits of
information.
The acquired information is stored in one (1)
of the two (2) B-Scan memory buffers. At
any time during the acquisition, the content
on one memory buffer is displayed while the
other is updated with new data from the
amplifier.
The display control maps data in the
memory buffer onto the LCD screen, thus
showing sector scans. The microprocessor
periodically checks touch screen controls
and calculates mapping coefficients, to pan
the image.
There is a simultaneous vector display that
shows the signal along a selected ray,
represented as an amplitude waveform. This
supplemental information enables the doctor
to differentiate between the two (2) signals
that have the closest intensities of grey.
Doc# 0300-1903-3A page 4