Page 9 of 87 DGH 55B-INS-OMENG Rev: 3
4. Use Of Ultrasound In Ophthalmic Measurement
4.1 Introduction To Ultrasound
Ultrasound offers a non-invasive method to examine the interior of solid objects. Ultrasonic
pulses consist of sound waves of a frequency level too high to be heard by the human ear. When
a sound impulse strikes an interface, some sound is reflected, and some sound is transmitted.
Because some sound will pass through the surface and be reflected by the next surface, complex
structures can be examined with ultrasound. When ultrasound penetrates an object with several
interfaces, the reflected ultrasound can be observed as a waveform with peaks that are related to
the positions of the interfaces.
The DGH 55B transducer emits ultrasound pulses and detects ultrasound signals that have been
reflected back. The time delay between the echoes is used to calculate distances between
surfaces in the eye.
NOTE: Ultrasound cannot travel through air because air is not dense enough for the high
frequency waves to propagate. Ultrasonic measurements must therefore be performed by direct
contact or through a denser medium such as water.
4.2 Using Ultrasound To Ascertain Correct Probe Alignment
Sound travels in straight lines, so the direction of reflected sound is based solely on its angle of
incidence. Sound hitting an interface perpendicularly will reflect back along the same path that it
approached (Figure 4.2.1). Sound hitting an interface at an angle will reflect at an angle away
from the source (Figure 4.2.2). The transmitted sound will continue on at a lesser amplitude
because of reflected energy lost at the interface.
When reflected ultrasound is shown as a two-dimensional waveform, the peaks are related to the
positions of the interfaces. By comparing the relative height (intensity) of the peaks, one can
determine the angle at which the sound is striking it. Steadily diminishing peaks are an indicator
that the ultrasound is not perpendicular to the interfaces
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