Image Processing Reference
In-Depth Information
Figure5.5 Another sonar image of theEmpireKnight. (CourtesyofGarryKozak,L-3Klein)
Some high-resolution sonar images are particularly eerie, especially when the
imaged objects are located on a bottom that is otherwise devoid of features.
Figure 5.6(a) is a sonar image of the bottom of an inland lake that is 60 feet deep.
The drowning victim can be seen lying on the bottom. The shape of the victim's
body is clearly visible in Fig. 5.6(b). This sonar image was made with 455 kHz
sound waves, corresponding to a wavelength in water of 3 mm.
The Japanese were the first to use acoustic imaging for medical applications. Their
early work, which took place after World War II, started with simple ultrasonic
devices that acted like depth finders, reading out a distance to an interface or
boundary within tissue. The boundary might be something like the surface of
a tumor, which is often denser than surrounding tissue and will reflect back an
acoustic signal to a transducer. These devices were the precursors to modern
ultrasound imaging systems that use intensive computer processing to produce
startling imagery. Ultrasound imaging is very useful for examination of a fetus
inutero(i.e., in the womb). One can non-invasively image the fetus through
layers of the mother's tissue without potentially harmful radiation. The imaging
system generates acoustic waves with wavelengths of a few millimeters or shorter,
yielding very high-resolution imagery. The waves emerge from a transducer placed
against the mother's abdomen and propagate inward, reflecting off of various
boundaries, for example, the boundary between the amniotic fluid and the fetus'
skin. Figure 5.7 shows a fetusinuteroat eight months, imaged with sound waves
with frequencies varying between 4 and 7 MHz. 4 This frequency range corresponds
4 1 MHz = 1 million cycles per second.
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