Sound (or pressure) waves in the 3 MHz to 10 MHz fre-quency range are used for imaging the body by detecting the intensity of the reflected waves from various organs and dis-playing this reflected intensity as a gray-scale (or color) image. The sound waves are generated by applying an electri-cal pulse to a piezoelectric crystal. This crystal also acts as a receiver of the reflected waves after the transmitter pulse is terminated. A typical ultrasound transducer contains a linear array of such crystals, which can be fired in sequence or op-erated as a phased array to cause the ultrasound beam to rap-idly scan across an area 5 to 10 cm in width for real-time imaging. The useful imaging depth is determined by the fre-quency; the higher frequencies (shorter wavelengths) have less penetrability. For example, at 10 MHz the imaging depth is limited to a few centimeters. Unfortunately, the lower the frequency, the poorer the axial resolution, because objects that are closer together than a wavelength cannot be sepa-rated. Hence, there is a tradeoff between axial resolution and penetration depth. Because ultrasound radiation is nonioniz-ing, no adverse biological effects have been observed at diag-nostic power levels.
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