Page 40 - January 2006

The newsletter of
The Acoustical Society of America
Biomedical Ultrasound– Past, Present, and Future
E. Carr Everbach
Swarthmore College Swarthmore, Pennsylvania 19081
Historical evidence sug- gests that as far back as 250 BCE, captains of Ancient Greek ships would drop lead weights called “sounders” overboard and count up until a thud was heard. The counts provided a measurement of water depth; from this practice we have an early use of propagation time to estimate depth and the ori- gin of the phrase “sounding something out.”
Sonar developed soon after
the sinking of the Titanic, and
was given a boost by the subse-
quent discovery of piezoelectric materials that could func- tion as acoustic sources as well as receivers. The familiar sonar “ping” allowed vessels to use time-of-flight measure- ments of echoes, as bats and dolphins do, to estimate tar- get range and size. Modern biomedical ultrasound got its start during World War II as an extension of sonar to high- er frequencies and to the human body. (See Fig. 1)
Early ultrasound technology of the 1950s utilized a single piezoelectric crystal in contact with layers of human tissue. An acoustic wave was launched into the tissue con- sisting of several cycles at a frequency above human hear- ing (~20 kHz). A series of reflections returned to the crys- tal due to mismatches of acoustic impedance and the resulting reflections at each interface. The echoes were
Fig. 2. Schematic description of early ultrasound technology of the 1950s. A sin- gle piezoelectric crystal in contact with layers of human tissue launches an acoustic wave consisting of several cycles at a frequency above human hearing (20 kHz).
represented in “A-mode” (amplitude) displays as a verti- cal deflection on an oscillo- scope screen. If the speed of sound in human soft tissues is assumed to be constant at 1540 m/s, the times between reflec- tions could be interpreted as the distances between layers as shown in Fig. 2.
If A-mode amplitudes, however, are represented by the brightness of dots on a phosphor screen and the transducer is scanned across the tissue, a “B-mode” (bright- ness) image results (Fig. 3).
Assuming a constant speed of sound allows quantitative
Fig. 1. Medical ultrasound is increasingly using the information from tissue nonlinearities to help diagnose illness. Shown is an ultrasound scan of a kidney.
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38 Acoustics Today, January 2006