Biomedical Engineering Reference
In-Depth Information
FIGURE 16.1 A plot of a series of transmitted acoustic pulses repeated at intervals called a pulse repetition
period (PRP). Typically, the -3 dB width of a pulse is only a few microseconds in order to resolve different tissue
interfaces, whereas the PRP is about a millisecond. For the purposes of illustration, the PRP has been compressed to
enhance the presentation of pulses. The horizontal, or time, axis represents ambient pressure. Positive values on the
vertical axis show compressional acoustic pressure, and negative values show rarefactional pressure.
FIGURE 16.2 Basic echo ranging system consisting of a transmitter, a transducer, a receiver, an amplifier, and an
oscilloscope display. Pulse echoes are shown below at delays corresponding to depths of reflecting objects above.
the delay,
t
, to each echo is the round-trip distance to the object, 2
z
, divided by the speed of
sound in the material,
. The echo range instrument consisted of a piezoelectric
transducer that converted the electrical pulses from the transmitter to acoustic pulses and
reconverted received echoes from targets into electrical signals. These signals were then
amplified and displayed as a time record on an oscilloscope. This type of display was
known as an A-mode display, with A signifying amplitude.
In 1949, U.S. Naval doctor G. Ludwig reported his measurements of sound speed in parts
of the body, and the results showed they had an average value of
c
,or
t ¼
2
z
/
c
1540 m/s. More
recent precise measurements showed that sound speed for most tissues varied by only a
few percent, as listed in Table 16.1. This important finding meant that the location of organs
c 0 ¼
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