Biomedical Engineering Reference
In-Depth Information
Then from the Fourier transform pair expressed in Eq. (16.2),
` 1
½ G ðoÞ ¼ g ð t Þ¼ð i =
2
Þ½dð t t
Þdð t þ t
Þ
1
1
From Eq. (16.4b) with
a ¼
3 and
b ¼ o 0 , find
r
(
t
)as
d
t t 1
3
t þ t 1
3
` 1
½ R ðoÞ ¼ ð e i o 0 t =
3
Þð i =
2
Þ d
and from scaling in the impulse function in Eq. (16.5),
r ð t Þ¼ð e i o 0 t Þ¼ð i =
2
Þ½dð t t 1 Þdð t þ t 1 Þ
16.2 DIAGNOSTIC ULTRASOUND IMAGING
Diagnostic ultrasound imaging is the most widely used form of medical imaging. Its abil-
ity to reveal body structures and dynamics and blood flow in real time at low cost in a safe
manner (using non-ionizing radiation) has expanded its growth and utility for over fifty
years. Improvements in technology and electronic miniaturization have decreased the cost
and size of ultrasound systems and made highly portable systems possible. In this section,
the history and principles of diagnostic ultrasound are discussed.
16.2.1 Origins of Ultrasound Imaging
Though it has long been known that bats use sound for echolocation, the intentional use
of ultrasound (sound with frequencies above our range of hearing) for this purpose began,
surprisingly, with the sinking of the
Titanic
in 1913. L. F. Richardson, a British scientist,
filed patents within months of the
disaster for echolocation of icebergs (and other
objects) using sound in either water or air. By the end of World War I, C. Chilowsky and
P. Langevin in France invented practical implementations of echolocation with high-
powered electronic transmitters and piezoelectric transducers for locating submarines and
echo ranging. These principles of echo ranging were applied much later to electromagnetic
waves to create RADAR (RAdio Detection And Ranging). The circular sweep of a RADAR
echo ranging line is displayed on PPI (plan position indicator) monitors, an early example
of pulse-echo imaging. This type of display and technology was in turn adopted by under-
water investigators to develop SONAR (SOund Navigation And Ranging).
After experiencing these technologies during World War II, several doctors wanted to
apply echolocation principles to the interior of the human body. Fortunately, a device—
the supersonic reflectoscope—that was developed for finding defects in solid objects by
ultrasound echolocation became available in addition to other surplus wartime equipment.
This type of equipment sent out short pulses a few microseconds in length, repeated at lon-
ger intervals of a millisecond, as shown in Figure 16.1.
Short pulses were needed to determine the location of tissue boundaries. A typical setup
is shown in Figure 16.2, along with a record of echoes displayed as a function of time. Here,
Titanic
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