Biomedical Engineering Reference
In-Depth Information
TABLE 16.1
Acoustic Properties of Tissue
(dB/MHz
y
-cm)
(kg/m
3
)
Material
C (m/s)
a
y
r
Z (MegaRayls)
Air
343
1.21
0.0004
Bone
3,360
3.54
0.9
1,789
6.00
Blood
1,550
0.14
1.21
1,039
1.61
Fat
1,450
0.6
1.0
952
1.38
Honey
2,030
1,420
2.89
Liver
1,570
0.45
1.05
1,051
1.65
Muscle
1,580
0.57
1.0
1,041
1.645
10
3
Water @ 20
C
1,482.3
2.17
2.0
1,000
1.482
could be determined by the simple delay equation with a single value for sound speed,
c
0
,
which is still used today in modern imaging systems. In other words, the geometric accu-
racy of the placement of organs in an ultrasound image largely depends on the uniformity
of the sound speed in the field of view. Ludwig also measured the characteristic acoustic
impedance of tissues,
represent the density and speed of sound
of each tissue, respectively. He found that at the boundary of two different tissues, the
reflection factor was related to the impedances of the individual tissues,
Z
¼ r
c
, in which
r
and
c
Z
1
and
Z
2
:
RF
¼
Z
2
Z
1
Z
ð
16
:
6a
Þ
þ
Z
2
1
Independently, Dr. D. Howry, another U.S. doctor, found that because the reflection fac-
tors were small, ultrasound penetrated through multiple layers of soft tissue with ease
(except for regions with gas or bone). In 1956, he was able to make detailed anatomical
maps of the body with sound and showed that they corresponded with known locations
and sizes of organs and tissues. A graph of reflection factors in dB (decibels),
¼
20 log
10
ð
RF
Þ
ð
16
:
6b
Þ
RF
dB
all with reference to
for blood is shown in Figure 16.3. These factors are different enough to pro-
vide adequate discrimination, an important factor in differentiating among tissues in imaging.
Dr. J. J. Wild, an English surgeon, and J. Reid, an electrical engineer (now professor emer-
itus) working in Minnesota with a surplus 15 MHz radar simulator in 1951, recognized the
value of ultrasound for diagnosis and attempted to use it to detect cancer in the stomach.
They also made calculations on pulse-echo waveforms to infer properties of healthy and
diseased tissue and began a new field now called tissue characterization. In the process,
Wild and Reid developed near real-time ultrasound imaging systems and ways of placing
transducers (that transmit sound and receive pulse echoes and convert them to electrical
signals) directly on the skin.
In the 1950s, a number of groups around the world became interested in the diagnostic
possibilities of ultrasound. S. Satomura, Y. Nimura, and T. Yoshida in Japan detected blood
Z
1
