A typical ultrasound transducer used for medicaldiagnosis produces a beam of ultrasound with a frequency of 1.0 MHz.The beam travels from the transducer through tissue and partially reflectswhen it encounters different structures in the tissue. The same transducerthat produces the ultrasound also detects the reflections.The transduceremits a short pulse of ultrasound and waits to receive the reflected echoesbefore emitting the next pulse. By measuring the time between the initialpulse and the arrival of the reflected signal, we can use the speed of ultrasoundin tissue, 1540 m>s, to determine the distance from the transducerto the structure that produced the reflection.As the ultrasound beam passes through tissue, the beam is attenuatedthrough absorption. Thus deeper structures return weaker echoes. A typicalattenuation in tissue is -100 dB/m. MHz; in bone it is -500 dB/m . MHz.In determining attenuation, we take the reference intensity to be the intensityproduced by the transducer Because the speed of ultrasound in bone is about twice the speedin soft tissue, the distance to a structure that lies beyond a bone can bemeasured incorrectly. If a beam passes through 4 cm of tissue, then 2 cmof bone, and then another 1 cm of tissue before echoing off a cyst andreturning to the transducer, what is the difference between the truedistance to the cyst and the distance that is measured by assuming thespeed is always 1540 m>s? Compared with the measured distance,the structure is actually (a) 1 cm farther; (b) 2 cm farther; (c) 1 cmcloser; (d) 2 cm closer.
A typical ultrasound transducer used for medical
diagnosis produces a beam of ultrasound with a frequency of 1.0 MHz.
The beam travels from the transducer through tissue and partially reflects
when it encounters different structures in the tissue. The same transducer
that produces the ultrasound also detects the reflections.
The transducer
emits a short pulse of ultrasound and waits to receive the reflected echoes
before emitting the next pulse. By measuring the time between the initial
pulse and the arrival of the reflected signal, we can use the speed of ultrasound
in tissue, 1540 m>s, to determine the distance from the transducer
to the structure that produced the reflection.
As the ultrasound beam passes through tissue, the beam is attenuated
through absorption. Thus deeper structures return weaker echoes. A typical
attenuation in tissue is -100 dB/m. MHz; in bone it is -500 dB/m . MHz.
In determining attenuation, we take the reference intensity to be the intensity
produced by the transducer Because the speed of ultrasound in bone is about twice the speed
in soft tissue, the distance to a structure that lies beyond a bone can be
measured incorrectly. If a beam passes through 4 cm of tissue, then 2 cm
of bone, and then another 1 cm of tissue before echoing off a cyst and
returning to the transducer, what is the difference between the true
distance to the cyst and the distance that is measured by assuming the
speed is always 1540 m>s? Compared with the measured distance,
the structure is actually (a) 1 cm farther; (b) 2 cm farther; (c) 1 cm
closer; (d) 2 cm closer.
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