Chapter 12, Problem 76PP

After a beam passes through 10 cm of tissue, what is the beam’s intensity as a fraction of its initial intensity from the transducer?

1. A.    1 × 10⁻¹¹
2. B.     0.001
3. C.     0.01
4. D.    0.1

BIO Ultrasound imaging. 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 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, technicians 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 the determination of attenuation, the reference intensity is taken to be the intensity produced by the transducer.