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5 Acoustics in Biology and Medicine
5.42 How much time elapses between the moment when an ultrasonic wave enters
the body, is reflected by the mitral valve 4.5 cm away, and its echo returns?
5.43 What is the wavelength of a 2 MHz US wave in tissue if the wave has a speed
of 1.4 × 10 5 cm/s? How small an object is resolvable by such a wave?
5.44 What is the smallest wavelength of ultrasound used currently in medical
diagnosis? Why not smaller?
5.45 Ultrasonic probes can measure blood flow in the arteries of retina, and fetal
heart pumping rate. Describe how you might construct and operate such probes.
5.46 The sound wave from a 1 MHz US probe is reflected 180 ◦ from a mitral
valve moving away from the source at 3.5 cm/s. What will be the shift in frequency
detected at the probe?
5.47 If the intensity of the received sound at a 1 MHz probe must be greater than
1% of the probe’s emitted sound, what is the greatest depth that sound reflections
can be detectable? Take the attenuation to be 0.5 dB/cm at 1 MHz.
5.48 If all the sound energy of an 1 MHz US continuous emitter with an intensity
of 1 W/cm 2 is deposited into an isolated cubic centimeter of tissue, with a density of
1.1 g/cc, specific heat of 1.2 calories per gm-degree and sound velocity of 1.54 m/s,
how much would the tissue rise in temperature after 1 min, assuming no heat
escapes?
5 Acoustics in Biology and Medicine
5.42 How much time elapses between the moment when an ultrasonic wave enters
the body, is reflected by the mitral valve 4.5 cm away, and its echo returns?
5.43 What is the wavelength of a 2 MHz US wave in tissue if the wave has a speed
of 1.4 × 10 5 cm/s? How small an object is resolvable by such a wave?
5.44 What is the smallest wavelength of ultrasound used currently in medical
diagnosis? Why not smaller?
5.45 Ultrasonic probes can measure blood flow in the arteries of retina, and fetal
heart pumping rate. Describe how you might construct and operate such probes.
5.46 The sound wave from a 1 MHz US probe is reflected 180 ◦ from a mitral
valve moving away from the source at 3.5 cm/s. What will be the shift in frequency
detected at the probe?
5.47 If the intensity of the received sound at a 1 MHz probe must be greater than
1% of the probe’s emitted sound, what is the greatest depth that sound reflections
can be detectable? Take the attenuation to be 0.5 dB/cm at 1 MHz.
5.48 If all the sound energy of an 1 MHz US continuous emitter with an intensity
of 1 W/cm 2 is deposited into an isolated cubic centimeter of tissue, with a density of
1.1 g/cc, specific heat of 1.2 calories per gm-degree and sound velocity of 1.54 m/s,
how much would the tissue rise in temperature after 1 min, assuming no heat
escapes?