For cranial ultrasound, why is it advantageous to use frequencies in the kHZ range rather than the MHz range?

(a) The antinodes of the standing waves will be closer together at the lower frequencies than at the higher frequencies;

(b) There will be no standing waves at the lower frequencies;

(c) Cranial bones will attenuate the ultrasound more at the lower frequencies than at the higher frequencies;

(d) Cranial bones will attenuate the ultrasound less at the lower frequencies than at the higher frequencies.

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.