A beam of \(1.0 \mathrm{MHz}\) ultrasound begins with an intensity of \(1000 \mathrm{~W} / \mathrm{m}^{2}\). After traveling \(12 \mathrm{~cm}\) through tissue with no significant reflection, the intensity is about A. \(750 \mathrm{~W} / \mathrm{m}^{2}\)

B. \(500 \mathrm{~W} / \mathrm{m}^{2}\)

C. \(250 \mathrm{~W} / \mathrm{m}^{2}\)

D. \(125 \mathrm{~W} / \mathrm{m}^{2}\)

Ultrasound is absorbed in the body; this complicates the use of ultrasound to image tissues. The intensity of a beam of ultrasound decreases by a factor of 2 after traveling a distance of 40 wavelengths. Each additional travel of 40 wavelengths results in a decrease by another factor of 2 .