Which statement about depth of penetration in ultrasound is true?

• A. 3 MHz penetrates less deeply than 1 MHz
• B. 3 MHz penetrates more deeply than 1 MHz
• C. They penetrate equally
• D. Depth is unrelated to frequency

Answer: (A)

Higher ultrasound frequency is attenuated more quickly by tissue, so it deposits most of its energy in superficial layers. A 3 MHz setting therefore reaches a shallower depth than a 1 MHz setting. In practice, 1 MHz can treat deeper tissues (several centimeters), while 3 MHz is used for superficial structures (about 1–2 cm). So, 3 MHz penetrates less deeply than 1 MHz. The idea that depth is unrelated to frequency or that both frequencies penetrate equally isn’t correct, and 1 MHz clearly reaches deeper tissues than 3 MHz.