Why does a 3 MHz ultrasound beam heat tissue more quickly than a 1 MHz beam?

• A. It travels deeper into tissue
• B. It produces superficial heating
• C. It uses continuous output only
• D. It uses lower energy

Answer: (B)

Higher-frequency ultrasound deposits energy more readily in tissue because attenuation (absorption) increases with frequency. The 3 MHz beam is absorbed more at the surface, so more energy is converted to heat in the superficial layers, leading to a faster temperature rise there. In contrast, lower-frequency ultrasound penetrates deeper with less energy absorbed per millimeter near the surface, so surface heating is slower. The heating rate is driven by how much energy is absorbed, not by continuous versus pulsed output or by a universally lower energy.