SPL is directly proportional to which factor?

• A. Number of cycles in the pulse
• B. Both the number of cycles in the pulse and the wavelength
• C. Wavelength
• D. Frequency

Answer: (B)

Spatial pulse length is the distance the ultrasound pulse occupies in tissue. It depends on two things: how many cycles are in the transmitted pulse and the wavelength of those cycles. Each cycle has a length equal to the wavelength in the tissue, so adding more cycles lengthens the overall pulse, and using a longer wavelength (which happens at lower frequency or slower speed) also lengthens each cycle. Multiply the two together and you get SPL. In soft tissue, wavelength is about c/f, with c ≈ 1540 m/s, so higher frequency shortens the wavelength and can shorten SPL for a fixed number of cycles, while increasing the number of cycles linearly increases SPL.