Which of the following is determined by the source and the medium?

• A. Frequency
• B. Period
• C. Propagation Speed
• D. Range Resolution

Answer: (D)

Axial (range) resolution is governed by the spatial pulse length, which is how long the transmitted pulse sits in space. This length equals the number of cycles in the pulse times the wavelength of sound. The wavelength comes from the medium through λ = c/f, where c is the speed of sound in that medium and f is the transmitted frequency. The number of cycles (pulse duration) is determined by the source. So, how finely you can distinguish two reflectors along the beam depends on both how the pulse is generated by the transducer and how fast sound travels in the tissue. That’s why range resolution is determined by the source and the medium.

In contrast, frequency and period are set by the source (with period being the inverse of frequency), while propagation speed is a property of the medium itself.