The resolving power of an ultrasonic search unit is directly proportional to which parameter?

• A. Diameter
• B. Bandwidth
• C. Pulse repetition
• D. None of the above

Answer: (B)

Resolving power in ultrasound is about distinguishing two reflectors that lie close to each other along the beam path, which is governed by the axial resolution. Axial resolution depends on the spatial pulse length—the distance the pulse occupies in the material—which is essentially the pulse duration multiplied by the material’s sound speed. Shorter pulses give shorter spatial pulse lengths, so two nearby echoes can be separated more clearly.

Bandwidth controls how short the emitted pulse is. A transducer with wide bandwidth produces briefer, more broadband pulses, reducing the pulse length and increasing axial resolution. In other words, increasing bandwidth directly improves the ability to resolve two close features along the beam, making resolving power proportional to bandwidth.

Diameter mainly affects beam spread and lateral resolution, not the depth-direction resolving power. Pulse repetition frequency affects how fast you can send pulses and the maximum depth range you can image, but not the inherent ability to separate closely spaced reflectors in the depth direction.