Which probe would provide the best axial resolution to measure the thickness of a small structure?

• A. 5 MHz linear array
• B. 7 MHz linear array
• C. 10 MHz linear array
• D. 5 MHz curved array

Answer: (C)

Axial resolution depends on how short the ultrasound pulse is along the beam path. Shorter pulses come from higher frequencies because they have shorter wavelengths, which shortens the spatial pulse length. With a shorter spatial pulse length, the system can distinguish two closely spaced interfaces along the beam, such as the front and back surfaces of a small structure.

Since the 10 MHz linear array operates at a higher frequency than the others, it produces the shortest wavelength and the finest axial resolution. This makes it the best choice for accurately measuring a small structure’s thickness. Keep in mind that higher frequency waves attenuate more quickly and don’t penetrate as deeply, but for a small, superficial target, the 10 MHz linear array provides the clearest axial detail.