Which formula represents axial resolution in soft tissue?

Which formula represents axial resolution in soft tissue?

• A. 0.77 x cycles x Frequency
• B. 0.77 x cycles / Frequency
• C. 0.77 / (Frequency x cycles)
• D. 0.77 x Frequency / cycles

Answer: (B)

Axial resolution is determined by how long the ultrasound pulse is along the beam direction—the shorter the spatial pulse length, the finer the detail you can resolve. The spatial pulse length equals the number of cycles in the pulse times the wavelength (SPL = n × λ), and the wavelength in tissue is λ = c/f, with c about 1540 m/s. The axial resolution is about half of the SPL, so d ≈ SPL/2 = (n × c)/(2f). When frequency is expressed in MHz and distance in millimeters, this simplifies to roughly d ≈ 0.77 × n / f_MHz. That makes the correct formula 0.77 × cycles divided by frequency. The constant 0.77 comes from using c ≈ 1540 m/s and converting units to millimeters and MHz. So increasing the number of cycles worsens axial resolution, while increasing frequency improves it. For example, a 2-cycle pulse at 3 MHz gives about 0.51 mm.

Axial resolution is determined by how long the ultrasound pulse is along the beam direction—the shorter the spatial pulse length, the finer the detail you can resolve. The spatial pulse length equals the number of cycles in the pulse times the wavelength (SPL = n × λ), and the wavelength in tissue is λ = c/f, with c about 1540 m/s. The axial resolution is about half of the SPL, so d ≈ SPL/2 = (n × c)/(2f). When frequency is expressed in MHz and distance in millimeters, this simplifies to roughly d ≈ 0.77 × n / f_MHz. That makes the correct formula 0.77 × cycles divided by frequency. The constant 0.77 comes from using c ≈ 1540 m/s and converting units to millimeters and MHz. So increasing the number of cycles worsens axial resolution, while increasing frequency improves it. For example, a 2-cycle pulse at 3 MHz gives about 0.51 mm.