Which equation expresses axial resolution in ultrasound imaging?

Which equation expresses axial resolution in ultrasound imaging?

• A. Axial resolution (mm) = spatial pulse length (mm) x 2
• B. Axial resolution (mm) = spatial pulse length (mm)/2
• C. Axial resolution (mm) = frequency (MHz) x depth / 2
• D. Axial resolution (mm) = wavelength (mm) / number of cycles

Answer: (B)

Axial resolution is about how close two reflectors along the ultrasound beam can be and still be distinguished. It depends on how long the emitted pulse is in tissue, the spatial pulse length. Because two echoes must be separated by at least about half of that pulse length to appear as separate reflections, axial resolution equals spatial pulse length divided by 2. Spatial pulse length itself is the product of how many cycles are in the pulse and the wavelength. Shorter pulses—fewer cycles or shorter wavelengths—yield smaller spatial pulse length and thus better axial resolution. The other formulations don’t reflect this relationship: they either scale the pulse length incorrectly, or mix in depth or frequency in ways that don’t directly define the resolving length along the beam.

Axial resolution is about how close two reflectors along the ultrasound beam can be and still be distinguished. It depends on how long the emitted pulse is in tissue, the spatial pulse length. Because two echoes must be separated by at least about half of that pulse length to appear as separate reflections, axial resolution equals spatial pulse length divided by 2. Spatial pulse length itself is the product of how many cycles are in the pulse and the wavelength. Shorter pulses—fewer cycles or shorter wavelengths—yield smaller spatial pulse length and thus better axial resolution. The other formulations don’t reflect this relationship: they either scale the pulse length incorrectly, or mix in depth or frequency in ways that don’t directly define the resolving length along the beam.