Which factor most directly determines axial resolution?

Which factor most directly determines axial resolution?

• A. Beam width
• B. Spatial pulse length
• C. Lateral beam width
• D. Pixel size

Answer: (B)

Axial resolution—the ability to distinguish two structures that lie along the direction of the ultrasound beam—depends on how long the pulse is in tissue. This is governed by the spatial pulse length, which is the physical length of the emitted pulse in space (the number of cycles in the pulse multiplied by the wavelength). A shorter spatial pulse length means echoes from two nearby interfaces return separately rather than merge, so two closely spaced reflectors along the beam can be distinguished. In practice, axial resolution is about half the spatial pulse length, so reducing SPL directly improves axial detail. Higher frequency can shorten wavelength and, with the same pulse structure, shorten SPL, further enhancing axial resolution. Beam width affects lateral resolution (how well you separate objects side by side across the image), not along the beam. Pixel size relates to how finely the image is sampled for display, not the inherent ability to separate closely spaced reflectors along the beam.

Axial resolution—the ability to distinguish two structures that lie along the direction of the ultrasound beam—depends on how long the pulse is in tissue. This is governed by the spatial pulse length, which is the physical length of the emitted pulse in space (the number of cycles in the pulse multiplied by the wavelength). A shorter spatial pulse length means echoes from two nearby interfaces return separately rather than merge, so two closely spaced reflectors along the beam can be distinguished. In practice, axial resolution is about half the spatial pulse length, so reducing SPL directly improves axial detail. Higher frequency can shorten wavelength and, with the same pulse structure, shorten SPL, further enhancing axial resolution.

Beam width affects lateral resolution (how well you separate objects side by side across the image), not along the beam. Pixel size relates to how finely the image is sampled for display, not the inherent ability to separate closely spaced reflectors along the beam.