Which factor least influences axial resolution?

Which factor least influences axial resolution?

• A. Damping
• B. Beam width
• C. Frequency
• D. Pulse duration

Answer: (B)

Axial resolution is determined by how long the ultrasound pulse lasts along the beam path. The shorter the pulse along the axis, the better the ability to distinguish two reflectors that are close together in depth. Spatial pulse length (SPL) is the key quantity: SPL = number of cycles in the pulse × wavelength. Since wavelength shrinks as frequency increases, higher frequency shortens SPL and improves axial resolution. Pulse duration also matters because a shorter pulse (fewer cycles or shorter period) reduces SPL, enhancing axial resolution. Damping affects how long the pulse rings after emission; more damping shortens the pulse, improving axial resolution, while less damping lengthens the pulse and worsens it. Beam width, on the other hand, governs lateral resolution—the ability to distinguish reflectors side-to-side across the beam—rather than along the beam axis. It does not change the length of the pulse along the axis. Therefore, beam width least influences axial resolution.

Axial resolution is determined by how long the ultrasound pulse lasts along the beam path. The shorter the pulse along the axis, the better the ability to distinguish two reflectors that are close together in depth.

Spatial pulse length (SPL) is the key quantity: SPL = number of cycles in the pulse × wavelength. Since wavelength shrinks as frequency increases, higher frequency shortens SPL and improves axial resolution. Pulse duration also matters because a shorter pulse (fewer cycles or shorter period) reduces SPL, enhancing axial resolution. Damping affects how long the pulse rings after emission; more damping shortens the pulse, improving axial resolution, while less damping lengthens the pulse and worsens it.

Beam width, on the other hand, governs lateral resolution—the ability to distinguish reflectors side-to-side across the beam—rather than along the beam axis. It does not change the length of the pulse along the axis.

Therefore, beam width least influences axial resolution.