Which will not improve axial resolution?

• A. Increased damping
• B. Increased transducer frequency
• C. Increased bandwidth
• D. Increased focusing

Answer: (D)

Axial resolution is determined by the spatial pulse length—the shorter the pulse length along the beam axis, the better the ability to distinguish two reflectors in the direction of the beam. Increasing damping reduces ringing and shortens the emitted pulse, which directly shortens the spatial pulse length and improves axial resolution. Higher transducer frequency lowers the wavelength; since spatial pulse length is the number of cycles times the wavelength, a higher frequency tends to shorten SPL and thus improve axial resolution. Broader bandwidth allows the impulse to be briefer in time and space, also sharpening axial resolution. Increasing focusing, however, narrows the beam laterally to improve lateral resolution and does not shorten the pulse along the axis, so it does not improve axial resolution.