Axial resolution is improved with:

• A. Increased bandwidth
• B. Shorter pulse length
• C. Shorter wavelength
• D. All of the above

Answer: (D)

Axial resolution depends on the spatial pulse length, which is the physical length of the emitted pulse along the beam. A shorter spatial pulse length means echoes from two closely spaced reflectors along the axis will be separated in time enough to be distinguished, so the image resolves better in the axial direction.

Spatial pulse length is set by the wavelength and the number of cycles in the pulse. Shorter wavelength reduces the distance per cycle, and fewer cycles shorten the overall pulse length. Bandwidth plays a role because a wider bandwidth allows the emitted pulse to be shorter in time; broader frequency content equates to a shorter temporal pulse, which translates into a shorter spatial pulse length.

So, using a shorter pulse length, a shorter wavelength, and a wider bandwidth all contribute to improved axial resolution.