Axial resolution is improved with:

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

Answer: (D)

Axial resolution improves when the spatial pulse length is shorter, because a shorter length confines the ultrasound energy more tightly along the beam direction, allowing two closely spaced reflectors to be distinguished.

Spatial pulse length depends on how long the emitted pulse is in time, which is the number of cycles in the pulse multiplied by the wavelength. You can shorten SPL by emitting fewer cycles in the pulse (shorter pulse length) or by using a shorter wavelength, which comes from a higher frequency transducer. Broader bandwidth also helps because it enables the generation of shorter, more transient pulses, effectively reducing the pulse duration and thus the SPL.

Since increasing bandwidth, shortening the pulse length, and using shorter wavelengths each contribute to a shorter spatial pulse length, all of these factors improve axial resolution.