How does increasing the number of cycles in a pulse affect SPL and axial resolution?

• A. Fewer cycles broaden bandwidth
• B. More cycles increase SPL and worsen axial resolution
• C. Fewer cycles increase SPL
• D. More cycles improve axial resolution

Answer: (B)

Spatial pulse length (SPL) is the physical length of the ultrasound pulse along the beam and equals the number of cycles in the pulse times the wavelength. When you increase the number of cycles, you lengthen the pulse in space, so SPL becomes larger. Axial resolution—the ability to distinguish structures along the beam direction—improves with shorter SPL and worsens with longer SPL. So adding more cycles makes the pulse longer and degrades axial resolution. For context, shorter pulses (fewer cycles) have broader bandwidth, which is why they tend to provide better axial detail.