Spatial pulse length is determined by which of the following?

• A. Wavelength times the number of cycles in the pulse
• B. Wavelength minus the number of cycles in the pulse
• C. Wavelength times the axial resolution
• D. Wavelength times the beam width

Answer: (A)

Spatial pulse length is the physical length of the ultrasound pulse along the beam axis. It equals the distance the sound travels during the transmission of the pulse, which is the number of cycles in the pulse times one wavelength. In other words, SPL = λ × N, where λ is the wavelength in the tissue and N is how many cycles are in the pulse. This is why this option is correct: every cycle spans one wavelength in space, so stacking N cycles covers Nλ of distance.

For intuition, if the wavelength is 0.3 mm and there are 3 cycles, the spatial pulse length is 0.9 mm. Shorter SPL (fewer cycles or shorter wavelength) improves axial resolution, because you can distinguish structures that are closer along the beam path.

The other options don’t describe the true length of the pulse along the axis: subtracting a number of cycles from a wavelength isn’t a meaningful physical quantity for SPL; axial resolution relates to SPL but is not itself the length of the pulse; beam width describes the lateral size of the beam, not the along-axis pulse length.