Shorter pulses _____ axial resolution.

• A. Improve
• B. Increase penetration
• C. Worsen
• D. Have no effect

Answer: (A)

Shorter pulses improve axial resolution because axial resolution depends on how short the spatial pulse length is along the beam. Spatial pulse length is the physical length of the ultrasound pulse in tissue, which equals the number of cycles in the pulse times the wavelength. Fewer cycles, or a shorter wavelength (higher frequency), shorten this length. When the pulse is shorter, two reflectors that are close together along the beam can produce distinct echoes instead of one blurred echo, so structures along the axis can be resolved more precisely. Shorter pulses don’t increase penetration; in fact, using higher frequency to shorten the pulse often reduces penetration. They do affect resolution, making axial resolution better.