If the number of cycles in the transmitted pulsed wave is increased, axial resolution is degraded.

• A. Lateral resolution is degraded
• B. Penetration is decreased
• C. Axial resolution is degraded
• D. Spatial pulse length becomes shorter

Answer: (C)

Axial resolution is determined by how long the transmitted pulse is along the beam, called the spatial pulse length. Spatial pulse length equals the number of cycles in the pulse multiplied by the wavelength. Increasing the number of cycles makes the pulse longer in space, so echoes from closely spaced structures along the beam axis appear merged rather than distinct. That reduces the ability to separate two objects that lie one after the other along the axis, i.e., axial resolution is degraded.

Lateral resolution depends on beam width and focusing, not on how many cycles are in the pulse. Penetration is influenced more by attenuation and frequency content; while longer pulses can affect energy, simply increasing cycles doesn’t directly determine depth of penetration. Shorter spatial pulse length would improve axial resolution, which is the opposite effect of increasing cycles.