Why does increasing the number of cycles in the transmitted pulse degrade axial resolution?

• A. Because spatial pulse length increases
• B. Because lateral resolution improves
• C. Because there is higher attenuation
• D. Because deeper depth of penetration is achieved

Answer: (A)

Axial resolution is governed by how long the transmitted pulse sits in space—the spatial pulse length. The spatial pulse length is the number of cycles in the pulse times the wavelength, so adding more cycles lengthens the pulse in space. A longer pulse means echoes from two reflectors that are close along the beam axis overlap in time, making it harder to distinguish them as separate objects. So increasing the number of cycles directly increases spatial pulse length and degrades axial resolution. Lateral resolution depends on beam width, while attenuation and depth of penetration relate more to frequency and tissue properties, not the number of cycles.