If the frequency is decreased, the numerical value of the radial resolution will be:

• A. increased
• B. decreased
• C. unchanged
• D. varies

Answer: (A)

Radial (range) resolution is how closely two reflectors along the beam can be distinguished. It is determined by the spatial pulse length, which is the physical length of one ultrasound pulse. Spatial pulse length depends on wavelength and the number of cycles in the pulse; wavelength is inversely related to frequency (lambda = c/f). When the frequency is lowered, the wavelength grows, so the pulse length becomes longer. A longer pulse length means you need a larger separation in depth to distinguish two echoes, and range resolution is roughly tied to half of that pulse length. So decreasing frequency makes the radial resolution value larger (worse), because echoes from two close reflectors will blur together over a greater distance.