Which statement best describes the f-number in ultrasound instrumentation?

• A. Focal length divided by the aperture
• B. Aperture multiplied by the number of elements in the transducer
• C. Number of transducer elements divided by the Fraunhofer zone
• D. Beam width 1 cm from the transducer surface

Answer: (A)

The f-number describes how tightly a beam is focused. It is defined as the focal length divided by the transducer’s aperture (the width of the emitting surface). This dimensionless ratio tells you how convergent the beam will be: a smaller f-number (short focal length for a given aperture or a larger aperture for a given focal length) produces a tighter beam at focus and better lateral resolution there, while a larger f-number broadens the beam and increases the depth of field but reduces lateral resolution in the focal zone. At a given wavelength, the beam width near focus is roughly proportional to the f-number, so this ratio is a handy design guide for predicting focusing behavior. The other statements don’t define f-number: they either mix in the number of elements, refer to diffraction distances, or describe a specific beam width rather than the defining ratio.