Which option best describes the F-number?

Which option best describes the F-number?

• 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 1cm from the transducer surface

Answer: (A)

The main idea here is how focusing is described in ultrasound. The F-number is a dimensionless ratio that tells you how tightly the beam is focused, and it’s defined as the focal length divided by the aperture width (the active width of the transducer that is used to form the beam). In other words, F-number = focal length / aperture. This ratio matters because a smaller F-number (a larger aperture relative to the focal length) yields a more tightly focused beam and better lateral resolution near the focal point. A larger F-number means a less tightly focused beam and wider beamwidth at the focus. The other options don’t describe this ratio: - The product of aperture and number of elements isn’t the F-number. - Dividing the number of elements by the Fraunhofer zone mixes concepts from different areas and isn’t how F-number is defined. - Beam width measured at a fixed distance describes an outcome influenced by focusing, not the defining ratio itself. So the definition is focal length divided by aperture.

The main idea here is how focusing is described in ultrasound. The F-number is a dimensionless ratio that tells you how tightly the beam is focused, and it’s defined as the focal length divided by the aperture width (the active width of the transducer that is used to form the beam). In other words, F-number = focal length / aperture.

This ratio matters because a smaller F-number (a larger aperture relative to the focal length) yields a more tightly focused beam and better lateral resolution near the focal point. A larger F-number means a less tightly focused beam and wider beamwidth at the focus.

The other options don’t describe this ratio:

• The product of aperture and number of elements isn’t the F-number.

• Dividing the number of elements by the Fraunhofer zone mixes concepts from different areas and isn’t how F-number is defined.

• Beam width measured at a fixed distance describes an outcome influenced by focusing, not the defining ratio itself.

So the definition is focal length divided by aperture.