What adjustment would increase the depth of field in ultrasound imaging?

Increasing the depth of field in ultrasound imaging can be effectively achieved by using a higher focal number. The focal number, or f-number, is a measure of the aperture size relative to the focal length of the transducer. A higher focal number corresponds to a smaller aperture, which leads to a more significant depth of field. This means that more structures at varying depths can be rendered in acceptable focus, especially beneficial when imaging deeper anatomical features.

In contrast, decreasing the transducer frequency would typically increase the wavelength of the ultrasound waves and could lead to decreased resolution at greater depths but does not fundamentally affect the depth of field in a beneficial way. Increasing the output power does not influence the depth of field directly; instead, it affects the amplitude of the returning echoes, potentially increasing the signal-to-noise ratio. Adjusting gain settings primarily impacts the brightness of the image but does not alter the physical characteristics that determine the depth of field. Therefore, using a higher focal number is the mechanism that primarily enhances depth of field in ultrasound imaging.