Which parameter does NOT affect the spatial resolution in ultrasound imaging?

The parameter that does not affect spatial resolution in ultrasound imaging is pupil diameter. Spatial resolution in ultrasound is primarily influenced by factors such as transducer frequency, focal length, and lens curvature.

Transducer frequency directly impacts spatial resolution because higher frequencies provide better axial resolution, allowing for finer detail to be visualized. Similarly, focal length plays a role in how focused the ultrasound beam is; a shorter focal length can lead to improved lateral resolution, allowing for the visualization of smaller structures.

Lens curvature can affect the focusing of the ultrasound beam, and thus, impact spatial resolution as well. In contrast, pupil diameter is a concept relevant in optics, particularly related to the eye and not directly applicable to ultrasound imaging, where spatial resolution is determined by the characteristics of the ultrasound system and the properties of the sound waves used. Therefore, pupil diameter does not have an effect on spatial resolution in the context of ultrasound imaging.