In a linear array ultrasound, which factor primarily determines the slice thickness?

• A. Elevation
• B. Mechanical focus
• C. Axial resolution
• D. Penetration

Answer: (B)

Slice thickness is the thickness of the imaging plane in the elevational (out-of-plane) direction, and it is set by how the beam is focused in that plane. In a linear array, this elevational beam profile is controlled primarily by mechanical focusing, such as a curved lens or a fixed elevation focal length. A tighter elevational focus yields a thinner slice at the focal depth, which is why mechanical focus determines slice thickness. Axial resolution depends on pulse length and wavelength along the beam, not the thickness of the imaging plane. Penetration relates to how deep the ultrasound can image, and elevation describes the direction of the slice, not its thickness.