Beam Width (Slice Thickness)

Ultrasound systems assume all reflectors lie directly along the main axis of the ultrasound beam (i.e., the acoustic axis or central ray)¹; however, ultrasound beams have a finite size. The out-of-plane beam width (slice thickness) can be measured with a diffuse scattering plane.² The plane is oriented at a 45-degree angle so that the displayed echoes are equal to the out-of-plane echoes. Ultrasound beams can be focused to reduce the out-of-plane beam width and thereby improve image quality.

FIGURE 5-1 Out-of-plane slice thickness. Ultrasound scan of a diffuse scattering plane (a sheet of sandpaper).

FIGURE 5-2 The beam profile is shown as a function of the distance from the central ray. Because needle diameters are substantially less than those of the slice plane, a strong relationship between needle diameter and visibility is expected.