MUSCULOSKELETAL COMPUTED RADIOGRAPHY IN CHILDREN - SCATTER REDUCTION AND IMPROVEMENT IN BONY TRABECULAR SHARPNESS USING AIR-GAP PLACEMENT OF THE IMAGING PLATE

The effect of various air gaps on computed radiographic musculoskeleta l images was investigated using a knee phantom. Scatter to primary rad iation ratios were measured using the beam stop method at air gaps ran ging from 0 to 30 in. (0-762-mm). Bony trabecular sharpness, line pair resolution, quantum mottle and visualization of low-contrast beads in the soft tissues were evaluated. A significant reduction of scatter t o primary radiation ratio, from a value of almost 1 at table top to ab out 0.4 at 10-in. (254-mm) air gap and about 0.2 at 25-in. (635-mm) ai r gap placement of the computed radiography (CR) imaging plate, was ob tained. A progressive improvement in bony trabecular sharpness and lin e pair resolution, compared with the table top and Bucky images was ob served on 10-in. (254-mm) through 25-in. (635-mm) air gap images. Shar pness of the bony trabeculae and line pair resolution were best on the 25-in. (635-mm) air gap images. The skin entrance radiation dose does not have to be increased for air gap digital radiography. The radiogr aphic noise or quantum mottle is highest on the Bucky image, higher on air gap images and minimal on the table top images, despite a high sc atter to primary radiation ratio at the table top. The lower quantum m ottle on the table top images allowed for maximal visualization of low contrast densities in the soft tissues. Air gap radiography further i mproves musculoskeletal computed imaging by reducing the scatter to pr imary radiation ratio without an increase in the skin entrance dose. F or significant reduction of the scatter to primary radiation ratio and best evaluation of line pair spatial resolution and bony trabeculae, a 25-in. (635-mm) air gap with digital radiography would be optimal. F or evaluation of low contrast densities in the soft tissues, table top placement would be the technique of choice.