A limited, low-dose computed tomography protocol to examine the sacroiliac joints.

Limited, low-dose, three-scan computed tomography (CT) was shown to be as accurate as a complete CT series in examining the sacroiliac joints and is suggested as an effective alternative to plain radiography as the primary means to detect sacroiliitis. The advantages include the brevity of the examination, a 2-fold to 4-fold reduction in radiation exposure relative to conventional radiography and a 20-fold to 30-fold reduction relative to a full CT series. The technique was developed from studies of anatomic specimens in which the articular surfaces were covered with a film of barium to show clearly the synovial surfaces and allow the choice of the most appropriate levels of section. From the anteroposterior scout view the following levels were defined: at the first sacral foramen, between the first and second sacral foramina and at the third sacral foramen. In the superior section a quarter of the sacroiliac joint is synovial, whereas in the inferior section the entire joint is synovial. The three representative cuts and the anteroposterior scout view are displayed on a single 14 x 17 in. (36 x 43 cm) film. Comparative images at various current strengths showed that at lower currents than conventionally used no diagnostic information was lost, despite a slight increase in noise. The referring physicians at the authors' institution prefer this protocol to the imaging routine previously used.