Sacrum and Sacroiliac Joints

For the sake of integrity, the pinhole scan anatomy of the sacrum and the sacroiliac joints are described together. The posterior pinhole scin-tigraph of the sacrum taken at a relatively low magnification level reveals accumulation of tracer in the sacroiliac joints with mild uptake in the sacral body and lateral parts. The scan resolution at this magnification level is such that the small anatomical structures of the sacrum are not discerned. At a higher magnification levels, however, the intermediate and lateral sacral crests as well as the individual sacral foramina are portrayed (Fig. 4.22). Typically, the sacral foramina are presented as trans-

Fig. 4.22 Posterior view of the sacrum. Higher magnification posterior pinhole scintigraph of the sacrum demonstrates the individual sacral foramina f) as ovoid photopenic areas surrounded by sacral crests (5c), which accumulate tracer minimally (S1 first segment of the sacrum). The sacrococcygeal joint concentrates tracer modestly due to articular motion (5cj)

Fig. 4.22 Posterior view of the sacrum. Higher magnification posterior pinhole scintigraph of the sacrum demonstrates the individual sacral foramina f) as ovoid photopenic areas surrounded by sacral crests (5c), which accumulate tracer minimally (S1 first segment of the sacrum). The sacrococcygeal joint concentrates tracer modestly due to articular motion (5cj)

versely ovoid photopenic areas. Occasionally, the sacrococcygeal joint stands out due to increased uptake caused by articular movement. Characteristically, the tracer uptake in the sac-roiliac joints is more intense in the synovial lower compartment than in the ligamental upper compartment (Fig. 4.23A). The sacrum and ilia overlap at the sacroiliac joints in the straight posterior view, requiring the butterfly view for their separation (Fig. 4.23B, C). In this special view, the upper compartment is portrayed as a vertical, wedge-shaped photopenia between the tuberosities of the ilium and sacrum, whereas the lower compartment is only incompletely separated. Tracer is accumulated more intensely in the iliac auricular surface than in the sacral because greater articular movement occurs in the former. It is to be underscored that, contrary to the traditional description, the lower compartment more avidly accumulates tracer than the upper. The upper and lower compartments of the sacroiliac joints are separated from each other by an oblique

Joints Sacrum

Fig. 4.23A-C Posterior view of the sacroiliac joints. A Lower magnification, posterior pinhole scintigraph of the sacrum portrays intense, triangular tracer uptake symmetrically in both sacroiliac joints. The uptake in the sacrum is relatively minimal, revealing no detail. B Posterior tangential or butterfly pinhole scintigraph of the right sacroiliac joint separates the joint space, which is wedge-shaped in the upper compartment (arrows). The separation is incomplete in the synovial, lower compartment and complete in the ligamentous, upper compartment. The prominent tracer uptake is localized on the iliac side. C Tangential radiograph identifies the joint space between the sacrum and ilium (arrows)

photopenic band, featuring increased uptake in the quadrilateral auricular surface of the lower compartment (Fig. 4.23B). The posterior iliac crest accumulates tracer only modestly.

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