**4. Sacroilitis**

Sacroiliac joint involvement is a common finding in the spondyloarthropathies (SpA) group and it is an important parameter included in the diagnostic criteria. Conventional radiology is essential in the evaluation of the sacroiliac joints, but it does not detect early abnormalities and, therefore, the use of other complementary techniques currently available, such as CT, MRI, US, and bone scintigraphy, is necessary to avoid delaying the diagnosis.

Until only a few years ago, bone scintigraphy was the gold standard technique for early diagnosis of inflammatory processes at this joint. The assessment of sacroilitis by scintigraphy is based on the quantification of radiotracer uptake in the sacrum and sacroiliac joints. This technique can help to differentiate between degenerative and inflammatory disorders in patients with nonespecific radiological changes. Both conventional bone scintigraphy and SPECT mode are sensitive techniques for early detection of SpA in patients with low back pain and who still do not have typical radiological changes, because the uptake of radiopharmaceutical at sacroiliac joints is produced before structural damage happens. However, in the sclerotic phase of evolved SpA, scintigraphy frequently does not detect any abnormalities. MRI, more sensitive in detecting early changes and more specific, has shifted to the bone scan. In this regard, a published review (Schneider, 2006; Song et al., 2008) realized with the aim to assess the diagnostic value of scintigraphy for detecting sacroilitis in patients with established and/or probable ankylosing spondylitis, has proven limited value of scintigraphy both in the early detection of sacroilitis and in patients with established ankylosing spondylitis. In patients with SpA and gastrointestinal symptoms but negative endoscopic or radiological test results, abdominal scintigraphy with labelled leukocytes can be used to assess the abdominal involvement (Colamussi et al., 2004; as cited in Elkayam et al., 2002).
