Conventional radiography has for many
years been used to diagnose sacroiliitis and the findings by radiography are
still part of the accepted international diagnostic criteria for ankylosing
spondylitis (radiographic axSpA). However, radiography can only show permanent
(structural) damage of the joints and/or bone. The diagnosis can therefore be
difficult to obtain and may be delayed for many years unless radiography early
in the disease is replaced or supplemented by cross sectional imaging (CT or
MRI). In later stages of the disease detection of both sacroiliitis and spinal
changes can often be obtained by radiography.
Computer tomography (CT) is more
sensitive than radiography for detecting structural changes in the form of erosion
of joint facets, joint space alteration and increased bone density (sclerosis).
However, CT cannot visualise early active inflammation in the subchondral bone
unless using modern Dual-energy CT-technique. Nevertheless, CT can be
of help in the diagnosis of minor erosive SIJ changes, which can be difficult
to detect by MRI, and in the diagnosis of anatomical variation at the SIJ such
as accessory joints. In later stages of the disease, CT can be valuable for the
detection of spinal fractures etc.
Magnetic resonance imaging (MRI) has been proven superior to CT and radiography for detecting
early sacroiliitis because MRI can show active osseous and joint space inflammation
in addition to structural damage. In later stages MR can provide valuable
information about disease activity corresponding to the SIJ as well as the
spine. MRI is therefore increasingly used in the diagnosis and follow-up of SpA
being appropriate for measuring the disease activity and thereby assess the
effect of therapy.