NORMAL VARIATION IN THE MAGNETIC-RESONANCE-IMAGING APPEARANCES OF THESACROILIAC JOINTS - PITFALLS IN THE DIAGNOSIS OF SACROILIITIS

The purpose of this paper is to define the normal and variable appeara nces of the sacroiliac joints and adjacent subchondral marrow on unenh anced and enhanced Magnetic Resonance Imaging (MRI). Twenty subjects w ere imaged with spin-echo T1-weighted (T1), fast spin-echo T2-weighted (T2), T1-weighted with fat suppression (T1FS) and fast short tau inve rsion recovery (Fast STIR) sequences. Five of these subjects were imag ed following intravenous Gd-DTPA administration, and the enhancement f actor of the synovial compartment of the sacroiliac joint and subchond ral marrow was calculated. The appearance of the cartilage of the syno vial compartment on T1 and T2 images is of an intermediate signal boun ded by signal void of bone cortex. On TIFS and Fast STIR images the ca rtilage has an intermediate to high signal. The marrow on T1, T2 and T 1FS images has a homogeneous intermediate signal. T1FS images demonstr ate the synovial compartment with greater clarity than T1 images. Cort ical erosions and subchondral sclerosis were not demonstrated in our s ubjects, and partial volume artifact between the synovial and ligament ous compartments should not be interpreted as erosions. On Fast STIR i mages there is normally a region of high signal from the immediate sub chondral marrow, which should not be interpreted as early sacroiliitis . Seven subjects demonstrated a patchy distribution of fat within the bone marrow, an appearance which alone does not indicate sacroiliitis. One subject had an accessory articular facet of the sacroiliac joint. The percentage maximal enhancement factor of the synovial compartment of the sacroiliac joint and adjacent subchondral marrow is 52% and 94 % on T1FS images respectively. These figures redefine the normal maxim al enhancement factors in this region of the body.