Normal variation in the magnetic resonance imaging appearances of the sacroiliac joints: pitfalls in the diagnosis of sacroiliitis.

The purpose of this paper is to define the normal and variable appearances of the sacroiliac joints and adjacent subchondral marrow on unenhanced and enhanced Magnetic Resonance Imaging (MRI). Twenty subjects were imaged with spin-echo T1-weighted (T1), fast spin-echo T2-weighted (T2), T1-weighted with fat suppression (T1FS) and fast short tau inversion recovery (Fast STIR) sequences. Five of these subjects were imaged following intravenous Gd-DTPA administration, and the enhancement factor of the synovial compartment of the sacroiliac joint and subchondral marrow was calculated. The appearance of the cartilage of the synovial compartment on T1 and T2 images is of an intermediate signal bounded by signal void of bone cortex. On T1FS and Fast STIR images the cartilage has an intermediate to high signal. The marrow on T1, T2 and T1FS images has a homogeneous intermediate signal. T1FS images demonstrate the synovial compartment with greater clarity than T1 images. Cortical erosions and subchondral sclerosis were not demonstrated in our subjects, and partial volume artifact between the synovial and ligamentous compartments should not be interpreted as erosions. On Fast STIR images there is normally a region of high signal from the immediate subchondral 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 maximal enhancement factors in this region of the body.