Optimization of MRI pulse sequences to visualize the normal pars interarticularis

Magnetic resonance imaging (MRI) is a potentially useful means of detecting acute pars stress fractures. However, recent publications have highlighted the deficiencies of routine MRI in evaluating the pars interarticularis, Twenty-nine volunteers underwent thin section, multi-planar MR examinations of the lumbar spine to evaluate whether the normal pars could be more reliably demonstrated. MRI examinations were performed with sagittal and reserve angle oblique axial T1W images, and also 3D sagittal gradient echo images with reverse angle reconstructions. Sagittal STIR images were also acquired in 14 cases. Two hundred and ninety pars were evaluated, of which 66% were deemed definitely intact (type I) on the sagittal T1W images alone (continuous marrow throughout the pars). However 93% were deemed intact when all images were reviewed together, The majority of pars defects occur at L4/L5, and 74% of these were intact on the sagittal T1W sequence alone, and 90% on combined sequences. Eighteen pars (6%) were considered to be hypointense (type II) after review of all sequences, but appeared otherwise intact. One volunteer had bilateral definite pars defects (type IV) at L5 on all sequences. This study indicates that improved visualization of the normal pars interarticularis can be achieved with optimized MRI.