Evaluating PARS on MRI

- Traditional thin cut multi-detector CT (1-2 mm) enables multi-planar reformats and is excellent for detection of pars defects

- However in young patients, MRI could be considered first

- Generally, the acceptable slice thickness is should be no larger than 3 slip 1 mm. Using this slice thickness, the facets can be seen. Note the L5 pars interarticularis (arrow on superior facet).

- Due to the slight obliquity & size of the facets and slice thickness, the facets can sometimes be challenging to evaluate if the cortices are so close.

- A 3D sequence with isotropic thin voxels can be extremely helpful. Here is a coronal T2 non fat sat CUBE sequence acquired with overlapping 1.2 mm thickness skip 0.5 mm. Note the intact L3 pars (inferior facet of L3 show here). 

- Scroll through the sequence to appreciate the incredible in-plane resolution.

- This allows appreciation of the nerve roots passing through the lateral recess; here it is located just before the foramina. Note the iliolumbar ligament and regional soft tissues.

- These thin slices allow for reformats, such as the typical sagittal view or even coronal oblique (not shown).

- Generally T2 non fat sat is preferred which allows visualization of CSF and fat to distinguish from the bone cortex.

- Other isotropic high-spatial-resolution data sequences include CUBE, VIBE, SPACE, CUBE, FLEX, dependent on vendor.

- If 3D is not available, consider no larger than 3 skip 1 mm, or skip 0 using interleave technique.

- Pars defects can also be evaluated in the axial plane. The typical articulation should be smooth.