MRI Protocol for Piriformis Syndrome
Order a pelvic MRI with specific sequences including axial and coronal T1-weighted, T2-weighted FSE, and fat-suppressed FSE images through the entire piriformis muscle and sciatic nerve course, with 3-mm slice thickness.
Recommended Protocol
Based on the available evidence, the optimal MRI approach for suspected piriformis syndrome should include:
Core Sequences
- Axial and coronal T1-weighted images through the pelvic region
- Fast spin-echo (FSE) T2-weighted sequences
- Fat-suppressed FSE sequences
- 3-mm section thickness with 0.5-mm gap to visualize the entire piriformis muscle and sciatic nerve pathway 1
Key Imaging Considerations
Start with lumbar spine radiographs first 2, as the ACR Appropriateness Criteria emphasize that radiographs should be obtained in most cases to exclude other pathology and guide subsequent imaging selection. However, for soft tissue evaluation specifically, MRI is superior.
The pelvic MRI sequences can be added to routine lumbar MRI scans 3. A 2019 study demonstrated that adding just three specific pelvic sequences to routine lumbar MRI was sufficient to accurately delineate piriformis muscle pathologies and sciatic nerve abnormalities, identifying piriformis syndrome in 8.2% of patients who underwent both studies.
What to Measure and Assess
Anatomical Features to Evaluate:
- Piriformis muscle cross-sectional area (PMCSA): Most sensitive parameter, with cutoff of 611.67 mm² (75% sensitivity and specificity) 4
- Piriformis muscle thickness (PMT): Cutoff of 14.24 mm (70% sensitivity and specificity) 4
- Muscle asymmetry: Greater than 3 mm asymmetry may be significant, though up to 8 mm can occur normally 5
- Sciatic nerve signal intensity and caliber: Look for enlargement or increased signal 1, 6
- Anatomical variants: Bigastric muscle appearance, nerve coursing through muscle, or accessory muscle bellies 1, 7
Advanced Sequences (When Available)
Consider diffusion tensor imaging (DTI) with tractography for objective assessment of sciatic nerve injury 6. This technique can quantify:
- Fractional anisotropy (FA) values: Lower in affected nerves
- Apparent diffusion coefficient (ADC): Higher distal to the piriformis muscle in symptomatic patients
DTI provides objective evidence of nerve injury and can track improvement post-treatment, though it's not routinely available at all centers.
Critical Pitfalls to Avoid
Don't limit imaging to the ipsilateral hip alone 2. Include views of the pelvis and proximal femur to avoid missing referred pain sources or bilateral pathology.
Normal MRI doesn't exclude piriformis syndrome. In surgical series, 30% of patients with confirmed piriformis syndrome had no apparent MRI abnormalities 1, emphasizing that this remains primarily a clinical diagnosis.
Recognize normal anatomical variations: The S2 nerve roots traverse the piriformis muscle in 75% of normal patients, and S3 roots in 97% 5. These are not pathological findings by themselves.
Complementary Imaging
Ultrasound has a role for dynamic assessment and can identify snapping iliopsoas tendon or guide therapeutic injections 2, 8. Dynamic ultrasound during hip rotation may demonstrate motion-related changes at the piriformis enthesis that aren't visible on static MRI.
CT may be needed if bony abnormalities like enthesopathy with cortical spurs are suspected, particularly when MRI findings are inconclusive 8.