From the Guidelines
The recommended protocol for a sacral MRI includes T1-weighted and short tau inversion recovery (STIR) sequences, as well as T2-weighted images, to identify both active inflammatory lesions and chronic structural lesions of the sacroiliac joints. This protocol is based on the most recent evidence from the American College of Radiology, which suggests that MRI of the sacroiliac joints is the next imaging technique of choice in the evaluation of patients with suspected axial spondyloarthritis (axSpA) after radiography is performed 1. The use of STIR and T1-weighted images allows for the identification of active inflammatory lesions, such as bone marrow edema, capsulitis, synovitis, and enthesitis, as well as chronic structural lesions, such as sclerosis, erosions, fat deposition, and ankylosis.
When performing a sacral MRI, it is essential to consider the following:
- Patients should remove all metal objects before the scan and inform technologists about any implants, pregnancy, or claustrophobia.
- The procedure generally takes 30-45 minutes, during which the patient must lie still on the examination table.
- No special preparation is typically required, though some facilities may request fasting for 4-6 hours if contrast is planned.
- Sedation may be offered for claustrophobic patients.
- The use of contrast enhancement with gadolinium-based agents may be considered, although it has not been shown to significantly increase the diagnostic accuracy of MRI for sacroiliitis 1.
The recommended protocol provides optimal visualization of sacral anatomy, including bone marrow, nerve roots, and surrounding soft tissues, allowing for accurate diagnosis of conditions like sacral fractures, tumors, infections, or degenerative changes. The multiplanar imaging capability of MRI makes it superior to CT scans for evaluating the complex anatomy of the sacral region and detecting subtle abnormalities in soft tissues. According to the most recent study, the sensitivity and specificity of MRI for axSpA depend on the imaging criteria used, patient population, and reference standard for diagnosis, with a sensitivity of 79% and a specificity of 89% for the diagnosis of axSpA 1.
From the Research
Sacral MRI Protocol
The recommended protocol for a sacral MRI (Magnetic Resonance Imaging) involves the use of specific sequences and imaging planes to evaluate the sacrum and surrounding structures.
- The use of T1-weighted and T2-weighted spin-echo images, as well as T1-weighted gradient-echo sequences with fat suppression, can help identify fracture lines, sclerotic areas, and bone marrow edema in cases of osteoporotic insufficiency fractures 2.
- For the evaluation of sacral plexus nerve sheath tumors, MR imaging utilizing a 1.5T magnet and spin echo pulse sequences can provide homogeneous signal intensity and isointense with adjacent muscle on T1 weighted images, and markedly increased signal intensity on T2 weighted images 3.
- The optimal imaging planes for MR assessment of the sacral plexus and proximal sciatic nerve include direct coronal and direct axial planes, which enable thorough evaluation of all components of the sacral plexus and proximal sciatic nerve 4.
- For the imaging of sacroiliitis in early seronegative spondylarthropathy, MR sequences such as semicoronal T1, semicoronal and semiaxial STIR, and semicoronal T1 FS before and after i.v. contrast can provide visualization and grading of active inflammatory changes in the subchondral bone and surrounding ligaments 5.
Key Considerations
- The choice of MRI protocol may depend on the specific clinical indication, such as the evaluation of sacral tumors, osteoporotic insufficiency fractures, or sacroiliitis.
- The use of contrast agents, such as Gadolinium DTPA, may be necessary to enhance the visibility of certain lesions or structures 2, 3.
- The interpretation of sacral MRI images requires a thorough understanding of the anatomy and pathology of the sacrum and surrounding structures, as well as the characteristic imaging findings of various conditions 6.