MRI OF SACRAL SPINE

MRI of the Sacral Spine is a specialized imaging technique that uses powerful magnetic fields and radiofrequency pulses to obtain high-resolution, cross-sectional images of the sacral vertebrae, sacroiliac joints, spinal canal, nerve roots, and surrounding soft tissues. This region, located at the base of the spine between the lumbar vertebrae and coccyx, plays a crucial role in supporting body weight, connecting the spine to the pelvis, and facilitating lower limb movement. MRI is the preferred imaging modality for assessing sacral bone and soft tissue pathologies because it offers superior contrast resolution without ionizing radiation.

Purpose of MRI of Sacral Spine

1. Pain evaluation

• Investigation of persistent lower back or sacral pain not explained by X-rays or CT.
• Differentiation between musculoskeletal, nerve-related, or joint-related causes.

2. Nerve root assessment

• Identification of nerve root compression due to disc bulge, tumor, or cyst.
• Evaluation of sacral plexus lesions.

3. Trauma evaluation

• Detection of sacral fractures, especially insufficiency or occult fractures.
• Identification of bone marrow edema or ligament injuries.

4. Tumors and masses

• Detection of primary sacral tumors such as chordomas or giant cell tumors.
• Identification of metastatic lesions from cancers like prostate, breast, or lung.

5. Infections and inflammation

• Diagnosis of osteomyelitis, septic sacroiliitis, or tuberculosis.
• Assessment of inflammatory sacroiliitis seen in ankylosing spondylitis.

6. Congenital anomalies

• Identification of spina bifida, sacral agenesis, or tethered cord syndrome.

7. Postoperative follow-up

• Evaluation of surgical sites for tumor recurrence, infection, or scar formation.

Procedure for MRI of Sacral Spine

Before the scan

• Detailed history of symptoms and previous treatments is taken.
• Patient must inform about any metallic implants, pacemakers, or prior injuries with retained metal.
• All metallic objects such as jewelry, watches, and belts must be removed.
• Contrast studies may require fasting for a few hours before the procedure.
• Patients with claustrophobia may be given sedation if necessary.

During the scan

• Patient lies supine with the sacral region centered within the MRI coil.
• Positioning aids like cushions or straps help reduce motion.
• Images are acquired in multiple planes — axial, sagittal, and coronal.
• Fat suppression techniques may be used to enhance lesion visibility.
• If contrast is used, it is injected intravenously mid-procedure.
• Scan duration is typically 20 to 40 minutes.

After the scan

• No restrictions unless sedation was used.
• If contrast was administered, hydration is encouraged.
• A radiologist interprets the images and forwards the report to the referring physician.

MRI Sequences and Techniques for Sacral Spine

T1-weighted sequences

• Provide clear detail of bone marrow and anatomical structures.
• Useful for identifying fatty marrow, fractures, or marrow replacement lesions.

T2-weighted sequences

• Highlight cerebrospinal fluid and soft tissue changes.
• Detect edema, cysts, and inflammatory lesions.

STIR (Short Tau Inversion Recovery)

• Suppresses fat to reveal subtle bone marrow or soft tissue edema.
• Ideal for detecting inflammation, trauma, or metastatic spread.

Contrast-enhanced T1 sequences

• Highlight tumors, infections, and inflammatory changes.
• Differentiate between post-treatment scar tissue and recurrent disease.

Diffusion-weighted imaging (DWI)

• Helps in detecting small tumors or abscesses based on water molecule movement.

Common Conditions Diagnosed with MRI of Sacral Spine

Sacral insufficiency fractures

• Common in elderly or osteoporotic patients.
• Appear as bone marrow edema and fracture lines.

Chordoma

• Slow-growing malignant tumor often arising in the midline sacrum.
• MRI shows destructive bone lesion with soft tissue extension.

Metastatic disease

• Spread from cancers like breast, prostate, or lung.
• MRI detects lesions earlier than X-ray or CT.

Ankylosing spondylitis

• Chronic inflammatory disease affecting sacroiliac joints.
• Early detection by MRI before X-ray changes appear.

Infectious sacroiliitis

• Bacterial or tubercular infection causing joint swelling, marrow edema, and abscess.

Tarlov cysts

• Fluid-filled sacs at nerve root sheaths.
• Usually asymptomatic but may cause nerve compression.

Syringomyelia or tethered cord

• Congenital anomalies that may extend to sacral region and cause neurological symptoms.

Advantages of MRI of Sacral Spine

• Superior soft tissue contrast compared to CT and X-ray.
• No ionizing radiation exposure.
• Multiplanar imaging for complete anatomical visualization.
• Early detection of inflammatory and neoplastic conditions.
• Differentiation between active inflammation and chronic changes.

Limitations and Considerations

Motion artifacts

• Movement can reduce image clarity, particularly in restless patients.

Metallic implants

• Can cause distortion of images near the sacral area.

Contrast-related risks

• Rare allergic reactions to gadolinium-based agents.
• Kidney function evaluation needed before contrast use in at-risk individuals.

Claustrophobia

• Some patients may require sedation to tolerate the scan.

Patient Preparation Tips

• Wear comfortable clothing free of metal.
• Follow fasting instructions if contrast is planned.
• Inform the radiology team about implants, surgeries, or prior spinal injuries.
• Stay as still as possible during imaging for optimal results.

Conclusion

MRI of the Sacral Spine is an essential diagnostic tool for evaluating the bony, neural, and soft tissue structures of the sacral region. It offers unparalleled image detail, enabling early and accurate diagnosis of fractures, tumors, infections, inflammatory conditions, and congenital anomalies. Its ability to image in multiple planes without radiation makes it a preferred choice for both initial evaluation and follow-up of sacral pathologies. By detecting disease processes at an early stage, MRI plays a key role in guiding timely treatment, monitoring therapeutic progress, and improving patient outcomes.