What does an MRI show that a CT scan doesn't in a patient with Systemic Lupus Erythematosus (SLE) presenting with unilateral facial numbness?

MRI Advantages Over CT in SLE Patient with Unilateral Facial Numbness

In an SLE patient with unilateral facial numbness, MRI directly visualizes the trigeminal nerve itself, detects inflammatory neuritis, identifies demyelinating lesions, and reveals perineural tumor spread—none of which CT can adequately demonstrate. 1

Direct Nerve Visualization

MRI provides direct visualization of the trigeminal nerve (CN V) along its entire course from the brainstem through Meckel's cave to the peripheral branches, which is impossible with CT. 1 This is critical because:

• The nerve itself cannot be directly imaged with CT, which only shows surrounding bone and soft tissue 2
• High-resolution thin-cut heavily T2-weighted sequences combined with contrast-enhanced imaging provide detailed cranial nerve anatomy that CT cannot match 1
• MRI allows assessment of the brainstem trigeminal nuclei, which can be affected by SLE-related demyelinating disease 2

Inflammatory and Demyelinating Disease Detection

MRI offers superior soft tissue contrast to demonstrate neuritis and demyelinating lesions—both highly relevant in SLE patients. 1 Specifically:

• Multiple sclerosis and other demyelinating conditions can result in trigeminal neuropathy, and MRI is essential for identifying brainstem demyelinating plaques 2
• SLE itself can cause CNS inflammation and vasculitis that manifests as facial numbness, visible only on MRI 3, 4
• Subtle nerve inflammation or enhancement patterns indicating neuritis are invisible on CT 1

Perineural Tumor Spread Assessment

MRI is the gold standard for detecting perineural spread of malignancy with sensitivities of 73-100%, whereas CT provides limited evaluation. 1 This matters because:

• Contrast-enhanced MRI identifies microscopic perineural tumor extension along trigeminal nerve branches that CT cannot visualize 1
• Pre- and postcontrast MRI imaging provides optimal characterization of lesions along the nerve pathway 1
• Small intrinsic nerve tumors and early perineural malignancy spread are frequently missed on CT 1

Recommended Imaging Protocol

The American College of Radiology recommends MRI head without and with IV contrast OR MRI orbits, face, and neck without and with IV contrast as the primary imaging modality for unilateral facial numbness. 1 The protocol should include:

• Thin-cut high-resolution sequences through the entire trigeminal nerve course 2
• Pre- and postcontrast imaging to identify and characterize lesions optimally 2, 1
• Coverage of brainstem, Meckel's cave, and peripheral nerve branches 2, 1

Critical Pitfall to Avoid

Do not rely on CT alone for evaluating facial numbness in SLE patients, as it will miss inflammatory conditions, demyelinating disease, subtle tumors, and perineural spread. 1 CT should be reserved for:

• Patients with absolute MRI contraindications 1
• Complementary assessment of bony anatomy when fracture or osseous pathology is specifically suspected 2