What can an MRI (Magnetic Resonance Imaging) show that a CT (Computed Tomography) scan cannot in cases of unilateral facial numbness?

MRI Advantages Over CT in Unilateral Facial Numbness

MRI provides superior soft tissue contrast that allows direct visualization of the trigeminal nerve itself, detection of subtle nerve inflammation or neuritis, identification of demyelinating lesions, and assessment of perineural tumor spread—all of which CT cannot adequately demonstrate. 1

Key Diagnostic Capabilities Unique to MRI

Direct Nerve Visualization

• MRI can directly visualize 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
• High-resolution thin-cut heavily T2-weighted sequences combined with contrast-enhanced imaging provide detailed visualization of cranial nerve anatomy that CT cannot match 1
• The trigeminal nerve root exit zone and pontine tegmentum lesions causing facial numbness are best seen on MRI 2

Soft Tissue Pathology Detection

• MRI offers superior soft tissue contrast to demonstrate denervation changes and directly image features of neuritis, which are invisible on CT 1
• Inflammatory and infectious conditions affecting the nerve without causing significant expansion are only detectable on MRI 3
• Demyelinating lesions (such as in multiple sclerosis) causing facial numbness appear on MRI but are typically missed on CT 2

Perineural Tumor Spread

• MRI is the gold standard for detecting perineural spread of malignancy, with sensitivities ranging from 73-100%, whereas CT provides limited evaluation of this critical finding 1
• Contrast-enhanced MRI can identify microscopic perineural tumor extension along the trigeminal nerve branches that CT cannot visualize 1
• Pre- and postcontrast MRI imaging provides optimal characterization of lesions along the nerve pathway 1

Brainstem and Central Pathology

• MRI excels at imaging the brainstem nuclei and pontine tegmentum where central causes of facial numbness originate, areas poorly visualized on CT 1, 2
• Pontine lesions causing facial numbness are consistently identified on MRI and correlate with neurophysiological findings 2
• Vascular compression syndromes (such as in trigeminal neuralgia) are best evaluated with MRI/MRA, showing neurovascular contact at the nerve root exit zone 4

What CT Can Show (But MRI Shows Better)

Limited CT Capabilities

• CT primarily demonstrates bony anatomy of neurovascular foramina and canals but cannot visualize the nerve itself 3, 5
• CT with contrast may show large space-occupying lesions or muscle atrophy but misses subtle nerve pathology 1
• CT provides inferior soft tissue evaluation compared to MRI, even with contrast enhancement 1, 6

Clinical Implications

Recommended Imaging Approach

• 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
• These MRI protocols may be equivalent alternatives or complementary depending on institutional protocols 1
• CT should be reserved for patients with MRI contraindications or when bony anatomy assessment is specifically needed 1

Common Pitfalls to Avoid

• Do not rely on CT alone for evaluating facial numbness, as it will miss inflammatory conditions, demyelinating disease, subtle tumors, and perineural spread 1, 3
• Small intrinsic nerve tumors and early perineural malignancy spread are frequently missed on CT 1, 3
• Neurovascular compression causing trigeminal neuralgia requires MRI/MRA for proper visualization 4