Why would a neurologist order a Magnetic Resonance Imaging (MRI) scan for a patient presenting with neurological symptoms?

Why Neurologists Order MRI Scans

Neurologists order MRI primarily to detect structural brain pathology that impacts morbidity and mortality—including stroke, tumors, infections, inflammatory conditions, and vascular malformations—with MRI being superior to CT for identifying small infarcts, posterior fossa lesions, encephalitis, and subtle hemorrhages that directly influence treatment decisions and patient outcomes. 1

Primary Diagnostic Indications

Suspected Ischemic Stroke or TIA

• MRI detects acute ischemia in 31% of patients with probable stroke/TIA symptoms and 17% of those with possible stroke/TIA, even when symptoms are transient or minor. 2
• Critically, 70% of missed ischemic stroke diagnoses present with altered mental status rather than classic focal deficits, making MRI essential when CT is unrevealing. 1
• Patients with MRI evidence of acute ischemia have a 13-fold increased risk of subsequent stroke within 18 months compared to those without (18% vs 1%), directly impacting secondary prevention strategies. 2
• Diffusion-weighted imaging (DWI) identifies acute ischemic changes that CT misses, particularly in posterior circulation and small vessel territories. 1, 3

Seizures and Epilepsy

• MRI is the imaging study of choice for all patients with epilepsy in nonemergent settings, serving to identify causative lesions, determine prognosis, and guide treatment strategy. 4
• MRI detects structural abnormalities in focal epilepsies with 70% success rate compared to only 30% for CT. 4
• High-resolution protocols identify hippocampal sclerosis (the most common cause of temporal lobe seizures), focal cortical dysplasia, and other malformations of cortical development that may be surgically treatable. 4
• Priority for MRI includes patients with focal neurological findings, persistent headache, recent head trauma, or EEG abnormalities, as these correlate with high probability of structural pathology. 4

Suspected Brain Tumors

• MRI provides superior gray-white matter differentiation and multiplanar imaging capability for detecting and characterizing brain tumors, including low-grade gliomas that may present with chronic seizures. 4
• The classic MRI finding of glioblastoma—an irregularly shaped rim-enhancing lesion with central necrosis—correlates directly with histological features and guides surgical planning. 5
• MRI detects tumors in patients with epilepsy history exceeding 20 years, identifying lesions that CT misses. 4
• Contrast-enhanced sequences are essential when neoplasm is suspected, improving sensitivity for smaller lesions and characterizing tumor extent. 4

Suspected CNS Infections

• Brain MRI with DWI, ADC sequences, and gadolinium-enhanced T1-weighted imaging achieves 92% sensitivity and 91% specificity for distinguishing CNS abscess from stroke or mass. 6
• MRI should be performed within 24-48 hours for suspected encephalitis, ideally within 24 hours of hospital admission. 1, 3
• Ring-enhancing lesions with restricted diffusion (central hyperintensity on DWI with low ADC values) are pathognomonic for abscess, distinguishing it from other ring-enhancing lesions. 6
• Contrast-enhanced MRI is indicated for suspected meningitis with persistent neurological symptoms, particularly in immunocompromised patients. 3

Suspected Inflammatory Conditions

• MRI is the definitive imaging modality for multiple sclerosis diagnosis and follow-up, with 3D T2-FLAIR sequences detecting periventricular and juxtacortical lesions that establish dissemination in space and time. 7
• Brain MRI with gadolinium demonstrates ongoing clinically silent disease activity, evaluates unexpected clinical worsening, and establishes baseline before starting or modifying therapy. 7
• Routine brain MRI every 6 months to 2 years is recommended for all patients with relapsing MS to monitor disease progression and treatment response. 7

When MRI is Preferred Over CT

Superior Sensitivity for Specific Pathologies

• MRI has 95% sensitivity for subtle subarachnoid hemorrhage when CT is negative. 1, 3
• T2* gradient-echo and susceptibility-weighted imaging (SWI) sequences are 3-6 times more sensitive than CT for detecting microhemorrhages and blood products of various stages. 3
• MRI detects small infarcts, encephalitis, and posterior fossa lesions that CT routinely misses due to beam-hardening artifact. 4, 1

Evaluation of Underlying Vascular Lesions

• MRI is complementary to CT for evaluating intracranial hemorrhage when an underlying lesion is suspected, including arteriovenous malformations, cavernous malformations, or hemorrhagic tumors. 4, 1
• MR angiography evaluates for vasculopathy, aneurysms, and vascular malformations without radiation or iodinated contrast. 3

Altered Mental Status with Unrevealing CT

• When patients with acute mental status changes do not respond to initial management and CT is unrevealing, MRI is usually appropriate to evaluate for occult neurological pathology. 4
• MRI leads to changes in clinical management in 76% of patients with acute disorders of consciousness, including revised diagnoses, revised levels of care, and improved prognostication. 1, 3
• MRI detects posterior reversible encephalopathy syndrome, Wernicke encephalopathy, carbon monoxide poisoning, and metronidazole toxicity that CT cannot identify. 4

Critical Clinical Caveats

When CT Remains First-Line

• CT is the first-line imaging modality for suspected acute intracranial hemorrhage, mass effect, or hydrocephalus in emergent settings due to rapid acquisition and ability to maintain patient access during scanning. 4, 1, 3
• CT is appropriate when patients are unstable or require continuous monitoring during imaging. 3
• In immediate post-traumatic seizures, noncontrast CT rapidly identifies acute hemorrhage, skull fractures, and mass effect requiring urgent intervention. 4

Low-Yield Scenarios

• The diagnostic yield of MRI in new-onset delirium is low in the absence of focal neurologic deficits or history of recent falls. 1, 3
• In new-onset psychosis without neurologic deficits, the yield of neuroimaging for detecting pathology responsible for symptoms is very low. 3
• Some epilepsy forms have low yield on MRI, including typical primary generalized epilepsy and benign focal epilepsies of childhood with characteristic EEG features. 4

Timing Considerations

• For suspected encephalitis, MRI should be performed within 24 hours of hospital admission, but certainly within 48 hours. 3
• For traumatic brain injury with persistent unexplained neurologic findings despite normal CT, MRI should be performed as soon as clinically feasible. 3
• Prolonged antibiotic therapy (several weeks) reduces MRI sensitivity for abscess detection. 6

Optimal MRI Protocol Selection

Standard Brain MRI Protocol

• MRI head without IV contrast is typically the initial study for evaluating brain parenchyma and detecting hemorrhage. 1, 3
• T2* gradient-echo and susceptibility-weighted imaging sequences optimally detect microhemorrhages. 1, 3
• Diffusion-weighted imaging identifies acute ischemic changes and axonal injuries. 1, 3
• 3D T2-FLAIR sequences detect periventricular lesions and subtle cortical abnormalities. 7

When to Add Contrast

• Gadolinium-enhanced sequences are indicated when intracranial infection, tumor, inflammatory lesions, or vascular pathologies are suspected. 4, 1
• Contrast is useful when images without IV contrast are insufficient or if neoplasm or inflammatory condition is suspected. 4
• Post-contrast T1-weighted imaging detects ring-enhancing lesions characteristic of abscess, active MS plaques, and tumor enhancement. 6, 7

Specialized Sequences

• For epilepsy evaluation, protocols should include coronal T1-weighted (3 mm) imaging perpendicular to the hippocampus, high-resolution 3D T1-weighted gradient echo (1-mm isotropic voxels), and coronal/axial FLAIR to assess hippocampal pathology and cortical dysplasia. 4
• For MS surveillance, progressive multifocal leukoencephalopathy protocol includes FLAIR and DWI sequences only. 7