What are the indications for an MRI (Magnetic Resonance Imaging) of the brain?

Indications for MRI Brain

MRI brain is indicated when CT is unrevealing or when higher sensitivity is required for specific pathologies including small infarcts, encephalitis, posterior fossa lesions, subtle subarachnoid hemorrhage, and suspected intracranial masses, infections, or inflammatory conditions. 1, 2

Primary Clinical Scenarios Requiring MRI Brain

Acute Neurological Presentations

Altered Mental Status with Negative or Non-Diagnostic CT

• MRI serves as the second-line test when occult pathology is suspected after unrevealing head CT, with superior sensitivity for detecting small ischemic infarcts, encephalitis, and subtle cases of subarachnoid hemorrhage 1
• Notably, 70% of patients with missed ischemic stroke diagnoses presented with altered mental status, emphasizing MRI's critical role in this population 1, 2
• MRI may be considered as first-line imaging in clinically stable patients with suspected occult CNS malignancy, inflammatory disorder, or CNS infection, though diagnostic yield may be low 1

New Onset Seizures

• MRI is the imaging study of choice in the non-emergent setting for all patients with epilepsy, with success rates of detecting lesions in focal epilepsies far exceeding CT (MRI detects causative lesions versus only 30% detection rate with CT) 1
• MRI established the proximate cause for acute seizures in 44% of patients, and CT failed to detect the cause in 19% of patients where subsequent MRI identified the etiology 3
• Priority for MRI includes patients with focal neurologic findings, persistent headache, recent head trauma, and EEG abnormalities 1

Subacute or Chronic Head Trauma

• MRI is the most useful initial imaging for subacute or chronic head trauma with unexplained cognitive or neurologic deficits, as it is more sensitive than CT for subtle findings adjacent to calvarium or skull base and for small white matter lesions (microbleeds) from traumatic axonal injury 1
• Brain MRI should include susceptibility-weighted imaging (SWI), which is 3-6 times more sensitive than conventional T2* gradient-echo sequences for detecting hemorrhagic lesions 2

Suspected Vascular Pathologies

Stroke and Transient Ischemic Attack

• MRI evidence of acute ischemia was found in 42% of patients meeting National Institute of Neurological Disorders and Stroke criteria for TIA/stroke, and importantly, in 16% of patients with non-TIA focal symptoms 4
• Patients with MRI evidence of acute brain ischemia had an 18-month stroke risk of 18% versus only 1% in those without MRI evidence (age-adjusted hazard ratio 13) 4
• MRI should be performed for transient or minor neurological symptoms even when clinical probability of TIA/stroke is uncertain or when stroke is not the most likely differential diagnosis 4

Intracranial Hemorrhage Evaluation

• MRI is complementary to CT for evaluating intracranial hemorrhage when an underlying lesion is suspected, including hemorrhagic primary or secondary brain mass, arteriovenous malformation, or cavernous venous malformation 1
• For suspected subtle cases of subarachnoid hemorrhage with negative CT, MRI has 95% sensitivity 2

Posterior Reversible Encephalopathy Syndrome

• Noncontrast MRI examinations are usually sufficient for assessing intracranial complications related to hypertensive emergency, including posterior reversible encephalopathy syndrome 1

Suspected Infectious or Inflammatory Conditions

CNS Infections

• MRI should be performed within 24-48 hours for suspected encephalitis, ideally within 24 hours of hospital admission 2
• Contrast-enhanced MRI examinations (with and without IV contrast) should be performed if intracranial infection, tumor, inflammatory lesions, or vascular pathologies are suspected 1
• MRI is indicated for suspected meningitis with persistent neurological symptoms and for suspected CNS infection in immunocompromised patients 2

Demyelinating Disease

• In patients with suspected multiple sclerosis and negative or minimal brain MRI abnormalities, spinal MRI can be of considerable diagnostic value, particularly in those presenting with symptoms referable to spinal cord or optic nerves 5

Suspected Neoplastic Conditions

Brain Tumors

• Clinically stable patients with known malignancy and new neurological symptoms should undergo MRI brain 2
• MRI is superior to CT for characterizing intracranial mass lesions, with advantages in detecting lesions where X-ray attenuation differs little from normal parenchyma, including low-grade infiltrating neoplasms 6
• Further characterization of suspected intracranial mass lesions identified on initial CT requires MRI 1, 2

New Onset Psychosis

Organic Causes of Psychosis

• MRI brain (with contrast for definitive characterization) may be performed for new onset psychosis to evaluate for organic causes including tumors, infarcts in specific brain areas (particularly temporal lobe), systemic lupus erythematosus, encephalitis, multiple sclerosis, Wilson disease, Huntington disease, or metachromatic leukodystrophy 1
• However, the diagnostic yield is very low in the absence of focal neurologic deficits 2

Progressive or Known Intracranial Pathology

Worsening Mental Status with Known Intracranial Process

• MRI without and with IV contrast, or MRI without IV contrast, is usually appropriate for acute or progressively worsening mental status in patients with known intracranial process (mass, recent hemorrhage, recent infarct, CNS infection) 1
• MRI is complementary to CT for further evaluation of suspected intracranial mass lesions, intracranial infection, and nonspecific regions of edema 1

Optimal MRI Protocol Considerations

Standard Protocol Elements

• MRI head without IV contrast is typically the initial study for evaluating brain parenchyma and detecting hemorrhage 2
• T2* gradient-echo (GRE) and susceptibility-weighted imaging (SWI) sequences optimally detect microhemorrhages 2
• Diffusion-weighted imaging (DWI) identifies acute ischemic changes and axonal injuries 2
• T2-weighted FLAIR imaging detects perilesional edema and subtle abnormalities 2

Contrast Enhancement Indications

• Gadolinium-based contrast agents are generally not necessary for initial evaluation of traumatic or hemorrhagic lesions 2
• Contrast-enhanced MRI should be performed for definitive characterization of focal lesions, suspected autoimmune disorders (multiple sclerosis, neuropsychiatric lupus), intracranial infection, tumor, or inflammatory conditions 1

Seizure-Specific Protocol

• Protocols should include coronal T1-weighted (3 mm) imaging perpendicular to the long axis of the hippocampus for evaluating hippocampal sclerosis (the most common cause of temporal lobe seizures) 1
• High-resolution volume (3-D) acquisition with 1-mm isotropic voxels is essential for assessing malformations of cortical development such as focal cortical dysplasia 1

Critical Caveats and Limitations

When CT Remains First-Line

• CT remains 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 1, 2
• CT is preferred when there is increased risk for intracranial bleeding (anticoagulant use, coagulopathy), hypertensive emergency, or clinical suspicion for elevated intracranial pressure 1

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 2
• In patients with new-onset psychosis without neurologic deficits, the yield of neuroimaging in detecting pathology responsible for symptoms is very low 2
• MRI may not be feasible in unstable patients or those requiring continuous monitoring 2

Impact on Clinical Management

Decision-Making Value

• In patients with acute disorders of consciousness, MRI leads to changes in clinical management in 76% of cases, including revised diagnoses, revised levels of care, improved diagnostic confidence, and improved prognostication 2
• After MRI, an additional 5% of patients with transient or minor neurological symptoms would be treated with antiplatelet drugs compared to the treatment plan before MRI 4