Brain MRI: What It Shows, When You Need One

What can a brain MRI detect?

MRI is the most detailed structural imaging tool available for the brain. It is used to evaluate:

• Stroke and ischemic damage — areas of brain tissue that lost blood supply show as bright or dark regions depending on the MRI sequence
• Brain tumors — both primary brain tumors and metastases from cancers elsewhere in the body
• Multiple sclerosis — white matter lesions that are characteristic of MS appear distinctly on MRI
• Aneurysms and vascular malformations — MR angiography (MRA), a variant of brain MRI, images blood vessels
• Encephalitis and inflammation — infections or autoimmune conditions affecting brain tissue
• Structural abnormalities — congenital malformations, hydrocephalus, and atrophy
• Dementia workup — to rule out structural causes of cognitive decline and characterize patterns of brain volume loss
• Epilepsy localization — to identify structural causes of seizures such as cortical dysplasia or hippocampal sclerosis
• Head trauma — detecting hemorrhage and contusions (though CT is faster in emergencies) 3Ref 3National Institute of Neurological Disorders and Stroke (2023).Peripheral Neuropathy.Role of MRI and neuroimaging in evaluating structural causes of neurological symptoms

How is a brain MRI different from a CT scan?

Both produce cross-sectional images of the brain, but they differ in important ways:

When CT is preferred: In an emergency — a sudden severe headache, acute stroke symptoms, head trauma — CT is chosen because it is fast and reliably detects acute hemorrhage 1Ref 1Wolf SJ, et al. (ACEP Clinical Policies Subcommittee on Acute Headache) (2019).Clinical Policy: Critical Issues in the Evaluation and Management of Adult Patients Presenting to the Emergency Department With Acute Headache.CT as first-line imaging for sudden severe headache (thunderclap headache) to evaluate for subarachnoid hemorrhage before MRI. Brain MRI follows once the immediate emergency is addressed.

When MRI is preferred: For chronic or subacute neurological symptoms, suspected MS, tumor follow-up, dementia workup, or epilepsy evaluation, MRI provides far more detail.

When do you need a brain MRI for headaches?

Most headaches — including tension headaches and common migraine — do not require imaging. For primary migraine or tension-type headache with a normal neurological examination, MRI is generally not appropriate 2Ref 2Utukuri PS, Shih RY, Ajam AA, et al.; Expert Panel on Neurological Imaging (2023).ACR Appropriateness Criteria® Headache: 2022 Update.Evidence-based indications for brain imaging in headache; primary migraine or tension headache with normal neurological exam is usually not appropriate for MRI. Clinical guidelines reserve brain imaging for headaches that have features suggesting a structural or dangerous cause. These include 1Ref 1Wolf SJ, et al. (ACEP Clinical Policies Subcommittee on Acute Headache) (2019).Clinical Policy: Critical Issues in the Evaluation and Management of Adult Patients Presenting to the Emergency Department With Acute Headache.CT as first-line imaging for sudden severe headache (thunderclap headache) to evaluate for subarachnoid hemorrhage before MRI2Ref 2Utukuri PS, Shih RY, Ajam AA, et al.; Expert Panel on Neurological Imaging (2023).ACR Appropriateness Criteria® Headache: 2022 Update.Evidence-based indications for brain imaging in headache; primary migraine or tension headache with normal neurological exam is usually not appropriate for MRI:

• The worst headache of your life (thunderclap onset) — this requires emergency CT first
• Headache with fever, neck stiffness, or rash
• New headache in a person over 50
• Headache progressively worsening over days to weeks
• Headache associated with neurological symptoms (weakness, vision changes, speech problems)
• Headache following head trauma
• Headache in a person with a known cancer or compromised immune system
• Headache not fitting a recognized primary headache pattern after clinical evaluation

What happens during a brain MRI?

You lie on a table that slides into a cylindrical scanner. The machine makes loud knocking and humming sounds during image acquisition — earplugs or headphones are offered. You must remain still; movement blurs the images.

A standard brain MRI uses several different sequences (T1, T2, FLAIR) that highlight different tissues. With contrast (gadolinium dye injected through an IV), areas of blood-brain barrier breakdown light up, which helps identify tumors, active inflammation, and some vascular abnormalities.

The scan itself takes 30 to 60 minutes. Results are read by a neuroradiologist and sent to the ordering clinician.

Metal and MRI: Before the scan, the technician screens for metal implants (pacemakers, cochlear implants, certain aneurysm clips) because the magnetic field is powerful. Most modern implants are MRI-compatible but must be verified.

What specialist orders a brain MRI?

A primary care clinician, neurologist, or emergency physician can order a brain MRI depending on the clinical situation. A neuroradiologist reads the images and writes a report. A neurologist interprets findings in the context of your symptoms and examination.

Gale does not directly order imaging, but a Gale primary care clinician can evaluate your neurological symptoms, determine whether imaging is warranted, and coordinate a referral to neurology when needed.