Brain MRI Basics: Understanding Common Neurological Findings

When your doctor orders a brain MRI, it’s not because they’re being overly cautious-it’s because they need to see what’s happening inside your skull in a way no other test can. Unlike X-rays or CT scans, MRI doesn’t use radiation. Instead, it uses powerful magnets and radio waves to create detailed pictures of your brain’s soft tissues. This makes it the most reliable tool for spotting problems like tumors, strokes, multiple sclerosis, and even subtle changes tied to memory loss or seizures. But what do those images actually show? And how do doctors tell the difference between normal aging and something serious?

What Brain MRI Actually Shows

A brain MRI doesn’t just take a photo-it captures how water behaves in different tissues. That’s why it’s so good at showing damage. For example, when brain tissue swells from inflammation or a stroke, water builds up. On MRI, that swelling lights up like a beacon. Normal brain tissue looks gray and white in clear layers. When those layers blur or show bright spots, it’s a red flag.

There are four main types of MRI scans used in neurology, each highlighting something different:

• T1-weighted images give you the clearest view of anatomy. Fat and certain proteins look bright white. Cerebrospinal fluid (CSF), which surrounds the brain, looks dark. This is how radiologists check if the brain’s structure is in the right place.
• T2-weighted images make water shine. Swelling, infections, and old injuries show up as bright spots. But here’s the catch: CSF also looks bright on T2, so it’s easy to mistake a fluid-filled space for a lesion if you’re not careful.
• FLAIR sequences fix that problem. They turn CSF dark while keeping abnormal tissue bright. That’s why FLAIR is the go-to for spotting multiple sclerosis plaques, which cluster around the brain’s ventricles. If you see bright spots near the fluid spaces but not inside them, it’s likely MS.
• Diffusion-weighted imaging (DWI) is the fastest way to catch a stroke. Within minutes of a blood vessel blocking, water can’t move normally in brain cells. DWI picks this up right away-often before symptoms even fully develop. A low ADC value (below 600 x 10^-6 mm²/s) confirms an acute stroke.

These sequences work together. A radiologist doesn’t look at one image-they compare them like puzzle pieces. A bright spot on T2 and FLAIR that’s also bright on DWI? That’s a fresh stroke. A bright spot on T2 and FLAIR but normal on DWI? Likely an old injury or something benign.

Common Findings and What They Mean

Not every bright spot on an MRI means something dangerous. Many are harmless, especially as people age. Here are the most common findings and how to interpret them:

• White matter hyperintensities (WMHs): These are small bright spots, often near the brain’s fluid spaces. They’re seen in 15% of people under 50 and up to 90% of those over 70. In older adults, they’re usually linked to tiny blood vessel changes from high blood pressure or aging. But if they’re large, asymmetrical, or appear in unusual places, they could signal MS or another inflammatory condition.
• Small lacunar infarcts: These are tiny, old strokes-often less than 5mm. They show up as small, round bright spots in deep brain areas like the thalamus or basal ganglia. Many people have them without ever knowing. They’re a sign of long-term vascular risk, not an emergency.
• Enlarged ventricles: The brain’s fluid-filled spaces can grow with age. But if they’re enlarged along with thinning of the brain’s outer layer, it could mean dementia. The key is comparing ventricle size to the surrounding brain tissue. If the brain looks shrunken, it’s likely atrophy-not a tumor or blockage.
• Incidental meningiomas or schwannomas: Sometimes, MRI picks up slow-growing, noncancerous tumors. A vestibular schwannoma, for example, grows on the hearing nerve. These are often found by accident during scans for headaches or dizziness. If they’re under 2mm and not growing, they’re usually monitored, not treated.
• Microbleeds: These are tiny spots of old bleeding, seen best on susceptibility-weighted imaging. They’re common in older adults with high blood pressure or amyloid angiopathy. More than five microbleeds increases stroke risk. But one or two? Often just a sign of aging.

Location matters more than size. A lesion in the temporal lobe? Think herpes encephalitis. One in the parietal or occipital lobe? Could be posterior reversible encephalopathy syndrome (PRES), often linked to high blood pressure or kidney disease. The brain isn’t random-certain diseases favor certain areas.

When MRI Is the Best Choice (and When It’s Not)

MRIs are powerful, but they’re not always the first step. In emergency situations-like sudden weakness, slurred speech, or head trauma-CT is faster. A CT scan takes 5 minutes. An MRI takes 30 to 45. In stroke cases, every minute counts. If a clot needs to be removed or dissolved, CT gives you the answer fast.

But for everything else, MRI wins. It’s the only test that can reliably spot:

• Early multiple sclerosis plaques (97% accurate vs. 65% for CT)
• Small tumors like acoustic neuromas as tiny as 2mm
• Subtle signs of dementia before memory loss becomes obvious
• Inflammation from infections like meningitis or encephalitis

CT still has its place. It’s better at spotting fresh bleeding, broken bones, or swelling from trauma. But if you’re having seizures, unexplained dizziness, or progressive memory issues, MRI is the only test that gives you the full picture.

What You Might Not Realize About MRI

Many people worry about the noise, the tight space, or whether they’ll be able to stay still. But the bigger issue is overuse. A 2021 JAMA Neurology study found that only 1.3% of brain MRIs done for routine headaches without neurological symptoms showed anything meaningful. The American College of Radiology says MRI is usually not appropriate for uncomplicated migraines. That’s why doctors now use strict guidelines: if you have a normal neurological exam and no red flags (like vision loss, weakness, or seizures), an MRI is rarely needed.

Another myth: MRI can’t tell you how old a lesion is. A bright spot could be from a stroke that happened yesterday-or 10 years ago. That’s why radiologists compare images over time. If a spot has stayed the same for three years, it’s probably not dangerous. If it’s growing, that’s a red flag.

And then there’s cost. A brain MRI can cost between $1,200 and $3,500 in the U.S., while a CT runs $500-$1,500. In rural areas, only 42% of hospitals have MRI machines. That’s why access isn’t equal. But for the right patient, the value is undeniable.

How Radiologists Read an MRI-Step by Step

Reading an MRI isn’t guesswork. It’s a system. Experienced radiologists follow a strict order:

1. Check the midline. Is the brain centered? If the ventricles are pushed to one side, there’s a mass or swelling.
2. Look at the ventricles. Are they enlarged? Is the tissue around them thinning?
3. Examine the basal ganglia and thalamus. Small bright spots here? Likely old lacunar strokes.
4. Scan the cerebral lobes. Are there lesions in the white matter? Are they symmetrical? MS lesions tend to be scattered and asymmetrical.
5. Check the cerebellopontine angle. This tiny space behind the ear is where acoustic neuromas hide. Miss it, and you might miss a treatable tumor.
6. Look at the meninges and skull base. Infection or cancer can spread here.

One pro tip: Never rely on T2 alone to judge ventricle size. CSF looks bright on T2, making ventricles appear larger than they are. Always cross-check with FLAIR-CSF is dark there, so the real size becomes clear.

What’s Next for Brain MRI?

Technology is moving fast. New 7.0T MRI machines-still rare but growing-can show brain layers as thin as 0.5mm. That’s enough to see individual nerve pathways. AI tools are cutting scan times in half without losing detail. And researchers are developing ways to measure brain chemistry directly from MRI-like myelin content or blood flow-turning images into biological reports.

By 2027, these quantitative techniques could become routine. Imagine an MRI that doesn’t just show a lesion, but tells you if it’s active, healing, or stable. That’s the future.

For now, brain MRI remains the gold standard. It’s not perfect. It’s expensive. It takes time. But when it comes to understanding what’s wrong inside your brain, nothing else comes close.