Brain MRI Basics: Understanding Common Neurological Findings

When your doctor orders a brain MRI, it’s not just a picture-it’s a detailed map of what’s happening inside your head. Unlike X-rays or CT scans, MRI doesn’t use radiation. Instead, it uses powerful magnets and radio waves to show soft tissues with incredible clarity. This makes it the brain MRI gold standard for spotting problems like tumors, strokes, multiple sclerosis, and even subtle changes linked to dementia. But what do those images actually mean? And why do some areas look bright while others are dark? You don’t need to be a radiologist to understand the basics.

What You’re Seeing: The Three Key MRI Sequences

Not all brain MRI images are the same. Three main types of scans-T1-weighted, T2-weighted, and FLAIR-are used together to give a full picture. Each one highlights different tissues based on how water and fat behave in magnetic fields.

T1-weighted images are like a high-resolution anatomy textbook. Fat and structures with lots of fat-like the fatty marrow in skull bones-show up bright white. Cerebrospinal fluid (CSF), which fills the spaces around your brain and spinal cord, looks dark. This makes it easy to see the shape of your brain, the boundaries between gray and white matter, and any abnormal growths that might push or distort those structures.

T2-weighted images flip the script. Water shows up bright. That means areas with swelling, inflammation, or fluid buildup-like a stroke or infection-light up like a beacon. But here’s the catch: CSF is also bright on T2. So if you see a bright spot near a ventricle, is it a lesion… or just fluid? That’s where FLAIR comes in.

FLAIR (Fluid-Attenuated Inversion Recovery) is T2’s smarter cousin. It cancels out the bright CSF signal, making it go dark again. Now, any bright spots near the ventricles or in the brain’s white matter? Those are real abnormalities. This is why FLAIR is the go-to sequence for spotting multiple sclerosis plaques. Those tiny white matter lesions? They glow against the now-dark CSF background. Without FLAIR, they’d be easy to miss.

What Common Findings Actually Look Like

Once you know how the sequences work, you can start recognizing the most frequent findings. Here’s what they typically mean:

• White matter hyperintensities: These are small, bright spots on T2 and FLAIR scans, often near the ventricles. They’re extremely common in people over 60. In many cases, they’re just signs of small vessel disease-tiny blood vessels in the brain that have narrowed over time due to high blood pressure or aging. But if they’re numerous, widespread, or located in unusual areas, they could point to multiple sclerosis or other inflammatory conditions.
• Acute stroke: On a DWI (diffusion-weighted imaging) scan, a stroke shows up as a bright spot within minutes of the event. This is the most critical sequence in emergency neurology. If you see a bright spot on DWI that’s dark on the ADC map (a companion image), it means water movement is restricted-classic for a fresh blockage. CT scans can miss this for hours; MRI catches it right away.
• Brain tumors: Tumors often appear as irregular masses with mixed signals. They may enhance (get brighter) after contrast dye is injected, showing where the blood-brain barrier is broken. The location matters too: tumors in the pituitary gland affect hormones, while those near the motor cortex can cause weakness.
• Microbleeds: These tiny spots of old bleeding show up as dark dots on SWI (susceptibility-weighted imaging) or gradient echo sequences. They’re common in people with high blood pressure or amyloid angiopathy. A few are often harmless. Dozens? That’s a red flag.
• Atrophy: Brain shrinkage appears as enlarged ventricles and widened grooves on the surface. It’s normal with aging, but if it’s much worse than expected for someone’s age-especially in the hippocampus-it can signal early Alzheimer’s disease.

Why MRI Beats CT for Most Brain Issues

Many people wonder: why not just do a CT scan? It’s faster, cheaper, and more available. And for trauma or acute bleeding, that’s true. CT can spot a large hemorrhage in under a minute. But when it comes to the brain’s soft tissues, MRI is in a different league.

CT shows about 4 to 5 shades of gray. MRI shows 30 to 40. That means MRI can tell the difference between a normal nerve and a tiny tumor, or between a new stroke and an old one. It also doesn’t use radiation-critical for kids, pregnant women, or people needing repeated scans over years.

CT struggles with the back of the brain-the cerebellum and brainstem. Bone blocks the X-rays, creating blurry artifacts. MRI sees right through. That’s why a person with dizziness and double vision gets an MRI, not a CT, when a brainstem stroke is suspected.

For multiple sclerosis, MRI detects plaques with 97% accuracy. CT? Only 65%. For small tumors like acoustic neuromas-tumors on the hearing nerve-MRI picks up lesions as small as 2mm. CT needs at least 5mm to even notice them.

What MRI Can’t Tell You (And What You Shouldn’t Panic About)

Even the best tool has limits. One big issue: MRI can’t always tell how old a lesion is. A bright spot on FLAIR could be from a stroke that happened yesterday-or one from ten years ago. That’s why doctors look at the whole picture: symptoms, history, and sometimes follow-up scans.

Another common source of confusion: incidental findings. About 1 in 10 brain MRIs done for headaches or dizziness without neurological signs show something unusual-like a small meningioma or a cyst. Most are harmless. But they cause anxiety and lead to more tests. The American College of Radiology says MRI is usually not appropriate for routine migraines without warning signs. Overuse drives up costs and creates false alarms.

And yes, some things look scary but aren’t. Flow voids-dark lines in vessels like the basilar artery-are normal. They’re just fast-moving blood. Novices often mistake them for blockages. Also, periventricular hyperintensities are present in 15% of people under 50 and 90% of those over 70. They’re not automatically a sign of disease.

How Radiologists Read an MRI: A Simple System

Reading an MRI isn’t random. Experts follow a checklist:

1. Start with the midline. Is the brain centered? Is the third ventricle symmetric? Shifts can mean swelling or a mass.
2. Check the ventricles. Are they enlarged? Is there fluid buildup?
3. Look at the basal ganglia and thalamus. Small bright spots here often mean old tiny strokes.
4. Scan the white matter. Look for FLAIR hyperintensities. Are they symmetrical? Are they near the ventricles or spread out?
5. Examine the cortex. Any swelling, thinning, or abnormal signal?
6. Check the posterior fossa. This is where CT fails. Look for brainstem or cerebellar lesions.
7. Finally, the cerebellopontine angles. Tiny tumors here-like vestibular schwannomas-can cause hearing loss. They’re easy to miss if you don’t look.

It takes months of training to spot these patterns reliably. Residents learn to recognize normal gray-white contrast-a 30-40% difference on T1-and know that anything outside that range needs attention.

What’s New in Brain MRI

The field isn’t standing still. Newer machines-3.0 Tesla and even 7.0 Tesla-give sharper images. Some research centers now use AI to cut scan times in half without losing detail. That’s huge for people who struggle to stay still, like those with Parkinson’s or dementia.

Quantitative MRI is also growing. Instead of just saying “there’s a bright spot,” doctors can now measure things like myelin content or blood flow. These numbers help track multiple sclerosis progression or Alzheimer’s changes more precisely than before.

Diffusion tensor imaging (DTI), which maps nerve fiber pathways, is now standard in 68% of MS protocols. It shows how damage to white matter tracts affects movement or speech-linking structure to function.

But access remains uneven. In rural Australia, only 42% of hospitals have MRI machines. Urban centers have near-universal access. That gap means delays in diagnosis for people living outside major cities.

What to Do After Your Scan

If you’ve had a brain MRI, don’t try to read it yourself. Radiologists spend years learning this. But you can ask smart questions:

• Is this finding normal for my age?
• Is this something that needs follow-up?
• Could this explain my symptoms?
• Do I need another scan in the future?

Don’t panic over a vague term like “white matter changes.” Ask what it means for your health. Most findings are not emergencies. But knowing what’s normal and what’s not helps you partner with your doctor-and avoid unnecessary worry.

If your doctor says your brain MRI showed something unusual, don’t assume the worst. Many findings are incidental, slow-growing, or unrelated to your symptoms. The goal isn’t to find every tiny change-it’s to find the ones that matter. And with modern MRI, doctors have more tools than ever to do just that.