Brain MRI Basics: Understanding Common Neurological Findings

When a doctor orders a brain MRI, it’s usually because something doesn’t add up - a sudden headache, memory lapses, numbness, or vision changes. But what you see on the scan isn’t always what it seems. A bright spot on an MRI doesn’t automatically mean a tumor. A dark area isn’t always a stroke. Understanding the basics of brain MRI can help you make sense of the results, ask better questions, and avoid unnecessary panic.

What a Brain MRI Actually Shows

Unlike X-rays or CT scans, MRI doesn’t use radiation. Instead, it uses powerful magnets and radio waves to map the water content in your brain tissue. Different types of tissue - gray matter, white matter, cerebrospinal fluid, blood vessels - hold water differently. The machine picks up these differences and turns them into detailed images.

Modern brain MRIs are done on machines with 1.5T or 3.0T magnetic strength. The 3.0T machines give sharper images, especially for tiny structures like cranial nerves. A typical scan takes 30 to 45 minutes. You lie still in a narrow tube while the machine makes loud knocking sounds. It’s not painful, but it can feel claustrophobic. If you’re anxious, talk to your doctor about sedation options.

The real power of MRI comes from its ability to show soft tissue contrast - about 100 times better than CT scans. That means it can spot subtle changes in the brain long before they show up on other tests. That’s why it’s the gold standard for diagnosing conditions like multiple sclerosis, brain tumors, and early strokes.

The Five Key MRI Sequences You Need to Know

Doctors don’t rely on just one image. They look at a series of different scans, called sequences. Each one highlights different things. Here are the five you’ll see most often:

• T1-weighted: This is the anatomical map. Fat and some types of tissue appear bright white. Cerebrospinal fluid (CSF) looks dark. It’s great for seeing the shape of brain structures - ventricles, brainstem, cortex. If your brain looks shrunken on T1, that’s atrophy, which can happen with aging or dementia.
• T2-weighted: Water shows up bright here. Swelling, inflammation, and most lesions glow white. But here’s the catch: CSF also looks bright. That’s why you can’t tell if a bright spot is a lesion or just fluid without looking at another sequence.
• FLAIR: This is T2 with the CSF signal turned off. It’s the most important sequence for spotting brain disease. MS plaques, small strokes, and infections show up as bright spots against a dark background. If you see bright dots around the ventricles on FLAIR, they’re likely tiny blood vessel changes - common in older adults and not always serious.
• Diffusion-weighted imaging (DWI): This picks up acute strokes within minutes. When brain cells die from lack of oxygen, water can’t move freely. DWI shows that restriction as bright spots. If DWI is normal, an acute stroke is very unlikely. ADC maps confirm it - if the numbers are below 600 x 10^-6 mm²/s, it’s an active stroke.
• SWI or Gradient Echo: This finds blood. Tiny bleeds, even those too small to see on CT, show up as dark spots. These are common in older people with high blood pressure or after head injuries. Finding them doesn’t always mean something’s wrong - but they’re clues.

Doctors always compare these sequences together. A spot that’s bright on T2 and FLAIR but dark on DWI? Probably old. Bright on DWI and FLAIR? That’s a new stroke. Bright on SWI? Could be a bleed. The pattern tells the story.

Common Findings - And What They Really Mean

Not every abnormality on an MRI is dangerous. Here are five findings you’re likely to hear about:

• White matter hyperintensities: These are bright spots on FLAIR or T2, often near the ventricles. They’re called “leukoaraiosis.” In people over 60, they’re extremely common - up to 90% of those over 70 have them. They’re linked to small vessel disease, high blood pressure, or aging. Unless they’re massive or worsening fast, they’re usually not a cause for alarm.
• Small lacunar infarcts: These are tiny (3-5mm) strokes in deep brain areas like the thalamus or basal ganglia. They often cause no symptoms. Many people live with them without knowing. But if you have several, it raises your risk for future stroke or dementia.
• Brain atrophy: The brain naturally shrinks with age. But if it’s happening faster than expected - especially in the hippocampus - it could point to Alzheimer’s or other dementias. The key is comparing it to your age group. A 75-year-old with mild atrophy is normal. A 50-year-old with severe atrophy isn’t.
• Incidental meningiomas or schwannomas: These are slow-growing, usually benign tumors. A 2mm vestibular schwannoma (on the hearing nerve) might be found on an MRI done for dizziness. Most never grow or cause problems. Doctors often just watch them with repeat scans.
• Flow voids: These are dark lines on T2 or FLAIR. They’re not lesions - they’re blood vessels. Beginners often mistake them for damage. Experienced radiologists know they’re normal. Look for symmetry. If both sides have the same dark lines, it’s just blood flow.

One of the biggest mistakes is overinterpreting. A 2021 JAMA Neurology study found that only 1.3% of brain MRIs done for routine headaches without neurological symptoms showed anything serious. Most people get scared by normal aging changes.

When MRI Is the Best Choice - And When It’s Not

MRIs aren’t always the right first test. In trauma, stroke, or emergency situations, CT is faster - done in 5 minutes versus 45 for MRI. If someone’s having a stroke and needs clot-busting drugs, waiting for an MRI could cost them brain function.

Here’s when MRI wins:

• Multiple sclerosis - MRI finds plaques with 97% accuracy, while CT misses 35%.
• Early ischemic stroke - DWI detects changes in under 30 minutes. CT often looks normal for hours.
• Brain tumors - MRI shows size, location, and how it’s affecting surrounding tissue better than any other test.
• Seizure disorders - Especially temporal lobe epilepsy, where MRI can find subtle scarring invisible on CT.
• Chronic conditions like dementia or Parkinson’s - MRI helps rule out other causes and track progression.

CT still wins for:

• Acute head trauma - It’s fast and shows skull fractures and bleeding clearly.
• Patients with metal implants - Pacemakers, cochlear implants, or certain aneurysm clips make MRI unsafe.
• Emergency settings - If you’re unstable, you get CT first.

What Your Doctor Isn’t Telling You

There’s a lot of pressure to order MRIs - from patients, from fear, from defensive medicine. But not every headache needs a scan. The American College of Radiology says MRI is “usually not appropriate” for uncomplicated migraines.

Doctors are trained to look for “red flags”: sudden severe headache (thunderclap), new seizures, weakness on one side, vision loss, confusion, or fever. If you have those, MRI is urgent. If you have a 10-year history of migraines with the same pattern? Probably not.

Also, MRI can’t tell you how old a lesion is without contrast or advanced techniques. A bright spot on FLAIR could be from a stroke that happened yesterday - or 10 years ago. That’s why your doctor will ask about your symptoms, not just look at the scan.

And yes, MRIs are expensive - $1,200 to $3,500 in the U.S. In rural areas, only 42% of hospitals have one. That’s why access isn’t equal. But in places where it’s available, it’s often the only way to see what’s really going on inside the brain.

How Radiologists Read an MRI - A Simple System

Experts don’t just glance. They follow a routine. Here’s how they do it:

1. Start with the midline. Is the brain centered? Is the third ventricle symmetrical? Shifts can mean swelling or a mass.
2. Check the ventricles. Are they enlarged? That’s atrophy. Are they compressed? That’s a tumor or bleeding.
3. Look at the basal ganglia and thalamus. Bright spots here? Old small strokes.
4. Scan the cortex. Is there swelling? Is the gray-white border sharp? Blurry borders can mean infection or inflammation.
5. Check the cerebellopontine angle. This tiny space behind the ear is where acoustic neuromas hide. Miss it, and you miss a 2mm tumor.
6. Review FLAIR and DWI together. That’s where most pathology hides.

One radiologist I spoke with said: “If you check the cerebellopontine angle on every scan, you’ll find something unusual once every 50 scans. Most of the time, it’s nothing. But sometimes, it’s everything.”

What’s Next for Brain MRI?

Technology is moving fast. 7.0T MRI machines - still mostly in research - can show brain layers thinner than a human hair. AI is cutting scan times in half without losing detail. Quantitative MRI, which measures actual tissue properties like water content or blood flow, is starting to replace guesswork with numbers.

By 2027, doctors may use MRI to measure myelin loss in MS or blood flow changes in early Alzheimer’s - not just see the shape, but measure the damage. That’s the future: not just pictures, but biomarkers.

For now, though, the basics still matter most. Knowing what T2, FLAIR, and DWI show can turn confusion into clarity. You don’t need to be a radiologist to understand that a bright spot isn’t always bad - and a dark spot isn’t always normal. Ask your doctor: “Which sequence shows this finding? What does it mean in context of my symptoms?” That’s how you take back control.

What to Do After Your MRI

Don’t panic over a report. Don’t Google every bright spot. Ask your doctor:

• Which sequence shows the finding?
• Is it new or old?
• Does it match my symptoms?
• Do I need another scan in the future?

Most findings are normal variants or slow-progressing changes. Only a small fraction require treatment. The goal of MRI isn’t to find something wrong - it’s to find out what’s really going on. And sometimes, the answer is: nothing serious at all.