Imaging Nerd - Radiology Made Simple

Key Points

The brain has three compartments — supratentorial, posterior fossa, and the ventricles full of cerebrospinal fluid (CSF) — and most findings make sense once you know which one you're in.
The four lobes plus the cerebellum and brainstem are the neighborhoods; learn where they live so you can say where something is, not just that it's there.
CSF flows from the lateral ventricles, through the middle, out the back of the brainstem, and around the outside — block it anywhere and pressure backs up.
Gray matter lives on the outside (cortex) and in a few deep clusters; white matter is the wiring in between. Strokes and many diseases respect this border.
The two big arterial systems — front (carotid) and back (vertebrobasilar) — divide the brain into territories that explain why stroke symptoms come in patterns.

The brain looks like an intimidating gray walnut, and the first time you scroll through a head CT it feels like flying over a foreign city at night with no map. The good news: it's a city with a very logical layout. Once you know the neighborhoods, the highways, and where the water runs, you can navigate almost any scan. Let me hand you that map.

This page is the anatomy you lean on for everything else — when you approach a head CT or approach a brain MRI, you're really just touring these same landmarks looking for trouble.

Three compartments: the floor plan

Picture the skull as a house with a sturdy shelf partway down the back. That shelf is the tentorium, a tent of dura that splits the brain into two stories.

Supratentorial — upstairs. The big cerebral hemispheres and their lobes.
Infratentorial / posterior fossa — downstairs and in the back. The cerebellum and brainstem, packed into a small bony room.
The ventricles — the plumbing. A connected set of fluid-filled spaces deep in the middle.

Why care about the shelf? Because the downstairs is cramped. A mass in the tiny posterior fossa runs out of room fast and starts squeezing the brainstem, which is the part you really, really need. That's the setup for the herniation syndromes — the brain getting shoved through doorways it was never meant to use.

The neighborhoods: lobes, cerebellum, brainstem

Upstairs, each hemisphere is carved into four lobes, and they're easier to remember by job than by name.

Lobe	Roughly where	Headline job
Frontal	Front	Movement, planning, personality
Parietal	Top-middle/back	Sensation, spatial sense
Temporal	Sides, by the ears	Hearing, memory, language
Occipital	Very back	Vision

Downstairs, the cerebellum is the little ridged hindbrain that fine-tunes balance and coordination — think of it as the brain's stability control. The brainstem is the stalk connecting brain to spinal cord, and it runs the unglamorous-but-essential stuff: breathing, heart rate, consciousness. It's small, central, and absolutely not optional.

Gray on the outside, white on the inside

Slice the brain and you'll see two shades. The gray matter is the wrinkled rind on the surface (the cortex) plus a few deep clusters called the deep gray nuclei — the basal ganglia and thalamus. The white matter is everything in between: the wiring, the cables connecting one region to another.

The color difference is real and it's load-bearing for imaging. On a normal CT, gray matter is slightly denser (brighter) than white matter, and you can trace the boundary between them. When that crisp border goes blurry — the dreaded "loss of gray-white differentiation" — something is wrong, and it's one of the early tells of ischemic stroke. On MRI the contrast flips and shifts depending on the sequence, which is the whole point of understanding T1 and T2 weighting.

Figure · CT

Axial non-contrast head CT at the level of the basal ganglia, normal: label the slightly brighter cortex (gray matter), the darker subcortical white matter, the deep gray nuclei (basal ganglia and thalami), and the dark CSF in the frontal horns of the lateral ventricles.

The plumbing: CSF and the ventricles

The brain floats in cerebrospinal fluid, a clear shock-absorbing liquid that's made deep inside and circulates on a one-way tour. The path is worth memorizing because blockages cause real problems:

CSF is produced in the lateral ventricles (one in each hemisphere), drains through narrow channels into the midline third ventricle, squeezes through a famously skinny tunnel to the fourth ventricle behind the brainstem, then exits to flow around the outside of the brain and spinal cord before being reabsorbed.

Here's the analogy that sticks: it's a series of ponds connected by streams, with the streams getting narrower as you go. Plug a stream and every pond upstream swells. That backed-up swelling is hydrocephalus, and on imaging it shows up as ballooned ventricles. On any scan, the ventricles are dark on CT and follow fluid on MRI — a handy internal landmark.

Note

The ventricles are your friends on a scan. They're symmetric, predictably shaped, and central — so when one is squashed, shifted off the midline, or blown up like a water balloon, your eye should snap to it. Asymmetry of the normally symmetric is one of the most reliable alarm bells in neuroradiology.

The highways: blood supply in two systems

The brain is fed by two arterial systems that meet in a ring at the base.

The front is supplied by the internal carotid arteries, which branch into the anterior and middle cerebral arteries feeding most of the cerebral hemispheres.
The back is supplied by the vertebral arteries, which join into the basilar artery and feed the brainstem, cerebellum, and the posterior cerebral arteries to the occipital lobes.

Each major artery waters its own territory, like a city zoned by which reservoir serves it. This is why stroke symptoms cluster into recognizable patterns rather than random deficits — and why knowing the territories lets you predict the vessel from the symptoms, and vice versa.

Pitfall

Don't assume every dark patch on a CT is a stroke or every bright one is blood. Normal CSF spaces, prominent perivascular spaces, and age-related volume loss can all fool a fresh eye. Anatomy first: confirm where you are and whether the finding respects a known territory or border before you commit to calling it pathology. (Bleeding gets its own full treatment in intracranial hemorrhage.)

Putting the map to work