Imaging Nerd - Radiology Made Simple

Key Points

The non-contrast head CT is the workhorse of emergency brain imaging: fast, everywhere, and brilliant at finding fresh blood and big bleeds.
Read it the same way every single time. A fixed checklist beats raw genius, because the brain only has so many ways to go wrong and they all hide in the same handful of places.
Fresh blood is bright (high attenuation); old blood, fluid, and most early strokes are dark. That brightness scale is the whole language of the study.
"Blood, blip, bone, brain" plus the spaces in between — symmetry is your best friend, and the corners are where things go to hide.

It's 3 a.m., someone's been found confused on a bathroom floor, and the question is brutally simple: is there blood in their head, and is their brain getting squeezed? The non-contrast head CT exists to answer exactly that, and it does it in about the time it takes to microwave a burrito. Your job is to not fumble the read.

Why the head CT, and why non-contrast

CT builds its picture from how much of the X-ray beam each tissue eats on the way through — the radiologists call this attenuation. Dense stuff (bone, fresh clotted blood) eats a lot and shows up white; watery stuff (fluid, edema) lets the beam sail through and shows up dark. Brain sits in the gray middle, which is convenient, because that's literally what it's called.

We skip contrast for the emergency study on purpose. Acute blood is already bright on its own, and squirting in bright contrast would just be camouflage — like trying to find a snowball in a blizzard. Save the contrast for hunting tumors and infection later.

Note

A head CT is fantastic at fresh blood, mass effect, and bone. It is bad at the back of the brain (the posterior fossa, where bone causes streaky artifact) and at very early strokes. "Normal CT" never means "normal brain."

Read it the same way every time

The single most useful habit in all of radiology is a fixed search pattern — you run the same loop so you never skip the thing that kills the patient. A friendly mnemonic for the head is Blood, Blip, Bone, Brain, but here's a fuller checklist:

Step	What you're checking	The big "uh-oh"
Blood	The CSF spaces and brain surface for bright material	Intracranial hemorrhage, extra-axial collections
Cisterns	The fluid-filled basal cisterns at the skull base	Effaced cisterns = pressure; blood in them = aneurysmal SAH
Ventricles	Size and symmetry of the fluid chambers	Hydrocephalus, or a trapped/compressed ventricle
Brain	Gray-white differentiation, symmetry	Early ischemic stroke, mass, edema
Bone & soft tissue	Windowed for the skull and scalp	Fracture, scalp swelling pointing you to the injury

The trick is to run every row, every time — even when the answer screams at you from row one. Tunnel vision on the obvious bleed is how the second, quieter finding gets missed.

Symmetry is your superpower

The brain is gloriously symmetric, which makes it easy to police. Flip your eyes left-right across each slice like you're playing spot-the-difference in a kids' magazine. A sulcus that's fatter on one side, a ventricle that's squashed, a midline that's shoved off-center — these asymmetries are often louder and more reliable than the actual lesion. The midline structures should sit, well, in the middle; when they don't, something is pushing.

Key Point

Midline shift means one side is taking up room the other side wants. Trace the falx and the septum — if they're bowed across the middle, go find what's doing the shoving, and check the cisterns for herniation.

Windows: same data, different glasses

Here's a thing that trips up beginners: the same CT is viewed through different "windows," which is just a way of telling the screen which slice of the brightness scale to spread across black-to-white. Brain window makes subtle gray-white differences pop. Bone window makes the skull crisp so you can catch a hairline fracture. Subdural (intermediate) window widens things so a thin sliver of blood hugging the skull doesn't blend into the bone next to it. Looking at only one window is like inspecting a car with one eye closed — technically possible, professionally embarrassing.

Pitfall

A thin subdural hematoma can vanish against the adjacent bright skull on standard brain window. Always sweep through a wider (subdural) window, and check both sides — a small bleed loves to hide right up against the inner table.

Figure · CT

Axial non-contrast head CT, brain window, at the level of the lateral ventricles, showing normal symmetric gray-white differentiation and midline structures for comparison/baseline.

What "acute" looks like

Timing changes the color of blood, which is oddly poetic. Fresh, clotted blood is bright (hyperdense). Over days to weeks it breaks down and fades, passing through a phase where it matches brain (isodense — sneaky, because it can hide) and eventually going dark (hypodense), like old fluid. So a bright crescent on the brain surface is recent; a dark one is old. An early ischemic stroke does the opposite of a bleed — instead of bright, the affected brain subtly loses its crisp gray-white border and looks a touch too dark and swollen.

Figure · CT

Axial non-contrast head CT showing a hyperdense (bright) crescentic extra-axial collection along the cerebral convexity with mild effacement of the underlying sulci — acute subdural hematoma.

Critical

The findings that need a phone call right now: a large or expanding bleed, blood in the basal cisterns, effaced cisterns, significant midline shift, or signs of herniation. These don't wait for the formal report.

The takeaway