Reading a head CT scan comes down to a systematic approach: you look at the same structures in the same order every time so you don’t miss anything. The most widely taught method uses the mnemonic “Blood Can Be Very Bad,” which walks you through five categories: Blood, Cisterns, Brain, Ventricles, and Bone. Before diving into each step, it helps to understand what you’re actually seeing on the screen.
What the Shades of Gray Mean
A CT scan measures how dense each tiny block of tissue is, then assigns it a shade on a grayscale. Dense materials like bone appear bright white. Air appears black. Everything else falls somewhere in between, and the exact position on that scale is measured in Hounsfield Units (HU). Knowing a few key values gives you an anchor for identifying what you’re looking at:
- Air: -1000 HU (black)
- Fat: around -50 HU (dark gray)
- Cerebrospinal fluid (CSF): +15 HU (dark gray)
- White matter: about +25 to +28 HU (medium gray)
- Gray matter: about +35 to +40 HU (slightly lighter gray)
- Fresh blood: +30 to +45 HU initially, but clotted blood is brighter, often +50 to +70 HU
- Bone: +1000 HU and above (bright white)
The practical takeaway: anything that looks unexpectedly bright (hyperdense) could be blood or calcification. Anything unexpectedly dark (hypodense) could be swelling, dead tissue, or fluid where it shouldn’t be. Your eye should always be comparing one side of the brain to the other, looking for asymmetry.
Contrast vs. Non-Contrast Scans
Most head CTs you’ll encounter are non-contrast scans. That’s the standard for acute trauma, suspected stroke in the first few hours, and any situation where the main question is “is there bleeding?” Contrast dye is injected through an IV when the clinical question involves tumors, infections like abscesses, or staging cancer. It makes blood vessels and abnormal tissue light up, which helps distinguish a mass from surrounding brain. If someone hands you a head CT from the emergency department after a fall or sudden neurological change, it’s almost certainly non-contrast.
Step 1: Blood
Fresh blood clots are denser than brain tissue, so they appear as bright white areas where white shouldn’t be. You’re scanning for bleeding in several locations: on the surface of the brain, within the brain tissue itself, and inside the ventricles (the fluid-filled chambers). The location and shape of the bleeding tell you what type it is.
An epidural hematoma, bleeding between the skull and the tough outer membrane covering the brain, appears as a lens-shaped (biconvex) bright white collection pressed against the inner skull. It has clean, rounded edges and does not cross the lines where skull bones meet (suture lines). A subdural hematoma, bleeding just beneath that outer membrane, looks different. It spreads out in a crescent shape that drapes along the curve of the brain and can extend across the entire hemisphere. One useful detail: subdural collections often have a thin crescent-shaped “tail” trailing along the edge, and the angle where the blood meets the skull is wider (obtuse) rather than the tight curve of an epidural.
Blood within the brain tissue itself (intracerebral hemorrhage) appears as a bright white blob surrounded by normal gray brain. Blood inside the ventricles shows up as bright white layering within spaces that should contain only dark CSF.
Step 2: Cisterns
The cisterns are CSF-filled spaces at the base of the brain, sitting around the brainstem. On a normal scan, they appear as dark (fluid-density) spaces with clear outlines. The ones you can most reliably identify are the spaces surrounding the brainstem and the space behind the cerebellum (the cisterna magna).
Their clinical importance is straightforward: when the brain swells, these spaces get compressed first. If the cisterns look squeezed, partially filled in, or completely gone, intracranial pressure is rising. Effaced (flattened) basal cisterns in a trauma patient is an ominous sign that the brain is running out of room inside the skull. Compare what you see to the normal appearance of open, dark, well-defined spaces. If they look tight or asymmetric, something is pushing on the brain.
Step 3: Brain
This is the most complex step, and the key principle is symmetry. You’re comparing the left hemisphere to the right, looking for any area that’s brighter or darker than its mirror image on the other side.
Gray-White Differentiation
On a normal head CT, you can see a subtle but real difference between the slightly lighter gray matter (the brain’s outer cortex and deep nuclei) and the slightly darker white matter beneath it. The difference is only about 7 to 8 Hounsfield Units, so it’s subtle. When that distinction disappears and everything blurs into one uniform shade, it signals serious pathology: diffuse brain swelling, loss of oxygen to the brain, or severe trauma. This “loss of gray-white differentiation” is one of the most important things to recognize because it indicates widespread injury.
Early Signs of Stroke
Ischemic stroke (a blocked blood vessel) is notoriously hard to see on CT in the first several hours. The brain tissue hasn’t swollen enough yet to look obviously different. But there are early clues. The most specific is the “hyperdense MCA sign,” where the middle cerebral artery itself appears as a bright white line because clotted blood inside the vessel is denser than flowing blood. This shows up in roughly 13% of early stroke CTs. Other early findings include subtle darkening (hypodensity) of the brain tissue supplied by the blocked vessel, loss of the normal distinction between gray and white matter in the region near the ear called the insular cortex (the “insular ribbon sign,” present in about 29% of early cases), and flattening of the normal grooves (sulci) on the brain surface, visible in about 41% of cases. These changes are easy to miss if you’re not specifically looking for asymmetry between the two sides.
Step 4: Ventricles
The ventricles are butterfly-shaped chambers filled with CSF that appear dark on CT. You’re looking at three things: size, symmetry, and position.
Enlarged ventricles can mean hydrocephalus, where CSF isn’t draining properly and is building up. Compressed or slit-like ventricles suggest the surrounding brain is swollen and squeezing them shut. But the most critical finding is whether the ventricles have shifted to one side.
This is called midline shift, and it’s measured at the level of a thin membrane called the septum pellucidum that sits between the two front horns of the lateral ventricles. To measure it, you find the midpoint of the skull’s inner width, then measure how far the septum pellucidum has moved from that midpoint. Shifts are classified as mild (under 5 mm), moderate (5 to 10 mm), or severe (over 10 mm). A shift greater than 5 mm from a blood collection is generally considered a surgical emergency. In stroke patients, a shift of just 3.7 mm or more within 24 hours can predict a life-threatening degree of swelling. The average shift in patients who are comatose with unequal pupils is about 10 mm.
Step 5: Bone
Most CT viewing software lets you switch to a “bone window,” which adjusts the contrast so bone detail is visible rather than soft tissue. This is where you look for fractures, and it’s where a common mistake happens: confusing a normal suture line (the natural joints between skull bones) with a fracture.
The differences are reliable once you know what to look for. Fractures appear as sharp, clean lines with smooth, non-sclerotic (non-white) edges. Sutures have a zigzag pattern with interlocking fingers and slightly bright (sclerotic) borders, similar to a zipper. Fractures tend to widen as they approach a suture line and can cross from one bone to another. Sutures merge smoothly with adjacent sutures and don’t create widening at the junction. Fractures are usually on one side; when sutures appear as subtle lines, they’re typically present symmetrically on both sides, especially in the parietal bones at the top of the skull.
One practical clue: acute skull fractures are almost always accompanied by at least 4 mm of soft tissue swelling on the outside of the skull at the fracture site. If you see a questionable line in the bone but no overlying scalp swelling, it’s more likely a suture. Certain normal sutures in children can also mimic fractures. The occipital suture at the base of the skull, for instance, closes by age 4. A lucency in that location after age 4 that extends more than 2 cm from the edge of the large opening at the skull base (the foramen magnum) is a fracture, not a persistent suture.
Putting It All Together
The value of “Blood Can Be Very Bad” is that it forces a complete review. It’s tempting to spot one obvious finding, like a large bright subdural collection, and stop looking. But injuries and pathologies often coexist. A subdural hematoma may come with a skull fracture. A stroke may cause enough swelling to compress the ventricles and shift the midline. Running through all five steps every time takes under a minute once you’re practiced, and it catches the findings you’d otherwise miss.
Start by scrolling through the entire scan from bottom to top, getting an overall impression. Then go back through systematically: scan for anything bright white that shouldn’t be there (blood), check that the dark spaces at the base of the brain are open (cisterns), compare both sides of the brain for symmetry and preserved gray-white contrast (brain), confirm the ventricles are normal in size and centered (ventricles), then switch to bone windows and trace the skull looking for sharp lines with soft tissue swelling (bone). Each pass builds on the last, and the pattern becomes second nature with repetition.