A CT scanner is a large, donut-shaped machine with an opening in the center, and the images it produces are detailed black-and-white cross-sections of your body that look like slices taken through your anatomy. If you searched this question, you’re probably curious about one or both of those things: what you’ll see walking into the scan room, and what the actual images look like on screen.
The Machine Itself
The most prominent part of a CT scanner is the gantry, a large ring (often white or beige plastic) with a circular opening in the center called the bore. The bore is typically around 70 centimeters wide, roughly 27 inches, which is large enough for most adults to fit through comfortably. Attached to the gantry is a flat, motorized table that slides you in and out of the opening.
Inside the gantry, hidden behind the plastic housing, an X-ray tube and a detector array spin rapidly around you. You never see these components. From your perspective, you’re simply lying on a table that glides smoothly into a relatively short tunnel. Unlike an MRI machine, which encloses you in a long tube, a CT scanner’s ring is only about two feet deep. Most people find it far less claustrophobic.
The room also has a control station, usually behind a glass window in an adjacent area. A technologist operates the scanner from there, communicating with you through a speaker. The whole appointment usually takes about an hour including preparation, but the actual scanning portion is often under five minutes.
What the Images Look Like
CT images appear as grayscale pictures, where different shades of white, gray, and black represent different types of tissue. The brightness of each pixel is determined by how much the tissue absorbs X-rays. Dense materials like bone appear bright white. Air (in your lungs, for example) appears black. Soft tissues like muscle and organs fall somewhere in the middle as various shades of gray.
Radiologists use a standardized brightness scale called Hounsfield units to interpret these shades. Water sits at zero on this scale. Air is at negative 1,000 (the darkest). Dense bone can reach positive 1,000 to 3,000 (the brightest). Fat registers around negative 60, muscle around 40, and brain tissue falls between 30 and 40. These precise values help doctors distinguish between tissues that might look similar to the naked eye.
The most common view is the axial plane, which shows your body as if someone sliced through you horizontally, like cutting a loaf of bread. If you looked at an axial image of your abdomen, you’d see a roughly oval cross-section with your spine at the back (bright white), organs in the middle (gray), and a rim of fat and skin around the outside (darker gray). Modern scanners also reconstruct images in two other planes: sagittal (a side view, as if splitting you left from right) and coronal (a front-to-back view, as if splitting you into front and back halves). Viewing all three planes helps doctors assess the full shape and extent of anything abnormal.
How Contrast Changes the Picture
Some CT scans use contrast agents to make certain structures stand out more clearly. You might drink a liquid before the scan that coats and highlights your stomach, small intestines, and bowel. Or a technologist might inject an iodine-based dye into a vein, which travels through your bloodstream and makes blood vessels and highly vascular organs (like the liver and kidneys) appear much brighter on the images.
On a contrast-enhanced scan, blood vessels that would normally blend into surrounding tissue suddenly pop as bright white lines and circles. Tumors, which often have their own blood supply, can light up differently than the tissue around them. Areas of inflammation or infection also tend to absorb contrast in distinctive ways, making them easier to spot against normal tissue.
How Doctors Read What They See
Radiologists look for breaks in the expected pattern. A fracture shows up as a dark line through bright white bone. Fluid collections, like blood pooling in the brain after an injury, appear brighter than surrounding brain tissue because blood is denser than normal neural tissue. A kidney stone stands out as a small, intensely bright spot because calcium is highly dense.
Tumors can appear as masses that are lighter or darker than the tissue they’re sitting in, sometimes with irregular edges. Bleeding within the brain or abdomen shows up as a bright (hyperdense) area on a non-contrast scan because clotted blood is denser than the tissue around it. Swelling or fluid buildup, on the other hand, tends to appear darker (closer to water’s density) than normal tissue.
Doctors can also adjust the “window” settings on screen to optimize visibility for specific tissues. A lung window makes the air-filled lung tissue and its fine structures visible by stretching out the darker end of the grayscale. A bone window cranks up the contrast at the bright end so fractures and bone lesions become obvious. The underlying data is the same; the display is just tuned differently, similar to adjusting brightness and contrast on a photograph.
3D Reconstructions
Because a CT scan captures hundreds of thin slices (sometimes less than a millimeter thick), software can stack those slices together to build three-dimensional models. These reconstructions look dramatically different from the flat grayscale slices. A 3D bone reconstruction of a skull, for example, looks like a photograph of an actual skull that you can rotate on screen. Surgeons use these models to plan operations, and vascular specialists use them to map out blood vessel anatomy before procedures.
Some reconstructions strip away everything except a single tissue type. A CT angiogram, for instance, can isolate blood vessels and display them as a branching tree structure, color-coded to show narrowing or blockages. These images are visually striking and look nothing like the standard grayscale slices most people picture when they think of a CT scan.
Newer Scanners, Sharper Images
The latest generation of CT scanners uses photon-counting detectors instead of the traditional technology. These newer detectors measure the energy of each individual X-ray photon, which eliminates electronic noise and improves spatial resolution. The practical result is sharper, cleaner images at a lower radiation dose. Photon-counting scanners can also produce color-coded images that distinguish between different materials (like calcium, iodine, and soft tissue) in a single scan, adding a layer of visual information that traditional grayscale images can’t provide.