CT Outputs
A CT scan builds its image from a series of X-rays taken from many angles around the body. As the scanner emits X-rays toward the patient, detectors on the opposite side measure how much of that radiation is absorbed versus transmitted through the body — a process called attenuation. The degree of attenuation depends on how dense the imaged tissue is, and each tissue is assigned a value called a Hounsfield Unit, or CT number.
Why bone looks white and soft tissue looks darker
- Dense tissue, like bone, absorbs radiation strongly, so less makes it through to the detector on the other side — this shows up as white or light gray on the image.
- Less dense tissue, like the lungs, absorbs less radiation, so more of the signal reaches the detector — this shows up darker.
- Because CT relies on ionizing radiation and density differences, structures like the brain, blood vessels, and digestive organs don't show up with as much natural contrast as bone does.
From slices to a 3D image
Unlike a conventional X-ray, which produces a single two-dimensional image and requires the patient to be repositioned to capture a different angle, CT collects projections from many angles automatically. A single scan can produce anywhere from a handful up to several hundred individual image slices. Through a mathematical process called reconstruction, those slices are stacked and combined into a three-dimensional dataset that can be viewed, rotated, and examined from any plane on a monitor. This reconstructed data is never a perfect replica of the body being scanned, but it's accurate enough to serve as a reliable basis for diagnosis.
Common types of CT exams
| Exam | What it's used for |
|---|---|
| Calcium score test | Assesses coronary artery risk for a future heart attack |
| Chest CT | Investigates chest pain or coughing; spots lung nodules or early-stage cancers |
| CT angiography | Uses contrast material to examine the arteries and veins supplying the heart, brain, kidneys, and other organs |
| CT-guided biopsy | Guides a needle precisely to sample abnormal tissue in the lungs, lymph nodes, or other organs |
| CT enterography | Tailored to assess the small bowel |
| Heart CT | Uses contrast dye to detail heart valves, chambers, and vessels like the aorta and pulmonary veins |
| Virtual colonoscopy (CT colonography) | Examines the colon using X-rays from outside the body, without a scope |
| PET/CT | Combines CT with PET to pinpoint molecular activity, helping identify disease |
Example: what an abdominal CT can reveal
An abdominal CT uses X-rays to capture detailed images of the stomach, intestines, liver, pancreas, kidneys, bladder, and other abdominal organs from many angles. It's commonly used to identify:
- Abscesses
- Appendicitis
- Bile duct blockages
- Cholecystitis (gallbladder irritation)
- Crohn's disease
- Hernias
- Injuries to abdominal organs
- Kidney problems, including stones, blockages, swelling, and damage
- Masses and tumors, including cancers of the colon, kidneys, pancreas, and ovaries
- Pancreatitis
- Ulcerative colitis