A computed tomography scanner, or CT scanner, uses X-rays to make detailed images of the inside of the body. Unlike a single flat X-ray image, a CT scan combines many measurements taken from different angles. This lets doctors see thin cross sections of organs, bones, blood vessels, and injuries.
CT matters because it can quickly reveal problems that may be hidden in ordinary exams or simple X-rays.
Inside the circular gantry, an X-ray tube rotates around the patient while a detector array measures how much radiation passes through each path. Dense materials such as bone absorb more X-rays, while air and soft tissue absorb less, so each projection carries information about internal structure. A computer uses reconstruction algorithms to turn the many angled projections into slices, then stacks slices to form a 3D view.
The patient table moves through the gantry so the scanner can image a volume of the body.
Key Facts
- CT stands for computed tomography, meaning computer-made slice imaging.
- X-ray attenuation follows I = I0e^(-mu x), where mu is the attenuation coefficient and x is thickness.
- A CT scanner collects many projections as the X-ray tube rotates around the patient.
- Each reconstructed slice represents a thin cross section of the body.
- CT number in Hounsfield units is HU = 1000(mu_tissue - mu_water) / mu_water.
- More projections usually improve image detail, but scan settings must balance image quality and radiation dose.
Vocabulary
- Gantry
- The circular housing of a CT scanner that contains the rotating X-ray tube, detector array, and support systems.
- X-ray tube
- The device that produces X-rays by accelerating electrons into a metal target.
- Detector array
- A curved set of sensors that measures the X-rays that pass through the patient from many angles.
- Projection
- A set of detector measurements taken from one viewing angle during a CT scan.
- Reconstruction
- The computer process that converts many X-ray projections into cross-sectional images.
Common Mistakes to Avoid
- Thinking a CT scanner takes one picture, because it actually collects many projections from many angles and reconstructs them into slices.
- Confusing CT with MRI, because CT uses ionizing X-rays while MRI uses magnetic fields and radio waves.
- Assuming brighter always means denser, because CT image brightness depends on the display window and level chosen by the operator.
- Forgetting that the patient table moves during many scans, because table motion lets the scanner build images across a volume rather than just one thin circle.
Practice Questions
- 1 A CT tube completes 2 rotations per second. How long does it take to complete 10 rotations?
- 2 An X-ray beam has initial intensity I0 = 100 units. After passing through tissue, the detector measures I = 25 units. What fraction of the original intensity reached the detector, and what percent was attenuated?
- 3 Explain why a CT scanner needs measurements from many angles instead of only one front-facing X-ray image to create a cross-sectional slice.