CT Physics
CT imaging uses X-ray beams and a computer to build cross-sectional images of the body. Each slice represents a specific level of anatomy, with slice thickness chosen to minimize scatter radiation and overlapping structures, using devices called collimators to shape the beam.
How the image is built
CT data is divided into a grid of pixels, with each pixel representing a small detail within the image. Structures appear in varying shades of gray based on how much they attenuate, or weaken, the X-ray beam as it passes through.
- The linear attenuation coefficient quantifies how X-ray photons interact with matter, and varies with a material's density, atomic number, and the beam's energy.
- Higher density and atomic number cause more attenuation, which is what drives image contrast.
- Denser materials appear white on a CT image; less dense materials appear black.
- Contrast agents, such as iodine, temporarily change a structure's density to make it stand out more clearly.
Attenuation is quantified using Hounsfield units (HU), which help characterize different tissue types, though HU values can be slightly inaccurate due to various factors. Because CT beams contain a mix of X-ray energies ("polychromatic"), low-energy photons tend to get absorbed preferentially, which can create an artifact called beam hardening. Filtering the beam with a material like aluminum reduces this artifact, improves image quality, and lowers the patient's radiation dose.
Slice thickness is chosen based on the anatomy and suspected pathology. Thinner slices are better at catching small details and reduce volume averaging, where normal and abnormal tissue blend together within a single slice.
How the image is captured
X-ray photons are generated in a tube mounted on the gantry, with voltage and tube current controlling their intensity. Detectors convert the transmitted X-rays into an electric current, which the data acquisition system (DAS) processes and the central processing unit (CPU) converts into an image, typically expressed in Hounsfield units and displayed through pixel values.
The three imaging phases
| Phase | What happens |
|---|---|
| Data acquisition | Raw X-ray data is collected as the beam passes through the body |
| Image reconstruction | The collected data is processed and assigned pixel values |
| Image display | The processed data is shown as shades of gray for viewing |
Detectors
CT scanners use detectors — either single elements or arrays of many — along with reference detectors that help with calibration and reducing artifacts. The size of the fan-shaped X-ray beam and the number of active detector elements depend on the scan's selected field of view. An ideal detector captures transmitted photons efficiently, has minimal afterglow (lingering scintillation after exposure), suppresses scattered radiation effectively, and stays stable without needing frequent recalibration.
Overall detector performance depends on several combined factors:
- Stopping power of the detector material
- Scintillator efficiency, for solid-state detectors
- Charge collection efficiency, for xenon-based detectors
- Geometric efficiency — the ratio of collimator plate area to detector surface area
- Scatter rejection — how well scattered radiation is filtered out
- Capture efficiency — the ability to acquire photons
- Absorption efficiency — the proportion of photons actually absorbed
- Response time — how quickly the signal returns to zero after X-ray exposure
- Dynamic range — the ratio between the strongest and weakest measurable signals
Scanner generations and detector types
Most modern CT scanners use solid-state crystal detectors; older systems sometimes used xenon gas detectors, though these have become less common due to limitations in multidetector-row CT systems.
| Design | How it works | Trade-off |
|---|---|---|
| Third-generation | A detector array and X-ray tube rotate together, generating a fan-shaped beam without needing to translate the beam or detector | Faster scans with fewer motion artifacts and better image quality, but can introduce ring artifacts |
| Fourth-generation | A fixed detector array sits within the gantry while only the tube rotates | More detectors, but more prone to motion artifacts; addressed with over-scans, which increase patient radiation exposure |
| Electron beam CT (EBCT) | Uses a fixed electron gun and anode target instead of a rotating tube | Very fast, but limited clinical use due to spatial resolution, cost, and insurance reimbursement challenges; largely superseded by newer multidetector-row technology |
Data acquisition and reconstruction
The gantry's data acquisition system (DAS) converts the analog signals from detectors into digital signals. As continuous X-rays generate rays read by the DAS, the system correlates each ray's attenuation with its position, building a profile for each view. These profiles are projected onto a matrix, which can produce streak artifacts, so the data is run through filtered mathematical operations to reduce them.
A newer approach, iterative reconstruction, builds the image by repeatedly comparing computed projections against the original data, refining the result each pass. This can reduce image noise and lower radiation dose by up to 50%.
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