Explain the working principle and applications of CT scan.

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Accepted Answer

## CT Scan: Principle and Applications

**Computed Tomography** uses X-rays and computer reconstruction to produce cross-sectional (axial) images of the body.

### Principle
A thin fan-shaped X-ray beam passes through the patient while the X-ray tube and detector array rotate around the patient. Different tissues attenuate the beam by different amounts: $I = I_0 e^{-\mu x}$. Detectors record many projections, and the computer reconstructs the image (filtered back projection or iterative reconstruction).

```mermaid
flowchart LR
  A[X-ray tube rotates] --> B[Beam through patient]
  B --> C[Detector array measures attenuation]
  C --> D[Data acquisition system]
  D --> E[Reconstruction computer]
  E --> F[Cross-sectional image]
```

**CT number (Hounsfield unit):** $HU = 1000 	imes (\mu - \mu_{water})/\mu_{water}$. Water = 0, air = -1000, bone > +400.

### Components
Gantry (tube, detectors, slip ring), patient table, high-voltage generator, DAS, computer and console.

### Generations
Spiral/helical CT with multi-slice detectors scans the volume quickly in one breath-hold.

### Applications
| Area | Use |
|---|---|
| Head | Stroke, haemorrhage, tumours |
| Chest | Lung nodules, pulmonary embolism |
| Abdomen | Tumours, stones, trauma |
| Bone | Complex fractures |
| Cardiac | Coronary calcium, CT angiography |
| Radiotherapy | Treatment planning, image-guided biopsy |

### Limitations
Higher radiation dose, contrast reactions, cost.

### Exam tip
Write attenuation law, HU scale, the block diagram and four applications.

CT Scan: Principle and Applications

Principle

Components

Generations

Applications

Limitations

Exam tip

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