# CT scanning

> CIE A-Level Physics · 9702
> Source: https://www.owlsprep.com/study/cie-9702-u28-ct-scanning/

This sub-topic covers the core principles of computed tomography (CT) scanning, how 3D body images are constructed from multiple X-ray measurements, and compares CT scanning to conventional planar X-ray imaging for clinical diagnosis.

**Prerequisites:** [X-ray production and attenuation](https://www.owlsprep.com/study/cie-9702-u28-x-ray-production-attenuation/)

## Learning objectives

- Describe the basic operating principles of CT scanning
- Explain how CT images are constructed from X-ray data
- Compare CT scanning with conventional planar X-ray imaging
- Evaluate advantages and disadvantages of CT for clinical use

## Basic Principles of CT Scanning

**Computed Tomography (CT) Scanning** — A medical imaging technique that uses hundreds of narrow X-ray beams taken from different angles around the body to produce detailed cross-sectional images of internal structures.

*Example:* CT is commonly used to detect and stage tumours, image brain injuries, and plan radiotherapy treatment.

Unlike conventional X-ray imaging that produces a single 2D projection of all tissues along the X-ray path, CT scanning isolates individual thin slices of the body to eliminate overlap of structures. The X-ray source and detector array rotate around the patient as they move slowly through the scanner gantry, collecting absorption data from every angle.

**Worked example:** Explain why a conventional X-ray cannot easily distinguish a small chest tumour from overlapping healthy tissue, while a CT scan can.

1. A conventional X-ray is a projection image: all attenuation from tissues along the entire X-ray path is summed onto a single pixel.
2. Overlapping healthy tissue and a small tumour combine their attenuation, so the difference between them is too small to detect.
3. CT scanning produces a cross-sectional slice of the chest, so only tissues within that thin slice contribute to each pixel in the final image.
4. No overlap from structures above or below the slice occurs, so small attenuation differences between the tumour and healthy tissue are clearly visible.

> **tip**
>
> Always mention elimination of overlapping structures when comparing CT to conventional X-rays — this is a common 1-mark exam point.

## CT Image Reconstruction

Rotation of the X-ray source and detector collects thousands of individual attenuation measurements for each patient slice. Each measurement gives the total attenuation along one X-ray beam path. Algorithms combine all these measurements to calculate the attenuation coefficient for every small volume element (voxel) in the slice.

**Voxel** — A 3D volume pixel, the smallest unit of a CT scan, representing the attenuation of a small cube of tissue in the scanned slice.

Each voxel's attenuation coefficient is converted to a value on the Hounsfield scale, defined as:

$$\text{HU} = 1000 \times \frac{\mu - \mu_{\text{water}}}{\mu_{\text{water}}}$$

On this scale, water is 0 HU, air is -1000 HU, and dense bone is ~+1000 HU. Values are mapped to greyscale to produce the final 2D cross-section, and multiple slices can be combined to make a 3D reconstruction.

**Worked example:** A CT voxel has an attenuation coefficient of 0.21 cm⁻¹. Water has an attenuation coefficient of 0.19 cm⁻¹ at the X-ray energy used. Calculate the Hounsfield unit value for the voxel.

1. Write down known values:
2. $$\mu = 0.21 \text{ cm}^{-1}, \mu_{\text{water}} = 0.19 \text{ cm}^{-1}$$
3. Substitute into the Hounsfield unit formula:
4. $$\text{HU} = 1000 \times \frac{0.21 - 0.19}{0.19}$$
5. Calculate the final result:
6. $$\text{HU} \approx 105$$
7. This value falls in the expected range for soft tissue (-100 HU to +200 HU).

## Advantages and Disadvantages

CT scanning has significant clinical benefits compared to conventional X-ray, but also has important drawbacks:

- Much higher soft-tissue contrast than conventional X-rays, allowing detection of small early-stage tumours.
- Eliminates overlap of structures, so lesions are not hidden by surrounding tissue.
- Can produce 3D images from multiple slices for accurate surgical and radiotherapy planning.
- Much higher radiation dose: a full-body CT delivers ~10 mSv, compared to ~0.02 mSv for a chest X-ray.
- More expensive and slower to perform than conventional X-rays.
- Requires contrast dyes for some scans that carry a small risk of allergic reaction.

**Check your understanding**

Test your understanding:

1. Which of the following is a major advantage of CT scanning over conventional X-ray imaging?

   - A: Lower radiation dose per scan
   - B: No overlap of internal structures
   - C: Cheaper to perform
   - D: Faster scan time

   *Why:* Correct. CT eliminates overlap of structures, but it has higher radiation dose, higher cost, and longer scan times than conventional X-rays.

## Common pitfalls

- **Wrong:** Claiming CT scanning uses gamma rays instead of X-rays
  - Why it fails: Students confuse CT with PET scanning, which does use gamma rays from radiotracers
  - Correct: CT scanning uses X-ray beams from an X-ray source rotating around the patient
- **Wrong:** Confusing voxels with pixels in CT imaging
  - Why it fails: Students mix up 2D image units and 3D body volume units
  - Correct: A pixel is the 2D unit in the displayed image; a voxel is the 3D volume element in the patient's body
- **Wrong:** Claiming CT scanning gives a lower radiation dose than conventional X-rays
  - Why it fails: Students assume newer technology means lower dose, but CT uses many more X-ray measurements
  - Correct: CT scanning gives a significantly higher effective radiation dose than most conventional X-ray scans
- **Wrong:** Forgetting CT produces cross-sectional images rather than a single projection
  - Why it fails: Students remember CT produces 3D images but forget the core difference from conventional X-ray
  - Correct: The key difference between CT and conventional X-ray is that CT produces thin cross-sectional slices, rather than one projection of all tissues along the beam path

## Cheatsheet

| Feature | CT Scanning | Conventional X-ray |
| --- | --- | --- |
| Image type | Cross-sectional slices / 3D | 2D projection |
| Structure overlap | No overlap | Overlap occurs |
| Soft tissue contrast | High | Low |
| Typical effective dose | 1–10 mSv | < 0.1 mSv |
| Cost | Higher | Lower |

## What's next

CT scanning builds on the core concepts of X-ray production and attenuation that underpin all X-ray-based medical imaging. For CIE A-Level exams, you are often required to compare CT scanning with other diagnostic imaging techniques, so understanding its core properties is key to scoring full marks on comparison questions. Next, explore the other major imaging techniques covered in the medical imaging unit to build your ability to compare and contrast different methods for exam questions.

- [Ultrasound Imaging](https://www.owlsprep.com/study/cie-9702-u28-ultrasound-imaging/)
- [Nuclear medicine imaging](https://www.owlsprep.com/study/cie-9702-u28-nuclear-medicine-imaging/)
- [Astronomy and cosmology](https://www.owlsprep.com/study/cie-9702-u29-overview/)

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