CT scanner (A-level only) — AQA A-Level Physics

A CT (computed tomography) scanner is an advanced medical imaging technique that uses X-rays to produce detailed cross-sectional images of the body. Unlike a standard X-ray, which gives a 2D projection, a CT scanner rotates an X-ray source and detector array around the patient, capturing multiple projections from different angles. A computer then reconstructs these projections into a 3D image, allowing doctors to visualise internal structures with high precision.

This topic is part of the AQA A-Level Physics specification under Medical Physics. It builds on your understanding of X-ray production, attenuation, and the interaction of radiation with matter. Understanding CT scanners is crucial because it combines physics principles with real-world medical applications, and it often appears in exam questions that test your ability to explain the process and perform calculations involving attenuation coefficients.

• →CT scanners use X-rays because their attenuation depends on tissue density and atomic number, allowing differentiation between bone, soft tissue, and air.
• →The X-ray source and detector array rotate around the patient, taking multiple projections from different angles.
• →The intensity of transmitted X-rays is measured by detectors and follows the equation I = I0 e^(-mu x), where mu is the linear attenuation coefficient.
• →A computer reconstructs cross-sectional images by calculating attenuation coefficients for each voxel, and multiple slices form a 3D image.
• →CT scans provide higher contrast and detail than plain X-rays but involve a higher radiation dose to the patient.

• Describes a narrow, monochromatic X-ray beam produced by an X-ray tube and collimated to pass through a thin slice of the patient.
• Explains that the X-ray tube and the detector array move around the patient, and that the beam is translated across the slice, so attenuation is measured along many different paths.
• States that detectors measure the transmitted intensity and that comparison with the incident intensity gives the attenuation along each beam path.
• Explains that a computer processes the many attenuation readings and reconstructs a cross-sectional visual image of the slice.
• Relates image contrast to differences in attenuation between tissues, for example bone attenuating more strongly than soft tissue.
• Recognises that using many directions avoids the ambiguity of a single projection, where overlapping tissues could give the same total attenuation.
• Identifies image resolution as the ability to distinguish two close objects or fine detail, and links it to the imaging modality's wavelength or physical interaction mechanism.
• Explains that CT uses X-rays and so involves ionising radiation, giving a patient dose that must be justified against clinical benefit.
• Compares cost in terms of equipment purchase, installation shielding, running and maintenance, and staff training, not just the scanner price.
• Discusses safety for both patient and operator, including dose, exposure time, need for contrast agents and any non-ionising alternatives.
• Reaches a justified conclusion for a stated context, such as trauma, soft-tissue injury or pregnancy, rather than listing generic points.
• Uses correct terminology such as spatial resolution, ionising radiation, absorbed dose and contrast, avoiding vague words like 'better' or 'clearer'.

• 💡Use the sequence tube, collimation, transmission, detection, computer reconstruction, image when describing how a CT scan is produced.
• 💡Name the components explicitly, such as X-ray tube, collimators, detector array and computer, to show precise knowledge.
• 💡Explain why many directions are needed, linking the number of attenuation measurements to the ability to reconstruct a cross-section.
• 💡Keep the description of the beam consistent: narrow and monochromatic, with attenuation measured along each path through the slice.
• 💡Read the context in the question and tailor the comparison to that patient or scenario rather than writing a generic list.
• 💡Structure each paragraph around one criterion, giving an advantage and a disadvantage for each technique before concluding.
• 💡Use comparative language such as 'higher resolution than', 'lower dose than' and 'more expensive to install than' to make the comparison explicit.
• 💡Always use the correct terminology: 'attenuation', 'transmitted intensity', 'detector array', 'cross-sectional image', 'voxel'.
• 💡When explaining the process, structure your answer logically: rotation, multiple projections, measurement, computer reconstruction, 3D image.
• 💡For calculations, remember to convert units if necessary (e.g., cm to m) and use the exponential attenuation equation correctly.

• Saying the patient moves through the scanner while the tube and detectors stay fixed; in the basic description the tube and detector array rotate around the patient.
• Describing a broad white beam rather than a narrow, monochromatic beam; the narrow beam defines the slice and monochromatic photons make attenuation depend on path length and material.
• Claiming the computer takes a single photograph; the computer reconstructs an image from many separate attenuation measurements taken along different paths.
• Confusing CT with a simple X-ray radiograph, where overlapping structures are superimposed rather than separated into a cross-sectional image.
• Treating resolution as brightness or contrast: correct this by defining resolution as the smallest separation at which two points remain distinguishable.
• Claiming CT is always safest because it is quick: correct this by noting the ionising dose and the need for justification, especially in pregnancy or repeated scans.
• Comparing only purchase price and ignoring running costs, shielding, maintenance and staffing: correct this by treating cost as a total-lifecycle comparison.
• Students often think CT scans use a radioactive tracer, confusing them with PET or gamma cameras. Correction: CT uses X-rays, not radioactive tracers.
• Students may believe that CT scans produce images directly from X-ray film. Correction: CT uses electronic detectors and computer reconstruction, not film.
• Students sometimes state that the patient moves through the scanner while the X-ray source is stationary. Correction: In most CT scanners, the X-ray source and detectors rotate around the patient, who lies still on a moving table.

1. 1Day 1-2: Review the basics of X-ray production and attenuation. Ensure you understand the equation I = I0 e^(-mu x) and can rearrange it.
2. 2Day 3-4: Learn the components of a CT scanner and how they work together. Draw a labelled diagram and annotate it with the function of each part.
3. 3Day 5-6: Practice explaining the image reconstruction process in your own words. Write a 6-mark answer and compare it to the mark scheme.
4. 4Day 7-8: Work through calculation questions involving attenuation coefficients and transmitted intensity. Check your units and significant figures.
5. 5Day 9-10: Complete past paper questions on CT scanners. Review examiner reports for common mistakes and refine your answers.

• 📋Explanation questions: Describe how a CT scanner produces an image, or explain the advantage of CT over plain X-ray. Advice: Use precise terminology and structure your answer in logical steps.
• 📋Calculation questions: Calculate transmitted intensity, attenuation coefficient, or ratio of intensities. Advice: Write down the equation, substitute values, and show all steps clearly.
• 📋Comparison questions: Compare CT with other imaging techniques like MRI or ultrasound. Advice: Focus on physical principles, image quality, and safety (radiation dose).
• 📋Diagram questions: Label a diagram of a CT scanner or describe the path of X-rays. Advice: Ensure you can identify the X-ray source, detectors, and patient position.