CT Scanner Principles Revision Notes | A Level AQA Physics

1. Why CT Scanning Changed Medicine

A conventional X-ray collapses a three-dimensional body into a flat shadow — useful, but limited. Overlapping structures blur together, and soft tissues with similar densities become almost indistinguishable. CT scanning was developed to solve exactly this problem.

The key insight behind CT is this: if you collect X-ray transmission data from many different angles around the body, a computer can mathematically reconstruct what the internal cross-section must look like. The result is a detailed slice-by-slice map of the body's interior — something a single projection X-ray can never provide.

Every component of a CT scanner — the rotating tube, the narrow beam, the detector array, and the reconstruction software — exists to make that mathematical reconstruction as accurate as possible. Understanding why each component is designed the way it is will give you the depth of explanation that AQA mark schemes reward.

2. The X-ray Tube and Its Rotation

Picture the patient lying inside a large ring — the gantry. Mounted on the inside of that ring, directly opposite a curved bank of detectors, is the X-ray tube. The tube and detector array rotate together around the patient, completing a full 360° sweep (or sometimes 180° depending on the system design).

At each angular position during the rotation, the tube fires a narrow X-ray beam through the patient's body. The detectors on the opposite side record how much of that beam has been transmitted — in other words, how much has been attenuated by the tissues it passed through. This is repeated at hundreds of different angles as the tube sweeps around.

Modern CT scanners also move the patient table slowly through the gantry during rotation, producing a helical (spiral) path of data collection. This allows a continuous volume of the body to be imaged rather than a single isolated slice, but the fundamental principle — rotating tube, rotating detectors, multiple angular measurements — remains the same.

3. The Narrow, Monochromatic X-ray Beam

Two features of the X-ray beam in a CT scanner are critical: it is narrow, and it is monochromatic. Each feature serves a distinct purpose, and examiners frequently ask you to explain both.

The beam is kept narrow — collimated to a thin fan or pencil shape — so that it illuminates only a well-defined slice through the patient at any one moment. A broad, diverging beam would irradiate tissue above and below the plane of interest, increasing the patient's radiation dose without contributing useful image data. The narrow beam also reduces scattered radiation reaching the detectors, which would otherwise degrade image quality.

The beam is monochromatic — meaning it contains X-ray photons of a single, well-defined energy rather than a broad spectrum. This matters because the degree to which a tissue attenuates X-rays depends on the photon energy. If the beam contains a mixture of energies, lower-energy photons are preferentially absorbed as the beam passes through the body (a phenomenon called beam hardening), and the effective energy of the beam changes with depth. This makes it impossible to assign a consistent attenuation value to each tissue type. A monochromatic beam eliminates this problem: every photon has the same energy, so the attenuation coefficient of each tissue remains constant and well-defined throughout the measurement.

In practice, producing a truly monochromatic beam is not straightforward. Clinical CT scanners use metal filters (typically aluminium or copper) to absorb lower-energy photons, narrowing the spectrum to produce an approximately monochromatic beam. This is sufficient for consistent attenuation measurements in clinical use, but the beam retains a small spread of energies. Genuinely monochromatic beams require synchrotron sources, which are used in research settings but are not found in clinical scanners. The result of filtration is a beam whose energy spread is narrow enough that attenuation measurements remain consistent across all angular positions of the rotating tube.

4. The Array of Detectors

Directly opposite the X-ray tube, curving around the far side of the patient, sits the detector array. Rather than a single detector, this is a bank of many individual detector elements — often hundreds — arranged in a curved row (or multiple rows in modern multi-slice scanners).

Each detector element records the intensity of the X-ray beam that has passed through the patient along a specific path. Because the beam fans out slightly from the tube, each detector element samples a slightly different ray through the cross-section. Together, the full array captures a complete projection — a set of intensity measurements across the entire width of the patient — at each angular position of the rotating tube.

The detectors convert the incoming X-ray photons into electrical signals. The magnitude of each signal is proportional to the transmitted beam intensity at that detector's position. A high signal means little attenuation (the beam passed through low-density tissue such as air or fat); a low signal means high attenuation (the beam passed through dense tissue such as bone or a tumour). These electrical signals are then passed to the computer for processing.

5. Computer Processing and Image Reconstruction

The detector signals on their own are not images — they are a large set of attenuation measurements, each corresponding to a specific ray path through the patient at a specific angle. Turning this raw data into a recognisable cross-sectional image is the job of the computer, and the mathematical technique used is called filtered back-projection (or more generally, tomographic reconstruction).

Here is the core idea. Imagine shining a torch through a frosted glass box and recording the shadow on the far wall. One shadow tells you very little about what is inside the box. But if you rotate the torch and record shadows from hundreds of different angles, you accumulate enough information to work backwards and deduce the internal structure. The computer does exactly this, using the attenuation data from every angular position to calculate the attenuation coefficient at each tiny volume element (voxel) within the slice.

Each voxel is assigned a CT number (also called a Hounsfield unit, HU), which quantifies its attenuation relative to water. Water is defined as 0 HU; air is approximately −1000 HU; dense bone is approximately +1000 HU; soft tissues fall in between. The computer maps these CT numbers onto a greyscale, producing the familiar cross-sectional image where bright regions represent dense, highly attenuating structures and dark regions represent low-density structures.

Because the computer reconstructs the image mathematically from multiple projections, it can also apply image processing techniques — such as windowing, where the displayed greyscale range is adjusted to highlight specific tissue types — and generate three-dimensional renderings by stacking multiple adjacent slices. This computational flexibility is one of the key advantages of CT over conventional X-ray imaging.

Worked example: Tracing the CT Imaging Process from Tube to Image

Describe, in sequence, how a CT scanner produces a cross-sectional image of a patient's abdomen. [6 marks]

1. The X-ray tube emits a collimated, narrow fan-beam of monochromatic X-rays directed through the patient's abdomen at the initial angular position (e.g. 0°). — [B1] Narrow beam reduces dose and scatter; monochromatic beam ensures consistent attenuation coefficients. Both features must be stated for full credit.
2. As the beam passes through the abdomen, different tissues (fat, muscle, organs, bone) absorb and scatter X-ray photons to different extents. Dense structures attenuate more; low-density structures attenuate less. — [B1] Attenuation varies by tissue density and atomic number — this is the physical basis of contrast in the image.
3. The curved detector array on the opposite side of the patient records the transmitted beam intensity at each detector element. Each element produces an electrical signal proportional to the transmitted intensity. — [B1] The array captures a complete projection across the full width of the abdomen simultaneously at this angle.
4. The X-ray tube and detector array rotate together around the patient (through 360°). At each new angular position, the beam fires and the detectors record a new projection. Hundreds of projections are collected in total. — [B1] Multiple angular projections are essential — the computer needs data from many directions to reconstruct the internal structure unambiguously.
5. The computer applies a reconstruction algorithm (filtered back-projection) to the full set of projection data. For each voxel in the cross-section, it calculates the attenuation coefficient and converts this to a CT number (Hounsfield unit). — [B1] The mathematical reconstruction is what distinguishes CT from a conventional X-ray — it recovers three-dimensional structural information from two-dimensional projections.
6. Each CT number is assigned a greyscale value: high HU (bone, ~+1000 HU) appears bright white; low HU (air, ~−1000 HU) appears black; soft tissues appear in intermediate grey shades. The result is a cross-sectional image displayed on screen. — [B1] The final image can be windowed by the radiologist to emphasise specific tissue types — a further advantage of computer-based reconstruction.

Answer: A 6-mark answer should cover: narrow monochromatic beam from rotating tube; attenuation by different tissues; detector array recording transmitted intensities at each angle; rotation through 360° collecting multiple projections; computer reconstruction (filtered back-projection) calculating attenuation per voxel; CT numbers mapped to greyscale cross-sectional image.

6. Comparing CT with Conventional X-ray Imaging

Both CT and conventional X-ray imaging use the same underlying physics — X-ray attenuation by tissue — but they differ fundamentally in what information they extract and how they present it. Understanding this comparison is a common AQA exam demand.

The higher radiation dose of CT is a genuine clinical concern. Because the tube rotates and fires repeatedly, the patient receives significantly more ionising radiation than from a single X-ray. This is why CT is used when the diagnostic benefit clearly outweighs the risk — for example, when investigating a suspected tumour or internal haemorrhage — rather than as a routine first-line investigation.

7. Exam Technique and Mark-Winning Strategies

CT scanner questions appear regularly in the medical physics section of AQA A Level papers. The marks are available across several command words — describe, explain, compare, and evaluate — and each demands a different response structure. Here is how to approach each one precisely.

For describe questions about CT scanning, work through the process in sequence: tube fires narrow monochromatic beam → beam attenuated by tissues → detector array records transmitted intensities → tube and array rotate → process repeats at each angle → computer reconstructs image. Sequence and completeness earn the marks; you do not need to explain why at this stage.

For explain questions, the word "because" is your best tool. "The beam is monochromatic because a single-energy beam ensures each tissue has a consistent attenuation coefficient, making the reconstruction mathematically reliable." Each mark point needs a cause linked to a consequence.

For evaluate questions about CT versus conventional X-ray, structure your answer as: advantage of CT → evidence → limitation of CT → evidence → conclusion with judgement. A common trap is listing only advantages and forgetting to address the higher radiation dose — examiners specifically look for balanced treatment.

8. Bringing It All Together

Every component of a CT scanner connects back to the same goal: collecting enough angular information about X-ray attenuation within the body that a computer can reconstruct a faithful cross-sectional image. The rotating tube provides the multiple viewpoints; the narrow beam defines the slice and minimises scatter; the monochromatic beam ensures attenuation measurements are consistent; the detector array converts transmitted intensities into electrical signals; and the computer stitches all of this together into a clinically useful image.

When you encounter a CT question in the exam, ask yourself which component is being targeted. If the question asks about image quality or reconstruction accuracy, the answer almost always involves the monochromatic beam or the computer processing. If it asks about how different tissue types are distinguished, the answer involves attenuation coefficients and CT numbers. If it asks about the mechanical design, the answer involves the rotating tube and detector array.