Comparing Imaging Techniques Revision Notes | A Level AQA Physics

1. Why Comparing Imaging Techniques Matters

Every day, clinicians face a decision: which imaging technique gives the most useful picture of what is happening inside a patient, with the least risk and the greatest practicality? Getting that decision wrong can mean a missed diagnosis, an unnecessary radiation dose, or a scan that takes so long the patient cannot tolerate it.

AQA asks you to compare imaging techniques across three specific dimensions: image resolution, convenience, and safety. These three criteria form the framework for every comparison question you will encounter, so building a precise, evidence-based understanding of each one is the goal of this lesson.

2. The Imaging Techniques You Need to Know

Before comparing techniques, you need a clear picture of what each one actually does. The main modalities covered at A-level are X-ray (including CT), MRI, ultrasound, and radionuclide imaging (including PET and gamma camera scintigraphy). Each exploits a different physical phenomenon to generate an image.

Notice that X-ray, CT, and radionuclide imaging all involve ionising radiation, while MRI and ultrasound do not. That distinction is central to the safety comparison. Keep it in mind as you work through the next sections.

3. Image Resolution — How Much Detail Can You See?

Resolution describes the smallest separation between two structures that the technique can distinguish. A high-resolution image shows fine anatomical detail; a low-resolution image blurs nearby structures together. Resolution is not fixed — it depends on the wavelength or frequency of the probe, the detector design, and the processing algorithm used.

X-ray and CT produce high spatial resolution because X-ray wavelengths are extremely short (around $10^{-10}$ m), allowing fine structural detail to be captured. CT improves on plain X-ray by reconstructing cross-sectional slices, giving three-dimensional anatomical information with sub-millimetre resolution in modern scanners.

MRI also achieves high spatial resolution — comparable to CT for soft tissue — and has a particular advantage: it distinguishes between different soft tissues far better than X-ray does, because different tissues contain different concentrations of hydrogen nuclei and have different relaxation times. This makes MRI the preferred choice for imaging the brain, spinal cord, and joints.

Ultrasound resolution depends on the frequency of the sound wave used. Higher frequency gives shorter wavelength and therefore better resolution, but the wave is absorbed more quickly, limiting penetration depth. A 10 MHz probe resolves structures around 0.15 mm apart but cannot image deep organs effectively. A 3.5 MHz probe reaches deeper but with coarser resolution. There is always a trade-off between resolution and penetration depth in ultrasound.

Radionuclide imaging — including gamma camera scintigraphy and PET — has the lowest spatial resolution of the common modalities, typically 4–10 mm for PET and coarser still for planar scintigraphy. This is because the image is built from detected gamma photons emitted in all directions from within the body; collimators and coincidence detection improve localisation, but the fundamental limit is the statistical nature of radioactive decay and the relatively large detector elements used.

The key insight here is that radionuclide imaging sacrifices spatial resolution in exchange for something X-ray and MRI cannot provide: functional information. A PET scan shows where glucose metabolism is highest, revealing tumour activity or brain function rather than just anatomy. Resolution and function are different dimensions of image quality, and examiners expect you to acknowledge both.

4. Convenience — Practicality in a Clinical Setting

Convenience covers everything that affects how practical a scan is to perform and tolerate. AQA groups several factors under this heading: scan duration, patient preparation required, whether the patient must remain still, equipment size and availability, cost, and whether the technique can be used at the bedside or requires a specialist facility.

Plain X-ray is the most convenient modality by almost every measure. The equipment is widely available, the scan takes seconds, no special preparation is needed, and the patient can be positioned in almost any orientation. This is why X-ray remains the first-line investigation for suspected fractures and chest pathology despite its relatively limited soft-tissue contrast.

Ultrasound is also highly convenient. The equipment is portable — it can be taken to the patient's bedside or used in an outpatient clinic — and there is no ionising radiation, so it can be repeated freely. The scan is performed in real time, which is particularly useful for guiding needle biopsies or assessing foetal development during pregnancy. The main limitation is operator dependence: image quality varies significantly with the skill of the sonographer.

CT is less convenient than plain X-ray. The scanner is large, fixed in a dedicated room, and the patient must lie still inside a narrow bore for several minutes. However, a full-body CT can be completed in under 10 minutes with modern multi-slice scanners, and no special preparation is usually required beyond removing metal objects. CT is available in most district general hospitals.

MRI is the least convenient of the anatomical techniques. Scan times are long — typically 20 to 60 minutes — and the patient must remain completely still inside a loud, narrow bore magnet. Patients with certain metallic implants (pacemakers, some surgical clips) cannot safely enter the magnetic field. The equipment is expensive and available only in larger hospitals. Claustrophobic patients may require sedation.

Radionuclide imaging presents a different kind of inconvenience. The patient must be injected with a radioactive tracer and then wait for it to distribute through the body before imaging begins — this uptake period can be 30 minutes to several hours depending on the tracer. The patient is also mildly radioactive during and after the scan, which requires careful management of contact with others, particularly pregnant women and young children. PET scanners are expensive and available only in specialist centres.

5. Safety — Weighing the Risks

Safety in medical imaging centres on two main concerns: the biological effects of ionising radiation, and any non-radiation hazards specific to the technique. You need to be precise about which techniques carry which risks, and why.

Ionising radiation — used in X-ray, CT, and radionuclide imaging — can cause DNA damage. Most damage is repaired by cellular mechanisms, but a small residual risk of inducing cancer remains. The risk is proportional to the dose received, quantified in millisieverts (mSv). A chest X-ray delivers approximately 0.02 mSv; a CT of the abdomen delivers around 10 mSv; a radionuclide bone scan delivers approximately 3–4 mSv. These doses are small compared with the natural background radiation of around 2.7 mSv per year in the UK, but they are not zero, and cumulative exposure across multiple scans must be considered.

Because of this risk, ionising radiation techniques follow the ALARA principle — doses should be kept As Low As Reasonably Achievable. Particular caution applies to children (whose dividing cells are more radiosensitive) and to pregnant patients (where the foetus is especially vulnerable). Radionuclide imaging adds a further consideration: the patient continues to emit radiation after the scan until the tracer has decayed or been excreted, requiring temporary precautions around vulnerable individuals.

MRI carries no ionising radiation risk. The strong magnetic field and radiofrequency pulses used are not known to cause biological harm at diagnostic field strengths. However, the magnetic field creates a different safety concern: ferromagnetic objects become dangerous projectiles near the scanner, and implanted devices such as pacemakers or cochlear implants may malfunction or heat up. Careful screening of patients before entering the MRI suite is therefore essential.

Ultrasound is generally considered the safest modality. It uses non-ionising sound waves, carries no known biological risk at diagnostic intensities, and can be used throughout pregnancy. This is why ultrasound is the standard technique for foetal imaging — the safety profile is well established over decades of clinical use.

6. Putting It Together — Structured Comparison

Now that you have a clear understanding of each dimension, the skill the exam tests is combining them into a coherent, justified comparison. The table below gives you a reference framework. Use it to structure your answers, but always adapt your reasoning to the specific clinical context in the question.

A common exam scenario presents a specific patient — perhaps a pregnant woman with abdominal pain, or a child with a suspected bone tumour — and asks you to justify a choice of imaging technique. The answer must address all three criteria: what resolution is needed for the diagnosis, how practical the scan is for this patient, and what the safety implications are given the patient's circumstances.

Worked example: Structuring a Comparison Answer

A 28-year-old pregnant patient (12 weeks) presents with right iliac fossa pain. The clinician considers ultrasound or CT to investigate possible appendicitis. Compare these two techniques in terms of resolution, convenience, and safety for this patient. [6 marks]

1. CT: very high spatial resolution, sub-millimetre slices, can clearly visualise the appendix and surrounding structures. Ultrasound: moderate resolution, dependent on operator skill and patient body habitus; the appendix may not always be visualised, particularly in obese patients or when obscured by bowel gas. — [B1] for stating CT has higher spatial resolution. [B1] for acknowledging ultrasound resolution is frequency-dependent and may be insufficient to visualise the appendix reliably.
2. Both are relatively convenient. CT is fast (minutes) and widely available. Ultrasound is portable, requires no special preparation, and can be performed at the bedside. Neither requires a long uptake period or specialist centre. — [B1] for a valid convenience comparison — accept any accurate point distinguishing the two (e.g. ultrasound is portable; CT requires a fixed scanner).
3. CT uses ionising radiation (approximately 10 mSv for abdominal CT). Ionising radiation poses a risk of DNA damage to the developing foetus, particularly in the first trimester when organogenesis is occurring. Ultrasound uses non-ionising sound waves and has no known harmful effects on the foetus at diagnostic intensities. — [B1] for identifying that CT uses ionising radiation and stating the risk to the foetus. [B1] for stating ultrasound is non-ionising and safe in pregnancy.
4. Ultrasound should be the first-line investigation. Although CT offers superior resolution, the ionising radiation risk to the foetus outweighs this advantage. Ultrasound is safe, convenient, and sufficient for initial assessment. CT may be used if ultrasound is inconclusive and the clinical need is urgent, as the risk of untreated appendicitis to both mother and foetus exceeds the radiation risk. — [B1] for a balanced conclusion that weighs safety against diagnostic need — this is the mark-winning evaluative judgement.

Answer: Ultrasound is preferred as first-line imaging due to its non-ionising nature and safety in pregnancy, despite lower resolution than CT. CT is reserved for cases where ultrasound is inconclusive and clinical urgency demands it.

7. Radionuclide Imaging — Resolution and Safety in Context

Radionuclide imaging deserves particular attention because its resolution and safety profile are both distinctive and frequently misunderstood. Recall from Section 3 that PET and gamma camera imaging have the lowest spatial resolution of the common modalities. This is not a flaw to be apologised for — it reflects the fundamental physics of detecting gamma photons emitted from within the body.

What radionuclide imaging provides instead is functional information: it shows where a particular biological process is occurring, not just where a structure is located. A PET scan using fluorodeoxyglucose (FDG) highlights regions of high glucose uptake, which correspond to metabolically active tumour tissue. This functional sensitivity can detect disease at a stage when the anatomy looks normal on CT or MRI — making radionuclide imaging complementary to, rather than competing with, high-resolution anatomical techniques.

In clinical practice, PET is frequently combined with CT in a single hybrid scanner (PET-CT). The CT component provides the anatomical map with high spatial resolution; the PET component overlays the functional signal. This combination addresses the resolution limitation of PET directly, and it is worth mentioning in exam answers when discussing how the limitations of radionuclide imaging are managed in practice.

On the safety side, radionuclide imaging involves administering a radioactive substance internally. The tracer emits ionising radiation from within the patient's body until it decays or is excreted. The effective dose varies with the tracer and the organ being imaged, but is typically in the range of 2–10 mSv — comparable to CT. Crucially, the patient remains a source of ionising radiation after leaving the scanner, which has implications for contact with pregnant women and young children in the hours following the scan.

8. Exam Technique — How to Score Full Marks on Comparison Questions

Comparison questions are among the most predictable in medical physics, but they are also among the most poorly answered. The most common reason for losing marks is addressing only one or two of the three criteria (resolution, convenience, safety) rather than all three. Examiners award marks for each criterion addressed, so a complete answer must cover all three explicitly.

A second common error is making vague comparative statements without scientific justification. Writing "MRI is safer than X-ray" earns no marks on its own. Writing "MRI does not use ionising radiation, so there is no risk of DNA damage, whereas X-ray delivers a dose of approximately 0.02 mSv which carries a small but non-zero risk of inducing cancer" earns the mark. Precision and mechanism are what distinguish a B-grade answer from an A-grade one.

For evaluate or justify questions, you must reach a conclusion. State which technique is more appropriate for the given scenario and explain why the advantages outweigh the disadvantages in that specific context. A conclusion that restates the comparison without making a judgement will not access the top mark band.