Diagnostic methods in medicine — OCR A-Level Physics
Test yourself on Diagnostic methods in medicine with OCR A-Level practice questions.
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Diagnostic methods in medicine explained
A medical tracer is a radioactive substance introduced into the body so that its emitted radiation can be detected outside and used to image or monitor an organ.
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The tracer must be taken up by the target tissue, emit radiation that escapes the body, and have a suitable half-life: long enough to complete the investigation but short enough to limit the patient's dose. Technetium–99m is widely used because it emits gamma radiation, has a half-life of about 6 hours, and can be attached to many pharmaceuticals that target specific organs. Fluorine–18 is used in positron emission tomography because it emits positrons; when a positron meets an electron, two gamma photons are produced in opposite directions and detected in coincidence. Its half-life of about 110 minutes suits imaging of metabolic activity, for example in the brain or in tumours.
(b) gamma camera; components – collimator, scintillator, photomultiplier tubes, computer and display; formation of image
A gamma camera images a patient after a gamma-emitting tracer is administered. The collimator is a lead block with parallel holes that admits only near-parallel gamma rays, so each point on the detector maps to a known direction. Gamma photons strike a large scintillator crystal, commonly sodium iodide doped with thallium, producing many visible-light photons. These reach an array of photomultiplier tubes; each tube's photocathode releases electrons by the photoelectric effect, and dynodes multiply the signal by secondary emission. The computer compares pulse heights across tubes to calculate the interaction position, building a two-dimensional image of tracer distribution. The display shows counts as brightness or colour, so regions of high uptake appear bright. The collimator limits sensitivity but preserves spatial resolution.
(c) diagnosis using gamma camera
Diagnosis with a gamma camera begins by administering a radiopharmaceutical whose gamma-emitting radionuclide is chosen to accumulate in the organ of interest. For example, technetium-99m labelled to a carrier may concentrate in bone or kidneys. Gamma photons escaping the patient are detected by the camera, and the computer builds a two-dimensional map of count rate against position. A healthy organ shows a characteristic pattern of uptake; disease may appear as a cold spot where uptake is reduced, or a hot spot where uptake is increased. The clinician compares the image with normal expectations and may quantify counts in regions of interest. The technique gives functional information about physiology rather than sharp anatomical detail, and the collimator limits sensitivity, so image quality depends on tracer activity, acquisition time and patient positioning.
(d) positron emission tomography (PET) scanner; annihilation of positron–electron pairs; formation of image
A PET scanner images the distribution of a positron-emitting radionuclide, often fluorine-18 in fluorodeoxyglucose, which is taken up by metabolically active cells. When a positron is emitted, it travels a short distance and annihilates with an electron. The annihilation converts the total rest mass into two gamma photons of equal energy, each about 511 keV, emitted in almost opposite directions. A ring of detectors surrounds the patient. When two detectors on opposite sides register photons within a very short coincidence time window, the annihilation event is assumed to lie on the line joining them. The computer collects many such coincidence events and reconstructs a three-dimensional image of tracer concentration. The technique gives functional information about metabolism and is often combined with CT for anatomical reference.
(e) diagnosis using PET scanning.
PET scanning is used to diagnose and monitor conditions by imaging metabolic activity. A positron-emitting tracer such as fluorine-18 labelled fluorodeoxyglucose is injected; cells with high metabolic demand, including many cancer cells, take up more of it. The scanner records coincidence gamma pairs and reconstructs a three-dimensional map of tracer concentration. Areas of high uptake appear as hot spots and may indicate tumours, active inflammation or specific brain activity. Because PET shows function rather than fine anatomy, it is often combined with CT or MRI so that metabolic hot spots can be located precisely. Diagnosis relies on comparing the pattern and intensity of uptake with normal expectations, and on clinical context. The technique exposes the patient to ionising radiation, so benefit must be weighed against risk.
Your focus
- Define a medical tracer and state the properties required of one.
- Describe the use of technetium–99m and fluorine–18 in diagnostic imaging.
- Explain how the half-life and emission type of a tracer affect its suitability for a given investigation.
Show all 15 objectives
- Identify the collimator, scintillator, photomultiplier tubes, computer and display in a gamma camera.
- Describe the function of each component in the formation of an image.
- Explain how the position of a gamma interaction is determined from photomultiplier tube signals.
- Describe how a gamma camera is used to obtain diagnostic information.
- Interpret simple gamma camera images in terms of tracer uptake.
- Explain the difference between functional information and anatomical detail.
- Describe the annihilation of a positron–electron pair and the production of two gamma photons.
- Explain how coincidence detection in a PET scanner locates an annihilation event.
- Outline how a PET image is formed from many coincidence events.
- Describe how PET scanning is used to diagnose disease.
- Interpret PET images in terms of metabolic uptake.
- Explain why PET is often combined with anatomical imaging techniques.
Diagnostic methods in medicine exam tips
Marking Points
- A medical tracer is a radioactive substance introduced into the body whose emitted radiation is detected externally to image or monitor an organ.
- Technetium–99m emits gamma radiation, has a half-life of about 6 hours, and can be attached to pharmaceuticals that target specific organs.
- Fluorine–18 emits positrons and is used in positron emission tomography, where annihilation produces two gamma photons detected in coincidence.
- A suitable tracer must be taken up by the target tissue and must emit radiation that can escape the body.
- The half-life must be long enough for the investigation but short enough to limit the radiation dose to the patient.
- The collimator is a lead grid with parallel channels that absorbs oblique gamma rays so only near-parallel rays reach the scintillator, defining the line of sight.
- The scintillator, often NaI(Tl), absorbs a gamma photon and emits a burst of visible-light photons whose number is proportional to the deposited energy.
- Photomultiplier tubes convert light into an electrical pulse: the photocathode emits electrons, and dynodes produce secondary emission, multiplying the signal.
- The computer uses the relative pulse sizes from several photomultiplier tubes to locate each scintillation event and accumulates counts to form the image.
- The display presents the count distribution as brightness or colour, allowing regions of high and low tracer uptake to be distinguished.
- A gamma-emitting radiopharmaceutical is administered and is designed to accumulate in the target organ or tissue.
- Gamma photons emitted from within the patient are detected externally by the gamma camera.
- The computer forms a two-dimensional image showing the distribution of tracer uptake.
- Areas of abnormally high or low uptake are interpreted as hot spots or cold spots that may indicate disease.
- The image provides functional information about the organ, and diagnosis depends on comparing the pattern with normal uptake.
- A positron-emitting radionuclide is administered and accumulates in metabolically active tissue.
- A positron emitted from the nucleus travels a short distance before annihilating with an electron.
- Annihilation converts mass into two gamma photons of equal energy, each about 511 keV, emitted in nearly opposite directions.
- A ring of detectors records coincident gamma photons within a short time window, defining a line of response through the annihilation point.
- The computer reconstructs a three-dimensional image of tracer distribution from many coincidence events.
- A positron-emitting radiopharmaceutical is administered and its uptake reflects metabolic activity.
- The PET scanner detects coincidence gamma pairs and reconstructs a three-dimensional distribution of tracer.
- Regions of abnormally high uptake, called hot spots, may indicate tumours, inflammation or active brain regions.
- PET provides functional information and is often combined with CT or MRI for anatomical localisation.
- Diagnosis involves comparing the uptake pattern with normal expectations and considering the clinical context and radiation risk.
Examiner Tips
- 💡Match each tracer to its emission and imaging method: technetium–99m to gamma imaging, fluorine–18 to positron emission tomography.
- 💡Justify the half-life in terms of completing the investigation while limiting the patient's radiation dose.
- 💡State that the tracer must be taken up by the target organ so that the image shows that organ.
- 💡Link each component to its function in one sentence: collimator selects direction, scintillator converts gamma to light, photomultiplier tubes amplify and convert to electrical signal, computer locates events, display shows the image.
- 💡Use the correct order of energy conversion: gamma photon to visible photons to electrons to electrical pulse.
- 💡When explaining image formation, state that the computer uses the relative signals from several photomultiplier tubes to calculate the position of each event.
- 💡State the clinical purpose: to assess organ function by mapping tracer uptake.
- 💡Use the terms hot spot and cold spot correctly and link each to increased or decreased uptake.
- 💡Mention that the choice of radiopharmaceutical depends on the organ being investigated.
- 💡State the annihilation process clearly: positron plus electron gives two gamma photons of about 511 keV each.
- 💡Explain coincidence detection: two detectors on opposite sides must register photons within a very short time window.
- 💡Link the line of response to the reconstruction of a three-dimensional image.
- 💡State the clinical question PET can answer: where is metabolic activity abnormally high or low?
- 💡Use the term hot spot for high uptake and link it to possible pathology.
- 💡Mention that PET is often combined with CT or MRI to provide anatomical reference.
Common Mistakes
- Thinking the tracer must emit alpha or beta radiation that is detected outside the body: gamma radiation is used because it penetrates tissue and can be detected externally.
- Believing a longer half-life is always better: a half-life that is too long leaves the patient exposed for longer, so it must be matched to the investigation.
- Confusing technetium–99m with fluorine–18: technetium–99m is a gamma emitter used in single-photon imaging, while fluorine–18 is a positron emitter used in PET.
- Assuming any radioactive substance can be a tracer: it must be chemically taken up by the target tissue and must not be toxic.
- Thinking the collimator focuses gamma rays like a lens; it does not refract or focus, it absorbs oblique rays so only near-parallel rays pass.
- Confusing the scintillator with the photomultiplier tube; the scintillator converts gamma photons to visible light, while the photomultiplier tube converts light to an electrical pulse.
- Believing the gamma camera detects positrons or emits radiation; it passively detects gamma photons emitted from within the patient.
- Assuming the computer produces a three-dimensional image; a standard gamma camera produces a two-dimensional projection image.
- Thinking the gamma camera itself emits radiation into the patient; the radiation comes from the administered radiopharmaceutical.
- Confusing a cold spot with a hot spot; a cold spot has reduced tracer uptake, while a hot spot has increased uptake.
- Assuming the gamma camera gives detailed anatomical images like a CT scan; it primarily shows functional distribution of tracer.
- Believing any gamma emitter is suitable; the radionuclide must emit gamma rays of suitable energy and have a half-life long enough for imaging but short enough to limit dose.
- Thinking the positron itself is detected; the positron annihilates and the two gamma photons are detected.
- Believing the two gamma photons have different energies; they each have about 511 keV, equal to the electron rest energy.
- Assuming the photons are emitted exactly 180° apart in every case; they are emitted in nearly opposite directions, and small deviations affect resolution.
- Confusing PET with gamma camera imaging; PET detects coincidence gamma pairs from annihilation, while a gamma camera detects single gamma photons with a collimator.
- Thinking PET gives sharp anatomical detail on its own; it primarily shows metabolic function and is often fused with CT or MRI.
- Assuming all high uptake is cancer; normal organs such as the brain, heart and kidneys can also show high uptake.
- Ignoring the radiation dose to the patient; PET uses ionising radiation and the risk must be justified.
- Confusing the role of the tracer with the scanner; the tracer accumulates in tissue, while the scanner detects the emitted gamma photons.