24.3.3Medical physics and astronomy (A Level)
Detecting Gamma-Rays from PET Scanning
5 min read 11 parts Lesson 13 of 22 1 quiz
Lesson summary
24.3.3 Detecting Gamma-Rays from PET Scanning — AS PHYSICS 9702 notes: Explain how scintillators, photomultipliers and coincidence detection locate tracer in a PET scan and form an image.Layout — The patient lies stationary in a tube surrounded by a ring of detectors.Images — Ima…
- Absorption
- Faint light
- Photocathode
- Pulse size
- Different jobs
- Time window
Section 1 of 11The detector ring
The detector ring
Layout
- The patient lies stationary in a tube surrounded by a ring of detectors.
Images
- Images of slices of the body show the position of the radioactive tracer.
Two parts
- Each detector has a crystal scintillator and a photomultiplier.
Section 2 of 11Scintillator
Absorption
- A gamma photon hitting the crystal excites an electron to a higher energy state.
Cascade
- The excited electron excites more electrons as it travels through the crystal.
Light
- When the excited electrons fall back, the lost energy is emitted as visible light photons.
Materials
- Examples are bismuth germanate and lutetium oxyorthosilicate.
Section 3 of 11Photomultiplier
Faint light
- The scintillator's photons are very faint and must be amplified.
Photocathode
- The light releases electrons from a photocathode.
Dynodes
- A series of dynodes multiplies the electrons in a cascade, giving a measurable electrical signal.
Pulse size
- It shows the photon energy, so electronics accept only pulses near 511 keV and reject scattered photons.
Different jobs
- The scintillator converts gamma to light and the photomultiplier converts light to an electrical signal.
Section 4 of 11Coincidence detection
Time window
- Electronics look for two pulses from different detectors within a few nanoseconds.
Accepted event
- Such a coincidence is taken as one annihilation.
Single pulses
- A pulse with no partner is ignored, since it may be background or a photon whose partner was absorbed.
Line of response
- Each coincidence defines a straight line joining the two detectors, along which the annihilation occurred.
- The photons travel in a line because of conservation of momentum.
No collimator
- Unlike a gamma camera, PET needs no lead collimator, which gives it high sensitivity.
Section 5 of 11Locating the annihilation
Intersections
- The computer finds where lines of response intersect, the regions with the highest activity.
Time of flight
- If the annihilation is exactly mid-way, both photons arrive together.
- If it is a distance x closer to one detector, the path difference is 2x, so Δt = 2x/c.
- The displacement from the centre is half of cΔt.
Accuracy
- Timing to a few hundred picoseconds locates the event to within several centimetres along the line.
Section 6 of 11Image construction
Counts
- A scan collects millions of coincidence events over several minutes.
Voxels
- The computer divides the body into voxels and finds the tracer concentration from lines passing through each.
Brightness
- The more photon pairs formed at a point, the more tracer is present, appearing as a brighter point.
False colour
- Hot areas show high uptake, such as a tumour, and cold regions show low activity.
Attenuation correction
- Data are corrected for absorption of gamma rays, often using a CT scan in the same session.
Section 7 of 11Strengths and limitations
Strengths
- It shows metabolic function that other imaging cannot, can detect disease early and is quantitative.
Limitations
- It is costly, needs a nearby cyclotron, gives a dose from the tracer and any CT, and has a resolution of several millimetres.
- Resolution is limited by positron range and detector size.
Patient
- The patient must stay still.
Activity
- Enough counts are needed for an image, but the activity should be as low as possible.
Section 8 of 11Comparison with other techniques
Comparison with other techniques
Function
- PET shows how an organ is working rather than how it is built.
Radiation
- It uses ionising radiation, unlike ultrasound.
Resolution
- It costs more and has lower spatial resolution than CT.
Section 9 of 11More from the specification
More from the specification
Positron Emission and Radiotracer Decay
- In Positron Emission Tomography (PET), a patient is injected with a biologically active radiotracer incorporating an isotope with a short half-life that undergoes beta-plus (β⁺) nuclear decay, emitting a positron inside body tissues and metabolic organs.
- (such as fluorodeoxyglucose, ¹⁸F-FDG)
Positron-Electron Annihilation and Gamma Photon Emission
- When the emitted positron encounters an electron in surrounding tissue, they undergo mutual annihilation;
- their rest masses convert into electromagnetic energy, releasing two identical gamma-ray photons travelling in opposite directions (180° apart) governed by the mass-energy equation E = m·c² = h·f.
Coincidence Detection and PET Image Reconstruction
- Ring detectors surrounding the patient detect pairs of gamma photons arriving simultaneously;
- calculating the slight difference in arrival times (time-of-flight) pinpoints the exact site of annihilation,
- allowing computer algorithms to map metabolic activity and disease across patient tissues.
Positron Emission and Radiotracer Decay
- In Positron Emission Tomography (PET), a patient is injected with a biologically active radiotracer incorporating an isotope with a short half-life that undergoes beta-plus (β⁺) nuclear decay, emitting a positron inside body tissues and metabolic organs.
- (such as fluorodeoxyglucose, ¹⁸F-FDG)
Positron-Electron Annihilation and Gamma Photon Emission
- When the emitted positron encounters an electron in surrounding tissue, they undergo mutual annihilation;
- their rest masses convert into electromagnetic energy, releasing two identical gamma-ray photons travelling in opposite directions (180° apart) governed by the mass-energy equation E = m·c² = h·f.
Coincidence Detection and PET Image Reconstruction
- Ring detectors surrounding the patient detect pairs of gamma photons arriving simultaneously;
- calculating the slight difference in arrival times (time-of-flight) pinpoints the exact site of annihilation,
- allowing computer algorithms to map metabolic activity and disease across patient tissues.
Nuclear equation
- Equation in which nucleon number and charge are conserved.
Half-life
- Time for half the nuclei of an isotope to decay.
Frequency
- Revolutions per second, f = 1/T.What the symbols mean: T = period, the time for one complete wave (s) · f = frequency (Hz)
E
E hf = mec2
What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
E = hf
What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Scintillator | A crystal that emits visible light when a gamma-ray photon is absorbed. |
| Photomultiplier | A device that amplifies faint light into an electrical signal using a photocathode and dynodes. |
| Coincidence detection | Registering two pulses from different detectors within a few nanoseconds as one annihilation. |
| Line of response | The straight line joining two detectors that register a coincidence, along which the annihilation occurred. |
| Time of flight | Using the difference in arrival times of two photons to find the position of the annihilation along the line. |
Section 11 of 11Exam technique
1
Thinking PET uses a lead collimator in front of the detectors — coincidence timing replaces it.
2
Saying one detector firing is enough to register an event — two simultaneous detections are needed.
3
Using the full path difference as the displacement — the displacement is half of cΔt.
4
Confusing a scintillator with a photomultiplier — the scintillator turns gamma into light and the photomultiplier turns light into an electrical signal.
5
Forgetting that the scanner shows function rather than anatomy — PET gives metabolic information.