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PET and PET/CT physics
A positron travels a short distance, annihilates with an electron and creates two 511 keV photons in nearly opposite directions; the scanner detects them simultaneously (coincidence) and draws a line between them — "electronic collimation" that gives far higher sensitivity than SPECT.
Why it matters in the exam
The exam asks about event types: true — both photons from the same event, without scatter; scatter — one photon was deflected and the line is misplaced; and random — two photons from different events detected by chance in the same timing window. Random events rise roughly with the square of activity and are corrected with a delayed window. Factors that limit resolution: positron range (a higher-energy positron travels further — Rb-82 and Ga-68 are less sharp than F-18), slight non-collinearity of the two photons, and detector size.
Detectors: LSO or LYSO crystals are fast and bright, and BGO is dense; in "digital" PET the PMTs have been replaced by silicon photomultipliers (SiPM). Time of flight measures the difference in arrival time of the two photons, narrows the event position on the line and improves the signal-to-noise ratio, mainly in large patients. Attenuation correction is CT-based; so metal, concentrated contrast and respiratory motion (misregistration at the lung base and liver dome) create artefacts — and the non-corrected image should always be reviewed too.
SUV = tissue activity concentration divided by (injected activity divided by body weight); it depends on calibration, time from injection, glucose and lesion size (partial volume effect in lesions smaller than about twice the resolution). CT radiation dose is measured by CTDIvol (average dose in a slice) and by DLP, which is CTDIvol multiplied by the scan length and therefore accounts for length; effective dose is estimated from it. Low-dose CT for attenuation correction only reduces radiation when diagnostic CT is not needed. Daily quality control includes a detector check and calibration with a source or phantom.
Key points
- PET — two 511 keV photons in coincidence.
- Events: true, scatter and random.
- Long positron range — poorer resolution.
- CT attenuation correction — check metal and motion.
- DLP = CTDIvol × scan length.
Summary table
| Concept | Definition | Effect |
|---|---|---|
| True event | Two photons from the same event | The wanted signal |
| Scatter event | A photon was deflected | Wrong position, lower contrast |
| Random event | Photons from different events | Rises roughly with the square of activity |
| Positron range | Distance to annihilation | High energy — poorer resolution |
| Time of flight | Difference in arrival times | Better signal-to-noise ratio |
| DLP | CTDIvol multiplied by scan length | Accounts for length |
A free exam question
במדידת Hounsfield units של ממצא ב- CT מתקבל ערך של אפס . מהו היחס בין מקדם הבליעה של הקרינה של מים ( m ) לזו של הממצא ( =? m / m )? water water (the question as examined, in Hebrew)
- א. 0
- ב. 1
- ג. 2
- ד. 1000
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Sources
- Physics in Nuclear Medicine (Cherry, Sorenson, Phelps), 4th edition (2012) — Positron emission tomography
- EANM Procedure Guidelines for Tumour Imaging: FDG PET/CT Version 2.0, 2015 — Scanner quality control and harmonisation
- AAPM Report: Radiation Dose from CT — CTDI and DLP, 2008 — Dose descriptors
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.