Nuclear Medicine · Imaging physics
Gamma Camera, SPECT and PET Physics One-Liners
High-yield one-liners on gamma camera components, SPECT reconstruction, PET coincidence detection and key physics principles underlying nuclear medicine imaging.
Rapid-revision one-liners covering the physics of gamma camera detection, SPECT acquisition and reconstruction, PET coincidence detection and key quality-control principles.
- Gamma camera detector
- The gamma camera uses a thallium-activated sodium iodide [NaI(Tl)] scintillation crystal, typically 9.5 mm (3/8 inch) thick, which converts incident gamma photons into visible light detected by photomultiplier tubes.
- Collimator function
- The collimator is the primary determinant of spatial resolution and sensitivity in planar imaging; it absorbs obliquely incident photons, allowing only photons travelling perpendicular to the detector to form an image.
- Low-energy high-resolution collimator
- LEHR collimators have small holes and long septa, providing high spatial resolution at the expense of sensitivity; used for Tc-99m imaging at 140 keV.
- High-energy collimator
- High-energy collimators have thicker septa to prevent septal penetration by high-energy photons (e.g., I-131 at 364 keV); using an LEHR collimator for I-131 causes severe star artefact from septal penetration.
- SPECT acquisition
- SPECT acquires multiple planar projections over 180 or 360 degrees; typical protocols use 60-64 projections over 360 degrees with 20-30 seconds per projection.
- Filtered back projection versus iterative reconstruction
- Iterative reconstruction (e.g., OSEM) is now standard for SPECT, modelling photon attenuation, scatter and detector response; it produces fewer artefacts and better contrast recovery than filtered back projection.
- SPECT/CT attenuation correction
- The CT component provides an attenuation map (mu-map) for correction of photon attenuation, improving quantitative accuracy and reducing artefacts in deep structures such as the inferior wall on myocardial perfusion imaging.
- PET coincidence detection
- PET detects pairs of 511 keV annihilation photons arriving simultaneously (within 6-12 nanoseconds) at opposing detectors; the line connecting the two detection points is the line of response.
- PET spatial resolution
- Clinical PET spatial resolution is 4-6 mm FWHM, limited by positron range before annihilation, non-collinearity of annihilation photons (0.5 degrees deviation) and detector crystal size.
- PET versus SPECT sensitivity
- PET is 10-20 times more sensitive than SPECT because it uses electronic collimation (coincidence detection) rather than physical collimators, which reject over 99.9% of photons.
- Time-of-flight PET
- Time-of-flight (TOF) PET measures the small difference in arrival times of the two annihilation photons, localising the event along the line of response and improving signal-to-noise ratio, particularly in large patients.
- PET attenuation correction
- CT-based attenuation correction in PET/CT uses a bilinear transformation to convert CT Hounsfield units to 511 keV attenuation coefficients; PET/MRI uses segmentation or atlas-based methods.
- Scatter correction
- Compton scatter degrades contrast and quantitative accuracy; energy window-based methods (e.g., dual or triple energy window) and model-based iterative corrections are used in both SPECT and PET.
- Dead time
- At high count rates, detector electronics cannot process events fast enough (dead time), causing count loss and image degradation; PET scanners are more susceptible due to higher photon flux.
- Uniformity quality control
- Daily intrinsic uniformity flood with a point source or weekly extrinsic flood with a sheet source is required; non-uniformity greater than 3% coefficient of variation warrants investigation.
- Centre of rotation
- SPECT centre-of-rotation (COR) alignment must be verified regularly; COR misalignment causes ring artefacts and degrades resolution, particularly in small hot lesions.
- Partial volume effect
- Lesions smaller than twice the system spatial resolution (approximately 8-12 mm for PET) suffer underestimation of true activity due to partial volume averaging; correction algorithms are increasingly applied.
- Random coincidences
- Random coincidences occur when two unrelated photons are detected within the coincidence timing window; they increase with the square of activity and are estimated using a delayed coincidence window.
Caution
Specific resolution values, timing windows and QC thresholds vary by scanner manufacturer and model. Always reference institutional physics protocols and current IAEA or ACR quality-control guidelines.
Exam pearl
Remember that collimator choice is the dominant factor in SPECT resolution versus sensitivity trade-off, whereas PET resolution is fundamentally limited by positron range and non-collinearity.
Viva questions
- Why is PET more sensitive than SPECT?
- PET uses electronic collimation via coincidence detection, which does not reject the vast majority of photons as physical collimators do in SPECT. This gives PET approximately 10-20 times higher sensitivity.
- What limits PET spatial resolution?
- PET resolution is limited by positron range before annihilation, non-collinearity of the two 511 keV photons (approximately 0.5 degrees deviation from 180 degrees), and detector crystal element size. Clinical systems achieve 4-6 mm FWHM.
- What happens if you image I-131 with a low-energy collimator?
- Severe septal penetration occurs because I-131 emits 364 keV photons that penetrate the thin septa of a low-energy collimator, producing a star artefact and degrading image quality. A high-energy collimator with thicker septa is required.
- How does time-of-flight PET improve image quality?
- TOF PET measures the small arrival-time difference between the two annihilation photons, localising the event along the line of response rather than uniformly distributing it. This improves signal-to-noise ratio, particularly beneficial in large patients.
- What is the partial volume effect and why does it matter clinically?
- Lesions smaller than twice the system resolution (approximately 8-12 mm for PET) have their activity spread over a larger volume, causing underestimation of true SUV. This can lead to under-staging of small lesions or underestimation of treatment response.