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The gamma camera, collimators and SPECT
A gamma camera converts a gamma photon into light in an NaI(Tl) crystal, light into an electrical signal in photomultiplier tubes (PMT), and accepts only photons of the right energy (PHA) — and the collimator, which accepts only photons travelling in one direction, sets the balance between resolution and sensitivity.
Why it matters in the exam
The exam asks about each component's role. The parallel-hole collimator defines direction and without it there is no image; its resolution is better with longer, narrower holes, but sensitivity then falls; and the most important factor is distance from the patient — the closer the camera, the sharper the image. High-energy isotopes (I-131, In-111, Ga-67) need a collimator with thicker septa. A pinhole collimator magnifies small organs such as the thyroid; a converging (fan beam) collimator improves brain SPECT.
The PHA (pulse-height analyser) accepts only signals within an energy window (for example 20% around 140 keV) and rejects scattered photons that lost energy. A semiconductor detector (CZT) converts a photon directly into an electrical signal, without a light stage, so its energy resolution is better; in dedicated cardiac cameras the high sensitivity also comes from geometry focused on the heart. SPECT: the camera rotates around the patient and collects projections — usually 60–128 over 360°, and for the heart usually 180°. A body-contour orbit keeps the distance short and improves resolution compared with a circular orbit.
Reconstruction: filtered back-projection (FBP) is fast; iterative reconstruction (OSEM) is more accurate and reduces artefacts, but entered routine use late because of computation time. A low-pass filter smooths noise; a high-pass filter enhances edges and also noise. SPECT/CT adds anatomical localisation and attenuation correction. Quality control: daily uniformity — non-uniformity causes ring artefacts in SPECT; resolution and linearity with a bar phantom weekly; centre of rotation for SPECT; and daily energy peaking. A failed PMT appears as a round cold spot.
Key points
- The collimator sets direction and the resolution–sensitivity balance.
- The PHA rejects scattered photons by energy window.
- A body-contour orbit improves resolution.
- OSEM is more accurate than FBP.
- QC: daily uniformity, weekly bars, centre of rotation for SPECT.
Summary table
| Component | Role | Key point |
|---|---|---|
| Parallel collimator | Selecting direction | Long and narrow — good resolution, low sensitivity |
| NaI(Tl) crystal | Converting gamma to light | Thick crystal — high sensitivity, lower resolution |
| PMT | Converting light to an electrical signal | Failed PMT — cold spot |
| PHA | Selecting energy | Rejects scattered photons |
| CZT | Direct conversion to an electrical signal | Good energy resolution |
| CT in a hybrid system | Localisation and attenuation correction | Metal and motion artefact |
In the full version for subscribers
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Sources
- Physics in Nuclear Medicine (Cherry, Sorenson, Phelps), 4th edition (2012) — The gamma camera: basic principles and performance; Tomographic reconstruction
- IAEA Quality Control Atlas for Scintillation Camera Systems, 2003 — Uniformity, resolution and centre of rotation
- EANM Guideline for Routine Quality Control of Gamma Cameras and SPECT, 2019 — Test frequencies and tolerances
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.
See also: Basics of diagnostic nuclear medicine