What is a gamma camera?
A gamma camera is a medical imaging device used in nuclear medicine to detect gamma photons emitted from radiopharmaceuticals within the body and to form planar images of their distribution. It converts incoming gamma radiation into visible light, then into electrical signals, which are processed to determine the location of each detected photon.
A gamma camera detects gamma photons emitted from within the patient and converts them into spatially localised electrical signals to form nuclear medicine images.
The gamma camera does not produce radiation. Instead, it detects radiation emitted from within the patient following administration of a radioactive tracer. The spatial distribution of detected photons reflects tracer uptake in tissues, allowing functional imaging.
Modern gamma cameras are also used as the detector systems in SPECT (Single Photon Emission Computed Tomography) imaging.
Understanding the physics
A conventional gamma camera consists of several key components arranged in sequence:
Collimator – A lead structure containing many parallel holes. It allows only photons travelling in certain directions to reach the detector, providing spatial localisation.
Scintillation crystal – Typically sodium iodide doped with thallium (NaI(Tl)). When a gamma photon interacts with the crystal, it produces a brief flash of light (a scintillation).
Photomultiplier tubes (PMTs) – These convert the scintillation light into electrical signals and amplify them.
Positioning electronics (Anger logic) – These determine the x–y location of each detected event based on the distribution of signals across the PMTs.
Energy discrimination circuitry – This applies an energy window to reject scattered photons that do not fall within the expected photopeak range.
When a gamma photon emitted from the patient passes through the collimator and interacts in the crystal, a light pulse is generated. The amount of light is proportional to the photon’s energy. The PMTs detect this light and convert it into electrical signals. By analysing the relative signal strength in adjacent PMTs, the system calculates the interaction position.
Each valid event contributes a count at a specific location in a digital matrix. Over time, the accumulation of many such events forms an image representing tracer distribution.
The gamma camera’s spatial resolution and sensitivity are largely determined by the collimator design, while energy resolution depends on crystal and electronics performance.
Where this matters clinically
The gamma camera is the foundational device for SPECT imaging. Its design determines image quality, spatial resolution, and sensitivity. Understanding how it works clarifies many trade-offs in nuclear medicine imaging, including resolution–sensitivity balance and scatter rejection.