Gamma Camera

Overview

A gamma camera, also called an Anger camera after Hal Anger who invented it in 1957, images the inside of the body by detecting gamma rays emitted from a radioactive tracer the patient has been given. The tracer is chosen so it concentrates in a tissue or process of interest, bone turnover, blood flow, tumour metabolism, and the camera builds a map of where the radiation is coming from. Unlike X-ray imaging, which shows anatomy from an external source, nuclear medicine shows physiology from a source inside the patient.

The machine is built around one or two Scintillation Detector Heads carrying a large scintillation crystal and an array of photomultipliers, fronted by a Collimator Set and mounted on a rotating Rotating Gantry over a low-attenuation Patient Table. Events are processed by the Acquisition Electronics and reconstructed on the Reconstruction Workstation.

Detecting a gamma photon

Each gamma that reaches a Scintillation Detector Head first passes through the collimator, then strikes the NaI(Tl) Scintillation Crystal, a single thallium-doped sodium iodide crystal about 9.5 mm thick. The absorbed gamma produces a brief flash of visible light proportional to its energy. The Optical Light Guide spreads that flash across the Photomultiplier Array, roughly 59 Photomultiplier Tube photomultiplier tubes packed in a hexagonal pattern. Each tube turns the few photons it collects into a measurable pulse through dynode multiplication, biased by its Dynode HV Divider. Tubes nearer the flash see more light, so the relative pulse sizes encode where in the crystal the event happened. A Mu-metal Shield keeps stray magnetic fields from deflecting the electrons inside the tubes, and a Head Lead Shielding lead jacket stops gammas arriving from any direction other than through the collimator.

Forming the image

The crystal and tubes alone cannot make an image, because a scintillation tells you a gamma arrived but not which way it was travelling. The Collimator Set supplies that information. The Low-Energy High-Resolution Collimator is a slab of lead drilled with thousands of fine parallel holes; only gammas travelling nearly perpendicular to the crystal pass through, so each point on the crystal corresponds to a known line in space. Different studies use different collimators, the Medium-Energy Collimator for higher-energy isotopes, the Pinhole Collimator for magnified views of small organs, and they are swapped with the Collimator Exchange Cart because each weighs many kilograms of lead.

In the Acquisition Electronics, the Position Logic performs Anger logic: it weights the Photomultiplier Tube signals to compute the event's X and Y coordinates, while the Energy Discriminator measures the total light to estimate the gamma's energy. Only events inside a narrow window around the tracer's photopeak, for example 140 keV for technetium-99m, are accepted, which rejects gammas that have scattered in the body and lost energy and would otherwise blur the image. The Correction Board applies stored energy, linearity, and uniformity maps so the picture is geometrically true and evenly responsive across the whole field.

Planar and tomographic imaging

With the heads held still, the camera produces a flat projection image, as in a bone scan. For three-dimensional imaging (SPECT, single-photon emission computed tomography) the Rotating Gantry rotates the heads around the patient, the Rotation Drive stepping them through many angles while the Head Positioner keeps each detector as close to the body as the Body-Contour Sensor allows, since resolution falls with distance. The set of projections is reconstructed into cross-sectional slices on the Workstation Computer using iterative algorithms, much as CT reconstructs from X-ray projections. Dual heads halve the acquisition time by collecting two projections at once.

Supporting systems

Gain stability depends on the High-Voltage Supply, a low-ripple 800–1,200 V source feeding every photomultiplier, because a small drift in high voltage shifts the apparent energy of every event. The Reconstruction Workstation reconstructs and displays the images and sends them over DICOM to the hospital archive. The carbon-fibre Carbon-Fibre Table Top and thin Detector Head Cover are kept as thin as possible so they attenuate the gammas as little as possible, while the Housing and Covers presents a smooth, patient-friendly surface over the heavy shielded heads.