The Gamma Camera

A radionuclide tracer accumulated inside the body is only clinically useful once its gamma emission has actually been detected and turned into an image. The gamma camera is the instrument that does this job — and, unlike a simple sensor that just counts photons, it has to work out where each one came from.

The problem: detecting gamma rays and locating them

Detecting a gamma photon is not, on its own, very useful — a bare detector placed anywhere near the patient will register photons arriving from all directions, with no way to tell which part of the body they came from. To build a genuinely useful image showing where a radionuclide tracer (such as Tc-99m, covered in the first lesson of this topic) has accumulated, the camera must associate each detected photon with a specific direction, and therefore a specific location in the patient. This is exactly the job split between the gamma camera's three main components, arranged in the order gamma photons actually pass through the device: the collimator, the scintillator, and the photomultiplier tube array.

The collimator: selecting direction

The collimator is a thick sheet of dense, gamma-absorbing material — typically lead — pierced by a very large number of narrow, parallel channels, positioned as the first component the emitted gamma photons encounter. Its role is purely geometric. Only photons travelling very close to the exact direction the channels are aligned in can pass straight through a channel and reach the detector behind; a photon arriving at any other angle strikes the lead between or along a channel and is absorbed before it ever reaches the detector.

This is what allows the camera to build a real image rather than simply detecting "some gamma radiation from somewhere": a photon that makes it through a given channel must have travelled along (very nearly) that channel's direction, so wherever it is detected on the far side directly corresponds to a specific line back through the patient.

The scintillator: converting gamma photons to light

Immediately behind the collimator sits the scintillator, a single large, flat crystal. When a gamma photon that has passed through the collimator strikes this crystal, its energy is absorbed and converted into a small flash of visible light. This is directly analogous to the scintillator layer used in a flat panel X-ray detector, covered earlier in this module — the same basic principle (high-energy photon in, visible light flash out) applies, just now detecting gamma rays emitted from inside the patient rather than X-rays transmitted through them.

The photomultiplier tube array: detecting and amplifying the light

Behind the scintillator crystal sits an array of many individual photomultiplier tubes (PMTs), each capable of detecting extremely faint light and amplifying it into a measurable electrical signal.

A photomultiplier tube operates in stages: incoming light strikes a photosensitive surface at the front of the tube, releasing a small number of electrons. These electrons are then accelerated by a potential difference toward a series of further electrode stages inside the tube, and at each stage the arriving electrons knock out several more electrons than struck it — a cascade that multiplies the original handful of electrons into a very much larger number by the time they reach the far end of the tube. The result is a sizeable, easily measured electrical pulse produced from what began as a single very weak flash of light.

Position logic: from many signals to one location

A single scintillation flash in the crystal is rarely detected by only one PMT — because the flash of light spreads out somewhat before reaching the PMT array, several neighbouring PMTs typically each register some signal simultaneously, with the strength of each PMT's signal depending on how close that PMT is to the actual point where the flash occurred (a PMT directly above the flash registers a strong signal; one further away registers a weaker one).

Electronic processing circuitry compares the relative signal strengths recorded across the whole PMT array for each individual detected event, and from this pattern calculates the most likely original position of the scintillation flash in the crystal — and therefore the position on the collimator (and ultimately the direction back into the patient) that the original gamma photon came from. Repeating this position calculation for very many individual gamma photons detected over the course of a scan, and plotting each calculated position, builds up a complete image showing where the radionuclide tracer has accumulated within the patient.

Mark as complete