A gamma camera consists of a large diameter (25-40 cm) NaI(Tl) crystal, ~1 cm thick;
The crystal is viewed by an array of 37-91 PM tubes;
PM tubes signals are processed by a position circuit which generates +/- X and +/- Y signals;
These position signals are summed to form a Z signal which is fed to a pulse height analyser;
The +/- X, +/- Y and discriminated Z signals are sent to a computer for digital image processing;
A collimator is used to improve the spatial resolution of a gamma-camera;
Collimators typically consist of a Pb plate containing a large number of small holes;
The most common type is a parallel multi-hole collimator;
The most resolvable area is directly in front of a collimator;
Parallel-hole collimators vary in terms of the number of holes, the hole diameter, the length of each hole and the septum thickness - the combination of which affect the sensitivity and spatial resolution of the imaging system;
Other types include the diverging-hole collimator (which generates minified images), the converging-hole collimator (which generates magnified images) and the pin-hole collimator (which generates magnified inverted images);
Conventional imaging with a gamma camera is referred to as Planar Imaging, i.e. a 2D image portraying a 3D object giving superimposed details and no depth information;
Single Photon Emission Computed Tomography (SPECT) produces images of slices through the body;
SPECT uses a gamma camera to record images at a series of angles around the patient;
The resultant data can be processed using Filtered Back Projection and Iterative Reconstruction;
SPECT gamma-cameras can have one, two or three camera heads;
Positron Emission Tomography (PET) also produces images of slices through the body;
PET exploits the positron annihilation process where two 0.51 MeV back-to-back gamma-rays are produced;
If these gamma-rays are detected, their origin will lie on a line joining two of the detectors of the ring of detectors which encircles the patient;
A Time-of-Flight method can be used to localise their origin;
PET systems require on-site or nearby cyclotron to produce short-lived radioisotopes, such as C-11, N-13, O-15 and F-18.
