Imaging Techniques

Abstract

Almost without exception, modern nuclear medicine images are acquired using a gamma camera based on the principles first suggested by Anger in 1957 (Fig. 4.1).¹ The camera consists of a scintillation detector made of a single sodium iodide crystal which is typically between 200 mm and 500 mm diameter and between 6 and 20 mm thick. Behind the crystal are a number of photomultiplier tubes (up to 90 or 100) which are optically coupled to the surface of the crystal and which sense the scintillations arising whenever a gamma ray is absorbed by the crystal. The areas of crystal seen by the tubes overlap, but the location of each scintillation can be computed from the relative responses in each tube. The energy of the photons can also be measured from the response of the tubes, and the electrical output from the camera head to the imaging computer consists of x and y coordinates and photon energy (z) for each scintillation that is detected. In front of the crystal is a collimator which consists of a disc of lead penetrated by a honeycomb of holes. The holes allow only gamma rays that are travelling perpendicularly to the crystal face to enter the crystal and in this way the gamma photons absorbed by the crystal form an image of the distribution of radionuclide in front of the camera.