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Imaging in medicine
Imaging in medicine

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6.3.1 Collimator

Without a collimator, gamma rays from all directions would be collected by the crystal and no useful image could be obtained. Gamma rays cannot be focused by a lens but a collimator consisting of a series of holes in a lead plate can be used to select the direction of the rays falling on the crystal. Most collimators in use today are parallel hole collimators. A parallel hole collimator is shown schematically in Figure 16.

Figure 16
Figure 16: Schematic diagram of a parallel hole collimator. Gamma rays are shown in purple. Obliquely incident gamma rays are absorbed in the collimator septa

The resolution and sensitivity of a collimator depend on a number of factors including:

  • hole size (h);

  • the thickness of the septa (s), the lead between the holes;

  • the length of the holes (l);

  • the energy of the gamma rays.

Different collimators are selected for different procedures – e.g. Low Energy High Resolution.