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The scintigraphic image corresponds to the projection of the distribution of radioactivity on
the crystal detector. Gamma rays cannot be focused using lenses as in the case of light. The use
of a special kind of collimator can permit just to one direction gamma rays to reach the crystal,
the most common being perpendicular to the crystal. A collimator is a wafer usually lead
wherein cylindrical or conical holes are drilled along a system axes determined. Gamma-ray
where the path does not borrow these directions is absorbed by the collimator before reaching
the crystal. The partition (wall) separating two adjacent holes i called "septa". The thickness of
lead is calculated to cause an attenuation of at least 95% of the energy of the photons passing
through the septa. The most commonly used collimator is the parallel holes. It retains the
dimensions of the image. For non-parallel collimators, the dimensions of the image depend on
the geometrical disposition and the divergence or convergence nature of the collimator. This
leads to a geometric distortion must be taken into account. The efficiency of a collimator is the
fraction of radiation passing through the collimator (without any interaction), reaching the
crystal and effectively participating in the image formation. The collimator resolution corresponds
to the accuracy of the image formed in the detector. Resolution improves with
increasing thickness of the septa at the expense of collimator efficiency. A good compromise
is to find the realization of a collimator performance depends on the intrinsic characteristics
of the detector and the use we want to make [2].
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