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[Determination of focal distance and convergence assessment in collimators with focalized geometry].

OBJECTIVE
A method to calculate intrinsic parameters of fan beam collimators is presented. This method allows us to determine focal length and to assess the convergence of the collimator on a single focus line.


MATERIAL AND METHODS
Images of a radioactive source located successively in a series of known coordinate positions are acquired. To determine the focal distance, a method has been developed that minimizes the sum of the distances between the experimental image point and the theoretical ones. Collimator convergence was evaluated by the intersections between all the pairs of lines drawn between the source location and its experimental image point.


RESULTS
The results found between the two collimators showed that the actual focal length deviates 1.5% from the real focal value and the nominal focal length. The results also indicate that the collimator holes do not converge on a single focal line but rather on a focal area, with average deviations in regards to the real focus being *x = 2.5 mm and *z = 7.1 mm.


CONCLUSIONS
The errors in regards to the original values and the extension of the focal zone make it possible to conclude that the quality of the collimators used is acceptable.