llimator explain</h3>
In a protocol the collimator is usually presented in a string like '16x0.55' or '32x1.2', 
where the '16x0.55' means there are 16 slices and the thickness of each slice is 0.55mm. 
We strongly suggest to define the collimator in a string but NOT two numbers 16 and 0.55, 
there are some reasons, one reason is the nominal thickness is not always real, e.g. the 0.55 could be really 0.551024mm which shall be calculated by the 
detectors size and system geometry but not laid down priorly.
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