Radiation imaging systems, nuclear medicine diagnosis apparatus and positioning adjusting mechanism

A radiation imaging system includes a detecting unit including a plurality of radiation detecting elements arranged in a plane for radiation detection, a collimator provided with through holes respectively aligned with the radiation detecting elements and opening in an entrance surface such that radiations from a specified direction are selectively made to fall on the radiation detecting elements, a second case joined to a first case fixed to the side surface of the collimator to define a holding chamber G, a holder holding the detecting unit and placed in the holding chamber G such that spaces that allow the holder to be moved in a plane parallel to the entrance surface are formed between the holder and the second case, and a position adjusting mechanism for adjusting the positional relation between the second case and the holder by moving the holder relative to the second case in distance ranges defined by the spaces.