Radiation imaging system, 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 radiation from a specified direction is 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 defines a holding chamber G, and a holder holding the detecting unit is 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. 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.