Couch movements

 
Scales of the lateral and longitudinal movement of the couch should be checked with marks made on a sheet of paper on the couch. The check of the couch vertical movement should be made by noting the position of the crosswires at different couch heights. Gantry sag can cause an apparent misalignment of the couch and this can be checked using a plumb-line. The scales indicate absolute distances from a settled point and must be within 2 mm of their established value. In practice relative movements of the couch must be more accurate than the absolute indication (depending the method of the equipment is used).

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