1. An X-ray detector apparatus ( 14 ) comprising an array of detector pixels ( 20 ), each pixel comprising a conversion element ( 26 , 260 ) for converting incident radiation into a charge flow, a charge storage element ( 28 ) and a switching arrangement ( 50 ) enabling the charge stored to be provided to an output of the pixel, wherein the array of pixels is arranged into a plurality of sub-arrays ( 40 ), each sub array ( 40 ) comprising a plurality of pixels, the pixels in each sub-array sharing a common dose sensing output provided to a dose sensing output conductor ( 42 ; 72 ) which extends to a periphery of the pixel array, wherein the dose sensing output conductor ( 42 ; 72 ) for one sub-array of pixels passes through the area occupied by another sub-array of pixels, wherein a plurality of additional screening electrodes ( 120 ) are provided, with a screening electrode ( 120 ) substantially adjacent the dose sensing output conductor ( 72 ) for each sub-array of pixels.