Fig. 1

Schematic representation of the N _grid × N _grid grid points (black dots) centred about the detector plane intercept point (yellow square). The grid is, in this case, distributed over four pixels (whose borders are represented by the grey lines), and the numbers of grid points in the respective pixels are 4, 8, 8, and 16 for quadrants 1, 2, 3, and 4 (Q1, Q2, Q3, and Q4), respectively. The signal contribution to a voxel from a specific pixel is the product of the pixel value and a normalization factor, that is, the number of grid points in the pixel divided by the total number of grid points in all of the detectors for the current voxel