Fanbeam detector distance from vertex

I am trying to set up a fan beam system for reconstruction. I do understand that "D" is defined as the vertex from the source to the center of rotation. My question is how do we define the distance from the detector to the vertex or center of rotation? Most of the images I see, the radius of the detector arc appears to be larger than the radius (D) of the source arc. The angles can be the same but the difference in distances may need to be accounted for?

Our data ends up a column of numbers for the detector positions at each souce projection. This results in a 2D matrix of positive counts. Is there any processing that needs to be done to this matrix before entering it into Matlab? Adjusting for each detector distance from the source with the inverse square law? Taking a log of each count?

The basic code we are using is shown below. We are using the value of D as the physical measurement in mm.

D=400

ssensor=5;

dsensor=5;

output_size = D*2;

Ifan1 = ifanbeam(F,D, 'FanSensorGeometry', 'arc', 'FanCoverage', 'cycle', 'FanSensorSpacing',dsensor, 'FanRotationIncrement', ssensor , 'Filter', 'Ram-Lak', 'FrequencyScaling',1, 'Interpolation', 'linear', 'OutputSize',output_size);

figure; imagesc(Ifan1); colormap(jet); colorbar;