Unit Conversion#
In order to simplify usage, reconstructions are done using arbitrary length units (ALU). In this system, 1 ALU can correspond to any convenient measure of distance chosen by the user.
Default Parameter Example:
By default, when the CT model is constructed, the spacing between detector channels is set to 1 ALU.
However, the detector channel spacing can be changed by setting the delta_det_channel parameter.
So if we reconstruct CT data from a scanner with detector channel spacing of 0.25 mm, with this default behavior,
then delta_det_channel=1.0 and we interpret this as 1 ALU = 0.25 mm.
In this case, the MBIRJAX reconstruction will be a 3D JAX array recon in units of \(\mbox{ALU}^{-1}\).
However, this can be converted to conventional units of \(\mbox{mm}^{-1}\) with the following scaling:
NSI Reconstruction Example: The NSI preprocessing functions in Preprocessing set all the parameters to the
units used by NSI which are in mm.
In this case, 1 ALU = 1 mm and the reconstructions will have units of \(\mbox{mm}^{-1}\).
In order to convert to units of \(\mbox{cm}^{-1}\), we use the following scaling:
Zeiss Reconstruction Example: The Zeiss Versa preprocessing functions in Preprocessing allow the user
to select units of either “um”, “mm”, “cm”, or “m”, with the default being “mm”.
In this default case, 1 ALU = 1 mm and reconstructions will have units of \(\mbox{mm}^{-1}\).
Emission CT Example: Once again, we assume that the channel spacing in the detector is 0.25 mm,
and we again assume the default reconstruction parameters of delta_det_channel=1.0.
So we have that 1 ALU = 0.25 mm.
Using this convention, the 3D array, recon, will be in units of photons/AU.
However, the image can be converted to units of photons/mm using the following equation: