[Castor-users] Units of reconstructed images

[Thibaut Merlin]

  Dear Jan,

I think I already answered to your student but I will copy my answer here
in case someone from the list has a similar question:

By default, the units in the reconstructed images are basically
count/voxel. The calibration factor (defined in the histogram) is to
normalize the sensitivity image values in order to get the resulting images
in kBq/mL or any meaningful value. One potential way to compute such factor
is to use a uniform object such as a cylinder with uniform activity
concentration which is known. From the estimated value in the reconstructed
images and the known activity concentration, one could derive a calibration
factor.
The sensitivity image values are computed by projecting all the line of
response of a normalization or histogram datafile (if provided) or all the
available lines of response from the geometry (default). Due to the number
of lines, the sensitivity image values tend to be rather high for usual
systems (>1e5), resulting in very low reconstructed image values if the
calibration factor is not fixed.

Regarding the mailing-list, one must suscribe to it (either during
registration or from this page:https://castor-project.org/mailing-list),
and use the same email address used for the subscription in order to use
it. There were however some issues with the mailing-list which caused
several emails to be stucked.

Best regards,
Thibaut

"Grahe, Jan" <jan.grahe at pmi.rwth-aachen.de> a écrit :

> Dear CASToR users,
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>    Could you help us answer a question regarding the reconstructed image
> units in CASToR?
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>    Basically, a student of mine reconstructed a Monte-Carlo simulation
> of a NEMA IQ phantom which should exhibit activities from 5.3kBq/ml in
> the background to around 21.2kBq/ml in the hot spheres. The
> reconstruction is performed using CASToR with a voxel size of 2x2x2mm3.
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>    When visualizing the reconstructed images in amide we obtain values
> in the hot spheres in the order of magnitude of 4e-4, but we do not know
> which unit the images have and whether they are dependent on the voxel
> size. The 4e-4 should match the input of 21.2kBq/ml if we did not make a
> mistake using either GATE or CASToR.
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>    Therefore, how can we convert the values of the reconstructed images
> into values of activity per ml (kBq/ml)?
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>    While we are asking about units, what is the unit of the sensitivity
> map? The sensitivity map has values of about 1e5-1e6.
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>    Thanks for your help and best regards,
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>    Jan
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>    P.S.: Seems like my student cannot post to the mailing list, so I
> tried on his behalf. If you need the exact command line arguments, I
> will ask my student and deliver them subsequently.
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