[Castor-users] Scatter rate estimation for LM-TOF-MLEM

[jbaran]

Hi Simon,

thanks for your prompt reply. and sorry for the late response.

I did as you suggest. I remove the -ignore-corr fdur flag from the 
reconstruction. I attached the results for both, NEMA IEC phantom 
(scatter fraction calculated by ourselves) - NEMA_IEC_fdur.png - and the 
cuboid phantom (scatter correction calculated by CASToR) - 
cuboid_fdur.png.

For the NEMA IEC plot, all presented images are reconstructed with 
scatter correction implemented and tested by ourselves. The difference 
is in an applied -ignore-corr fdur flag and scaling factor of the 
scatter rates and the legend is as follows:
TOP LEFT: scalling factor = 20000 with -ignore-corr fdur
TOP RIGHT: scalling factor = 1  (without -ignore-corr fdur frame 
duration is incorporated)
BOTTOM LEFT: scalling factor = 20000 superimposed on scalling factor = 1 
with -ignore-corr fdur (the images looks the same)
BOTTOM RIGHT: profiles along the orange line from previous plots

You can see that three of the reconstructed images looks very similar. 
The most significant difference is in the image on TOP LEFT so the one 
with scalling factor = 20000 with -ignore-corr fdur. But still the 
effect of the scatters is observable.

Regarding the way of scatter rate estimation we did in principle as 
follows. We calculate the scatter corrections for each TOF bin. Then we 
merge some LORs to calculate the let say downsampled scatter rate. Then 
we divided this number by the number of LORs and time. In principle you 
can find some similarity to the SSRB procedure.

For the cuboid study where the CASToR calculate the scatter rate factors 
internally, the results doesn't change. For this case I reconstruct 
images once again but without -ignore-corr fdur flag. Incorporation of 
the scatter rate does not give any improvement of the image quality. It 
seems that even with nonTOF data these is something wrong with scatter 
correction.

The legend for the plot (cuboid_fdu.png) is as follows:
TOP LEFT: true + scatter coincidences scatter corrected
TOP RIGHT: true + scatter coincidences
BOTTOM LEFT: true coincidences
BOTTOM RIGHT: profiles along the orange line from previous plots.

All the best,
Jakub



On 2021-12-07 11:51, STUTE Simon wrote:
> Hi,
> 
> OK thanks for the explanations.
> 
> Why ignoring the frame duration in the reconstruction ?
> 
> As the scatter correction is supplied as a rate, you really need the
> frame duration to rescale the scatters to the prompt coincidences.
> 
> Could you try to reconstruct with the frame duration taken into account 
> ?
> 
> How did you translate the estimation of the scatter rates based on a
> sinogram to the list-mode data ? Did you take the binning size into
> account ? You need to divide your scatter rate for a TOF bin by the
> size of the TOF bin to get an "almost" continuous measurement.
> 
> Simon
> 
> ________________________________________
> De : jbaran <jbaran at ifj.edu.pl>
> Envoyé : mardi 7 décembre 2021 10:59
> À : STUTE Simon
> Cc : Castor Users
> Objet : Re: [Castor-users] Scatter rate estimation for LM-TOF-MLEM
> 
> Hi Simon,
> 
> many thanks for the quick response. The answers are as follows:
> 
> 
> On 2021-12-07 09:56, STUTE Simon wrote:
>> Hello Jakub,
>> 
>> Thanks for the detailed email !
>> 
>> Do you take the scan duration into account in the datafile header ?
> 
> Yes, I added the information about the scan duration in the header. On
> the other hand in the reconstruction I used -ignore-corr fdur option.
> 
>> 
>> You say that you estimated the scatter rate for each TOF bin
>> independently to be 1-2. Is it "1 or 2" (which would be insanely high)
>> or "e-2" ?
> 
> I was not explicit here. The scatter rate passed to the castor lm is
> ranging from 0 to 2. However most values are in the range from 1e-3 to
> 1e-1.
> 
>> 
>> How did you compute this estimated scatter rate ?
> 
> We made basic estimation based on Monte Carlo simulations - NEMA IEC
> study. For the cubic study we use -sc flag during the conversion of the
> ROOT data to CASToR. For this the scatter rate is at the level of  
> about
> 1e-4.
> 
>> 
>> And how did you translate this information into your list-mode file ?
> 
> We adapt the CASToR code to read the scatter correction rate factor 
> from
> the matrix provided by ourselves.
> 
>> 
>> Best
>> Simon
> 
> All the best,
> Jakub
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