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<p>Dear Christopher Zhao,</p>
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<p>Glad to know that you are happy with CASToR.</p>
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<p>In CASToR, the system matrix is factorized using two components: a projector and an image-based PSF. We did it that way because it is generic and efficient and it allows to adapt to any geometry.<br>
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<p>1) The projector makes the link between the image space and the data space. We provide few geometric projector implementations that compute the elements on-the-fly, based on the scanner description. You can have a brief description with reference articles
of these projectors by running castor with the option '-help-projm'.</p>
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<p>2) As mentioned, you can consider the system matrix as a product of two components: the projector and the image-based PSF. This means that the PSF is applied each time the system matrix appears: in the forward and backward projection processes. When you
create a convolver with CASToR using the option "-conv gaussian,X,X,X::when", you need to specify when the convolution is applied. In your case, you likely used the "psf" keyword. If you look at the inline documentation using the option "-help-conv", you will
have all explanations about when the convolution is applied. For "psf", it says "(include both 'forward' and 'backward'; the standard image-based PSF modelling)".</p>
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<p>Auto-citing could be blamed, but have a look at Stute and Comtat 2013 "Practical considerations for image-based PSF and blobs reconstruction in PET" in Phys Med Biol. You will find some explanations about this but most importantly many relevant references
to the literature.</p>
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<p>Best</p>
<p>Simon<br>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>De :</b> Castor-users <castor-users-bounces@lists.castor-project.org> de la part de ÕÔöÎ <zhaoxin@szbl.ac.cn><br>
<b>Envoy¨¦ :</b> mardi 22 f¨¦vrier 2022 05:40<br>
<b>À :</b> castor-users<br>
<b>Objet :</b> [Castor-users] question about System Matrix & PSF in CASToR iteration</font>
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<span style="margin:.0px; padding:.0px; border:.0px; outline:.0px; font-family:Arial,sans-serif; font-size:15.0px; text-align:justify">Dear CASToR Users and Developers,</span><br>
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We have using CASToR to recon our PET data for half a year, and it shows impressive ability to provide the image that we urgently needed. But there are two problems that confused me£º</div>
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1£©How does CASToR generate its System Matrix when running OSEM? I used to know that before running MLEM iterative reconstruction, one should precompute the system matrix for the current specific PET scanner system, which will cost a lot, and must re-generate
it when changing different PET scanner geometry. But CASToR needs no precomputation and can recon any kind of PET geometry immediately, how does it happen?</div>
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2£©When does Point Spread Function calculation be used in MLEM iteration? I compared MLEM reconstruction with and without PSF calculation, and find out that the MLEM with PSF provides far better image results than another. <span style="font-family:Tahoma,Arial,STHeiti,SimSun">In
the attachment, I showed the reconstructed image of Derenzo by CASToR</span><span style="font-family:Tahoma,Arial,STHeiti,SimSun"> with PSF</span><span style="font-family:Tahoma,Arial,STHeiti,SimSun">, its image quality is good! I just what to in which step
does PSF work in CASToR MLEM iteration? Does anyone know which <span style="color:#000000; font-family:Tahoma,Arial,STHeiti,SimSun; font-size:14.0px; font-style:normal; font-weight:400; text-align:start; text-indent:.0px; text-transform:none; widows:2; background-color:#ffffff; float:none; display:inline">article</span> I should
read£¿</span></div>
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<span class=" __aliyun_node_has_color __aliyun_node_has_color __aliyun_node_has_color" style="margin:.0px; padding:.0px; border:.0px; outline:.0px; font-family:Tahoma,Arial,STHeiti,SimSun; font-size:14.0px; color:#000000"><span class=" __aliyun_node_has_color __aliyun_node_has_bgcolor __aliyun_node_has_color __aliyun_node_has_bgcolor __aliyun_node_has_color __aliyun_node_has_bgcolor" style="margin:.0px; padding:.0px; border:.0px; outline:.0px; color:#000000; font-family:Tahoma,Arial; font-size:14.0px; font-style:normal; font-weight:400; text-align:start; text-indent:.0px; text-transform:none; widows:2; background-color:#ffffff; float:none; display:inline">Thank you
a lot </span></span>for your help, any answer will be helpful.</div>
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<div style="clear:both">Christopher Zhao</div>
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