Issue using detectors with negative radius

[Lorenzo Spinelli]

Dear Dr. Fang,

trying to using the new feature of negative radius detector to construct a ring detector, I noticed an issue in calculating the time point spread functions: the TPSFs calculated with the new feature result about twice larger than the TPSFs calculated by approximating the ring detector with a series of circular detectors. From a comparison with the solution of the diffusion equation, the latter TPSFs seem the correct ones.

Thanks a lot for your attention.

Best regards,

Lorenzo Spinelli

[Lorenzo Spinelli]

Dear Qianqian,

thank you so much for the new release of MCX.

However, I have to came back to an issue I noticed about the detector with negative radius to create ring detectors.

In particular, I would like to have 3 ring detectors of radius 10, 20 and 30 mm, with thickness 2, 4 and 6 mm, respectively.

Then, I created the detector array as:

cfg.detpos(:, :) = [

    50    50     0    -9

    50    50     0    11

    50    50     0   -18

    50    50     0    22

    50    50     0   -27

    50    50     0    33

]

My volume is a 100x100x10 box with the source at [50 50 0].

If it is valid that detectors are checked in order to establish to which detector the detected photon has to be assigned, I think this detector array is correct.

Indeed,  the detected photons have only the labels 2, 4 or 6 for detector, as these are the detectors with positive radius. 

However, when I reconstruct the normalized distributions of time of flight, that is the time point spread functions, they result different from same TPSFs calculated by approximating the ring detectors with a series of circular detectors: it seems that the number of detected photons is over estimated.

Can you please help me, indicating if there is some error in the definition of the detector array?

Best,

lorenzo

---

Da: mcx-...@googlegroups.com <mcx-...@googlegroups.com> per conto di Lorenzo Spinelli <lspin...@gmail.com>
Inviato: mercoledì 8 gennaio 2025 15:21
A: mcx-users <mcx-...@googlegroups.com>
Oggetto: [mcx-users] Issue using detectors with negative radius

 

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[Qianqian Fang]

hi Lorenzo,

when I ran a test using your described domain/detector settings, I saw the detected photon positions are distributed in the way shown in the attached figure

this is expected because mcx detects photons at the intersections between the spherical detector and the boundary. because the thickness of the domain, your detectors capture photons from both z=0 and z=10 planes.

I suppose the presence of the photons on the transmission side may skew your TPSF.

if you want to limit the detected photons only on the z=0 plane, you can do so by using the exit position (cfg.issaveexit=1, or cfg.savedetflag contains flag 'x')

plotmesh(detp.p(detp.p(:,3)<1,:), '.')

let me know if this addresses your issue.

Qianqian

[Lorenzo Spinelli]

Dear Qianqian,

thanks for the fast reply. 

I understood the problem and applied your suggestion to fix it. 

Now, everything it is ok. 

Thank you again. 

Bye,

Lorenzo