detector inside the medium

[zhang haohui]

Dear Prof. Fang,

I am currently using your mcxlab package and would like to detect the number of photons with a photodetector embedded inside the medium.

From the manual, I see that "mcx detectors captures photons at the "intersecting surface" between tissue-0-voxel boundary and a sphere of given radius". My question is: is it possible to define a small surface inside the medium and capture the photons passing through that surface?

Thank you very much for your help.

Best wishes

Haohui

[Qianqian Fang]

hi Haohui,

the reason that mcx imposes this restriction (detection only happens before a photon exiting the domain) is to prevent continued propagation after detection - which is not physically possible.

because of that, detectors must be either located on the bounding box of the domain or the interface between a zero/non-zero voxel.

if your detector are indeed located inside the domain, you will have to create a small "cavity" region filled with 0-valued voxels. for example, you can do

cfg.issrcfrom0 = 1;
cfg.vol=ones(60, 60, 60);
cfg.vol(28:32, 28:32, 10) = 0;
cfg.detpos = [30,30,10, 4];

this should be able to capture all photons exiting from the voxel boundaries of those set to 0. As you can see, a limitation of this approach is that it must be a gap at least 1 voxel wide.

Qianqian

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[Qianqian Fang]

no, they won't. any photon leaving from a non-zero voxel to the background will be terminated.

On 9/1/25 19:07, zhang haohui wrote:

Hello, Prof. Fang,

Thank you very much for your reply. I find your method very clever and now have a follow-up question. For photons that leave the medium and enter the small void, will they be able to propagate back into the medium?  

Best regards

Haohui

[zhang haohui]

Hello, Prof. Fang

Thank you for your explanation. 

Best regards

Haohui