Isotropic source point in a homogeneous slab resulting in a non gaussian green function

[Pascal Nguyen]

Dear Prof. Fang,

Firstly, thank you for your amazing software.

I am attempting to simulate an isotropic source point inside a slab using "pattern3d" so that I can extend it to an isotropic pattern of fluorescence. However, with the source type "pattern3d" and the isotropic source point, when I attempt to examine the last slice of the green function computed via MCX, it should exhibit a Gaussian profile. Unfortunately, it does not.

I have tested with an another Monte Carlo software which gave me a gaussian profile, did I configure MCX wrongly ?

Here is a part of my MCX configuration using your python wrapper :

# Volume Load
vol = np.ones((51,41,16))
#MIDDLE
vol[26, 21, 8] = 2

cfg={}
cfg['vol'] = vol
cfg['prop'] = np.array([[0,0,1,1], #mm-1
[0.2,0.05,0.9,1.37],
[0.2,0.05,0.9,1.37]])

mask_pattern = copy.deepcopy(cfg['vol'])
mask_pattern[mask_pattern != 2] = 0
mask_pattern[mask_pattern == 2] = 1

cfg['srctype'] = 'pattern3d'
cfg['srcpattern'] = mask_pattern
cfg['srcpos'] = [0, 0, 0];
cfg['srcdir'] = [0, 0, 1, float("nan")] # nan means random direction
cfg['srcparam1'] = list(cfg['srcpattern'].shape) + [0]

cfg['nphoton'] = float(1e7)

cfg['tstart'] = 0
cfg['tend'] = 5e-9
cfg['tstep'] = 5e-9
cfg['bc'] = '______000001'
cfg['savedetflag'] = 'xv'
cfg['maxdetphoton'] = 1E8
cfg['srctype'] = "pencil"
cfg['autopilot'] = 1
cfg['gpuid'] = 1
cfg['unitinmm'] = 1
cfg['debuglevel'] = 'P'
cfg['outputtype'] = 'fluence'

Thank you in advance,

Best regards,

Pascal Nguyen

[Qianqian Fang]

hi Pascal,

I believe what you saw was the voxel shape effect - your domain has very low scattering, so the light intensity across the domain is related to the light emitted along each direction. by setting one voxel as emitter, you can see that angular distribution of the emitted rays are not spatially uniform.

either you replace it by an 'isotropic' source or increase your medium scattering should give you more uniform angular distribution.

Qianqian

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[Pascal Nguyen]

Hi Prof. Fang,

Using isotropic source works perfectly. Thank you very much Prof. Fang.

Best regards,

Pascal Nguyen