Photon absorption
An absorption event requires that the absorbed photon fraction be added
to a matrix in which different elements correspond to different positions in the
tissue. For example, if a weighting technique is used with the variable stepsize
method, the appropriate element of the absorption matrix is incremented by
(1-a)w. The number of bins in the absorption matrix is determined by the spatial
resolution desired. Increasing the number of entries increases the spatial
resolution, but also increases the absorption uncertainty in each element
(because fewer absorption events will take place in each element and the error is
inversely proportional to the square root of the number of absorption events).
The fluence rate is obtained by dividing the final value of each matrix element by
(1) the equivalent spatial volume of the element, (2) the absorption coefficient,
(3) the total number of photons propagated, and (4) the initial weight of each
photon.
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