Photons Count Estimator

One of the most complex parameters to define in the RT and CT modules of CIVA is the number of photons that are used for the Monte-Carlo modelling algorithm. This number must be high enough to get a relevant and realistic result (as for other statistical models, it requires a significant number of photons to be computed to obtain reliable results), but too many photons will make the computation time prohibitive.

This number of photons to be modelled depends mainly of the following parameters:

The X-Ray (or gamma Ray) energy source
The inspected material
The thickness of the material

Since CIVA 2015, an estimator of the minimum number of photons to be simulated has been included, to help the user in preparing the configuration. This option has been developed to choose an adapted number of photons for each configuration.

To illustrate how this tool is operating and useful, we present here an example of a simulation of a pipe inspection with the so-called “ellipse” technique:

3D vizualisation of the pipe

With an insufficient number of Monte-Carlo photons defined in the configuration and performed without this estimator tool, the simulation provides the result displayed hereafter:

Full simulation (including direct and scattered radiation) with a low number of photons

We can clearly see that the result is not satisfactory, as most of the pixels on the detector were not impacted by any photons. It is nearly impossible to recognize the irradiated object: not enough Monte-Carlo photons have been simulated to converge towards a correct result. Without this estimator tool it is therefore necessary to relaunch as many simulations as needed increasing the number of photons to be simulated till a relevant result is achieved.

A click on the “Estimation” button launches a quick Monte-Carlo simulation. At the end of this simulation, CIVA proposes in the GUI a number of photons to consider in order to perform a correct simulation.