Study of X-ray scattered in a phantom as function of its maximum energy during bimodal PET/CT imaging.

In a simultaneous Bimodal System with Positron Emission Tomography and Computed Tomography, the X-rays from a collimated X-ray tube are scattered over an anatomical region on the patient being scanning and cause a contamination effect on the signal received by the detectors distributed in the shape of a ring of the PET system. To study this phenomenon, each of the components of a bimodal system was modeled and simulated by the Monte Carlo method in Geant4 an X-ray beam produced in a RTW tube MCBM 65B with typical fluences used in medical diagnosis of small animals hitting a cylindrical phantom with a diameter greater than Field of View (FOV) in the tomographic center. The number of ionization events that occur in each of the phoswich detectors of the PET were obtained, the spatial distribution of the scattered X-rays was studied according to three maximum energies of the spectrum and calculation was made to find a filter that was located at the input of the phoswich detector that attenuates the X-rays by 98% and that would allow transmitting the gamma rays of annihilation also by 98%. For this, simulations were carried out using various filter materials, finding that copper is an excellent candidate and were found an optimum thicknesses between 0.5 mm and 1.3 mm, according to the average energy of the X-rays used in the different exploration techniques.

Optimization by Monte Carlo method of photon fluence from the X-ray beam spectrum in a bimodal tomographic system.

A bimodal tomographic system with a RTW MCBM 65B-50Mo X-ray tube and a XPAD3s semiconductor camera that contains 8 bars, each one with 67,200 hybrid pixels are modeled in GEANT4 simulation code. Several conical X-ray spectra were simulated, particularly a spectrum with a peak energy of 17.4 keV used in tomography on small animals. Three phantoms located in the tomographic center were added to the simulation to evaluate the image quality and its magnification based on the simulation of different photon fluences and the rotation effect of the tomographic system with an average angular velocity of 360o per minute. The images were recorded and analyzed in 2D through ROOT software toolkit in virtual XPAD3 detector. The quantitative method 20-80% of the maximum intensity of radiation was used for obtain the contouring of the phantoms, this method is used in radiotherapy and radiodiagnosis imaging. For this purpose, the images were taken to DICOM format in order to estimate the optical density of the contours and to evaluate the optimum and minimum photon fluence to be used in the tomographic system in order to reduce the absorbed doses in the individuals. This study allowed to determine the optimal fluence to validate it with realistic fluences used in the tomographic prototype ClearPET /XPAD-CT and to make an intercomparison with the absorbed doses measured with detectors located in the tomographic center.