L. Montani

Differences among Monte Carlo codes in the calculations of voxel S values for radionuclide targeted therapy and analysis of their impact on absorbed dose evaluations

Several updated Monte Carlo (MC) codes are available to perform calculations of voxel S values for radionuclide targeted therapy. The aim of this work is to analyze the differences in the calculations obtained by different MC codes and their impact on absorbed dose evaluations performed by voxel dosimetry. Voxel S values for monoenergetic sources (electrons and photons) and different radionuclides (90Y, 131I, and 188Re) were calculated. Simulations were performed in soft tissue. Three general-purpose MC codes were employed for simulating radiation transport: MCNP4C, EGSnrc, and GEANT4. The data published by the MIRD Committee in Pamphlet No. 17, obtained with the EGS4 MC code, were also included in the comparisons. The impact of the differences (in terms of voxel S values) among the MC codes was also studied by convolution calculations of the absorbed dose in a volume of interest. For uniform activity distribution of a given radionuclide, dose calculations were performed on spherical and elliptical volumes, varying the mass from 1 to 500 g. For simulations with monochromatic sources, differences for self-irradiation voxel S values were mostly confined within 10% for both photons and electrons, but with electron energy less than 500 keV, the voxel S values referred to the first neighbor voxels showed large differences (up to 130%, with respect to EGSnrc) among the updated MC codes. For radionuclide simulations, noticeable differences arose in voxel S values, especially in the bremsstrahlung tails, or when a high contribution from electrons with energy of less than 500 keV is involved. In particular, for 90Y the updated codes showed a remarkable divergence in the bremsstrahlung region (up to about 90% in terms of voxel S values) with respect to the EGS4 code. Further, variations were observed up to about 30%, for small source-target voxel distances, when low-energy electrons cover an important part of the emission spectrum of the radionuclide (in our case, for 131I). For 90Y and 188Re, the differences among the various codes have a negligible impact (within few percents) on convolution calculations of the absorbed dose; thus either one of the MC programs is suitable to produce voxel S values for radionuclide targeted therapy dosimetry. However, if a low-energy beta-emitting radionuclide is considered, these differences can affect also dose depositions at small source-target voxel distances, leading to more conspicuous variations (about 9% for 1311) when calculating the absorbed dose in the volume of interest.

Factors affecting flat panel PMT calibration for gamma ray imaging

Hamamatsu H8500 Flat Panel PMT represents the last technological advancement in gamma ray imaging. Compact size makes it attractive for medical imaging application. To study and compare image performance two Flat Panel PMTs were coupled to CsI(Tl) and NaI(Tl) scintillation arrays with 3 mm and 1.8 mm pixel size respectively and they were connected to multi-anode electronic readout (64 channel). Furthermore a pulsed blue LED coupled to an optical fiber was utilized to scan the tube with different light distribution spreading. The study took into account how PMT anode gain uniformity response, light distribution and intensity, influence spatial resolution, position linearity and image noise. Gain calibration was firstly studied because of PMT gain anode non uniformity response, which range between 27:100 and between 45:100 respectively. Furthermore each crystal pixel produces different charge distribution and this depends on the matching between anode and scintillation array lattice. The amount of anode charge can change more than a factor five for narrow light distributions. Tube gain setting results critical, in fact because of energy resolution of each anode spectra, only a factor five pulse height variation can be adequately converted by ADC. In addition there is a further gain anode variation due to PMT non uniformity response of a factor 3. This mentioned two elements, do not allow to convert all pulses in the useful pulse height ADC range. As a consequence image position distortion and background are produced. Flat Panel shows good image performance. However, because of the big anode size and PMT gain non uniformity response, the gain setting can be critical to obtain the best image performance for scintillation light distribution comparable with anode size.

CsI(Tl) Micro-Pixel Scintillation Array for Ultra-high Resolution Gamma-ray Imaging

The aim of this paper is to investigate the intrinsic spatial resolution limit by coupling a CsI(Tl) micro-pixel scintillation array to position sensitive photomultipliers (PSPMTs) for ultra-high resolution gamma-ray imaging. On this purpose, 1 mm thick array with 0.2 mm pixel side, 0.4 mm pitch has been realized by Spectra Physics (Hilger). The present scintillation arrays technology is suitable to produce larger crystal areas. In this paper we present spatial resolution and positioning results obtained by coupling the micro-pixel scintillation array to Hamamatsu square PSPMTs: 1rdquo R8520-C12, 1rdquo R5900-L16 and 2rdquo H8500 Flat panel PMT. Preliminary measurements demonstrate better performance in term of uniformity response when micro-pixel array is coupled to a H8500 PSPMT model. This setup carries out an intrinsic spatial resolution lower limit of about 0.6 mm FWHM at 50% FWHM energy resolution, defining it as the minimum scintillation array pitch detectable at 122 keV. The results obtained by R5900-L16 with a better sampling of the scintillation light has shown an improvement of the position linearity in spite of a worse spatial resolution due to the poor light output of scintillation array.

CATHECTOR: A GAMMA-RAY DETECTOR IN A CATHETER

This work is aimed to study the feasibility of a low energy gamma-ray detector for nuclear medicine to be inserted into a body cavity, duct, or vessel. The small dimension detector concept is based on a CsI:Tl scintillation crystal coupled to a Si Avalanche Photodiode (APD) mounted in a catheter. Due to the availability of Tc in nuclear medicine departments, 140 keV photons have been considered for detector design. At this energy the response concerns a more extended volume nearby the cathector than in the case of beta emitting radiotracers. Monte Carlo simulations have been performed and results have then been compared to experimental ones. The latter have been obtained using a Co radioisotopic source and a laboratory setup including a detector having dimensions not optimized coupled to a low-noise conventional electronics. Count rates and sensitivities have been measured approaching a point source to the detector along some paths and considering different background irradiation levels.

DISIS - a computer simulation code for discrete scintillation imagers

A computer simulation code has been developed in order to foresee the response of discrete scintillation imaging devices. Discrete Scintillation Imager Simulator (DISIS) has been designed for imagers based on a scintillation array coupled to a position sensitive light sensor (like position sensitive photomultiplier tube or avalanche photodiode array) by a planar light guide. The simulator is a deterministic code that uses: (i) a model describing the single photon light distribution emerging from a crystal pixel for charge integrals evaluation; (ii) the assigned algorithm for centroid calculation; and (iii) the Gaussian spread for localizing, crystal by crystal, the events on the image. In particular DISIS allows us to study the spatial response over the imager field of view changing parameters individually. The imager optimization can be obtained searching an acceptable pixel identification. To this aim a good trade-off between the spread of light distribution, the light sampling capability over the light-sensor area and the centroiding algorithm has to be found.