Rault, Erwann

English

Ghent University Academic Bibliography

Optimization of Yttrium-90 BremsstrahlungImaging with Monte Carlo SimulationsErwann RaultSupervisor(s): Ignace Lemahieu, Stefaan Vandenberghe, Steven StaelensI. INTRODUCTIONTARGETED radionuclide therapy (TRT) isan emerging technique for cancer treat-ment. It uses the properties of some pharma-ceuticals to fix in specific region of the body todeliver locally a lethal dose of radiation. Due toits physical properties, 90Y is a good candidatefor this therapy. Its high incidence of β− emis-sion optimizes the dose deposition within thetargeted cells while its low incidence of pho-tons emission minimizes the dose received byother organs. This low gamma emission limitshowever the imaging of the spatial distributionof the pharmaceutical in the body.The bremsstrahlung interactions of the emit-ted β− electrons in the body produces a con-tinuous spectrum of photons that can be usedfor imaging [1]. Previous results show the im-portance of using a medium energy (MEGP)collimator together with a low energy window(50 to 150 keV) to minimize contaminationand maximize the sensitivity while imagingbremsstrahlung photons. The aim of this studywas, based on Monte Carlo simulations, toevaluate the efficiency of basic correction tech-niques, for contamination and detector blur-ring, in terms of contrast and noise.II. MATERIALS AND METHODSA. SimulationsThe Philips AXIS systemwas simulated withthe GATE [2] (Geant4 Application for Tomo-—————————————————–E. Rault is with the Electronic and InformationSystems Department, Ghent University (UGent),Gent, Belgium. E-mail: Erwann.Rault@UGent.be .graphic Emission) package. A previously de-veloped fast bremsstrahlung photons generator[3] was used in this study to speed up the emis-sion of the bremsstrahlung photons.B. Image reconstructionAll images were reconstructed with theMLEM algorithm from 90 projections of thephantom. The images were reconstructed in128*128*128 matrices and the pixel size wasset to 3.67 mm.Photons that scatter in the phantom or in thedetector, as well as photons penetrating the col-limator are going to induce false information inthe sinogram and need to be corrected for. Thebackground subtraction technique (Bgd. Sub.),used in PET and previously described by Karpet al. [4], was investigated in this study.The distance dependent blurring of the de-tector (Det. Mod.) was also included in thereconstruction process. A system matrix wasestimated by the simulation of multiple pointsources at increasing distances from the colli-mator, and was then incorporated in both for-ward and back projections of the MLEM algo-rithm.C. Jaszczak phantomThe Jaszczak phantom was used to evalu-ate the noise and the contrast recovery for thedifferent reconstruction techniques. The simu-lated phantom was a cylinder homogeneouslyfilled with activity and containing five hot le-sions of respectively 19.1, 25.4, 31.8, 38 and45.6 mm diameter and a cold lesion of 45.6 mmdiameter (contrast: 8:1).The noise was defined as the normalizedstandard deviation of the pixels in the back-ground region. It was computed on homoge-neous slices of the phantom and from pixelssituated at the same distance from the center ofthe phantom than the lesions to avoid the effectof attenuation.The contrast recovery (CR) was computedusing the following equation:CR =µlesion−µbgdµbgd∗ 1C−1where µlesion represents the mean value in thelesion and mubgd the mean value in the back-ground. C is the theoretical contrast.III. RESULTSFigure 1. Reconstructed images.Figure 2. Contrast to Noise curves for the hotspot of31.8 mm.Reconstructed images of the phantom areshown in Fig. 1. The contrast recovery curvesshow the same behavior for all lesion sizes (hotand cold lesions). The curves corresponding tothe 31.8mm hot lesion are shown in Fig. 2. Forthis lesion, the detector modeling (Det Mod)improves the contrast recovery of about 13%for 25% noise in the background. The back-ground subtraction method (Bgd Sub) improvesthe contrast recovery of 30%, while the use ofboth methods rises the contrast by 52%. For thecold lesion, the use of both methods improvesthe contrast of 82%.IV. DISCUSSIONThe background subtraction technique givesthe best improvement in terms of contrast re-covery. This method is however enhancing thenoise in the images. This is the drawback ofsinogram based scatter subtraction techniques.One possible solution could be to integrate thescatter estimation inside the forward projectionof MLEM. This would ensure a better noise be-havior during the reconstruction process.All results presented in this study are basedon a simple geometry of the phantom. With-out any homogeneity, the contamination is wellapproximated with the background subtractiontechnique. For more complex geometries, thespatial distribution of the scatter would be moredifficult to assess.V. CONCLUSIONThis study presented an accurate method forthe reconstruction of bremsstrahlung images.This method, based on a contamination back-ground removal and on a resolution modelingof the camera in both forward and back projec-tions, can improve the contrast of 52% for a hotlesion of 31.8 mm and of 82% for a cold lesiongiven the same amount of noise in the images.REFERENCES[1] S. Shen, G. L. DeNardo, A. Yuan, D. A.DeNardo, and S. J. DeNardo, “Planar gammacamera imaging and quantitation of yttrium-90bremsstrahlung.,” J Nucl Med, vol. 35, no. 8, pp.1381–1389, Aug 1994.[2] S. Jan, G. Santin, D. Strul, and S. Staelenset al., “GATE: a simulation toolkit for PET andSPECT.,” Phys Med Biol, vol. 49, no. 19, pp.4543–4561, Oct 2004.[3] E. Rault, S. Vandenberghe, S. Staelens, J. DeBeenhouwer, , and I. Lemahieu, “Fast simula-tion of yttrium-90 bremsstrahlung with gate,” tobe published in nimA, 2008.[4] J. S. Karp, G. Muehllehner, D. A. MankofF,C. E. Ordonez, J. M. Ollinger, M. E. Daube-Witherspoon, A. T. Haigh, and D. J. Beerbohm,“Continuous-slice penn-pet: a positron tomo-graph with volume imaging capability.,” J NuclMed, vol. 31, no. 5, pp. 617–627, May 1990.

Optimization of Yttrium-90 Bremsstrahlung imaging with Monte Carlo simulations

https://biblio.ugent.be/publication/679165/file/751400

Optimization of Yttrium-90 Bremsstrahlung imaging with Monte Carlo simulations

Abstract

Similar works

Full text

Available Versions

Ghent University Academic Bibliography