Characterizing the modulation transfer function (MTF) of proton/carbon radiography using Monte Carlo simulations

Authors

  • Seco Joao,

    1. Department of Radiation Oncology, Francis H. Burr Proton Therapy Center, Massachusetts General Hospital (MGH), Boston, Massachusetts 02114
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  • Oumano Michael,

    1. Department of Physics, University of Massachusetts at Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02125
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  • Depauw Nicolas,

    1. Department of Radiation Oncology, Francis H. Burr Proton Therapy Center, Massachusetts General Hospital (MGH), Boston, Massachusetts 02114 and Center for Medical Radiation Physics (CMRP), University of Wollongong (UoW), NSW 2522, Australia
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  • Dias Marta F.,

    1. Department of Biomedical Engineering and Biophysics, University of Lisbon, Alameda da Universidade, 1649-004 Lisboa, Portugal
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  • Teixeira Rui P.,

    1. Department of Biomedical Engineering and Biophysics, University of Lisbon, Alameda da Universidade, 1649-004 Lisboa, Portugal
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  • Spadea Maria F.

    1. Department of Experimental and Clinical Medical, Magna Graecia University, Catanzaro 88100, Italy
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Abstract

Purpose:

To characterize the modulation transfer function (MTF) of proton/carbon radiography using Monte Carlo simulations. To assess the spatial resolution of proton/carbon radiographic imaging.

Methods:

A phantom was specifically modeled with inserts composed of two materials with three different densities of bone and lung. The basic geometry of the phantom consists of cube-shaped inserts placed in water. The thickness of the water, the thickness of the cubes, the depth of the cubes in the water, and the particle beam energy have all been varied and studied. There were two phantom thicknesses considered 20 and 28 cm. This represents an average patient thickness and a thicker sized patient. Radiographs were produced for proton beams at 230 and 330 MeV and for a carbon ion beam at 400 MeV per nucleon. The contrast-to-noise ratio (CNR) was evaluated at the interface of two materials on the radiographs, i.e., lung-water and bone-water. The variation in CNR at interface between lung-water and bone-water were study, where a sigmoidal fit was performed between the lower and the higher CNR values. The full width half-maximum (FWHM) value was then obtained from the sigmoidal fit. Ultimately, spatial resolution was defined by the 10% point of the modulation-transfer-function (MTF10%), in units of line-pairs per mm (lp/mm).

Results:

For the 20 cm thick phantom, the FWHM values varied between 0.5 and 0.7 mm at the lung-water and bone-water interfaces, for the proton beam energies of 230 and 330 MeV and the 400 MeV/n carbon beam. For the 28 cm thick phantom, the FWHM values varied between 0.5 and 1.2 mm at the lung-water and bone-water interface for the same inserts and beam energies. For the 20 cm phantom the MTF10% for lung-water interface is 2.3, 2.4, and 2.8 lp/mm, respectively, for 230, 330, and 400 MeV/n beams. For the same 20 cm thick phantom but for the bone-water interface the MTF10% yielded 1.9, 2.3, and 2.7 lp/mm, respectively, for 230, 330, and 400 MeV/n beams. In the case of the thicker 28 cm phantom, the authors observed that at the lung-water interface the MTF10% is 1.6, 1.9, and 2.6 lp/mm, respectively, for 230, 330, and 400 MeV/n beams. While for the bone-water interface the MTF10% was 1.4, 1.9, and 2.9 lp/mm, respectively, for 230, 330, and 400 MeV/n beams.

Conclusions:

Carbon radiography (400 MeV/n) yielded best spatial resolution, with MTF10% = 2.7 and 2.8 lp/mm, respectively, at the lung-water and bone-water interfaces. The spatial resolution of the 330 MeV proton beam was better than the 230 MeV proton, because higher incident proton energy suffer smaller deflections within the patient and thus yields better proton radiographic images. The authors also observed that submillimeter resolution can be obtained with both proton and carbon beams.

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