Effect of Lower-energy Source on the Tumor Representation in Neutron Stimulated Emission Computed Tomography: An Evaluation Study

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2017

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Abstract

Proposed is an investigation into the effect of lower-energy source on the tumor representation of an image acquired by a neutron-based spectroscopic imaging modality, Neutron Stimulated Emission Computed Tomography (NSECT).

The NSECT experiments were performed at a shielded neutron source of the Triangle Universities Nuclear Laboratory (TUNL), which was proficient at creating neutron beams of energy up to 20MeV. However, this neutron generator is not feasible for clinical use due to its large size. Smaller compact sources such as deuterium-deuterium (DD) neutron generators are attractive alternatives that can produce neutrons of sufficient energy to stimulate isotopes of interest in the human body. However, DD generator is not competent at producing neutrons of high energy. Thus, the focus of this work is to evaluate the effect of lower-energy neutrons, such as 2.5MeV and 3.2MeV, on the NSECT images.

The experiments were modeled and simulated in this work using a Monte Carlo toolkit, Geant4. In Geant4 space, an anthropomorphic phantom of cancerous tissue was scanned by a simulated neutron source. During scanning, the phantom was translated to cover the whole field of view (FOV) and rotated over 180 degrees for the purpose of tomographic imaging. Neutrons and gammas emitted were captured by a virtual detector, which could identify the energy and position of each particle. Information of position and energy of gammas detected resulted in a sinogram for an array of energies, created by selecting the energy characteristic to a specific element. Using the sinograms, two-dimensional maps of the spatial concentration of the element could be reconstructed through a reconstruction algorithm and the elemental concentration revealed the internal geometry of the phantom.

Images were generated when the phantom was scanned by 5MeV, 3.2MeV and 2.5MeV neutron sources. Comparison of tumor parameters in these images indicates that a neutron source of lower energy could degrade the tumor representation in a NSECT image on the aspects of concentration, brightness and underestimation of the tumor size. Then further investigations with 50,000, 100,000, 150,000 and 200,000 neutron events were performed respectively in the 3.2MeV-source case and 2.5MeV-source case in order to test whether the number of neutrons is correlated to the quality of the reconstructed images. Improvement of tumor representation, for example, a clearer tumor region and more accurate tumor size information, shows that increase in the number of incident neutrons has a positive effect on the reconstructed image. This work demonstrates the effect that low energy neutrons have on the image and verifies the feasibility of using low-energy neutrons as the source in NSECT breast imaging.

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Du, Yixiao (2017). Effect of Lower-energy Source on the Tumor Representation in Neutron Stimulated Emission Computed Tomography: An Evaluation Study. Master's thesis, Duke University. Retrieved from https://hdl.handle.net/10161/16410.

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