Real time image guided radiotherapy has been proposed by integrating an in-line 6 MV linear accelerator (linac) to a magnetic resonance (MR) imager in either a parallel or transverse configuration. In either configuration, magnetic interference in the linac is caused by its immersion in the magnetic fringe fields of the MR imager. Thus in order to minimize the effect of the magnetic interference, investigations on linac performance in external magnetic fields was completed through various simulations. Finite difference and finite element methods as well as particle simulations were performed in order to design an electron gun and an in-line 6 MV linac waveguide. Monte Carlo simulations provided calculations of dose distributions in a water tank from the derived electron phase space at the linac target. The entire simulation was validated against measurements taken from a commercial medical in-line 6 MV linac, other simulation programs, and theory. The validated linac simulation was used to investigate linac performance in external magnetic fields. The results of this investigation showed that the linac had a much lower tolerance to transverse magnetic fields compared to longitudinal fields. While transverse magnetic fields caused a global deflection of the electron beam away from the central axis of the waveguide, longitudinal fields changed the optics of the electron gun in a suboptimal way. Both transverse and longitudinal magnetic fields caused excessive beam loss if the field strength was large enough. Heating caused by excessive beam loss in external magnetic fields was shown to have little effect on the resonant frequency of the waveguide, and any change in dosimetry, if it existed, was shown to be easily corrected using the jaws or multileaf collimators (MLCs). It was determined that the low-field parallel configuration linac-MR system investigated did not require any magnetic shielding, so the focus was on shielding the transverse configuration. Using beam loss, MLC motor tolerance to magnetic fields, and MR imager homogeneity as constraints, passive and active magnetic shielding was designed and optimized. Thus through the parallel configuration, or using magnetic shielding, magnetic interference has been reduced to within the linac operational tolerance. / Medical Physics

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This study examined the short term memory (STM) difference of magnetic resonance imaging (MRI) technologists versus non-MRI technologists. Human and animal studies have indicated that residual magnetic fields have caused changes within the cerebral structure. Research on residual magnetic fields and their effect on STM is still at its infancy. A quasi-experimental design was used to determine if any significant difference existed between the STM of MRI technologists (n = x) and a control population sample (n = x). The STM of both groups was assessed with the use of the Rivermead Behavioural Memory Test-Third Edition. Solicitation of the participants was from a national MRI organization, the American Society of Radiologic Technologists, and community workers within the profession. The control group of participants was solicited through community board postings. Only the New York/New Jersey metro area and the New Hampshire/Maine area participants were used for this study. These participants were of various age ranges, genders, and educational levels. ANOVA and regression analyses were used to analyze the data. The study showed mixed results, indicating no significant STM difference in the overall memory scores of both groups F (1, 80) =3.061, p =..084, but it did show a significant difference in STM when it came to prospective memory, memory of planned events. These findings illustrate a need for further research in this area. Expanding the geographical reach and sample size could clarify the role of MRI on STM. The results of this study suggest that procedures that limit the exposure of the MRI technologists to the residual magnetic fields surrounding MRI machines could yield a reduction in loss of prospective memory.