In this study, we investigate the effect of dental filling materials (DFM) on RapidArcTM treatment plans and delivery in a
patient undergoing radiotherapy treatment. The presence of DFM creates uncertainties in CT number and causes long
streaking artifacts in the reconstructed images which greatly affect the dose distribution inside the oral cavity. The
influence of extensive dental filling artifacts on dose distribution was performed using a geometrically well defined head
and neck IMRT verification phantom (PTW, Freiburg, Germany) together with inserts from DFM (Amalgam, 11.3
g/cm3). The phantom was scanned using Siemens SOMATOM Sensation CT simulator (Siemens AG, Germany) under
standard head and neck imaging protocol (120 kV, 120 mAs, voxel size 1×1×2 mm3). Three RapidArcTM plans were
created in the Varian Eclipse treatment planning System (TPS) to treat oral cavity using the same CT dataset including;
1) raw CT image, 2) streaking artifacts replaced with a mask of 10 HU and 3) 2 cm thick 6000 HU virtual filter (a
volume around the teeth in TPS to mimic extra attenuation). The virtual filter thickness optimization was purely based on
measured PDD data acquired with DFM and the calculation in Eclipse Planning System using direct beam. The dose
delivery and distribution for the three plans was verified using Gafchromic EBT2 (International Specialty Product,
Wayne, NJ, USA) film measurements. The artifact mask and virtual filter around the teeth in the planning was found
very useful to reduce the discrepancies between the dose plan and delivery. From clinical point of view, these results can
be helpful to understand the increase of mucositis in patient having DFM, and further investigation is underway for
clinical solution.
Monochromatic beams produced with synchrotron sources are known to give higher contrast for mammography than clinical broadband sources. Monochromatic beams could also be achieved with clinical x-ray sources, by diffraction off of flat monochromator crystals, but monochromatic intensities are too low for imaging because only a small fraction of the incident beam is at the right energy and angle. With the use of polycapillary optics, monochromatic intensities could be increased. Two different x-ray optics schemes were tested to provide high monochromatic intensity from conventional divergent sources. A polycapillary collimating optic was employed to collect a large solid angle and redirect it into a parallel beam, which can be efficiently diffracted from a flat crystal. Measurements were performed for crystals of varying angular acceptance because there is a trade-off between intensity and resolution. Alternatively, doubly curved crystal (DCC) optics can be used to collect and focus monochromatic x rays from a divergence source. Higher monochromatic intensity can be obtained because the DCC optic diffracts and focuses the incident beam across the whole area of the crystal. For both methods, monochromatization occurs before the patient, resulting in a potential dose reduction as well as significant measured contrast enhancement. Measurements were made of contrast, resolution and intensity for the two techniques, and were compared to each other and to theoretical calculations.
Polycapillary x-ray optics are bundles of micron size hollow tubes, inside of which x rays are propagated by total reflection much like visible light in solid fiber optics. The small critical angle for total reflection from the glass walls of the tubes, 0.06° at 27 keV, results in very high angular selectivity. The field of view of each capillary tube is limited by this angular acceptance to less than 50 microns at a source-to-optic distance of 2 cm. Each adjacent tube works in parallel so that a large area can be covered at this resolution with much higher count rate than for a single collimator. Measurements have been performed using 125I brachytherapy seeds in Lucite phantoms using the optics and imaging detectors. Measured resolutions were detector-limited at better than 0.1 mm. Calculations for expected sensitivity and signal-to-background ratios were developed from geometrical models and show good agreement with measurements. Results indicate that the optics provide superior signal count rates to conventional collimators for geometries such as small animal imaging in which sub millimeter resolution with inch-wide or larger fields of view are desirable.
Three dimensional focusing of characteristic x-rays can be achieved by diffraction from doubly curved crystals (DCC). A focused beam total reflection x-ray fluorescence technique was developed based on these optics. This technique provides good detection sensitivity and spatial resolution for localized detection of surface deposits. Compact low power small spot x-ray sources were used to demonstrate the benefit of the x-ray optics for focusing Cr Kα, Cu Kα and Mo Kα radiation.
The DCC optic was also applied to monochromatic micro x-ray fluorescence (MMXRF), providing good detection sensitivity and spatial resolution for deeper impurities. The detection capability of the focused beam TXRF and MMXRF systems was investigated with dried droplets of calibrated low concentration solutions.
Additionally, the implementation of high contrast monochromatic imaging with a very low power source was demonstrated using the DCC optics at mammographic energies. Images of contrast phantoms were obtained with high clarity.
Polycapillary optics can be employed as efficient low pass devices in conjunction with simple absorption filters to produce narrow band radiation from conventional broadband x-ray tube sources. Narrow band filtration has been shown to be adequate for low-resolution protein crystallography without a monochromator and for investigating energy-dependent phenomena such as Compton scatter production. For applications that would benefit from more monochromatic or more parallel input beams, polycapillary collimating optics can be used to collect divergent radiation and redirect it towards a monochromatizing crystal to produce orders of magnitude higher diffracted intensity than from pinhole collimation. The implementation of high contrast monochromatic and refractive index imaging with a very low power source has been demonstrated. Polycapillary optics can also be used to provide spatial resolution for inherently monochromatic applications such as microfluorescence and radioscintigraphy.
The potential use of polycapillary optics could provide extraordinarily high spatial resolution imaging of radioactive sources for a new generation of gamma cameras is being investigated. A series of images from 125I brachytherapy seeds in Lucite phantoms display resolution better than 0.1 mm and signal to noise ratios in excess of a factor of 10. Such "cellular" level resolution might allow early stage location of prostate tumors, and be used to study their size, shape, and rate of growth. Even before being developed into the compact size needed for transrectal investigation of prostate cancer, imaging detectors using such high spatial resolution polycapillary angular filters may be valuable for small animal model studies and other research.
X rays emitted over a large angular range from conventional, laboratory-based sources can be transformed into a parallel beam or focused onto a small sample area to give efficient utilization of small sources for powder diffraction. For optimal system design, it is important that source parameters be well characterized. Source to window distance, spot size, intensity, and uniformity were measured for an Oxford Ultrabright Molybdenum source. Two polycapillary optics, a weakly focusing, and a collimating, optic were also characterized in detail. Measurements of x-ray diffraction data have been assessed for silicon and organic powders, and agree well with parameters predicted from the source and lens characterization.
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