The Experts below are selected from a list of 99189 Experts worldwide ranked by ideXlab platform
H. Delis - One of the best experts on this subject based on the ideXlab platform.
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Monte Carlo simulation of breast imaging using synchrotron radiation
Medical physics, 2012Co-Authors: Niki T. Fitousi, H. Delis, George PanayiotakisAbstract:Purpose: Synchrotron radiation (SR), being the brightest artificial source of x-rays with a very promising geometry, has raised the scientific expectations that it could be used for breast imaging with optimized results. The ''in situ'' evaluation of this technique is difficult to perform, mostly due to the limited available SR facilities worldwide. In this study, a simulation model for SR breast imaging was developed, based on Monte Carlo simulation techniques, and validated using data acquired in the SYRMEP beamline of the Elettra facility in Trieste, Italy. Furthermore, primary results concerning the performance of SR were derived. Methods: The developed model includes the exact setup of the SR beamline, considering that the x-ray source is located at almost 23 m from the slit, while the photon energy was considered to originate from a very narrow Gaussian spectrum. Breast phantoms, made of Perspex and filled with air cavities, were irradiated with energies in the range of 16-28 keV. The model included a Gd{sub 2}O{sub 2}S detector with the same characteristics as the one available in the SYRMEP beamline. Following the development and validation of the model, experiments were performed in order to evaluate the Contrast resolution of SR. A phantom made ofmore » adipose tissue and filled with inhomogeneities of several compositions and sizes was designed and utilized to simulate the irradiation under conventional mammography and SR conditions. Results: The validation results of the model showed an excellent agreement with the experimental data, with the correlation for Contrast being 0.996. Significant differences only appeared at the edges of the phantom, where phase effects occur. The initial evaluation experiments revealed that SR shows very good performance in terms of the image quality indices utilized, namely Subject Contrast and Contrast to noise ratio. The response of Subject Contrast to energy is monotonic; however, this does not stand for Contrast to noise ratio, since there is a range of optimal performance for SR (18-21 keV). In comparison to conventional mammography, SR shows improved Subject Contrast for energies lower than the mean energy of each spectrum. Conclusions: The comparison of the results of the two models, conventional and SR, proved that SR exhibits better performance in the majority of cases. The proposed simulation model offers the possibility to perform exhaustive search to evaluate the performance of SR in clinical applications such as breast imaging.« less
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Suitability of new anode materials in mammography: Dose and Subject Contrast considerations using Monte Carlo simulation
Medical physics, 2006Co-Authors: H. Delis, George M. Spyrou, Lena Costaridou, G. Tzanakos, G.s. PanayiotakisAbstract:Mammography is the technique with the highest sensitivity and specificity, for the early detection of nonpalpable lesions associated with breast cancer. As screening mammography refers to asymptomatic women, the task of optimization between the image quality and the radiation dose is critical. A way toward optimization could be the introduction of new anode materials. A method for producing the x-ray spectra of different anode/filter combinations is proposed. The performance of several mammographic spectra, produced by both existing and theoretical anode materials, is evaluated, with respect to their dose and Subject Contrast characteristics, using a Monte Carlo simulation.The mammographic performance is evaluated utilizing a properly designed mathematical phantom with embedded inhomogeneities, irradiated with different spectra, based on combinations of conventional and new (Ru, Ag) anode materials, with several filters (Mo, Rh, Ru, Ag, Nb, Al). An earlier developed and validated Monte Carlo model, for deriving both image and dose characteristics in mammography, was utilized and overall performance results were derived in terms of Subject Contrast to dose ratio and squared Subject Contrast to dose ratio. Results demonstrate that soft spectra, mainly produced from Mo, Rh, and Ru anodes and filtered with k-edge filters, provide increased Subject Contrast for inhomogeneities of bothmore » small size, simulating microcalcifications and low density, simulating masses. The harder spectra (W and Ag anode) come short in the discrimination task but demonstrate improved performance when considering the dose delivered to the breast tissue. As far as the overall performance is concerned, new theoretical spectra demonstrate a noticeable good performance that is similar, and in some cases better compared to commonly used systems, stressing the possibility of introducing new materials in mammographic practice as a possible contribution to its optimization task. In the overall optimization task in terms of Subject Contrast to dose ratio, tube voltage was found to have a minor effect, while with respect to the filter material, a lesion specific performance was noticed, with Al filtered spectra showing improved characteristics in case of the inhomogeneities simulating microcalcifications, while softer k-edge filtered spectra are more suitable for the discrimination of inhomogeneities simulating masses.« less
John W. Wong - One of the best experts on this subject based on the ideXlab platform.
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a radiographic and tomographic imaging system integrated into a medical linear accelerator for localization of bone and soft tissue targets
International Journal of Radiation Oncology Biology Physics, 1999Co-Authors: Douglas G Drake, Michel Moreau, John W. Wong, Alvaro Martinez, David A. JaffrayAbstract:Abstract Purpose: Dose escalation in conformal radiation therapy requires accurate field placement. Electronic portal imaging devices are used to verify field placement but are limited by the low Subject Contrast of bony anatomy at megavoltage (MV) energies, the large imaging dose, and the small size of the radiation fields. In this article, we describe the in-house modification of a medical linear accelerator to provide radiographic and tomographic localization of bone and soft-tissue targets in the reference frame of the accelerator. This system separates the verification of beam delivery (machine settings, field shaping) from patient and target localization. Materials and Methods: A kilovoltage (kV) x-ray source is mounted on the drum assembly of an Elekta SL-20 medical linear accelerator, maintaining the same isocenter as the treatment beam with the central axis at 90° to the treatment beam axis. The x-ray tube is powered by a high-frequency generator and can be retracted to the drum-face. Two CCD-based fluoroscopic imaging systems are mounted on the accelerator to collect MV and kV radiographic images. The system is also capable of cone-beam tomographic imaging at both MV and kV energies. The gain stages of the two imaging systems have been modeled to assess imaging performance. The Contrast-resolution of the kV and MV systems was measured using a Contrast-detail (C-D) phantom. The dosimetric advantage of using the kV imaging system over the MV system for the detection of bone-like objects is quantified for a specific imaging geometry using a C-D phantom. Accurate guidance of the treatment beam requires registration of the imaging and treatment coordinate systems. The mechanical characteristics of the treatment and imaging gantries are examined to determine a localizing precision assuming an unambiguous object. MV and kV radiographs of patients receiving radiation therapy are acquired to demonstrate the radiographic performance of the system. The tomographic performance is demonstrated on phantoms using both the MV and the kV imaging system, and the visibility of soft-tissue targets is assessed. Results and Discussion: Characterization of the gains in the two systems demonstrates that the MV system is x-ray quantum noise-limited at very low spatial frequencies; this is not the case for the kV system. The estimates of gain used in the model are validated by measurements of the total gain in each system. Contrast-detail measurements demonstrate that the MV system is capable of detecting Subject Contrasts of less than 0.1% (at 6 and 18 MV). A comparison of the kV and MV Contrast-detail performance indicates that equivalent bony object detection can be achieved with the kV system at significantly lower doses (factors of 40 and 90 lower than for 6 and 18 MV, respectively). The tomographic performance of the system is promising; soft-tissue visibility is demonstrated at relatively low imaging doses (3 cGy) using four laboratory rats. Conclusions: We have integrated a kV radiographic and tomographic imaging system with a medical linear accelerator to allow localization of bone and soft-tissue structures in the reference frame of the accelerator. Modeling and experiments have demonstrated the feasibility of acquiring high-quality radiographic and tomographic images at acceptable imaging doses. Full integration of the kV and MV imaging systems with the treatment machine will allow on-line radiographic and tomographic guidance of field placement. Keywords:
G.s. Panayiotakis - One of the best experts on this subject based on the ideXlab platform.
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Suitability of new anode materials in mammography: Dose and Subject Contrast considerations using Monte Carlo simulation
Medical physics, 2006Co-Authors: H. Delis, George M. Spyrou, Lena Costaridou, G. Tzanakos, G.s. PanayiotakisAbstract:Mammography is the technique with the highest sensitivity and specificity, for the early detection of nonpalpable lesions associated with breast cancer. As screening mammography refers to asymptomatic women, the task of optimization between the image quality and the radiation dose is critical. A way toward optimization could be the introduction of new anode materials. A method for producing the x-ray spectra of different anode/filter combinations is proposed. The performance of several mammographic spectra, produced by both existing and theoretical anode materials, is evaluated, with respect to their dose and Subject Contrast characteristics, using a Monte Carlo simulation.The mammographic performance is evaluated utilizing a properly designed mathematical phantom with embedded inhomogeneities, irradiated with different spectra, based on combinations of conventional and new (Ru, Ag) anode materials, with several filters (Mo, Rh, Ru, Ag, Nb, Al). An earlier developed and validated Monte Carlo model, for deriving both image and dose characteristics in mammography, was utilized and overall performance results were derived in terms of Subject Contrast to dose ratio and squared Subject Contrast to dose ratio. Results demonstrate that soft spectra, mainly produced from Mo, Rh, and Ru anodes and filtered with k-edge filters, provide increased Subject Contrast for inhomogeneities of bothmore » small size, simulating microcalcifications and low density, simulating masses. The harder spectra (W and Ag anode) come short in the discrimination task but demonstrate improved performance when considering the dose delivered to the breast tissue. As far as the overall performance is concerned, new theoretical spectra demonstrate a noticeable good performance that is similar, and in some cases better compared to commonly used systems, stressing the possibility of introducing new materials in mammographic practice as a possible contribution to its optimization task. In the overall optimization task in terms of Subject Contrast to dose ratio, tube voltage was found to have a minor effect, while with respect to the filter material, a lesion specific performance was noticed, with Al filtered spectra showing improved characteristics in case of the inhomogeneities simulating microcalcifications, while softer k-edge filtered spectra are more suitable for the discrimination of inhomogeneities simulating masses.« less
Carolyn A. Macdonald - One of the best experts on this subject based on the ideXlab platform.
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Phase Contrast imaging with polycapillary optics
Penetrating Radiation Systems and Applications XIII, 2012Co-Authors: Hassan Abbas, Tianxi Sun, Carolyn A. MacdonaldAbstract:Conventional diagnostic radiography is limited by the similarity between x-ray absorption coefficients of normal tissue and carcinoma, which results in poor inherent Subject Contrast. Differences in x-ray refractive indices are much larger, so phase imaging has the potential for higher Contrast. Unfortunately, the spatial coherence necessary for simple in-line phase Contrast requires small sources at large distances, and hence excessive exposure times. Other schemes such as grating techniques require multiple images and complex alignment. In this work, polycapillary optics were employed to increase the intensity of the x-ray beam for simple propagation in-line imaging. Focusing through pinhole apertures created a small virtual source of high intensity from which phase Contrast edge effects were observed with tissueequivalent phantoms.
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Beam collimation with polycapillary x-ray optics for high Contrast high resolution monochromatic imaging
Medical physics, 2004Co-Authors: Francisca R. Sugiro, Carolyn A. MacdonaldAbstract:Monochromatic imaging can provide better Contrast and resolution than conventional broadband radiography. In broadband systems, low energy photons do not contribute to the image, but are merely absorbed, while high energy photons produce scattering that degrades the image. By tuning to the optimal energy, one can eliminate undesirable lower and higher energies. Monochromatization is achieved by diffraction from a single crystal. A crystal oriented to diffract at a particular energy, in this case the characteristic line energy, diffracts only those photons within a narrow range of angles. The resultant beam from a divergent source is nearly parallel, but not very intense. To increase the intensity, collimation was performed with polycapillary x-ray optics, which can collect radiation from a divergent source and redirect it into a quasi parallel beam. Contrast and resolution measurements were performed with diffracting crystals with both high and low angular acceptance. Testing was first done at 8 keV with an intense copper rotating anode x-ray source, then 17.5 keV measurements were made with a low power molybdenum source. At 8 keV, Subject Contrast was a factor of five higher than for the polychromatic case. At 17.5 keV, monochromatic Contrast was two times greater than the conventional polychromatic Contrast. The Subject Contrasts measured at both energies were in good agreement with theory. An additional factor of two increase in Contrast, for a total gain of four, is expected at 17.5 keV from the removal of scatter. Scatter might be simply removed using an air gap, which does not degrade resolution with a parallel beam.
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Monochromatic imaging with a conventional source using polycapillary x-ray optics
Medical Imaging 2001: Physics of Medical Imaging, 2001Co-Authors: Francisca R. Sugiro, Carolyn A. MacdonaldAbstract:Monochromatic parallel beam imaging produces high Subject Contrast, high resolution, and low patient dose. Polycapillary collimating optics can be used to create a beam of sufficient intensity for monochromatization from a conventional source. Monochromatization is achieved by diffraction from a single crystal. Contrast, resolution, and intensity measurements were performed with both high and low angular acceptance crystals. Testing was first done at 8 keV with an intense copper rotating anode, then preliminary 17.5 keV measurements were made with a low power molybdenum source. At 8 keV, Contrast enhancement was a factor of 5 relative to the polychromatic case, in good agreement with theoretical values. At 17.5 keV, monochromatic Subject Contrast is a factor of 2 times greater than the conventional polychromatic Contrast. An additional factor of two increase in Contrast is expected from the removal of scatter obtained from using the air gap which is allowable from the parallel beam. The measured angular resolution after the crystal was 0.6 mrad for a silicon crystal. The use of polycapillary collimating optics allowed monochromatic imaging measurements using a conventional rotating anode source and computed radiography plate in 300 mAs.
George Panayiotakis - One of the best experts on this subject based on the ideXlab platform.
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Monte Carlo simulation of breast imaging using synchrotron radiation
Medical physics, 2012Co-Authors: Niki T. Fitousi, H. Delis, George PanayiotakisAbstract:Purpose: Synchrotron radiation (SR), being the brightest artificial source of x-rays with a very promising geometry, has raised the scientific expectations that it could be used for breast imaging with optimized results. The ''in situ'' evaluation of this technique is difficult to perform, mostly due to the limited available SR facilities worldwide. In this study, a simulation model for SR breast imaging was developed, based on Monte Carlo simulation techniques, and validated using data acquired in the SYRMEP beamline of the Elettra facility in Trieste, Italy. Furthermore, primary results concerning the performance of SR were derived. Methods: The developed model includes the exact setup of the SR beamline, considering that the x-ray source is located at almost 23 m from the slit, while the photon energy was considered to originate from a very narrow Gaussian spectrum. Breast phantoms, made of Perspex and filled with air cavities, were irradiated with energies in the range of 16-28 keV. The model included a Gd{sub 2}O{sub 2}S detector with the same characteristics as the one available in the SYRMEP beamline. Following the development and validation of the model, experiments were performed in order to evaluate the Contrast resolution of SR. A phantom made ofmore » adipose tissue and filled with inhomogeneities of several compositions and sizes was designed and utilized to simulate the irradiation under conventional mammography and SR conditions. Results: The validation results of the model showed an excellent agreement with the experimental data, with the correlation for Contrast being 0.996. Significant differences only appeared at the edges of the phantom, where phase effects occur. The initial evaluation experiments revealed that SR shows very good performance in terms of the image quality indices utilized, namely Subject Contrast and Contrast to noise ratio. The response of Subject Contrast to energy is monotonic; however, this does not stand for Contrast to noise ratio, since there is a range of optimal performance for SR (18-21 keV). In comparison to conventional mammography, SR shows improved Subject Contrast for energies lower than the mean energy of each spectrum. Conclusions: The comparison of the results of the two models, conventional and SR, proved that SR exhibits better performance in the majority of cases. The proposed simulation model offers the possibility to perform exhaustive search to evaluate the performance of SR in clinical applications such as breast imaging.« less
