Zdenka Kuncic | The University of Sydney (original) (raw)

Papers by Zdenka Kuncic

IEEE Transactions on Nuclear Science, 2000

Medical Imaging 2013: Physics of Medical Imaging, 2013

ABSTRACT Current electronic portal imaging devices (EPIDs) are generally used for megavoltage ima... more ABSTRACT Current electronic portal imaging devices (EPIDs) are generally used for megavoltage imaging in radiotherapy and employ a thin Cu plate/ phosphor screen to convert x-ray energies into optical photons . In order to achieve a high spatial resolution, thin screens are used which subsequently results in low x-ray absorption and thus a low detective quantum efficiency (DQE) for megavoltage x-rays. Additionally, the high atomic number Cu/ phosphor screen materials is not ideal for dosimetric applications. To improve the imaging and dosimetry dual-functionality of EPIDs, water equivalent plastic scintillators have been proposed. Plastic scintillator fibers may however be susceptible to radiation damage caused primarily by ionizations from low energy secondary electrons. An accumulation or clustering of these ionization events, within regions corresponding to the volume of the plastic polymer chains, may lead to chain breaks. This could result in changes to the optical photon absorption properties and optical yield of the fiber, affecting the overall imaging performance of the detector. Here we used Monte Carlo radiation transport simulations for a preliminary investigation into the distribution of ionizations within a single plastic fiber. We find a large number of ionization events can accumulate along the fiber length, which over repeated exposures could lead to damage. To determine the effect of damage on the imaging performance, two fiber arrays were modeled with and without areas of damage. The damaged fiber array was found to produce approximately half the number of counts as the undamaged array.

Astrophysical Journal, 2009

Based on our long (~300 ks) 2007 XMM-Newton observation of the Seyfert galaxy NGC 1365, we report... more Based on our long (~300 ks) 2007 XMM-Newton observation of the Seyfert galaxy NGC 1365, we report here on the spectral and timing behavior of two ultraluminous X-ray sources (ULXs), which had previously reached isotropic X-ray luminosities L X ≈ 4 × 1040 erg s-1 (0.3-10 keV band). In 2007, they were in a lower state (L X ≈ 5

Space Science Reviews - SPACE SCI REV, 2003

Recent data and theory for type II solar radio bursts are reviewed, focusing on a recent analytic... more Recent data and theory for type II solar radio bursts are reviewed, focusing on a recent analytic quantitative theory for interplanetary type II bursts. The theory addresses electron reflection and acceleration at the type II shock, formation of electron beams in the foreshock, and generation of Langmuir waves and the type II radiation there. The theory's predictions as functions of the shock and plasma parameters are summarized and discussed in terms of space weather events. The theory is consistent with available data, has explanations for radio-loud/quiet coronal mass ejections (CMEs) and why type IIs are bursty, and can account for empirical correlations between type IIs, CMEs, and interplanetary disturbances.

Physics in Medicine and Biology, 2010

Backscatter factors are important parameters in the determination of dose for kilovoltage x-ray b... more Backscatter factors are important parameters in the determination of dose for kilovoltage x-ray beams. However, backscatter factors are difficult to measure experimentally, and tabulated values are based largely on Monte Carlo calculations. In this study we have determined new backscatter factors by both experimental and Monte Carlo methods, and compared them with existing backscatter factors published in the AAPM TG-61 protocol. The purpose of this study is twofold: (1) to evaluate the overall effectiveness of using Gafchromic EBT film for backscatter factor measurements and (2) to determine whether existing Monte Carlo-calculated backscatter factors need to be updated. We measured backscatter factors using Gafchromic EBT film for three field sizes (2, 4 and 6 cm diameter cones) and three kilovoltage beam qualities, including 280 kVp for which similar measurements have not previously been reported. We also present new Monte Carlo-calculated backscatter factors obtained using the EGSnrc/BEAMnrc code system to simulate the Pantak kilovoltage x-ray unit used in our measurements. The results were compared with backscatter factors tabulated in the AAPM TG-61 protocol for kilovoltage x-ray dosimetry. The largest difference between our measured and calculated backscatter factors and the AAPM TG-61 values was found to be 2.5%. This agreement is remarkably good, considering that the AAPM TG-61 values consist of a combination of experimental and Monte Carlo calculations obtained over 20 years ago using different measurement techniques, as well as older Monte Carlo code and cross-section data. Furthermore, our Monte Carlo-calculated backscatter factors agree within 1% with the AAPM TG-61 values for all beam qualities and field sizes. Our Gafchromic film measurements had slightly larger differences with the AAPM TG-61 backscatter factors, up to approximately 2% for the 6 cm diameter cone at a beam quality of 50 kVp. The largest difference in backscatter factors, of 2.5%, was found between Monte Carlo-calculated and Gafchromic film-measured data for the 100 kVp x-ray beam with the 4 cm diameter cone. The differences in backscatter factors between the three data sets (measurements, calculations and published values) are all within the uncertainties from our Gafchromic film measurements and Monte Carlo calculations. Our results demonstrate the suitability of using Gafchromic EBT film to measure equipment-specific backscatter factors for kilovoltage x-ray beams over the entire energy range and also confirm that backscatter factors published in kilovoltage dosimetry protocols still remain valid.

Monthly Notices of the Royal Astronomical Society, 2009

Monthly Notices of the Royal Astronomical Society, 2008

Monthly Notices of the Royal Astronomical Society, 2006

We calculate the broad-band radio-X-ray spectra predicted by microblazar and microquasar models f... more We calculate the broad-band radio-X-ray spectra predicted by microblazar and microquasar models for ultraluminous X-ray sources (ULXs), exploring the possibility that their dominant power-law component is produced by a relativistic jet, even at near-Eddington mass accretion rates. We do this by first constructing a generalized disc-jet theoretical framework in which some fraction of the total accretion power, Pa, is efficiently removed from the accretion disc by a magnetic torque responsible for jet formation. Thus, for different black hole masses, mass accretion rates and magnetic coupling strength, we self-consistently calculate the relative importance of the modified disc spectrum, as well as the overall jet emission due to synchrotron and Compton processes. In general, transferring accretion power to a jet makes the disc fainter and cooler than a standard disc at the same mass accretion rate; this may explain why the soft spectral component appears less prominent than the dominant power-law component in most bright ULXs. We show that the apparent X-ray luminosity and spectrum predicted by the microquasar model are consistent with the observed properties of most ULXs. We predict that the radio synchrotron jet emission is too faint to be detected at the typical threshold of radio surveys to date. This is consistent with the high rate of non-detections over detections in radio counterpart searches. Conversely, we conclude that the observed radio emission found associated with a few ULXs cannot be due to beamed synchrotron emission from a relativistic jet.

Monthly Notices of the Royal Astronomical Society, 1998

Medical Physics, 2014

Respiratory signal, binning method, and reconstruction algorithm are three major controllable fac... more Respiratory signal, binning method, and reconstruction algorithm are three major controllable factors affecting image quality in thoracic 4D cone-beam CT (4D-CBCT), which is widely used in image guided radiotherapy (IGRT). Previous studies have investigated each of these factors individually, but no integrated sensitivity analysis has been performed. In addition, projection angular spacing is also a key factor in reconstruction, but how it affects image quality is not obvious. An investigation of the impacts of these four factors on image quality can help determine the most effective strategy in improving 4D-CBCT for IGRT. Fourteen 4D-CBCT patient projection datasets with various respiratory motion features were reconstructed with the following controllable factors: (i) respiratory signal (real-time position management, projection image intensity analysis, or fiducial marker tracking), (ii) binning method (phase, displacement, or equal-projection-density displacement binning), and (iii) reconstruction algorithm [Feldkamp-Davis-Kress (FDK), McKinnon-Bates (MKB), or adaptive-steepest-descent projection-onto-convex-sets (ASD-POCS)]. The image quality was quantified using signal-to-noise ratio (SNR), contrast-to-noise ratio, and edge-response width in order to assess noise/streaking and blur. The SNR values were also analyzed with respect to the maximum, mean, and root-mean-squared-error (RMSE) projection angular spacing to investigate how projection angular spacing affects image quality. The choice of respiratory signals was found to have no significant impact on image quality. Displacement-based binning was found to be less prone to motion artifacts compared to phase binning in more than half of the cases, but was shown to suffer from large interbin image quality variation and large projection angular gaps. Both MKB and ASD-POCS resulted in noticeably improved image quality almost 100% of the time relative to FDK. In addition, SNR values were found to increase with decreasing RMSE values of projection angular gaps with strong correlations (r ≈ -0.7) regardless of the reconstruction algorithm used. Based on the authors&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; results, displacement-based binning methods, better reconstruction algorithms, and the acquisition of even projection angular views are the most important factors to consider for improving thoracic 4D-CBCT image quality. In view of the practical issues with displacement-based binning and the fact that projection angular spacing is not currently directly controllable, development of better reconstruction algorithms represents the most effective strategy for improving image quality in thoracic 4D-CBCT for IGRT applications at the current stage.

Journal of Physics: Conference Series, 2010

Journal of Geophysical Research, 2007