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Sökning: WFRF:(Poludniowski G)

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1.
  • Omar, A., et al. (författare)
  • A framework for organ dose estimation in x-ray angiography and interventional radiology based on dose-related data in DICOM structured reports
  • 2016
  • Ingår i: Physics in Medicine and Biology. - Stockholm : Institute of Physics Publishing (IOPP). - 0031-9155 .- 1361-6560. ; 61:8, s. 3063-3083
  • Tidskriftsartikel (refereegranskat)abstract
    • Although interventional x-ray angiography (XA) procedures involve relatively high radiation doses that can lead to deterministic tissue reactions in addition to stochastic effects, convenient and accurate estimation of absorbed organ doses has traditionally been out of reach. This has mainly been due to the absence of practical means to access dose-related data that describe the physical context of the numerous exposures during an XA procedure. The present work provides a comprehensive and general framework for the determination of absorbed organ dose, based on non-proprietary access to dose-related data by utilizing widely available DICOM radiation dose structured reports. The framework comprises a straightforward calculation workflow to determine the incident kerma and reconstruction of the geometrical relation between the projected x-ray beam and the patient's anatomy. The latter is difficult in practice, as the position of the patient on the table top is unknown. A novel patient-specific approach for reconstruction of the patient position on the table is presented. The proposed approach was evaluated for 150 patients by comparing the estimated position of the primary irradiated organs (the target organs) with their position in clinical DICOM images. The approach is shown to locate the target organ position with a mean (max) deviation of 1.3 (4.3), 1.8 (3.6) and 1.4 (2.9) cm for neurovascular, adult and paediatric cardiovascular procedures, respectively. To illustrate the utility of the framework for systematic and automated organ dose estimation in routine clinical practice, a prototype implementation of the framework with Monte Carlo simulations is included.
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2.
  • Merzan, D., et al. (författare)
  • Evaluating the impact of scan settings on automatic tube current modulation in CT using a novel phantom
  • 2017
  • Ingår i: British Journal of Radiology. - : British Institute of Radiology. - 0007-1285 .- 1748-880X. ; 90:1069
  • Tidskriftsartikel (refereegranskat)abstract
    • Objective: The aim of this study was to make a comprehensive evaluation of how variable scan settings can affect the performance of automatic tube current modulation (ATCM) in recent CT scanners from the four major manufacturers. Methods: A phantom was designed and manufactured for the purpose of evaluating ATCM. The phantom was scanned with four categories of systematically varied settings (scan projection radiograph, technique and reconstruction parameters and phantom miscentring). The performance of ATCM, in terms of applied tube current and noise uniformity, for the scans with varied settings was compared with a reference scan using subjective and quantitative approaches. Results: The ATCM implemented by each manufacturer is based on different principles and any affect to the performance of the ATCM, when varying scan settings, will manifest differently among the vendors. The results are summarized in four tables corresponding to the categories of varied settings. Conclusion: The developed phantom proved useful for evaluating the ATCM. It is important to understand how different implementations (vendor specific) of ATCM perform in order to make informed decisions about the selection of scan settings when designing protocols. The resulting tables can serve as a reference for understanding the different implementations of ATCM and highlight settings that should be taken into consideration when adjusting an imaging protocol. Advances in knowledge: The results from this work can serve as a reference for how changes in geometry or scan settings can affect the performance of ATCM, in terms of tube current and noise. © 2016 The Authors.
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3.
  • Nowik, P., et al. (författare)
  • The synthetic localizer radiograph – A new CT scan planning method
  • 2019
  • Ingår i: Physica medica (Testo stampato). - : Elsevier BV. - 1120-1797 .- 1724-191X. ; 61, s. 58-63
  • Tidskriftsartikel (refereegranskat)abstract
    • Objective: To investigate if the conventional localizer radiograph (LR)can be replaced by a synthetic LR (SLR), generated from a low-dose spiral CT scan, for CT scan planning with minimal changes to current clinical workflows. Methods: A dosimetric comparison of SLRs and LRs was made using Monte Carlo methods. Water equivalent diameters (WEDs)of a centered and mis-centered phantom were estimated from low-dose spiral CT scans and LRs acquired at different angles. Body sizes, in the form of two lengths and two diameters obtained from SLRs and LRs, were compared for 10 patients (4 men and 6 women with a mean age of 74.8 and 76.2 years respectively)undergoing CT of thorax and abdomen. The image quality of SLRs for CT scan planning relative to LRs was rated using a 5-grade scale by four radiologists and two CT radiographers. Results: An SLR can be obtained at a comparable effective dose to that of traditionally acquired LRs: 0.14 mSv. WEDs from LRs were more affected by mis-centering than WEDs calculated from low-dose spiral scans. One significant discrepancy of estimated body sizes was observed, the broadest part of the patient that on lateral localizers showed a mean deviation of 17.7 mm (range: 7.3–28.7 mm, p < 0.001). The anteroposterior/posteroanterior SLR image quality was assessed as better compared to an LR while the same could not be shown for lateral localizers. Conclusions: SLRs based on low-dose spiral scans can replace LRs for CT planning.
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4.
  • Ardenfors, Oscar, et al. (författare)
  • Organ doses from a proton gantry-mounted cone-beam computed tomography system characterized with MCNP6 and GATE
  • 2018
  • Ingår i: Physica medica (Testo stampato). - 1120-1797 .- 1724-191X. ; 53, s. 56-61
  • Tidskriftsartikel (refereegranskat)abstract
    • PurposeTo determine organ doses from a proton gantry-mounted cone-beam computed tomography (CBCT) system using two Monte Carlo codes and to study the influence on organ doses from different acquisition modes and repeated imaging.MethodsThe CBCT system was characterized with MCNP6 and GATE using measurements of depth doses in water and spatial profiles in air. The beam models were validated against absolute dose measurements and used to simulate organ doses from CBCT imaging with head, thorax and pelvis protocols. Anterior and posterior 190° scans were simulated and the resulting organ doses per mAs were compared to those from 360° scans. The influence on organ doses from repeated imaging with different imaging schedules was also investigated.ResultsThe agreement between MCNP6, GATE and measurements with regard to depth doses and beam profiles was within 4% for all protocols and the corresponding average agreement in absolute dose validation was 4%. Absorbed doses for in-field organs from 360° scans ranged between 6 and 8 mGy, 15–17 mGy and 24–54 mGy for the head, thorax and pelvis protocols, respectively. Cumulative organ doses from repeated CBCT imaging ranged between 0.04 and 0.32 Gy for weekly imaging and 0.2–1.6 Gy for daily imaging. The anterior scans resulted in an average increase in dose per mAs of 24% to the organs of interest relative to the 360° scan, while the posterior scan showed a 37% decrease.ConclusionsA proton gantry-mounted CBCT system was accurately characterized with MCNP6 and GATE. Organ doses varied greatly depending on acquisition mode, favoring posterior scans.
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5.
  • Bujila, Robert, et al. (författare)
  • Practical approaches to approximating MTF and NPS in CT with an example application to task-based observer studies
  • 2017
  • Ingår i: Physica medica (Testo stampato). - : Elsevier BV. - 1120-1797 .- 1724-191X. ; 33, s. 16-25
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: To investigate two methods of approximating the Modulation Transfer Function (MTF) and Noise Power Spectrum (NPS) in computed tomography (CT) for a range of scan parameters, from limited image acquisitions. Methods: The two methods consist of 1) using a linear systems approach to approximate the NPS for different filtered backprojection (FBP) kernels with a filter function derived from the kernel ratio of determined MTFs and 2) using an empirical fitted model to approximate the MTF and NPS. In both cases a scaling function accounts for variations in mAs and kV. The two methods of approximating the MTF/ NPS are further investigated by comparing image quality figure of merits (FOM) d' and AUC calculated using approximations of the MTF/NPS and MTF/NPS that have been determined for different mAs/kV levels and reconstruction kernels. Results: The greatest RMSE for NPS approximated for a range of mAs/kVp/convolution kernels using both methods and compared to determined NPS was 0.05 of the peak value. The RMSE for FOM with the kernel ratio method were at most 0.1 for d' and 0.01 for the AUC. Using the empirical model method, the RMSE for FOM were at most 0.02 for d' and 0.001 for the AUC. Conclusions: The two methods proposed in this paper can provide a convenient way of approximating the MTF and NPS for use in, among other things, mathematical observer studies. Both methods require a relatively small number of direct determinations of NPS from scan acquisitions to model the NPS/MTF for arbitrary mAs and kV.
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6.
  • Holstensson, Maria, et al. (författare)
  • Comparison of acquisition protocols for ventilation/perfusion SPECT - a Monte Carlo study
  • 2019
  • Ingår i: Physics in Medicine and Biology. - : Institute of Physics (IOP). - 0031-9155 .- 1361-6560. ; 64:23
  • Tidskriftsartikel (refereegranskat)abstract
    • One of the most commonly used imaging techniques for diagnosing pulmonary embolism (PE) is ventilation/perfusion (V/P) scintigraphy. The aim of this study was to evaluate the performance of the currently used imaging protocols for V/P single photon emission computed tomography (V/P SPECT) at two nuclear medicine department sites and to investigate the effect of altering important protocol parameters. &#13; &#13; The Monte Carlo technique was used to simulate 4D digital phantoms with perfusion defects. Six imaging protocols were included in the study and a total of 72 digital patients were simulated. Six dually trained radiologists/nuclear medicine physicians reviewed the images and reported all perfusion mismatch findings. The radiologists also visually graded the image quality. &#13; &#13; No statistically significant differences in diagnostic performance were found between the studied protocols, but visual grading analysis pointed out one protocol as significantly superior to four of the other protocols. Considering the study results, we have decided to harmonize our clinical protocols for imaging patients with suspected PE. The administered Technegas and macro aggregated albumin activities have been altered, a low energy all purpose collimator is used instead of a low energy high resolution collimator and the acquisition times have been lowered.
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7.
  • Lindstrom, Jan, et al. (författare)
  • Experimental assessment of a phosphor model for estimating the relative extrinsic efficiency in radioluminescent detectors
  • 2020
  • Ingår i: Physica medica (Testo stampato). - : Elsevier. - 1120-1797 .- 1724-191X. ; 76, s. 117-124
  • Tidskriftsartikel (refereegranskat)abstract
    • Optimising phosphor screens in dose detectors or imaging sensor designs is a cumbersome and time- consuming work normally involving specialised measuring equipment and advanced modelling. It is known that crucial optical parameters of the same phosphor may vary within a wide range of values. The aim of this work was to experimentally assess a simple previously published model where the case specific optical parameters (scattering and absorption) are instead represented by a fixed, single parameter, the light extinction factor, xi. The term extrinsic efficiency, N, of a phosphor is also introduced, differing from the common denotation "absolute efficiency", after noting that unknown factors (such as temperature dependence) can have an influence during efficiency estimations and hence difficult to claim absoluteness. N is expressed as the ratio of light energy emitted per unit area at the phosphor surface to incident x-ray energy fluence. By focusing on ratios and relative changes in this study, readily available instruments in a Medical Physics Department (i.e. a photometer) could be used. The varying relative extrinsic efficiency for an extended range of particle sizes (7.5 and 25 mu m) and layer thicknesses (220 to 830 mu m) were calculated in the model from the input parameters: the mean particle size of the phosphor, the layer thickness, the light extinction factor and the calculated energy imparted to the layer. In-house manufactured screens (Gd2O2S:Tb) were used for better control of design parameters. The model provided good qualitative agreement to experiment with quantitative deviations in relative extrinsic efficiency within approximately 2%.
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8.
  • Nowik, Patrik, et al. (författare)
  • The dosimetric impact of including the patient table in CT dose estimates
  • 2017
  • Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 62:23, s. 538-547
  • Tidskriftsartikel (refereegranskat)abstract
    • The purpose of this study was to evaluate the dosimetric impact of including the patient table in Monte Carlo CT dose estimates for both spiral scans and scan projection radiographs (SPR). CT scan acquisitions were simulated for a Siemens SOMATOM Force scanner (Siemens Healthineers, Forchheim, Germany) with and without a patient table present. An adult male, an adult female and a pediatric female voxelized phantom were simulated. The simulated scans included tube voltages of 80 and 120 kVp. Spiral scans simulated without a patient table resulted in effective doses that were overestimated by approximately 5 % compared to the same simulations performed with the patient table present. Doses in selected individual organs (breast, colon, lung, red bone marrow and stomach) were overestimated by up to 8 %. Effective doses from SPR acquired with the X-ray tube stationary at 6 o'clock (posterior-anterior) were overestimated by 14-23 % when the patient table was not included, with individual organ dose discrepancies (breast, colon, lung red bone marrow and stomach) all exceeding 13%. The reference entrance skin dose to the back were in this situation overestimated by 6-15 %. These results highlight the importance of including the patient table in patient dose estimates for such scan situations.
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9.
  • Oden, Jakob, et al. (författare)
  • Technical Note : On the calculation of stopping-power ratio for stoichiometric calibration in proton therapy
  • 2015
  • Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 42:9, s. 5252-5257
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: The quantitative effects of assumptions made in the calculation of stopping-power ratios (SPRs) are investigated, for stoichiometric CT calibration in proton therapy. The assumptions investigated include the use of the Bethe formula without correction terms, Bragg additivity, the choice of I-value for water, and the data source for elemental I-values. Methods: The predictions of the Bethe formula for SPR (no correction terms) were validated against more sophisticated calculations using the SRIM software package for 72 human tissues. A stoichiometric calibration was then performed at our hospital. SPR was calculated for the human tissues using either the assumption of simple Bragg additivity or the Seltzer-Berger rule (as used in ICRU Reports 37 and 49). In each case, the calculation was performed twice: First, by assuming the I-value of water was an experimentally based value of 78 eV (value proposed in Errata and Addenda for ICRU Report 73) and second, by recalculating the I-value theoretically. The discrepancy between predictions using ICRU elemental I-values and the commonly used tables of Janni was also investigated. Results: Errors due to neglecting the correction terms to the Bethe formula were calculated at less than 0.1% for biological tissues. Discrepancies greater than 1%, however, were estimated due to departures from simple Bragg additivity when a fixed I-value for water was imposed. When the I-value for water was calculated in a consistent manner to that for tissue, this disagreement was substantially reduced. The difference between SPR predictions when using Janni's or ICRU tables for I-values was up to 1.6%. Experimental data used for materials of relevance to proton therapy suggest that the ICRU-derived values provide somewhat more accurate results (root-mean-square-error: 0.8% versus 1.6%). Conclusions: The conclusions from this study are that (1) the Bethe formula can be safely used for SPR calculations without correction terms; (2) simple Bragg additivity can be reasonably assumed for compound materials; (3) if simple Bragg additivity is assumed, then the I-value for water should be calculated in a consistent manner to that of the tissue of interest (rather than using an experimentally derived value); (4) the ICRU Report 37 I-values may provide a better agreement with experiment than Janni's tables.
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10.
  • Ödén, Jakob, et al. (författare)
  • Comparison of CT-number parameterization models for stoichiometric CT calibration in proton therapy
  • 2018
  • Ingår i: Physica medica (Testo stampato). - 1120-1797 .- 1724-191X. ; 47, s. 42-49
  • Tidskriftsartikel (refereegranskat)abstract
    • Purpose: This study compares the predictions of three parameterization models used in previously published works, implementing the stoichiometric CT calibration for proton therapy, and a further two alternative parameterizations suggested here. Methods: Stoichiometric calibrations of patient CT-number to stopping-power ratio (SPR) were performed for four CT protocols using tissue substitutes supplied by CIRS (CIRS Inc., Norfolk, VA, USA). To evaluate robustness of the five models (Sch96/Sch00/Mar12/Karol/Spek), the calibration was repeatedly simulated by randomly perturbing the measured CT-numbers of the tissue substitutes (1 sigma:10HU). The impact of high-Z content was assessed through calibrations where the two substitutes with barium content were replaced by hypothetical materials without barium. Results: The stoichiometric calibrations generally agreed within 1% between the models, for non-bony tissues. For higher CT-numbers, a well-known 2-parameter model (Sch00) generated larger SPRs compared to the other models, with inter-model discrepancies of up to 3%. The 95% coverage interval of the calibrations obtained from the robustness analysis varied substantially. The well-known 2- and 3-parameter models (Sch00/Sch96) had the largest intervals. However, the partly-hypothetical (i.e. no barium) input data generated calibrations that agreed within 1% over the whole CT scale for all models and improved the 95% coverage interval of the well-known models (Sch00/Sch96). Conclusion: All parameterization models performed comparably if the scanned materials only contained elements with Z <= 20. However, the two alternative models proposed here (Karol/Spek), together with a previously published 1-parameter model (Mar12), generated robust calibrations in close agreement even when tissue substitutes contain elements with higher atomic number.
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