| 1. |
- Slatkin, Daniel, et al.
(författare)
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Microbeam Radiation Therapy
- 1992
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405 .- 2473-4209. ; 19, s. 1395-1400
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Tidskriftsartikel (refereegranskat)abstract
- It is proposed to carry out radiotherapy and radiosurgery for brain lesions by crossfiring an array of parallel, closely spaced microbeams of synchrotron-generated x rays several times through an isocentric target, each microbeam in the array having an 25-µm-wide adjustable-height rectangular cross section. The following inferences from the known tissue sparing of 22-MeV deuteron microbeams in the mouse brain and the following exemplary Monte Carlo computations indicate that endothelial cells in the brain that are lethally irradiated by any microbeam in an array of adequately spaced microbeams outside an isocentric target will be replaced by endothelial cells regenerated from microscopically contiguous, minimally irradiated endothelium in intermicrobeam segments of brain vasculature. Endothelial regeneration will prevent necrosis of the nontargeted parenchymal tissue. However, neoplastic and/or nonneoplastic targeted tissues at the isocenter will be so severely depleted of potentially mitotic endothelial and parenchymal cells by multiple overlapping microbeams that necrosis will ensue. The Monte Carlo computations simulate microbeam irradiations of a 16-cm diameter, 16-cm-long cylindrical human head phantom using 50-, 100-, and 150-keV monochromatic x rays.
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| 2. |
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| 3. |
- Malusek, Alexandr, et al.
(författare)
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A model-based iterative reconstruction algorithm DIRA using patient-specific tissue classification via DECT for improved quantitative CT in dose planning
- 2017
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Ingår i: Medical physics (Lancaster). - : WILEY. - 0094-2405. ; 44:6, s. 2345-2357
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To develop and evaluate-in a proof-of-concept configuration-a novel iterative reconstruction algorithm (DIRA) for quantitative determination of elemental composition of patient tissues for application to brachytherapy with low energy (amp;lt; 50 keV) photons and proton therapy. Methods: DIRA was designed as a model-based iterative reconstruction algorithm, which uses filtered backprojection, automatic segmentation and multimaterial tissue decomposition. The evaluation was done for a phantom derived from the voxelized ICRP 110 male phantom. Soft tissues were decomposed to the lipid, protein and water triplet, bones were decomposed to the compact bone and bone marrow doublet. Projections were derived using the Drasim simulation code for an axial scanning configuration resembling a typical DECT (dual-energy CT) scanner with 80 kV and Sn140 kV x-ray spectra. The iterative loop produced mono-energetic images at 50 and 88 keV without beam hardening artifacts. Different noise levels were considered: no noise, a typical noise level in diagnostic imaging and reduced noise level corresponding to tenfold higher doses. An uncertainty analysis of the results was performed using type A and B evaluations. The two approaches were compared. Results: Linear attenuation coefficients averaged over a region were obtained with relative errors less than 0.5% for all evaluated regions. Errors in average mass fractions of the three-material decomposition were less than 0.04 for no noise and reduced noise levels and less than 0.11 for the typical noise level. Mass fractions of individual pixels were strongly affected by noise, which slightly increased after the first iteration but subsequently stabilized. Estimates of uncertainties in mass fractions provided by the type B evaluation differed from the type A estimates by less than 1.5% for most cases. The algorithm was fast, the results converged after 5 iterations. The algorithmic complexity of forward polyenergetic projection calculation was much reduced by using material doublets and triplets. Conclusions: The simulations indicated that DIRA is capable of determining elemental composition of tissues, which are needed in brachytherapy with low energy (amp;lt; 50 keV) photons and proton therapy. The algorithm provided quantitative monoenergetic images with beam hardening artifacts removed. Its convergence was fast, image sharpness expressed via the modulation transfer function was maintained, and image noise did not increase with the number of iterations. c 2017 American Association of Physicists in Medicine
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| 4. |
- Sandborg, Michael, et al.
(författare)
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Comparison of clinical and physical measures of image quality in chest and pelvis computed radiography at different tube voltages
- 2006
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 33:11, s. 4169-4175
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this work was to study the dependence of image quality in digital chest and pelvis radiography on tube voltage, and to explore correlations between clinical and physical measures of image quality. The effect on image quality of tube voltage in these two examinations was assessed using two methods. The first method relies on radiologists' observations of images of an anthropomorphic phantom, and the second method was based on computer modeling of the imaging system using an anthropomorphic voxel phantom. The tube voltage was varied within a broad range (50-150 kV), including those values typically used with screen-film radiography. The tube charge was altered so that the same effective dose was achieved for each projection. Two x-ray units were employed using a computed radiography (CR) image detector with standard tube filtration and antiscatter device. Clinical image quality was assessed by a group of radiologists using a visual grading analysis (VGA) technique based on the revised CEC image criteria. Physical image quality was derived from a Monte Carlo computer model in terms of the signal-to-noise ratio, SNR, of anatomical structures corresponding to the image criteria. Both the VGAS (visual grading analysis score) and SNR decrease with increasing tube voltage in both chest PA and pelvis AP examinations, indicating superior performance if lower tube voltages are employed. Hence, a positive correlation between clinical and physical measures of image quality was found. The pros and cons of using lower tube voltages with CR digital radiography than typically used in analog screen-film radiography are discussed, as well as the relevance of using VGAS and quantum-noise SNR as measures of image quality in pelvis and chest radiography.
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| 5. |
- Sandborg, Michael, 1961-, et al.
(författare)
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Schemes for the optimization of chest radiography using a computer model of the patient and x-ray imaging system
- 2001
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 28:10, s. 2007-2019
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Tidskriftsartikel (refereegranskat)abstract
- A computer program has been developed to model chest radiography. It incorporates a voxel phantom of an adult and includes antiscatter grid, radiographic screen, and film. Image quality is quantified by calculating the contrast (?OD) and the ideal observer signal-to-noise ratio (SNRI) for a number of relevant anatomical details at various positions in the anatomy. Detector noise and system unsharpness are modeled and their influence on image quality is considered. A measure of useful dynamic range is computed and defined as the fraction of the image that is reproduced at an optical density such that the film gradient exceeds a preset value. The effective dose is used as a measure of the radiation risk for the patient. A novel approach to patient dose and image quality optimization has been developed and implemented. It is based on a reference system acknowledged to yield acceptable image quality in a clinical trial. Two optimizations schemes have been studied, the first including the contrast of vessels as measure of image quality and the second scheme using also the signal-to-noise ratio of calcifications. Both schemes make use of our measure of useful dynamic range as a key quantity. A large variety of imaging conditions was simulated by varying the tube voltage, antiscatter device, screen-film system, and maximum optical density in the computed image. It was found that the optical density is crucial in screen-film chest radiography. Significant dose savings (30%-50%) can be accomplished without sacrificing image quality by using low-atomic-number grids with a low grid ratio or an air gap and more sensitive screen-film system. Dose-efficient configurations proposed by the model agree well with the example of good radiographic technique suggested by the European Commission. ⌐ 2001 American Association of Physicists in Medicine.
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| 6. |
- Tapiovaara, M, et al.
(författare)
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How should low-contrast detail detectability be measured in fluoroscopy?
- 2004
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 31:9, s. 2564-2576
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Tidskriftsartikel (refereegranskat)abstract
- The relationship and precision of four methods for measuring the low-contrast detail detectability in fluoroscopic imaging were studied. These included the physical measurement of the accumulation rate of the square of the signal-to-noise ratio (SNRrate2), two-alternative forced-choice (2-AFC) experiments, sixteen-alternative forced-choice (16-AFC) experiments and subjective determination of the threshold contrast. The precision and sensitivity of the threshold contrast measurement were seen to be modest in the constancy testing of fluoroscopic equipment: only large changes in system performance could be reliably detected by that method. The measurement of the SNRrate2 is suggested instead. The relationship between the results of the various methods were studied, and it was found that human performance can be related to SNRrate2 by introducing the concept of the effective image information integration time (teff). When measured for an unlimited observation time, it depicts the saturation of human performance in detecting a static low-contrast detail in dynamic image noise. Here, teff was found to be about 0.6 s in 2-AFC tests and 0.3 s in 16-AFC tests. © 2004 American Association of Physicists in Medicine.
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