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Mar 2013

Volume 40, Issue 3, pp. 030401-037302-1

Spotlight Figure

Med. Phys. 40, 032305 (2013); http://dx.doi.org/10.1118/1.4790466 (12 pages)

Shannon C. Agner, Jun Xu, and Anant Madabhushi
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EDITORIAL: The importance of adaptive response in cancer prevention and therapy

Mohan Doss

Med. Phys. 40, 030401 (2013); http://dx.doi.org/10.1118/1.4773027 (2 pages)

Online Publication Date: 8 February 2013

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Abstract Unavailable
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87.55.D- Treatment planning
87.19.xj Cancer
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POINT/COUNTERPOINT: Therapeutic rather than diagnostic medical physicists should lead the development and clinical implementation of image-guided nonionizing therapeutic modalities such as MR-guided high-intensity ultrasound

Wolfgang A. Tomé, Ph.D., R. Jason Stafford, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 40, 030601 (2013); http://dx.doi.org/10.1118/1.4789481 (4 pages)

Online Publication Date: 11 February 2013

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Abstract Unavailable
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87.61.Qr Functional imaging
87.63.dh Ultrasonographic imaging
87.50.yt Therapeutic applications
87.59.B- Radiography
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MEDICAL PHYSICS LETTER: Development of XFCT imaging strategy for monitoring the spatial distribution of platinum-based chemodrugs: Instrumentation and phantom validation

Yu Kuang, Guillem Pratx, Magdalena Bazalova, Jianguo Qian, Bowen Meng, and Lei Xing

Med. Phys. 40, 030701 (2013); http://dx.doi.org/10.1118/1.4789917 (7 pages)

Online Publication Date: 13 February 2013

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Purpose: Developing an imaging method to directly monitor the spatial distribution of platinum-based (Pt) drugs at the tumor region is of critical importance for early assessment of treatment efficacy and personalized treatment. In this study, the authors investigated the feasibility of imaging platinum (Pt)-based drug distribution using x-ray fluorescence (XRF, a.k.a. characteristic x ray) CT (XFCT).
Methods: A 5-mm-diameter pencil beam produced by a polychromatic x-ray source equipped with a tungsten anode was used to stimulate emission of XRF photons from Pt drug embedded within a water phantom. The phantom was translated and rotated relative to the stationary pencil beam in a first-generation CT geometry. The x-ray energy spectrum was collected for 18 s at each position using a cadmium telluride detector. The spectra were then used for the K-shell XRF peak isolation and sinogram generation for Pt. The distribution and concentration of Pt were reconstructed with an iterative maximum likelihood expectation maximization algorithm. The capability of XFCT to multiplexed imaging of Pt, gadolinium (Gd), and iodine (I) within a water phantom was also investigated.
Results: Measured XRF spectrum showed a sharp peak characteristic of Pt with a narrow full-width at half-maximum (FWHM) (FWHMKα1 = 1.138 keV, FWHMKα2 = 1.052 keV). The distribution of Pt drug in the water phantom was clearly identifiable on the reconstructed XRF images. Our results showed a linear relationship between the XRF intensity of Pt and its concentrations (R2 = 0.995), suggesting that XFCT is capable of quantitative imaging. A transmission CT image was also obtained to show the potential of the approach for providing attenuation correction and morphological information. Finally, the distribution of Pt, Gd, and I in the water phantom was clearly identifiable in the reconstructed images from XFCT multiplexed imaging.
Conclusions: XFCT is a promising modality for monitoring the spatial distribution of Pt drugs. The technique may be useful in tailoring tumor treatment regimen in the future.
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87.57.Q- Computed tomography
87.59.B- Radiography
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): Focus Series Introduction

Frédéric Noo and Andrew Karellas

Med. Phys. 40, 031001 (2013); http://dx.doi.org/10.1118/1.4794317 (1 page)

Online Publication Date: 28 February 2013

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Abstract Unavailable
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01.30.Cc Conference proceedings
87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): Interesting detector shapes for third generation CT scanners

Marc Kachelrieß

Med. Phys. 40, 031101 (2013); http://dx.doi.org/10.1118/1.4789588 (14 pages)

Online Publication Date: 28 February 2013

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Purpose:To discuss options in designing detector shapes in third generation CT and to quantify potential cost savings for compact third generation CT systems, and to extend the work from two-dimensional fan-beam CT to three-dimensional cone-beam CT for circular, sequential, and spiral scan trajectories.
Methods: Third generation CT scanners typically comprise detectors which are flat or whose shape is the segment of a cylinder or a sphere that is focused onto the focal spot of the x-ray source. There appear to be two design criteria that favor this choice of detector shape. One is the possibility of performing fan-beam and cone-beam filtered backprojection in the native geometry (without rebinning) and the other criterion could be to enable the early use of focused antiscatter grids. It is less known, however, that other detector shapes may also have these properties. While these designs have been evaluated for 2D CT from a theoretical standpoint more than one decade ago the authors revisit and generalize these considerations, extend them to 3D circular, sequential, and spiral cone-beam CT and propose an optimal design in terms of detector costs while keeping image quality constant. Their considerations and conclusions are based on considering the sampling density of the x-rays, including the effects of finite focal spot and finite detector element size. Proposing image reconstruction algorithms or numerically evaluating the results by reconstructing simulated projection data is not within the scope of this work.
Results: If the detector arc is curved to be nearly concentric with the circle describing the edge of the field of measurement significantly less detector area and detector pixels are required compared to today's third generation CT systems where the detector arc is centered about the focal spot. Combined with a detector that just covers the spiral Tam window cost savings of 60% or more are possible in compact CT systems. In terms of practicability the new designs appear to be nearly as easy to realize as today's third generation systems.
Conclusions: Compact CT systems, which require the focal spot to be mounted close to the edge of the field of measurement, may significantly benefit from using detector shapes other than the typical equiangular detector that is focused onto the focal spot.
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87.57.Q- Computed tomography
87.57.C- Image quality
87.57.nf Reconstruction
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): A low dose simulation tool for CT systems with energy integrating detectors

Stanislav Žabić, Qiu Wang, Thomas Morton, and Kevin M. Brown

Med. Phys. 40, 031102 (2013); http://dx.doi.org/10.1118/1.4789628 (14 pages)

Online Publication Date: 28 February 2013

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Purpose: This paper introduces a new strategy for simulating low-dose computed tomography (CT) scans using real scans of a higher dose as an input. The tool is verified against simulations and real scans and compared to other approaches found in the literature.
Methods: The conditional variance identity is used to properly account for the variance of the input high-dose data, and a formula is derived for generating a new Poisson noise realization which has the same mean and variance as the true low-dose data. The authors also derive a formula for the inclusion of real samples of detector noise, properly scaled according to the level of the simulated x-ray signals.
Results: The proposed method is shown to match real scans in number of experiments. Noise standard deviation measurements in simulated low-dose reconstructions of a 35 cm water phantom match real scans in a range from 500 to 10 mA with less than 5% error. Mean and variance of individual detector channels are shown to match closely across the detector array. Finally, the visual appearance of noise and streak artifacts is shown to match in real scans even under conditions of photon-starvation (with tube currents as low as 10 and 80 mA). Additionally, the proposed method is shown to be more accurate than previous approaches (1) in achieving the correct mean and variance in reconstructed images from pure-Poisson noise simulations (with no detector noise) under photon-starvation conditions, and (2) in simulating the correct noise level and detector noise artifacts in real low-dose scans.
Conclusions: The proposed method can accurately simulate low-dose CT data starting from high-dose data, including effects from photon starvation and detector noise. This is potentially a very useful tool in helping to determine minimum dose requirements for a wide range of clinical protocols and advanced reconstruction algorithms.
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87.57.Q- Computed tomography
87.59.-e X-ray imaging
87.53.Bn Dosimetry/exposure assessment
87.64.Aa Computer simulation
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): Filtered backprojection proton CT reconstruction along most likely paths

Simon Rit, George Dedes, Nicolas Freud, David Sarrut, and Jean Michel Létang

Med. Phys. 40, 031103 (2013); http://dx.doi.org/10.1118/1.4789589 (9 pages)

Online Publication Date: 28 February 2013

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Purpose: Proton CT (pCT) has the potential to accurately measure the electron density map of tissues at low doses but the spatial resolution is prohibitive if the curved paths of protons in matter is not accounted for. The authors propose to account for an estimate of the most likely path of protons in a filtered backprojection (FBP) reconstruction algorithm.
Methods: The energy loss of protons is first binned in several proton radiographs at different distances to the proton source to exploit the depth-dependency of the estimate of the most likely path. This process is named the distance-driven binning. A voxel-specific backprojection is then used to select the adequate radiograph in the distance-driven binning in order to propagate in the pCT image the best achievable spatial resolution in proton radiographs. The improvement in spatial resolution is demonstrated using Monte Carlo simulations of resolution phantoms.
Results: The spatial resolution in the distance-driven binning depended on the distance of the objects from the source and was optimal in the binned radiograph corresponding to that distance. The spatial resolution in the reconstructed pCT images decreased with the depth in the scanned object but it was always better than previous FBP algorithms assuming straight line paths. In a water cylinder with 20 cm diameter, the observed range of spatial resolutions was 0.7 − 1.6 mm compared to 1.0 − 2.4 mm at best with a straight line path assumption. The improvement was strongly enhanced in shorter 200° scans.
Conclusions: Improved spatial resolution was obtained in pCT images with filtered backprojection reconstruction using most likely path estimates of protons. The improvement in spatial resolution combined with the practicality of FBP algorithms compared to iterative reconstruction algorithms makes this new algorithm a candidate of choice for clinical pCT.
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87.53.Kn Conformal radiation treatment
87.55.K- Monte Carlo methods
87.57.cf Spatial resolution
87.57.nf Reconstruction
87.57.Q- Computed tomography
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): Comparison of human and Hotelling observer performance for a fan-beam CT signal detection task

Adrian A. Sanchez, Emil Y. Sidky, Ingrid Reiser, and Xiaochuan Pan

Med. Phys. 40, 031104 (2013); http://dx.doi.org/10.1118/1.4789590 (9 pages)

Online Publication Date: 28 February 2013

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Purpose: A human observer study was performed for a signal detection task for the case of fan-beam x-ray computed tomography. Hotelling observer (HO) performance was calculated for the same detection task without the use of efficient channels. By considering the full image covariance produced by the filtered backprojection (FBP) algorithm and avoiding the use of channels in the computation of HO performance, the authors establish an absolute upper bound on signal detectability. Therefore, this study serves as a baseline for relating human and ideal observer performance in the case of fan-beam CT.
Methods: Eight human observers participated in a two-alternative forced choice experiment where the signal of interest was a small simulated ellipsoid in the presence of independent, identically distributed Gaussian detector noise. Theoretical performance of the HO, which is equivalent to the ideal observer in this case (see Sec. 13.2.12 in Barrett and Myers [Foundations of Image Science (Wiley, Hoboken, NJ, 2004)] , was also computed and compared to the performance of the human observers. In addition to a reference FBP implementation, two FBP implementations with inherent loss of HO signal detectability (e.g., by apodizing the ramp filter) were also investigated. Each of these latter two implementations takes the form of a discrete-to-discrete linear operator (i.e., a matrix), which has a nontrivial null-space resulting in the loss of detectability.
Results: Estimated observer detectability index (mathA) values for the human observers and SNR values for the HO were obtained. While Hanning filtering in the FBP implementation with a cutoff frequency of 1/4 of the Nyquist frequency reduces HO SNR (due to the reconstruction matrix's nontrivial null-space), this filtering was shown to consistently improve human observer performance. By contrast, increasing the image pixel size was seen to have a comparable effect on both the HO and the human observers, degrading performance.
Conclusions: These results, which characterize HO and human observer performance for a signal detection task in fan-beam FBP noise, form a basis for applying model observer metrics to fan-beam CT when knowledge of the full image-domain noise statistics is important. Further, by calculating HO performance without relying on channels, these results are particularly relevant when an information theoretic approach is considered, e.g., in optimization of the image reconstruction algorithm with respect to preservation of signal detectability. Finally, the HO (which is here equivalent to the ideal observer) provides an absolute upper bound on detection performance, and our results therefore provide insight into the performance of human observers relative to the optimum for two different cases wherein ideal observer performance is compromised through degradation of the data quality. In one case (regularization), human performance is improved to practically ideal performance, and in the other (larger pixel size), ideal and human observer performance are approximately degraded equivalently.
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87.57.Q- Computed tomography
87.57.nf Reconstruction
87.57.cm Noise
02.60.Pn Numerical optimization
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): Patchwork reconstruction with resolution modeling for digital breast tomosynthesis

Koen Michielsen, Katrien Van Slambrouck, Anna Jerebko, and Johan Nuyts

Med. Phys. 40, 031105 (2013); http://dx.doi.org/10.1118/1.4789591 (10 pages)

Online Publication Date: 28 February 2013

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Purpose: Digital breast tomosynthesis is a relatively new diagnostic x-ray modality that allows high resolution breast imaging while suppressing interference from overlapping anatomical structures. However, proper visualization of microcalcifications remains a challenge. For the subset of systems considered by the authors, the main cause of deterioration is movement of the x-ray source during exposures. They propose a modified grouped coordinate ascent algorithm that includes a specific acquisition model to compensate for this deterioration.
Methods: A resolution model based on the movement of the x-ray source during image acquisition is created and combined with a grouped coordinate ascent algorithm. Choosing planes parallel to the detector surface as the groups enables efficient implementation of the position dependent resolution model. In the current implementation, the resolution model is approximated by a Gaussian smoothing kernel. The effect of the resolution model on the iterative reconstruction is evaluated by measuring contrast to noise ratio (CNR) of spherical microcalcifications in a homogeneous background. After this, the new reconstruction method is compared to the optimized filtered backprojection method for the considered system, by performing two observer studies: the first study simulates clusters of spherical microcalcifications in a power law background for a free search task; the second study simu-lates smooth or irregular microcalcifications in the same type of backgrounds for a classification task.
Results: Including the resolution model in the iterative reconstruction methods increases the CNR of microcalcifications. The first observer study shows a significant improvement in detection of microcalcifications (p = 0.029), while the second study shows that performance on a classification task remains the same (p = 0.935) compared to the filtered backprojection method.
Conclusions: The new method shows higher CNR and improved visualization of microcalcifications in an observer experiment on synthetic data. Further study of the negative results of the classification task showed performance variations throughout the volume linked to the changing noise structure introduced by the combination of the resolution model and the smoothing prior.
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87.59.bd Computed radiography
87.57.nf Reconstruction
87.57.np Smoothing
87.57.cf Spatial resolution
87.57.cm Noise
02.50.-r Probability theory, stochastic processes, and statistics
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X-RAY COMPUTED TOMOGRAPHY: 2012 ADVANCES IN IMAGE FORMATION (Online only): Weighted simultaneous algebraic reconstruction technique for tomosynthesis imaging of objects with high-attenuation features

Y. M. Levakhina, J. Müller, R. L. Duschka, F. Vogt, J. Barkhausen, and T. M. Buzug

Med. Phys. 40, 031106 (2013); http://dx.doi.org/10.1118/1.4789592 (12 pages)

Online Publication Date: 28 February 2013

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Purpose: This paper introduces a nonlinear weighting scheme into the backprojection operation within the simultaneous algebraic reconstruction technique (SART). It is designed for tomosynthesis imaging of objects with high-attenuation features in order to reduce limited angle artifacts.
Methods: The algorithm estimates which projections potentially produce artifacts in a voxel. The contribution of those projections into the updating term is reduced. In order to identify those projections automatically, a four-dimensional backprojected space representation is used. Weighting coefficients are calculated based on a dissimilarity measure, evaluated in this space. For each combination of an angular view direction and a voxel position an individual weighting coefficient for the updating term is calculated.
Results: The feasibility of the proposed approach is shown based on reconstructions of the following real three-dimensional tomosynthesis datasets: a mammography quality phantom, an apple with metal needles, a dried finger bone in water, and a human hand. Datasets have been acquired with a Siemens Mammomat Inspiration tomosynthesis device and reconstructed using SART with and without suggested weighting. Out-of-focus artifacts are described using line profiles and measured using standard deviation (STD) in the plane and below the plane which contains artifact-causing features. Artifacts distribution in axial direction is measured using an artifact spread function (ASF). The volumes reconstructed with the weighting scheme demonstrate the reduction of out-of-focus artifacts, lower STD (meaning reduction of artifacts), and narrower ASF compared to nonweighted SART reconstruction. It is achieved successfully for different kinds of structures: point-like structures such as phantom features, long structures such as metal needles, and fine structures such as trabecular bone structures.
Conclusions: Results indicate the feasibility of the proposed algorithm to reduce typical tomosynthesis artifacts produced by high-attenuation features. The proposed algorithm assigns weighting coefficients automatically and no segmentation or tissue-classification steps are required. The algorithm can be included into various iterative reconstruction algorithms with an additive updating strategy. It can also be extended to computed tomography case with the complete set of angular data.
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87.59.-e X-ray imaging
87.57.cp Artifacts and distortion
87.57.nf Reconstruction
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
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© 2013, American Association of Physicists in Medicine. Individual readers of this journal, and nonprofit libraries acting for them, are freely permited to make fair use of the material in it, such as to copy an article for use in teaching or research. (For other kinds of copying see "Copying Fees.") Permission is granted to quote from this journal in scientific works with the customary acknowledgment of the source. To reprint a figure, table, or other excerpt requires, in addition, AAPM may require that permission also be obtained from one of the authors. Address inquiries and notices to Penny Slattery, Journal Manager, Medical Physics Journal, AAPM, One Physics Ellipse, College Park, MD 20740-3846; email: journal@aapm.org.

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