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May 2010

Volume 37, Issue 5, pp. 1939-2397

Spotlight Figure

Med. Phys. 37, 1976 (2010); http://dx.doi.org/10.1118/1.3368603 (11 pages)

Angelique Ale, Ralf B. Schulz, Athanasios Sarantopoulos, and Vasilis Ntziachristos
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POINT/COUNTERPOINT: Most residency programs for radiation oncology physicists do not reflect the heightened importance of medical imaging

X. Ronald Zhu, Ph.D., Rupak K. Das, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 1939 (2010); http://dx.doi.org/10.1118/1.3355935 (3 pages)

Online Publication Date: 8 April 2010

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Abstract Unavailable
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87.53.Kn Conformal radiation treatment
87.56.-v Radiation therapy equipment
87.85.Pq Biomedical imaging
87.19.xj Cancer

RADIATION THERAPY PHYSICS: Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies

Bijan Arjomandy, Ramesh Tailor, Aman Anand, Narayan Sahoo, Michael Gillin, Karl Prado, and Milos Vicic

Med. Phys. 37, 1942 (2010); http://dx.doi.org/10.1118/1.3373523 (6 pages) | Cited 21 times

Online Publication Date: 8 April 2010

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Purpose: Since the Gafchromic film EBT has been recently replaced by the newer model EBT2, its characterization, especially energy dependence, has become critically important. The energy dependence of the dose response of Gafchromic EBT2 film is evaluated for a broad range of energies from different radiation sources used in radiation therapy.
Methods: The beams used for this study comprised of kilovoltage x rays (75, 125, and 250 kVp), 137Cs gamma (662 KeV), 60Co gamma (1.17–1.33 MeV), megavoltage x rays (6 and 18 MV), electron beams (6 and 20 MeV), and proton beams (100 and 250 MeV). The film’s response to each of the above energies was measured over the dose range of 0.4–10 Gy, which corresponds to optical densities ranging from 0.05 to 0.74 for the film reader used.
Results: The energy dependence of EBT2 was found to be relatively small within measurement uncertainties (1σ = ±4.5%) for all energies and modalities.
Conclusion: For relative and absolute dosimetry of radiation therapy beams, the weak energy dependence of the EBT2 makes it most suitable for clinical use compared to other films.
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87.57.uq Dosimetry
87.56.-v Radiation therapy equipment

RADIATION IMAGING PHYSICS: Anatomical background and generalized detectability in tomosynthesis and cone-beam CT

G. J. Gang, D. J. Tward, J. Lee, and J. H. Siewerdsen

Med. Phys. 37, 1948 (2010); http://dx.doi.org/10.1118/1.3352586 (18 pages) | Cited 16 times

Online Publication Date: 9 April 2010

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Purpose: Anatomical background presents a major impediment to detectability in 2D radiography as well as 3D tomosynthesis and cone-beam CT (CBCT). This article incorporates theoretical and experimental analysis of anatomical background “noise” in cascaded systems analysis of 2D and 3D imaging performance to yield “generalized” metrics of noise-equivalent quanta (NEQ) and detectability index as a function of the orbital extent of the (circular arc) source-detector orbit.
Methods: A physical phantom was designed based on principles of fractal self-similarity to exhibit power-law spectral density (κ/fβ) comparable to various anatomical sites (e.g., breast and lung). Background power spectra [SB(f)] were computed as a function of source-detector orbital extent, including tomosynthesis ( ∼ 10°–180°) and CBCT (180°+fan to 360°) under two acquisition schemes: (1) Constant angular separation between projections (variable dose) and (2) constant total number of projections (constant dose). The resulting SB was incorporated in the generalized NEQ, and detectability index was computed from 3D cascaded systems analysis for a variety of imaging tasks.
Results: The phantom yielded power-law spectra within the expected spatial frequency range, quantifying the dependence of clutter magnitude (κ) and correlation (β) with increasing tomosynthesis angle. Incorporation of SB in the 3D NEQ provided a useful framework for analyzing the tradeoffs among anatomical, quantum, and electronic noise with dose and orbital extent. Distinct implications are posed for breast and chest tomosynthesis imaging system design—applications varying significantly in κ and β, and imaging task and, therefore, in optimal selection of orbital extent, number of projections, and dose. For example, low-frequency tasks (e.g., soft-tissue masses or nodules) tend to benefit from larger orbital extent and more fully 3D tomographic imaging, whereas high-frequency tasks (e.g., microcalcifications) require careful, application-specific selection of orbital extent and number of projections to minimize negative effects of quantum and electronic noise.
Conclusions: The complex tradeoffs among anatomical background, quantum noise, and electronic noise in projection imaging, tomosynthesis, and CBCT can be described by generalized cascaded systems analysis, providing a useful framework for system design and optimization.
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87.59.bf Digital radiography
87.57.Q- Computed tomography
87.57.cm Noise
87.57.N- Image analysis

RADIATION THERAPY PHYSICS: Four-dimensional intensity-modulated radiation therapy planning for dynamic tracking using a direct aperture deformation (DAD) method

Minzhi Gui, Yuanming Feng, Byongyong Yi, Anil Arvind Dhople, and Cedric Yu

Med. Phys. 37, 1966 (2010); http://dx.doi.org/10.1118/1.3319498 (10 pages) | Cited 6 times

Online Publication Date: 9 April 2010

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Purpose: Planning for the delivery of intensity-modulated radiation therapy (IMRT) to a moving target, referred to as four-dimensional (4D) IMRT planning, is a crucial step for achieving the treatment objectives for sites that move during treatment delivery. The authors proposed a simplistic method that accounts for both rigid and nonrigid respiration-induced target motion based on 4D computed tomography (4DCT) data sets.
Methods: A set of MLC apertures and weights was first optimized on a reference phase of a 4DCT data set. At each beam angle, the apertures were morphed from the reference phase to each of the remaining phases according to the relative shape changes in the beam’s eye view of the target. Three different planning schemes were evaluated for two lung cases and one pancreas patient: (1) Individually optimizing each breathing phase; (2) optimizing the reference phase and shifting the optimized apertures to other breathing phases based on a rigid-body image registration; and (3) optimizing the reference phase and deforming the optimized apertures to the other phases based on the deformation and translation of target contours. Planning results using scheme 1 serves as the “gold standard” for plan quality assessment; scheme 2 is the method previously proposed in the literature; and scheme 3 is the method the authors proposed in this article. The optimization results were compared between the three schemes for all three cases.
Results: The proposed scheme 3 is comparable to scheme 1 in plan quality, and provides improved target coverage and conformity with similar normal tissue dose compared with scheme 2.
Conclusions: Direct aperture deformation method for 4D IMRT planning improves upon methods that only consider rigid-body motion and achieves a plan quality close to that optimized for each of the phases.
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87.57.Q- Computed tomography
87.57.nj Registration
87.19.Wx Pneumodyamics, respiration
87.55.dk Dose-volume analysis
87.59.-e X-ray imaging

RADIATION MEASUREMENT PHYSICS: Imaging performance of a hybrid x-ray computed tomography-fluorescence molecular tomography system using priors

Angelique Ale, Ralf B. Schulz, Athanasios Sarantopoulos, and Vasilis Ntziachristos

Med. Phys. 37, 1976 (2010); http://dx.doi.org/10.1118/1.3368603 (11 pages) | Cited 10 times

Online Publication Date: 9 April 2010

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Purpose: The performance is studied of two newly introduced and previously suggested methods that incorporate priors into inversion schemes associated with data from a recently developed hybrid x-ray computed tomography and fluorescence molecular tomography system, the latter based on CCD camera photon detection. The unique data set studied attains accurately registered data of high spatially sampled photon fields propagating through tissue along 360° projections.
Methods: Approaches that incorporate structural prior information were included in the inverse problem by adding a penalty term to the minimization function utilized for image reconstructions. Results were compared as to their performance with simulated and experimental data from a lung inflammation animal model and against the inversions achieved when not using priors.
Results: The importance of using priors over stand-alone inversions is also showcased with high spatial sampling simulated and experimental data. The approach of optimal performance in resolving fluorescent biodistribution in small animals is also discussed.
Conclusions: Inclusion of prior information from x-ray CT data in the reconstruction of the fluorescence biodistribution leads to improved agreement between the reconstruction and validation images for both simulated and experimental data.
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87.57.Q- Computed tomography
87.63.L- Visual imaging
87.57.nf Reconstruction
87.59.-e X-ray imaging
42.30.Wb Image reconstruction; tomography

RADIATION PROTECTION PHYSICS: A comparative study on the risk of second primary cancers in out-of-field organs associated with radiotherapy of localized prostate carcinoma using Monte Carlo-based accelerator and patient models

Bryan Bednarz, Basit Athar, and X. George Xu

Med. Phys. 37, 1987 (2010); http://dx.doi.org/10.1118/1.3367012 (8 pages) | Cited 1 time

Online Publication Date: 9 April 2010

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Purpose: A physician’s decision regarding an ideal treatment approach (i.e., radiation, surgery, and/or hormonal) for prostate carcinoma is traditionally based on a variety of metrics. One of these metrics is the risk of radiation-induced second primary cancer following radiation treatments. The aim of this study was to investigate the significance of second cancer risks in out-of-field organs from 3D-CRT and IMRT treatments of prostate carcinoma compared to baseline cancer risks in these organs.
Methods: Monte Carlo simulations were performed using a detailed medical linear accelerator model and an anatomically realistic adult male whole-body phantom. A four-field box treatment, a four-field box treatment plus a six-field boost, and a seven-field IMRT treatment were simulated. Using BEIR VII risk models, the age-dependent lifetime attributable risks to various organs outside the primary beam with a known predilection for cancer were calculated using organ-averaged equivalent doses.
Results: The four-field box treatment had the lowest treatment-related second primary cancer risks to organs outside the primary beam ranging from 7.3×10−9 to 2.54×10−5%/MU depending on the patients age at exposure and second primary cancer site. The risks to organs outside the primary beam from the four-field box and six-field boost and the seven-field IMRT were nearly equivalent. The risks from the four-field box and six-field boost ranged from 1.39×10−8 to 1.80×10−5%/MU, and from the seven-field IMRT ranged from 1.60×10−9 to 1.35×10−5%/MU. The second cancer risks in all organs considered from each plan were below the baseline risks.
Conclusions: The treatment-related second cancer risks in organs outside the primary beam due to 3D-CRT and IMRT is small. New risk assessment techniques need to be investigated to address the concern of radiation-induced second cancers from prostate treatments, particularly focusing on risks to organs inside the primary beam.
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87.53.Kn Conformal radiation treatment
87.53.Bn Dosimetry/exposure assessment
87.55.kh Applications
87.19.xj Cancer

MAGNETIC RESONANCE PHYSICS: A robust coregistration method for in vivo studies using a first generation simultaneous PET/MR scanner

Thomas S. C. Ng, Daniel Procissi, Yibao Wu, and Russell E. Jacobs

Med. Phys. 37, 1995 (2010); http://dx.doi.org/10.1118/1.3369447 (9 pages) | Cited 2 times

Online Publication Date: 12 April 2010

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Purpose: Hybrid positron emission tomography (PET)/magnetic resonance (MR) imaging systems have recently been built that allow functional and anatomical information obtained from PET and MR to be acquired simultaneously. The authors have developed a robust coregistration scheme for a first generation small animal PET/MR imaging system and illustrated the potential of this system to study intratumoral heterogeneity in a mouse model.
Methods: An alignment strategy to fuse simultaneously acquired PET and MR data, using the MR imaging gradient coordinate system as the reference basis, was developed. The fidelity of the alignment was evaluated over multiple study sessions. In order to explore its robustness in vivo, the alignment strategy was applied to explore the heterogeneity of glucose metabolism in a xenograft tumor model, using 18F-FDG-PET to guide the acquisition of localized 1H MR spectra within a single imaging session.
Results: The alignment method consistently fused the PET/MR data sets with subvoxel accuracy (registration error mean = 0.55 voxels, <0.28 mm); this was independent of location within the field of view. When the system was used to study intratumoral heterogeneity within xenograft tumors, a correlation of high 18F-FDG-PET signal with high choline/creatine ratio was observed.
Conclusions: The authors present an implementation of an efficient and robust coregistration scheme for multimodal noninvasive imaging using PET and MR. This setup allows time-sensitive, multimodal studies of physiology to be conducted in an efficient manner.
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87.19.lf MRI: anatomic, functional, spectral, diffusion
87.57.uk Positron emission tomography (PET)
87.15.R- Reactions and kinetics
87.61.-c Magnetic resonance imaging

RADIATION THERAPY PHYSICS: Beam-centric algorithm for pretreatment patient position correction in external beam radiation therapy

Supratik Bose, Himanshu Shukla, and Jonathan Maltz

Med. Phys. 37, 2004 (2010); http://dx.doi.org/10.1118/1.3327457 (13 pages) | Cited 3 times

Online Publication Date: 12 April 2010

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Purpose: In current image guided pretreatment patient position adjustment methods, image registration is used to determine alignment parameters. Since most positioning hardware lacks the full six degrees of freedom (DOF), accuracy is compromised. The authors show that such compromises are often unnecessary when one models the planned treatment beams as part of the adjustment calculation process. The authors present a flexible algorithm for determining optimal realizable adjustments for both step-and-shoot and arc delivery methods.
Methods: The beam shape model is based on the polygonal intersection of each beam segment with the plane in pretreatment image volume that passes through machine isocenter perpendicular to the central axis of the beam. Under a virtual six-DOF correction, ideal positions of these polygon vertices are computed. The proposed method determines the couch, gantry, and collimator adjustments that minimize the total mismatch of all vertices over all segments with respect to their ideal positions. Using this geometric error metric as a function of the number of available DOF, the user may select the most desirable correction regime.
Results: For a simulated treatment plan consisting of three equally weighted coplanar fixed beams, the authors achieve a 7% residual geometric error (with respect to the ideal correction, considered 0% error) by applying gantry rotation as well as translation and isocentric rotation of the couch. For a clinical head-and-neck intensity modulated radiotherapy plan with seven beams and five segments per beam, the corresponding error is 6%. Correction involving only couch translation (typical clinical practice) leads to a much larger 18% mismatch. Clinically significant consequences of more accurate adjustment are apparent in the dose volume histograms of target and critical structures.
Conclusions: The algorithm achieves improvements in delivery accuracy using standard delivery hardware without significantly increasing total treatment session duration. It encourages parsimonious utilization of all available DOF. Finally, in certain cases, it obviates the need of a robotic couch having six DOF for the correction of patient displacement and rotations.
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87.56.nk Collimators
87.59.-e X-ray imaging
87.53.Bn Dosimetry/exposure assessment
87.57.nj Registration
87.55.dk Dose-volume analysis
87.55.Gh Simulation

RADIATION IMAGING PHYSICS: Contrast-enhanced spectral mammography with a photon-counting detector

Erik Fredenberg, Magnus Hemmendorff, Björn Cederström, Magnus Åslund, and Mats Danielsson

Med. Phys. 37, 2017 (2010); http://dx.doi.org/10.1118/1.3371689 (13 pages) | Cited 12 times

Online Publication Date: 12 April 2010

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Purpose: Spectral imaging is a method in medical x-ray imaging to extract information about the object constituents by the material-specific energy dependence of x-ray attenuation. The authors have investigated a photon-counting spectral imaging system with two energy bins for contrast-enhanced mammography. System optimization and the potential benefit compared to conventional non-energy-resolved absorption imaging was studied.
Methods: A framework for system characterization was set up that included quantum and anatomical noise and a theoretical model of the system was benchmarked to phantom measurements.
Results: Optimal combination of the energy-resolved images corresponded approximately to minimization of the anatomical noise, which is commonly referred to as energy subtraction. In that case, an ideal-observer detectability index could be improved close to 50% compared to absorption imaging in the phantom study. Optimization with respect to the signal-to-quantum-noise ratio, commonly referred to as energy weighting, yielded only a minute improvement. In a simulation of a clinically more realistic case, spectral imaging was predicted to perform approximately 30% better than absorption imaging for an average glandularity breast with an average level of anatomical noise. For dense breast tissue and a high level of anatomical noise, however, a rise in detectability by a factor of 6 was predicted. Another ∼ 70%–90% improvement was found to be within reach for an optimized system.
Conclusions: Contrast-enhanced spectral mammography is feasible and beneficial with the current system, and there is room for additional improvements. Inclusion of anatomical noise is essential for optimizing spectral imaging systems.
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87.59.E- Mammography
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
87.59.B- Radiography

RADIATION IMAGING PHYSICS: A signature dissimilarity measure for trabecular bone texture in knee radiographs

T. Woloszynski, P. Podsiadlo, G. W. Stachowiak, and M. Kurzynski

Med. Phys. 37, 2030 (2010); http://dx.doi.org/10.1118/1.3373522 (13 pages) | Cited 4 times

Online Publication Date: 14 April 2010

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Purpose: The purpose of this study is to develop a dissimilarity measure for the classification of trabecular bone (TB) texture in knee radiographs. Problems associated with the traditional extraction and selection of texture features and with the invariance to imaging conditions such as image size, anisotropy, noise, blur, exposure, magnification, and projection angle were addressed.
Methods: In the method developed, called a signature dissimilarity measure (SDM), a sum of earth mover’s distances calculated for roughness and orientation signatures is used to quantify dissimilarities between textures. Scale-space theory was used to ensure scale and rotation invariance. The effects of image size, anisotropy, noise, and blur on the SDM developed were studied using computer generated fractal texture images. The invariance of the measure to image exposure, magnification, and projection angle was studied using x-ray images of human tibia head. For the studies, Mann–Whitney tests with significance level of 0.01 were used. A comparison study between the performances of a SDM based classification system and other two systems in the classification of Brodatz textures and the detection of knee osteoarthritis (OA) were conducted. The other systems are based on weighted neighbor distance using compound hierarchy of algorithms representing morphology (WND-CHARM) and local binary patterns (LBP).
Results: Results obtained indicate that the SDM developed is invariant to image exposure (2.5–30 mA s), magnification (×1.00–×1.35), noise associated with film graininess and quantum mottle (<25%), blur generated by a sharp film screen, and image size (>64×64 pixels). However, the measure is sensitive to changes in projection angle (>5°), image anisotropy (>30°), and blur generated by a regular film screen. For the classification of Brodatz textures, the SDM based system produced comparable results to the LBP system. For the detection of knee OA, the SDM based system achieved 78.8% classification accuracy and outperformed the WND-CHARM system (64.2%).
Conclusions: The SDM is well suited for the classification of TB texture images in knee OA detection and may be useful for the texture classification of medical images in general.
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87.59.-e X-ray imaging
87.59.B- Radiography
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