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Jan 2010

Volume 37, Issue 1, pp. 1-407

Spotlight Figure

Med. Phys. 37, 329 (2010); http://dx.doi.org/10.1118/1.3273034 (10 pages)

Alexander D. Klose, Bradley J. Beattie, Hamid Dehghani, Lena Vider, Carl Le, Vladimir Ponomarev, and Ronald Blasberg
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POINT/COUNTERPOINT: Medical physicists should be allowed by States to image and treat, just like radiologic technologists

Michael S. Gossman, M.S., Lisa A. Burgess, M.S., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 1 (2010); http://dx.doi.org/10.1118/1.3253993 (3 pages) | Cited 2 times

Online Publication Date: 4 December 2009

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Abstract Unavailable
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87.57.-s Medical imaging
87.85.Pq Biomedical imaging
87.55.-x Treatment strategy
87.56.-v Radiation therapy equipment

RADIATION THERAPY PHYSICS: Evaluation of the interplay effect when using RapidArc to treat targets moving in the craniocaudal or right-left direction

Laurence Court, Matthew Wagar, Ross Berbeco, Adam Reisner, Brian Winey, Debbie Schofield, Dan Ionascu, Aaron M. Allen, Richard Popple, and Tania Lingos

Med. Phys. 37, 4 (2010); http://dx.doi.org/10.1118/1.3263614 (8 pages) | Cited 5 times

Online Publication Date: 4 December 2009

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Purpose: We have investigated the dosimetric errors caused by the interplay between the motions of the LINAC and the tumor during the delivery of a volume modulated arc therapy treatment. This includes the development of an IMRT QA technique, applied here to evaluate RapidArc plans of varying complexity.
Methods: An IMRT QA technique was developed, which involves taking a movie of the delivered dose (0.2 s frames) using a 2D ion chamber array. Each frame of the movie is then moved according to a respiratory trace and the cumulative dose calculated. The advantage of this approach is that the impact of turning the beam on at different points in the respiratory trace, and of different types of motion, can be evaluated using data from a single irradiation. We evaluated this technique by comparing with the results when we actually moved the phantom during irradiation. RapidArc plans were created to treat a 62 cc spherical tumor in a lung phantom (16 plans) and a 454 cc irregular tumor in an actual patient (five plans). The complexity of each field was controlled by adjusting the MU (312–966 MU). Each plan was delivered to a phantom, and a movie of the delivered dose taken using a 2D ion chamber array. Patient motion was modeled by shifting each dose frame according to a respiratory trace, starting the motion at different phases. The expected dose distribution was calculated by blurring the static dose distribution with the target motion. The dose error due to the interplay effect was then calculated by comparing the delivered dose with the expected dose distribution. Peak-to-peak motion of 0.5, 1.0, and 2.0 cm in the craniocaudal and right-left directions, with target periods of 3 and 5 s, were evaluated for each plan (252 different target motion/plan combinations).
Results: The daily dose error due to the interplay effect was less than 10% for 98.4% of all pixels in the target for all plans investigated. The percentage of pixels for which the daily dose error could be larger than 5% increased with increasing plan complexity (field MU), but was less than 15% for all plans if the motion was 1 cm or less. For 2 cm motion, the dose error could be larger than 5% for 40% of pixels, but was less than 5% for more than 80% of pixels for MU<550, and was less than 10% for 99% of all pixels. The interplay effect was smaller for 3 s periods than for 5 s periods.
Conclusions: The interplay between the motions of the LINAC and the target can result in an error in the delivered dose. This effect increases with plan complexity, and with target magnitude and period. It may average out after many fractions.
Show PACS
87.53.Kn Conformal radiation treatment
29.20.Ej Linear accelerators
87.53.Bn Dosimetry/exposure assessment

RADIATION IMAGING PHYSICS: CT colonography: Advanced computer-aided detection scheme utilizing MTANNs for detection of “missed” polyps in a multicenter clinical trial

Kenji Suzuki, Don C. Rockey, and Abraham H. Dachman

Med. Phys. 37, 12 (2010); http://dx.doi.org/10.1118/1.3263615 (10 pages) | Cited 2 times

Online Publication Date: 4 December 2009

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Purpose: The purpose of this study was to develop an advanced computer-aided detection (CAD) scheme utilizing massive-training artificial neural networks (MTANNs) to allow detection of “difficult” polyps in CT colonography (CTC) and to evaluate its performance on false-negative (FN) CTC cases that radiologists “missed” in a multicenter clinical trial.
Methods: The authors developed an advanced CAD scheme consisting of an initial polyp-detection scheme for identification of polyp candidates and a mixture of expert MTANNs for substantial reduction in false positives (FPs) while maintaining sensitivity. The initial polyp-detection scheme consisted of (1) colon segmentation based on anatomy-based extraction and colon-based analysis and (2) detection of polyp candidates based on a morphologic analysis on the segmented colon. The mixture of expert MTANNs consisted of (1) supervised enhancement of polyps and suppression of various types of nonpolyps, (2) a scoring scheme for converting output voxels into a score for each polyp candidate, and (3) combining scores from multiple MTANNs by the use of a mixing artificial neural network. For testing the advanced CAD scheme, they created a database containing 24 FN cases with 23 polyps (range of 6–15 mm; average of 8 mm) and a mass (35 mm), which were “missed” by radiologists in CTC in the original trial in which 15 institutions participated.
Results: The initial polyp-detection scheme detected 63% (15/24) of the missed polyps with 21.0 (505/24) FPs per patient. The MTANNs removed 76% of the FPs with loss of one true positive; thus, the performance of the advanced CAD scheme was improved to a sensitivity of 58% (14/24) with 8.6 (207/24) FPs per patient, whereas a conventional CAD scheme yielded a sensitivity of 25% at the same FP rate (the difference was statistically significant).
Conclusions: With the advanced MTANN CAD scheme, 58% of the polyps missed by radiologists in the original trial were detected and with a reasonable number of FPs. The results suggest that the use of an advanced MTANN CAD scheme may potentially enhance the detection of “difficult” polyps.
Show PACS
87.57.Q- Computed tomography
87.63.lm Image enhancement
87.57.nm Segmentation

MAGNETIC RESONANCE PHYSICS: Detection of longitudinal lung structural and functional changes after diagnosis of radiation-induced lung injury using hyperpolarized math magnetic resonance imaging

Lindsay Mathew, Stewart Gaede, Andrew Wheatley, Roya Etemad-Rezai, George B. Rodrigues, and Grace Parraga

Med. Phys. 37, 22 (2010); http://dx.doi.org/10.1118/1.3263616 (10 pages) | Cited 7 times

Online Publication Date: 4 December 2009

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Purpose: Therapeutic radiation doses for thoracic tumors are significantly restricted to decrease the risk of nontumor tissue damage, yet radiation-induced lung injury (RILI) still occurs in over 1/3 of thoracic radiation treatment cases. Although RILI can be clinically monitored using pulmonary function measurements, the regional functional effects of the injury are not well understood. Hyperpolarized math magnetic resonance imaging provides measurements of regional lung function and structure with high spatial and temporal resolution; the authors use this tool longitudinally for the first time in seven subjects after clinical diagnosis of RILI in order to better understand regional changes in lung function and structure post-RILI.
Methods: All subjects underwent spirometry, plethysmography, and MRI at 3.0 T 35.1±12.2 weeks after radiation therapy commenced. Thoracic math, static math ventilation, and math diffusion-weighted images were acquired to generate the math apparent diffusion coefficient (ADC) and math percent ventilated volume (PVV). Four subjects returned 22.0±0.8 weeks after baseline imaging for follow-up spirometry and math MRI measurements of ADC and PVV.
Results: At baseline, PVV was significantly different (p = 0.025) and lower in the ipsilateral diseased lung (55±29%) compared to the contralateral nondiseased lung (88±5%). Longitudinally, significant increases were observed for math MRI PVV (16%±6%, p = 0.012) and math MRI ADC (0.02±0.01 cm2/s, p = 0.003) in the contralateral lung only, in the four subjects who returned for follow-up, while no changes in the ipsilateral lung were reported.
Conclusions: Hyperpolarized math MRI was well tolerated in all subjects with moderate to severe RILI. Functional improvements and microstructural changes were observed in the contralateral lung, while the ipsilateral lung remained stable, suggesting that functional compensatory changes may have occurred in the contralateral lung due to ipsilateral lung radiation-induced injury.
Show PACS
87.61.-c Magnetic resonance imaging
87.55.dk Dose-volume analysis
87.19.U- Hemodynamics
87.19.Wx Pneumodyamics, respiration

RADIATION IMAGING PHYSICS: A BPF-FBP tandem algorithm for image reconstruction in reverse helical cone-beam CT

Seungryong Cho, Dan Xia, Charles A. Pellizzari, and Xiaochuan Pan

Med. Phys. 37, 32 (2010); http://dx.doi.org/10.1118/1.3263618 (8 pages)

Online Publication Date: 4 December 2009

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Purpose: Reverse helical cone-beam computed tomography (CBCT) is a scanning configuration for potential applications in image-guided radiation therapy in which an accurate anatomic image of the patient is needed for image-guidance procedures. The authors previously developed an algorithm for image reconstruction from nontruncated data of an object that is completely within the reverse helix. The purpose of this work is to develop an image reconstruction approach for reverse helical CBCT of a long object that extends out of the reverse helix and therefore constitutes data truncation.
Methods: The proposed approach comprises of two reconstruction steps. In the first step, a chord-based backprojection-filtration (BPF) algorithm reconstructs a volumetric image of an object from the original cone-beam data. Because there exists a chordless region in the middle of the reverse helix, the image obtained in the first step contains an unreconstructed central-gap region. In the second step, the gap region is reconstructed by use of a Pack–Noo-formula-based filteredbackprojection (FBP) algorithm from the modified cone-beam data obtained by subtracting from the original cone-beam data the reprojection of the image reconstructed in the first step.
Results: The authors have performed numerical studies to validate the proposed approach in image reconstruction from reverse helical cone-beam data. The results confirm that the proposed approach can reconstruct accurate images of a long object without suffering from data-truncation artifacts or cone-angle artifacts.
Conclusions: They developed and validated a BPF-FBP tandem algorithm to reconstruct images of a long object from reverse helical cone-beam data. The chord-based BPF algorithm was utilized for converting the long-object problem into a short-object problem. The proposed approach is applicable to other scanning configurations such as reduced circular sinusoidal trajectories.
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87.57.Q- Computed tomography
87.53.Jw Therapeutic applications, including brachytherapy
87.57.nf Reconstruction

RADIATION IMAGING PHYSICS: Method for evaluating bow tie filter angle-dependent attenuation in CT: Theory and simulation results

John M. Boone

Med. Phys. 37, 40 (2010); http://dx.doi.org/10.1118/1.3264616 (9 pages) | Cited 7 times

Online Publication Date: 4 December 2009

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Purpose: Dosimetry in computed tomography (CT) is increasingly based on Monte Carlo studies that define the dose in the patient (in mGy) as a function of air kerma (free in air) at isocenter (mGy). The accuracy of Monte Carlo studies depends in part on the accuracy of the characterization of the bow tie filter for a given CT scanner model. A simple method for characterizing the bow tie filter attenuation profile in CT scanners would therefore be very useful. The theory behind such a method is proposed.
Methods: A measurement protocol is discussed mathematically and demonstrated using computer simulation. The proposed method requires the placement of a radiation monitor at the periphery of the CT field, and the time domain signal (kerma rate versus time) is measured with good temporal resolution ( ∼ 200 Hz or better) and with all other objects (e.g., patient couch) retracted from the field of view. Knowledge of the source to isocenter distance (or alternately, the isocenter to probe distance) is required. The stationary detector records the kerma rate versus time signal as the gantry rotates through several revolutions. From this temporal data, signal processing techniques are used to extract in-phase peaks, as well as out-of-phase kerma rate levels. From these data, the distance from isocenter to the probe can be determined (or, alternatively, the source to isocenter distance), and the angle-dependent bow tie filter attenuation can be computed. By measuring the angle-dependent bow tie filter attenuation at several kVp settings, the bow tie composition versus fan angle can be computed using basis decomposition techniques.
Results: The simulations illustrated that with 2% added noise in the kerma rate versus time signal, the attenuation properties of a hypothetical two component (aluminum and polymethyl methacrylate) bow tie filter could be determined (r2>0.99). Although the computed basis material thicknesses were not exactly equal to the actual thicknesses, their combined attenuation factors matched that of the actual filter across kVp’s to within an average of 0.057%.
Conclusions: It is concluded that the proposed method may provide a simple noninvasive approach to characterizing the performance of bow tie filters in CT systems; however, experimental validation is necessary.
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87.57.Q- Computed tomography
87.53.Bn Dosimetry/exposure assessment
87.55.dk Dose-volume analysis
87.55.kh Applications
87.57.cf Spatial resolution
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)

RADIATION THERAPY PHYSICS: Photon beam quality variations of a flattening filter free linear accelerator

Dietmar Georg, Gabriele Kragl, Sacha af Wetterstedt, Patrick McCavana, Brendan McClean, and Tommy Knöös

Med. Phys. 37, 49 (2010); http://dx.doi.org/10.1118/1.3264617 (5 pages) | Cited 10 times

Online Publication Date: 4 December 2009

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Purpose: Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams.
Methods: Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke’s Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270° with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm).
Results: For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(θ) varies by about 11% for an off-axis ray angle θ = 10°. These results agree within ±2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10°. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV.
Conclusions: Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified.
Show PACS
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
29.20.Ej Linear accelerators
29.27.-a Beams in particle accelerators
87.53.Bn Dosimetry/exposure assessment

RADIATION IMAGING PHYSICS: Lung perfusion imaging in small animals using 4D micro-CT at heartbeat temporal resolution

Cristian T. Badea, Samuel M. Johnston, Ergys Subashi, Yi Qi, Laurence W. Hedlund, and G. Allan Johnson

Med. Phys. 37, 54 (2010); http://dx.doi.org/10.1118/1.3264619 (9 pages) | Cited 8 times

Online Publication Date: 4 December 2009

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Purpose: Quantitative in vivo imaging of lung perfusion in rodents can provide critical information for preclinical studies. However, the combined challenges of high temporal and spatial resolution have made routine quantitative perfusion imaging difficult in small animals. The purpose of this work is to demonstrate 4D micro-CT for perfusion imaging in rodents at heartbeat temporal resolution and isotropic spatial resolution.
Methods: We have recently developed a dual tube/detector micro-CT scanner that is well suited to capture first pass kinetics of a bolus of contrast agent used to compute perfusion information. Our approach is based on the paradigm that similar time density curves can be reproduced in a number of consecutive, small volume injections of iodinated contrast agent at a series of different angles. This reproducibility is ensured by the high-level integration of the imaging components of our system with a microinjector, a mechanical ventilator, and monitoring applications. Sampling is controlled through a biological pulse sequence implemented in LABVIEW. Image reconstruction is based on a simultaneous algebraic reconstruction technique implemented on a graphic processor unit. The capabilities of 4D micro-CT imaging are demonstrated in studies on lung perfusion in rats.
Results: We report 4D micro-CT imaging in the rat lung with a heartbeat temporal resolution (approximately 150 ms) and isotropic 3D reconstruction with a voxel size of 88 μm based on sampling using 16 injections of 50 μL each. The total volume of contrast agent injected during the experiments (0.8 mL) was less than 10% of the total blood volume in a rat. This volume was not injected in a single bolus, but in multiple injections separated by at least 2 min interval to allow for clearance and adaptation. We assessed the reproducibility of the time density curves with multiple injections and found that these are very similar. The average time density curves for the first eight and last eight injections are slightly different, i.e., for the last eight injections, both the maximum of the average time density curves and its area under the curve are decreased by 3.8% and 7.2%, respectively, relative to the average time density curves based on the first eight injections. The radiation dose associated with our 4D micro-CT imaging is 0.16 Gy and is therefore in the range of a typical micro-CT dose.
Conclusions: 4D micro-CT-based perfusion imaging demonstrated here has immediate application in a wide range of preclinical studies such as tumor perfusion, angiogenesis, and renal function. Although our imaging system is in many ways unique, we believe that our approach based on the multiple injection paradigm can be used with the newly developed flat-panel slip-ring-based micro-CT to increase their temporal resolution in dynamic perfusion studies.
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87.57.Q- Computed tomography
87.57.cf Spatial resolution
87.57.nf Reconstruction
87.19.rh Fluid transport and rheology
47.63.Jd Microcirculation and flow through tissues

RADIATION IMAGING PHYSICS: Automated estimation of progression of interstitial lung disease in CT images

Yulia Arzhaeva, Mathias Prokop, Keelin Murphy, Eva M. van Rikxoort, Pim A. de Jong, Hester A. Gietema, Max A. Viergever, and Bram van Ginneken

Med. Phys. 37, 63 (2010); http://dx.doi.org/10.1118/1.3264662 (11 pages) | Cited 4 times

Online Publication Date: 4 December 2009

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Purpose: A system is presented for automated estimation of progression of interstitial lung disease in serial thoracic CT scans.
Methods: The system compares corresponding 2D axial sections from baseline and follow-up scans and concludes whether this pair of sections represents regression, progression, or unchanged disease status. The correspondence between serial CT scans is achieved by intrapatient volumetric image registration. The system classification function is trained with two different feature sets. Features in the first set represent the intensity distribution of a difference image between the baseline and follow-up CT sections. Features in the second set represent dissimilarities computed between the baseline and follow-up images filtered with a bank of general purpose texture filters.
Results: In an experiment on 74 scan pairs, the system classification accuracies were 76.1% and 79.5% for the two feature sets, respectively, while the accuracies of two observer radiologist were 78.5% and 82%, respectively. The agreements of the system with the reference standard, measured by weighted kappa statistics, were 0.611 and 0.683 for the two feature sets, respectively.
Conclusions: The system employing the second feature set showed good agreement with the reference standard, and its accuracy approached that of two radiologists.
Show PACS
87.57.Q- Computed tomography
87.57.nj Registration
02.50.-r Probability theory, stochastic processes, and statistics
87.19.X- Diseases

RADIATION IMAGING PHYSICS: Accurate color measurement methods for medical displays

Anindita Saha, Edward F. Kelley, and Aldo Badano

Med. Phys. 37, 74 (2010); http://dx.doi.org/10.1118/1.3265879 (8 pages)

Online Publication Date: 4 December 2009

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Purpose: The necessity for standard instrumentation and measurements of color that are repeatable and reproducible is the major motivation behind this work. Currently, different instrumentation and methods can yield very different results when measuring the same feature such as color uniformity or color difference. As color increasingly comes into play in medical imaging diagnostics, display color will have to be quantified in order to assess whether the display should be used for imaging purposes. The authors report on the characterization of three novel probes for measuring display color with minimal contamination from screen areas outside the measurement spot or from off-normal emissions. They compare three probe designs: A modified small-spot luminance probe and two conic probe designs based on black frusta.
Methods: To compare the three color probe designs, spectral and luminance measurements were taken with specialized instrumentation to determine the luminance changes and color separation abilities of the probes. The probes were characterized with a scanning slit method, veiling glare, and a moving laser and LED arrangement. The scanning slit measurement was done using a black slit plate over a white line on an LCD monitor. The luminance was measured in 1 mm increments from the center of the slit to ±15 mm above and below the slit at different distances between the probe and the slit. The veiling glare setup consisted of measurements of the luminance of a black spot pattern with a white disk of radius of 100 mm as the black spot increases in 1 mm radius increments. The moving LED and laser method consisted of a red and green light orthogonal to the probe tip for the light to directly shine into the probe. The green light source was moved away from the red source in 1 cm increments to measure color stray-light contamination at different probe distances.
Results: The results of the color testing using the LED and laser methods suggest a better performance of one of the frusta probes at shorter distances between the light sources, which translates to less contamination. The tails of the scans indicate the magnitude of the spread in signal due to light from areas outside the intended measurement spot. The measurements indicate a corresponding glare factor for a large spot of 140, 500, and 2000 for probe A, B1, and B2, respectively. The dual-laser setup suggests that color purity can be maintained up to a few tens of millimeters outside the measurement spot.
Conclusions: The comparison shows that there are significant differences in the performance of each probe design, and that those differences have an effect on the measured quantity used to quantify display color. Different probe designs show different measurements of the level of light contamination that affects the quantitative color determination.
Show PACS
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
42.79.Kr Display devices, liquid-crystal devices
85.60.Pg Display systems
87.63.L- Visual imaging
07.60.Dq Photometers, radiometers, and colorimeters
42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy
42.72.-g Optical sources and standards
85.60.Jb Light-emitting devices
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