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Aug 2010

Volume 37, Issue 8, pp. 3915-4522

Spotlight Figure

Med. Phys. 37, 3918 (2010); http://dx.doi.org/10.1118/1.3455790 (1 page)

 
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POINT/COUNTERPOINT: Authorization to practice as a medical physicist is sometimes better achieved by registration rather than licensure

Douglas E. Pfeiffer, MS, Jeffrey P. Masten, MSJD, and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 3915 (2010); http://dx.doi.org/10.1118/1.3456440 (3 pages)

Online Publication Date: 12 July 2010

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01.78.+p Science and government (funding, politics, etc.)

OBITUARY: Gail Adams, Ph.D. 1918–2010

 

Med. Phys. 37, 3918 (2010); http://dx.doi.org/10.1118/1.3455790 (1 page)

Online Publication Date: 12 July 2010

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01.60.+q Biographies, tributes, personal notes, and obituaries
87.90.+y Other topics in biological and medical physics (restricted to new topics in section 87)

RADIATION MEASUREMENT PHYSICS: Dosimetric effects of an air cavity for the SAVI™ partial breast irradiation applicator

Susan L. Richardson and Ramiro Pino

Med. Phys. 37, 3919 (2010); http://dx.doi.org/10.1118/1.3457328 (8 pages)

Online Publication Date: 12 July 2010

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Purpose: To investigate the dosimetric effect of the air inside the SAVI™ partial breast irradiation device.
Methods: The authors have investigated how the air inside the SAVI™ partial breast irradiation device changes the delivered dose from the homogeneously calculated dose. Measurements were made with the device filled with air and water to allow comparison to a homogenous dose calculation done by the treatment planning system. Measurements were made with an ion chamber, TLDs, and film. Monte Carlo (MC) simulations of the experiment were done using the EGSnrc suite. The MC model was validated by comparing the water-filled calculations to those from a commercial treatment planning system.
Results: The magnitude of the dosimetric effect depends on the size of the cavity, the arrangement of sources, and the relative dwell times. For a simple case using only the central catheter of the largest device, MC results indicate that the dose at the prescription point 1 cm away from the air-water boundary is about 9% higher than the homogeneous calculation. Independent measurements in a water phantom with a similar air cavity gave comparable results. MC simulation of a realistic multidwell position plan showed discrepancies of about 5% on average at the prescription point for the largest device.
Conclusions: The dosimetric effect of the air cavity is in the range of 3%–9%. Unless a heterogeneous dose calculation algorithm is used, users should be aware of the possibility of small treatment planning dose errors for this device and make modifications to the treatment delivery, if necessary.
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87.53.Bn Dosimetry/exposure assessment
87.85.-d Biomedical engineering
87.56.-v Radiation therapy equipment
87.53.Jw Therapeutic applications, including brachytherapy
87.55.dk Dose-volume analysis

RADIATION THERAPY PHYSICS: Effects of breathing variation on gating window internal target volume in respiratory gated radiation therapy

Jing Cai, Robert McLawhorn, Paul W. Read, James M. Larner, Fang-fang Yin, Stanley H. Benedict, and Ke Sheng

Med. Phys. 37, 3927 (2010); http://dx.doi.org/10.1118/1.3457329 (8 pages) | Cited 4 times

Online Publication Date: 12 July 2010

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Purpose: To investigate the effects of breathing variation on gating window internal target volume (ITVGW) in respiratory gated radiation therapy.
Method and Materials: Two-dimensional dynamic MRI (dMRI) of lung motion was acquired in ten volunteers and eight lung cancer patients. Resorted dMRI using 4DCT acquisition method (RedCAM) was generated for selected subjects by simulating the image rebinning process. A dynamic software generated phantom (dSGP) was created by moving a solid circle (to mimic the “tumor”) with dMRI-determined motion trajectories. The gating window internal target area (ITAGW, 2D counterpart of ITVGW) was determined from both RedCAM and dSGP/dMRI. Its area (A), major axis (L1), minor axis (L2), and similarity (S) were calculated and compared.
Results: In the phantom study of 3 cm tumor, measurements of the ITAGW from dSGP (A = 10.0±1.3 cm2, L1 = 3.8±0.4 cm, and L2 = 3.3±0.1 cm) are significantly (p<0.001) greater than those from RedCAM (A = 8.5±0.7 cm2, L1 = 3.5±0.2 cm, and L2 = 3.1±0.1 cm). Similarly, the differences are significantly greater (p<0.001) for the 1 cm tumor (A = 1.9±0.5 cm2, L1 = 1.9±0.4 cm, and L2 = 1.3±0.1 cm in dSGP; A = 1.3±0.1 cm2, L1 = 1.5±0.2 cm, and L2 = 1.1±0.1 cm in RedCAM). In patient studies, measurements of the ITAGW from dMRI (A = 15.5±8.2 cm2, L1 = 5.0±1.1 cm, and L2 = 3.8±1.2 cm) are also significantly greater (p<0.05) than those from RedCAM (A = 13.2±8.5 cm2, L1 = 4.3±1.4 cm, and L2 = 3.7±1.2 cm). Similarities were 0.9±0.1, 0.8±0.1, and 0.8±0.1 in the 3 cm tumor phantom, 1 cm tumor phantom, and patient studies, respectively.
Conclusion: ITVGW can be underestimated by 4DCT due to breathing variations. An additional margin may be needed to account for this potential error in generating a PTVGW. Cautions need to be taken when generating ITVGW from 4DCT in respiratory gated radiation therapy, especially for small tumors (<3 cm) with a large motion range (>1 cm).
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87.53.Jw Therapeutic applications, including brachytherapy
87.57.N- Image analysis
87.57.Q- Computed tomography
87.57.nm Segmentation
87.61.-c Magnetic resonance imaging
87.19.xj Cancer
87.19.Wx Pneumodyamics, respiration

RADIATION THERAPY PHYSICS: Enhanced epidermal dose caused by localized electron contamination from lead cutouts used in kilovoltage radiotherapy

J. E. Lye, D. J. Butler, and D. V. Webb

Med. Phys. 37, 3935 (2010); http://dx.doi.org/10.1118/1.3458722 (5 pages) | Cited 2 times

Online Publication Date: 12 July 2010

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Purpose: To investigate and quantify electron contamination from the lead cutouts used in kilovoltage x-ray radiotherapy.
Methods: The lead cutouts were modeled with the Monte Carlo EGSnrc user codes DOSXYZnrc and DOSRZnrc for x-ray beams ranging from 50 to 300 kVp. The results from the model were confirmed with Gafchromic film measurements. The model and measurements investigated the dose distribution with and without gladwrap™ shielding under the lead, and dose distributions with round, square, and serrated edge cutouts.
Results: Large dose enhancement near the edges of the lead was observed due to electron contamination. At the epidermal/dermal border, there is double the dose at the edge of the lead compared to the central dose due to electron contamination for a 150 kVp beam and three times the dose for a 300 kVp beam. gladwrap™ shielding effectively removes the contaminant dose enhancement using ten and four layers for 300 and 150 kVp beams, respectively.
Conclusions: The contaminant dose enhancement is undesirable as it could cause unnecessary erythema and hyperpigmentation at the border of the treated and untreated skin and lead to a poorer cosmetic outcome. The contamination is easily removed by gladwrap™ shielding placed under or around the lead cutout.
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87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.53.Kn Conformal radiation treatment

TISSUE MEASUREMENTS: Multilevel analysis of spatiotemporal association features for differentiation of tumor enhancement patterns in breast DCE-MRI

Sang Ho Lee, Jong Hyo Kim, Nariya Cho, Jeong Seon Park, Zepa Yang, Yun Sub Jung, and Woo Kyung Moon

Med. Phys. 37, 3940 (2010); http://dx.doi.org/10.1118/1.3446799 (17 pages) | Cited 2 times

Online Publication Date: 12 July 2010

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Purpose: Analyzing spatiotemporal enhancement patterns is an important task for the differential diagnosis of breast tumors in dynamic contrast-enhanced MRI (DCE-MRI), and yet remains challenging because of complexities in analyzing the time-series of three-dimensional image data. The authors propose a novel approach to breast MRI computer-aided diagnosis (CAD) using a multilevel analysis of spatiotemporal association features for tumor enhancement patterns in DCE-MRI.
Methods: A database of 171 cases consisting of 111 malignant and 60 benign tumors was used. Time-series contrast-enhanced MR images were obtained from two different types of MR scanners and protocols. The images were first registered for motion compensation, and then tumor regions were segmented using a fuzzy c-means clustering-based method. Spatiotemporal associations of tumor enhancement patterns were analyzed at three levels: Mapping of pixelwise kinetic features within a tumor, extraction of spatial association features from kinetic feature maps, and extraction of kinetic association features at the spatial feature level. A total of 84 initial features were extracted. Predictable values of these features were evaluated with an area under the ROC curve, and were compared between the spatiotemporal association features and a subset of simple form features which do not reflect spatiotemporal association. Several optimized feature sets were identified among the spatiotemporal association feature group or among the simple feature group based on a feature ranking criterion using a support vector machine based recursive feature elimination algorithm. A least-squares support vector machine (LS-SVM) classifier was used for tumor differentiation and the performances were evaluated using a leave-one-out testing.
Results: Predictable values of the extracted single features ranged in 0.52–0.75. By applying multilevel analysis strategy, the spatiotemporal association features became more informative in predicting tumor malignancy, which was shown by a statistical testing in ten spatiotemporal association features. By using a LS-SVM classifier with the optimized second and third level feature set, the CAD scheme showed Az of 0.88 in classification of malignant and benign tumors. When this performance was compared to the same LS-SVM classifier with simple form features which do not reflect spatiotemporal association, there was a statistically significant difference (0.88 vs 0.79, p<0.05), suggesting that the multilevel analysis strategy yields a significant performance improvement.
Conclusions: The results suggest that the multilevel analysis strategy characterizes the complex tumor enhancement patterns effectively with the spatiotemporal association features, which in turn leads to an improved tumor differentiation. The proposed CAD scheme has a potential for improving diagnostic performance in breast DCE-MRI.
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87.61.-c Magnetic resonance imaging
87.57.R- Computer-aided diagnosis

RADIATION IMAGING PHYSICS: An analytical model of the effects of pulse pileup on the energy spectrum recorded by energy resolved photon counting x-ray detectors

Katsuyuki Taguchi, Eric C. Frey, Xiaolan Wang, Jan S. Iwanczyk, and William C. Barber

Med. Phys. 37, 3957 (2010); http://dx.doi.org/10.1118/1.3429056 (13 pages) | Cited 14 times

Online Publication Date: 12 July 2010

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Purpose: Recently, novel CdTe photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed. When such detectors are operated under a high x-ray flux, however, coincident pulses distort the recorded energy spectrum. These distortions are called pulse pileup effects. It is essential to compensate for these effects on the recorded energy spectrum in order to take full advantage of spectral information PCXDs provide. Such compensation can be achieved by incorporating a pileup model into the image reconstruction process for computed tomography, that is, as a part of the forward imaging process, and iteratively estimating either the imaged object or the line integrals using, e.g., a maximum likelihood approach. The aim of this study was to develop a new analytical pulse pileup model for both peak and tail pileup effects for nonparalyzable detectors.
Methods: The model takes into account the following factors: The bipolar shape of the pulse, the distribution function of time intervals between random events, and the input probability density function of photon energies. The authors used Monte Carlo simulations to evaluate the model.
Results: The recorded spectra estimated by the model were in an excellent agreement with those obtained by Monte Carlo simulations for various levels of pulse pileup effects. The coefficients of variation (i.e., the root mean square difference divided by the mean of measurements) were 5.3%–10.0% for deadtime losses of 1%–50% with a polychromatic incident x-ray spectrum.
Conclusions: The proposed pulse pileup model can predict recorded spectrum with relatively good accuracy.
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87.57.Q- Computed tomography
87.57.nf Reconstruction
87.10.Rt Monte Carlo simulations

MAGNETIC RESONANCE PHYSICS: The performance of steady-state harmonic magnetic resonance elastography when applied to viscoelastic materials

Marvin M. Doyley, Irina Perreard, Adam. J. Patterson, John B. Weaver, and Keith M. Paulsen

Med. Phys. 37, 3970 (2010); http://dx.doi.org/10.1118/1.3454738 (10 pages) | Cited 4 times

Online Publication Date: 12 July 2010

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Purpose: The clinical efficacy of breast elastography may be limited when the authors employ the assumption that soft tissues exhibit linear, frequency-independent isotropic mechanical properties during the recovery of shear modulus. Thus, the purpose of this research was to evaluate the degradation in performance incurred when linear-elastic MR reconstruction methods are applied to phantoms that are fabricated using viscoelastic materials.
Methods: To develop phantoms with frequency-dependent mechanical properties, the authors measured the complex modulus of two groups of cylindrical-shaped gelatin samples over a wide frequency range (up to 1 kHz) with the established principles of time-temperature superposition (TTS). In one group of samples, the authors added varying amounts of agar (1%–4%); in the other group, the authors added varying amounts of sucrose (2.5%–20%). To study how viscosity affected the performance of the linear-elastic reconstruction method, the authors constructed an elastically heterogeneous MR phantom to simulate the case where small viscoelastic lesions were surrounded by relatively nonviscous breast tissue. The breast phantom contained four linear, viscoelastic spherical inclusions (10 mm diameter) that were embedded in normal gelatin. The authors imaged the breast phantom with a clinical prototype of a MRE system and recovered the shear-modulus distribution using the overlapping-subzone-linear-elastic image-reconstruction method. The authors compared the recovered shear modulus to that measured using the TTS method.
Results: The authors demonstrated that viscoelastic phantoms could be fabricated by including sucrose in the gelation process and that small viscoelastic inclusions were visible in MR elastograms recovered using a linear-elastic MR reconstruction process; however, artifacts that degraded contrast and spatial resolution were more prominent in highly viscoelastic inclusions. The authors also established that the accuracy of the MR elastograms depended on the degree of viscosity that the inclusion exhibited.
Conclusions: The results demonstrated that reconstructing shear modulus from other constitutive laws, such as viscosity, should improve both the accuracy and quality of MR elastograms of the breast.
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87.57.rh Mammography
87.61.Tg Clinical applications
87.19.rd Elastic properties
87.57.nf Reconstruction

RADIATION IMAGING PHYSICS: Design and optimization of large area thin-film CdTe detector for radiation therapy imaging applications

E. Ishmael Parsai, Diana Shvydka, and Jun Kang

Med. Phys. 37, 3980 (2010); http://dx.doi.org/10.1118/1.3438082 (15 pages)

Online Publication Date: 13 July 2010

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Purpose: The authors investigate performance of thin-film cadmium telluride (CdTe) in detecting high-energy (6 MV) x rays. The utilization of this material has become technologically feasible only in recent years due to significant development in large area photovoltaic applications.
Methods: The CdTe film is combined with a metal plate, facilitating conversion of incoming photons into secondary electrons. The system modeling is based on the Monte Carlo simulations performed to determine the optimized CdTe layer thickness in combination with various converter materials.
Results: The authors establish a range of optimal parameters producing the highest DQE due to energy absorption, as well as signal and noise spatial spreading. The authors also analyze the influence of the patient scatter on image formation for a set of detector configurations. The results of absorbed energy simulation are used in device operation modeling to predict the detector output signal. Finally, the authors verify modeling results experimentally for the lowest considered device thickness.
Conclusions: The proposed CdTe-based large area thin-film detector has a potential of becoming an efficient low-cost electronic portal imaging device for radiation therapy applications.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
87.56.-v Radiation therapy equipment
87.85.J- Biomaterials

RADIATION IMAGING PHYSICS: Kinetic model-based factor analysis of dynamic sequences for 82-rubidium cardiac positron emission tomography

R. Klein, R. S. Beanlands, R. W. Wassenaar, S. L. Thorn, M. Lamoureux, J. N. DaSilva, A. Adler, and R. A. deKemp

Med. Phys. 37, 3995 (2010); http://dx.doi.org/10.1118/1.3438474 (16 pages)

Online Publication Date: 13 July 2010

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Purpose: Factor analysis has been pursued as a means to decompose dynamic cardiac PET images into different tissue types based on their unique temporal signatures to improve quantification of physiological function. In this work, the authors present a novel kinetic model-based (MB) method that includes physiological models of factor relationships within the decomposition process. The physiological accuracy of MB decomposed 82Rb cardiac PET images is evaluated using simulated and experimental data. Precision of myocardial blood flow (MBF) measurement is also evaluated.
Methods: A gamma-variate model was used to describe the transport of 82Rb in arterial blood from the right to left ventricle, and a one-compartment model to describe the exchange between blood and myocardium. Simulations of canine and rat heart imaging were performed to evaluate parameter estimation errors. Arterial blood sampling in rats and 11CO blood pool imaging in dogs were used to evaluate factor and structure accuracy. Variable infusion duration studies in canine were used to evaluate MB structure and global MBF reproducibility. All results were compared to a previously published minimal structure overlap (MSO) method.
Results: Canine heart simulations demonstrated that MB has lower root-mean-square error (RMSE) than MSO for both factor (0.2% vs 0.5%, p<0.001 MB vs MSO, respectively) and structure (3.0% vs 4.7%, p<0.001) estimations, as with rat heart simulations (factors: 0.2% vs 0.9%, p<0.001 and structures: 3.0% vs 6.7%, p<0.001). MB blood factors compared to arterial blood samples in rats had lower RMSE than MSO (1.6% vs 2.2%, p = 0.025). There was no difference in the RMSE of blood structures compared to a 11CO blood pool image in dogs (8.5% vs 8.8%, p = 0.23). Myocardial structures were more reproducible with MB than with MSO (RMSE = 3.9% vs 6.2%, p<0.001), as were blood structures (RMSE = 4.9% vs 5.6%, p = 0.006). Finally, MBF values tended to be more reproducible with MB compared to MSO (CV = 10% vs 18%, p = 0.16). The execution time of MB was, on average, 2.4 times shorter than MSO (p<0.001) due to fewer free parameters.
Conclusions: Kinetic model-based factor analysis can be used to provide physiologically accurate decomposition of 82Rb dynamic PET images, and may improve the precision of MBF quantification.
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87.57.uk Positron emission tomography (PET)
87.85.gf Fluid mechanics and rheology
47.63.Cb Blood flow in cardiovascular system
87.19.Hh Cardiac dynamics
87.19.rh Fluid transport and rheology
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