bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling Grain Boundary Scattering and Thermal Conductivity of Polysilicon Using an Effective Medium Approach
Authors: English Timothy S. , Smoyer Justin L. , Duda John C. , Norris Pamela M. , Beecham Thomas E. , Hopkins Patrick E.
Published: March 2011

This work develops a new model for calculating the thermal conductivity of polycrystalline silicon using an effective medium approach which discretizes the contribution to thermal conductivity into that of the grain and grain boundary regions. While the Boltzmann transport equation under the relaxation time approximation is used to model the grain thermal conductivity, a lower limit thermal conductivity model for disordered layers is applied in order to more accurately treat phonon scattering in the grain boundary regions, which simultaneously removes the need for fitting parameters frequently used in the traditional formation of grain boundary scattering times. The contributions of the grain and grain boundary regions are then combined using an effective medium approach to compute the total thermal conductivity. The model is compared to experimental data from literature for both undoped and doped polycrystalline silicon films. In both cases, the new model captures the correct temperature dependent trend and demonstrates good agreement with experimental thermal conductivity data from 20 to 300K.
Three-Dimensional Numerical Study on Freezing Phase Change Heat Transfer in Biological Tissue Embedded With Two Cryoprobes
Authors: Zhao Fang , Chen Zhenqian , Shi Mingheng
Published: March 2011

A mathematical model for phase change heat transfer in cryosurgery was established. In this model, a fractal tree-like branched network was used to describe the complicated geometrical frame of blood vessel. The temperature distribution and ice crystal growth process in biological tissue including normal tissue and tumor embedded with two cryoprobes were numerically simulated. The effects of cooling rate, initial temperature and distance of two cryoprobes on freezing process of tissue were also studied. The results show that the ice crystal grows more rapidly in the initial freezing stage and then slows down in the following process, and the pre-cooling of cryoprobes has no obvious effect on freezing rate of tissue. It also can be seen that the distance of 10 mm between two cryoprobes is the most appropriate choice for operation effect in the range of operating conditions presented in this study.
Effects of Surface Treatment on Frost Formation and Defrosting
Authors: Kim Kyoungmin , Lee Kwan-Soo
Published: March 2011

Frost formation and defrosting experiments were conducted to investigate the effect of surface treatment by using hydrophilic, bare and hydrophobic fins. The three surface treated fins have 2.5°, 75°, and 142° of static surface contact angles, respectively. Early stages of frosting, the frost surfaces showed differing frost formations and the onset of frosting varied according to surface treatment. Hydrophobic fin shows frost retardation, but the effect was insignificant. Moreover, the sizes of the frost crystals and water droplets were affected by the surface treatment. In the result of 120 min frosting experiments, hydrophilic fin had the lowest frost thickness and the highest frost density among the surface-treated fins. The effect of surface treatment on defrosting time was found to be insignificant.
Status and potential of atmospheric plasma processing of materials
Authors: Pappas Daphne
Published: March 2011

This paper is a review of the current status and potential of atmospheric plasma technology for materials processing. The main focus is the recent developments in the area of dielectric barrier discharges with emphasis in the functionalization of polymers, deposition of organic and inorganic coatings, and plasma processing of biomaterials. A brief overview of both the equipment being used and the physicochemical reactions occurring in the gas phase is also presented. Atmospheric plasma technology offers major industrial, economic, and environmental advantages over other conventional processing methods. At the same time there is also tremendous potential for future research and applications involving both the industrial and academic world.
Effects of nonlinear ultrasound propagation on high intensity brain therapy
Authors: Pinton Gianmarco , Aubry Jean‐Francois , Fink Mathias , Tanter Mickael
Published: March 2011

Purpose:

As an ultrasound wave propagates nonlinearly, energy is transferred to higher frequencies where it is more strongly attenuated. Compared to soft tissue, the skull has strongly heterogeneous material parameters. The authors characterize with experiments and establish a numerical method that can describe the effects of the skull on the nonlinear components of ultrasonic wave propagation for application to high intensity focused ultrasound (HIFU) therapy in the brain. The impact of nonlinear acoustic propagation on heat deposition and thermal dose delivery is quantified and compared to linear assumptions by coupling an acoustic simulation with a heating model for brain tissue.

Methods:

A degassed dessicated human skull was placed in a water tank and insonified at 1 MPa with 7 mm transducer from a custom array designed for HIFU treatment. Two dimensional scans were performed preceding and following propagation through the skull with a calibrated hydrophone. Data from the scan preceding the skull were used as an input to a three dimensional finite difference time domain (FDTD) simulation that calculates the effects of diffraction, density, attenuation with linear dependence on frequency via relaxation mechanisms, and second order nonlinearity. A measured representation of the skull was used to determine the skull's acoustic properties. The validated acoustic model was used to determine the loss due to nonlinear propagation and then coupled to a finite difference simulation of the bioheat equation for two focal configurations at 3 and 7.5 cm from the skull surface.

Results:

Prior to propagation through the skull, the second harmonic component was 19 dB lower than the fundamental, and the third harmonic component was 37 dB lower. Following the skull, the second harmonic component was 35 dB lower and the third harmonic was 55 dB lower. The simulation is in agreement with the measurements to within 0.5 dB across the considered frequency range and shows good agreement across the two dimensional scan. It is then shown that the volume of treated brain is at least twice as large when assuming nonlinear acoustics.

Conclusions:

The authors have established a three dimensional FDTD simulation that accurately models the effects of nonlinearity and attenuation for propagation through the skull. Experimental validation shows good agreement across a broad frequency range and spatial extent. The nonlinear thermal dose was over an order of magnitude larger at the focus than the linear thermal dose and the necrotic volume was larger by at least a factor of 2. These results have particular applications to treatment planning.
Fast, limited‐data photoacoustic imaging for multiplexed systems using a frequency‐domain estimation technique
Authors: Gamelin John K. , Aguirre Andres , Zhu Quing
Published: March 2011

Purpose:

A new frequency‐domain estimation algorithm has been developed that usesa priori information to simultaneously improve imaging quality and time resolution in photoacoustic tomography with incomplete data sets.

Methods:

The method involves application of a single‐stage Wiener optimal filter to augment data sets by interpolation between measurement locations using relationships determined in a reference scan. The filter can be applied in real‐time using FFT methods using either fixed or dynamic references and used with any imaging algorithm. The performance of the method is compared to a modified version of constrained backprojection algorithms using simulations and experimental investigations.

Results:

Simulations demonstrate the effectiveness of the approach for tracking dynamic photoacoustic activity for data sets with limited views (90°) or tomographic views with a reduced number of acquisition angles at any given time. Experimental data of contrast uptake and washout using a 512‐element curved transducer with 8:1 electronic multiplexing with the algorithm demonstrate full two‐dimensional tomographic imaging with a temporal resolution better than 130 ms.

Conclusions:

The estimation algorithm enables high spatial resolution, real‐time imaging of dynamic physiological events or volumetric regions for photoacoustic systems employing multiplexing or scanning.
CT effective dose per dose length product using ICRP 103 weighting factors
Authors: Huda Walter , Magill Dennise , He Wenjun
Published: March 2011

Purpose:

To generate effective dose per unit dose length product (E/DLP) conversion factors incorporating ICRP Publication 103 tissue weighting factors.

Methods:

Effective doses for CT examinations were obtained using theIMPACT Dosimetry Calculator using all 23 dose data sets that are offered by this spreadsheet. CT examinations were simulated for scans performed along the patient long axis for each dosimetry data set using a 4 cm beam width ranging from the upper thighs to top of the head. Five basic body regions (head, neck, chest, abdomen, and pelvis), as well as combinations of the regions (head/neck, chest/abdomen, abdomen/pelvis, and chest/abdomen/pelvis) and whole body CT scans were investigated. Correction factors were generated that can be applied to convert E/DLP conversion factors based on ICRP 60 data to conversion factors that are valid for ICRP 103 data (i.e., ).

Results:

Use of ICRP 103 weighting factors increase effective doses for head scans by, for chest scans by , and decrease effective doses for pelvis scans by . Current E/DLP conversion factors are estimated to be for head CT examinations and range between 14 and for body CT examinations.

Conclusions:

Factors that enable patient CT doses to be adjusted to account for ICRP 103 tissue weighting factors are provided, which result in E/DLP factors that were increased in head and chest CT, reduced in pelvis CT, and showed no marked change in neck and abdomen CT.
Four‐dimensional computed tomography pulmonary ventilation images vary with deformable image registration algorithms and metrics
Authors: Yamamoto Tokihiro , Kabus Sven , Klinder Tobias , von Berg Jens , Lorenz Cristian , Loo Billy W. , Keall Paul J.
Published: March 2011

Purpose:

A novel pulmonary ventilation imaging technique based on four‐dimensional (4D) CT has advantages over existing techniques and could be used for functional avoidance in radiotherapy. There are various deformable image registration (DIR) algorithms and two classes of ventilation metric that can be used for 4D‐CT ventilation imaging, each yielding different images. The purpose of this study was to quantify the variability of the 4D‐CT ventilation to DIR algorithms and metrics.

Methods:

4D‐CT ventilation images were created for 12 patients using different combinations of two DIR algorithms, volumetric and surface‐based , yielding two displacement vector fields (DVFs) per patient ( and ), and two metrics, Hounsfield unit (HU) change and Jacobian determinant of deformation , yielding four ventilation image sets (, , , and ). First, and were compared visually and quantitatively to the length of 3D displacement vector difference. Second, four ventilation images were compared based on voxel‐based Spearman's rank correlation coefficients and coefficients of variation as a measure of spatial heterogeneity. was chosen as the reference for the comparison.

Results:

The mean length of 3D vector difference between and was on average, which was smaller than the voxel dimension of the image set and the variations. Visually, the reference demonstrated similar regional distributions with ; the reference, however, was markedly different from and . The correlation coefficients of with , , and were , , and , respectively, indicating that the metric introduced larger variations in the ventilation images than the DIR algorithm. The spatial heterogeneities for , , , and were , , , and , respectively, also demonstrating that the metric introduced larger variations.

Conclusions:

4D‐CT pulmonary ventilation images vary widely with DIR algorithms and metrics. Careful physiologic validation to determine the appropriate DIR algorithm and metric is needed prior to its applications.
Accounting for the tongue‐and‐groove effect using a robust direct aperture optimization approach
Authors: Salari Ehsan , Men Chunhua , Romeijn H. Edwin
Published: March 2011

Purpose:

Traditionally, the tongue‐and‐groove effect due to the multileaf collimator architecture in intensity‐modulated radiation therapy (IMRT) has typically been deferred to the leaf sequencing stage. The authors propose a new direct aperture optimization method for IMRT treatment planning that explicitly incorporates dose calculation inaccuracies due to the tongue‐and‐groove effect into the treatment plan optimization stage.

Methods:

The authors avoid having to accurately estimate the dosimetric effects of the tongue‐and‐groove architecture by using lower and upper bounds on the dose distribution delivered to the patient. They then develop a model that yields a treatment plan that is robust with respect to the corresponding dose calculation inaccuracies.

Results:

Tests on a set of ten clinical head‐and‐neck cancer cases demonstrate the effectiveness of the new method in developing robust treatment plans with tight dose distributions in targets and critical structures. This is contrasted with the very loose bounds on the dose distribution that are obtained by solving a traditional treatment plan optimization model that ignores tongue‐and‐groove effects in the treatment planning stage.

Conclusions:

A robust direct aperture optimization approach is proposed to account for the dosimetric inaccuracies caused by the tongue‐and‐groove effect. The experiments validate the ability of the proposed approach in designing robust treatment plans regardless of the exact consequences of the tongue‐and‐groove architecture.
The difference of scoring dose to water or tissues in Monte Carlo dose calculations for low energy brachytherapy photon sources
Authors: Landry Guillaume , Reniers Brigitte , Pignol Jean‐Philippe , Beaulieu Luc , Verhaegen Frank
Published: March 2011

Purpose:

The goal of this work is to compare (radiation transported in medium; dose scored in medium) and (radiation transported in medium; dose scored in water) obtained from Monte Carlo (MC) simulations for a subset of human tissues of interest in low energy photon brachytherapy. Using low dose rate seeds and an electronic brachytherapy source (EBS), the authors quantify the large cavity theory conversion factors required. The authors also assess whether applying large cavity theory utilizing the sources' initial photon spectra and average photon energy induces errors related to spatial spectral variations. First, ideal spherical geometries were investigated, followed by clinical brachytherapy LDR seed implants for breast and prostate cancer patients.

Methods:

Two types of dose calculations are performed with theGEANT4 MC code. (1) For several human tissues, dose profiles are obtained in spherical geometries centered on four types of low energy brachytherapy sources: , , and seeds, as well as an EBS operating at 50 kV. Ratios of over are evaluated in the 0–6 cm range. In addition to mean tissue composition, compositions corresponding to one standard deviation from the mean are also studied. (2) Four clinical breast (using ) and prostate (using ) brachytherapy seed implants are considered. MC dose calculations are performed based on postimplant CT scans using prostate and breast tissue compositions. PTV values are compared for and .

Results:

(1) Differences of −3% to 70% are observed for the investigated tissues. For a given tissue, is similar for all sources within 4% and does not vary more than 2% with distance due to very moderate spectral shifts. Variations of tissue composition about the assumed mean composition influence the conversion factors up to 38%. (2) The ratio of over for clinical implants matches at 1 cm from the single point sources.

Conclusions:

Given the small variation with distance, using conversion factors based on the emitted photon spectrum (or its mean energy) of a given source introduces minimal error. The large differences observed between scoring schemes underline the need for guidelines on choice of media for dose reporting. Providing such guidelines is beyond the scope of this work.
Interior region‐of‐interest reconstruction using a small, nearly piecewise constant subregion
Authors: Taguchi Katsuyuki , Xu Jingyan , Srivastava Somesh , Tsui Benjamin M. W. , Cammin Jochen , Tang Qiulin
Published: March 2011

Purpose:

To develop a method to reconstruct an interior region‐of‐interest (ROI) image with sufficient accuracy that uses differentiated backprojection (DBP) projection onto convex sets (POCS) [H. Kudo et al., “Tiny a priori knowledge solves the interior problem in computed tomography,” Phys. Med. Biol., – (2008)] and a tiny knowledge that there exists a nearly piecewise constant subregion.

Methods:

The proposed method first employs filtered backprojection to reconstruct an image on which a tiny region with a small variation in the pixel values is identified inside the ROI. Total variation minimization [H. Yu and G. Wang, “Compressed sensing based interior tomography,” Phys. Med. Biol., – (2009); W. Han et al., “A general total variation minimization theorem for compressed sensing based interior tomography,” Int. J. Biomed. Imaging, Article (2009)] is then employed to obtain pixel values in the subregion , which serve as a priori knowledge in the next step. Finally, DBP‐POCS is performed to reconstruct inside the ROI. Clinical data and the reconstructed image obtained by an x‐ray computed tomography system (SOMATOM Definition; Siemens Healthcare) were used to validate the proposed method. The detector covers an object with a diameter of . The projection data were truncated either moderately to limit the detector coverage to of the object or severely to cover . Images were reconstructed using the proposed method.

Results:

The proposed method provided ROI images with correct pixel values in all areas except near the edge of the ROI. The coefficient of variation, i.e., the root mean square error divided by the mean pixel values, was less than 2.0% or 4.5% with the moderate or severe truncation cases, respectively, except near the boundary of the ROI.

Conclusions:

The proposed method allows for reconstructing interior ROI images with sufficient accuracy with a tiny knowledge that there exists a nearly piecewise constant subregion.
Experience of micromultileaf collimator linear accelerator based single fraction stereotactic radiosurgery: Tumor dose inhomogeneity, conformity, and dose fall off
Authors: Hong Linda X. , Garg Madhur , Lasala Patrick , Kim Mimi , Mah Dennis , Chen Chin‐Cheng , Yaparpalvi Ravindra , Mynampati Dinesh , Kuo Hsiang‐Chi , Guha Chandan , Kalnicki Shalom
Published: March 2011

Purpose:

Sharp dose fall off outside a tumor is essential for high dose single fraction stereotactic radiosurgery (SRS) plans. This study explores the relationship among tumor dose inhomogeneity, conformity, and dose fall off in normal tissues for micromultileaf collimator (mMLC) linear accelerator (LINAC) based cranial SRS plans.

Methods:

Between January 2007 and July 2009, 65 patients with single cranial lesions were treated with LINAC‐based SRS. Among them, tumors had maximum diameters: 31; between 20 and 30 mm: 21; and : 13. All patients were treated with 6 MV photons on a Trilogy® linear accelerator (Varian Medical Systems, Palo Alto, CA) with a tertiary m3® high‐resolution mMLC (Brainlab, Feldkirchen, Germany), using either noncoplanar conformal fixed fields or dynamic conformal arcs. The authors also created retrospective study plans with identical beam arrangement as the treated plan but with different tumor dose inhomogeneity by varying the beam margins around the planning target volume (PTV). All retrospective study plans were normalized so that the minimum PTV dose was the prescription dose (PD). Isocenter dose, mean PTV dose, RTOG conformity index (CI), RTOG homogeneity index (HI), dose gradient index (defined as the difference between equivalent sphere radius of 50% isodose volume and prescription isodose volume), and normal tissue volume (as a ratio to PTV volume) receiving 50% prescription dose were calculated.

Results:

HI was inversely related to the beam margins around the PTV. CI had a “V” shaped relationship with HI, reaching a minimum when HI was approximately 1.3. Isocenter dose and mean PTV dose (as percentage of PD) increased linearly with HI. and initially declined with HI and then reached a plateau when HI was approximately 1.3. These trends also held when tumors were grouped according to their maximum diameters. The smallest tumor group (maximum diameters ) had the most HI dependence for dose fall off. For treated plans, CI averaged with HI ; the average was , , and , respectively, for tumors , between 20 and 30 mm, and .

Conclusions:

Tumor dose inhomogeneity can be used as an important and convenient parameter to evaluate mMLC LINAC‐based SRS plans. Sharp dose fall off in the normal tissue is achieved with sufficiently high tumor dose inhomogeneity. By adjusting beam margins, a homogeneity index of approximately 1.3 would provide best conformity for the authors' SRS system.
Low‐dose cardio‐respiratory phase‐correlated cone‐beam micro‐CT of small animals
Authors: Sawall Stefan , Bergner Frank , Lapp Robert , Mronz Markus , Karolczak Marek , Hess Andreas , Kachelrieß Marc
Published: March 2011

Purpose:

Micro‐CT imaging of animal hearts typically requires a double gating procedure because scans during a breath‐hold are not possible due to the long scan times and the high respiratory rates. Simultaneous respiratory and cardiac gating can either be done prospectively or retrospectively. True five‐dimensional information can be either retrieved with retrospective gating or with prospective gating if several prospective gates are acquired. In any case, the amount of information available to reconstruct one volume for a given respiratory and cardiac phase is orders of magnitude lower than the total amount of information acquired. For example, the reconstruction of a volume from a 10% wide respiratory and a 20% wide cardiac window uses only 2% of the data acquired. Achieving a similar image quality as a nongated scan would therefore require to increase the amount of data and thereby the dose to the animal by up to a factor of 50.

Methods:

To achieve the goal of low‐dose phase‐correlated (LDPC) imaging, the authors propose to use a highly efficient combination of slightly modified existing algorithms. In particular, the authors developed a variant of the McKinnon–Bates image reconstruction algorithm and combined it with bilateral filtering in up to five dimensions to significantly reduce image noise without impairing spatial or temporal resolution.

Results:

The preliminary results indicate that the proposed LDPC reconstruction method typically reduces image noise by a factor of up to 6 (e.g., from 170 to 30 HU), while the dose values lie in a range from 60 to 500 mGy. Compared to other publications that apply 250–1800 mGy for the same task [C. T. Badea et al., “4D micro‐CT of the mouse heart,” Mol. Imaging (), – (2005); M. Drangova et al., “Fast retrospectively gated quantitative four‐dimensional (4D) cardiac micro computed tomography imaging of free‐breathing mice,” Invest. Radiol. (), – (2007); S. H. Bartling et al., “Retrospective motion gating in small animal CT of mice and rats,” Invest. Radiol. (), – (2007)], the authors' LDPC approach therefore achieves a more than tenfold dose usage improvement.

Conclusions:

The LDPC reconstruction method improves phase‐correlated imaging from highly undersampled data. Artifacts caused by sparse angular sampling are removed and the image noise is decreased, while spatial and temporal resolution are preserved. Thus, the administered dose per animal can be decreased allowing for long term studies with reduced metabolic inference.
Treatment planning of a skin‐sparing conical breast brachytherapy applicator using conventional brachytherapy software
Authors: Yang Yun , Melhus Christopher S. , Sioshansi Shirin , Rivard Mark J.
Published: March 2011

Purpose:

AccuBoost is a noninvasive image‐guided technique for the delivery of partial breast irradiation to the tumor bed and currently serves as an alternate to conventional electron beam boost. To irradiate the target volume while providing dose sparing to the skin, the round applicator design was augmented through the addition of an internally truncated conical shield and the reduction of the source to skin distance.

Methods:

Brachytherapy dose distributions for two types of conical applicators were simulated and estimated using Monte Carlo (MC) methods for radiation transport and a conventional treatment planning system (TPS). MC‐derived and TPS‐generated dose volume histograms (DVHs) and dose distribution data were compared for both the conical and round applicators for benchmarking purposes.

Results:

Agreement using the gamma‐index test was ⩾99.95% for distance to agreement and dose accuracy criteria of 2 mm and 2%, respectively. After observing good agreement, TPS DVHs and dose distributions for the conical and round applicators were obtained and compared. Brachytherapy dose distributions generated using Pinnacle3 for ten CT data sets showed that the parallel‐opposed beams of the conical applicators provided similar PTV coverage to the round applicators and reduced the maximum dose to skin, chest wall, and lung by up to 27%, 42%, and 43%, respectively.

Conclusions:

Brachytherapy dose distributions for the conical applicators have been generated using MC methods and entered into the Pinnacle3 TPS via the Tufts technique. Treatment planning metrics for the conical AccuBoost applicators were significantly improved in comparison to those for conventional electron beam breast boost.
Tolerance levels of EPID‐based quality control for volumetric modulated arc therapy
Authors: Jørgensen M. K. , Hoffmann L. , Petersen J. B. B. , Præstegaard L. H. , Hansen R. , Muren L. P.
Published: March 2011

Purpose:

Volumetric modulated arc therapy (VMAT) includes features such as a variable dose rate and gantry speed in addition to the beam modulation achieved with multileaf collimator (MLC) motion patterns employed in intensity modulated radiotherapy. Three tests have previously been proposed for the evaluation of the performance of VMAT delivery. In order to enable a convenient and accurate routine machine quality control (QC) program, the present study proposes tolerance levels for these tests based on a department‐wide implementation of an electronic portal imaging device (EPID)‐based QC.

Methods:

Three different VMAT tests—a picket fence (PF) test, a dose rate versus gantry speed (DRGS) test, and a dose rate versus MLC leaf speed (DRMLC) test—were performed on nine accelerators using two different EPIDs (aS1000 and aS500, Varian Medical Systems). All tests were repeated six times for each accelerator. The images were analyzed using an in‐house‐developed software. For the PF test, the positions and widths of individual MLC leaf gaps were compared to the mean value. In the DRGS and DRMLC tests, different combinations of dose rate, gantry speed, and MLC leaf speed were used to deliver identical doses to separate parts of the EPID. The tests were evaluated by looking for deviations in the constancy of the measured dose for the preset combinations of dose rate, gantry speed, and MLC leaf speed.

Results:

For the PF test, a 0.3 mm tolerance level was suggested for the positioning of the MLC leaves. The tolerance level for the gap width was 0.5 mm. For the DRGS and DRMLC tests, a 3% tolerance level was proposed.

Conclusions:

With the adapted levels of tolerance for an EPID‐based approach, the PF, the DRGS, and the DRMLC tests offer a convenient and accurate machine QC program for linear accelerators used for VMAT.
Open design eight‐channel transmit/receive coil for high‐resolution and real‐time ankle imaging at 7 T
Authors: Orzada Stephan , Bitz Andreas K. , Schäfer Lena C. , Ladd Susanne C. , Ladd Mark E. , Maderwald Stefan
Published: March 2011

Purpose:

At 1.5 T, real‐time MRI of joint movement has been shown to be feasible. However, 7 T, provides higher SNR and thus an improved potential for parallel imaging acceleration. The purpose of this work was to build an open, U‐shaped eight‐channel transmit/receive microstrip coil for 7 T MRI to enable high‐resolution and real‐time imaging of the moving ankle joint.

Methods:

A U‐shaped eight‐channel transmit/receive array for the human ankle was built.‐parameters and ‐factor were measured. SAR calculations of different ankle postures were performed to ensure patient safety. Inhomogeneities in the transmit field consequent to the open design were compensated for by the use of static RF shimming. High‐resolution and real‐time imaging was performed in human volunteers.

Results:

The presented array showed good performance with regard to patient comfort and image quality. High acceleration factors of up to 4 are feasible without visible acceleration artifacts. Reasonable image homogeneity was achieved with RF shimming.

Conclusions:

Open, noncylindrical designs for transmit/receive coils are practical at 7 T and real‐time imaging of the moving joint is feasible with the presented coil design.
Dosimetric verification of surface and superficial doses for head and neck IMRT with different PTV shrinkage margins
Authors: Shiau An‐Cheng , Lai Pei‐Ling , Liang Ji‐An , Shueng Pei‐Wei , Chen Wei‐Li , Kuan Wei‐Peng
Published: March 2011

Purpose:

Dosimetric uncertainty in the surface and superficial regions is still a major concern for radiation therapy and becomes more important when using the inverse planning algorithm for IMRT. The purpose of this study was to measure dose distributions and to evaluate the calculation accuracy in the superficial region for different planning target volume (PTV) shrinkage methods for head and neck IMRT plans.

Methods:

A spherical polystyrene phantom 160 mm in diameter (ball phantom) was used to simulate the shape of the head. Strips of superflab bolus with thicknesses of 3.5 and 7.0 mm were spread on the surface of the ball phantom. Three sets of CT images were acquired for the ball phantom without and with the bolus. The hypothetical clinical target volume (CTV) and critical structures (spinal cord and parotid glands) were outlined on each set of CT images. The PTVs were initially created by expanding an isotropic 3 mm margin from the CTV and then margins of 0, 3, and 5 mm were shrunk from the phantom surface for dosimetric analysis. Seven‐field IMRT plans with a prescribed dose of 180 cGy and same dose constraints were designed using an Eclipse treatment planning system. Superficial doses at depths of 0, 3.5, and 7.0 mm and at seven beam axis positions (gantry angles of 0°, 30°, 60°, 80°, 330°, 300°, and 280°) were measured for each PTV shrinkage margin using 0.1 mm ultrathin thermoluminescent dosimeters. For each plan, the measured doses were compared to the calculated doses.

Results:

The PTV without shrinkage had the highest intensity and the steepest dose gradient in the superficial region. The mean measured doses for different positions at depths of 0, 3.5, and 7.0 mm were, , and , respectively. For a PTV with 3 mm shrinkage, the mean measured doses were , , and . For a PTV with 5 mm shrinkage, the mean measured doses were , , and . The comparisons indicated that more than 73.3% of the calculated points are with doses lower than the measured points and the difference of the dose becomes more significant in the shallower region. At 7.0 mm depth, the average difference between calculations and measurements was 2.5% (maximum 5.5%).

Conclusions:

Application of the PTV shrinkage method should take into account the calculation inaccuracy, tumor coverage, and possible skin reaction. When the tumor does not invade the superficial region, an adequate shrinkage margin from the surface is helpful for reducing the skin reaction. As the tumor invades the superficial region, adding a bolus is a method better than only contouring PTV with skin inclusion.
Beam hardening effects in grating‐based x‐ray phase‐contrast imaging
Authors: Chabior Michael , Donath Tilman , David Christian , Bunk Oliver , Schuster Manfred , Schroer Christian , Pfeiffer Franz
Published: March 2011

Purpose:

In this work, the authors investigate how beam hardening affects the image formation in x‐ray phase‐contrast imaging and consecutively develop a correction algorithm based on the results of the analysis.

Methods:

The authors' approach utilizes a recently developed x‐ray imaging technique using a grating interferometer capable of visualizing the differential phase shift of a wave front traversing an object. An analytical description of beam hardening is given, highlighting differences between attenuation and phase‐contrast imaging. The authors present exemplary beam hardening artifacts for a number of well‐defined samples in measurements at a compact laboratory setup using a polychromatic source.

Results:

Despite the differences in image formation, the authors show that beam hardening leads to a similar reduction of image quality in phase‐contrast imaging as in conventional attenuation‐contrast imaging. Additionally, the authors demonstrate that for homogeneous objects, beam hardening artifacts can be corrected by a linearization technique, applicable to all kinds of phase‐contrast methods using polychromatic sources.

Conclusions:

The evaluated correction algorithm is shown to yield good results for a number of simple test objects and can thus be advocated in medical imaging and nondestructive testing.
Study of the response of plastic scintillation detectors in small‐field 6 MV photon beams by Monte Carlo simulations
Authors: Wang Lilie L. W. , Beddar Sam
Published: March 2011

Purpose:

To investigate the response of plastic scintillation detectors (PSDs) in a 6 MV photon beam of various field sizes using Monte Carlo simulations.

Methods:

Three PSDs were simulated: A BC‐400 and a BCF‐12, each attached to a plastic‐core optical fiber, and a BC‐400 attached to an air‐core optical fiber. PSD response was calculated as the detector dose per unit water dose for field sizes ranging from down to for both perpendicular and parallel orientations of the detectors to an incident beam. Similar calculations were performed for a CC01 compact chamber. The off‐axis dose profiles were calculated in the photon beam and were compared to the dose profile calculated for the CC01 chamber and that calculated in water without any detector. The angular dependence of the PSDs' responses in a small photon beam was studied.

Results:

In the perpendicular orientation, the response of the BCF‐12 PSD varied by only 0.5% as the field size decreased from to , while the response of BC‐400 PSD attached to a plastic‐core fiber varied by more than 3% at the smallest field size because of its longer sensitive region. In the parallel orientation, the response of both PSDs attached to a plastic‐core fiber varied by less than 0.4% for the same range of field sizes. For the PSD attached to an air‐core fiber, the response varied, at most, by 2% for both orientations.

Conclusions:

The responses of all the PSDs investigated in this work can have a variation of only 1%–2% irrespective of field size and orientation of the detector if the length of the sensitive region is not more than 2 mm long and the optical fiber stems are prevented from pointing directly to the incident source.
Full‐field 3D photoacoustic imaging based on plane transducer array and spatial phase‐controlled algorithm
Authors: Zhou Qiuzhen , Ji Xuanrong , Xing Da
Published: March 2011

Purpose:

Photoacoustic imaging (PAI) used for noninvasive imaging of biological tissue has been reported in many literature. However, there are still some disadvantages in the novel technique, such as the poor efficiency of imaging. In the current PAI, multiple excitation of laser and multiple acquisition of signal are necessary for image reconstruction. In this case, laser pulses may injure biological tissue due to energy accumulation. To popularize PAI in clinical applications, it is necessary to develop a new imaging approach to increase the efficiency of PAI.

Methods:

A spatial phase‐controlled algorithm is presented for full‐field three‐dimensional (3D) image reconstruction. By using the algorithm, photoabsorption sources at different depths can be reconstructed using just one set of data acquired in single laser shot. Unfocused plane transducer array and parallel data‐acquisition (PDA) equipment are used for real‐time photoacoustic (PA) signal detection and acquisition.

Results:

The spatial resolution of the 3D PAI system was analyzed. Two graphite rods at various positions in a simulation model and two bifurcate vessels in the ear of rabbit were imaged. In addition, the motion trace of one particle flowing at constant velocity was captured dynamically. Experimental results showed that spatial phase‐controlled algorithm based on plane transducer array and PDA system was capable of static and dynamic 3D PAI.

Conclusions:

Spatial phase‐controlled algorithm is introduced for 3D image reconstruction. The PA signals are collected by plane transducer array and PDA system in single pulse excitation. The acquired volumetric data are sufficient for 3D image reconstruction. Therefore, tissue can avoid the long‐term exposure to light source and it is safer than the current PAI forin vivo imaging. With an increase in the repetition rate of laser pulse and speed of image display, the imaging method will realize real‐time 3D imaging, which will be significant in clinical detection and medical diagnosis.
A hybrid approach for rapid, accurate, and direct kilovoltage radiation dose calculations in CT voxel space
Authors: Kouznetsov Alexei , Tambasco Mauro
Published: March 2011

Purpose:

To develop and validate a fast and accurate method that uses computed tomography (CT) voxel data to estimate absorbed radiation dose at a point of interest (POI) or series of POIs from a kilovoltage (kV) imaging procedure.

Methods:

The authors developed an approach that computes absorbed radiation dose at a POI by numerically evaluating the linear Boltzmann transport equation (LBTE) using a combination of deterministic and Monte Carlo (MC) techniques. This hybrid approach accounts for material heterogeneity with a level of accuracy comparable to the general MC algorithms. Also, the dose at a POI is computed within seconds using the Intel® Core™ i7 CPU 920 2.67 GHz quad core architecture, and the calculations are performed using CT voxel data, making it flexible and feasible for clinical applications. To validate the method, the authors constructed and acquired a CT scan of a heterogeneous block phantom consisting of a succession of slab densities: Tissue (1.29 cm), bone (2.42 cm), lung (4.84 cm), bone (1.37 cm), and tissue (4.84 cm). Using the hybrid transport method, the authors computed the absorbed doses at a set of points along the central axis and direction of the phantom for an isotropic 125 kVp photon spectral point source located along the central axis 92.7 cm above the phantom surface. The accuracy of the results was compared to those computed with MCNP, which was cross‐validated with EGSnrc, and served as the benchmark for validation.

Results:

The error in the depth dose ranged from −1.45% to with a mean and standard deviation of −0.12% and 0.66%, respectively. The error in the profile ranged from −1.3% to , with standard deviations of −0.3% and 0.5%, respectively. The number of photons required to achieve these results was .

Conclusions:

The voxel‐based hybrid method evaluates the LBTE rapidly and accurately to estimate the absorbed x‐ray dose at any POI or series of POIs from a kV imaging procedure.
Confidence limit variation for a single IMRT system following the TG119 protocol
Authors: Gordon J. D. , Krafft S. P. , Jang S. , Smith‐Raymond L. , Stevie M. Y. , Hamilton R. J.
Published: March 2011

Purpose:

To evaluate the robustness of TG119‐based quality assurance metrics for an IMRT system.

Methods:

Four planners constructed treatment plans for the five IMRT test cases described in TG119. All plans were delivered to a solid water phantom in one treatment session in order to minimize session‐dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized film and an ionization chamber. Per‐field measurements were collected using a diode array device at an effective depth of 5 cm. All data collected were analyzed using the TG119 specifications to determine the confidence limit values for each planner separately and then compared.

Results:

The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. The resulting confidence limits were 3.13%, 1.98%, 3.65%, and 4.39%. Confidence limit values determined by composite film analysis were 8.06%, 13.4%, 9.30%, and 16.5%. Confidence limits from per‐field measurements were 1.55%, 0.00%, 0.00%, and 2.89%.

Conclusions:

For a single IMRT system, the accuracy assessment provided by TG119‐based quality assurance metrics showed significant variations in the confidence limits between planners across all composite and per‐field evaluations. This observed variation is likely due to the different levels of modulation between each planner's set of plans. Performing the TG119 evaluation using plans produced by a single planner may not provide an adequate estimation of IMRT system accuracy.
A GMM‐based breast cancer risk stratification using a resonance‐frequency electrical impedance spectroscopy
Authors: Lederman Dror , Zheng Bin , Wang Xingwei , Sumkin Jules H. , Gur David
Published: March 2011

Purpose:

The authors developed and tested a multiprobe‐based resonance‐frequency‐based electrical impedance spectroscopy (REIS) system. The purpose of this study was to preliminarily assess the performance of this system in classifying younger women into two groups, those ultimately recommended for biopsy during imaging‐based diagnostic workups that followed screening and those rated as negative during mammography.

Methods:

A seven probe‐based REIS system was designed, assembled, and is currently being tested in the breast imaging facility. During an examination, contact is made with the nipple and six concentric points on the breast skin. For each measurement channel between the center probe and one of the six external probes, a set of electrical impedance spectroscopy (EIS) signal sweeps is performed and signal outputs ranging from 200 to 800 kHz at 5 kHz interval are recorded. An initial subset of 174 examinations from an ongoing prospective clinical study was selected for this preliminary analysis. An initial set of 35 features, 33 of which represented the corresponding EIS signal differences between the left and right breasts, was established. A Gaussian mixture model (GMM) classifier was developed to differentiate between “positive” (biopsy recommended) cases and “negative” (nonbiopsy) cases. Selecting an optimal feature set was performed using genetic algorithms with an area under a receiver operating characteristic curve (AUC) as the fitness criterion.

Results:

The recorded EIS signal sweeps showed that, in general, negative (nonbiopsy) examinations have a higher level of electrical impedance symmetry between the two breasts than positive (biopsy) examinations. Fourteen features were selected by genetic algorithm and used in the optimized GMM classifier. Using a leave‐one‐case‐out test, the GMM classifier yielded a performance level of, which compared favorably to other three widely used classifiers including support vector machine, classification tree, and linear discriminant analysis. These results also suggest that the REIS signal based GMM classifier could be used as a prescreening tool to correctly identify a fraction of younger women at higher risk of developing breast cancer (i.e., 47% sensitivity at 90% specificity).

Conclusions:

The study confirms that asymmetry in electrical impedance characteristics between two breasts provides valuable information regarding the presence of a developing breast abnormality; hence, REIS data may be useful in classifying younger women into two groups of “average” and “significantly higher than average” risk of having or developing a breast abnormality that would ultimately result in a later imaging‐based recommendation for biopsy.
The influence of the field setup on the dosimetry of abutted fields in single‐isocenter half‐beam techniques
Authors: Hernandez V. , Sempau J.
Published: March 2011

Purpose:

To study the influence of the field setup on the dosimetry at the junction in single‐isocenter half‐beam techniques.

Methods:

The dosimetry at the junction for a two‐field setup with the gantry at zero was first evaluated with radiochromic films. A three‐field setup, with an anterior field and two opposed lateral fields, was also analyzed for two different relative positions of the fields involved. In all cases, the dose increase at the central axis, called the junction dose, was measured.

Results:

Junction doses varied greatly with the setup. For the three‐field setup, the junction dose differed from that obtained with the two‐field setup, and it greatly depended on the relative position of the fields. When the anterior field was closer to the gantry than the lateral fields, a field gap occurred and the junction dose was negative. When the anterior field was farther from the gantry than the lateral fields, a field overlap was obtained and the junction dose was positive. The difference in the junction dose between the three‐field setups was around 18% for the three accelerators evaluated.

Conclusions:

Having a uniform dose distribution for two fields at gantry 0° does not guarantee a uniform distribution at other gantry angles. Junction doses are largely affected by the relative position of the radiation fields, which may have an impact in clinical practice. Therefore, any method aiming to assess or to optimize the dose homogeneity at the junction should take this effect into account.
Dutch and English listeners’ interpretation of vowel duration
Authors: van der Feest Suzanne V. H. , Swingley Daniel
Published: March 2011

Dutch and English listeners’ interpretation of vowel duration changes was examined in a word transcription task. Listeners were presented with spoken words realized with canonical or altered vowel durations. Dutch listeners often misperceived lengthened short vowels and shortened long vowels, identifying them as the short/long counterpart of the target, whereas English listeners more rarely misidentified words with altered vowel duration. Although Dutch and English are similar prosodically and phonologically, listeners’ treatment of vowel duration in clear speech is different across the two languages.
Impact of wall rotation on supraglottal jet stability in voiced speech
Authors: D. Erath Byron , W. Plesniak Michael
Published: March 2011

Supraglottal jet variability was investigated in a scaled-up flow facility incorporating driven vocal fold models with and without wall rotation. Principle component analysis was performed on the experimental supraglottal flow fields to ascertain the roll of glottal wall motion on the development of the supraglottal jet. It is shown that intraglottal flow asymmetries that develop due to wall rotation are not the primary mechanism for generating large-scale cycle-to-cycle deflection of the supraglottal jet. However, wall rotation does decrease the energy content of the first mode, redistributing it to the higher modes through an increase in unstructured flow variability.