bluuebunny/crossref_metadata_2025 huggingface dataset:
A silicon strip detector dose magnifying glass for IMRT dosimetry
Authors: Wong J. H. D. , Carolan M. , Lerch M. L. F. , Petasecca M. , Khanna S. , Perevertaylo V. L. , Metcalfe P. , Rosenfeld A. B.
Published: January 2010

Purpose:

Intensity modulated radiation therapy (IMRT) allows the delivery of escalated radiation dose to tumor while sparing adjacent critical organs. In doing so, IMRT plans tend to incorporate steep dose gradients at interfaces between the target and the organs at risk. Current quality assurance (QA) verification tools such as 2D diode arrays, are limited by their spatial resolution and conventional films are nonreal time. In this article, the authors describe a novel silicon strip detector (CMRP DMG) of high spatial resolution suitable for measuring the high dose gradients in an IMRT delivery.

Methods:

A full characterization of the detector was performed, including dose per pulse effect, percent depth dose comparison with Farmer ion chamber measurements, stem effect, dose linearity, uniformity, energy response, angular response, and penumbra measurements. They also present the application of the CMRP DMG in the dosimetric verification of a clinical IMRT plan.

Results:

The detector response changed by 23% for a 390‐fold change in the dose per pulse. A correction function is derived to correct for this effect. The strip detector depth dose curve agrees with the Farmer ion chamber within 0.8%. The stem effect was negligible (0.2%). The dose linearity was excellent for the dose range of. A uniformity correction method is described to correct for variations in the individual detector pixel responses. The detector showed an over‐response relative to tissue dose at lower photon energies with the maximum dose response at nominal photon energy. Penumbra studies using a Varian Clinac 21EX at 1.5 and depths were measured to be 2.77 and for the secondary collimators, 3.52 and for the multileaf collimator rounded leaf ends, respectively. Point doses measured with the strip detector were compared to doses measured with EBT film and doses predicted by the Philips Pinnacle treatment planning system. The differences were and , respectively. They demonstrated the high temporal resolution capability of the detector readout system, which will allow one to investigate the temporal dose pattern of IMRT and volumetric modulated arc therapy (VMAT) deliveries.

Conclusions:

The CMRP silicon strip detector dose magnifying glass interfaced to a TERA ASIC DAQ system has high spatial and temporal resolution. It is a novel and valuable tool for QA in IMRT dose delivery and for VMAT dose delivery.
Investigation of the effects of treatment planning variables in small animal radiotherapy dose distributions
Authors: Motomura Amy R. , Bazalova Magdalena , Zhou Hu , Keall Paul J. , Graves Edward E.
Published: January 2010

Purpose:

Methods used for small animal radiation treatment have yet to achieve the same dose targeting as in clinical radiation therapy. Toward understanding how to better plan small animal radiation using a system recently developed for this purpose, the authors characterized dose distributions produced from conformal radiotherapy of small animals in a microCT scanner equipped with a variable‐aperture collimator.

Methods:

Dose distributions delivered to a cylindrical solid water phantom were simulated using a Monte Carlo algorithm. Phase‐space files for 120 kVp x‐ray beams and collimator widths of 1–10 mm at isocenter were generated using BEAMnrc software, and dose distributions for evenly spaced beams numbered from 5 to 80 were generated in DOSXYZnrc for a variety of targets, including centered spherical targets in a range of sizes, spherical targets offset from centered by various distances, and various ellipsoidal targets. Dose distributions were analyzed using dose volume histograms. The dose delivered to a mouse bearing a spontaneous lung tumor was also simulated, and dose volume histograms were generated for the tumor, heart, left lung, right lung, and spinal cord.

Results:

Results indicated that for centered, symmetric targets, the number of beams required to achieve a smooth dose volume histogram decreased with increased target size. Dose distributions for noncentered, symmetric targets did not exhibit any significant loss of conformality with increasing offset from the phantom center, indicating sufficient beam penetration through the phantom for targeting superficial targets from all angles. Even with variable collimator widths, targeting of asymmetric targets was found to have less conformality than that of spherical targets. Irradiation of a mouse lung tumor with multiple beam widths was found to effectively deliver dose to the tumor volume while minimizing dose to other critical structures.

Conclusions:

Overall, this method of generating and analyzing dose distributions provides a quantitative method for developing practical guidelines for small animal radiotherapy treatment planning. Future work should address methods to improve conformality in asymmetric targets.
A new metric for assessing IMRT modulation complexity and plan deliverability
Authors: McNiven Andrea L. , Sharpe Michael B. , Purdie Thomas G.
Published: January 2010

Purpose:

To evaluate the utility of a new complexity metric, the modulation complexity score (MCS), in the treatment planning and quality assurance processes and to evaluate the relationship of the metric with deliverability.

Methods:

A multisite (breast, rectum, prostate, prostate bed, lung, and head and neck) and site‐specific (lung) dosimetric evaluation has been completed. The MCS was calculated for each beam and the overall treatment plan. A 2D diode array (MapCHECK™, Sun Nuclear, Melbourne, FL) was used to acquire measurements for each beam. The measured and planned dose (PINNACLE3, Phillips, Madison, WI) was evaluated using different percent differences and distance to agreement (DTA) criteria (3%/3 mm and 2%/1 mm) and the relationship between the dosimetric results and complexity (as measured by the MCS or simple beam parameters) assessed.

Results:

For the multisite analysis (243 plans total), the mean MCS scores for each treatment site were breast (0.92), rectum (0.858), prostate (0.837), prostate bed (0.652), lung (0.631), and head and neck (0.356). The MCS allowed for compilation of treatment site‐specific statistics, which is useful for comparing different techniques, as well as for comparison of individual treatment plans with the typical complexity levels. For the six plans selected for dosimetry, the average diode percent pass rate was 98.7% (minimum of 96%) for 3%/3 mm evaluation criteria. The average difference in absolute dose measurement between the planned and measured dose was 1.7 cGy. The detailed lung analysis also showed excellent agreement between the measured and planned dose, as all beams had a diode percentage pass rate for 3%/3 mm criteria of greater than 95.9%, with an average pass rate of 99.0%. The average absolute maximum dose difference for the lung plans was 0.7 cGy. There was no direct correlation between the MCS and simple beam parameters which could be used as a surrogate for complexity level (i.e., number of segments or MU). An evaluation criterion of 2%/1 mm reliably allowed for the identification of beams that are dosimetrically robust. In this study we defined a robust beam or plan as one that maintained a diode percentage pass rate greater than 90% at 2%/1 mm, indicating delivery that was deemed accurate when compared to the planned dose, even under stricter evaluation criterion. MCS and MU threshold criteria were determined by defining a required specificity of 1.0. A MCS threshold of 0.8 allowed for identification of robust deliverability with a sensitivity of 0.36. In contrast, MU had a lower sensitivity of 0.23 for a threshold of 50 MU.

Conclusions:

The MCS allows for a quantitative assessment of plan complexity, on a fixed scale, that can be applied to all treatment sites and can provide more information related to dose delivery than simple beam parameters. This could prove useful throughout the entire treatment planning and QA process.
Development and validation of segmentation and interpolation techniques in sinograms for metal artifact suppression in CT
Authors: Veldkamp Wouter J. H. , Joemai Raoul M. S. , van der Molen Aart J. , Geleijns Jacob
Published: January 2010

Purpose:

Metal prostheses cause artifacts in computed tomography (CT) images. The purpose of this work was to design an efficient and accurate metal segmentation in raw data to achieve artifact suppression and to improve CT image quality for patients with metal hip or shoulder prostheses.

Methods:

The artifact suppression technique incorporates two steps: metal object segmentation in raw data and replacement of the segmented region by new values using an interpolation scheme, followed by addition of the scaled metal signal intensity. Segmentation of metal is performed directly in sinograms, making it efficient and different from current methods that perform segmentation in reconstructed images in combination with Radon transformations. Metal signal segmentation is achieved by using a Markov random field model (MRF). Three interpolation methods are applied and investigated. To provide a proof of concept, CT data of five patients with metal implants were included in the study, as well as CT data of a PMMA phantom with Teflon, PVC, and titanium inserts. Accuracy was determined quantitatively by comparing mean Hounsfield (HU) values and standard deviation (SD) as a measure of distortion in phantom images with titanium (original and suppressed) and without titanium insert. Qualitative improvement was assessed by comparing uncorrected clinical images with artifact suppressed images.

Results:

Artifacts in CT data of a phantom and five patients were automatically suppressed. The general visibility of structures clearly improved. In phantom images, the technique showed reduced SD close to the SD for the case where titanium was not inserted, indicating improved image quality. HU values in corrected images were different from expected values for all interpolation methods. Subtle differences between interpolation methods were found.

Conclusions:

The new artifact suppression design is efficient, for instance, in terms of preserving spatial resolution, as it is applied directly to original raw data. It successfully reduced artifacts in CT images of five patients and in phantom images. Sophisticated interpolation methods are needed to obtain reliable HU values close to the prosthesis.
The design of a simulated in‐line side‐coupled 6 MV linear accelerator waveguide
Authors: St. Aubin Joel , Steciw Stephen , Fallone B. G.
Published: January 2010

Purpose:

The design of a 3D in‐line side‐coupled 6 MV linac waveguide for medical use is given, and the effect of the side‐coupling and port irises on the radio frequency (RF), beam dynamics, and dosimetric solutions is examined. This work was motivated by our research on a linac‐MR hybrid system, where accurate electron trajectory information for a clinical medical waveguide in the presence of an external magnetic field was needed.

Methods:

For this work, the design of the linac waveguide was generated using the finite element method. The design outlined here incorporates the necessary geometric changes needed to incorporate a full‐end accelerating cavity with a single‐coupling iris, a waveguide‐cavity coupling port iris that allows power transfer into the waveguide from the magnetron, as well as a method to control the RF field magnitude within the first half accelerating cavity into which the electrons from the gun are injected.

Results:

With the full waveguide designed to resonate at, a full 3D RF field solution was obtained. The accuracy of the 3D RF field solution was estimated through a comparison of important linac parameters ( factor, shunt impedance, transit time factor, and resonant frequency) calculated for one accelerating cavity with the benchmarked program SUPERFISH. It was found that the maximum difference between the 3D solution and SUPERFISH was less than 0.03%. The eigenvalue solver, which determines the resonant frequencies of the 3D side‐coupled waveguide simulation, was shown to be highly accurate through a comparison with lumped circuit theory. Two different waveguide geometries were examined, one incorporating a 0.5 mm first side cavity shift and another with a 1.5 mm first side cavity shift. The asymmetrically placed side‐coupling irises and the port iris for both models were shown to introduce asymmetries in the RF field large enough to cause a peak shift and skewing (center of gravity minus peak shift) of an initially cylindrically uniform electron beam accelerating within the waveguide. The shifting and skewing of the electron beam were found to be greatest due to the effects of the side‐coupling irises on the RF field. A further Monte Carlo study showed that this effect translated into a 1% asymmetry in a field dose profile.

Conclusions:

A full 3D design for an in‐line side‐coupled 6 MV linear accelerator that emulates a common commercial waveguide has been given. The effect of the side coupling on the dose distribution has been shown to create a slight asymmetry, but overall does not affect the clinical applicability of the linac. The 3D in‐line side‐coupled linac model further provides a tool for the investigation of linac performance within an external magnetic field, which exists in an integrated linac‐MR system.
Development of a population‐based model of surface segmentation uncertainties for uncertainty‐weighted deformable image registrations
Authors: Wu Jian , Murphy Martin J. , Weiss Elisabeth , Sleeman William C. , Williamson Jeffrey
Published: January 2010

Purpose:

To develop a population‐based model of surface segmentation uncertainties for uncertainty‐weighted surface‐based deformable registrations.

Methods:

The contours of the prostate, the bladder, and the rectum were manually delineated by five observers on fan beam CT images of four prostate cancer patients. First, patient‐specific representations of structure segmentation uncertainties were derived by determining the interobserver variability (i.e., standard deviation) of the structure boundary delineation. This was achieved by (1) generating an average structure surface mesh from the structure contours drawn by different observers, and (2) calculating three‐dimensional standard deviation surface meshes (SDSMs) based on the perpendicular distances from the individual boundary surface meshes to the average surface mesh computed above. Then an average structure surface mesh was constructed to be the reference mesh for the population‐based model. The average structure meshes of the other patients were deformably registered to the reference mesh. The calculated deformable vector fields were used to map the patient‐specific SDSMs to the reference mesh to obtain the registered SDSMs. Finally, the population‐based SDSM was derived by taking the average of the registered SDSMs in quadrature.

Results:

Population‐based structure surface statistical models of the prostate, the bladder, and the rectum were created by mapping the patient‐specific SDSMs to the population surface model. Graphical visualization indicates that the boundary uncertainties are dependent on anatomical location.

Conclusions:

The authors have developed and demonstrated a general method for objectively constructing surface maps of uncertainties derived from topologically complex structure boundary segmentations from multiple observers. The computed boundary uncertainties have significant spatial variations. They can be used as weighting factors for surface‐based probabilistic deformable registration.
Quantifying the effect of anode surface roughness on diagnostic x‐ray spectra using Monte Carlo simulation
Authors: Mehranian A. , Ay M. R. , Alam N. Riyahi , Zaidi H.
Published: January 2010

Purpose:

The accurate prediction of x‐ray spectra under typical conditions encountered in clinical x‐ray examination procedures and the assessment of factors influencing them has been a long‐standing goal of the diagnostic radiology and medical physics communities. In this work, the influence of anode surface roughness on diagnostic x‐ray spectra is evaluated using MCNP4C‐based Monte Carlo simulations.

Methods:

An image‐based modeling method was used to create realistic models from surface‐cracked anodes. An in‐house computer program was written to model the geometric pattern of cracks and irregularities from digital images of focal track surface in order to define the modeled anodes into MCNP input file. To consider average roughness and mean crack depth into the models, the surface of anodes was characterized by scanning electron microscopy and surface profilometry. It was found that the average roughness in the most aged tube studied is about . The correctness of MCNP4C in simulating diagnostic x‐ray spectra was thoroughly verified by calling its Gaussian energy broadening card and comparing the simulated spectra with experimentally measured ones. The assessment of anode roughness involved the comparison of simulated spectra in deteriorated anodes with those simulated in perfectly plain anodes considered as reference. From these comparisons, the variations in output intensity, half value layer (HVL), heel effect, and patient dose were studied.

Results:

An intensity loss of 4.5% and 16.8% was predicted for anodes aged by 5 and deep cracks (50 kVp, 6° target angle, and 2.5 mm Al total filtration). The variations in HVL were not significant as the spectra were not hardened by more than 2.5%; however, the trend for this variation was to increase with roughness. By deploying several point detector tallies along the anode‐cathode direction and averaging exposure over them, it was found that for a 6° anode, roughened by deep cracks, the reduction in exposure is 14.9% and 13.1% for 70 and 120 kVp tube voltages, respectively. For the evaluation of patient dose, entrance skin radiation dose was calculated for typical chest x‐ray examinations. It was shown that as anode roughness increases, patient entrance skin dose decreases averagely by a factor of 15%.

Conclusions:

It was concluded that the anode surface roughness can have a non‐negligible effect on output spectra in aged x‐ray imaging tubes and its impact should be carefully considered in diagnostic x‐ray imaging modalities.
Automated segmentation and quantification of liver and spleen from CT images using normalized probabilistic atlases and enhancement estimation
Authors: Linguraru Marius George , Sandberg Jesse K. , Li Zhixi , Shah Furhawn , Summers Ronald M.
Published: January 2010

Purpose:

To investigate the potential of the normalized probabilistic atlases and computer‐aided medical image analysis to automatically segment and quantify livers and spleens for extracting imaging biomarkers (volume and height).

Methods:

A clinical tool was developed to segment livers and spleen from 257 abdominal contrast‐enhanced CT studies. There were 51 normal livers, 44 normal spleens, 128 splenomegaly, 59 hepatomegaly, and 23 partial hepatectomy cases. 20 more contrast‐enhanced CT scans from a public site with manual segmentations of mainly pathological livers were used to test the method. Data were acquired on a variety of scanners from different manufacturers and at varying resolution. Probabilistic atlases of livers and spleens were created using manually segmented data from ten noncontrast CT scans (five male and five female). The organ locations were modeled in the physical space and normalized to the position of an anatomical landmark, the xiphoid. The construction and exploitation of liver and spleen atlases enabled the automated quantifications of liver/spleen volumes and heights (midhepatic liver height and cephalocaudal spleen height) from abdominal CT data. The quantification was improved incrementally by a geodesic active contour, patient specific contrast‐enhancement characteristics passed to an adaptive convolution, and correction for shape and location errors.

Results:

The livers and spleens were robustly segmented from normal and pathological cases. For the liver, the Dice/Tanimoto volume overlaps were 96.2%/92.7%, the volume/height errors were 2.2%/2.8%, the root‐mean‐squared error (RMSE) was 2.3 mm, and the average surface distance (ASD) was 1.2 mm. The spleen quantification led to 95.2%/91% Dice/Tanimoto overlaps, 3.3%/1.7% volume/height errors, 1.1 mm RMSE, and 0.7 ASD. The correlations with clinical/manual height measurements were 0.97 and 0.93 for the spleen and liver, respectively . No significant difference was found comparing interobserver and automatic‐manual volume/height errors for liver and spleen.

Conclusions:

The algorithm is robust to segmenting normal and enlarged spleens and livers, and in the presence of tumors and large morphological changes due to partial hepatectomy. Imaging biomarkers of the liver and spleen from automated computer‐assisted tools have the potential to assist the diagnosis of abdominal disorders from routine analysis of clinical data and guide clinical management.
Comparison of different computed radiography systems: Physical characterization and contrast detail analysis
Authors: Rivetti Stefano , Lanconelli Nico , Bertolini Marco , Nitrosi Andrea , Burani Aldo , Acchiappati Domenico
Published: January 2010

Purpose:

In this study, five different units based on three different technologies—traditional computed radiography (CR) units with granular phosphor and single‐side reading, granular phosphor and dual‐side reading, and columnar phosphor and line‐scanning reading—are compared in terms of physical characterization and contrast detail analysis.

Methods:

The physical characterization of the five systems was obtained with the standard beam condition RQA5. Three of the units have been developed by FUJIFILM (FCR ST‐VI, FCR ST‐BD, and FCR Velocity U), one by Kodak (Direct View CR 975), and one by Agfa (DX‐S). The quantitative comparison is based on the calculation of the modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). Noise investigation was also achieved by using a relative standard deviation analysis. Psychophysical characterization is assessed by performing a contrast detail analysis with an automatic reading of CDRAD images.

Results:

The most advanced units based on columnar phosphors provide MTF values in line or better than those from conventional CR systems. The greater thickness of the columnar phosphor improves the efficiency, allowing for enhanced noise properties. In fact, NPS values for standard CR systems are remarkably higher for all the investigated exposures and especially for frequencies up to 3.5 lp/mm. As a consequence, DQE values for the three units based on columnar phosphors and line‐scanning reading, or granular phosphor and dual‐side reading, are neatly better than those from conventional CR systems. Actually, DQE values of about 40% are easily achievable for all the investigated exposures.

Conclusions:

This study suggests that systems based on the dual‐side reading or line‐scanning reading with columnar phosphors provide a remarkable improvement when compared to conventional CR units and yield results in line with those obtained from most digital detectors for radiography.
Filtered region of interest cone‐beam rotational angiography
Authors: Schafer Sebastian , Noël Peter B. , Walczak Alan M. , Hoffmann Kenneth R.
Published: January 2010

Purpose:

Cone‐beam rotational angiography (CBRA) is widely used in the modern clinical settings. In a number of procedures, the area of interest is often considerably smaller than the field of view (FOV) of the detector, subjecting the patient to potentially unnecessary x‐ray dose. The authors therefore propose a filter‐based method to reduce the dose in the regions of low interest, while supplying high image quality in the region of interest (ROI).

Methods:

For such procedures, the authors propose a method of filtered region of interest (FROI)‐CBRA. In the authors’ approach, a gadolinium filter with a circular central opening is placed into the x‐ray beam during image acquisition. The central region is imaged with high contrast, while peripheral regions are subjected to a substantial lower intensity and dose through beam filtering. The resulting images contain a high contrast/intensity ROI, as well as a low contrast/intensity peripheral region, and a transition region in between. To equalize the two regions’ intensities, the first projection of the acquisition is performed with and without the filter in place. The equalization relationship, based on Beer's law, is established through linear regression using corresponding filtered and nonfiltered data. The transition region is equalized based on radial profiles.

Results:

Evaluations in 2D and 3D show no visible difference between conventional FROI‐CBRA projection images and reconstructions in the ROI. CNR evaluations show similar image quality in the ROI, with a reduced CNR in the reconstructed peripheral region. In all filtered projection images, the scatter fraction inside the ROI was reduced. Theoretical and experimental dose evaluations show a considerable dose reduction; using a ROI half the original FOV reduces the dose by 60% for the filter thickness of 1.29 mm.

Conclusions:

These results indicate the potential of FROI‐CBRA to reduce the dose to the patient while supplying the physician with the desired image detail inside the ROI.
Comparison of Monte Carlo collimator transport methods for photon treatment planning in radiotherapy
Authors: Schmidhalter D. , Manser P. , Frei D. , Volken W. , Fix M. K.
Published: January 2010

Purpose:

The aim of this work was a Monte Carlo (MC) based investigation of the impact of different radiation transport methods in collimators of a linear accelerator on photon beam characteristics, dose distributions, and efficiency. Thereby it is investigated if it is possible to use different simplifications in the radiation transport for some clinical situations in order to save calculation time.

Methods:

Within the Swiss Monte Carlo Plan, a GUI‐based framework for photon MC treatment planning, different MC methods are available for the radiation transport through the collimators [secondary jaws and multileaf collimator (MLC)]: EGSnrc (reference),, and Pin (an in‐house developed MC code). Additional nonfull transport methods were implemented in order to provide different complexity levels for the MC simulation: Considering collimator attenuation only, considering Compton scatter only or just the firstCompton process, and considering the collimators as totally absorbing. Furthermore, either a simple or an exact geometry of the collimators can be selected for the absorbing or attenuation method. Phasespaces directly above and dose distributions in a water phantom are analyzed for academic and clinical treatment fields using 6 and 15 MV beams, including intensity modulated radiation therapy with dynamic MLC.

Results:

For all MC transport methods, differences in the radial mean energy and radial energy fluence are within 1% inside the geometric field. Below the collimators, the energy fluence is underestimated for nonfull MC transport methods ranging from 5% for Compton to 100% for Absorbing. Gamma analysis using EGSnrc calculated doses as reference shows that the percentage of voxels fulfilling a 1% /1 mm criterion is at least 98% when using, Compton, or firstCompton transport methods. When using the methods Pin, Transmission, Flat‐Transmission, Flat‐Absorbing or Absorbing, the mean value of points fulfilling this criterion over all tested cases is 97%, 88%, 74%, 68%, or 65%, respectively. However, compared to EGSnrc calculations, the gain in efficiency is a factor of up to 10 for and up to 48 for the absorbing method.

Conclusions:

The results of this investigation suggest that it is an option to use a simple transport technique in the initial treatment planning process and use more accurate transport methods for the final dose calculation accepting longer calculation times.
A residual correction method for high‐resolution PET reconstruction with application to on‐the‐fly Monte Carlo based model of positron range
Authors: Fu Lin , Qi Jinyi
Published: January 2010

Purpose:

The quality of tomographic images is directly affected by the system model being used in image reconstruction. An accurate system matrix is desirable for high‐resolution image reconstruction, but it often leads to high computation cost. In this work the authors present a maximum a posteriori reconstruction algorithm with residual correction to alleviate the tradeoff between the model accuracy and the computation efficiency in image reconstruction.

Methods:

Unlike conventional iterative methods that assume that the system matrix is accurate, the proposed method reconstructs an image with a simplified system matrix and then removes the reconstruction artifacts through residual correction. Since the time‐consuming forward and back projection operations using the accurate system matrix are not required in every iteration, image reconstruction time can be greatly reduced.

Results:

The authors apply the new algorithm to high‐resolution positron emission tomography reconstruction with an on‐the‐fly Monte Carlo (MC) based positron range model. Computer simulations show that the new method is an order of magnitude faster than the traditional MC‐based method, whereas the visual quality and quantitative accuracy of the reconstructed images are much better than that obtained by using the simplified system matrix alone.

Conclusions:

The residual correction method can reconstruct high‐resolution images and is computationally efficient.
The effects of dose calculation resolution on dose accuracy for radiation therapy treatments of the lung. Part II. A comparison of dose distributions from an explicit lung model to dose distributions derived from a CT representation
Authors: Babcock Kerry , Sidhu Narinder
Published: January 2010

Purpose:

Due to limitations in computer memory and computation time, typical radiation therapy treatments are calculated with a voxel dimension on the order of several millimeters. The anatomy below this practical resolution is approximated as a homogeneous region uniform in atomic composition and density. The purpose of this article is to examine whether the exclusion of anatomic structure below the practical dose calculation resolution produces deviations in the resulting dose distributions.

Methods:

EGSnrc calculated dose distributions from the BRANCH lung model of Part I are compared and contrasted to dose distributions from a CT representation of the same BRANCH model for three different phases of the respiration cycle.

Results:

The exclusion of branching structures below a CT resolution of resulted in a deviation in dose. The deviation in dose was as high as 14% but was localized around the branching structures. There was no significant variation in the dose deviation as a function of either field size or lung density.

Conclusions:

The exclusion of explicit branching structures of the lung in a CT representation creates localized deviations in dose. To ensure accurate dose calculations, CT resolution must be increased.
Investigation of energy dependence of EBT and EBT‐2 Gafchromic film
Authors: Lindsay Patricia , Rink Alexandra , Ruschin Mark , Jaffray David
Published: January 2010

Purpose:

The purpose of this study was to quantify the extent of energy dependence of Gafchromic film to x‐ray energies ranging in quality from 105 kVp to 6 MV, and relate this dependency to the film's chemical composition and date of production.

Methods:

Lots of Gafchromic EBT film manufactured in 2004 and 2005 together with more recent batches produced in 2007 were evaluated for energy dependence. Multiple batches of EBT‐2 film were also evaluated. Energy dependence was quantified as—the ratio of net optical density (netOD) measured at a given energy relative to the netOD measured at 6 MV, as measured on a linear accelerator. was evaluated for beam qualities of 105 and 220 kVp on a clinical orthovoltage unit using two separate techniques—a flatbed scanner (Epson) and a real‐time fiber‐optic readout system. Neutron activation analysis for chlorine and bromine content was performed on all the films to determine whether the composition of the film had changed between batches of film exhibiting different energy dependence responses.

Results:

For batches of EBT manufactured in 2007, was 0.75 and was 0.85, indicating an under‐response at orthovoltage energies. These results were confirmed using both the Epson flatbed scanner as well as the real‐time readout system. For batches of EBT film manufactured before 2006, ranged from 0.9 to 1.0. The results from the neutron activation analysis confirmed a direct relationship between the concentration of chlorine and the magnitude of under‐response at orthovoltage energies. EBT‐2 film exhibited values ranging from 0.79 (under‐response) to 1.20 (over‐response) among batches containing varying concentrations of bromine, chlorine, and potassium.

Conclusions:

The results of this study indicated that differences in energy response of EBT and EBT‐2 films were due to differences in the chemical composition and therefore the effective atomic number of the film, which have changed over time. To achieve an energy independent dosimeter over a range of kilovoltage energies, the effective atomic number of the dosimeter must be closely matched to that of water. Small deviations in chemical composition can lead to large deviations in response as a function of energy.
Comparison of organ doses for patients undergoing balloon brachytherapy of the breast with HDR or electronic sources using Monte Carlo simulations in a heterogeneous human phantoma)
Authors: Mille Matthew M. , Xu X. George , Rivard Mark J.
Published: January 2010

Purpose:

Accelerated partial breast irradiation via interstitial balloon brachytherapy is a fast and effective treatment method for certain early stage breast cancers. The radiation can be delivered using a conventional high‐dose rate (HDR) gamma‐emitting source or a novel electronic brachytherapy (eBx) source which uses lower energy x rays that do not penetrate as far within the patient. A previous study [A. Dickler, M. C. Kirk, N. Seif, K. Griem, K. Dowlatshahi, D. Francescatti, and R. A. Abrams, “A dosimetric comparison of MammoSite high‐dose‐rate brachytherapy and Xoft Axxent electronic brachytherapy,” Brachytherapy 6, 164–168 (2007)] showed that the target dose is similar for HDR and eBx. This study compares these sources based on the dose received by healthy organs and tissues away from the treatment site.

Methods:

A virtual patient with left breast cancer was represented by a whole‐body, tissue‐heterogeneous female voxel phantom. Monte Carlo methods were used to calculate the dose to healthy organs in a virtual patient undergoing balloon brachytherapy of the left breast with HDR or eBx sources. The dose‐volume histograms for a few organs which received large doses were also calculated. Additional simulations were performed with all tissues in the phantom defined as water to study the effect of tissue inhomogeneities.

Results:

For both HDR and eBx, the largest mean organ doses were received by the ribs, thymus gland, left lung, heart, and sternum which were close to the brachytherapy source in the left breast. eBx yielded mean healthy organ doses that were more than a factor of smaller than for HDR for all organs considered, except for the three closest ribs. Excluding these ribs, the average and median dose‐reduction factors were and , respectively. The volume distribution of doses in nearby soft tissue organs that were outside the PTV were also improved with eBx. However, the maximum dose to the closest rib with the eBx source was 5.4 times greater than that of the HDR source. The ratio of tissue‐to‐water maximum rib dose for the eBx source was .

Conclusions:

The results of this study indicate that eBx may offer lower toxicity to most healthy tissues, except nearby bone. TG‐43 methods have a tendency to underestimate dose to bone, especially the ribs. Clinical studies evaluating the negative health effects caused by irradiating healthy organs are needed so that physicians can better understand when HDR or eBx might best benefit a patient.
Focal beam distortion and treatment planning for transrib focused ultrasound thermal therapy: A feasibility study using a two‐dimensional ultrasound phased array
Authors: Liu Hao‐Li , Hsu Chia‐Lung , Huang Sheng‐Min , Hsi Yi‐Wen
Published: January 2010

Purpose:

The purpose of this study is to numerically investigate the feasibility of employing a spherical‐section ultrasound phased array for transrib thermal ablation of liver tumors.

Methods:

Based on CT images, the authors performed a 3D reconstruction of the ribs and the surrounding soft tissues. A 3D pseudospectral time‐domain (PSTD) solver was used to assess wave propagation and the distribution of pressure, with the aim of determining the specific absorption rate (SAR) and the resulting thermal doses and dynamics. Phase aberrations caused by the interposed ribs were corrected to assess the efficacy of the device in improving the SAR gain between the ribs and the target positions.

Results:

Experimental results supported the usefulness of the PSTD solver for predicting the pressure distribution due to the interfering obstacle. In addition, the method allowed the correction of phase aberrations caused by the ribs, and a significant improvement (176%) in the SAR gain between the ribs and the target points was observed at specific frequencies.

Conclusions:

The method allowed successful tissue targeting without causing overheating of the ribs. One main advantage of this approach is the accurate estimation of phase aberration caused by heterogeneously porous ribs and other interposed tissues. This strategy might prove useful to assess the effectiveness and safety of focused ultrasound thermal ablation prior to transrib treatment.
Sensitivity calibration procedures in optical‐CT scanning of BANG®3 polymer gel dosimeters
Authors: Xu Y. , Wuu Cheng‐Shie , Maryanski Marek J.
Published: January 2010

The dose response of the BANG®3 polymer gel dosimeter (MGS Research Inc., Madison, CT) was studied using the OCTOPUS™ laser CT scanner (MGS Research Inc., Madison, CT). Six 17 cm diameter and 12 cm high Barex cylinders, and 18 small glass vials were used to house the gel. The gel phantoms were irradiated with 6 and 10 MV photons, as well as 12 and 16 MeV electrons using a Varian Clinac 2100EX. Three calibration methods were used to obtain the dose response curves: (a) Optical density measurements on the 18 glass vials irradiated with graded doses from 0 to 4 Gy using 6 or 10 MV large field irradiations; (b) optical‐CT scanning of Barex cylinders irradiated with graded doses (0.5, 1, 1.5, and 2 Gy) from four adjacent photon fields or electron fields; and (c) percent depth dose (PDD) comparison of optical‐CT scans with ion chamber measurements for , 12 and 16 MeV electron fields. The dose response of the BANG®3 gel was found to be linear and energy independent within the uncertainties of the experimental methods (about 3%). The slopes of the linearly fitted dose response curves (dose sensitivities) from the four field irradiations (, , , and ) and the PDD matching methods ( and ) agree within 2.2%, indicating a good reproducibility of the gel dose response within phantoms of the same geometry. The dose sensitivities from the glass vial approach are different from those of the cylindrical Barex phantoms by more than 30%, owing probably to the difference in temperature inside the two types of phantoms during gel formation and irradiation, and possible oxygen contamination of the glass vial walls. The dose response curve obtained from the PDD matching approach with 16 MeV electron field was used to calibrate the gel phantom irradiated with the 12 MeV, electron field. Three‐dimensional dose distributions from the gel measurement and the Eclipse planning system (Varian Corporation, Palo Alto, CA) were compared and evaluated using 3% dose difference and 2 mm distance‐to‐agreement criteria.
Susceptibility gradient quantization by MRI signal response mapping (SIRMA) to dephaser
Authors: Franconi F. , Chapon C. , Le Jeune J. J. , Richomme P. , Lemaire L.
Published: January 2010

Purpose:

Susceptibility effects are a very efficient source of contrast in magnetic resonance imaging. However, detection is hampered by the fact the induced contrast is negative. In this work, the SIgnal Response MApping (SIRMA) to dephaser method is proposed to map susceptibility gradient to improve visualization.

Methods:

In conventional gradient echo acquisitions, the echo formation of susceptibility affected spins is shifted in‐space, the shift being proportional to the susceptibility gradient. Susceptibility gradients map can be produced by measuring this induced shifts. The SIRMA method measures these shifts from a series of dephased images collected with additional incremental dephasers. These additional dephasers correspond either to a slice refocusing gradient offset or to a reconstruction window off‐centering. The signal intensity profile as a function of the additional dephaser was determined on a pixel‐by‐pixel basis from the ensemble of dephased images. Susceptibility affected voxels presented a signal response profile maximum shifted compared to nonaffected voxels ones. Shift magnitude and sign were measured for each pixel to determine susceptibility gradients and produce a susceptibility gradient map.

Results:

In vitro experiments demonstrated the ability of the method to map gradient inhomogeneities induced by a cylinder. Quantization accuracy was evaluated comparing SIRMA images and simulations performed on the well‐characterized air filled cylinder model. Performances of the SIRMA method, evaluated in vitro on cylinders filled with various superparamagnetic iron oxide SPIO concentrations, showed limited influence of acquisition parameters. Robustness of the method was then assessed in vivo after an infusion of SPIO‐loaded nanocapsules into the rat brain using a convection‐enhanced drug delivery approach. The region of massive susceptibility gradient induced by the SPIO‐loaded nanocapsules was clearly delineated on SIRMA maps and images were compared to weighted images, Susceptibility Gradient Map (SGM), and histological Perl's staining slice. The potential for quantitative evaluation of SPIO distribution volume was demonstrated.

Conclusions:

The proposed method is a promising technique for a wide range of applications especially in molecular or cellular imaging with respect to its quantitative nature and its computational simplicity.
Decorrelated Neuronal Firing in Cortical Microcircuits
Authors: Ecker Alexander S. , Berens Philipp , Keliris Georgios A. , Bethge Matthias , Logothetis Nikos K. , Tolias Andreas S.
Published: January 2010

Columns, Connections, and Correlations

What is the nature of interactions between neurons in neural circuits? The prevalent hypothesis suggests that dense local connectivity causes nearby cortical neurons to receive substantial amounts of common input, which in turn leads to strong correlations between them. Now two studies challenge this view, which impacts our fundamental understanding of coding in the cortex.

Ecker
et al.

(p.
584
) investigated the statistics of correlated firing in pairs of neurons from area V1 of awake macaque monkeys. In contrast to previous studies, correlations turned out to be very low, irrespective of the stimulus being shown to the animals, the distances of the recording sites, and the similarity of the neuron's receptive fields or response properties. In an accompanying modeling and recording paper,

Renart
et al.

(p.
587
) demonstrate how it is possible to have zero noise correlation, even among cells with common input.
The Free-Energy Landscape of Clusters of Attractive Hard Spheres
Authors: Meng Guangnan , Arkus Natalie , Brenner Michael P. , Manoharan Vinothan N.
Published: January 2010

Packing Puzzle

The packing of a large number of spheres is a well-studied problem with maximal packing based on the arrangement of nearest neighbors. With much smaller numbers of particles, it is the free energy that governs which packing arrangements dominate.

Meng
et al.

(p.
560
; see the Perspective by

Crocker

) looked at the assembly of colloidal clusters where the number of particles was limited from 2 to 10. For five particles or fewer, only one packing arrangement was found. For six or more particles, while a number of similar energy structures could form, the probability of formation was biased toward those structures with the greater number of nearest-neighbor connections.
Phase Transitions of Adsorbed Atoms on the Surface of a Carbon Nanotube
Authors: Wang Zenghui , Wei Jiang , Morse Peter , Dash J. Gregory , Vilches Oscar E. , Cobden David H.
Published: January 2010

Phase Transitions on Carbon Nanotubes

The nature of phase transitions changes with system dimensionality. Many aspects of two-dimensional systems have been explored by adsorbing rare gases on graphite surfaces.

Wang
et al.

(p.
552
) reduce the dimensionality further by examining phase transitions of argon and krypton on single-walled carbon nanotubes, following the extent of surface coverage and mapping out phase transitions by using the nanotube as a resonator. Changing the conductance and thus the density of surface electrons also allowed exploration of the effect of adsorbate-surface interactions.
Combined Effects on Selectivity in Fe-Catalyzed Methylene Oxidation
Authors: Chen Mark S. , White M. Christina
Published: January 2010

Secondary Selectivity

Organic molecules consist principally of rings and chains of methylene (secondary) CH
2
groups, intermittently adorned with oxygen or nitrogen centers and more heavily substituted carbons at the junctions. Synthetic transformations would be most efficient if the C–H bonds in any particular methylene group along the framework could be targeted for selective modification. However, for the most part, these carbon centers prove remarkably hard to differentiate for reactive purposes.
Chen and White
(p.
566
) now show that an iron catalyst can direct peroxide to oxidize specific secondary C–H bonds preferentially, and with reasonable efficiency, in a range of complex molecules. The observed selectivities follow predictable trends correlated with the electronic and steric environment of the target site.
Turning Away from High Symmetry
Authors: Crocker John C.
Published: January 2010

Light microscopy studies of cluster formation by colloidal particles show that less-symmetrical structures are favored under equilibrium conditions.
A Test for Geoengineering?
Authors: Robock Alan , Bunzl Martin , Kravitz Ben , Stenchikov Georgiy L.
Published: January 2010

Stratospheric geoengineering cannot be tested in the atmosphere without full-scale implementation.
Haiti's Quake Shifts Clinic's Focus From AIDS to Aid
Authors: Cohen Jon
Published: January 2010

Since the 12 January earthquake, GHESKIO (the Haitian Group for the Study of Kaposi's Sarcoma and Opportunistic Infections) has converted itself into a hospital with a surgical unit and a makeshift outdoor home for thousands of refugees. On 21 January, GHESKIO leader Jean "Bill" Pape responded to questions from
Science
in an e-mail.
Did a Battering Rain of Comets Bring Ganymede to Geologic Life?
Authors: Kerr Richard A.
Published: January 2010

A study published this week offers a possible explanation for why jovian moon Ganymede evolved so much more than Callisto did: Ganymede suffered a far worse beating 3.9 billion years ago when a rain of comets and asteroids battered much of the solar system.
Faintest Thrum Heralds Quantum Machines
Authors: Cho Adrian
Published: January 2010

After years of trying, physicists are on the verge of making tiny vibrating devices that make the slightest possible movement. Far weirder mechanical devices could follow.