bluuebunny/crossref_metadata_2025 huggingface dataset:
Singularity Equations of Gough–Stewart Platforms Using a Minimal Set of Geometric Parameters
Authors: Jiang Qimi , Gosselin Clément M.
Published: October 2008

So far, in the derivation of the singularity equations of Gough–Stewart platforms, all researchers defined the mobile frame by making its origin coincide with the considered point on the platform. One problem can be that the obtained singularity equation contains too many geometric parameters and is not convenient for singularity analysis, especially not convenient for geometric optimization. Another problem can be that the obtained singularity equation cannot be used directly in practice. To solve these problems, this work presents a new approach to derive the singularity equation of the Gough–Stewart platform. The main point is that the origin of the mobile frame is separated from the considered point and chosen to coincide with a special point on the platform in order to minimize the geometric parameters defining the platform. Similarly, by defining a proper fixed frame, the geometric parameters defining the base can also be minimized. In this way, no matter which practical point of the platform is chosen as the considered point, the obtained singularity equation contains only a minimal set of geometric parameters and becomes a solid foundation for the geometric optimization based on singularity analysis.
The Maximal Singularity-Free Workspace of the Gough–Stewart Platform for a Given Orientation
Authors: Jiang Qimi , Gosselin Clément M.
Published: October 2008

The maximal singularity-free workspace of parallel mechanisms is a desirable criterion in robot design. However, for a 6DOF parallel mechanism, it is very difficult to find an analytic method to determine the maximal singularity-free workspace around a prescribed point for a given orientation. Hence, a numerical algorithm is presented in this paper to compute the maximal singularity-free workspace as well as the corresponding leg length ranges of the Gough–Stewart platform. This algorithm is based on the relationship between the maximal singularity-free workspace and the singularity surface. Case studies with different orientations are performed to demonstrate the presented algorithm. The obtained results can be applied to the geometric design or parameter (leg length) setup of this type of parallel robots.
A Decision-Analytic System Concept Selection for a Public Project
Authors: Takai Shun , Ishii Kosuke
Published: October 2008

This paper presents a decision-analytic system concept selection for a public project. In the proposed approach, the decision-maker (engineer) chooses a system concept and requests a budget for building the system. The unique customer (government) decides whether to approve the project but also has the option to cancel the project if the cost of the project exceeds the budget. Thus, at the time of system concept selection, the cost of the system, the government’s criterion to approve the project, and its criterion to continue/cancel the project when the cost exceeds the budget are modeled as uncertainties. To model the uncertainty of project continuation, the engineer assumes that the government’s criterion to continue/cancel the project is a linear combination of its initial criterion to approve the project and the budget. By analyzing the government’s option to cancel the project, the engineer can optimize his or her choice of a system concept (technology and target system requirements) and budget.
Theoretical and Experimental Analysis of a Cycloidal Speed Reducer
Authors: Gorla Carlo , Davoli Piermaria , Rosa Francesco , Longoni Claudio , Chiozzi Franco , Samarani Alessandro
Published: October 2008

In this paper, a theoretical and experimental investigation on an innovative cycloidal speed reducer is presented. The typical cycloid drive has a planet wheel, the profile of which is the internal offset of an epitrochoid meshing with cylindrical rollers connected to the case. This reducer, on the contrary, has an external ring gear, the transverse profile of which is the external offset of an epitrochoid and engages with the planet wheel by means of cylindrical rollers. This paper investigates the structural characteristics and the kinematic principles of this type of reducer. A theoretical approach based on the theory of gearing (following Litvin’s approach) is developed and compared to a development of Blanche and Yang’s approach. Furthermore, a simplified procedure to calculate the force distribution on cycloid drive elements, its power losses, and theoretical mechanical efficiency is presented. The effects of design parameters on the values of forces are studied for an optimal design of this type of reducer. The theoretical model is tuned on the basis of the results of tests made on purpose. The mechanical efficiency dependency on speed and torque is described. The main aim of this work is to tune a theoretical model in order to predict the operating behavior of the cycloid drive and to improve its design procedure.
Towards new functional nanostructures for medical imaging
Authors: Matsuura Naomi , Rowlands J. A.
Published: October 2008

Nanostructures represent a promising new type of contrast agent for clinical medical imaging modalities, including magnetic resonance imaging, x‐ray computed tomography, ultrasound, and nuclear imaging. Currently, most nanostructures are simple, single‐purpose imaging agents based on spherical constructs (e.g., liposomes, micelles, nanoemulsions, macromolecules, dendrimers, and solid nanoparticle structures). In the next decade, new clinical imaging nanostructures will be designed as multi‐functional constructs, to both amplify imaging signals at disease sites and deliver localized therapy. Proposals for nanostructures to fulfill these new functions will be outlined. New functional nanostructures are expected to develop in five main directions: Modular nanostructures with additive functionality; cooperative nanostructures with synergistic functionality; nanostructures activated by theirin vivoenvironment; nanostructures activated by sources outside the patient; and novel, nonspherical nanostructures and components. The development and clinical translation of next‐generation nanostructures will be facilitated by a combination of improved clarity of thein vivoimaging and biological challenges and the requirements to successfully overcome them; development of standardized characterization and validation systems tailored for the preclinical assessment of nanostructure agents; and development of streamlined commercialization strategies and pipelines tailored for nanostructure‐based agents for their efficient translation to the clinic.
Analytical calculation of central‐axis dosimetric data for a dedicated radiosurgery linear accelerator
Authors: Yang James N. , Pino Ramiro
Published: October 2008

Narrow beams are extensively used in stereotactic radiosurgery. The accuracy of treatment planning dose calculation depends largely on how well the dosimetric data are measured during the machine commissioning. Narrow beams are characterized by the lack of lateral electronic equilibrium. The lateral electronic disequilibrium in the radiation field and detector's finite size are likely to compromise the accuracy in dose measurements in these beams. This may have a profound impact on outcome in patients who undergo stereotactic radiosurgery. To confirm the measured commissioning data for a dedicated linear accelerator‐based radiosurgery system, we developed an analytical model to calculate the narrow photon beam central‐axis dose. This model is an extension of a previously reported method of Nizin and Mooij for the calculation of the absorbed dose under lateral electronic disequilibrium conditions at depth of or greater. The scatter factor and tissue‐maximum ratio were calculated for narrow beams using the parametrized model and compared to carefully measured results for the same beams. For narrow beam radii ranging from , the differences between the analytical and measured scatter factors were no greater than 1.4%. In addition, the differences between the analytical and measured tissue‐maximum ratios were within 3.3% for regions greater than the maximum dose depth. The estimated error of this analytical calculation was less than 2%, which is sufficient to validate measurement results.
Automated 2D–3D registration of portal images and CT data using line‐segment enhancement
Authors: Munbodh Reshma , Chen Zhe , Jaffray David A. , Moseley Douglas J. , Knisely Jonathan P. S. , Duncan James S.
Published: October 2008

In prostate radiotherapy, setup errors with respect to the patient's bony anatomy can be reduced by aligning 2D megavoltage (MV) portal images acquired during treatment to a reference 3D kilovoltage (kV) CT acquired for treatment planning purposes. The purpose of this study was to evaluate a fully automated 2D–3D registration algorithm to quantify setup errors in 3D through the alignment of line‐enhanced portal images and digitally reconstructed radiographs computed from the CT. The line‐enhanced images were obtained by correlating the images with a filter bank of short line segments, or “sticks” at different orientations. The proposed methods were validated on (1) accurately collected gold‐standard data consisting of a 3D kV cone‐beam CT scan of an anthropomorphic phantom of the pelvis and 2D MV portal images in the anterior–posterior (AP) view acquired at 15 different poses and (2) a conventional 3D kV CT scan and weekly 2D MV AP portal images of a patient over . The mean (and standard deviation) of the absolute registration error for rotations around the right‐lateral (RL), inferior–superior (IS), and posterior–anterior (PA) axes were 0.212° (0.214°), 0.055° (0.033°) and 0.041° (0.039°), respectively. The corresponding registration errors for translations along the RL, IS, and PA axes were , , and . The mean (and standard deviation) of the total registration error was . Registration on the patient images was successful in all eight cases as determined visually. The results indicate that it is feasible to automatically enhance features in MV portal images of the pelvis for use within a completely automated 2D–3D registration framework for the accurate determination of patient setup errors. They also indicate that it is feasible to estimate all six transformation parameters from a 3D CT of the pelvis and a single portal image in the AP view.
The accuracy of MU calculations for dynamic wedge with the Varian's Anisotropic Analytical Algorithm
Authors: Salomons G. J. , Kerr A. T. , Mei X. , Patel D.
Published: October 2008

In November 2007 Varian Medical Systems released a new version of ECLIPSE (version 8.1.17), which includes a correction to its Anisotropic Analytical Algorithm affecting monitor unit calculations for Enhanced Dynamic Wedge. The purpose of this study is to evaluate and compare the accuracy of the MU calculations for version 8.1.17 with that from the previous version (8.0.05).
Transient noise characterization and filtration in CCD cameras exposed to stray radiation from a medical linear accelerator
Authors: Archambault Louis , Briere Tina Marie , Beddar Sam
Published: October 2008

Charge coupled devices (CCDs) are being increasingly used in radiation therapy for dosimetric purposes. However, CCDs are sensitive to stray radiation. This effect induces transient noise. Radiation‐induced noise strongly alters the image and therefore limits its quantitative analysis. The purpose of this work is to characterize the radiation‐induced noise and to develop filtration algorithms to restore image quality. Two models of CCD were used for measurements close to a medical linac. The structure of the transient noise was first characterized. Then, four methods of noise filtration were compared: median filtering of a time series of identical images, uniform median filtering of single images, an adaptive filter with switching mechanism, and a modified version of the adaptive switch filter. The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: The average noise cluster size was and for the U2000 and the Luca, respectively. The median of a time series of images resulted in the best filtration and minimal image distortion. For applications where time series is impractical, the adaptive switch filter must be used to reduce image distortion. Our modified version of the switch filter can be used in order to handle nonisolated groups of noisy pixels.
The replacement correction factor for the BIPM flat cavity ion chamber and the value of
Authors: Wang L. L. W. , Rogers D. W. O.
Published: October 2008

A graphite flat cavity ionization chamber is used at the BIPM in France to determine the absorbed dose to graphite in a photon beam and thereby used to determine the product of the value of , the average energy required to produce an ion pair in dry air, and the value of , the mean restricted mass collision stopping‐power ratio for graphite to air in a beam. The accuracy of the value thus determined depends upon the accuracy of the perturbation correction factors adopted for this chamber. The perturbation effect of this chamber was accounted for by the replacement correction factor whose value was calculated by an analytical method and confirmed by an EGS4 Monte Carlo calculation. The purpose of this study is to investigate the validity of the analytical and the EGS4 calculations by using recently established methods and the EGSnrc Monte Carlo code, a much improved version of EGS4, to calculate the replacement correction factors for the graphite chamber. It is found that the replacement correction factors used for the BIPM chamber are not correct: the values used are smaller than they should be by about 1%. This leads to a 1% overestimation of the value determined by using this chamber. This implies that air kerma standards that are directly proportional to this product need to be reduced by 1%. Based on the values of the replacement correction factors calculated in this study, and on the value of evaluated from ICRU Report No. 37 stopping power for graphite, the value of determined by using the BIPM chamber should be . If a more recent value of mean excitation energy for graphite and grain density are used to evaluate the graphite stopping power, then the value obtained for is .
Prevention of post‐focal thermal damage by formation of bubbles at the focus during high intensity focused ultrasound therapy
Authors: Zderic Vesna , Foley Jessica , Luo Wenbo , Vaezy Shahram
Published: October 2008

Safety concerns exist for potential thermal damage at tissue‐air or tissue‐bone interfaces located in the post‐focal region during high intensity focused ultrasound (HIFU) treatments. We tested the feasibility of reducing thermal energy deposited at the post‐focal tissue‐air interfaces by producing bubbles (due to acoustic cavitation and/or boiling) at the HIFU focus. HIFU (in‐situ intensities of , frequencies of ) was applied for to produce lesions (in turkey breast in‐vitro , and rabbit liver and thigh muscle in‐vivo ). Tissue temperature was measured at the tissue‐air interface using a thermal (infrared) camera. Ultrasound imaging was used to detect bubbles at the HIFU focus, appearing as a hyperechoic region. In‐vitro results showed that when no bubbles were present at the focus (at lower intensities of ), the temperature at the interface increased continuously, up to above the baseline by the end of treatment. When bubbles formed immediately after the start of HIFU treatment (at the high intensity of ), the temperature increased briefly for above the baseline temperature and then decreased to above the baseline by the end of treatment. Similar results were obtained in in‐vivo experiments with the temperature increases (above the baseline temperature) at the muscle‐air and liver‐air interfaces at the end of the high intensity treatment lower by and , respectively, as compared to the low intensity treatment. Thermal effects of HIFU at post‐focal tissue‐air interfaces, such as in bowels, could result in clinically significant increases in temperature. Bubble formation at the HIFU focus may provide a method for shielding the post‐focal region from potential thermal damage.
Benchmarking of Monte Carlo simulation of bremsstrahlung from thick targets at radiotherapy energies
Authors: Faddegon Bruce A. , Asai Makoto , Perl Joseph , Ross Carl , Sempau Josep , Tinslay Jane , Salvat Francesc
Published: October 2008

Several Monte Carlo systems were benchmarked against published measurements of bremsstrahlung yield from thick targets for beams. The quantity measured was photon fluence at per unit energy per incident electron (spectra), and total photon fluence, integrated over energy, per incident electron (photon yield). Results were reported at on the beam axis for Al and Pb targets and at at angles out to 90° for Be, Al, and Pb targets. Beam energy was revised with improved accuracy of 0.5% using an improved energy calibration of the accelerator. Recently released versions of the Monte Carlo systems EGSNRC, GEANT4, and PENELOPE were benchmarked against the published measurements using the revised beam energies. Monte Carlo simulation was capable of calculation of photon yield in the experimental geometry to 5% out to 30°, 10% at wider angles, and photon spectra to 10% at intermediate photon energies, 15% at lower energies. Accuracy of measured photon yield from 0 to 30° was 5%, , increasing to 7% for the larger angles. EGSNRC and PENELOPE results were within of the measured photon yield at all beam energies and angles, GEANT4 within Photon yield at nonzero angles for angles covering conventional field sizes used in radiotherapy (out to 10°), measured with an accuracy of 3%, was calculated within of measurement for EGSNRC, for PENELOPE and GEANT4. Calculated spectra closely matched measurement at photon energies over . Photon spectra near were underestimated by as much as 10% by all three codes. The photon spectra below for the Be and Al targets and small angles were overestimated by up to 15% when using EGSNRC and PENELOPE, 20% with GEANT4. EGSNRC results with the NIST option for the bremsstrahlung cross section were preferred over the alternative cross section available in EGSNRC and over EGS4. GEANT4 results calculated with the “low energy” physics list were more accurate than those calculated with the “standard” physics list.
Novel x‐ray attenuation mechanism: Role of interatomic distance
Authors: Poelzing Steven , Smoot Adam F. , Veeraraghavan Rengasayee
Published: October 2008

Little progress has been made over the last for improving attenuation by x‐ray contrast agents, in part because the mechanisms of x‐ray attenuation are thought to be well understood. We hypothesized that x‐ray absorbance can be modulated by altering the interatomic spacing between K‐edge attenuating atoms. Iodomethane, diiodomethane, 2,6‐diiodo‐4‐nitroanaline, and diiodobenzene isomers were dissolved in DMSO and imaged with an OEC Compact 7600 fluoroscope. At a tube voltage of , absorbance of equimolar diiodomethane was significantly greater than iodomethane by 45%. Interestingly, diiodomethane absorbance was significantly greater than iodomethane (by 5%, ) despite equal amounts of iodine in both solutions. 1,3‐diiodobenzene absorbance was significantly greater than 1,2‐ and 1,4‐diiodobenzene . However, 2,6‐diiodo‐4‐nitroanaline absorbance was similar to 1,3‐diiodobenzene. When a linear model was fit for absorbance as a function of density and harmonic error (the fractional remainder of the inter‐iodine distance and the K‐shell ionizing wavelength) at different beam energies, a significant overall fit was obtained for both unfiltered and hardened beams . While the slope of absorbance as a function of harmonic error was significant for all conditions , the slope with respect to density was significant only when the beam was unfiltered . Also harmonic error, but not density, displayed significant energy‐dependent effects on absorbance . These data suggest that harmonic error is a strong determinant of absorbance, particularly when the beam energy is concentrated near the iodine K‐edge and is likely a descriptor of K‐characteristic photon interactions. Therefore, x‐ray absorbance may be modulated by the distance between covalently linked x‐ray K‐edge attenuating atoms. This finding has important implications for increasing contrast agent absorbance as well as for designing molecular transducers capable of modulating K‐edge attenuating atomic distances and thereby x‐ray absorbance.
Improving quality assurance for assembled COMS eye plaques using a pinhole gamma camera
Authors: Beiki‐Ardakani Akbar , Jezioranski John , Jaffray David A. , Yeung Ivan
Published: October 2008

A quality assurance system has been designed to verify the location and strength of seeds loaded in a brachytherapy eye plaque. This system consists of (1) a pinhole camera in conjunction with a Lumisys ACR‐2000i computed radiography (CR) unit to image the location and measure the relative strength of the seeds with autoradiography, and (2) a source strength jig with a survey meter to estimate the total activity of the seeds in the plaque. Five holders of different sizes were made for fixation of the COMS (Collaborative Ocular Melanoma Study) plaques (12, 14, 16, 18, and ) in the camera. The plaque‐to‐pinhole distance has been optimized to be to give approximately uniform intensity on the CR image for uniformly loaded COMS plaques. The pinhole‐to‐detector distance can be kept at either for 1:1 scale, or at larger distances for higher magnification. For a 1:1 scaling and pinhole diameter of , useful images are obtained with time‐activity product (mCi sec) ranging from . Within this range, the pinhole system is able to differentiate seed activities of . The resulting pinhole autoradiograph is able to (1) confirm the correct number of seeds loaded in the plaque, (2) verify the proper sitting of the seeds in the silastic carrier and the plaque, (3) verify the relative activity distribution of the seeds loaded in the plaque, and (4) potentially evaluate the integrity of the seed. The source strength measurement system is able to measure the total strength of seeds in the plaque ranging from with an uncertainty of 5%.
Singular value description of a digital radiographic detector: Theory and measurements
Authors: Kyprianou Iacovos S. , Badano Aldo , Gallas Brandon D. , Myers Kyle J.
Published: October 2008

The operator represents the deterministic performance of any imaging system. For a linear, digital imaging system, this system operator can be written in terms of a matrix, , that describes the deterministic response of the system to a set of point objects. A singular value decomposition of this matrix results in a set of orthogonal functions (singular vectors) that form the system basis. A linear combination of these vectors completely describes the transfer of objects through the linear system, where the respective singular values associated with each singular vector describe the magnitude with which that contribution to the object is transferred through the system. This paper is focused on the measurement, analysis, and interpretation of the matrix for digital x‐ray detectors. A key ingredient in the measurement of the matrix is the detector response to a single x ray (or infinitestimal x‐ray beam). The authors have developed a method to estimate the 2D detector shift‐variant, asymmetric ray response function (RRF) from multiple measured line response functions (LRFs) using a modified edge technique. The RRF measurements cover a range of x‐ray incident angles from 0° (equivalent location at the detector center) to 30° (equivalent location at the detector edge) for a standard radiographic or cone‐beam CT geometric setup. To demonstrate the method, three beam qualities were tested using the inherent, , and Yb beam filtration. The authors show that measures using the LRF, derived from an edge measurement, underestimate the system's performance when compared with the matrix derived using the RRF. Furthermore, the authors show that edge measurements must be performed at multiple directions in order to capture rotational asymmetries of the RRF. The authors interpret the results of the matrix SVD and provide correlations with the familiar MTF methodology. Discussion is made about the benefits of the matrix technique with regards to signal detection theory, and the characterization of shift‐variant imaging systems.
Using a Monte Carlo model to predict dosimetric properties of small radiotherapy photon fields
Authors: Scott Alison J. D. , Nahum Alan E. , Fenwick John D.
Published: October 2008

Accurate characterization of small‐field dosimetry requires measurements to be made with precisely aligned specialized detectors and is thus time consuming and error prone. This work explores measurement differences between detectors by using a Monte Carlo model matched to large‐field data to predict properties of smaller fields. Measurements made with a variety of detectors have been compared with calculated results to assess their validity and explore reasons for differences. Unshielded diodes are expected to produce some of the most useful data, as their small sensitive cross sections give good resolution whilst their energy dependence is shown to vary little with depth in a linac beam. Their response is shown to be constant with field size over the range , with a correction of 3% needed for a field size of . BEAMnrc has been used to create a beam model, matched to dosimetric data for square fields larger than , and producing small‐field profiles and percentage depth doses (PDDs) that agree well with unshielded diode data for field sizes down to . For fields sizes of and above, little detector‐to‐detector variation exists in measured output factors, however for a field a relative spread of 18% is seen between output factors measured with different detectors—values measured with the diamond and pinpoint detectors lying below that of the unshielded diode, with the shielded diode value being higher. Relative to the corrected unshielded diode measurement, the Monte Carlo modeled output factor is 4.5% low, a discrepancy that is probably due to the focal spot fluence profile and source occlusion modeling. The large‐field Monte Carlo model can, therefore, currently be used to predict small‐field profiles and PDDs measured with an unshielded diode. However, determination of output factors for the smallest fields requires a more detailed model of focal spot fluence and source occlusion.
Quantitative comparison of direct phase retrieval algorithms in in‐line phase tomography
Authors: Langer Max , Cloetens Peter , Guigay Jean‐Pierre , Peyrin Françoise
Published: October 2008

A well‐known problem in x‐ray microcomputed tomography is low sensitivity. Phase contrast imaging offers an increase of sensitivity of up to a factor of in the hard x‐ray region, which makes it possible to image soft tissue and small density variations. If a sufficiently coherent x‐ray beam, such as that obtained from a third generation synchrotron, is used, phase contrast can be obtained by simply moving the detector downstream of the imaged object. This setup is known as in‐line or propagation based phase contrast imaging. A quantitative relationship exists between the phase shift induced by the object and the recorded intensity and inversion of this relationship is called phase retrieval. Since the phase shift is proportional to projections through the three‐dimensional refractive index distribution in the object, once the phase is retrieved, the refractive index can be reconstructed by using the phase as input to a tomographic reconstruction algorithm. A comparison between four phase retrieval algorithms is presented. The algorithms are based on the transport of intensity equation (TIE), transport of intensity equation for weak absorption, the contrast transfer function (CTF), and a mixed approach between the CTF and TIE, respectively. The compared methods all rely on linearization of the relationship between phase shift and recorded intensity to yield fast phase retrieval algorithms. The phase retrieval algorithms are compared using both simulated and experimental data, acquired at the European Synchrotron Radiation Facility third generation synchrotron light source. The algorithms are evaluated in terms of two different reconstruction error metrics. While being slightly less computationally effective, the mixed approach shows the best performance in terms of the chosen criteria.
Magnetic resonance spectroscopy
Authors: Prost Robert W.
Published: October 2008

The nuclear magnetic resonance phenomenon has given rise to both magnetic resonance imaging, which yields morphologic data, and magnetic resonance spectroscopy (MRS), which yields chemical data. In humans these data are derived principally from the resonances of the hydrogen nucleus in the low molecular weight compounds in the body. Hydrogen MRS has become a routinely used clinical tool in the brain, prostate, and breast. Other nuclei also demonstrate this phenomenon but each of these comes with additional difficulties, including low abundance, low sensitivity, and/or low chemical concentrations. The future of MRS includes a drive to higher main magnetic field strengths and new methods to create 4–5 orders of magnitude greater signal. The future of MRS is bright, but in the United States it is endangered by overuse and misuse driven by the advent of reimbursement.
Design and feasibility of active matrix flat panel detector using avalanche amorphous selenium for protein crystallography
Authors: Sultana Afrin , Reznik Alla , Karim Karim S. , Rowlands J. A.
Published: October 2008

Protein crystallography is the most important technique for resolving the three‐dimensional atomic structure of protein by measuring the intensity of its x‐ray diffraction pattern. This work proposes a large area flat panel detector for protein crystallography based on direct conversion x‐ray detection technique using avalanche amorphous selenium as the high gain photoconductor, and active matrix readout using amorphous silicon thin film transistors. The detector employs avalanche multiplication phenomenon of to make the detector sensitive to each incident x ray. The advantages of the proposed detector over the existing imaging plate and charge coupled device detectors are large area, high dynamic range coupled to single x‐ray detection capability, fast readout, high spatial resolution, and inexpensive manufacturing process. The optimal detector design parameters (such as detector size, pixel size, and thickness of layer), and operating parameters (such as electric field across the layer) are determined based on the requirements for protein crystallography application. The performance of the detector is evaluated in terms of readout time , dynamic range , and sensitivity ( x‐ray photon), thus validating the detector's efficacy for protein crystallography.
Automatic exposure control in pediatric and adult multidetector CT examinations: A phantom study on dose reduction and image quality
Authors: Papadakis Antonios E. , Perisinakis Kostas , Damilakis John
Published: October 2008

The aim of this study was to assess the potential of a modern modulation‐based automatic exposure control system (AEC) for dose reduction in pediatric and adult multidetector CT (MDCT) imaging and evaluate the quality of the images obtained. Five physical anthropomorphic phantoms that simulate the average individual as neonate, 1‐, 5‐, old child, and adult were scanned with a MDCT scanner, equipped with a modern AEC system. Dose reduction (%DR) was calculated as the percentage difference of the mean modulated and the preset tube current‐time product that is prescribed for standard head and body scan protocols. The effect of the tube potential and the orientation of the topogram acquisition on dose reduction were assessed. Image quality was evaluated on the basis of image noise and signal to noise ratio (SNR). The dose reduction values achieved in pediatric phantoms were remarkably lower than those achieved for the adult. The efficiency of the AEC is decreased at compared to higher tube potentials and for helical scans following an anterior posterior (AP‐AEC) compared to a lateral (LAT‐AEC) topogram acquisition. In AP‐AEC scans, the dose reduction ranged between 4.7 and 34.7% for neonate, 15.4 and 30.9% for old, 3.1 and 26.7% for old, 1.2 and 58.7% for old, and 15.5 and 57.4% for adult. In LAT‐AEC scans, the corresponding dose reduction ranged between 11.0 and 36.5%, 27.2 and 35.7%, 11.3 and 35.6%, 0.3 and 67.0%, and 15.0 and 61.7%, respectively. AP‐AEC scans resulted in a 17.1% and 19.7% dose increase in the thorax of neonate and the pelvis of the old phantom, respectively. The variation in the measured noise among images obtained along the scanning axis was lower in AEC activated compared to fixed milliamperes scans. However, image noise was significantly increased and SNR significantly decreased in most AEC activated compared to fixed milliamperes scans. In conclusion, AEC resulted in a (i) substantial dose reduction, which is less pronounced in children compared to adult, (ii) higher dose reduction in scans following a lateral compared to scans following an anterior‐posterior topogram acquisition, (iii) increase of image noise and degradation of SNR in the obtained images compared to the fixed milliamperes technique.
Normalization of the modulation transfer function: The open‐field approach
Authors: Friedman S. N. , Cunningham I. A.
Published: October 2008

The modulation transfer function (MTF) is widely used to describe the spatial resolution of x‐ray imaging systems. The MTF is defined to have a zero‐frequency value of unity, and it is common practice to ensure this by normalizing a measured MTF curve by the zero‐frequency value. However, truncation of the line spread function (LSF) within a finite region of interest (ROI) results in spectral leakage and causes a reduction in the measured MTF zero‐frequency value equal to the area of truncated LSF tails. Subsequent normalization by this value may result in inflated MTF values. We show that open‐field normalization with the edge method produces accurate MTF values at all nonzero frequencies without need for further normalization by the zero‐frequency value, regardless of ROI size. While both normalization techniques are equivalent for a sufficiently large ROI, a 5% inflation in MTF values was observed for a CsI‐based flat‐panel system when using a ROI. Use of open‐field normalization avoids potential inflation caused by zero‐frequency normalization.
Anniversary Paper: Image processing and manipulation through the pages ofMedical Physics
Authors: Armato Samuel G. , van Ginneken Bram
Published: October 2008

The language of radiology has gradually evolved from “the film” (the foundation of radiology since Wilhelm Roentgen's 1895 discovery of x‐rays) to “the image,” an electronic manifestation of a radiologic examination that exists within the bits and bytes of a computer. Rather than simply storing and displaying radiologic images in a static manner, the computational power of the computer may be used to enhance a radiologist's ability to visually extract information from the image through image processing and image manipulation algorithms. Image processing tools provide a broad spectrum of opportunities for image enhancement. Gray‐level manipulations such as histogram equalization, spatial alterations such as geometric distortion correction, preprocessing operations such as edge enhancement, and enhanced radiography techniques such as temporal subtraction provide powerful methods to improve the diagnostic quality of an image or to enhance structures of interest within an image. Furthermore, these image processing algorithms provide the building blocks of more advanced computer vision methods. The prominent role of medical physicists and the AAPM in the advancement of medical image processing methods, and in the establishment of the “image” as the fundamental entity in radiology and radiation oncology, has been captured in 35 volumes ofMedical Physics.
Effects of tumor motion in GRID therapy
Authors: Naqvi Shahid A. , Mohiuddin Majid M. , Ha Jonathan K. , Regine William F.
Published: October 2008

Clinical and biological evidence suggest that the success of GRID therapy in debulking large tumors depends on the high peak‐to‐valley contrast in the dose distribution. In this study, we show that the peaks and valleys can be significantly blurred out by respiration‐induced tumor motion, possibly affecting the clinical outcome. Using a kernel‐based Monte Carlo dose engine that incorporates phantom motion, we calculate the dose distributions for a GRID with hexagonally arranged holes. The holes have a diameter of and a minimum center‐to‐center separation of (projected at the isocenter). The phantom moves either in the direction, which is parallel to a line joining any two nearest neighbors, or in the perpendicular direction. The displacement‐time waveform is modeled with a function, with assigned 1 for symmetric motion, or 6 to simulate a large inhale‐exhale asymmetry. Dose calculations are performed on a water phantom for a x‐ray beam. Near , the static valley dose is , where is the peak static dose. For motion in the direction, the peak and valley doses vary periodically with the amplitude of motion and the transverse dose profiles are maximally flat near and . For the cos waveform, the minimum peak dose is and the maximum valley dose is . Less dose blurring is seen with the waveform, with and . For motion in the direction, the maximum flattening of dose profiles occurs at . GRIDs with smaller hole separations produce similar blurring at proportionally smaller amplitudes. The reported clinical response data from GRID therapy seem to indicate that mobile tumors, such as those in the thorax and abdomen, respond worse to GRID treatments than stationary tumors, such as those in the head and neck. To establish a stronger correlation between clinical response and tumor motion, and possibly improve the clinical response rates, it is recommended that prospective GRID therapy trials be conducted with motion compensation strategies, such as respiratory gating.
Design and evaluation of a 3D transrectal ultrasound prostate biopsy system
Authors: Cool Derek , Sherebrin Shi , Izawa Jonathan , Chin Joseph , Fenster Aaron
Published: October 2008

Biopsy of the prostate using ultrasound guidance is the clinical gold standard for diagnosis of prostate adenocarinoma. The current prostate biopsy procedure is limited to using 2D transrectal ultrasound (TRUS) images to target biopsy sites and record biopsy core locations for postbiopsy confirmation. Localization of the 2D image in its actual 3D position is ambiguous and limits procedural accuracy and reproducibility. We have developed a 3D TRUS prostate biopsy system that provides 3D intrabiopsy information for needle guidance and biopsy location recording. The system conforms to the workflow and imaging technology of the current biopsy procedure, making it easier for clinical integration. In this paper, we describe the system design and validate the system accuracy by performing mock biopsies on US/CT multimodal patient‐specific prostate phantoms. Our biopsy system generated 3D patient‐specific models of the prostate with volume errors less than 3.5% and mean boundary errors of less than . Using the 3D biopsy system, needles were guided to within of 3D targets and with a high probability of biopsying clinically significant tumors. The positions of the actual biopsy sites were accurately localized to within .
The effect of breast compression on mass conspicuity in digital mammography
Authors: Saunders Robert S. , Samei Ehsan
Published: October 2008

This study analyzed how the inherent quality of diagnostic information in digital mammography could be affected by breast compression. A digital mammography system was modeled using a Monte Carlo algorithm based on the Penelope program, which has been successfully used to model several medical imaging systems. First, the Monte Carlo program was validated against previous measurements and simulations. Once validated, the Monte Carlo software modeled a digital mammography system by tracking photons through a voxelized software breast phantom, containing anatomical structures and breast masses, and following photons until they were absorbed by a selenium‐based flat‐panel detector. Simulations were performed for two compression conditions (standard compression and 12.5% reduced compression) and three photon flux conditions (constant flux, constant detector signal, and constant glandular dose). The results showed that reduced compression led to higher scatter fractions, as expected. For the constant photon flux condition, decreased compression also reduced glandular dose. For constant glandular dose, the SdNR for a breast was and under standard and reduced compressions, respectively. For the case with constant glandular dose, the SdNR was and under standard and reduced compressions, respectively. The results suggest that if a particular imaging system can handle an approximately 10% increase in total tube output and 10% decrease in detector signal, breast compression can be reduced by about 12% in terms of breast thickness with little impact on image quality or dose.
Mutual‐information‐based image to patient re‐registration using intraoperative ultrasound in image‐guided neurosurgery
Authors: Ji Songbai , Wu Ziji , Hartov Alex , Roberts David W. , Paulsen Keith D.
Published: October 2008

An image‐based re‐registration scheme has been developed and evaluated that uses fiducial registration as a starting point to maximize the normalized mutual information (nMI) between intraoperative ultrasound (iUS) and preoperative magnetic resonance images (pMR). We show that this scheme significantly reduces tumor boundary misalignment between iUS pre‐durotomy and pMR from an average of in six resection surgeries. The corrected tumor alignment before dural opening provides a more accurate reference for assessing subsequent intraoperative tumor displacement, which is important for brain shift compensation as surgery progresses. In addition, we report the translational and rotational capture ranges necessary for successful convergence of the nMI registration technique ( and , respectively). The proposed scheme is automatic, sufficiently robust, and computationally efficient , and holds promise for routine clinical use in the operating room during image‐guided neurosurgical procedures.
Streaking artifacts reduction in four‐dimensional cone‐beam computed tomography
Authors: Leng Shuai , Zambelli Joseph , Tolakanahalli Ranjini , Nett Brian , Munro Peter , Star‐Lack Joshua , Paliwal Bhudatt , Chen Guang‐Hong
Published: October 2008

Cone‐beam computed tomography (CBCT) using an “on‐board” x‐ray imaging device integrated into a radiation therapy system has recently been made available for patient positioning, target localization, and adaptive treatment planning. One of the challenges for gantry mounted image‐guided radiation therapy (IGRT) systems is the slow acquisition of projections for cone‐beam CT (CBCT), which makes them sensitive to any patient motion during the scans. Aiming at motion artifact reduction, four‐dimensional CBCT (4D CBCT) techniques have been introduced, where a surrogate for the target's motion profile is utilized to sort the cone‐beam data by respiratory phase. However, due to the limited gantry rotation speed and limited readout speed of the on‐board imager, fewer than 100 projections are available for the image reconstruction at each respiratory phase. Thus, severe undersampling streaking artifacts plague 4D CBCT images. In this paper, the authors propose a simple scheme to significantly reduce the streaking artifacts. In this method, a prior image is first reconstructed using all available projections without gating, in which static structures are well reconstructed while moving objects are blurred. The undersampling streaking artifacts from static structures are estimated from this prior image volume and then can be removed from the phase images using gated reconstruction. The proposed method was validated using numerical simulations, experimental phantom data, and patient data. The fidelity of stationary and moving objects is maintained, while large gains in streak artifact reduction are observed. Using this technique one can reconstruct 4D CBCT datasets using no more projections than are acquired in a scan. At the same time, a temporal gating window as narrow as was utilized. Compared to the conventional 4D CBCT reconstruction, streaking artifacts were reduced by 60% to 70%.
Three‐dimensional image registration of MR proximal femur images for the analysis of trabecular bone parameters
Authors: Blumenfeld Janet , Studholme Colin , Carballido‐Gamio Julio , Carpenter Dana , Link Thomas M. , Majumdar Sharmila
Published: October 2008

This study investigated the feasibility of automatic image registration of MR high‐spatial resolution proximal femur trabecular bone images as well as the effects of gray‐level interpolation and volume of interest (VOI) misalignment on MR‐derived trabecular bone structure parameters. For six subjects in a short‐term study, a baseline scan and a follow‐up scan of the proximal femur were acquired on the same day. For ten subjects in a long‐term study, a follow‐up scan of the proximal femur was acquired after the baseline. An automatic image registration technique, based on mutual information, utilized a baseline and a follow‐up scan to compute transform parameters that aligned the two images. In the short‐term study, these parameters were subsequently used to transform the follow‐up image with three different gray‐level interpolators. Nearest‐neighbor interpolation and ‐spline approximation did not significantly alter bone parameters, while linear interpolation significantly modified bone parameters . Improvement in image alignment due to the automatic registration for the long‐term and short‐term study was determined by inspecting difference images and 3D renderings. This work demonstrates the first application of automatic registration, without prior segmentation, of high‐spatial resolution trabecular bone MR images of the proximal femur. Additionally, inherent heterogeneity in trabecular bone structure and imprecise positioning of the VOI along the slice (anterior–posterior) direction resulted in significant changes in bone parameters . Results suggest that automatic mutual information registration using ‐spline approximation or nearest neighbor gray‐level interpolation to transform the final image ensures VOI alignment between baseline and follow‐up images and does not compromise the integrity of MR‐derived trabecular bone parameters used in this study.