Volume 37, Issue 5, May 2010
Index of content:
Most residency programs for radiation oncology physicists do not reflect the heightened importance of medical imaging37(2010); http://dx.doi.org/10.1118/1.3355935View Description Hide Description
- RADIATION THERAPY PHYSICS
Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies37(2010); http://dx.doi.org/10.1118/1.3373523View Description Hide DescriptionPurpose:
Since the Gafchromic film EBT has been recently replaced by the newer model EBT2, its characterization, especially energy dependence, has become critically important. The energy dependence of the dose response of Gafchromic EBT2 film is evaluated for a broad range of energies from different radiation sources used in radiation therapy.Methods:
The beams used for this study comprised of kilovoltage x rays (75, 125, and 250 kVp), gamma (662 KeV), gamma (1.17–1.33 MeV), megavoltage x rays (6 and 18 MV), electron beams (6 and 20 MeV), and protonbeams (100 and 250 MeV). The film’s response to each of the above energies was measured over the dose range of 0.4–10 Gy, which corresponds to optical densities ranging from 0.05 to 0.74 for the film reader used.Results:
The energy dependence of EBT2 was found to be relatively small within measurement uncertainties for all energies and modalities.Conclusion:
For relative and absolute dosimetry of radiation therapybeams, the weak energy dependence of the EBT2 makes it most suitable for clinical use compared to other films.
Four-dimensional intensity-modulated radiation therapy planning for dynamic tracking using a direct aperture deformation (DAD) method37(2010); http://dx.doi.org/10.1118/1.3319498View Description Hide DescriptionPurpose:
Planning for the delivery of intensity-modulated radiation therapy(IMRT) to a moving target, referred to as four-dimensional (4D) IMRT planning, is a crucial step for achieving the treatment objectives for sites that move during treatmentdelivery. The authors proposed a simplistic method that accounts for both rigid and nonrigid respiration-induced target motion based on 4D computed tomography (4DCT) data sets.Methods:
A set of MLC apertures and weights was first optimized on a reference phase of a 4DCT data set. At each beam angle, the apertures were morphed from the reference phase to each of the remaining phases according to the relative shape changes in the beam’s eye view of the target. Three different planning schemes were evaluated for two lung cases and one pancreas patient: (1) Individually optimizing each breathing phase; (2) optimizing the reference phase and shifting the optimized apertures to other breathing phases based on a rigid-body image registration; and (3) optimizing the reference phase and deforming the optimized apertures to the other phases based on the deformation and translation of target contours. Planning results using scheme 1 serves as the “gold standard” for plan quality assessment; scheme 2 is the method previously proposed in the literature; and scheme 3 is the method the authors proposed in this article. The optimization results were compared between the three schemes for all three cases.Results:
The proposed scheme 3 is comparable to scheme 1 in plan quality, and provides improved target coverage and conformity with similar normal tissue dose compared with scheme 2.Conclusions:
Direct aperture deformation method for 4D IMRT planning improves upon methods that only consider rigid-body motion and achieves a plan quality close to that optimized for each of the phases.
Beam-centric algorithm for pretreatment patient position correction in external beam radiation therapy37(2010); http://dx.doi.org/10.1118/1.3327457View Description Hide DescriptionPurpose:
In current image guided pretreatment patient position adjustment methods, image registration is used to determine alignment parameters. Since most positioning hardware lacks the full six degrees of freedom (DOF), accuracy is compromised. The authors show that such compromises are often unnecessary when one models the planned treatment beams as part of the adjustment calculation process. The authors present a flexible algorithm for determining optimal realizable adjustments for both step-and-shoot and arc delivery methods.Methods:
The beam shape model is based on the polygonal intersection of each beam segment with the plane in pretreatment image volume that passes through machine isocenter perpendicular to the central axis of the beam. Under a virtual six-DOF correction, ideal positions of these polygon vertices are computed. The proposed method determines the couch, gantry, and collimator adjustments that minimize the total mismatch of all vertices over all segments with respect to their ideal positions. Using this geometric error metric as a function of the number of available DOF, the user may select the most desirable correction regime.Results:
For a simulated treatment plan consisting of three equally weighted coplanar fixed beams, the authors achieve a 7% residual geometric error (with respect to the ideal correction, considered 0% error) by applying gantry rotation as well as translation and isocentric rotation of the couch. For a clinical head-and-neck intensity modulated radiotherapy plan with seven beams and five segments per beam, the corresponding error is 6%. Correction involving only couch translation (typical clinical practice) leads to a much larger 18% mismatch. Clinically significant consequences of more accurate adjustment are apparent in the dose volume histograms of target and critical structures.Conclusions:
The algorithm achieves improvements in delivery accuracy using standard delivery hardware without significantly increasing total treatment session duration. It encourages parsimonious utilization of all available DOF. Finally, in certain cases, it obviates the need of a robotic couch having six DOF for the correction of patient displacement and rotations.
Experimental and theoretical dosimetry of a new polymer encapsulated iodine-125 source—SmartSeed: Dosimetric impact of fluorescence x rays37(2010); http://dx.doi.org/10.1118/1.3377750View Description Hide Description
Purpose: The detailed study of a new permanent iodine-125 brachytherapy source, SmartSeed, is presented in this article. It is the first iodine seed made with biocompatible polymer and is manufactured by the IBt-Bebig group.
Methods: Three dosimetric studies have been performed: The first one used thermoluminescent detectors in a solid water phantom with NIST (National Institute of Standards and Technology, USA) calibrated seeds, and two separate studies were of Monte Carlophoton transport calculations (MCNP5 code). The TG-43U1 protocol was applied to derive dosimetric parameters for clinical applications.
Results: The radial dose function was determined at different distances ranging from 0.5 to 10 cm; and the anisotropy function at angles ranging from 0° to 350° in 10° increments. Monte Carlo calculations were performed in liquid water to obtain values for , , , and as recommended by the TG-43U1 protocol for use in treatment planning system software. SmartSeed’s biocompatible polymer capsule permits fluorescence x rays (3, 5, and 12 keV), generated by lead glass marker, to be present in the emission spectrum, influencing the dose rate constant. The impact on near field dosimetry in water from these x rays was also investigated and reported. The capsule also attenuates iodine-125 energies much less than typical titanium encased sources, resulting in a highly isotropic source.
Conclusions: SmartSeed has a dose rate constant of , a radial dose function nearly identical to the IBt-Bebig model I25.S06 seed, and a highly isotropic dose distribution. Fluorescence x rays account for the relatively low value of , yet their variable contribution to dosimetry arising from seed dimensional uncertainties is estimated to be .
37(2010); http://dx.doi.org/10.1118/1.3373519View Description Hide DescriptionPurpose:
The beam model in a three dimensional treatment planning system (TPS) defines virtually the mechanical and dosimetric characteristics of a treatment unit. The manual optimization of a beam model during commissioning can be a time consuming task due to its iterative nature. Furthermore, the quality of the beam model commissioning depends on the user’s ability to manage multiple parameters and assess their impact on the agreement between measured and calculated dose. The objective of this work is to develop and validate the performance of an automated beam model optimization system (ABMOS) based on intensity modulated radiotherapy(IMRT) beam measurements to improve beam model accuracy while streamlining the commissioning process.Methods:
The ABMOS was developed to adjust selected TPS beam model parameters iteratively to maximize the agreement between measured and calculated 2D dose maps obtained for an IMRT beam pattern. A 2D diode array with high spatial resolution detectors was used to sample the entire IMRT beam pattern in a single dose measurement. The use of an IMRT beam pattern with large number of monitor units was selected to highlight the difference between planned and delivered dose and improve the signal to noise ratio in the low dose regions. ABMOS was applied to the optimization of a beam model for an Elekta Synergy S treatment unit. The optimized beam model was validated for two anatomical sites (25 paraspinal and 25 prostate cases) using two independent patient-specific IMRT quality control (QC) methods based on ion chamber and 2D diode array measurements, respectively. The conventional approach of comparing calculated and measured beam profiles and percent-depth dose curves was also used to assess improvement in beam model after ABMOS optimization. Elements of statistical process control were applied to the process of patient-specific QC performed with the ion chamber and the 2D array to complement the model comparison.Results:
After beam model optimization with ABMOS, improvement in planned to delivered dose agreement was demonstrated with both patient-specific IMRT QC methods and the calculated to measured profile comparison. In terms of ion chamber measurements, the largest improvement was observed for the paraspinal cases with the mean measured to calculated dose difference at the low dose points decreasing from −13.8% to 2.0% with the optimized beam model. The 2D diode array patient-specific QC also demonstrated clearly the improvement in beam model for both paraspinal and prostate cases with, on average, more than 96% of the diodes satisfying tolerances of 3% of dose difference or 2 mm of distance to agreement after ABMOS optimization. The capability index for both patient-specific QC methods also increased with the optimized beam model.Conclusions:
In this work, ABMOS was developed to use 2D diode array measurements of an IMRT beam pattern for the automated multivariable optimization of a TPS beam model. Based on the observed improvements in patient-specific QC results for 25 paraspinal and 25 prostate plans, optimization of the remaining clinical beam models using ABMOS is now ongoing in the institution.
37(2010); http://dx.doi.org/10.1118/1.3392165View Description Hide DescriptionPurpose:
Linac-MR systems for real-time image-guidedradiotherapy will utilize the multileaf collimators (MLCs) to perform conformal radiotherapy and tumor tracking. The MLCs would be exposed to the external fringe magnetic fields of the linac-MR hybrid systems. Therefore, an experimental investigation of the effect of an external magnetic field on the brushed permanent magnet DC motors used in some MLC systems was performed.Methods:
The changes in motor speed and current were measured for varying external magnetic field strengths up to 2000 G generated by an EEV electromagnet. These changes in motor characteristics were measured for three orientations of the motor in the external magnetic field, mimicking changes in motor orientations due to installation and/or collimator rotations. In addition, the functionality of the associated magnetic motor encoder was tested. The tested motors are used with the Varian 120 leaf Millennium MLC (Maxon Motor half leaf and full leaf motors) and the Varian 52 leaf MKII MLC (MicroMo Electronics leaf motor) including a carriage motor (MicroMo Electronics).Results:
In most cases, the magnetic encoder of the motors failed prior to any damage to the gearbox or the permanent magnet motor itself. This sets an upper limit of the external magnetic field strength on the motor function. The measured limits of the external magnetic fields were found to vary by the motor type. The leaf motor used with a Varian 52 leaf MKII MLC system tolerated up to. The carriage motor tolerated up to field. The motors used with the Varian 120 leaf Millennium MLC system were found to tolerate a maximum of .Conclusions:
The current Varian MLC system motors can be used for real-time image-guidedradiotherapy coupled to a linac-MR system, provided the fringe magnetic fields at their locations are below the determined tolerance levels. With the fringe magnetic fields of linac-MR systems expected to be larger than the tolerance levels determined, some form of magnetic shielding would be required.
37(2010); http://dx.doi.org/10.1118/1.3388869View Description Hide DescriptionPurpose:
The aim of this study is to evaluate the dosimetric performance of a newly developed proton-sensitive polymergel formulation for proton therapydosimetry.Methods:
Using passive scattered modulated and nonmodulated proton beams, the dose response of the gel was assessed. A next-generation optical CT scanner is used as the readout mechanism of the radiation-induced absorbance in the gel medium. Comparison of relative dose profiles in the gel to ion chamber profiles in water is performed. A simple and easily reproducible calibration protocol is established for routine gel batch calibrations. Relative stopping power ratio measurement of the gel medium was performed to ensure accurate water-equivalent depth dose scaling. Measured dose distributions in the gel were compared to treatment planning system for benchmark irradiations and quality of agreement is assessed using clinically relevant gamma index criteria.Results:
The dosimetric response of the gel was mapped up to 600 cGy using an electron-based calibration technique. Excellent dosimetric agreement is observed between ion chamber data and gel. The most notable result of this work is the fact that this gel has no observed dose quenching in the Bragg peak region. Quantitative dose distribution comparisons to treatment planning system calculations show that most of the geldose maps pass the 3%/3 mm gamma criterion.Conclusions:
This study shows that the new proton-sensitive geldosimeter is capable of reproducing ion chamberdose data for modulated and nonmodulated Bragg peak beams with different clinical beam energies. The findings suggest that the geldosimeter can be used as QA tool for millimeter range verification of proton beam deliveries in the dosimeter medium.
Homogenized blocked arcs for multicriteria optimization of radiotherapy: Analytical and numerical solutions37(2010); http://dx.doi.org/10.1118/1.3377771View Description Hide DescriptionPurpose:
Homogenized blocked arcs are intuitively appealing as basis functions for multicriteria optimization of rotational radiotherapy. Such arcs avoid an organ-at-risk (OAR), spread dose out well over the rest-of-body (ROB), and deliver homogeneous doses to a planning target volume (PTV) using intensity modulated fluence profiles, obtainable either from closed-form solutions or iterative numerical calculations. Here, the analytic and iterative arcs are compared.Methods:
Dose-distributions have been calculated for nondivergent beams, both including and excluding scatter, beam penumbra, and attenuation effects, which are left out of the derivation of the analytic arcs. The most straightforward analytic arc is created by truncating the well-known Brahme, Roos, and Lax (BRL) solution, cutting its uniform dose region down from an annulus to a smaller nonconcave region lying beyond the OAR. However, the truncation leaves behind high dose hot-spots immediately on either side of the OAR, generated by very high BRL fluence levels just beyond the OAR. These hot-spots can be eliminated using alternative analytical solutions “C” and “L,” which, respectively, deliver constant and linearly rising fluences in the gap region between the OAR and PTV (before truncation).Results:
Measured in terms of PTV dose homogeneity, ROB dose-spread, and OAR avoidance, C solutions generate better arc dose-distributions than L when scatter, penumbra, and attenuation are left out of the dose modeling. Including these factors, L becomes the best analytical solution. However, the iterative approach generates better dose-distributions than any of the analytical solutions because it can account and compensate for penumbra and scatter effects. Using the analytical solutions as starting points for the iterative methodology, dose-distributions almost as good as those obtained using the conventional iterative approach can be calculated very rapidly.Conclusions:
The iterative methodology is appropriate and useful for computing homogenized blocked arcs, as it produces better dose-distributions than the analytic approaches and their obvious extensions, and can more straightforwardly be used to generate homogenized arcs for concave OARs. However, the analytical solutions provide promising starting points for the iterative algorithm, leading to fast convergence.
37(2010); http://dx.doi.org/10.1118/1.3395573View Description Hide DescriptionPurpose:
To develop tools to plan modulated electron radiotherapy (MERT) and to compare the MERT plans to conventional or intensity modulated radiotherapy(IMRT) treatment plans.Methods:
Monte Carlo dose calculations of electron fields shaped with the inherent photonmultileaf collimators(MLCs) were investigated in this study. Treatment plans for four postmastectomy breast cancer patients were generated using MERT. The distances from the patient skin surfaces to the distal planning target volume surfaces were computed along the beam axis direction to determine the physical depth. Electron beam energies were selected to provide target coverage at these depths and energy bins were generated. A custom built MERT treatment planning graphical user interface (MERTgui) was used to shape the electron bins into deliverable electron segments. Monte Carlo dose distribution simulations were performed using the MLC-defined segments generated from the MERTgui. A custom built superpositiongui was used to combine doses for each segment using relative weights and final MERT treatment plans were compared to the conventional or IMRT treatment plans. In addition, a demonstration of combined MERT and IMRT treatment plans was performed.Results:
The MERT treatment plans provided acceptable target organ coverage in all cases. Relative to 3D conventional or IMRT treatment plans, the MERT plans predicted lower heartdoses in all cases; average of the heart of all plans was reduced from 14.1 to 3.3 Gy. The contralateral breast and contralateral lungdoses decreased substantially with MERT planning compared to IMRT (on average, contralateral breast heart was reduced from 8.7 to 0.7 Gy and contralateral lung was reduced from 8.4 to 1.2 Gy with MERT). Ipsilateral lung was lower with MERT than with the conventional plans (44.6 vs 29.2 Gy with MERT), but greater when compared against IMRT treatment plans (25.4 vs 28.9 Gy with MERT). A MERT and IMRT combination plan was generated to benefit from the complementary advantages of MERT and IMRT, resulting in satisfactory target coverage and reduced organ at risk doses.Conclusions:
MERT tools can facilitate treatment planning and provide plans for treatment of shallow targets such as the postmastectomy chest wall.
A simple method to quantify the coincidence between portal image graticules and radiation field centers or radiation isocenter37(2010); http://dx.doi.org/10.1118/1.3397452View Description Hide DescriptionPurpose:
The aim of this study was to develop a computerized method to quantify the coincidence between portal image graticules and radiation field centers or radiation isocenter. Three types of graticules were included in this study: Megavoltage (MV) mechanical graticule, MV electronic portal imaging device digital graticule, and kilovoltage (kV) on-board imaging digital graticule.Methods:
A metal ball bearing (BB) was imaged with MV and kV x-ray beams in a procedure similar to a Winston–Lutz test. The radiation fields, graticules, and BB were localized in eight portal images using Hough transform-based computer algorithms. The center of the BB served as a static reference point in the 3D space so that the distances between the graticule centers and the radiation field centers were calculated. The radiation isocenter was determined from the radiation field centers at different gantry angles.Results:
Misalignments of MV and kV portal imaging graticules varied with the gantry or x-ray source angle as a result of mechanical imperfections of the linear accelerator and its imaging system. While the three graticules in this study were aligned to the radiation field centers and the radiation isocenter within 2.0 mm, misalignments of 1.5–2.0 mm were found at certain gantry angles. These misalignments were highly reproducible with the gantry rotation.Conclusions:
A simple method was developed to quantify the alignments of portal image graticules directly against the radiation field centers or the radiation isocenter. The advantage of this method is that it does not require the BB to be placed exactly at the radiation isocenter through a precalibrated surrogating device such as room lasers or light field crosshairs. The present method is useful for radiation therapy modalities that require high-precision portal imaging such as image-guidedstereotactic radiotherapy.
37(2010); http://dx.doi.org/10.1118/1.3369445View Description Hide DescriptionPurpose:
Amorphous siliconEPIDs have been used for planar dose verification in IMRT treatments for many years. The support arm used to attach some types of EPIDs to linear accelerators can introduce inaccuracies to dosimetry measurements due to the presence of metallic parts in their structures. It is demonstrated that this uncertainty may be as large as of maximum image signal for large fields. In this study, a method has been described to quantify, model and correct for the effect of backscattered radiation from the EPID support arm (E-Arm type, Varian Medical Systems).Methods:
Measurements of a support arm backscatter kernel were made using several 6 MV pencil beam irradiations at a sample of positions over the sensitive area of the EPID in standard clinical setup and repeated with the EPID removed from the support arm but at the same positions. A curve-fit to the subtraction of EPID response obtained on and off the arm was used to define the backscatter kernel. The measured kernel was compared with a backscatter kernel obtained by Monte Carlo simulations with EGS/BEAM code. A backscatterdose prediction using the measured backscatter kernel was added to an existing EPIDdose prediction model. The improvement in the agreement of the modified model predictions with EPID measurements for a number of open fields and IMRT beams were investigated by comparison to the original model results.Results:
Considering all functions tested to find the best functional fit to the data points, a broad Gaussian curve proved to be the optimum fit to the backscatter data. The best fit through the Monte Carlo simulated backscatter kernel was also found to be a Gaussian curve. The maximum decrease in normalized root mean squared deviation of the measured and modeled EPIDimage profiles for open fields was 13.7% for a field with no decrease observed for a (the smallest) field as it was not affected by the arm backscatter. Gamma evaluation (2%, 2 mm criteria) showed the improvement in agreement between the model and measurement results when the backscatter was incorporated. The average increase in Gamma pass rate was 2% for head and neck and 1.3% for prostate IMRT fields investigated in this study.Conclusions:
The application of the backscatter kernel determined in this study improved the accuracy of dosimetry using a Varian EPID with E-arm for open fields of different sizes: Eight head and neck and seven prostate IMRT fields. Further improvement in the agreement between the modelpredictions and EPID measurements requires more sophisticated modeling of the backscatter.
37(2010); http://dx.doi.org/10.1118/1.3397455View Description Hide Description
Purpose: The details of a full simulation of an inline side-coupled 6 MV linear accelerator(linac) from the electron gun to the target are presented. Commissioning of the above simulation was performed by using the derived electron phase space at the target as an input into Monte Carlo studies of dose distributions within a water tank and matching the simulation results to measurement data. This work is motivated by linac-MR studies, where a validated full linac simulation is first required in order to perform future studies on linac performance in the presence of an external magnetic field.
Methods: An electron gun was initially designed and optimized with a 2D finite difference program using Child’s law. The electron gun simulation served as an input to a 6 MV linac waveguide simulation, which consisted of a 3D finite element radio-frequency field solution within the waveguide and electron trajectories determined from particle dynamics modeling. The electron gun design was constrained to match the cathode potential and electron gun current of a Varian 600C, while the linac waveguide was optimized to match the measured target current. Commissioning of the full simulation was performed by matching the simulated Monte Carlodose distributions in a water tank to measured distributions.
Results: The full linac simulation matched all the electrical measurements taken from a Varian 600C and the commissioning process lead to excellent agreements in the dose profile measurements. Greater than 99% of all points met a 1%/1mm acceptance criterion for all field sizes analyzed, with the exception of the largest field for which 98% of all points met the 1%/1mm acceptance criterion and the depth dose curves matched measurement to within 1% deeper than 1.5 cm depth. The optimized energy and spatial intensity distributions, as given by the commissioning process, were determined to be non-Gaussian in form for the inline side-coupled 6 MV linac simulated.
Conclusions: An integrated simulation of an inline side-coupled 6 MV linac has been completed and benchmarked matching all electrical and dosimetricmeasurements to high accuracy. The results showed non-Gaussian spatial intensity and energy distributions for the linac modeled.
37(2010); http://dx.doi.org/10.1118/1.3402184View Description Hide DescriptionPurpose:
To quantify diaphragm motion in megavoltage (MV) cone-beam computed tomography(CBCT) projections.Methods:
User identified ipsilateral hemidiaphragm apex (IHDA) positions in two full exhale and inhale frames were used to create bounding rectangles in all other frames of a CBCT scan. The bounding rectangle was enlarged to create a region of interest (ROI). ROI pixels were associated with a cost function: The product of image gradients and a gradient direction matching function for an ideal hemidiaphragm determined from 40 training sets. A dynamic Hough transform (DHT) models a hemidiaphragm as a contour made of two parabola segments with a common vertex (the IHDA). The images within the ROIs are transformed into Hough space where a contour’s Hough value is the sum of the cost function over all contour pixels. Dynamic programming finds the optimal trajectory of the common vertex in Hough space subject to motion constraints between frames, and an active contour model further refines the result. Interpolated ray tracing converts the positions to room coordinates. Root-mean-square (RMS) distances between these positions and those resulting from an expert’s identification of the IHDA were determined for 21 Siemens MV CBCT scans.Results:
Computation time on a 2.66 GHz CPU was 30 s. The average craniocaudal RMS error was. While much larger errors occurred in a few near-sagittal frames on one patient’s scans, adjustments to algorithm constraints corrected them.Conclusions:
The DHT based algorithm can compute IHDA trajectories immediately prior to radiation therapy on a daily basis using localization MVCBCT projection data. This has potential for calibrating external motion surrogates against diaphragm motion.
37(2010); http://dx.doi.org/10.1118/1.3399777View Description Hide DescriptionPurpose:
Improved radiotherapy dose delivery techniques over the past decade have increased the necessity for accurate, independent verification of delivered dose. Compton camera imaging (CCI) systems may have the potential to quantitatively reconstruct three-dimensional dose delivered to the patient with little or noa priori information.Methods:
In this work, the adequacy of a Compton camera imagingsystem for application to radiotherapy dosereconstruction is explored using analytical models of systemspatial and dosimetric resolution. The effects of scatter and absorption detector energy resolution, initial photon energy, and detector separation distance on system performance were calculated with the goal of determining whether current detector technology is adequate for such an application.Results:
Results indicate that the energy and spatial resolutions associated with current Si and Ge double-sided strip detectors in a planar configuration is sufficient to determine dose deposition to within an average of 1.9 mm and 2.5%. Minimum values of less than 0.5 mm and 1% are achievable under certain conditions. As the energy of the photon incident on the patient increases from 1.0 to 10 MeV, system performance improves at the expense of the range of patient and detectorscattering angles over which the system is capable of reconstructingdose deposition to within the acceptable upper limits of 5 mm and 5%. System performance also improves with increasing distance between the scatter and absorption detectors, but is acceptable throughout the range of values likely to be associated with a gantry-mounted system (2–20 cm).Conclusions:
The results indicate that Compton camera imagingsystems based on current solid-state detector technology have the potential to provide independent verification of dose delivered to a patient during radiation therapy. Further consideration must be given to detector efficiency and image reconstruction algorithms for this application of CCI systems.
- RADIATION IMAGING PHYSICS
37(2010); http://dx.doi.org/10.1118/1.3352586View Description Hide DescriptionPurpose:
Anatomical background presents a major impediment to detectability in 2D radiography as well as 3D tomosynthesis and cone-beam CT(CBCT). This article incorporates theoretical and experimental analysis of anatomical background “noise” in cascaded systems analysis of 2D and 3D imaging performance to yield “generalized” metrics of noise-equivalent quanta (NEQ) and detectability index as a function of the orbital extent of the (circular arc) source-detector orbit.Methods:
A physical phantom was designed based on principles of fractal self-similarity to exhibit power-law spectral density comparable to various anatomical sites (e.g., breast and lung). Background power spectra were computed as a function of source-detector orbital extent, including tomosynthesis and CBCT ( to 360°) under two acquisition schemes: (1) Constant angular separation between projections (variable dose) and (2) constant total number of projections (constant dose). The resulting was incorporated in the generalized NEQ, and detectability index was computed from 3D cascaded systems analysis for a variety of imaging tasks.Results:
The phantom yielded power-law spectra within the expected spatial frequency range, quantifying the dependence of clutter magnitude and correlation with increasing tomosynthesis angle. Incorporation of in the 3D NEQ provided a useful framework for analyzing the tradeoffs among anatomical, quantum, and electronic noise with dose and orbital extent. Distinct implications are posed for breast and chest tomosynthesisimagingsystem design—applications varying significantly in and , and imaging task and, therefore, in optimal selection of orbital extent, number of projections, and dose. For example, low-frequency tasks (e.g., soft-tissue masses or nodules) tend to benefit from larger orbital extent and more fully 3D tomographic imaging, whereas high-frequency tasks (e.g., microcalcifications) require careful, application-specific selection of orbital extent and number of projections to minimize negative effects of quantum and electronic noise.Conclusions:
The complex tradeoffs among anatomical background, quantum noise, and electronic noise in projection imaging,tomosynthesis, and CBCT can be described by generalized cascaded systems analysis, providing a useful framework for system design and optimization.
37(2010); http://dx.doi.org/10.1118/1.3371689View Description Hide DescriptionPurpose:
Spectral imaging is a method in medical x-ray imaging to extract information about the object constituents by the material-specific energy dependence of x-ray attenuation. The authors have investigated a photon-counting spectral imagingsystem with two energy bins for contrast-enhanced mammography.System optimization and the potential benefit compared to conventional non-energy-resolved absorption imaging was studied.Methods:
A framework for system characterization was set up that included quantum and anatomical noise and a theoretical model of the system was benchmarked to phantom measurements.Results:
Optimal combination of the energy-resolved images corresponded approximately to minimization of the anatomical noise, which is commonly referred to as energy subtraction. In that case, an ideal-observer detectability index could be improved close to 50% compared to absorption imaging in the phantom study. Optimization with respect to the signal-to-quantum-noise ratio, commonly referred to as energy weighting, yielded only a minute improvement. In a simulation of a clinically more realistic case, spectral imaging was predicted to perform approximately 30% better than absorption imaging for an average glandularity breast with an average level of anatomical noise. For dense breast tissue and a high level of anatomical noise, however, a rise in detectability by a factor of 6 was predicted. Another improvement was found to be within reach for an optimized system.Conclusions:
Contrast-enhanced spectral mammography is feasible and beneficial with the current system, and there is room for additional improvements. Inclusion of anatomical noise is essential for optimizing spectral imagingsystems.
37(2010); http://dx.doi.org/10.1118/1.3373522View Description Hide DescriptionPurpose:
The purpose of this study is to develop a dissimilarity measure for the classification of trabecular bone (TB) texture in knee radiographs. Problems associated with the traditional extraction and selection of texture features and with the invariance to imaging conditions such as image size, anisotropy,noise, blur, exposure, magnification, and projection angle were addressed.Methods:
In the method developed, called a signature dissimilarity measure (SDM), a sum of earth mover’s distances calculated for roughness and orientation signatures is used to quantify dissimilarities between textures. Scale-space theory was used to ensure scale and rotation invariance. The effects of image size, anisotropy,noise, and blur on the SDM developed were studied using computer generated fractal texture images. The invariance of the measure to image exposure, magnification, and projection angle was studied using x-rayimages of human tibia head. For the studies, Mann–Whitney tests with significance level of 0.01 were used. A comparison study between the performances of a SDM based classification system and other two systems in the classification of Brodatz textures and the detection of knee osteoarthritis (OA) were conducted. The other systems are based on weighted neighbor distance using compound hierarchy of algorithms representing morphology (WND-CHARM) and local binary patterns (LBP).Results:
Results obtained indicate that the SDM developed is invariant to image exposure (2.5–30 mA s), magnification, noise associated with film graininess and quantum mottle , blur generated by a sharp film screen, and image size . However, the measure is sensitive to changes in projection angle , imageanisotropy, and blur generated by a regular film screen. For the classification of Brodatz textures, the SDM based system produced comparable results to the LBP system. For the detection of knee OA, the SDM based system achieved 78.8% classification accuracy and outperformed the WND-CHARM system (64.2%).Conclusions:
The SDM is well suited for the classification of TB texture images in knee OA detection and may be useful for the texture classification of medical images in general.
37(2010); http://dx.doi.org/10.1118/1.3378673View Description Hide Description
The noise power spectrum (NPS) is a useful metric for understanding the noise content in images. To examine some unique properties of the NPS of fan beam CT, the authors derived an analytical expression for the NPS of fan beam CT and validated it with computer simulations. The nonstationary noise behavior of fan beam CT was examined by analyzing local regions and the entire field-of-view (FOV). This was performed for cases with uniform as well as nonuniform noise across the detector cells and across views. The simulated NPS from the entire FOV and local regions showed good agreement with the analytically derived NPS. The analysis shows that whereas the NPS of a large FOV in parallel beam CT (using a ramp filter) is proportional to frequency, the NPS with direct fan beam FBP reconstruction shows a high frequency roll off. Even in small regions, the fan beam NPS can show a sharp transition (discontinuity) at high frequencies. These effects are due to the variable magnification and therefore are more pronounced as the fan angle increases. For cases with nonuniform noise, the NPS can show the directional dependence and additional effects.
Simulation of image detectors in radiology for determination of scatter-to-primary ratios using Monte Carlo radiation transport code MCNP/MCNPX37(2010); http://dx.doi.org/10.1118/1.3377773View Description Hide Description
Purpose: The purpose of this study was to compare and validate three methods to simulate radiographicimage detectors with the Monte Carlo software MCNP/MCNPX in a time efficient way.
Methods: The first detector model was the standard semideterministic radiography tally, which has been used in previous image simulation studies. Next to the radiography tally two alternative stochastic detector models were developed: A perfect energy integrating detector and a detector based on the energy absorbed in the detector material. Validation of three image detector models was performed by comparing calculated scatter-to-primary ratios (SPRs) with the published and experimentally acquired SPR values.
Results: For mammographic applications, SPRs computed with the radiography tally were up to 44% larger than the published results, while the SPRs computed with the perfect energy integrating detectors and the blur-free absorbed energy detector model were, on the average, 0.3% (ranging from −3% to 3%) and 0.4% (ranging from −5% to 5%) lower, respectively. For general radiography applications, the radiography tally overestimated the measured SPR by as much as 46%. The SPRs calculated with the perfect energy integrating detectors were, on the average, 4.7% (ranging from −5.3% to −4%) lower than the measured SPRs, whereas for the blur-free absorbed energy detector model, the calculated SPRs were, on the average, 1.3% (ranging from −0.1% to 2.4%) larger than the measured SPRs.
Conclusions: For mammographic applications, both the perfect energy integrating detector model and the blur-free energy absorbing detector model can be used to simulate image detectors, whereas for conventional x-ray imaging using higher energies, the blur-free energy absorbing detector model is the most appropriate image detector model. The radiography tally overestimates the scattered part and should therefore not be used to simulate radiographicimage detectors.