Volume 36, Issue 11, November 2009
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- optical physics
- ultrasound physics
- thermotherapy physics
- tissue measurements
- anatomy and physiology
- radiation protection physics
- books and publications
- task group report
Index of content:
36(2009); http://dx.doi.org/10.1118/1.3218765View Description Hide Description
- RADIATION THERAPY PHYSICS
36(2009); http://dx.doi.org/10.1118/1.3232000View Description Hide DescriptionPurpose:
The purpose of this work is to assess which energy in minibeam radiation therapy provides the best compromise between the deposited dose in the tumor and the sparing of the healthy tissues.Methods:
Monte Carlo simulations (PENELOPE 2006) have been used as a method to calculate the ratio of the peak-to-valley doses (PVDR) in the healthy tissues and in the tumor for different beam energies. The maximization of the ratio of PVDR in the healthy tissues and in the tumor has been used as a criterion.Results:
The main result of this work is that, for the parameters being used in preclinical trials (minibeam sizes of and center-to-center separation), the optimum beam energy is 375 keV.Conclusions:
The conclusion is that this is the energy of minibeams that should be used in the preclinical studies.
36(2009); http://dx.doi.org/10.1118/1.3238153View Description Hide DescriptionPurpose:
The Caribbean Radiation Oncology Center acquired a DELTA4® diode phantom for helical tomotherapy IMRT QA and presents the results of their first 264 clinical cases.Methods:
The validation consisted of several case studies comparing existing ionization chamber and Gafchromic film IMRT QA results to diode phantom results, along with a longitudinal study analyzing the IMRT QA results against other machine QA procedures for a complete sample of IMRT patients.Results:
The case studies resulted in a maximum observed difference of 0.7% between the diode phantom and the ionization chambermeasurements in low dose-gradient regions. Over the longitudinal study, every IMRT QA plan passed a gamma specification of a 3%/3 mm and 98% of the diodes yielded a value of less than 1. In addition, the mean 90% isodose absolute difference for all plans was 0.05% with a (1σ) standard deviation of 1.19%.Conclusions:
The phantom measurements closely match the planned dose distributions in high and low dose-gradient regions. In addition, a significant positive statistical correlation was determined between the IMRT QA, daily QA, and rotational variation output measurements. Together, these results signify high degree of accuracy of both the DELTA4 phantom as well as the TomoTherapy® Hi-Art® system.
Kilovoltage beam Monte Carlo dose calculations in submillimeter voxels for small animal radiotherapy36(2009); http://dx.doi.org/10.1118/1.3238465View Description Hide DescriptionPurpose:
Small animal conformal radiotherapy (RT) is essential for preclinical cancer research studies and therefore various microRT systems have been recently designed. The aim of this paper is to efficiently calculate the dose delivered using our microRT system based on a microCT scanner with the Monte Carlo(MC) method and to compare the MC calculations to film measurements.Methods:
Doses from diameter photon beams deposited in a solid water phantom with voxels are calculated using the latest versions of the EGSnrc codes BEAMNRC and DOSXYZNRC. Two dose calculation approaches are studied: a two-step approach using phase-space files and direct dose calculation with BEAMNRC simulation sources. Due to the small beam size and submillimeter voxel size resulting in long calculation times, variance reduction techniques are studied. The optimum bremsstrahlung splitting number (NBRSPL in BEAMNRC) and the optimum DOSXYZNRCphoton splitting number are examined for both calculation approaches and various beam sizes. The dose calculation efficiencies and the required number of histories to achieve 1% statistical uncertainty—with no particle recycling—are evaluated for beams. As a final step, film dose measurements are compared to MC calculated dose distributions.Results:
The optimum NBRSPL is approximately for both dose calculation approaches. For the dose calculations with phase-space files, varies only slightly for beams and is established to be 300. for the DOSXYZNRC calculation with the BEAMNRC source ranges from 300 for the beam to 4000 for the beam. The calculation time significantly increases for small beam sizes when the BEAMNRC simulation source is used compared to the simulations with phase-space files. For the 2 and beams, the dose calculations with phase-space files are more efficient than the dose calculations with BEAMNRC sources by factors of 54 and 1.6, respectively. The dose calculation efficiencies converge for beams with diameters larger than .Conclusions:
A very good agreement of MC calculated dose distributions to film measurements is found. The mean difference of percentage depth dose curves between calculated and measured data for 2, 5, 10, and beams is 1.8%.
Comparing the accuracy of four-dimensional photon dose calculations with three-dimensional calculations using moving and deforming phantoms36(2009); http://dx.doi.org/10.1118/1.3238482View Description Hide DescriptionPurpose:
Four-dimensional (4D) dose calculation algorithms, which explicitly incorporate respiratory motion in the calculation of doses, have the potential to improve the accuracy of dose calculations in thoracic treatment planning; however, they generally require greater computing power and resources than currently used for three-dimensional (3D) dose calculations. The purpose of this work was to quantify the increase in accuracy of 4D dose calculations versus 3D dose calculations.Methods:
The accuracy of each dose calculation algorithm was assessed using measurements made with two phantoms. Specifically, the authors used a rigid moving anthropomorphic thoracic phantom and an anthropomorphic thoracic phantom with a deformable lung insert. To incorporate a clinically relevant range of scenarios, they programed the phantoms to move and deform with two motion patterns: A sinusoidal motion pattern and an irregular motion pattern that was extracted from an actual patient’s breathing profile. For each combination of phantom and motion pattern, three plans were created: A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan. Doses were calculated using 4D dose calculation methods as well as conventional 3D dose calculation methods. The rigid moving and deforming phantoms were irradiated according to the three treatment plans and doses were measured using thermoluminescent dosimeters(TLDs) and radiochromic film. The accuracy of each dose calculation algorithm was assessed using measured-to-calculated TLDdoses and a analysis.Results:
No significant differences were observed between the measured-to-calculated TLD ratios among 4D and 3D dose calculations. The results revealed that 4D dose calculations had significantly greater percentage of pixels passing the criteria than 3D dose calculations.Conclusions:
These results indicate no significant differences in the accuracy between the 4D and the 3D dose calculation methods inside the gross tumor volume. On the other hand, the film results demonstrated that the 4D dose calculations provided greater accuracy than 3D dose calculations in heterogeneous dose regions. The increase in accuracy of the 4D dose calculations was evident throughout the planning target volume.
36(2009); http://dx.doi.org/10.1118/1.3243085View Description Hide DescriptionPurpose:
In the radionuclide treatment of some forms of braintumors such as craniopharyngiomas, the selection of the appropriate radionuclide for therapy is a key element in treatment planning. The aim was to study the influence by considering the beta-emitter radionuclide dose rate in an intracranial cyst.Methods:
Dosimetry was performed using theMCNP4Cradiationtransport code. Analytical dosimetry was additionally performed using the Loevinger and the Berger formulas in the MATLAB software. Each result was compared under identical conditions. The advantages and disadvantages of using versus and were investigated.Results:
The dose rate at the inner surface of the cyst wall was estimated to be for a concentration of . Under identical conditions of treatment, the corresponding dose rates were for and for . For a well-defined cyst radius and identical wall thickness, higher dose rates resulted for .Conclusions:
To achieve the same radiological burden, the required amount of physical activity of injectable solution is lower for. This is found to be a consequence of both the radionuclide physical half-life and the pattern of energy deposition from the emitted radiation. According to the half-life and dose-rate results, would be a good substitute for .
36(2009); http://dx.doi.org/10.1118/1.3232001View Description Hide DescriptionPurpose:
The AccuBoost brachytherapy system applies HDR beams peripherally to the breast using collimating applicators. The purpose of this study was to benchmark Monte Carlo simulations of the HDR source, to dosimetrically characterize the round applicators using established Monte Carlo simulation and radiation measurement techniques and to gather data for clinical use.Methods
: Dosimetric measurements were performed in a polystyrene phantom, while simulations estimated dose in air, liquid water, polystyrene and ICRU 44 breast tissue.Dose distribution characterization of the 4–8 cm diameter collimators was performed using radiochromic EBT film and air ionization chambers.Results:
The central axis dose falloff was steeper for the 4 cm diameter applicator in comparison to the 8 cm diameter applicator, with surface to 3 cm depth-dose ratios of 3.65 and 2.44, respectively. These ratios did not considerably change when varying the phantom composition from breast tissue to polystyrene, phantom thickness from 4 to 8 cm, or phantom radius from 8 to 15 cm. Dose distributions on the central axis were fitted to sixth-order polynomials for clinical use in a hand calculation spreadsheet (i.e., nomogram). Dose uniformity within the useful applicator apertures decreased as depth-dose increased.Conclusions:
Monte Carlo benchmarking simulations of the HDR source using the MCNP5 radiation transport code indicated agreement within 1% of the published results over the radial/angular region of interest. Changes in phantom size and radius did not cause noteworthy changes in the central axis depth-dose. Polynomial fit depth-dose curves provide a simple and accurate basis for a nomogram.
36(2009); http://dx.doi.org/10.1118/1.3240488View Description Hide DescriptionPurpose
: Volumetric modulated arc therapy (VMAT) is a specific type of intensity-modulated radiation therapy(IMRT) in which the gantry speed, multileaf collimator(MLC) leaf position, and dose rate vary continuously during delivery. A treatment planning system for VMAT is presented.Methods
: Arc control points are created uniformly throughout one or more arcs. An iterative least-squares algorithm is used to generate a fluence profile at every control point. The control points are then grouped and all of the control points in a given group are used to approximate the fluence profiles. A direct-aperture optimization is then used to improve the solution, taking into account the allowed range of leaf motion of the MLC.Dose is calculated using a fast convolution algorithm and the motion between control points is approximated by 100 interpolateddose calculation points. The method has been applied to five cases, consisting of lung, rectum, prostate and seminal vesicles, prostate and pelvic lymph nodes, and head and neck. The resulting plans have been compared with segmental (step-and-shoot) IMRT and delivered and verified on an Elekta Synergy to ensure practicality.Results:
For the lung, prostate and seminal vesicles, and rectum cases, VMAT provides a plan of similar quality to segmental IMRT but with faster delivery by up to a factor of 4. For the prostate and pelvic nodes and head-and-neck cases, the critical structure doses are reduced with VMAT, both of these cases having a longer delivery time than IMRT. The plans in general verify successfully, although the agreement between planned and measured doses is not very close for the more complex cases, particularly the head-and-neck case.Conclusions:
Depending upon the emphasis in the treatment planning, VMAT provides treatment plans which are higher in quality and/or faster to deliver than IMRT. The scheme described has been successfully introduced into clinical use.
36(2009); http://dx.doi.org/10.1118/1.3245877View Description Hide DescriptionPurpose:
A method for performing fast simulations of absorbed dose using a patient’s computerized tomography(CT) scan without explicitly relying on a calibration curve is presented.Methods:
The method is based on geometrical deformations performed on a standard voxelized human phantom. This involves spatially transforming the human phantom to align it with the patient CTimage. Since the chemical composition and density of each voxel are given in the phantom data, a calibration curve is not used in the proposed method. For this study, the Monte Carlo(MC) codePENELOPE has been used as the simulation of reference. The results obtained with PENELOPE simulations are compared to those obtained with PENFAST and with the collapsed cone convolution algorithm implemented in a commercial treatment planning system.Results:
The comparisons of the absorbed doses calculated with the different algorithms on two patient CTs and the corresponding deformed phantoms show a maximum distance to agreement of, and in general, the obtained absorbed dose distributions are compatible within the reached statistical uncertainty. The validity of the deformation method for a broad range of patients is shown using MC simulations in random density phantoms. A PENFAST simulation of a photon beam impinging on a patient CT reaches 2% statistical uncertainty in the absorbed dose, in a voxel along the central axis, in running on a single core of a CPU.Conclusions:
The proposed method of the absorbed dose calculation in a deformed voxelized phantom allows for dosimetric studies in the geometry of a patient CT scan. This is due to the fact that the chemical composition and material density of the phantom are known. Furthermore, simulation using the phantom geometry can provide dosimetric information for each organ. The method can be used for quality assurance procedures. In relation toPENFAST, it is shown that a purely condensed-history algorithm (class I) can be used for absorbed dose estimation in patient CTs.
A design methodology using signal-to-noise ratio for plastic scintillation detectors design and performance optimization36(2009); http://dx.doi.org/10.1118/1.3231947View Description Hide DescriptionPurpose
: The design of novel plastic scintillation detectors(PSDs) is impeded by the lack of a suitable framework to simulate and predict their performance. The authors propose to use the signal-to-noise ratio (SNR) to model the performance of PSDs that use charge-coupled devices(CCDs) as photodetectors.Methods
: In PSDs using CCDs, the SNR is inversely related to the normalized standard deviation of the dose measurement. Thus, optimizing the SNR directly optimizes the system’s precision. In this work, a model of SNR as a function of the system parameters is derived for optical fiber-based PSDsystems. Furthermore, this proposed model is validated using experimental results. A formula for the efficiency of fiber coupling to CCDs is derived and used to simulate the performance of a PSD under varying magnifications.Results
: The proposed model is shown to simulate the experimental performance of an actual PSD to a suitable degree of accuracy under various conditions.Conclusions
: The SNR constitutes a useful tool to simulate the dosimetric precision of PSDs. Using the SNR model, recommendations for the design and optimization of PSDs are provided. Using the same framework, recommendations for non-fiber-based PSDs are also provided.
Investigation of the dosimetric accuracy of the isocenter shifting method in prostate cancer patients with and without hip prostheses36(2009); http://dx.doi.org/10.1118/1.3245882View Description Hide DescriptionPurpose:
The use of image guided radiation therapy(IGRT) enables compensation for prostate movement by shifting the treatment isocenter to track the prostate on a daily basis. Although shifting the isocenter can alter the source to skin distances (SSDs) and the effective depth of the target volume, it is commonly assumed that these changes have a negligible dosimetric effect, and therefore, the number of monitor units delivered is usually not adjusted. However, it is unknown whether or not this assumption is valid for patient with hip prostheses, which frequently contain high density materials.Methods:
The authors conducted a retrospective study to investigate dosimetric effect of the isocenter shifting method for prostate patients with and without hip prostheses. For each patient, copies of the prostate volume were shifted by up to 1.5 cm from the original position to simulate prostate movement in 0.5 cm increments. Subsequently, 12 plans were created for each patient by creating a copy of the original plan for each prostate position with the isocenter shifted to track the position of the shifted prostate. The dose to the prostate was then recalculated for each plan. For patients with hip prostheses, plans were created both with and without lateral beam angles entering through the prostheses.Results:
Without isocenter shifting to compensate for prostate motion of 1.5 cm, the dose to the 95% of the prostate changed by an average of 30% and by up to 64%. This was reduced to less than 3% with the isocenter shifting method. It was found that for patients with hip prostheses, this technique worked best for treatment plans that avoided beam angles passing through the prostheses.Conclusions:
The results demonstrated that the isocenter shifting method can accurately deliver dose to the prostate even in patients with hip prostheses.
Clinical implementation of a digital tomosynthesis-based seed reconstruction algorithm for intraoperative postimplant dose evaluation in low dose rate prostate brachytherapy36(2009); http://dx.doi.org/10.1118/1.3245888View Description Hide DescriptionPurpose:
The low dose rate brachytherapy procedure would benefit from an intraoperative postimplant dosimetry verification technique to identify possible suboptimal dose coverage and suggest a potential reimplantation. The main objective of this project is to develop an efficient, operator-free, intraoperative seed detection technique using the imaging modalities available in a low dose rate brachytherapy treatment room.Methods:
This intraoperative detection allows a complete dosimetry calculation that can be performed right after an I-125 prostate seed implantation, while the patient is still under anesthesia. To accomplish this, a digital tomosynthesis-based algorithm was developed. This automatic filtered reconstruction of the 3D volume requires seven projections acquired over a total angle of 60° with an isocentric imaging system.Results:
A phantom study was performed to validate the technique that was used in a retrospective clinical study involving 23 patients. In the patient study, the automatic tomosynthesis-based reconstruction yielded seed detection rates of 96.7% and 2.6% false positives. The seed localization error obtained with a phantom study is. The average time needed for reconstruction is below 1 min. The reconstruction algorithm also provides the seed orientation with an uncertainty of . The seed detection algorithm presented here is reliable and was efficiently used in the clinic.Conclusions:
When combined with an appropriate coregistration technique to identify the organs in the seed coordinate system, this algorithm will offer new possibilities for a next generation of clinical brachytherapy systems.
More than 10 years experience of beam monitoring with the Gantry 1 spot scanning proton therapy facility at PSI36(2009); http://dx.doi.org/10.1118/1.3244034View Description Hide DescriptionPurpose:
The beam monitoring equipments developed for the first PSI spot scanning proton therapy facility, Gantry 1, have been successfully used for more than 10 years. The purpose of this article is to summarize the author’s experience in the beam monitoring technique for dynamic proton scanning.Methods:
The spot dose delivery and verification use two independent beam monitoring and computer systems. In this article, the detector construction, electronic system, dosimetry, and quality assurance results are described in detail. The beam flux monitor is calibrated with a Faraday cup. The beam position monitoring is realized by measuring the magnetic fields of deflection magnets with Hall probes before applying the spot and by checking the beam position and width with an ionization strip chamber after the spot delivery.Results:
The results of thimble ionization chamberdosimetry measurements are reproducible (with a mean deviation of less than 1% and a standard deviation of 1%). The resolution in the beam position measurement is of the order of a tenth of a millimeter. The tolerance of the beam position delivery and monitoring during scanning is less than 1.5 mm.Conclusions:
The experiences gained with the successful operation of Gantry 1 represent a unique and solid background for the development of a new system, Gantry 2, in order to perform new advanced scanning techniques.
- RADIATION IMAGING PHYSICS
36(2009); http://dx.doi.org/10.1118/1.3231814View Description Hide Description
Breast tomosynthesis has been an exciting new development in the field of breast imaging. While the diagnostic improvement via tomosynthesis is notable, the full potential of tomosynthesis has not yet been realized. This may be attributed to the dependency of the diagnostic quality of tomosynthesis on multiple variables, each of which needs to be optimized. Those include dose, number of angular projections, and the total angular span of those projections. In this study, the authors investigated the effects of these acquisition parameters on the overall diagnostic image quality of breast tomosynthesis in both the projection and reconstruction space. Five mastectomy specimens were imaged using a prototype tomosynthesissystem. 25 angular projections of each specimen were acquired at 6.2 times typical single-view clinical dose level. Images at lower dose levels were then simulated using a noise modification routine. Each projection image was supplemented with 84 simulated 3 mm 3D lesions embedded at the center of 84 nonoverlapping ROIs. The projection images were then reconstructed using a filtered backprojection algorithm at different combinations of acquisition parameters to investigate which of the many possible combinations maximizes the performance. Performance was evaluated in terms of a Laguerre–Gauss channelized Hotelling observer model-based measure of lesion detectability. The analysis was also performed without reconstruction by combining the model results from projection images using Bayesian decision fusion algorithm. The effect of acquisition parameters on projection images and reconstructed slices were then compared to derive an optimization rule for tomosynthesis. The results indicated that projection images yield comparable but higher performance than reconstructed images. Both modes, however, offered similar trends: Performance improved with an increase in the total acquisition dose level and the angular span. Using a constant dose level and angular span, the performance rolled off beyond a certain number of projections, indicating that simply increasing the number of projections in tomosynthesis may not necessarily improve its performance. The best performance for both projection images and tomosynthesis slices was obtained for 15–17 projections spanning an angular arc of ∼45°—the maximum tested in our study, and for an acquisition dose equal to single-view mammography. The optimization framework developed in this framework is applicable to other reconstruction techniques and other multiprojection systems.
Accelerating Monte Carlo simulations of photon transport in a voxelized geometry using a massively parallel graphics processing unit36(2009); http://dx.doi.org/10.1118/1.3231824View Description Hide DescriptionPurpose:
It is a known fact that Monte Carlo simulations of radiation transport are computationally intensive and may require long computing times. The authors introduce a new paradigm for the acceleration of Monte Carlo simulations: The use of a graphics processing unit (GPU) as the main computing device instead of a central processing unit (CPU).Methods:
A GPU-based Monte Carlo code that simulates photon transport in a voxelized geometry with the accurate physics models fromPENELOPE has been developed using the CUDA™ programming model (NVIDIA Corporation, Santa Clara, CA).Results:
An outline of the new code and a sample x-ray imaging simulation with an anthropomorphic phantom are presented. A remarkable 27-fold speed up factor was obtained using a GPU compared to a single core CPU.Conclusions:
The reported results show that GPUs are currently a good alternative to CPUs for the simulation of radiation transport. Since the performance of GPUs is currently increasing at a faster pace than that of CPUs, the advantages of GPU-based software are likely to be more pronounced in the future.
36(2009); http://dx.doi.org/10.1118/1.3232004View Description Hide DescriptionPurpose:
To investigate a novel locally adaptive projection space denoising algorithm for low-dose CT data.Methods:
The denoising algorithm is based on bilateral filtering, which smooths values using a weighted average in a local neighborhood, with weights determined according to both spatial proximity and intensity similarity between the center pixel and the neighboring pixels. This filtering is locally adaptive and can preserve important edge information in the sinogram, thus maintaining high spatial resolution. A CTnoise model that takes into account the bowtie filter and patient-specific automatic exposure control effects is also incorporated into the denoising process. The authors evaluated the noise-resolution properties of bilateral filtering incorporating such a CTnoise model in phantom studies and preliminary patient studies with contrast-enhanced abdominal CT exams.Results:
On a thin wire phantom, the noise-resolution properties were significantly improved with the denoising algorithm compared to commercial reconstruction kernels. The noise-resolution properties on low-dose (40 mA s) data after denoising approximated those of conventional reconstructions at twice the dose level. A separate contrast plate phantom showed improved depiction of low-contrast plates with the denoising algorithm over conventional reconstructions when noise levels were matched. Similar improvement in noise-resolution properties was found on CT colonography data and on five abdominal low-energy (80 kV) CT exams. In each abdominal case, a board-certified subspecialized radiologist rated the denoised 80 kV images markedly superior in image quality compared to the commercially available reconstructions, and denoising improved the image quality to the point where the 80 kV images alone were considered to be of diagnostic quality.Conclusions:
The results demonstrate that bilateral filtering incorporating a CTnoise model can achieve a significantly better noise-resolution trade-off than a series of commercial reconstruction kernels. This improvement in noise-resolution properties can be used for improving image quality in CT and can be translated into substantial dose reduction.
Enhanced imaging of microcalcifications in digital breast tomosynthesis through improved image-reconstruction algorithms36(2009); http://dx.doi.org/10.1118/1.3232211View Description Hide DescriptionPurpose:
The authors develop a practical, iterative algorithm for image-reconstruction in undersampled tomographic systems, such as digital breast tomosynthesis (DBT).Methods:
The algorithm controls image regularity by minimizing the image total variation (TpV), a function that reduces to the total variation when or the image roughness when . Constraints on the image, such as image positivity and estimated projection-data tolerance, are enforced by projection onto convex sets. The fact that the tomographic system is undersampled translates to the mathematical property that many widely varied resultant volumes may correspond to a given data tolerance. Thus the application of image regularity serves two purposes: (1) Reduction in the number of resultant volumes out of those allowed by fixing the data tolerance, finding the minimum image TpV for fixed data tolerance, and (2) traditional regularization, sacrificing data fidelity for higher image regularity. The present algorithm allows for this dual role of image regularity in undersampled tomography.Results:
The proposed image-reconstruction algorithm is applied to three clinical DBT data sets. The DBT cases include one with microcalcifications and two with masses.Conclusions:
Results indicate that there may be a substantial advantage in using the present image-reconstruction algorithm for microcalcification imaging.
Experimental validation of Monte Carlo (MANTIS) simulated x-ray response of columnar CsI scintillator screens36(2009); http://dx.doi.org/10.1118/1.3233683View Description Hide DescriptionPurpose:
MANTIS is a Monte Carlo code developed for the detailed simulation of columnar CsI scintillator screens in x-ray imaging systems. Validation of this code is needed to provide a reliable and valuable tool for system optimization and accurate reconstructions for a variety of x-ray applications. Whereas previous validation efforts have focused on matching of summary statistics, in this work the authors examine the complete point response function (PRF) of the detector system in addition to relative light output values.Methods:
Relative light output values and high-resolution PRFs have been experimentally measured with a custom setup. A corresponding set of simulated light output values and PRFs have also been produced, where detailed knowledge of the experimental setup and CsI:Tl screen structures are accounted for in the simulations. Four different screens were investigated with different thicknesses, column tilt angles, and substrate types. A quantitative comparison between the experimental and simulated PRFs was performed for four different incidence angles (0°, 15°, 30°, and 45°) and two different x-ray spectra (40 and 70 kVp). The figure of merit (FOM) used measures the normalized differences between the simulated and experimental data averaged over a region of interest.Results:
Experimental relative light output values ranged from 1.456 to 1.650 and were in approximate agreement for aluminum substrates, but poor agreement for graphite substrates. The FOMs for all screen types, incidence angles, and energies ranged from 0.1929 to 0.4775. To put these FOMs in context, the same FOM was computed for 2D symmetric Gaussians fit to the same experimental data. These FOMs ranged from 0.2068 to 0.8029. Our analysis demonstrates thatMANTIS reproduces experimental PRFs with higher accuracy than a symmetric 2D Gaussian fit to the experimental data in the majority of cases. Examination of the spatial distribution of differences between the PRFs shows that the main reason for errors between MANTIS and the experimental data is that MANTIS-generated PRFs are sharper than the experimental PRFs.Conclusions:
The experimental validation ofMANTIS performed in this study demonstrates that MANTIS is able to reliably predict experimental PRFs, especially for thinner screens, and can reproduce the highly asymmetric shape seen in the experimental data. As a result, optimizations and reconstructions carried out using MANTIS should yield results indicative of actual detector performance. Better characterization of screen properties is necessary to reconcile the simulated light output values with experimental data.
36(2009); http://dx.doi.org/10.1118/1.3233684View Description Hide DescriptionPurpose:
C-arm based cone-beam CT(CBCT)imaging enables thein situ acquisition of three dimensional images. In the context of image-guided interventions, this technology potentially reduces the complexity of a procedure’s workflow. Instead of acquiring the preoperative volumetric images in a separate location and transferring the patient to the interventional suite, both imaging and intervention are carried out in the same location. A key component in image-guided interventions is image to patient registration. The most common registration approach, in clinical use, is based on fiducial markers placed on the patient’s skin which are then localized in the volumetric image and in the interventional environment. When using C-arm CBCT, this registration approach is challenging as in many cases the small size of the volumetric reconstruction cannot include both the skin fiducials and the organ of interest.Methods:
In this article the author shows that fiducial localization outside the reconstructed volume is possible if the projection images from which the reconstruction was obtained are available. By replacing direct fiducial localization in the volumetric images with localization in the projection images, the author obtains the fiducial coordinates in the volume’s coordinate system even when the fiducials are outside the reconstructed region.Results:
The approach was evaluated using two types of spherical fiducials, clinically used 4 mm diameter markers and a custom phantom embedded with 6 mm diameter markers that is part of a commercial navigation system. In all cases, the method localized all fiducials, including those that were outside the reconstructed volume. The method’s mean (std) localization error as evaluated using fiducials that were directly localized in the CBCTreconstruction was 0.55(0.22) mm for the 4 mm markers and 0.51(0.18) mm for the 6 mm markers.Conclusions:
Based on the evaluations the author concludes that the proposed localization approach is sufficiently accurate to augment or replace direct volumetric fiducial localization for thoracic-abdominal interventions. This allows the physician to position fiducials in a more flexible manner, relaxing the requirement that both the organ of interest and skin surface be contained in the volumetric reconstruction.
36(2009); http://dx.doi.org/10.1118/1.3238154View Description Hide DescriptionPurpose:
Accurate characterization of diagnosis instruments is crucial in medical applications such as radiology and clinical neurosciences. While classical CRT medical displays have been replaced almost exclusively with liquid crystal devices(LCDs), the assessment of their temporal properties (response times) is still largely based on heuristic methods, which have not been evaluated thoroughly yet. The authors introduce a novel approach and show that it improves the accuracy and reliability compared to the common heuristic recommended by ISO 9241-305 substantially for a wide range of settings.Methods:
The approach is based on disentangling the signal from the modulatory backlight through division (division approach). They evaluated this method in two different ways: First, they applied both methods to luminance transition measurements of different LCD monitors. Second, they simulated LCD luminance transitions by modeling the LCD optical responses according to a physical liquid crystal director orientation model. The simulated data were generated for four different response times, each with four different backlight modulation frequencies. Both the novel and the ISO convolution method were applied to the data.Results:
Application of the methods to the simulated data shows a bias of up to 46% for the ISO approach, while the novel division approach is biased at most 2%. In accordance with the simulations, estimates for real measurements show differences in the two approaches of more than 200% for some LCDpanels.Conclusion:
The division approach is robust against periodic backlight fluctuations and can reliably estimate even very short response times or small transitions. Unlike the established method, it meets the accuracy requirements of medical applications. In contrast, the popular convolution approach for estimating response times is prone to misestimations of time by several orders of magnitude and tend to further worsen as advances in LCD technology lead to shorter response times.