Volume 38, Issue 9, September 2011
Index of content:
38(2011); http://dx.doi.org/10.1118/1.3636414View Description Hide Description
- MEDICAL PHYSICS LETTERS
38(2011); http://dx.doi.org/10.1118/1.3600524View Description Hide Description
This report on intensity-modulated radiation therapy(IMRT) is part of a series of white papers addressing patient safety commissioned by the American Society for RadiationOncology’s (ASTRO) Target Safely Campaign. The document has been approved by the ASTRO Board of Directors, endorsed by the American Association of Physicists in Medicine (AAPM) and American Association of MedicalDosimetrists (AAMD), and reviewed and accepted by the American College of Radiology’s Commission on RadiationOncology. This report is related to other reports of the ASTRO white paper series on patient safety which are still in preparation, and when appropriate it defers to guidance that will be published by those groups in future white papers. This document takes advantage of the large body of work on quality assurance and quality control principles within radiationoncology whenever possible. IMRT provides increased capability to conform isodose distributions to the shape of the target(s), thereby reducing dose to some adjacent critical structures. This promise of IMRT is one of the reasons for its widespread use. However, the promise of IMRT is counterbalanced by the complexity of the IMRT planning and delivery processes, and the associated risks, some of which have been demonstrated by the New York Times reports on serious accidents involving both IMRT and other radiation treatment modalities. This report provides an opportunity to broadly address safe delivery of IMRT, with a primary focus on recommendations for human error prevention and methods to reduce the occurrence of errors or machine malfunctions that can lead to catastrophic failures or errors.
- RADIATION THERAPY PHYSICS
Bridging the gap between IMRT and VMAT: Dense angularly sampled and sparse intensity modulated radiation therapy38(2011); http://dx.doi.org/10.1118/1.3618736View Description Hide DescriptionPurpose:
To propose an alternative radiation therapy (RT) planning and delivery scheme with optimal angular beam sampling and intrabeam modulation for improved dose distribution while maintaining high delivery efficiency.Methods:
In the proposed approach, coined as dense angularly sampled and sparse intensity modulated RT (DASSIM-RT), a large number of beam angles are used to increase the angular sampling, leading to potentially more conformal dose distributions as compared to conventional IMRT. At the same time, intensity modulation of the incident beams is simplified to eliminate the dispensable segments, compensating the increase in delivery time caused by the increased number of beams and facilitating the plan delivery. In a sense, the proposed approach shifts and transforms, in an optimal fashion, some of the beam segments in conventional IMRT to the added beams. For newly available digital accelerators, the DASSIM-RT delivery can be made very efficient by concatenating the beams so that they can be delivered sequentially without operator’s intervention. Different from VMAT, the level of intensity modulation in DASSIS-RT is field specific and optimized to meet the need of each beam direction. Three clinical cases (a head and neck (HN) case, a pancreas case, and a lung case) are used to evaluate the proposed RT scheme. DASSIM-RT, VMAT, and conventional IMRT plans are compared quantitatively in terms of the conformality index (CI) and delivery efficiency.Results:
Plan quality improves generally with the number and intensity modulation of the incident beams. For a fixed number of beams or fixed level of intensity modulation, the improvement saturates after the intensity modulation or number of beams reaches to a certain level. An interplay between the two variables is observed and the saturation point depends on the values of both variables. For all the cases studied here, the CI of DASSIM-RT with 15 beams and 5 intensity levels (0.90, 0.79, and 0.84 for the HN, pancreas, and lung cases, respectively) is similar with that of conventional IMRT with seven beams and ten intensity levels (0.88, 0.79, and 0.83) and is higher than that of single-arc VMAT (0.75, 0.75, and 0.82). It is also found that the DASSIM-RT plans generally have better sparing of organs-at-risk than IMRT plans. It is estimated that the dosedelivery time of DASSIM-RT with 15 beams and 5 intensity levels is about 4.5, 4.4, and 4.2 min for the HN, pancreas, and lung case, respectively, similar to that of IMRT plans with 7 beams and 10 intensity levels.Conclusion: DASSIS-RT bridges the gap between IMRT and VMAT and allows optimal sampling of angular space and intrabeam modulation, thus it provides improved conformity in dose distributions while maintaining high delivery efficiency.
38(2011); http://dx.doi.org/10.1118/1.3608908View Description Hide DescriptionPurpose:
In the quest of a curative radiotherapy treatment for gliomas, new delivery modes are being explored. At the Biomedical Beamline of the European Synchrotron Radiation Facility, a new spatially fractionated technique, called minibeam radiation therapy (MBRT), is under development. The aims of this work were to assess different dosimetric aspects and to establish a dosimetry protocol to be applied in the forthcoming animal (rat) studies in order to evaluate the therapeutic index of this new radiotherapy approach.Methods:
Absolute dosimetry was performed with a thimble ionization chamber (PTW semiflex 31010) whose center was positioned at 2 g cm−2 depth. To translate the dose measured in broad beam configuration to the dose deposited with a minibeam, the scatter factors were used. Those were assessed by using the Monte Carlo simulations and verified experimentally with Gafchromic films and a Bragg Peak chamber. The comparison of the theoretical and experimental data were used to benchmark the calculations. Finally, the dose distributions in a rat phantom were evaluated by using the validated Monte Carlo calculations.Results:
The absolute dosimetry in broad beam configuration was measured in reference conditions. The dose rate was in the range between 168 and 224 Gy/min, depending on the storage ring current. A scatter factor of 0.80 ± 0.04 was obtained. Percentage depth dose and lateral profiles were evaluated both in homogenous and heterogeneous slab phantoms. The general good agreement between Monte Carlo simulations and experimental data permitted the benchmark of the calculations. Finally, the peak doses in the rat head phantom were assessed from the measurements in reference conditions. In addition, the peak-to-valley dose ratio values as a function of depth in the rat head were evaluated.Conclusions:
A new promising radiotherapy approach is being explored at the ESRF: Minibeam Radiation Therapy. To assess the therapeutic index of this new modality, in vivo experiments are being planned, for which an accurate knowledge of the dosimetry is essential. For that purpose, a complete set of measurements and Monte Carlo simulations was performed. The first dosimetry protocol for preclinical trials in minibeam radiation therapy was established. This protocol allows to have reproducibility in terms of dose for the different biological studies.
Optimizing the accuracy of a helical diode array dosimeter: A comprehensive calibration methodology coupled with a novel virtual inclinometer38(2011); http://dx.doi.org/10.1118/1.3622823View Description Hide DescriptionPurpose:
The goal of any dosimeter is to be as accurate as possible when measuring absolute dose to compare with calculated dose. This limits the uncertainties associated with the dosimeter itself and allows the task of dose QA to focus on detecting errors in the treatment planning (TPS) and/or delivery systems. This work introduces enhancements to the measurement accuracy of a 3D dosimeter comprised of a helical plane of diodes in a volumetric phantom.Methods:
We describe the methods and derivations of new corrections that account for repetition rate dependence, intrinsic relative sensitivity per diode, field size dependence based on the dynamic field size determination, and positional correction. Required and described is an accurate “virtual inclinometer” algorithm. The system allows for calibrating the array directly against an ion chamber signal collected with high angular resolution. These enhancements are quantitatively validated using several strategies including ion chamber measurements taken using a “blank” plastic shell mimicking the actual phantom, and comparison to high resolution dose calculations for a variety of fields: static, simple arcs, and VMAT. A number of sophisticated treatment planning algorithms were benchmarked against ion chamber measurements for their ability to handle a large air cavity in the phantom.Results:
Each calibration correction is quantified and presented vs its independent variable(s). The virtual inclinometer is validated by direct comparison to the gantry angle vs time data from machine log files. The effects of the calibration are quantified and improvements are seen in the dose agreement with the ion chamber reference measurements and with the TPS calculations. These improved agreements are a result of removing prior limitations and assumptions in the calibration methodology. Average gamma analysis passing rates for VMAT plans based on the AAPM TG-119 report are 98.4 and 93.3% for the 3%/3 mm and 2%/2 mm dose-error/distance to agreement threshold criteria, respectively, with the global dose-error normalization. With the local dose-error normalization, the average passing rates are reduced to 94.6 and 85.7% for the 3%/3 mm and 2%/2 mm criteria, respectively. Some algorithms in the convolution/superposition family are not sufficiently accurate in predicting the exit dose in the presence of a 15 cm diameter air cavity.Conclusions:
Introduction of the improved calibration methodology, enabled by a robust virtual inclinometer algorithm, improves the accuracy of the dosimeter’s absolute dose measurements. With our treatment planning and delivery chain, gamma analysis passing rates for the VMAT plans based on the AAPM TG-119 report are expected to be above 91% and average at about 95% level for γ(3%/3 mm) with the local dose-error normalization. This stringent comparison methodology is more indicative of the true VMAT system commissioning accuracy compared to the often quoted dose-error normalization to a single high value.
38(2011); http://dx.doi.org/10.1118/1.3611044View Description Hide DescriptionPurpose:
In this paper, the authors assess the accuracy of the Brainlab ExacTrac system for frameless intracranial stereotactic treatments in clinical practice.Methods:
They recorded couch angle and imagefusion results (comprising lateral, longitudinal, and vertical shifts, and rotation corrections about these axes) for 109 stereotactic radiosurgery and 166 stereotactic radiotherapypatient treatments. Frameless stereotactic treatments involve iterative 6D imagefusion corrections applied until the results conform to customizable pass criteria, theirs being 0.7 mm and 0.5° for each axis. The planning CT slice thickness was 1.25 mm. It has been reported in the literature that the CT slices’ thickness impacts the accuracy of localization to bony anatomy. The principle of invariance with respect to patient orientation was used to determine spatial accuracy.Results:
The data for radiosurgery comprised 927 image pairs, of which 532 passed (pass ratio of 57.4%). The data for radiotherapy comprised 15983 image pairs, of which 10 050 passed (pass ratio of 62.9%). For stereotactic radiotherapy, the combined uncertainty of ExacTrac calibration,imagefusion, and intrafraction motion was (95% confidence interval) 0.290–0.302 and 0.306–0.319 mm in the longitudinal and lateral axes, respectively. The combined uncertainty of imagefusion and intrafraction motion in the anterior-posterior coordinates was 0.174–0.182 mm. For stereotactic radiosurgery, the equivalent ranges are 0.323–0.393, 0.337–0.409, and 0.231–0.281 mm. The overall spatial accuracy was 1.24 mm for stereotactic radiotherapy (SRT) and 1.35 mm for stereotactic radiosurgery(SRS).Conclusions:
The ExacTrac intracranial frameless stereotactic system spatial accuracy is adequate for clinical practice, and with the same pass criteria, SRT is more accurate than SRS. They now use frameless stereotaxy exclusively at their center.
38(2011); http://dx.doi.org/10.1118/1.3622612View Description Hide DescriptionPurpose:
A novel rotational IMRT (rIMRT) technique using burst delivery (continuous gantry rotation with beam off during MLC repositioning) is investigated. The authors evaluate the plan quality and delivery efficiency and accuracy of this dynamic technique with a conventional flat 6 MV photon beam.Methods:
Burst-delivery rIMRT was implemented in a planning system and delivered with a 160-MLC linac. Ten rIMRT plans were generated for five anonymized patient cases encompassing head and neck, brain, prostate, and prone breast. All plans were analyzed retrospectively and not used for treatment. Among the varied plan parameters were the number of optimization points, number of arcs, gantry speed, and gantry angle range (alpha) over which the beam is turned on at each optimization point. Combined rotational/step-and-shoot rIMRT plans were also created by superimposing multiple-segment static fields at several optimization points. The rIMRT trial plans were compared with each other and with plans generated using helical tomotherapy and VMAT. Burst-mode rotational IMRT plans were delivered and verified using a diode array, ionization chambers,thermoluminescent dosimeters, and film.Results:
Burst-mode rIMRT can achieve plan quality comparable to helical tomotherapy, while the former may lead to slightly better OAR sparing for certain cases and the latter generally achieves slightly lower hot spots. Few instances were found in which increasing the number of optimization points above 36, or superimposing step-and-shoot IMRT segments, led to statistically significant improvements in OAR sparing. Using an additional rIMRT partial arc yielded substantial OAR dose improvements for the brain case. Measured doses from the rIMRT plan delivery were within 4% of the plan calculation in low dose gradient regions. Delivery time range was 228–375 s for single-arc rIMRT 200-cGy prescription with a 300 MU/min dose rate, comparable to tomotherapy and VMAT.Conclusions:
Rotational IMRT with burst delivery, whether combined with static fields or not, yields clinically acceptable and deliverable treatment plans.
38(2011); http://dx.doi.org/10.1118/1.3626487View Description Hide DescriptionPurpose:
This study provides a simple method for improving precision of x-raycomputed tomography(CT) scans of irradiated polymergeldosimetry. The noise affecting CT scans of irradiated gels has been an impediment to the use of clinical CT scanners for geldosimetry studies.Methods:
In this study, it is shown that multiple scans of a single PAGAT geldosimeter can be used to extrapolate a “zero-scan” image which displays a similar level of precision to an image obtained by averaging multiple CTimages, without the compromised dose measurement resulting from the exposure of the gel to radiation from the CT scanner.Results:
When extrapolating the zero-scan image, it is shown that exponential and simple linear fits to the relationship between Hounsfield unit and scan number, for each pixel in the image, provide an accurate indication of gel density.Conclusions:
It is expected that this work will be utilized in the analysis of three-dimensional gel volumes irradiated using complex radiotherapy treatments.
Implementing RapidArc into clinical routine: A comprehensive program from machine QA to TPS validation and patient QA38(2011); http://dx.doi.org/10.1118/1.3622672View Description Hide DescriptionPurpose:
With the increased commercial availability of intensity modulated arc therapy (IMAT) comes the need for comprehensive QA programs, covering the different aspects of this newly available technology. This manuscript proposes such a program for the RapidArc (RA) (Varian Medical Systems, Palo Alto) IMAT solution.Methods:
The program was developed and tested out for a Millennium120 MLC on iX Clinacs and a HighDefinition MLC on a Novalis TX, using a variety of measurement equipment including Gafchromic film, 2D ion chamber arrays (Seven29 and StarCheck, PTW, Freiburg, Germany) with inclinometer and Octavius phantom, the Delta4 systam (ScandiDos, Uppsala, Sweden) and the portal imager(EPID). First, a number of complementary machine QA tests were developed to monitor the correct interplay between the accelerating/decelerating gantry, the variable dose rate and the MLC position, straining the delivery to the maximum allowed limits. Second, a systematic approach to the validation of the dose calculation for RA was adopted, starting with static gantry and RA specific static MLC shapes and gradually moving to dynamic gantry, dynamic MLC shapes. RA plans were then optimized on a series of artificial structures created within the homogeneous Octavius phantom and within a heterogeneous lung phantom. These served the double purpose of testing the behavior of the optimization algorithm (PRO) as well as the precision of the forward dose calculation. Finally, patient QA on a series of clinical cases was performed with different methods. In addition to the well established in-phantom QA, we evaluated the portal dosimetry solution within the Varian approach.Results:
For routine machine QA, the “Snooker Cue” test on the EPID proved to be the most sensitive to overall problem detection. It is also the most practical one. The “Twinkle” and “Sunrise” tests were useful to obtain well differentiated information on the individual treatment delivery components. The AAA8.9 dose calculations showed excellent agreement with all corresponding measurements, except in areas where the 2.5 mm fixed fluence resolution was insufficient to accurately model the tongue and groove effect or the dose through nearly closed opposing leafs. Such cases benefited from the increased fluence resolution in AAA10.0. In the clinical RA fields, these effects were smeared out spatially and the impact of the fluence resolution was considerably less pronounced. The RA plans on the artificial structure sets demonstrated some interesting characteristics of the PRO8.9 optimizer, such as a sometimes unexpected dependence on the collimator rotation and a suboptimal coverage of targets within lung tissue. Although the portal dosimetry was successfully validated, we are reluctant to use it as a sole means of patient QA as long as no gantry angle information is embedded.Conclusions:
The all-in validation program allows a systematic approach in monitoring the different levels of RA treatments. With the systematic approach comes a better understanding of both the capabilities and the limits of the used solution. The program can be useful for implementation, but also for the validation of major upgrades.
Dosimetric intercomparison for multicenter clinical trials using a patient-based anatomic pelvic phantom38(2011); http://dx.doi.org/10.1118/1.3626484View Description Hide DescriptionPurpose:
To assess dose delivery accuracy to clinically significant points in a realistic patient geometry for two separate pelvic radiotherapy scenarios.Methods:
An inhomogeneous pelvic phantom was transported to 36 radiotherapy centers in Australia and New Zealand. The phantom was treated according to Phase III rectal and prostate trial protocols. Point dose measurements were made with thermoluminescent dosimeters(TLDs) and an ionisation chamber. Comprehensive site-demographic, treatment planning, and physical data were collected for correlation with measurement outcomes.Results:
Dose delivery to the prescription point for the rectal treatment was consistent with planned dose (mean difference between planned and measured dose − 0.1 ± 0.3% std err). Dose delivery in the region of the sacral hollow was consistently higher than planned (+1.2 ± 0.2%). For the prostate treatment,dose delivery to the prostate volume was consistent with planned doses (−0.49 ± 0.2%) and planned dose uniformity, though with a tendency to underdose the PTV at the prostate-rectal border. Measured out-of-field doses were significantly higher than planned.Conclusions:
A phantom based on realistic anatomy and heterogeneity can be used to comprehensively assess the influence of multiple aspects of the radiotherapytreatment process on dose delivery. The ability to verify dose delivery for two trials with a single phantom was advantageous.
Preliminary investigations for the option to use fast uniform scanning with compensators on a gantry designed for IMPT38(2011); http://dx.doi.org/10.1118/1.3626483View Description Hide DescriptionPurpose:
In this experimental study, the authors explored the possibility to deliver the dose for proton therapy with fast uniform scanning on a gantry primarily designed for the delivery of conformal beam scanning and IMPT. The uniform scanning submode has been realized without equipment modifications by using the same small pencil beam used for conformal scanning, resulting in reduced realization costs. Uniform scanning has recently been adopted in a few proton therapy centers, as a basic beam delivery solution, and as an alternative to the use of scattering foils. The option to use such a mode to mimic scattering on a full-fledged scanning gantry could be of interest for treating some specific indications and as a possible solution for treating moving targets.Methods:
Uniform iso-energy dose layers were painted by fast magnetic scanning alternated with fast energy changes with the gantry beam line. The layers were stacked and repainted appropriately to produce homogeneous three-dimensional dose distributions. A collimator/compensator was used to adjust the dose to coincide laterally/distally with the target volume. In addition, they applied volumetric repainting, since they are confident that this will further mitigate the effects of organ motion as compared with the presently used clinical scanning solutions. With the approach presented in this paper, they can profit from the higher flexibility of the scanning system to obtain additional advantages. For instance the shape of the energy layers can be adjusted to the projected target shape in order to reduce treatment time and neutrons produced in the collimator. The shape of the proximal layers can be shrunk, according to the cross section of the target at the corresponding range. This provides variable range modulation (proximal conformity) while standard scattering only provides fixed range modulation with unnecessary 100% dose proximal to the target. The field-specific hardware for a spherical target volume was mounted on the Gantry 2 nozzle. One field with proximal field size shrinking and one without, each of 1 Gy, were delivered. The dose distributions at different depths were recorded as CCD images of a scintillating screen.Results:
The time to scan the volume once was about 4 s and the total delivery time was approximately 30 s. For the field with proximal conformity, dose sparing of up to 25% was measured in the region proximal to the target. A repainting capability of 48 times was achieved on the most distal layer. The proximal layers were repainted more due to the contribution of the plateau dose from the deeper layers.Conclusions:
The flexibility of a fast scanning gantry with very fast energy changes can easily provide beam delivery by uniform layer stacking with a significant degree of volumetric repainting and with the benefit of a dose reduction proximal to the target volume.
38(2011); http://dx.doi.org/10.1118/1.3615622View Description Hide DescriptionPurpose:
In radiation therapytreatment planning, the clinical objectives of uniform high dose to the planning target volume (PTV) and low dose to the organs-at-risk (OARs) are invariably in conflict, often requiring compromises to be made between them when selecting the best treatment plan for a particular patient. In this work, the authors introduce Pareto-Aware Radiotherapy Evolutionary TreatmentOptimization (pareto), a multiobjective optimization tool to solve for beam angles and fluence patterns in intensity-modulated radiation therapy(IMRT)treatment planning.Methods:
pareto is built around a powerful multiobjective genetic algorithm (GA), which allows us to treat the problem of IMRTtreatment plan optimization as a combined monolithic problem, where all beam fluence and angle parameters are treated equally during the optimization. We have employed a simple parameterized beam fluence representation with a realistic dose calculation approach, incorporating patient scatter effects, to demonstrate feasibility of the proposed approach on two phantoms. The first phantom is a simple cylindrical phantom containing a target surrounded by three OARs, while the second phantom is more complex and represents a paraspinal patient.Results:
pareto results in a large database of Pareto nondominated solutions that represent the necessary trade-offs between objectives. The solution quality was examined for several PTV and OAR fitness functions. The combination of a conformity-based PTV fitness function and a dose-volume histogram (DVH) or equivalent uniform dose (EUD) -based fitness function for the OAR produced relatively uniform and conformal PTV doses, with well-spaced beams. A penalty function added to the fitness functions eliminates hotspots. Comparison of resulting DVHs to those from treatment plans developed with a single-objective fluence optimizer (from a commercial treatment planning system) showed good correlation. Results also indicated that pareto shows promise in optimizing the number of beams.Conclusions:
This initial evaluation of the evolutionary optimization software toolpareto for IMRTtreatment planning demonstrates feasibility and provides motivation for continued development. Advantages of this approach over current commercial methods for treatment planning are many, including: (1) fully automated optimization that avoids human controlled iterative optimization and potentially improves overall process efficiency, (2) formulation of the problem as a true multiobjective one, which provides an optimized set of Pareto nondominated solutions refined over hundreds of generations and compiled from thousands of parameter sets explored during the run, and (3) rapid exploration of the final nondominated set accomplished by a graphical interface used to select the best treatment option for the patient.
38(2011); http://dx.doi.org/10.1118/1.3626486View Description Hide DescriptionPurpose
: Original TomoTherapy systems may involve a trade-off between conformity and treatment speed, the user being limited to three slice widths (1.0, 2.5, and 5.0 cm). This could be overcome by allowing the jaws to define arbitrary fields, including very small slice widths (<1 cm), which are challenging for a beam model. The aim of this work was to incorporate the dynamic jaws feature into a Monte Carlo(MC) model called TomoPen, based on the MC codePENELOPE, previously validated for the original TomoTherapy system.Methods:
To keep the general structure of TomoPen and its efficiency, the simulation strategy introduces several techniques: (1) weight modifiers to account for any jaw settings using only the 5 cm phase-space file; (2) a simplified MC based model called FastStatic to compute the modifiers faster than pure MC; (3) actual simulation of dynamic jaws. Weight modifiers computed with both FastStatic and pure MC were compared. Dynamic jaws simulations were compared with the convolution/superposition (C/S) of TomoTherapy in the “cheese” phantom for a plan with two targets longitudinally separated by a gap of 3 cm. Optimization was performed in two modes: asymmetric jaws-constant couch speed (“running start stop,” RSS) and symmetric jaws-variable couch speed (“symmetric running start stop,” SRSS). Measurements with EDR2 films were also performed for RSS for the formal validation of TomoPen with dynamic jaws.Results
: Weight modifiers computed with FastStatic were equivalent to pure MC within statistical uncertainties (0.5% for three standard deviations). Excellent agreement was achieved between TomoPen and C/S for both asymmetric jaw opening/constant couch speed and symmetric jaw opening/variable couch speed, with deviations well within 2%/2 mm. For RSS procedure, agreement between C/S and measurements was within 2%/2 mm for 95% of the points and 3%/3 mm for 98% of the points, where dose is greater than 30% of the prescription dose (gamma analysis). Dose profiles acquired in transverse and longitudinal directions through the center of the phantom were also compared with excellent agreement (2%/2 mm) between all modalities.Conclusions:
The combination of weights modifiers and interpolation allowed implementing efficiently dynamic jaws and dynamic couch features into TomoPen at a minimal cost in terms of efficiency (simulation around 8 h on a single CPU).
- RADIATION IMAGING PHYSICS
38(2011); http://dx.doi.org/10.1118/1.3609096View Description Hide DescriptionPurpose:
The authors present a robust algorithm that removes the blurring and double-edge artifacts in high-resolution computed tomography(CT)images that are caused by misaligned scanner components. This alleviates the time-consuming process of physically aligning hardware, which is of particular benefit if components are moved or swapped frequently.Methods:
The proposed method uses the experimental data itself for calibration. A parameterized model of the scanner geometry is constructed and the parameters are varied until the sharpest 3D reconstruction is found. The concept is similar to passive auto-focus algorithms of digital optical instruments. The parameters are used to remap the projection data from thephysicaldetector to a virtual aligned detector. This is followed by a standard reconstruction algorithm, namely the Feldkamp algorithm. Feldkamp et al. [J. Opt. Soc. Am. A 1, 612–619 (1984)].Results:
An example implementation is given for a rabbit liver specimen that was collected with a circular trajectory. The optimal parameters were determined in less computation time than that for a full reconstruction. The example serves to demonstrate that (a) sharpness is an appropriate measure for projection alignment, (b) our parameterization is sufficient to characterize misalignments for cone-beam CT, and (c) the procedure determines parameter values with sufficient precision to remove the associated artifacts.Conclusions:
The algorithm is fully tested and implemented for regular use at The Australian National University micro-CT facility for both circular and helical trajectories. It can in principle be applied to more general imaging geometries and modalities. It is as robust as manual alignment but more precise since we have quantified the effect of misalignment.
Noise reduction in spectral CT: Reducing dose and breaking the trade-off between image noise and energy bin selection38(2011); http://dx.doi.org/10.1118/1.3609097View Description Hide DescriptionPurpose
: Our purpose was to reduce imagenoise in spectral CT by exploiting data redundancies in the energy domain to allow flexible selection of the number, width, and location of the energy bins.Methods
: Using a variety of spectral CTimaging methods, conventional filtered backprojection (FBP) reconstructions were performed and resulting images were compared to those processed using a Local HighlY constrained backPRojection Reconstruction (HYPR-LR) algorithm. The mean and standard deviation of CT numbers were measured within regions of interest (ROIs), and results were compared between FBP and HYPR-LR. For these comparisons, the following spectral CTimaging methods were used:(i) numerical simulations based on a photon-counting, detector-basedCTsystem, (ii) a photon-counting, detector-based micro CTsystem using rubidium and potassium chloride solutions, (iii) a commercial CTsystem equipped with integrating detectors utilizing tube potentials of 80, 100, 120, and 140 kV, and (iv) a clinical dual-energy CT examination. The effects of tube energy and energy bin width were evaluated appropriate to each CTsystem.Results
: The mean CT number in each ROI was unchanged between FBP and HYPR-LR images for each of the spectral CTimaging scenarios, irrespective of bin width or tube potential. However, imagenoise, as represented by the standard deviation of CT numbers in each ROI, was reduced by 36%–76%. In all scenarios, imagenoise after HYPR-LR algorithm was similar to that of composite images, which used all available photons. No difference in spatial resolution was observed between HYPR-LR processing and FBP. Dual energy patient data processed using HYPR-LR demonstrated reduced noise in the individual, low- and high-energy images, as well as in the material-specific basis images.Conclusions
: Noise reduction can be accomplished for spectral CT by exploiting data redundancies in the energy domain. HYPR-LR is a robust method for reducing imagenoise in a variety of spectral CTimagingsystems without losing spatial resolution or CT number accuracy. This method improves the flexibility to select energy bins in the manner that optimizes material identification and separation without paying the penalty of increased imagenoise or its corollary, increased patient dose.
Improved motion-compensated image reconstruction for PET using sensitivity correction per respiratory gate and an approximate tube-of-response backprojector38(2011); http://dx.doi.org/10.1118/1.3611041View Description Hide DescriptionPurpose:
One limitation of positron emission tomography(PET)imaging of the torso is patient motion. Motion-compensated image reconstruction (MCIR) is one method employed to reduce the deleterious effects of motion. Existing MCIR algorithms use a single sensitivity correction term, which provides inexact normalization for multigate data. Consequently, in this study, the authors derive and examine the performance of an MCIR algorithm with sensitivity correction per gate. In addition, they demonstrate an approximate tube-of-response (TOR) backprojector.Methods:
Simulated data from the NCAT phantom with six lesions added were used to compare MCIR algorithms with and without the incorporation of sensitivity correction per gate and TOR backprojection to postreconstruction registration (PRR) and imagesreconstructed without motion correction. To make the simulations more realistic, intragate motion was included. Deformation fields were determined from NCAT anatomical images using a free-form deformation approach with bending energy regularization.Results:
Sensitivity correction per gate and TOR backprojection improved mean lesion contrast-to-noise ratio by 6%–8%, with the maximum increase (21%–23%) found for the smallest lesion. These increases were obtained despite a small increase (3%) in noise as measured by standard deviation in a uniform lung region. Sensitivity correction per gate comes at no extra computational cost, whilst replacing line-of-response backprojection with TOR backprojection increased the overall computation time by ∼20%. In addition, MCIR was found to be superior to PRR, with one factor contributing to this difference being the differential impact of interpolation following deformation. MCIR was also shown to exhibit super-resolution.Conclusions:
Replacing a single sensitivity correction term in MCIR with sensitivity correction per gate improves lesion detectability. For a small increase in computational expense, further improvements are achieved using an approximate TOR backprojector rather than line-of-response backprojection.
38(2011); http://dx.doi.org/10.1118/1.3622600View Description Hide DescriptionPurpose:
We present a new morphometric measure of trabecular bone microarchitecture, calledmean node strength (NdStr), which is part of a newly developed approach called long range node-strut analysis. Our general aim is to describe and quantify the apparent “latticelike” microarchitecture of the trabecular bone network.Methods:
Similar in some ways to the topological node-strut analysis introduced by Garrahanet al. [J. Microsc. 142, 341–349 (1986)], our method is distinguished by an emphasis on long-range trabecular connectivity. Thus, while the topological classification of a pixel (after skeletonization) as a node, strut, or terminus, can be determined from the 3 × 3 neighborhood of that pixel, our method, which does not involve skeletonization, takes into account a much larger neighborhood. In addition, rather than giving a discrete classification of each pixel as a node, strut, or terminus, our method produces a continuous variable, node strength. The node strength is averaged over a region of interest to produce the mean node strength of the region.Results:
We have applied our long range node-strut analysis to a set of 26 high-resolution peripheral quantitative computed tomography (pQCT) axial images of human proximal tibiae acquired 17 mm below the tibial plateau. We found that NdStr has a strong positive correlation with trabecular volumetric bone mineral density (BMD). After an exponential transformation, we obtain a Pearson’s correlation coefficient ofr = 0.97. Qualitative comparison of images with similar BMD but with very different NdStr values suggests that the latter measure has successfully quantified the prevalence of the “latticelike” microarchitecture apparent in the image. Moreover, we found a strong correlation (r = 0.62) between NdStr and the conventional node-terminus ratio (Nd/Tm) of Garrahan et al. The Nd/Tm ratios were computed using traditional histomorphometry performed on bone biopsies obtained at the same location as the pQCT scans.Conclusions:
The newly introduced morphometric measure allows a quantitative assessment of the long-range connectivity of trabecular bone. One advantage of this method is that it is based on pQCT images that can be obtained noninvasively from patients, i.e., without having to obtain a bone biopsy from the patient.
38(2011); http://dx.doi.org/10.1118/1.3618731View Description Hide DescriptionPurpose:
The robustness of the phase retrieval methods is of critical importance for limiting and reducing radiation doses involved in x-ray phase contrastimaging. This work is to compare the robustness of two phase retrieval methods by analyzing the phase maps retrieved from the experimental images of a phantom.Methods:
Two phase retrieval methods were compared. One method is based on the transport of intensity equation (TIE) for phase contrast projections, and the TIE-based method is the most commonly used method for phase retrieval in the literature. The other is the recently developed attenuation-partition based (AP-based) phase retrieval method. The authors applied these two methods to experimental projection images of an air-bubble wrap phantom for retrieving the phase map of the bubble wrap. The retrieved phase maps obtained by using the two methods are compared.Results:
In the wrap’s phase map retrieved by using the TIE-based method, no bubble is recognizable, hence, this method failed completely for phase retrieval from these bubble wrap images. Even with the help of the Tikhonov regularization, the bubbles are still hardly visible and buried in the cluttered background in the retrieved phase map. The retrieved phase values with this method are grossly erroneous. In contrast, in the wrap’s phase map retrieved by using the AP-based method, the bubbles are clearly recovered. The retrieved phase values with the AP-based method are reasonably close to the estimate based on the thickness-based measurement. The authors traced these stark performance differences of the two methods to their different techniques employed to deal with the singularity problem involved in the phase retrievals.Conclusions:
This comparison shows that the conventional TIE-based phase retrieval method, regardless if Tikhonov regularization is used or not, is unstable against the noise in the wrap’s projection images, while the AP-based phase retrieval method is shown in these experiments to be superior to the TIE-based method for the robustness in performing the phase retrieval.
38(2011); http://dx.doi.org/10.1118/1.3622611View Description Hide DescriptionPurpose:
In dynamic contrast enhanced CT (DCE-CT) study, prolonged CT scanning with high temporal resolution is required to give accurate and precise estimates of kinetic parameters. However, such scanning protocol could lead to substantial radiationdose to the patient. A novel method is proposed to reduce radiationdose to patient, while maintaining high accuracy for kinetic parameter estimates in DCE-CT study.Methods:
The method is based on a previous investigation that the arterial impulse response (AIR) in DCE-CT study can be predicted using a population-based scheme. In the proposed method, DCE-CT scanning is performed with relatively low temporal resolution, hence, giving rise to reduction in patient dose. A novel method is proposed to estimate the arterial input function (AIF) based on the coarsely sampled AIF. By using the estimated AIF in the tracer kinetic analysis of the coarsely sampled DCE-CT study, the calculated kinetic parameters are able to achieve a high degree of accuracy. The method was tested on a DCE-CT data set of 48 patients with cervical cancer scanned at high temporal resolution. A random cohort of 34 patients was chosen to construct the orthonormal bases of the AIRs via singular value decomposition method. The determined set of orthonormal bases was used to fit the AIFs in the second cohort (14 patients) at varying levels of down sampling. For each dataset in the second cohort, the estimated AIF was used for kinetic analyses of the modified Tofts and adiabatic tissue homogeneity models for each of the down-sampling schemes between intervals from 2 to 15 s. The results were compared with analyses done with the “raw” down-sampled AIF.Results
: In the first group of 34 patients, there were 11 orthonormal bases identified to describe the AIRs. The AIFs in the second group were estimated in high accuracy based on the 11 orthonormal bases established in the first group along with down-sampled AIFs. Using the 11 orthonormal bases, the estimated AIFs for the second group were found to have an averaged maximal percentage error of 3.4% 7.5% in all sampling schemes up to 15 s. The results of kinetic analysis with the proposed method compared with down sampling alone showed that the proposed method is superior in maintaining the accuracy in volume transfer constant ( ) after 9 s down-sampling interval, blood volume ( ) for almost all down-sampling intervals, and blood flow (F) after 11 s down-sampling interval. The preliminary results suggested that the proposed method is able to support scanning intervals of 10–15 s at a cost of 6.2%–10.0% loss in accuracy of and 10.9%–19.4% in , and the scanning intervals of 12–15 s at a cost of 9.7%–14.6% for F in DEC-CT studies for patients with cervix cancer.Conclusions
: The proposed method of AIF estimation allows low scanning frequency in DCE-CT study to reduce radiationdose to patient, while maintaining relatively high accuracy in the kinetic parameter estimates. The initial results suggested that the method is applicable for DCE-CT studies for patients with cervical cancer.