Volume 32, Issue 4, April 2005
 RADIATION THERAPY PHYSICS


Retrospective analysis of 2D patientspecific IMRT verifications
View Description Hide DescriptionWe performed 858 twodimensional (2D) patientspecific intensity modulated radiotherapy verifications over a period of . Multifield, composite treatment plans were measured in phantom using calibrated Kodak EDR2 film and compared with the calculated dose extracted from two treatment planning systems. This research summarizes our findings using the normalized agreement test (NAT) index and the percent of pixels failing the gamma index as metrics to represent the agreement between measured and computed dose distributions. An inhouse dose comparison software package was used to register and compare all verifications. We found it was important to use an automatic positioning algorithm to achieve maximum registration accuracy, and that our automatic algorithm agreed well with anticipated results from known phantom geometries. We also measured absolute dose for each case using an ion chamber. Because the computed distributions agreed with ion chamber measurements better than the EDR2 film doses, we normalized EDR2 data to the computed distributions. The distributions of both the NAT indices and the percentage of pixels failing the gamma index were found to be exponential distributions. We continue to use both the NAT index and percent of pixels failing gamma with criteria to evaluate future verifications, as these two metrics were found to be complementary. Our data showed that using or criteria produces results similar to those using criteria. Normalized comparisons that have a NAT index greater than 45 and/or more than 20% of the pixels failing gamma for criteria represent outliers from our clinical data set and require further analysis. Because our QA verification results were exponentially distributed, rather than a tight grouping of similar results, we continue to perform patientspecific QA in order to identify and correct outliers in our verifications. The data from this work could be useful as a reference for other clinics to indicate anticipated trends in 2D verifications under various conditions.

Effects of xray and CT image enhancements on the robustness and accuracy of a rigid 3D/2D image registration
View Description Hide DescriptionA rigid body threedimensional/twodimensional (3D/2D) registration method has been implemented using mutual information, gradient ascent, and 3D texturemapbased digitally reconstructedradiographs. Nine combinations of commonly used xray and computed tomography(CT)image enhancement methods, including window leveling, histogram equalization, and adaptive histogram equalization, were examined to assess their effects on accuracy and robustness of the registration method. From a set of experiments using an anthropomorphic chest phantom, we were able to draw several conclusions. First, the CT and xray preprocessing combination with the widest attraction range was the one that linearly stretched the histograms onto the entire display range on both CT and xrayimages. The average attraction ranges of this combination were and in the translation and rotation dimensions, respectively, and the average errors were and . Second, the combination of the CTimage with tissue and bone information and the xrayimages with adaptive histogram equalization also showed subvoxel accuracy, especially the best in the translation dimensions. However, its attraction ranges were the smallest among the examined combinations (on average and ). Last the boneonly information on the CTimage did not show convergency property to the correct registration.

Quantitation of the reconstruction quality of a fourdimensional computed tomography process for lung cancer patients
View Description Hide DescriptionWe have developed a fourdimensional computed tomography (4D CT) technique for mapping breathing motion in radiotherapy treatment planning. A multislice CTscanner ( slices) operated in ciné mode was used to acquire 12 contiguous slices in each couch position for 15 consecutive scans ( rotation, between scans) while the patient underwent simultaneous quantitative spirometry measurements to provide a sorting metric. The spirometrysorted scans were used to reconstruct a 4D data set. A critical factor for 4D CT is quantifying the reconstructed data set quality which we measure by correlating the metric used relative to internalobject motion. For this study, the internal air content within the lung was used as a surrogate for internal motion measurements. Thresholding and image morphological operations were applied to delineate the aircontaining tissues(lungs, trachea) from each CT slice. The Hounsfield values were converted to the internal air content . The relationship between the air content and spirometermeasured tidal volume was found to be quite linear throughout the lungs and was used to estimate the overall accuracy and precision of tidal volumesorted 4D CT. Inspection of the CTscan air content as a function of tidal volume showed excellent correlations (typically ) throughout the lung volume. Because of the discovered linear relationship, the ratio of internal air content to tidal volume was indicative of the fraction of air change in each couch position. Theoretically, due to air density differences within the lung and in room, the sum of these ratios would equal 1.11. For 12 patients, the mean value was , indicating the high quality of spirometrybased image sorting. The residual of a firstorder fit between and was used to estimate the process precision. For all patients, the precision was better than 8%, with a mean value of . This quantitative analysis highlights the value of using spirometry as the metric in sorting CT scans. The 4D reconstruction provides the CT data required to measure the threedimensional trajectory of tumor and lungtissue during free breathing.

Monte Carlo simulation of a protontherapy platform devoted to ocular melanoma
View Description Hide DescriptionPatients with ocular melanoma have been treated since June 1991 at the medical cyclotron of the Centre Antoine Lacassagne (CAL). Positions and sizes of the ocular nozzle elements were initially defined based on experimental work, taking as a pattern functional existing facilities. Nowadays Monte Carlo (MC) calculation offers a tool to refine this geometry by adjusting size and place of beam modeling devices. Moreover, the MC tool is a useful way to calculate the dose and to evaluate the impact of secondary particles in the field of radiotherapy or radiation protection. Both LINAC and cyclotron producing x rays, electrons, protons, and neutrons are available in CAL, which suggests choosing MCNPX for its particle versatility. As a first step, the existing installation was input in MCNPX to check its aptitude to reproduce experimentally measured depthdose profile, lateral profile, outputfactor (OF), and absolute dose. The geometry was defined precisely and described from the last achromatic bending magnet of our proton beam line to the position of treated eyes. Relative comparisons of percentage depthdose and lateral profiles, performed between measured data and simulations, show an agreement of the order of 2% in dose and in range accuracy. These comparisons, carried out with and without beammodifying device, yield results compatible to the required precision in ocular melanoma treatments, as long as adequate choices are made on MCNPX input decks for physics card. Absolute dose and OF issued from calculations and measurements were also compared. Results obtained for these two kinds of data, carried out in the simplified situation of an unmodulated beam, indicate that MC calculation could effectively complement measurements. These encouraging results are a large source of motivation to promote further studies, first in a new design of the ocular nozzle, and second in the analysis of the influence of beammodifying devices attached to the final patient collimator, such as wedge or compensators, on dose values.

Investigation of using a power function as a cost function in inverse planning optimization
View Description Hide DescriptionThe purpose of this paper is to investigate the use of a power function as a cost function in inverse planning optimization. The cost function for each structure is implemented as an exponential power function of the deviation between the resultant dose and prescribed or constrained dose. The total cost function for all structures is a summation of the cost function of every structure. When the exponents of all terms in the cost function are set to 2, the cost function becomes a classical quadratic cost function. An independent optimization module was developed and interfaced with a research treatment planning system from the University of North Carolina for dose calculation and display of results. Three clinical cases were tested for this study with various exponents set for tumor targets and sensitive structures. Treatment plans with these exponent settings were compared, using dose volume histograms. The results of our study demonstrated that using an exponent higher than 2 in the cost function for the target achieved better dose homogeneity than using an exponent of 2. An exponent higher than 2 for serial sensitive structures can effectively reduce the maximum dose. Varying the exponent from 2 to 4 resulted in the most effective changes in dose volume histograms while the change from 4 to 8 is less drastic, indicating a situation of saturation. In conclusion, using a power function with exponent greater than 2 as a cost function can effectively achieve homogeneous dose inside the target and/or minimize maximum dose to the critical structures.

Determination of maximum leaf velocity and acceleration of a dynamic multileaf collimator: Implications for 4D radiotherapy
View Description Hide DescriptionThe dynamic multileaf collimator(MLC) can be used for fourdimensional (4D), or tumor tracking radiotherapy. However, the leaf velocity and acceleration limitations become a crucial factor as the MLC leaves need to respond in near real time to the incoming respiration signal. The aims of this paper are to measure maximum leaf velocity,acceleration, and deceleration to obtain the mechanical response times for the MLC, and determine whether the MLC is suitable for 4D radiotherapy.MLC leaf sequence files, requiring the leaves to reach maximum acceleration and velocity during motion, were written. The leaf positions were recorded every , from which the maximum leaf velocity,acceleration, and deceleration were derived. The dependence on the velocity and acceleration of the following variables were studied: leaf banks, inner and outer leaves, MLCMLC variations, gravity, friction, and the stability of measurements over time. Measurement results show that the two leaf banks of a MLC behave similarly, while the inner and outer leaves have significantly different maximum leaf velocities. The MLCMLC variations and the dependence of gravity on maximum leaf velocity are statistically significant. The average maximum leaf velocity at the isocenter plane of the MLC ranged from 3.3 to . The acceleration and deceleration at the isocenter plane of the MLC ranged from 50 to and 46 to , respectively. Interleaf friction had a negligible effect on the results, and the MLC parameters remained stable with time. Equations of motion were derived to determine the ability of the MLC response to fluoroscopymeasured diaphragm motion. Given the present MLC mechanical characteristics, 4D radiotherapy is feasible for up to 97% of respiratory motion. For the largest respiratory motionvelocities observed, beam delivery should be temporarily stopped (beam hold).

Fourdimensional radiotherapy planning for DMLCbased respiratory motion tracking
View Description Hide DescriptionFourdimensional (4D) radiotherapy is the explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy. Temporal anatomic changes can occur for many reasons, though the focus of the current investigation is respiration motion for lungtumors. The aim of this study was to develop 4D radiotherapy treatmentplanning methodology for DMLCbased respiratory motion tracking. A 4D computed tomography(CT) scan consisting of a series of eight 3D CTimage sets acquired at different respiratory phases was used for treatment planning. Deformable image registration was performed to map each CT set from the peakinhale respiration phase to the CTimage sets corresponding to subsequent respiration phases. Deformable registration allows the contours defined on the peakinhale CT to be automatically transferred to the other respiratory phase CTimage sets. Treatment planning was simultaneously performed on each of the eight 3D image sets via automated scripts in which the MLCdefined beam aperture conforms to the PTV (which in this case equaled the GTV due to CT scan length limitations) plus a penumbral margin at each respiratory phase. The dose distribution from each respiratory phase CTimage set was mapped back to the peakinhale CTimage set for analysis. The treatment intent of 4D planning is that the radiation beam defined by the DMLC tracks the respirationinduced target motion based on a feedback loop including the respiration signal to a realtime MLC controller. Deformation with respiration was observed for the lungtumor and normal tissues. This deformation was verified by examining the mapping of high contrast objects, such as the lungs and cord, between image sets. For the test case, dosimetric reductions for the cord, heart, and lungs were found for 4D planning compared with 3D planning. 4D radiotherapy planning for DMLCbased respiratory motion tracking is feasible and may offer tumordose escalation and/or a reduction in treatmentrelated complications. However, 4D planning requires new planning tools, such as deformable registration and automated treatment planning on multiple CTimage sets.

Radiological properties of normoxic polymer gel dosimeters
View Description Hide DescriptionThe radiological properties of the normoxic polymergeldosimeters MAGIC, MAGAS, and MAGAT [methacrylic and ascorbic acid in gelatin initiated by copper; methacrylic acid gelatine gel with ascorbic acid; and methacrylic acid gelatine and tetrakis (hydroxymethyl) phosphonium chloride, respectively] have been investigated. The radiological water equivalence was determined by comparing the polymergel macroscopic photon and electron interaction cross sections over the energy range from 10 keV to 20 MeV and by Monte Carlo modeling of depth doses. Normoxic polymergeldosimeters have a high gelatine and monomer concentration and therefore mass density up to 3.8% higher than water. This results in differences between the crosssection ratios of the normoxic polymergels and water of up to 3% for the attenuation, energy absorption, and collision stopping power coefficient ratios through the Compton dominant energy range. The mass crosssection ratios were within 2% of water except for the mass attenuation and energyabsorption coefficients ratios, which showed differences with water of up to 6% for energies less than 100 keV. Monte Carlo modeling was undertaken for the polymergeldosimeters to model the electron and photon transport resulting from a 6 MV photon beam. The absolute percentage differences between gel and water were within 1% and the relative percentage differences were within 3.5%. The results show that the MAGAT gel formulation is the most radiological water equivalent of the normoxic polymergeldosimeters investigated due to its lower mass density measurement compared with MAGAS and MAGIC gels.

Distributions of decayed nuclei generated in the and targets by the target nuclear fragment reaction using therapeutic MONO and SOBP proton beam
View Description Hide DescriptionIn protonradiotherapy, the irradiation dose can be concentrated on a tumor. To use this radiotherapy efficiently in the clinical field, it is necessary to evaluate the protonirradiated area and condition. The protonirradiated area can be confirmed by coincidence detection of pair annihilation gamma rays from decayed nuclei generated by target nuclear fragment reaction of irradiated proton nuclei and nuclei in the irradiation target. In this study, we performed experiments of proton irradiation to a polyethylene target containing nuclei, which is a major component of the human body, and a gelatinous water target containing nuclei at different proton irradiation energy levels under different beam conditions of monoenergetic Bragg peak and spreadout Bragg peak. The distribution of the activity in the target after proton irradiation was measured by a positron emission tomography(PET) apparatus, and compared with the calculated distribution. The temporal dependence of the activity distribution during the period between the completion of proton irradiation and the start of measurement by the PET apparatus was examined. The activity by clinical proton irradiation was in the PE target and in the water target, indicating that the intensity was sufficient for the evaluation of the distribution. The range of the activity distribution against the physical range was short (several millimeter water equivalent length), indicating the presence of target dependence. The range difference in the water target was slightly large with time dependence until the start of measurement. The difference of the lateral widths with full width half at maximum in the distributions of the measured irradiated dose and activity was within .

Comprehensive Monte Carlo calculation of the point spread function for a commercial Si EPID
View Description Hide DescriptionImages produced by commercial amorphous silicon electronic portal imaging devices (Si EPIDs) are subject to multiple blurring processes. Implementation of these devices for fluence measurement requires that the blur be removed from the images. A standard deconvolution operation can be performed to accomplish this assuming the blur kernel is spatially invariant and accurately known. This study determines a comprehensive blur kernel for the Varian aS500 EPID. Monte Carlo techniques are used to derive a dose kernel and an optical kernel, which are then combined to yield an overall blur kernel for both 6 and 15 MV photon beams. Experimental measurement of the line spread function (LSF) is used to verify kernel shape. Kernel performance is gauged by comparing EPIDimage profiles with inair dose profiles measured using a diamonddetector (approximating fluence) both before and after the EPIDimages have been deconvolved. Quantitative comparisons are performed using the metric, an extension of the wellknown metric, using acceptance criteria of 0.0784 cm (1 pixel width) distancetoagreement and 2% of the relative central axis fluence . Without incorporating any free parameters, acceptance was increased from 49.0% of pixels in a crossplane profile for a 6 MV open field to 92.0%. For a physically wedged field, acceptance increased from 40.3% to 73.9%. The effect of the optical kernel was found to be negligible for these acceptance parameters, however for , we observed an improvement from 66.1% (without) to 78.6% (with) of scores (from 20.6% before deconvolution). It is demonstrated that an empirical kernel having a triple exponential form or a semiempirical kernel based on a simplified model of the detector stack can match the performance of the comprehensive kernel.

A stochastic convolution/superposition method with isocenter sampling to evaluate intrafraction motion effects in IMRT
View Description Hide DescriptionCurrent methods to calculate dose distributions with organmotion can be broadly classified as “dose convolution” and “fluence convolution” methods. In the former, a static dose distribution is convolved with the probability distribution function (PDF) that characterizes the motion. However, artifacts are produced near the surface and around inhomogeneities because the method assumes shift invariance. Fluence convolution avoids these artifacts by convolving the PDF with the incident fluence instead of the patient dose. In this paper we present an alternative method that improves the accuracy, generality as well as the speed of dose calculation with organmotion. The algorithm starts by sampling an isocenter point from a parametrically defined space curve corresponding to the patientspecific motion trajectory. Then a photon is sampled in the linac head and propagated through the threedimensional (3D) collimator structure corresponding to a particular MLC segment chosen randomly from the planned IMRT leaf sequence. The photon is then made to interact at a point in the CTbased simulation phantom. Randomly sampled monoenergetic kernel rays issued from this point are then made to deposit energy in the voxels. Our method explicitly accounts for MLCspecific effects (spectral hardening, tongueandgroove, head scatter) as well as changes in SSD with isocentric displacement, assuming that the body moves rigidly with the isocenter. Since the positions are randomly sampled from a continuum, there is no motion discretization, and the computation takes no more time than a static calculation. To validate our method, we obtained ten separate film measurements of an IMRT plan delivered on a phantom moving sinusoidally, with each fraction starting with a random phase. For 2 cm motion amplitude, we found that a tenfraction average of the film measurements gave an agreement with the calculated infinite fraction average to within 2 mm in the isodose curves. The results also corroborate the existing notion that the interfraction dose variability due to the interplay between the MLCmotion and breathing motion averages out over typical multifraction treatments. Simulation with motion waveforms more representative of real breathing indicate that the motion can produce penumbral spreading asymmetric about the static dose distributions. Such calculations can help a clinician decide to use, for example, a larger margin in the superior direction than in the inferior direction. In the paper we demonstrate that a 15 min run on a single CPU can readily illustrate the effect of a patientspecific breathing waveform, and can guide the physician in making informed decisions about margin expansion and dose escalation.

Photonbeam subsource sensitivity to the initial electronbeam parameters
View Description Hide DescriptionOne limitation to the widespread implementation of Monte Carlo(MC) patient dosecalculation algorithms for radiotherapy is the lack of a general and accurate source model of the accelerator radiationsource. Our aim in this work is to investigate the sensitivity of the photonbeam subsource distributions in a MCsource model (with target, primary collimator, and flattening filter photon subsources and an electron subsource) for 6 and 18MV photon beams when the energy and radial distributions of initial electrons striking a linac target change. For this purpose, phasespace data (PSD) was calculated for various mean electron energies striking the target, various normally distributed electron energy spread, and various normally distributed electron radial intensity distributions. All PSD was analyzed in terms of energy, fluence, and energy fluence distributions, which were compared between the different parameter sets. The energy spread was found to have a negligible influence on the subsource distributions. The mean energy and radial intensity significantly changed the target subsource distribution shapes and intensities. For the primary collimator and flattening filter subsources, the distribution shapes of the fluence and energy fluence changed little for different mean electron energies striking the target, however, their relative intensity compared with the target subsource change, which can be accounted for by a scaling factor. This study indicates that adjustments to MCsource models can likely be limited to adjusting the target subsource in conjunction with scaling the relative intensity and energy spectrum of the primary collimator, flattening filter, and electron subsources when the energy and radial distributions of the initial electronbeam change.
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 RADIATION IMAGING PHYSICS


Cardiac conebeam CT volume reconstruction using ART
View Description Hide DescriptionModern computed tomography systems allow volume imaging of the heart. Up to now, approximately twodimensional (2D) and 3D algorithms based on filtered backprojection are used for the reconstruction. These algorithms become more sensitive to artifacts when the cone angle of the xray beam increases as it is the current trend of computed tomography(CT) technology. In this paper, we investigate the potential of iterative reconstruction based on the algebraic reconstruction technique (ART) for helical cardiac conebeam CT. Iterative reconstruction has the advantages that it takes the cone angle into account exactly and that it can be combined with retrospective cardiac gating fairly easily. We introduce a modified ART algorithm for cardiacCTreconstruction. We apply it to clinical cardiac data from a 16slice CT scanner and compare the images to those obtained with a current analytical reconstruction method. In a second part, we investigate the potential of iterative reconstruction for a large area detector with 256 slices. For the clinical cases, iterative reconstruction produces excellent images of diagnostic quality. For the large area detector, iterative reconstruction produces images superior to analytical reconstruction in terms of conebeam artifacts.

Fourdimensional computed tomography: Image formation and clinical protocol
View Description Hide DescriptionRespiratory motion can introduce significant errors in radiotherapy. Conventional CT scans as commonly used for treatment planning can include severe motion artifacts that result from interplay effects between the advancing scan plane and object motion. To explicitly include organ/target motion in treatment planning and delivery, timeresolved CTdata acquisition (4D Computed Tomography) is needed. 4DCT can be accomplished by oversampled CTdata acquisition at each slice. During several CT tube rotations projection data are collected in axial cine mode for the duration of the patient’s respiratory cycle (plus the time needed for a full CT gantry rotation). Multiple images are then reconstructed per slice that are evenly distributed over the acquisition time. Each of these images represents a different anatomical state during a respiratory cycle. After data acquisition at one couch position is completed, x rays are turned off and the couch advances to begin data acquisition again until full coverage of the scan length has been obtained. Concurrent to CTdata acquisition the patient’s abdominal surface motion is recorded in precise temporal correlation. To obtain CT volumes at different respiratory states, reconstructed images are sorted into different spatiotemporally coherent volumes based on respiratory phase as obtained from the patient’s surface motion. During binning, phase tolerances are chosen to obtain complete volumetric information since images at different couch positions are reconstructed at different respiratory phases. We describe 4DCT image formation and associated experiments that characterize the properties of 4DCT. Residual motion artifacts remain due to partial projection effects. Temporal coherence within resorted 4DCT volumes is dominated by the number of reconstructed images per slice. The more images are reconstructed, the smaller phase tolerances can be for retrospective sorting. From phantom studies a precision of about for quasiregular motion and typical respiratory periods could be concluded. A protocol for 4DCT scanning was evaluated and clinically implemented at the MGH. Patient data are presented to elucidate how additional patient specific parameters can impact 4DCT imaging.

Experimental validation of the Wigner distributions theory of phasecontrast imaging
View Description Hide DescriptionRecently, a new theory of phasecontrast imaging has been proposed by Wu and Liu [Med. Phys.31, 2378–2384 (2004)]. This theory, based upon Wigner distributions, provides a much stronger foundation for the evaluation of phasecontrast imaging systems than did the prior theories based upon FresnelKirchhoff diffraction theory. In this paper, we compare results of measurements made in our laboratory of phase contrast for different geometries and tube voltages to the predictions of the Wu and Liu model. In our previous publications, we have used an empirical measurement (the edge enhancement index) to parametrize the degree of phasecontrast effects in an image. While the Wu and Liu model itself does not predict imagecontrast, it does measure the degree of phase contrast that the system can image for a given spatial frequency. We have found that our previously published experimental results relating phasecontrast effects to geometry and xray tube voltage are consistent with the predictions of the Wu and Liu model.

A new data consistency condition for fanbeam projection data
View Description Hide DescriptionThe sum of all attenuation data acquired in one view of parallelbeam projections is a view angle independent constant. This fact is known as a data consistency condition on the twodimensional Radon transforms. It plays an important role in tomographic image reconstruction and artifact correction. In this paper, a novel fanbeam data consistency condition (FDCC) is derived and presented. Using the FDCC, individual projection data in one view of fanbeam projections can be estimated from filtering all other projection data measured from different view angles. Numerical simulations are performed to validate the new FDCC in correcting ring artifacts caused by malfunctioning detector cells.

Accurate technique for complete geometric calibration of conebeam computed tomography systems
View Description Hide DescriptionConebeam computed tomographysystems have been developed to provide in situimaging for the purpose of guiding radiation therapy. Clinical systems have been constructed using this approach, a clinical linear accelerator (Elekta Synergy RP) and an isocentric Carm. Geometric calibration involves the estimation of a set of parameters that describes the geometry of such systems, and is essential for accurate image reconstruction. We have developed a general analytic algorithm and corresponding calibration phantom for estimating these geometric parameters in conebeam computed tomography(CT)systems. The performance of the calibration algorithm is evaluated and its application is discussed. The algorithm makes use of a calibration phantom to estimate the geometric parameters of the system. The phantom consists of 24 steel ball bearings (BBs) in a known geometry. Twelve BBs are spaced evenly at 30 deg in two planeparallel circles separated by a given distance along the tube axis. The detector (e.g., a flat panel detector) is assumed to have no spatial distortion. The method estimates geometric parameters including the position of the xray source, position, and rotation of the detector, and gantry angle, and can describe complex sourcedetector trajectories. The accuracy and sensitivity of the calibration algorithm was analyzed. The calibration algorithm estimates geometric parameters in a high level of accuracy such that the quality of CTreconstruction is not degraded by the error of estimation. Sensitivity analysis shows uncertainty of 0.01° (around beam direction) to 0.3° (normal to the beam direction) in rotation, and 0.2 mm (orthogonal to the beam direction) to 4.9 mm (beam direction) in position for the medical linear accelerator geometry. Experimental measurements using a laboratory bench Conebeam CTsystem of known geometry demonstrate the sensitivity of the method in detecting small changes in the imaging geometry with an uncertainty of 0.1 mm in transverse and vertical (perpendicular to the beam direction) and 1.0 mm in the longitudinal (beam axis) directions. The calibration algorithm was compared to a previously reported method, which uses one ball bearing at the isocenter of the system, to investigate the impact of more precise calibration on the image quality of conebeam CTreconstruction. A thin steel wire located inside the calibration phantom was imaged on the conebeam CT lab bench with and without perturbations in source and detector position during the scan. The described calibration method improved the quality of the image and the geometric accuracy of the object reconstructed, improving the full width at half maximum of the wire by 27.5% and increasing contrast of the wire by 52.8%. The proposed method is not limited to the geometric calibration of conebeam CTsystems but can be used for many other systems, which consist of one or more point sources and area detectors such as calibration of megavoltage (MV) treatment system (focal spot movement during the beam delivery, MV source trajectory versus gantry angle, the axis of collimator rotation, and couch motion), cross calibration between Kilovolt imaging and MV treatment system, and cross calibration between multiple imagingsystems. Using the complete information of the system geometry, it was demonstrated that high image quality in CTreconstructions is possible even in systems with large geometric nonidealities.

Respiratory motion estimation from slowly rotating xray projections: Theory and simulation
View Description Hide DescriptionUnderstanding the movement of tumors caused by respiratory motion is very important for conformal radiatherapy. However, respiratory motion is very difficult to study by conventional xrayCTimaging since object motion causes inconsistent projection views, leading to artifacts in reconstructed images. We propose to estimate the parameters of a nonrigid, free breathing motion model from a set of projection views of the thorax that are acquired using a slowly rotating conebeam CTscanner. This approach involves deforming a motionfree reference thorax volume according to the estimated parameters and comparing its projections to the corresponding measured projection views. The parameters are optimized by minimizing a regularized squared error cost function. Simulation results with a fanbeam geometry show good agreement between the estimated motion and the true motion, which supports the potential of this approach for estimating fourdimensional (threedimensional spatial + temporal) respiratory motion.

Threedimensional fluorescence lifetime tomography
View Description Hide DescriptionNearinfraredfluorescencetomography using molecularly targeted lifetimesensitive, fluorescent contrast agents have applications for earlystage cancer diagnostics. Yet, although the measurement of fluorescent lifetime imaging microscopy (FLIM) is extensively used in microscopy and spectroscopy applications, demonstration of fluorescence lifetime tomography for medical imaging is limited to twodimensional studies. Herein, the feasibility of threedimensional fluorescencelifetime tomography on clinically relevant phantom volumes is established, using (i) a gainmodulated intensified charge coupled device(CCD) and modulated laser diode imaging system, (ii) two fluorescent contrast agents, e.g., Indocyanine green and 33’Diethylthiatricarbocyanine iodide differing in their fluorescence lifetime by 0.62 ns, and (iii) a two stage approximate extended Kalman filter reconstruction algorithm. Fluorescencemeasurements of phase and amplitude were acquired on the phantom surface under different target to background fluorescence absorption (70:1, 100:1) and fluorescence lifetime (1:1, 2.1:1) contrasts at target depths of 1.4–2 cm. The Bayesian tomography algorithm was employed to obtain threedimensional images of lifetime and absorption owing to the fluorophores.
