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At least 865 records · Page 48
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DOE OSTI2013

Four-dimensional dose evaluation using deformable image registration in radiotherapy for liver cancer

Purpose: In order to evaluate the dosimetric impact of respiratory motion on the dose delivered to the target volume and critical organs during free-breathing radiotherapy, a four-dimensional dose was evaluated using deformable image registration (DIR). Methods: Four-dimensional computed tomography (4DCT) images were acquired for 11 patients who were treated for liver cancer. Internal target volume-based treatment planning and dose calculation (3D dose) were performed using the end-exhalation phase images. The four-dimensional dose (4D dose) was calculated based on DIR of all phase images from 4DCT to the planned image. Dosimetric parameters from the 4D dose, were calculated and compared with those from the 3D dose. Results: There was no significant change of the dosimetric parameters for gross tumor volume (p > 0.05). The increase D{sub mean} and generalized equivalent uniform dose (gEUD) for liver were by 3.1%{+-} 3.3% (p= 0.003) and 2.8%{+-} 3.3% (p= 0.008), respectively, and for duodenum, they were decreased by 15.7%{+-} 11.2% (p= 0.003) and 15.1%{+-} 11.0% (p= 0.003), respectively. The D{sub max} and gEUD for stomach was decreased by 5.3%{+-} 5.8% (p= 0.003) and 9.7%{+-} 8.7% (p= 0.003), respectively. The D{sub max} and gEUD for right kidney was decreased by 11.2%{+-} 16.2% (p= 0.003) and 14.9%{+-} 16.8% (p= 0.005), respectively. For left kidney, D{sub max} and gEUD were decreased by 11.4%{+-} 11.0% (p= 0.003) and 12.8%{+-} 12.1% (p= 0.005), respectively. The NTCP values for duodenum and stomach were decreased by 8.4%{+-} 5.8% (p= 0.003) and 17.2%{+-} 13.7% (p= 0.003), respectively. Conclusions: The four-dimensional dose with a more realistic dose calculation accounting for respiratory motion revealed no significant difference in target coverage and potentially significant change in the physical and biological dosimetric parameters in normal organs during free-breathing treatment.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Collimator design for a dedicated molecular breast imaging-guided biopsy system: Proof-of-concept

Purpose: Molecular breast imaging (MBI) is a dedicated nuclear medicine breast imaging modality that employs dual-head cadmium zinc telluride (CZT) gamma cameras to functionally detect breast cancer. MBI has been shown to detect breast cancers otherwise occult on mammography and ultrasound. Currently, a MBI-guided biopsy system does not exist to biopsy such lesions. Our objective was to consider the utility of a novel conical slant-hole (CSH) collimator for rapid (<1 min) and accurate monitoring of lesion position to serve as part of a MBI-guided biopsy system. Methods: An initial CSH collimator design was derived from the dimensions of a parallel-hole collimator optimized for MBI performed with dual-head CZT gamma cameras. The parameters of the CSH collimator included the collimator height, cone slant angle, thickness of septa and cones of the collimator, and the annular areas exposed at the base of the cones. These parameters were varied within the geometric constraints of the MBI system to create several potential CSH collimator designs. The CSH collimator designs were evaluated using Monte Carlo simulations. The model included a breast compressed to a thickness of 6 cm with a 1-cm diameter lesion located 3 cm from the collimator face. The number of particles simulated was chosen to represent the count density of a low-dose, screening MBI study acquired with the parallel-hole collimator for 10 min after a {approx}150 MBq (4 mCi) injection of Tc-99m sestamibi. The same number of particles was used for the CSH collimator simulations. In the resulting simulated images, the count sensitivity, spatial resolution, and accuracy of the lesion depth determined from the lesion profile width were evaluated. Results: The CSH collimator design with default parameters derived from the optimal parallel-hole collimator provided 1-min images with error in the lesion depth estimation of 1.1 {+-} 0.7 mm and over 21 times the lesion count sensitivity relative to 1-min images acquired with the current parallel-hole collimator. Sensitivity was increased via more vertical cone slant angles, larger annular areas, thinner cone walls, shorter cone heights, and thinner radiating septa. Full width at half maximum trended in the opposite direction as sensitivity for all parameters. There was less error in the depth estimates for less vertical slant angles, smaller annular areas, thinner cone walls, cone heights near 1 cm, and generally thinner radiating septa. Conclusions: A Monte Carlo model was used to demonstrate the feasibility of a CSH collimator design for rapid biopsy application in molecular breast imaging. Specifically, lesion depth of a 1-cm diameter lesion positioned in the center of a typical breast can be estimated with error of less than 2 mm using circumferential count profiles of images acquired in 1 min.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Assessment of cardiac single-photon emission computed tomography performance using a scanning linear observer

Purpose: Single-photon emission computed tomography (SPECT) is widely used to detect myocardial ischemia and myocardial infarction. It is important to assess and compare different SPECT system designs in order to achieve the highest detectability of cardiac defects. Methods: Whitaker et al.'s study ['Estimating random signal parameters from noisy images with nuisance parameters: linear and scanning-linear methods,' Opt. Express 16(11), 8150-8173 (2008)] on the scanning linear observer (SLO) shows that the SLO can be used to estimate the location and size of signals. One major advantage of the SLO is that it can be used with projection data rather than with reconstruction data. Thus, this observer model assesses the overall hardware performance independent of any reconstruction algorithm. In addition, the computation time of image quality studies is significantly reduced. In this study, three systems based on the design of the GE cadmium zinc telluride-based dedicated cardiac SPECT camera Discovery 530c were assessed. This design, which is officially named the Alcyone Technology: Discovery NM 530c, was commercialized in August, 2009. The three systems, GE27, GE19, and GE13, contain 27, 19, and 13 detectors, respectively. Clinically, a human heart can be virtually segmented into three coronary artery territories: the left-anterior descending artery, left-circumflex artery, and right coronary artery. One of the most important functions of a cardiac SPECT system is to produce images from which a radiologist can accurately predict in which territory the defect exists [http://www.asnc.org/media/PDFs/PPReporting081511.pdf, Guideline from American Society of Nuclear Cardiology]. A good estimation of the extent of the defect from the projection images is also very helpful for determining the seriousness of the myocardial ischemia. In this study, both the location and extent of defects were estimated by the SLO, and the system performance was assessed by localization receiver operating characteristic (LROC) [P. Khurd and G. Gindi, 'Decision strategies maximizing the area under the LROC curve,' Proc. SPIE 5749, 150-161 (2005)] or estimation receiver operating characteristic (EROC) [E. Clarkson, 'Estimation receiver operating characteristic curve and ideal observers for combined detection/estimation tasks,' J. Opt. Soc. Am. A 24, B91-B98 (2007)] curves. Results: The area under the LROC/EROC curve (AULC/AUEC) and the true positive fraction (TPF) at a specific false positive fraction (FPF) can be treated as the figures of merit. For radii estimation with a 1 mm tolerance, the AUEC values of the GE27, GE19, and GE13 systems are 0.8545, 0.8488, and 0.8329, and the TPF at FPF = 5% are 77.1%, 76.46%, and 73.55%, respectively. The assessment of all three systems revealed that the GE19 system yields estimated information and cardiac defect detectability very close to those of the GE27 system while using eight fewer detectors. Thus, 30% of the expensive detector units can be removed with confidence. Conclusions: As the results show, a combination of the SLO and LROC/EROC curves can determine the configuration that yields the most relevant estimation/detection information. Thus, this is a useful method for assessing cardiac SPECT systems.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2013

Control over structure-specific flexibility improves anatomical accuracy for point-based deformable registration in bladder cancer radiotherapy

Purpose: Future developments in image guided adaptive radiotherapy (IGART) for bladder cancer require accurate deformable image registration techniques for the precise assessment of tumor and bladder motion and deformation that occur as a result of large bladder volume changes during the course of radiotherapy treatment. The aim was to employ an extended version of a point-based deformable registration algorithm that allows control over tissue-specific flexibility in combination with the authors' unique patient dataset, in order to overcome two major challenges of bladder cancer registration, i.e., the difficulty in accounting for the difference in flexibility between the bladder wall and tumor and the lack of visible anatomical landmarks for validation. Methods: The registration algorithm used in the current study is an extension of the symmetric-thin plate splines-robust point matching (S-TPS-RPM) algorithm, a symmetric feature-based registration method. The S-TPS-RPM algorithm has been previously extended to allow control over the degree of flexibility of different structures via a weight parameter. The extended weighted S-TPS-RPM algorithm was tested and validated on CT data (planning- and four to five repeat-CTs) of five urinary bladder cancer patients who received lipiodol injections before radiotherapy. The performance of the weighted S-TPS-RPM method, applied to bladder and tumor structures simultaneously, was compared with a previous version of the S-TPS-RPM algorithm applied to bladder wall structure alone and with a simultaneous nonweighted S-TPS-RPM registration of the bladder and tumor structures. Performance was assessed in terms of anatomical and geometric accuracy. The anatomical accuracy was calculated as the residual distance error (RDE) of the lipiodol markers and the geometric accuracy was determined by the surface distance, surface coverage, and inverse consistency errors. Optimal parameter values for the flexibility and bladder weight parameters were determined for the weighted S-TPS-RPM. Results: The weighted S-TPS-RPM registration algorithm with optimal parameters significantly improved the anatomical accuracy as compared to S-TPS-RPM registration of the bladder alone and reduced the range of the anatomical errors by half as compared with the simultaneous nonweighted S-TPS-RPM registration of the bladder and tumor structures. The weighted algorithm reduced the RDE range of lipiodol markers from 0.9-14 mm after rigid bone match to 0.9-4.0 mm, compared to a range of 1.1-9.1 mm with S-TPS-RPM of bladder alone and 0.9-9.4 mm for simultaneous nonweighted registration. All registration methods resulted in good geometric accuracy on the bladder; average error values were all below 1.2 mm. Conclusions: The weighted S-TPS-RPM registration algorithm with additional weight parameter allowed indirect control over structure-specific flexibility in multistructure registrations of bladder and bladder tumor, enabling anatomically coherent registrations. The availability of an anatomically validated deformable registration method opens up the horizon for improvements in IGART for bladder cancer.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

The effect of irregular breathing patterns on internal target volumes in four-dimensional CT and cone-beam CT images in the context of stereotactic lung radiotherapy

Purpose: Stereotactic lung radiotherapy is complicated by tumor motion from patient respiration. Four-dimensional CT (4DCT) imaging is a motion compensation method used in treatment planning to generate a maximum intensity projection (MIP) internal target volume (ITV). Image guided radiotherapy during treatment may involve acquiring a volumetric cone-beam CT (CBCT) image and visually aligning the tumor to the planning 4DCT MIP ITV contour. Moving targets imaged with CBCT can appear blurred and currently there are no studies reporting on the effect that irregular breathing patterns have on CBCT volumes and their alignment to 4DCT MIP ITV contours. The objective of this work was therefore to image a phantom moving with irregular breathing patterns to determine whether any configurations resulted in errors in volume contouring or alignment. Methods: A Perspex thorax phantom was used to simulate a patient. Three wooden 'lung' inserts with embedded Perspex 'lesions' were moved up to 4 cm with computer-generated motion patterns, and up to 1 cm with patient-specific breathing patterns. The phantom was imaged on 4DCT and CBCT with the same acquisition settings used for stereotactic lung patients in the clinic and the volumes on all phantom images were contoured. This project assessed the volumes for qualitative and quantitative changes including volume, length of the volume, and errors in alignment between CBCT volumes and 4DCT MIP ITV contours. Results: When motion was introduced 4DCT and CBCT volumes were reduced by up to 20% and 30% and shortened by up to 7 and 11 mm, respectively, indicating that volume was being under-represented at the extremes of motion. Banding artifacts were present in 4DCT MIP images, while CBCT volumes were largely reduced in contrast. When variable amplitudes from patient traces were used and CBCT ITVs were compared to 4DCT MIP ITVs there was a distinct trend in reduced ITV with increasing amplitude that was not seen when compared to true ITVs. Breathing patterns with a rest period following expiration resulted in well-defined superior edges and were better aligned using an edge-to-edge alignment technique. In most cases, sinusoidal motion patterns resulted in the closest agreements to true values and the smallest misalignments. Conclusions: Strategies are needed to compensate for volume losses at the extremes of motion for both 4DCT MIP and CBCT images for larger and varied amplitudes, and for patterns with rest periods following expiration. Lesions moving greater than 2 cm would warrant larger treatment margins added to the 4DCT MIP ITV to account for the volume being under-represented at the extremes of motion. Lesions moving with a rest period following expiration would be better aligned using an edge-to-edge alignment technique. Sinusoidal patterns represented the ideal clinical scenario, reinforcing the importance of investigating clinically relevant motions and their effects on 4DCT MIP and CBCT volumes. Since most patients do not breathe sinusoidally this may lead to misinterpretation of previous studies using only sinusoidal motion.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Characterization of statistical prior image constrained compressed sensing (PICCS): II. Application to dose reduction

Purpose: The ionizing radiation imparted to patients during computed tomography exams is raising concerns. This paper studies the performance of a scheme called dose reduction using prior image constrained compressed sensing (DR-PICCS). The purpose of this study is to characterize the effects of a statistical model of x-ray detection in the DR-PICCS framework and its impact on spatial resolution. Methods: Both numerical simulations with known ground truth and in vivo animal dataset were used in this study. In numerical simulations, a phantom was simulated with Poisson noise and with varying levels of eccentricity. Both the conventional filtered backprojection (FBP) and the PICCS algorithms were used to reconstruct images. In PICCS reconstructions, the prior image was generated using two different denoising methods: a simple Gaussian blur and a more advanced diffusion filter. Due to the lack of shift-invariance in nonlinear image reconstruction such as the one studied in this paper, the concept of local spatial resolution was used to study the sharpness of a reconstructed image. Specifically, a directional metric of image sharpness, the so-called pseudopoint spread function (pseudo-PSF), was employed to investigate local spatial resolution. Results: In the numerical studies, the pseudo-PSF was reduced from twice the voxel width in the prior image down to less than 1.1 times the voxel width in DR-PICCS reconstructions when the statistical model was not included. At the same noise level, when statistical weighting was used, the pseudo-PSF width in DR-PICCS reconstructed images varied between 1.5 and 0.75 times the voxel width depending on the direction along which it was measured. However, this anisotropy was largely eliminated when the prior image was generated using diffusion filtering; the pseudo-PSF width was reduced to below one voxel width in that case. In the in vivo study, a fourfold improvement in CNR was achieved while qualitatively maintaining sharpness; images also had a qualitatively more uniform noise spatial distribution when including a statistical model. Conclusions: DR-PICCS enables to reconstruct CT images with lower noise than FBP and the loss of spatial resolution can be mitigated to a large extent. The introduction of statistical modeling in DR-PICCS may improve some noise characteristics, but it also leads to anisotropic spatial resolution properties. A denoising method, such as the directional diffusion filtering, has been demonstrated to reduce anisotropy in spatial resolution effectively when it was combined with DR-PICCS with statistical modeling.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Tracking the dynamic seroma cavity using fiducial markers in patients treated with accelerated partial breast irradiation using 3D conformal radiotherapy

Purpose: The purpose of the present study was to perform an analysis of the changes in the dynamic seroma cavity based on fiducial markers in early stage breast cancer patients treated with accelerated partial breast irradiation (APBI) using three-dimensional conformal external beam radiotherapy (3D-CRT). Methods: A prospective, single arm trial was designed to investigate the utility of gold fiducial markers in image guided APBI using 3D-CRT. At the time of lumpectomy, four to six suture-type gold fiducial markers were sutured to the walls of the cavity. Patients were treated with a fractionation scheme consisting of 15 fractions with a fractional dose of 333 cGy. Treatment design and planning followed NSABP/RTOG B-39 guidelines. During radiation treatment, daily kV imaging was performed and the markers were localized and tracked. The change in distance between fiducial markers was analyzed based on the planning CT and daily kV images. Results: Thirty-four patients were simulated at an average of 28 days after surgery, and started the treatment on an average of 39 days after surgery. The average intermarker distance (AiMD) between fiducial markers was strongly correlated to seroma volume. The average reduction in AiMD was 19.1% (range 0.0%-41.4%) and 10.8% (range 0.0%-35.6%) for all the patients between simulation and completion of radiotherapy, and between simulation and beginning of radiotherapy, respectively. The change of AiMD fits an exponential function with a half-life of seroma shrinkage. The average half-life for seroma shrinkage was 15 days. After accounting for the reduction which started to occur after surgery through CT simulation and treatment, radiation was found to have minimal impact on the distance change over the treatment course. Conclusions: Using the marker distance change as a surrogate for seroma volume, it appears that the seroma cavity experiences an exponential reduction in size. The change in seroma size has implications in the size of the CTV, PTV, and percent of normal breast tissue irradiated when using 3D-CRT.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

First application of liquid-metal-jet sources for small-animal imaging: High-resolution CT and phase-contrast tumor demarcation

Purpose: Small-animal studies require images with high spatial resolution and high contrast due to the small scale of the structures. X-ray imaging systems for small animals are often limited by the microfocus source. Here, the authors investigate the applicability of liquid-metal-jet x-ray sources for such high-resolution small-animal imaging, both in tomography based on absorption and in soft-tissue tumor imaging based on in-line phase contrast. Methods: The experimental arrangement consists of a liquid-metal-jet x-ray source, the small-animal object on a rotating stage, and an imaging detector. The source-to-object and object-to-detector distances are adjusted for the preferred contrast mechanism. Two different liquid-metal-jet sources are used, one circulating a Ga/In/Sn alloy and the other an In/Ga alloy for higher penetration through thick tissue. Both sources are operated at 40-50 W electron-beam power with {approx}7 {mu}m x-ray spots, providing high spatial resolution in absorption imaging and high spatial coherence for the phase-contrast imaging. Results: High-resolution absorption imaging is demonstrated on mice with CT, showing 50 {mu}m bone details in the reconstructed slices. High-resolution phase-contrast soft-tissue imaging shows clear demarcation of mm-sized tumors at much lower dose than is required in absorption. Conclusions: This is the first application of liquid-metal-jet x-ray sources for whole-body small-animal x-ray imaging. In absorption, the method allows high-resolution tomographic skeletal imaging with potential for significantly shorter exposure times due to the power scalability of liquid-metal-jet sources. In phase contrast, the authors use a simple in-line arrangement to show distinct tumor demarcation of few-mm-sized tumors. This is, to their knowledge, the first small-animal tumor visualization with a laboratory phase-contrast system.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

2D/3D registration algorithm for lung brachytherapy

Purpose: A 2D/3D registration algorithm is proposed for registering orthogonal x-ray images with a diagnostic CT volume for high dose rate (HDR) lung brachytherapy. Methods: The algorithm utilizes a rigid registration model based on a pixel/voxel intensity matching approach. To achieve accurate registration, a robust similarity measure combining normalized mutual information, image gradient, and intensity difference was developed. The algorithm was validated using a simple body and anthropomorphic phantoms. Transfer catheters were placed inside the phantoms to simulate the unique image features observed during treatment. The algorithm sensitivity to various degrees of initial misregistration and to the presence of foreign objects, such as ECG leads, was evaluated. Results: The mean registration error was 2.2 and 1.9 mm for the simple body and anthropomorphic phantoms, respectively. The error was comparable to the interoperator catheter digitization error of 1.6 mm. Preliminary analysis of data acquired from four patients indicated a mean registration error of 4.2 mm. Conclusions: Results obtained using the proposed algorithm are clinically acceptable especially considering the complications normally encountered when imaging during lung HDR brachytherapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Combining prior day contours to improve automated prostate segmentation

Purpose: To improve the accuracy of automatically segmented prostate, rectum, and bladder contours required for online adaptive therapy. The contouring accuracy on the current image guidance [image guided radiation therapy (IGRT)] scan is improved by combining contours from earlier IGRT scans via the simultaneous truth and performance level estimation (STAPLE) algorithm. Methods: Six IGRT prostate patients treated with daily kilo-voltage (kV) cone-beam CT (CBCT) had their original plan CT and nine CBCTs contoured by the same physician. Three types of automated contours were produced for analysis. (1) Plan: By deformably registering the plan CT to each CBCT and then using the resulting deformation field to morph the plan contours to match the CBCT anatomy. (2) Previous: The contour set drawn by the physician on the previous day CBCT is similarly deformed to match the current CBCT anatomy. (3) STAPLE: The contours drawn by the physician, on each prior CBCT and the plan CT, are deformed to match the CBCT anatomy to produce multiple contour sets. These sets are combined using the STAPLE algorithm into one optimal set. Results: Compared to plan and previous, STAPLE improved the average Dice's coefficient (DC) with the original physician drawn CBCT contours to a DC as follows: Bladder: 0.81 {+-} 0.13, 0.91 {+-} 0.06, and 0.92 {+-} 0.06; Prostate: 0.75 {+-} 0.08, 0.82 {+-} 0.05, and 0.84 {+-} 0.05; and Rectum: 0.79 {+-} 0.06, 0.81 {+-} 0.06, and 0.85 {+-} 0.04, respectively. The STAPLE results are within intraobserver consistency, determined by the physician blindly recontouring a subset of CBCTs. Comparing plans recalculated using the physician and STAPLE contours showed an average disagreement less than 1% for prostate D98 and mean dose, and 5% and 3% for bladder and rectum mean dose, respectively. One scan takes an average of 19 s to contour. Using five scans plus STAPLE takes less than 110 s on a 288 core graphics processor unit. Conclusions: Combining the plan and all prior days via the STAPLE algorithm to produce treatment day contours is superior to the current standard of deforming only the plan contours to the daily CBCT. STAPLE also improves the precision, with a substantial decrease in standard deviation, a key for adaptive therapy. Geometrically and dosimetrically accurate contours can be automatically generated with STAPLE on prostate region kV CBCT in a time scale suitable for online adaptive therapy.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Improving best-phase image quality in cardiac CT by motion correction with MAM optimization

Purpose: Research in image reconstruction for cardiac CT aims at using motion correction algorithms to improve the image quality of the coronary arteries. The key to those algorithms is motion estimation, which is currently based on 3-D/3-D registration to align the structures of interest in images acquired in multiple heart phases. The need for an extended scan data range covering several heart phases is critical in terms of radiation dose to the patient and limits the clinical potential of the method. Furthermore, literature reports only slight quality improvements of the motion corrected images when compared to the most quiet phase (best-phase) that was actually used for motion estimation. In this paper a motion estimation algorithm is proposed which does not require an extended scan range but works with a short scan data interval, and which markedly improves the best-phase image quality. Methods: Motion estimation is based on the definition of motion artifact metrics (MAM) to quantify motion artifacts in a 3-D reconstructed image volume. The authors use two different MAMs, entropy, and positivity. By adjusting the motion field parameters, the MAM of the resulting motion-compensated reconstruction is optimized using a gradient descent procedure. In this way motion artifacts are minimized. For a fast and practical implementation, only analytical methods are used for motion estimation and compensation. Both the MAM-optimization and a 3-D/3-D registration-based motion estimation algorithm were investigated by means of a computer-simulated vessel with a cardiac motion profile. Image quality was evaluated using normalized cross-correlation (NCC) with the ground truth template and root-mean-square deviation (RMSD). Four coronary CT angiography patient cases were reconstructed to evaluate the clinical performance of the proposed method. Results: For the MAM-approach, the best-phase image quality could be improved for all investigated heart phases, with a maximum improvement of the NCC value by 100% and of the RMSD value by 81%. The corresponding maximum improvements for the registration-based approach were 20% and 40%. In phases with very rapid motion the registration-based algorithm obtained better image quality, while the image quality of the MAM algorithm was superior in phases with less motion. The image quality improvement of the MAM optimization was visually confirmed for the different clinical cases. Conclusions: The proposed method allows a software-based best-phase image quality improvement in coronary CT angiography. A short scan data interval at the target heart phase is sufficient, no additional scan data in other cardiac phases are required. The algorithm is therefore directly applicable to any standard cardiac CT acquisition protocol.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Objective models of compressed breast shapes undergoing mammography

Purpose: To develop models of compressed breasts undergoing mammography based on objective analysis, that are capable of accurately representing breast shapes in acquired clinical images and generating new, clinically realistic shapes. Methods: An automated edge detection algorithm was used to catalogue the breast shapes of clinically acquired cranio-caudal (CC) and medio-lateral oblique (MLO) view mammograms from a large database of digital mammography images. Principal component analysis (PCA) was performed on these shapes to reduce the information contained within the shapes to a small number of linearly independent variables. The breast shape models, one of each view, were developed from the identified principal components, and their ability to reproduce the shape of breasts from an independent set of mammograms not used in the PCA, was assessed both visually and quantitatively by calculating the average distance error (ADE). Results: The PCA breast shape models of the CC and MLO mammographic views based on six principal components, in which 99.2% and 98.0%, respectively, of the total variance of the dataset is contained, were found to be able to reproduce breast shapes with strong fidelity (CC view mean ADE = 0.90 mm, MLO view mean ADE = 1.43 mm) and to generate new clinically realistic shapes. The PCA models based on fewer principal components were also successful, but to a lesser degree, as the two-component model exhibited a mean ADE = 2.99 mm for the CC view, and a mean ADE = 4.63 mm for the MLO view. The four-component models exhibited a mean ADE = 1.47 mm for the CC view and a mean ADE = 2.14 mm for the MLO view. Paired t-tests of the ADE values of each image between models showed that these differences were statistically significant (max p-value = 0.0247). Visual examination of modeled breast shapes confirmed these results. Histograms of the PCA parameters associated with the six principal components were fitted with Gaussian distributions. The six-component model was also used to generate CC and MLO view mammogram breast shapes, using the mean PCA parameter values of these distributions and randomly generated values based on the fitted Gaussian distributions, which resemble clinically encountered breasts. A spreadsheet with the data necessary to apply this model is provided as the supplementary material. Conclusions: Our PCA models of breast shapes in both mammographic views successfully reproduce analyzed breast shapes and generate new clinically relevant shapes. This work can aid in research applications which incorporate breast shape modeling, such as x-ray scatter correction, dosimetry, and image registration.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Evaluation of interpolation methods for surface-based motion compensated tomographic reconstruction for cardiac angiographic C-arm data

Purpose: For interventional cardiac procedures, anatomical and functional information about the cardiac chambers is of major interest. With the technology of angiographic C-arm systems it is possible to reconstruct intraprocedural three-dimensional (3D) images from 2D rotational angiographic projection data (C-arm CT). However, 3D reconstruction of a dynamic object is a fundamental problem in C-arm CT reconstruction. The 2D projections are acquired over a scan time of several seconds, thus the projection data show different states of the heart. A standard FDK reconstruction algorithm would use all acquired data for a filtered backprojection and result in a motion-blurred image. In this approach, a motion compensated reconstruction algorithm requiring knowledge of the 3D heart motion is used. The motion is estimated from a previously presented 3D dynamic surface model. This dynamic surface model results in a sparse motion vector field (MVF) defined at control points. In order to perform a motion compensated reconstruction, a dense motion vector field is required. The dense MVF is generated by interpolation of the sparse MVF. Therefore, the influence of different motion interpolation methods on the reconstructed image quality is evaluated. Methods: Four different interpolation methods, thin-plate splines (TPS), Shepard's method, a smoothed weighting function, and a simple averaging, were evaluated. The reconstruction quality was measured on phantom data, a porcine model as well as on in vivo clinical data sets. As a quality index, the 2D overlap of the forward projected motion compensated reconstructed ventricle and the segmented 2D ventricle blood pool was quantitatively measured with the Dice similarity coefficient and the mean deviation between extracted ventricle contours. For the phantom data set, the normalized root mean square error (nRMSE) and the universal quality index (UQI) were also evaluated in 3D image space. Results: The quantitative evaluation of all experiments showed that TPS interpolation provided the best results. The quantitative results in the phantom experiments showed comparable nRMSE of Almost-Equal-To 0.047 {+-} 0.004 for the TPS and Shepard's method. Only slightly inferior results for the smoothed weighting function and the linear approach were achieved. The UQI resulted in a value of Almost-Equal-To 99% for all four interpolation methods. On clinical human data sets, the best results were clearly obtained with the TPS interpolation. The mean contour deviation between the TPS reconstruction and the standard FDK reconstruction improved in the three human cases by 1.52, 1.34, and 1.55 mm. The Dice coefficient showed less sensitivity with respect to variations in the ventricle boundary. Conclusions: In this work, the influence of different motion interpolation methods on left ventricle motion compensated tomographic reconstructions was investigated. The best quantitative reconstruction results of a phantom, a porcine, and human clinical data sets were achieved with the TPS approach. In general, the framework of motion estimation using a surface model and motion interpolation to a dense MVF provides the ability for tomographic reconstruction using a motion compensation technique.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Development of XFCT imaging strategy for monitoring the spatial distribution of platinum-based chemodrugs: Instrumentation and phantom validation

Purpose: Developing an imaging method to directly monitor the spatial distribution of platinum-based (Pt) drugs at the tumor region is of critical importance for early assessment of treatment efficacy and personalized treatment. In this study, the authors investigated the feasibility of imaging platinum (Pt)-based drug distribution using x-ray fluorescence (XRF, a.k.a. characteristic x ray) CT (XFCT). Methods: A 5-mm-diameter pencil beam produced by a polychromatic x-ray source equipped with a tungsten anode was used to stimulate emission of XRF photons from Pt drug embedded within a water phantom. The phantom was translated and rotated relative to the stationary pencil beam in a first-generation CT geometry. The x-ray energy spectrum was collected for 18 s at each position using a cadmium telluride detector. The spectra were then used for the K-shell XRF peak isolation and sinogram generation for Pt. The distribution and concentration of Pt were reconstructed with an iterative maximum likelihood expectation maximization algorithm. The capability of XFCT to multiplexed imaging of Pt, gadolinium (Gd), and iodine (I) within a water phantom was also investigated. Results: Measured XRF spectrum showed a sharp peak characteristic of Pt with a narrow full-width at half-maximum (FWHM) (FWHM{sub K{alpha}1}= 1.138 keV, FWHM{sub K{alpha}2}= 1.052 keV). The distribution of Pt drug in the water phantom was clearly identifiable on the reconstructed XRF images. Our results showed a linear relationship between the XRF intensity of Pt and its concentrations (R{sup 2}= 0.995), suggesting that XFCT is capable of quantitative imaging. A transmission CT image was also obtained to show the potential of the approach for providing attenuation correction and morphological information. Finally, the distribution of Pt, Gd, and I in the water phantom was clearly identifiable in the reconstructed images from XFCT multiplexed imaging. Conclusions: XFCT is a promising modality for monitoring the spatial distribution of Pt drugs. The technique may be useful in tailoring tumor treatment regimen in the future.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

Assessment and characterization of the total geometric uncertainty in Gamma Knife radiosurgery using polymer gels

Purpose: This work proposes and implements an experimental methodology, based on polymer gels, for assessing the total geometric uncertainty and characterizing its contributors in Gamma Knife (GK) radiosurgery. Methods: A treatment plan consisting of 26, 4-mm GK single shot dose distributions, covering an extended region of the Leksell stereotactic space, was prepared and delivered to a polymer gel filled polymethyl methacrylate (PMMA) head phantom (16 cm diameter) used to accurately reproduce every link in the GK treatment chain. The center of each shot served as a 'control point' in the assessment of the GK total geometric uncertainty, which depends on (a) the spatial dose delivery uncertainty of the PERFEXION GK unit used in this work, (b) the spatial distortions inherent in MR images commonly used for target delineation, and (c) the geometric uncertainty contributor associated with the image registration procedure performed by the Leksell GammaPlan (LGP) treatment planning system (TPS), in the case that registration is directly based on the apparent fiducial locations depicted in each MR image by the N-shaped rods on the Leksell localization box. The irradiated phantom was MR imaged at 1.5 T employing a T2-weighted pulse sequence. Four image series were acquired by alternating the frequency encoding axis and reversing the read gradient polarity, thus allowing the characterization of the MR-related spatial distortions. Results: MR spatial distortions stemming from main field (B{sub 0}) inhomogeneity as well as from susceptibility and chemical shift phenomena (also known as sequence dependent distortions) were found to be of the order of 0.5 mm, while those owing to gradient nonlinearities (also known as sequence independent distortions) were found to increase with distance from the MR scanner isocenter extending up to 0.47 mm at an Euclidean distance of 69.6 mm. Regarding the LGP image registration procedure, the corresponding average contribution to the total geometric uncertainty ranged from 0.34 to 0.80 mm. The average total geometric uncertainty, which also includes the GK spatial dose delivery uncertainty, was found equal to (0.88 {+-} 0.16), (0.88 {+-} 0.26), (1.02 {+-} 0.09), and (1.15 {+-} 0.24) mm for the MR image series acquired with the read gradient polarity (direction) set toward right, left, posterior, and anterior, respectively. Conclusions: The implemented methodology seems capable of assessing the total geometric uncertainty, as well as of characterizing its contributors, ascribed to the entire GK treatment delivery (i.e., from MR imaging to GK dose delivery) for an extended region of the Leksell stereotactic space. Results obtained indicate that the selection of both the frequency encoding axis and the read gradient polarity during MRI acquisition may affect the magnitude as well as the spatial components of the total geometric uncertainty.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2013

Analysis of a free-running synchronization artifact correction for MV-imaging with aSi:H flat panels

Purpose: Solid state flat panel electronic portal imaging devices (EPIDs) are widely used for megavolt (MV) photon imaging applications in radiotherapy. In addition to their original purpose in patient position verification, they are convenient to use in quality assurance and dosimetry to verify beam geometry and dose deposition or to perform linear accelerator (linac) calibration procedures. However, native image frames from amorphous silicon (aSi:H) detectors show a range of artifacts which have to be eliminated by proper correction algorithms. When a panel is operated in free-running frame acquisition mode, moving vertical stripes (periodic synchronization artifacts) are a disturbing feature in image frames. Especially for applications in volumetric intensity modulated arc therapy (VMAT) or motion tracking, the synchronization (sync) artifacts are the limiting factor for potential and accuracy since they become even worse at higher frame rates and at lower dose rates, i.e., linac pulse repetition frequencies (PRFs). Methods: The authors introduced a synchronization correction method which is based on a theoretical model describing the interferences of the panel's readout clocking with the linac's dose pulsing. Depending on the applied PRF, a certain number of dose pulses is captured per frame which is readout columnwise, sequentially. The interference of the PRF with the panel readout is responsible for the period and the different gray value levels of the sync stripes, which can be calculated analytically. Sync artifacts can then be eliminated multiplicatively in precorrected frames without additional information about radiation pulse timing. Results: For the analysis, three aSi:H EPIDs of various types were investigated with 6 and 15 MV photon beams at varying PRFs of 25, 50, 100, 200, and 400 pulses per second. Applying the sync correction at panels with gadolinium oxysulfide scintillators improved single frame flood field image quality drastically [improvement of the signal-to-noise ratio (SNR) up to 66.1 dB for 6 MV and 66.0 dB for 15 MV]. Also for the EPID with a caesium iodide scintillator, the noise for the lower PRFs could be reduced (SNR at 6 MV of up to 56.3 dB and at 15 MV up to 46.7 dB). However, the simplistic readout interference model fails at higher PRFs, where image lag and ghosting effects due to trapped charges in the thin film transistor and scintillator postglowing require additional corrections. Conclusions: The presented free-running sync correction method improves SNR of single frames and enables imaging applications, like low-dose rate imaging at increased image frame rates (e.g., to track moving gold fiducials in the lung). Adaptive image guided radiotherapy protocols become even feasible in VMAT plans. Also simultaneous kilovolt and MV imaging applications can benefit from new possibilities of MV scatter removal in x-ray images.

60 APPLIED LIFE SCIENCES↗