Edge Artifacts Research Articles

Resolution modeling in PET imaging: theory, practice, benefits, and pitfalls.

In this paper, the authors review the field of resolution modeling in positron emission tomography (PET) image reconstruction, also referred to as point-spread-function modeling. The review includes theoretical analysis of the resolution modeling framework as well as an overview of various approaches in the literature. It also discusses potential advantages gained via this approach, as discussed with reference to various metrics and tasks, including lesion detection observer studies. Furthermore, attention is paid to issues arising from this approach including the pervasive problem of edge artifacts, as well as explanation and potential remedies for this phenomenon. Furthermore, the authors emphasize limitations encountered in the context of quantitative PET imaging, wherein increased intervoxel correlations due to resolution modeling can lead to significant loss of precision (reproducibility) for small regions of interest, which can be a considerable pitfall depending on the task of interest.

Single Image Super-Resolution via Non Sub-sample Contourlet Transform based Learning and a Gabor Prior

Enhancing the quality of image is a continuous process in image processing related research activities. For some applications it becomes essential to have best quality of image such as in forensic department, where in order to retrieve maximum possible information, image has to be enlarged in terms of size, with higher resolution and other features associated with it. Such obtained high quality images have also a concern in satellite imaging, medical science, High Definition Television (HDTV), etc. In this paper a novel approach of getting high resolution image from a single low resolution image is discussed. The Non Sub-sampled Contourlet Transform (NSCT) based learning is used to learn the NSCT coefficients at the finer scale of the unknown high-resolution image from a dataset of high resolution images. The cost function consisting of a data fitting term and a Gabor prior term is optimized using an Iterative Back Projection (IBP). By making use of directional decomposition property of the NSCT and the Gabor filter bank with various orientations, the proposed method is capable to reconstruct an image with less edge artifacts. The validity of the proposed approach is proven through simulation on several images. RMS measures, PSNR measures and illustrations show the success of the proposed method.

Late gadolinium enhancement imaging using spiral readouts at 3T

Background Breath-hold late gadolinium enhancement (LGE) imaging is commonly performed as a stack of 2D short-axis slices through the ventricles. Standard inversion recovery with Cartesian k-space coverage requires long acquisition windows of 140 200 ms which have potential to introduce artefacts due to longitudinal magnetisation recovery and blurring due to cardiac motion. Ghosting in the phase encoding direction from beat-to-beat signal variations and due to poor breath-holding may mimic enhancement and a phase swapped (PS) stack of slices is also acquired to confirm diagnosis, although this doubles the scan time. Spiral data acquisition provides highly efficient coverage of k-space and would potentially allow a significant reduction of the acquisition window. This would reduce motion blurring and edge artefacts while at the same time, may eliminate the need for PS acquisitions. Its use for 2D LGE was investigated.

Edge artefacts and lost performance in national versus continental conservation priority areas

AbstractAimGlobal conservation policies, such as the Convention on Biological Diversity (CBD) decision to aim for the protection of 17% of the area of terrestrial ecosystems by 2020, are typically realized at national levels. We investigate the difference between continentally coordinated conservation versus nationally devolved conservation, in a manner relevant for the Nagoya resolution.LocationThe terrestrial areas of the Western Hemisphere.MethodsWe used IUCN distribution data for 8463 species of mammals, birds and amphibians in the Western Hemisphere. We investigated the consequences of prioritizing land at a continental scale, versus analysing priorities within each country separately. Spatial prioritization was performed using the Zonation software, which produces a complementarity‐based hierarchical priority ranking across the area of interest.ResultsWe found that coordinated continent‐wide priorities achieved > 50% higher mean protection levels than national analyses for the top 17% of land. National prioritizations also result in spatial priority patterns that can be considered as artefacts at the continental scale: in bands of high‐priority land concentrated at terrestrial political boundaries, such as at low‐latitude edges of temperate zone countries. We find that this edge artefact also correlates with the present distribution of conservation areas, with the density of conservation areas within 50 km of a national border being > 50% higher than the density of conservation areas away from national borders.Main conclusionsThe means by which national priorities are integrated with continental or global conservation prioritization will have considerable influence on how much is achieved by the CBD resolution. Focus on national species distributions and priorities will result in lost performance because of emphasis on nationally rare species that are comparatively common elsewhere. National borders intersect species distributions (and possibly diversity gradients), leading to clustering of nationally rare species and priority areas close to the border.

Image Quality and Quantitative Accuracy of Small Animal PET Images: Role of Physical Effects Correction and Reconstruction Algorithm

Positron emission tomography (PET) provides important metabolic information about tracer kinetics. Photon attenuation, scattered radiation, spatial resolution limitation and reconstruction algorithm parameters are variables that influence image quality and quantitative accuracy. The objective of this work was to examine the impact of the combined correction of photon attenuation and scatter on the quality and quantitative accuracy of small animal PET images reconstructed using the analytic filtered back projection algorithm (FBP) and the iterative maximum a posteriori probability (MAP) technique at different values of the smoothing parameter β.Methods. A homogenous water phantomandthree different configurations of NEMA image quality phantom were employed using a small animal PET scanner (InveonTM, Siemens medical solutions, Inc). A 20 min PET/CT scans were performed for all phantoms using a list mode acquisition. All Data were sorted into 3D sinograms and reconstructed using 2D FBP after Fourier rebinning and also reconstructed using OSEM3D/MAP (2 iteration OSEM3D and 18 iterations using MAP). Figures of merit used were error of tracer estimate, recovery coefficient, percentage standard deviation, spillover ratio and image noise. Results. Based on object geometry and size, FBP with attenuation and scatter correction provides accurate tracer concentration within the range of 5% and possible improvement can be achieved using MAP reconstruction. Without correction, errors of activity quantitation is significantly high, on average 34.5±7.2% Recovery coefficient was also found to improve with proper tuning of the β value using MAP reconstruction. Spillover ratio indicated some variations between FBP and MAP in air and water compartments, which might be attributed to the performance of the scatter correction. Noise measurements were superior with MAP reconstruction especially at high β values, but this occurs with edge artifacts and slight distortion of some geometric structures. Very small values of the beta have better resolution characteristics but high values have increased smoothing effect and blurring appearance. Conclusion: Attenuation correction is important in small animal PET and allows for reliable tracer estimate. Spatial resolution is improved by MAP technique and low β values provide high resolution capabilities and recovery coefficients but high values improve noise levels. For a given application, proper selection and employment of the smoothing parameter β should be followed. Scatter correction need to be improved especially in large objects or simultaneous acquisition of multiple numbers of animals.

Iterative method for the separation of blended seismic data: discussion on the algorithmic aspects

ABSTRACTRecently much attention has been given to the possibility of shooting in an overlapping fashion, the so‐called blended or simultaneous acquisition. In conventional acquisition the time intervals between successive sources are large enough to avoid interference in time. In blending, a temporal overlap between source responses is allowed. This additional degree of freedom in survey design has the potential to significantly reduce seismic acquisition costs while maintaining or improving the data quality. Deblending is the procedure of retrieving data as if they were acquired in the conventional, unblended way. This is an essential step in the case where standard processing flows are applied. Several methods have been proposed to perform data separation with the majority of them falling into two categories. The first category consists in methods that filter out the blending noise by arranging seismic data in some domain, whereas inversion techniques fall into the second category. We recently introduced an iterative estimation and subtraction algorithm that integrates elements of both categories. This method exploits the fact that the character of the blending noise differs in different domains, e.g., it is coherent in the common source domain but incoherent in the common receiver, common‐offset or common midpoint domains. Up to now, our method relies on the interaction with a human operator. The automation of the thresholding process is addressed in this paper, leading to a hands‐off algorithm for the separation of blended data, optimized for both efficiency and effectiveness. We found that one of the major limiting factors is the edge artefacts generated by the coherence‐pass filter. The effectiveness of the coherence‐pass filter has a considerable influence on the convergence of our deblending algorithm. This is shown by testing different coherence‐pass filters on marine data as well as introducing errors in the coherence‐pass process. We show that these data estimation errors can be handled properly and the best result is obtained using a τ‐p filter as a coherence‐pass filter. Furthermore, very promising results are obtained on numerically‐blended land data.

A comparison of four dimensional time-resolved with keyhole and three dimensional time-of-flight MR angiography for the evaluation of cerebral aneurysms

Objective To evaluate the accuracy and reliability of 4D time-resolved MRA with keyhole (4D-TRAK) for the detection and characterization of cerebral aneurysms ( CAs),with a comparison of 3D time-of-flight MRA (3D-TOF-MRA).Methods 3D-TOF-MRA,4D-TRAK and 3D-DSA were performed sequentially in 52 patients with suspected CAs.4D-TRAK was acquired using a combination of sensitivity encoding (SENSE) and contrast-enhanced (CE) timing robust angiography ( CENTRA ) k-space sampling techniques at a contrast dose of 10 ml at 3 T scanner. Accuracy,sensitivity,specificity of 4D-TRAK and 3D-TOF-MRA were calculated and compared for the detection of CAs on patient-based and aneurysm-based evaluation using 3D-DSA as a reference. Wilcoxon signed rank test were used. Results The overall image quality of 4D-TRAK was appropriate for the diagnostic purpose,but yet not comparable with that of 3D-TOF-MRA.In 52 patients with suspected GAs,58 CAs were confirmed on 3D-DSA finally.Fifty-one (with 2 false-positives and 9 false-negatives) and 58 (with 1 false-positive and 1 false-negative)CAs were visualized on 4D-TRAK and 3D-TOF-MRA,respectively.Accuracy,sensitivity and specificity on patient-based evaluation of 4D-TRAK and 3D-TOF-MRA were 92.31％ ( 48/52 ),93.33％ ( 42/45 ),85.71 ％ (6/7) and 98.08％ ( 51/52 ),100.00％ ( 45/45 ),85.71％ ( 6/7 ),respectively,and 74.07％(20/27),75.00％ ( 18/24),66.67％ (2/3) and 96.30％ (26/27),95.83％ (26/27),100.00％ (3/3)on aneurysm-based evaluation in patients with multiple CAs,respectively.Subgroup analysis revealed that for 19 very small CAs ( maximal diameter ＜3 mm,measured on 3D-DSA),9 were missed on 4D-TRAK and 1 on 3D-TOF-MRA( Z =- 2.464,P ＜ O.01 ). However,for 39 CAs with maximal diameter more than 3 mm,there was no significantly difference in the diagnostic accuracy (39 on 4D-TRAK vs.39 on 3D-TOFMRA) (Z =0.000,P ＞0.05).In 4 large CAs with maximal diameter more than 10 mm,4D-TRAK provided a better characterization of morphology than 3D-TOF-MRA.Conclusions 4D-TRAK with a combination of SENSE and CENTRA at 3 T shows potential value in the diagnosis of cerebral aneurysms.However,due to the compromise in spatial resolution and vascular edge artifacts,it does not yet have a diagnostic accuracy of CAs comparable with 3D-TOF-MRA.TRAK imaging can be of great help in patients with large-giant CAs to characterize the morphology of CAs and to diminish the risk of NSF in patients with renal impairment by using a lower-dose contrast. Key words: Magnetic resonance imaging; Angiography,digital subtraction; Comparative study

3T BOLD MRI with low intrascan variability and high reproducibility of limb oxygenation measurements

Current imaging methods cannot reliably quantify muscle oxygenationin patients with limb ischaemia. We propose a high-resolution BOLD sequence whereby edge artefacts and vessels may be excluded from measurements. CONCLUSIONS: The sequence and analysis proposed shows lowintrascan variability and high interscan reproducibility for measurement of T2*. The technique is therefore well suited for serial assessment of limb muscle oxygenation.

Image Compression using Space Adaptive Lifting Scheme

Problem statement: Digital images play an important role both in daily life applications as well as in areas of research and technology. Due to the increasing traffic caused by multimedia information and digitized form of representation of images; image compression has become a necessity. Approach: Wavelet transform has demonstrated excellent image compression performance. New algorithms based on Lifting style implementation of wavelet transforms have been presented in this study. Adaptively is introduced in lifting by choosing the prediction operator based on the local properties of the image. The prediction filters are chosen based on the edge detection and the relative local variance. In regions where the image is locally smooth, we use higher order predictors and near edges we reduce the order and thus the length of the predictor. Results: We have applied the adaptive prediction algorithms to test images. The original image is transformed using adaptive lifting based wavelet transform and it is compressed using Set Partitioning In Hierarchical Tree algorithm (SPIHT) and the performance is compared with the popular 9/7 wavelet transform. The performance metric Peak Signal to Noise Ratio (PSNR) for the reconstructed image is computed. Conclusion: The proposed adaptive algorithms give better performance than 9/7 wavelet, the most popular wavelet transforms. Lifting allows us to incorporate adaptivity and nonlinear operators into the transform. The proposed methods efficiently represent the edges and appear promising for image compression. The proposed adaptive methods reduce edge artifacts and ringing and give improved PSNR for edge dominated images.

Properties and Mitigation of Edge Artifacts in PSF-Based PET Reconstruction

PSF (point spread function) based image reconstruction causes an overshoot at sharp intensity transitions (edges) of the object. This edge artifact, or ringing, has not been fully studied. In this work, we analyze the properties of edge artifacts in PSF-based reconstruction in an effort to develop mitigation methods. Our study is based on 1D and 2D simulation experiments. Two approaches are adopted to analyze the artifacts. In the system theory approach, we relate the presence of edge artifacts to the null space and conditioning of the imaging operator. We show that edges cannot be accurately recovered with a practical number of image updates when the imaging matrices are poorly conditioned. In the frequency-domain analysis approach, we calculate the object-specific modulation transfer function (OMTF) of the system, defined as spectrum of the reconstruction divided by spectrum of the object. We observe an amplified frequency band in the OMTF of PSF-based reconstruction and that the band is directly related to the presence of ringing. Further analysis shows the amplified band is linearly related to kernel frequency support (the reciprocal of the reconstruction kernel FWHM), and the relation holds for different objects. Based on these properties, we develop a band-suppression filter to mitigate edge artifacts. We apply the filter to simulation and patient data, and compare its performance with other mitigation methods. Analysis shows the band-suppression filter provides better tradeoff of resolution and ringing suppression than a low-pass filter.

Spatially Adaptive Block-Based Super-Resolution

Super-resolution technology provides an effective way to increase image resolution by incorporating additional information from successive input images or training samples. Various super-resolution algorithms have been proposed based on different assumptions, and their relative performances can differ in regions of different characteristics within a single image. Based on this observation, an adaptive algorithm is proposed in this paper to integrate a higher level image classification task and a lower level super-resolution process, in which we incorporate reconstruction-based super-resolution algorithms, single-image enhancement, and image/video classification into a single comprehensive framework. The target high-resolution image plane is divided into adaptive-sized blocks, and different suitable super-resolution algorithms are automatically selected for the blocks. Then, a deblocking process is applied to reduce block edge artifacts. A new benchmark is also utilized to measure the performance of super-resolution algorithms. Experimental results with real-life videos indicate encouraging improvements with our method.

A local f-x Cadzow method for noise reduction of seismic data obtained in complex formations

A noise-reduction method with sliding windows in the frequency-space (f-x) domain, called the local f-x Cadzow noise-reduction method, is presented in this paper. This method is based on the assumption that the signal in each window is linearly predictable in the spatial direction while the random noise is not. For each Toeplitz matrix constructed by constant frequency slice, a singular value decomposition (SVD) is applied to separate signal from noise. To avoid edge artifacts caused by zero percent overlap between windows and to remove more noise, an appropriate overlap is adopted. Besides flat and dipping events, this method can enhance curved and conflicting events. However, it is not suitable for seismic data that contains big spikes or null traces. It is also compared with the SVD, f-x deconvolution, and Cadzow method without windows. The comparison results show that the local Cadzow method performs well in removing random noise and preserving signal. In addition, a real data example proves that it is a potential noise-reduction technique for seismic data obtained in areas of complex formations.

Preliminary Investigation of the Dosimetric Properties of 'RadGel'

A preliminary investigation into the efficacy of a new 3D dosimetry material, RadGelTM, for verification of radiation therapy dose distributions is presented. Small volumes of RadGelTM were found to exhibit a linear, reproducible response to dose. A gradual increase in optical-density (OD) with time was observed, suggesting scanning should be completed within 18 hours to keep a linear correlation of R2 > 0.99. A larger 10 cm diameter volume of RadGelTM was irradiated with a rotationally symmetric "spoke" plan designed to rigorously evaluate scanner/dosimeter combined performance. The dosimeter was imaged with the Duke Mid-sized Optical-CT Scanner (DMOS). Promising OD and corresponding dose maps were obtained. Edge artefacts were observed and are suspected to be exacerbated by the particular container used in this early study. Further studies will evaluate new containers and methods for refractive matching at the gel-container-fluid interface.

ScanSim: A tool for simulating optical-CT imaging

A software tool has been developed that can simulate image formation in a variety of optical-CT scanning configurations. The formalism of the simulation is introduced, including two main modes: a diverging point source mode, and a converging broad beam mode. Preliminary results are presented for scanning Presage dosimeters in both modes and immersed in refractive media of widely varying refractive index (RI), including air, water, and a fully matched medium. Pronounced differences in the edge artifacts and accuracy of reconstructed coefficients is observed. The ScanSim software is shown to be a useful tool to investigate and quantify many aspects of optical-CT image formation, including reducing dependence on matching fluids.

Multiscale Amplitude-Modulation Frequency-Modulation (AM–FM) Texture Analysis of Ultrasound Images of the Intima and Media Layers of the Carotid Artery

The intima-media thickness (IMT) of the common carotid artery (CCA) is widely used as an early indicator of cardiovascular disease (CVD). Clinically, there is strong interest in identifying how the composition and texture of the media layer (ML) can be associated with the risk of stroke. In this study, we use 2-D amplitude-modulation frequency-modulation (AM-FM) analysis of the intima-media complex (IMC), the ML, and intima layer (IL) of the CCA to detect texture changes as a function of age and sex. The study was performed on 100 ultrasound images acquired from asymptomatic subjects at risk of atherosclerosis. To investigate texture variations associated with age, we separated them into three age groups: 1) patients younger than 50; 2) patients aged between 50 and 60 years old; and 3) patients over 60 years old. We also separated the patients by sex. The IMC, ML, and IL were segmented manually by a neurovascular expert and also by a snake-based segmentation system. To reject strong edge artifacts, we prefilter with an AM-FM filterbank that is centered along the horizontal frequency axis (parallel to the long axis of the IMC, ML, and IL), while removing the low-pass filter estimates and frequency bands with large, vertical frequency components. To investigate significant texture changes, we extract the instantaneous amplitude (IA) and the magnitude of the instantaneous frequency (IF) over each layer component, for low-, medium-, and high-frequency AM-FM components. We detected significant texture differences between the higher risk age group of >60 years versus the lower risk age group of <50 and the 50-60 group. In particular, between the <50 and >60 groups, we found significant differences in the medium-scale IA extracted from the IMC. Between the >60 and the 50-60 groups, we found significant texture changes in the low-scale IA and high-scale IF magnitude extracted from the IMC, and the low-scale IA extracted from the IL. Also, we noted that the IA for the ML showed significant differences between males and females for all age groups. The AM--FM features provide complimentary information to classical texture analysis features like the gray-scale median, contrast, and coarseness. These findings provide evidence that AM--FM texture features can be associated with the progression of cardiovascular risk for disease and the risk of stroke with age. However, a larger scale study is needed to establish the application in clinical practice.

Application and Evaluation of a Measured Spatially Variant System Model for PET Image Reconstruction

Accurate system modeling in tomographic image reconstruction has been shown to reduce the spatial variance of resolution and improve quantitative accuracy. System modeling can be improved through analytic calculations, Monte Carlo simulations, and physical measurements. The purpose of this work is to improve clinical fully-3-D reconstruction without substantially increasing computation time. We present a practical method for measuring the detector blurring component of a whole-body positron emission tomography (PET) system to form an approximate system model for use with fully-3-D reconstruction. We employ Monte Carlo simulations to show that a non-collimated point source is acceptable for modeling the radial blurring present in a PET tomograph and we justify the use of a Na22 point source for collecting these measurements. We measure the system response on a whole-body scanner, simplify it to a 2-D function, and incorporate a parameterized version of this response into a modified fully-3-D OSEM algorithm. Empirical testing of the signal versus noise benefits reveal roughly a 15% improvement in spatial resolution and 10% improvement in contrast at matched image noise levels. Convergence analysis demonstrates improved resolution and contrast versus noise properties can be achieved with the proposed method with similar computation time as the conventional approach. Comparison of the measured spatially variant and invariant reconstruction revealed similar performance with conventional image metrics. Edge artifacts, which are a common artifact of resolution-modeled reconstruction methods, were less apparent in the spatially variant method than in the invariant method. With the proposed and other resolution-modeled reconstruction methods, edge artifacts need to be studied in more detail to determine the optimal tradeoff of resolution/contrast enhancement and edge fidelity.

Liver MRI at 3 T Using a Respiratory-Triggered Time-Efficient 3D T2-Weighted Technique: Impact on Artifacts and Image Quality

The purpose of this retrospective study was to qualitatively and quantitatively compare image quality of a time-efficient 3D T2-weighted sequence-the sampling perfection with application-optimized contrasts using different flip angle evolutions (SPACE) sequence-with a standard 2D T2-weighted turbo spin-echo (TSE) sequence for liver imaging at 3 T. Twenty patients underwent liver MRI at 3 T using T2-weighted breath-hold 2D TSE and respiratory-triggered SPACE sequences. Two radiologists independently assessed image quality for both sequences during separate sessions, followed by a side-by-side comparison. One reader performed a quantitative analysis of the estimated signal-to-noise ratio (SNR) and the relative contrast between the liver and other tissues. Image quality scores for the SPACE sequence were significantly better than those for the 2D TSE sequence for motion (p < 0.0001) and pulsation (p < 0.0001) artifact, flow signal suppression (p = 0.0015), sharpness of intrahepatic vessels (p < 0.0001), and sharpness of liver edge (p = 0.0015), with motion and pulsation artifacts being nearly eliminated using the SPACE sequence. However, the scores for B(1) inhomogeneity artifact were significantly worse for the SPACE sequence (p = 0.0117). Overall, both readers preferred SPACE sequence, although this difference was significant for only one reader (p = 0.025, p = 0.275). There was no significant difference between the sequences for estimated liver SNR (p = 0.1564), but the SPACE sequence showed significantly higher relative contrast between the liver and the kidney (p < 0.0001), gallbladder (p = 0.0476), and spleen (p < 0.0001). Relative contrast between the liver and parenchymal lesions was higher with the SPACE sequence than with the TSE sequence, although this difference was not statistically significant (p = 0.125). For T2-weighted liver imaging at 3 T, the respiratory-triggered SPACE sequence shows better image quality with near elimination of motion and pulsation artifacts and improved tissue contrast than the breath-hold 2D TSE sequence, but suffers from increased B(1) inhomogeneity artifact and longer scanning time.

CT Visualization of Cryoablation in Pulmonary Veins

Over 2 million adults in the United States are affected by atrial fibrillation (AF), a common cardiac arrhythmia that is associated with decreased survival, increased cardiovascular morbidities, and a decrease in quality of life. AF can be initiated by ectopic beats originating in the myocardial sleeves surrounding the pulmonary viens. Pulmonary vein (PV) isolation via radio frequency ablation is the current gold standard for treating patients with drug-refractory AF. However, cryoablation is emerging as a new minimally-invasive technique to achieve PV isolation. Cryoablation is fast gaining acceptance due to its minimal tissue disruption, decreased thrombogenicity, and reduced complications (RF can lead to low rate of stenosis). One important question in regard to this technology is whether the PV lesion is transmural and circumferential and to what extent adjacent tissues are involved in the freezing process. As ice formation lends itself to image contrast in the body, we hypothesized that intraprocedural CT visualization of the iceball formation would allow us to predict the extent of the cryolesion and provide us with a measure of the adjacent tissue damage. Cryoablation was performed using a prototype balloon catheter cryoablation system (Boston Scientific Corporation). CT visualization of iceball formation was assessed both in vitro and in vivo. Initial in vitro studies were performed in agarose gel phantoms immersed in a 37°C water bath. Subsequently, in vivo cryoablations were performed in 5 PV ostia in 3 crossbred farm swine. The catheters were positioned in the ostia under fluoroscopic guidance. CT scans of the thoracic region were obtained every 2.5 minutes. Animals were sacrified 6 days after the procedures. Gross pathology and histology of tissues in the region of interest were evaluated. Significant metal artifacts from the catheter and edge artifacts from the tissues surrounding the cryoballoon were observed under CT imaging both in vitro and in vivo. In vitro, it was found that the size of the iceball was comparable to that observed visually during freezing of agarose gel phantoms. In vivo, contrast change consistent with iceball formation was observed during the ablation in two out of five veins. The most clearly delineated iceball also yielded the clearest morbidity. In this case, esophageal injury on the anterior side proximal to the cryoablation site was noticed during necropsy of the animal in which the iceball was visualized. Transmural and circumferential lesions were obtained in all PVs ablated. We have shown that CT can be used to visualize iceball formation in vitro and in vivo (with limitations) using our cryoablation system. While the iceball in vitro is easily visualized, iceball growth in vivo is most evident once the iceball has grown beyond the PV into the adjacent tissues. This suggests that while CT cannot easily visualize iceball growth in the PV wall itself, it may still be an important tool to guide clinicians and reduce potential morbidities in adjacent tissues. The authors acknowledge Dan Busian (Fairview University Medical Center, Minneapolis, MN) and Dr. Erik Cressman for assistance with CT imaging.

Interpolation strategies for reducing IFOV artifacts in microgrid polarimeter imagery

Microgrid polarimeters are composed of an array of micro-polarizing elements overlaid upon an FPA sensor. In the past decade systems have been designed and built in all regions of the optical spectrum. These systems have rugged, compact designs and the ability to obtain a complete set of polarimetric measurements during a single image capture. However, these systems acquire the polarization measurements through spatial modulation and each measurement has a varying instantaneous field-of-view (IFOV). When these measurements are combined to estimate the polarization images, strong edge artifacts are present that severely degrade the estimated polarization imagery. These artifacts can be reduced when interpolation strategies are first applied to the intensity data prior to Stokes vector estimation. Here we formally study IFOV error and the performance of several bilinear interpolation strategies used for reducing it.

Investigating the feasibility of 3D dosimetry in the RPC IMRT H&N phantom

An urgent requirement for 3D dosimetry has been recognized because of high failure rate (∼25%) in RPC credentialing, which relies on point and 2D dose measurements. Comprehensive 3D dosimetry is likely to resolve more errors and improve IMRT quality assurance. This work presents an investigation of the feasibility of PRESAGE/optical-CT 3D dosimetry in the Radiologic Physics Center (RPC) IMRT H&N phantom. The RPC H&N phantom (with standard and PRESAGE dosimetry inserts alternately) was irradiated with the same IMRT plan. The TLD and EBT film measurement data from standard insert irradiation was provided by RPC. The 3D dose measurement data from PRESAGE insert irradiation was readout using the OCTOPUS™ 5X optical-CT scanner at Duke. TLD, EBT and PRESAGE dose measurements were inter-compared with Eclipse calculations to evaluate consistency of planning and delivery. Results showed that the TLD point dose measurements agreed with Eclipse calculations to within 5% dose-difference. Relative dose comparison between Eclipse dose, EBT dose and PRESAGE dose was conducted using profiles and gamma comparisons (4% dose-difference and 4 mm distance-to-agreement). Profiles showed good agreement between measurement and calculation except along steep dose gradient regions where Eclipse modelling might be inaccurate. Gamma comparisons showed that the measurement and calculation showed good agreement (>96%) if edge artefacts in measurements are ignored. In conclusion, the PRESAGE/optical-CT dosimetry system was found to be feasible as an independent dosimetry tool in the RPC IMRT H&N phantom.