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Results for "artifact reduction": 39 found

3D Wideband Late Gadolinium-Enhanced MRI for Radiotherapy of Ventricular Tachycardia: A Preliminary Study in Healthy Participants and Patient

Authors: Arash Bedayat, Jason Bradfield, Minsong Cao, Robert K Chin, Huiming Dong, J Paul Finn, Fei Han, Justin Hayase, Shu-Fu Shih, Xiaodong Zhong

Affiliation: Cardiac Electrophysiology, University of California, Los Angeles, Department of Radiation Oncology, University of California, Los Angeles, Department of Radiology, University of California, Los Angeles, Siemens Healthineers

Abstract Preview: Purpose: Cardiac SBRT is a promising treatment for ventricular tachycardia (VT). Success depends on accurate target delineation, for which 2D narrowband late gadolinium-enhanced (LGE) MRI offers valua...

A Novel Group-Wise Non-Rigid Iterative Closest Point Shape Registration Algorithm with Temporal Smoothness Regularizations for Computing the Respiratory Motion of the Heart in Respiratory 4DCTs and Avoiding Random Cardiac Motion Artifacts

Authors: Hongyu An, Phillip Cuculich, H Michael Gach, Yao Hao, Trevor McKeown, Clifford Robinson, Yuhao Wang, Deshan Yang

Affiliation: Washington University School of Medicine in St. Louis, Washington University School of Medicine, Duke University, Department of Radiation Oncology, Duke University, Washington University in St. Louis, Department of Radiation Oncology, Washington University School of Medicine in St. Louis

Abstract Preview: Purpose: Cardiorespiratory motion management is crucial in stereotactic arrhythmia radiotherapy to define target margins and minimize cardiac toxicity. While respiratory 4DCT (r4DCT) images can infer ...

A Novel Groupwise Shape Deformable Registration Algorithm to Quantify the Respiratory Motion of the Heart in Respiratory 4DCTs While Accounting for Random Cardiac Motion Artifacts in the Respiratory 4DCTs

Authors: Anna E. Rodrigues, Yuhao Wang, Deshan Yang

Affiliation: Duke University, Department of Radiation Oncology, Duke University

Abstract Preview: Purpose: Accurate motion margin definition in StereoTactic Arrhythmia Radiotherapy (STAR) requires accurate cardiorespiratory motion assessment. However, respiratory 4DCT (r4DCT) images are affected b...

A Novel Linac Onboard Dual-Layer Kv Imager Provides Increased Clinical CBCT Contrast and Metal Artifact Reduction Using Virtual Monoenergetic Images

Authors: Ross I. Berbeco, Vera Birrer, Raphael Bruegger, Pablo Corral Arroyo, Roshanak Etemadpour, Dianne M. Ferguson, Rony Fueglistaller, Thomas C. Harris, Yue-Houng Hu, Matthew W. Jacobson, Mathias Lehmann, Marios Myronakis

Affiliation: Brigham and Women's Hospital, Harvard Medial School, Dana-Farber Cancer Institute, Department of Radiation Oncology, Dana Farber/Brigham and Women's Cancer Center, Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Brigham and Women's Hospital, Varian Imaging Laboratory, Dana-Farber Cancer Institute

Abstract Preview: Purpose: Dual-energy imaging offers benefits, including metal artifact reductions (MAR) and improved contrast, that enhance visualization. Currently, dual-energy imaging is largely limited to CT scann...

A Quantitative Analysis of Hypersight CBCT Image Quality Using a Phantom-Based Approach Under Different Scatter Conditions

Authors: Denisa R. Goia, M. Saiful Huq, Ronald John Lalonde, Fang Li, Noor Mail, Joseph Shields, Christopher Tyerech

Affiliation: UPMC Hillman Cancer Center, UPMC Hillman Cancer Center and University of Pittsburgh School of Medicine, Department of Radiation Oncology, University of Pennsylvania, UPMC

Abstract Preview: Purpose: HyperSight is a new platform for image-guided radiation therapy, offering advanced reconstruction algorithms, a large field-of-view, and rapid acquisition times. To validate the performance o...

A Self-Supervised Deep Learning Approach for Automatic Identification and Metal Artifact Reduction in Cone-Beam CT for Brachytherapy

Authors: Rani Anne', Wenchao Cao, Yingxuan Chen, Wookjin Choi, Firas Mourtada, Yevgeniy Vinogradskiy

Affiliation: Thomas Jefferson University

Abstract Preview: Purpose: In-room mobile cone-beam CT (CBCT) is emerging to enhance high-dose-rate (HDR) brachytherapy workflow using on-demand imaging. However, metal artifacts from X-ray markers inside gynecological...

An Accelerated Scatter Estimation for Spectral CBCT Based on Linear Boltzmann Transport Equation

Authors: Zhiqiang Chen, Hewei Gao, Li Zhang, Guoxi Zhu

Affiliation: Tsinghua University

Abstract Preview: Purpose: Spectral cone-beam CT has better performance in material identification and quantitative analysis compared to single-energy cone-beam CT. However, X-ray scattering introduces significant scat...

Analysis of Orthopedic Metal Artifact Reduction’s (O-MAR’s) Effectiveness on Correcting Typical Anatomical Structure Average Volumetric Hounsfield Unit (AVHU) Values Due to High-Density Metal Artifacts

Authors: John T Barrett, Mehnaz Haque, Chulhaeng Huh, Shands James, Thomas B. Lavin, Anobel Maghsoodpour, Farshad Mostafaei, Austin Sanders

Affiliation: Department of Radiation Oncology, Augusta University, Department of Radiation Oncology, Medical College of Georgia, Augusta University, Georgia Radiation Therapy Center, Wellstar-MCG Health, Department of Radiation Oncology, Doctors Hospital of Augusta, Department of Radiology and Imaging, Augusta University

Abstract Preview: Purpose: This study assesses Philips’ O-MAR effectiveness in adjusting AVHU values of common anatomical materials affected by various high-density metal artifacts at varying distances.

Methods:...

BEST IN PHYSICS IMAGING: Volumetric Sampling Properties of Longitudinal Multiple X-Ray-Source Array Computed Tomography (MXA-CT)

Authors: Craig K Abbey, John M. Boone, Richard E. Colbeth, Andrew M. Hernandez, Vance Robinson, Paul Schwoebel, Jeffrey H. Siewerdsen, Alejandro Sisniega, Wojciech B. Zbijewski, Huanyi Zhou

Affiliation: UC Santa Barbara, University of California, UT MD Anderson Cancer Center, Johns Hopkins University, University of New Mexico Albuquerque, UC Davis Health, Varex Imaging Corporation

Abstract Preview: Purpose: A new CT configuration utilizing a multiple X-ray-source array (MXA) is under development to mitigate cone-beam (CB) artifacts in single-rotation wide coverage imaging. We investigate the sam...

Clinical Application of Biology-Guided Radiotherapy (BgRT) to Patient Having a Cardiac Implantable Electronic Device (CIED)

Authors: Girish Bal, Thomas I. Banks, Bin Cai, Yesenia Gonzalez, Zohaib Iqbal, Paul M. Medin, Rameshwar Prasad, Chenyang Shen, Robert Timmerman, Yuanyuan Zhang

Affiliation: Department of Radiation Oncology, UT Southwestern Medical Center, University of Texas Southwestern Medical Center, UT Southwestern Medical Center, RefleXion Medical

Abstract Preview: Purpose: Each year more than one million pacemakers and 200k cardioverter-defibrillators are implanted in patients worldwide. The presence of a CIED introduces challenges to the delivery of radiation ...

Comparative Study between Sparse Primary Sampling Grid Scatter Correction and Low-Count Monte Carlo-Based Scatter Reduction with 3-D Richardson-Lucy Denoising

Authors: Alan Rui Li, Qihui Lyu, Dan Ruan, Ke Sheng

Affiliation: Department of Radiation Oncology, University of California, Los Angeles, University of California San Francisco, Department of Radiation Oncology, University of California, San Francisco

Abstract Preview: Purpose:
The Sparse Primary Sampling (SPS) grid was shown in a previous computational study to improve image quality by correcting scatter-induced effects and artifacts in Cone-beam Computed Tomogr...

Comparison of CBCT Dose Calculation Accuracy between Truebeam Hypersight Image Reconstruction Algorithms

Authors: Sarah Aubert, Harald Keller, Jeff D. Winter

Affiliation: Princess Margaret Cancer Centre

Abstract Preview: Purpose: Support for treatment planning directly on CBCT imaging on C-arm linacs will enable adaptive radiation therapy (ART) capacity with increased treatment technique flexibility. The HyperSight im...

Comprehensive Evaluation of High-Performance Cone-Beam Computed Tomography on C-Arm and Ring-Gantry Linacs for Adaptive Radiation Therapy

Authors: Laura I. Cervino, Karen Episcopia, Hsiang-Chi Kuo, Sangkyu Lee, Seng Boh Gary Lim, Shih-Chi Lin, Grace Tang

Affiliation: Department of Medical Physics, Memorial Sloan Kettering Cancer Center

Abstract Preview: Purpose: This study evaluated the performance of the HyperSight Cone-Beam Computed Tomography (CBCT) system on a TrueBeam C-arm LINAC (TB) and two Ethos ring-gantry LINACs (ES) for adaptive radiation ...

Deep Autoencoder for Ring Artifact Denoising in Photon-Counting CT

Authors: Magdalena Bazalova-Carter, James Day, Xinchen Deng

Affiliation: University of Victoria

Abstract Preview: Purpose:
Ring artifacts in Photon-Counting Computed Tomography (PCCT) images can degrade image quality. this study aims to suppress ring artifacts with a novel autoencoder-based framework that leve...

Detector Physics-Incorporated Diffusion Denoising Models for Digital Breast Tomosynthesis with Dual-Layer Flat Panel Detectors

Authors: Alexander Bookbinder, Matthew Tivnan, Xiangyi Wu, Wei Zhao

Affiliation: Stony Brook Medicine, Massachusetts General Hospital

Abstract Preview: Purpose: To investigate and benchmark a system-adaptive diffusion-based digital breast tomosynthesis (DBT) denoising model for a direct-indirect dual-layer flat panel detector (DI-DLFPD) with a k-edge...

Development of a High-Speed Digital Breast Tomosynthesis System with a Two-Dimensional Multiple X-Ray-Source Array

Authors: John M. Boone, Andrew M. Hernandez, Paul Schwoebel, Jeffrey H. Siewerdsen, Alejandro Sisniega, Wojciech B. Zbijewski

Affiliation: Johns Hopkins University, University of California, UT MD Anderson Cancer Center, University of New Mexico Albuquerque, UC Davis Health

Abstract Preview: Purpose: To significantly improve image quality relative to clinically deployed digital breast tomosynthesis (DBT) systems, which use a 1D acquisition geometry (an arc), with a 2D image acquisition ge...

Dual Energy Cone Beam Computed Tomography for Artifact Reduction and Enhanced Image Quality Using Existing Hardware in Radiation Therapy

Authors: Michael J. Choi, Vindu Wathsala Kathriarachchi, Christopher L. Nelson, Andrew P. Soderstrom, Yawei Zhang

Affiliation: The University of Texas MD Anderson Cancer Center, MD Anderson, UF Health Proton Therapy Institute

Abstract Preview: Purpose: Cone Beam Computed Tomography (CBCT) is widely used in image-guided radiation therapy for patient positioning. While kV photons offer high image contrast, they are prone to artifacts caused b...

Dual-Branch Attention-Driven Network for Enhanced Sparse-View CBCT Reconstruction Using Planning CT As Prior Knowledge

Authors: Xiaoyi Dai, Manju Liu, Weiwei Sang, Pulin Sun, Fan Xia, Zhenyu Yang, Fang-Fang Yin, Chulong Zhang, Rihui Zhang

Affiliation: Jiahui International Hospital, Radiation Oncology, Duke Kunshan University, Medical Physics Graduate Program, Duke Kunshan University

Abstract Preview: Purpose:
Current deep learning-based sparse-view CBCT reconstruction methods are prone to hallucinatory artifacts, as they rely on inferred image details that may not correspond to true anatomical ...

Enhanced 3D Volumetric Denoising for Low-Dose CT Images Using Hformer

Authors: Edward Robert Criscuolo, Chenlu Qin, Deshan Yang, Zhendong Zhang

Affiliation: Duke University, Department of Radiation Oncology, Duke University

Abstract Preview: Purpose:
Low-dose CT (LDCT) imaging minimizes radiation exposure but introduces significant noise, compromising image quality. While deep learning-based denoising models such as HFormer achieve sta...

Evaluating the Capabilities of Hypersight CBCT for Advanced Dual-Energy CBCT Imaging in Online Adaptive Radiotherapy

Authors: Yi-Fang Wang, Yading Yuan

Affiliation: Columbia University Irving Medical Center, Department of Radiation Oncology, Columbia University Irving Medical Center

Abstract Preview: Purpose: HyperSight, the latest CBCT technology from Varian Medical Systems, integrates rapid 6-second data acquisition with advanced iterative reconstruction and upgraded hardware. Previous studies h...

Evaluation of a Commercial Metal Artifact Reduction Algorithm for IGRT in Patients with Hip Prostheses

Authors: Laura Bennett, Matthew Charles, Greg Cooley, John M. Floberg, Ryan Hutten, Elissa Khoudary, Nicholas Lynch, Kaili Ranta, Jordan M. Slagowski, Trevor Wilson

Affiliation: University of Pennsylvania, Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Department of Human Oncology, University of Wisconsin-Madison, University of Wisconsin-Madison

Abstract Preview: Purpose: To evaluate the impact of an iterative cone beam CT with metal artifact reduction (iCBCT+MAR) reconstruction algorithm (HyperSight TrueBeam 4.1, Varian Medical Systems) on image quality and s...

Feasibility of Efficient Offline Adaptive Replanning with Hypersight High-Performance Cone-Beam CT on Truebeam for Pelvis RT

Authors: Jochen Cammin, Shifeng Chen, Arun Gopal, Kai Huang, Jason K Molitoris, Amit Sawant, Kai Wang

Affiliation: University of Maryland, Department of Radiation Oncology, University of Maryland School of Medicine, Maryland University Baltimore, Department of Radiation Oncology, University of Maryland Medical Center

Abstract Preview: Purpose: The HyperSight CBCT optional feature on Varian TrueBeam linacs offers a larger field-of-view, improved Hounsfield units (HU) accuracy, and overall improved image quality, including metal arti...

Hybrid Prior-Enhanced Deep Image Prior (HPEDIP) Image Reconstruction for Ultra-Short Scans

Authors: Renee Farrell, Jinkoo Kim, Xin Qian, Ziyu Shu, Zhaozheng Yin, Tiezhi Zhang

Affiliation: Stony Brook Medicine, Washington University in St. Louis, Stony Brook University, Stony Brook University Hospital

Abstract Preview: Purpose: Ultra-short CT scan allows fast imaging speed, dose reduction, and compact system design. We developed a deep image prior (DIP) based reconstruction method named Hybrid Prior-Enhanced Deep Im...

Imaging Performance of Direct-Indirect Dual-Layer Flat-Panel-Detector Prototypes for Contrast-Enhanced Digital Mammography

Authors: Salman M. Arnab, Yves Chevalier, Samuel Gagné, Adrian F. Howansky, Luc Laperrière, Xiangyi Wu, Wei Zhao

Affiliation: Stony Brook Medicine, Analogic Canada

Abstract Preview: Purpose: A direct-indirect dual-layer flat-panel-detector (DI-DLFPD) is under development for patient motion artifact-free contrast-enhanced digital mammography (CEDM). DI-DLFPD comprises a direct fro...

In-Vivo Image Quality of Head/Neck and CNS with an Advanced C-Arm Linac CBCT Solution

Authors: Theodore Higgins Arsenault, Kenneth W. Gregg, Lauren E Henke, Rojano Kashani, Christian Erik Petersen, Alex T. Price, Atefeh Rezaei, Runyon C. Woods

Affiliation: University Hospitals Seidman Cancer Center

Abstract Preview: Purpose: CBCT is subject to more artifacts due to increased photon scatter, especially in areas of increased tissue heterogeneities compared to fan-beam CTs (FBCTs). Improved imaging panels combined w...

Initial Comparison of 3D CBCT Performance: New Mobile C-Arm Vs. O-Arm Unit

Authors: Wendy Siman, Wei Zhou

Affiliation: University of Colorado Anschutz Medical Campus, School of Medicine

Abstract Preview: Purpose:
To compare the image quality and radiation dose of 3D cone-beam CT modes of a mobile C-arm and an O-arm unit for intraoperative imaging.
Methods:
A 50-µm tungsten wire was imaged at ...

Investigation of Metallic Artefact Reduction Capabilities of Hypersight Enabled Platforms

Authors: Harald Keller, Iymad Mansour, Jeff D. Winter

Affiliation: Princess Margaret Cancer Centre

Abstract Preview: Purpose: The growing number of adaptive therapy applications is motivating image segmentation and direct dose calculation on CBCT. The purpose of this investigation is to evaluate the recently release...

Mask Guided Diffusion Model for Metal Artifacts Reduction

Authors: Shusen Jing, Qihui Lyu, Dan Ruan, Ke Sheng, Qifan Xu

Affiliation: Department of Radiation Oncology, University of California, Los Angeles, University of California San Francisco, Department of Radiation Oncology, University of California, San Francisco

Abstract Preview: Purpose: Metallic implants can significantly distort sinograms, leading to severe artifacts in computed tomography (CT) reconstructions. Reconstructing CT images containing metal is fundamentally an i...

Monte Carlo Simulation of a Prototype Fluence Field Modulated Cone Beam CT System

Authors: Gregory J Bootsma, Tunok Mondol

Affiliation: University Health Network, Princess Margaret Cancer Center

Abstract Preview: Purpose:
To evaluate the impact of prototype fluence field modulation (FFM) cone-beam CT (CBCT) on dose reduction and image quality using Monte Carlo simulations. This study particularly looks at t...

Multiple x-Ray Source Array (MXA) System Development for Better Ray Sampling in Large Coverage Axial CT Imaging

Authors: Craig K Abbey, John M. Boone, Richard E. Colbeth, Andrew M. Hernandez, Sarah E. McKenney, Vance Robinson, Paul Schwoebel, Jeffrey H. Siewerdsen, Alejandro Sisniega, Wojciech B. Zbijewski

Affiliation: UC Santa Barbara, University of California, UT MD Anderson Cancer Center, Johns Hopkins University, University of New Mexico Albuquerque, UC Davis Health, Varex Imaging Corporation

Abstract Preview: Purpose:
While wide coverage (160 mm) CT scanners can image whole organs (e.g. heart, brain, kidneys, etc.) in one axial scan, the peripheral regions of the field of view (along z) suffer from cons...

Novel 3D Imaging Systems for Surgical Guidance : Loop-X and Nview S1 in Comparison with O-Arm

Authors: Jia Wang, Vani Yadav

Affiliation: Stanford University

Abstract Preview: Purpose: To evaluate the radiation dose and imaging quality of two novel CBCT X-Ray imaging systems used in spine surgeries.
Methods: CBCT scan of ACR CT and CTDI phantoms was performed to acquire ...

Optimizing Design to Enhance the Energy Separation in a Kv Dual-Layer Imager (DLI)

Authors: Ross I. Berbeco, Vera Birrer, Raphael Bruegger, Pablo Corral Arroyo, Roshanak Etemadpour, Dianne M. Ferguson, Rony Fueglistaller, Thomas C. Harris, Yue-Houng Hu, Matthew W. Jacobson, Mathias Lehmann, Nicholas Lowther, Daniel Morf, Marios Myronakis

Affiliation: Brigham and Women's Hospital, Harvard Medial School, Dana-Farber Cancer Institute, Department of Radiation Oncology, Dana Farber/Brigham and Women's Cancer Center, Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute, Harvard Medical School, Brigham and Womens Hospital, Dana Farber Cancer Institute, Harvard Medical School, Brigham and Women's Hospital, Varian Imaging Laboratory, Dana-Farber Cancer Institute

Abstract Preview: Purpose: Multi-layer flat-panel imagers can improve for clinical image-guided radiotherapy applications, including the enhanced visualization of soft tissue and a reduction in image artifacts. Each im...

Patient-Specific Ultra-Sparse k-Space Reconstruction Using Motion Decomposition and Sinusoidal Representation Networks for Dynamic Volumetric MRI in Radiotherapy

Authors: Karyn A Goodman, Yang Lei, Tian Liu, Charlotte Elizabeth Read, Jing Wang, Qian Wang, Jiahan Zhang

Affiliation: Icahn School of Medicine at Mount Sinai, Department of Radiation Oncology, Beth Israel Deaconess Medical Center

Abstract Preview: Purpose: Accurate motion management in MRI-guided radiotherapy (MRIgRT) relies on real-time volumetric MRI to track intra-fractional anatomical changes. Dense k-space sampling, while capable of produc...

Phantom-Based Dosimetric Comparison of Multiple CT Metal Artifact Reduction Algorithms for Head-and-Neck Radiotherapy

Authors: Loren Bell, Charles S Cathcart, Brett William Eckroate, Harold Y Hu, Joshua Kilian-Meneghin, Ke Nie, Fredrick Warburton, Zhenyu Xiong, Keying (Karen) Xu, Ning J. Yue, Yin Zhang, Lili Zhou

Affiliation: RWJBarnabas Health, Basis Scottsdale, Rutgers Cancer Institute of New Jersey

Abstract Preview: Purpose: This study compares the dosimetric impact of multiple metal artifact reduction (MAR) algorithms from different CT simulator vendors using an anthropomorphic head phantom for head-and-neck (H/...

Robustness of Deep Learning-Based Motion Compensated 4D-CBCT Reconstruction to out-of-Distribution Data

Authors: Geoffrey D. Hugo, Eric Laugeman, Thomas R. Mazur, Pamela Samson, Kim A. Selting, Zhehao Zhang

Affiliation: University of Illinois, Washington University in St. Louis School of Medicine, WashU Medicine

Abstract Preview: Purpose: To investigate the robustness of a deep learning (DL)-based 4D-CBCT motion-compensated (MoCo) reconstruction method to out-of-distribution data.
Methods: Our developed 4D-CBCT reconstructi...

Simultaneous Motion Estimation and Image Reconstruction with Spatiotemporal Implicit Neural Representation Initial (STINR-SMEIR) for Gas Bubble Motion Artifact Reduction in on-Board CBCT Imaging

Authors: Hua-Chieh Shao, Shanshan Tang, Jing Wang, Kai Wang, You Zhang

Affiliation: Department of Radiation Oncology, UT Southwestern Medical Center, Medical Artificial Intelligence and Automation (MAIA) Lab, Department of Radiation Oncology, UT Southwestern Medical Center, Medical Artificial Intelligence and Automation (MAIA) Laboratory, Department of Radiation Oncology, UT Southwestern Medical Center, Department of Radiation Oncology, University of Maryland Medical Center

Abstract Preview: Purpose: Artifacts caused by gas bubble movement in the gastrointestinal tract can severely degrade the image quality of on-board abdominal cone-beam computed tomography (CBCT), impacting its utility ...

Tin-Filtered Spectral Shaping Technique in Clinical Photon-Counting CT: Impact on Noise Magnitude By Object Size, Radiation Dose, and Spectral Reconstruction Type

Authors: Xinhui Duan, Vivek Nair, Liqiang Ren, KyuHo Song, Kuan Zhang, Yue Zhang

Affiliation: Department of Radiology, UT Southwestern Medical Center

Abstract Preview: Purpose: To evaluate how tin-filtered spectral shaping in clinical photon-counting computed tomography (PCCT) affects noise magnitude across varying object sizes, radiation doses, and spectral reconst...

Twin Table-Tilt Imaging: Rapid Metal Artefact Reduction Intraoperative CBCT Imaging during Pedicle Screw Spinal Fixation

Authors: Owen Thomas Dillon, Andrew Kanawati, Tess Reynolds

Affiliation: Westmead Hospital, Image X Institute, Faculty of Medicine and Health, The University of Sydney

Abstract Preview: Purpose: Metal artefacts from implanted hardware and surgical tools remain a limitation for widespread adoption of intraoperative CBCT imaging. Gantry-based non-circular imaging trajectories have emer...

Using Higher Order Harmonics to Reduce Artifacts in Grating Interferometry

Authors: Les Butler, Joyoni Dey, Conner B Dooley, Victoria L Fontenot, Kyungmin Ham, Hunter Cole Meyer

Affiliation: Louisiana State University, Center for Advanced Microstructures and Devices, Linfield University

Abstract Preview: Purpose: X-ray interferometry is an emerging imaging modality with a wide variety of potential clinical applications, including lung and breast imaging. Small inaccuracies in grating position lead to ...