Validation of an optical flow method for tag displacement estimation

Activating deformation twinning in cubic boron nitride

Deformation twinning, a phenomenon primarily documented within metallic systems, has remained essentially unexplored in covalent materials due to the formidable challenges posed by their inherent extreme hardness and brittleness. Here, by employing a five-degree-of-freedom nano-manipulation stage inside a transmission electron microscope, we reveal a loading-specific twinning criterion for cubic boron nitride and successfully activate extensive deformation twinning with substantial improvements in mechanical properties in <100>-oriented cubic boron nitride submicrometre pillars at room temperature. Beyond cubic boron nitride, this criterion is also proven widely applicable across a spectrum of covalent materials. Investigations on the twinning dynamics at the atomic level in cubic boron nitride suggest a continuous-transition-mediated pathway. These findings substantially advance our comprehension of twinning mechanisms in covalent face-centred cubic materials, and herald a promising avenue for microstructural engineering aimed at enhancing the strength and toughness of these materials in their applications.

Computational Analysis of Cardiac Contractile Function

PURPOSE OF REVIEW

Heart failure results in the high incidence and mortality all over the world. Mechanical properties of myocardium are critical determinants of cardiac function, with regional variations in myocardial contractility demonstrated within infarcted ventricles. Quantitative assessment of cardiac contractile function is therefore critical to identify myocardial infarction for the early diagnosis and therapeutic intervention.

RECENT FINDINGS

Current advancement of cardiac functional assessments is in pace with the development of imaging techniques. The methods tailored to advanced imaging have been widely used in cardiac magnetic resonance, echocardiography, and optical microscopy. In this review, we introduce fundamental concepts and applications of representative methods for each imaging modality used in both fundamental research and clinical investigations. All these methods have been designed or developed to quantify time-dependent 2-dimensional (2D) or 3D cardiac mechanics, holding great potential to unravel global or regional myocardial deformation and contractile function from end-systole to end-diastole. Computational methods to assess cardiac contractile function provide a quantitative insight into the analysis of myocardial mechanics during cardiac development, injury, and remodeling.

Comparison of Diagnostic Value Between STE+LDDSE and CMR-FT for Evaluating Coronary Microvascular Obstruction in Post-PCI Patients for STEMI

Background

Coronary microvascular obstruction (CMVO) is closely associated with poor prognosis of ST-segment elevation myocardial infarction (STEMI) patients. However, data showing the comparison between cardiac magnetic resonance feature tracking (CMR-FT) and speckle tracking echocardiography (STE) combined with low-dose dobutamine stress echocardiography (LDDSE) in evaluating CMVO was scarcely available. We aimed to explore and compare the predictive value between CMR-FT and STE+LDDSE in detecting CMVO.

Methods

Sixty-one STEMI patients were executed cardiac magnetic resonance and echocardiography within the first 5–7 days after primary percutaneous coronary intervention (PCI). The myocardial strain analysis was performed in STE, STE+LDDSE, and CMR-FT, and strain parameters included radial strain (RS), circumferential strain (CS), and longitudinal strain (LS). ROC curves were performed to predict infarcted myocardium segments with CMVO.

Results

Finally, 324 infarcted myocardium segments were analyzed, including 100 infarcted segments with CMVO and 224 segments without CMVO by the gold standard assessment of late gadolinium-enhancement cardiac magnetic resonance imaging (LGE-CMR). The results showed that CS was generally superior to RS and LS in identifying CMVO. CS in CMR-FT facilitated the detection of CMVO, with a sensitivity, specificity, and accuracy of 78.00%, 81.25%, and 80.25%, respectively. The sensitivity, specificity, and accuracy of CS in STE combined with LDDSE were better than STE alone (76.00% vs 60.00%, 79.91% vs 64.29%, and 78.70% vs 62.96%, P < 0.05). In addition, CMR-FT is not superior to STE+LDDSE for detection of CMVO (P > 0.05).

Conclusion

Low-dose dobutamine can improve the clinical value of STE for evaluating CMVO in STEMI patients. Compared with CMR-FT, STE+LDDSE might be a better choice for STEMI patients because of its safety, convenience, and low-cost.

Quantification of Myocardial Deformation Applying CMR-Feature-Tracking-All About the Left Ventricle?

Purpose of Review

Cardiac magnetic resonance-feature-tracking (CMR-FT)-based deformation analyses are key tools of cardiovascular imaging and applications in heart failure (HF) diagnostics are expanding. In this review, we outline the current range of application with diagnostic and prognostic implications and provide perspectives on future trends of this technique.

Recent Findings

By applying CMR-FT in different cardiovascular diseases, increasing evidence proves CMR-FT-derived parameters as powerful diagnostic and prognostic imaging biomarkers within the HF continuum partly outperforming traditional clinical values like left ventricular ejection fraction. Importantly, HF diagnostics and deformation analyses by CMR-FT are feasible far beyond sole left ventricular performance evaluation underlining the holistic nature and accuracy of this imaging approach.

Summary

As an established and continuously evolving technique with strong prognostic implications, CMR-FT deformation analyses enable comprehensive cardiac performance quantification of all cardiac chambers.

HARP-I: A Harmonic Phase Interpolation Method for the Estimation of Motion From Tagged MR Images

We proposed a novel method called HARP-I, which enhances the estimation of motion from tagged Magnetic Resonance Imaging (MRI). The harmonic phase of the images is unwrapped and treated as noisy measurements of reference coordinates on a deformed domain, obtaining motion with high accuracy using Radial Basis Functions interpolations. Results were compared against Shortest Path HARP Refinement (SP-HR) and Sine-wave Modeling (SinMod), two harmonic image-based techniques for motion estimation from tagged images. HARP-I showed a favorable similarity with both methods under noise-free conditions, whereas a more robust performance was found in the presence of noise. Cardiac strain was better estimated using HARP-I at almost any motion level, giving strain maps with less artifacts. Additionally, HARP-I showed better temporal consistency as a new method was developed to fix phase jumps between frames. In conclusion, HARP-I showed to be a robust method for the estimation of motion and strain under ideal and non-ideal conditions.

Object Detection Based on Faster R-CNN Algorithm with Skip Pooling and Fusion of Contextual Information

Deep learning is currently the mainstream method of object detection. Faster region-based convolutional neural network (Faster R-CNN) has a pivotal position in deep learning. It has impressive detection effects in ordinary scenes. However, under special conditions, there can still be unsatisfactory detection performance, such as the object having problems like occlusion, deformation, or small size. This paper proposes a novel and improved algorithm based on the Faster R-CNN framework combined with the Faster R-CNN algorithm with skip pooling and fusion of contextual information. This algorithm can improve the detection performance under special conditions on the basis of Faster R-CNN. The improvement mainly has three parts: The first part adds a context information feature extraction model after the conv5_3 of the convolutional layer; the second part adds skip pooling so that the former can fully obtain the contextual information of the object, especially for situations where the object is occluded and deformed; and the third part replaces the region proposal network (RPN) with a more efficient guided anchor RPN (GA-RPN), which can maintain the recall rate while improving the detection performance. The latter can obtain more detailed information from different feature layers of the deep neural network algorithm, and is especially aimed at scenes with small objects. Compared with Faster R-CNN, you only look once series (such as: YOLOv3), single shot detector (such as: SSD512), and other object detection algorithms, the algorithm proposed in this paper has an average improvement of 6.857% on the mean average precision (mAP) evaluation index while maintaining a certain recall rate. This strongly proves that the proposed method has higher detection rate and detection efficiency in this case.

A Parallel Image Registration Algorithm Based on a Lattice Boltzmann Model

Image registration is a key pre-procedure for high level image processing. However, taking into consideration the complexity and accuracy of the algorithm, the image registration algorithm always has high time complexity. To speed up the registration algorithm, parallel computation is a relevant strategy. Parallelizing the algorithm by implementing Lattice Boltzmann method (LBM) seems a good candidate. In consequence, this paper proposes a novel parallel LBM based model (LB model) for image registration. The main idea of our method consists in simulating the convection diffusion equation through a LB model with an ad hoc collision term. By applying our method on computed tomography angiography images (CTA images), Magnet Resonance images (MR images), natural scene image and artificial images, our model proves to be faster than classical methods and achieves accurate registration. In the continuity of 2D image registration model, the LB model is extended to 3D volume registration providing excellent results in domain such as medical imaging. Our method can run on massively parallel architectures, ranging from embedded field programmable gate arrays (FPGAs) and digital signal processors (DSPs) up to graphics processing units (GPUs).

Magnetic Resonance Imaging of Contracting Ultrathin Cardiac Tissue

OBJECTIVE

Integrating cardiac-tissue patches into the beating heart and evaluating the long-term effects of such integration on cardiac contractility are two challenges in an emerging field of regenerative medicine. This pilot study presents tools for the imaging of contracting multicellular cardiac tissue constructs (MTCs) in vitro and demonstrates the feasibility of tracking the early development of strand geometry and contractions in ultrathin strands and layers of cardiac tissue using CINE MRI.

APPROACH

Cultured, ultrathin (~50-100-micron) MTCs of rat neonatal cardiomyocytes were plated in rectangular cell chambers (4.5 × 2.0 cm) with and without ultrathin, carbon EP electrodes embedded in the floor of the cell chamber. Two-dimensional, steady-state free precession (SSFP) CINE MRI, cell microscopy, and tissue photography were performed on Days 5-9 of cell development. Potential confounders and MRI artifacts were evaluated using non-contracting cardiac tissues and cell-free chambers filled with the cell-culture medium.

MAIN RESULTS

Synchronized contractions formed by Day 7; individual contracting tissue strands became identifiable by Day 9. The global patterns and details of the strand geometry and movement patterns in the SSFP images were in excellent agreement with microscopic and photographic images. No synchronized movement was identifiable by either microscopy or CINE MRI in the non-contracting MTCs or the cell-free medium. The EP recordings revealed well-defined depolarization and repolarization waveforms; the imaging artifacts generated by the carbon electrodes were small.

SIGNIFICANCE

This pilot study demonstrates the feasibility of imaging cardiac-strand patterns and contractile activity in ultrathin, two-dimensional cardiac tissue in commonly used clinical scanners.

Myocardial motion analysis based on an optical flow method using tagged MR images

We developed a method of velocimetry based on an optical flow method using quantitative analyses of tagged magnetic resonance (MR) images (tagged MR-optical flow velocimetry, tMR-O velocimetry). The purpose of our study was to examine the accuracy of measurement of the proposed tMR-O velocimetry. We performed retrospective pseudo-electrocardiogram (ECG) gating tagged cine MR imaging on a rotating phantom. We optimized imaging parameters for tagged MR imaging, and validated the accuracy of tMR-O velocimetry. Our results indicated that the difference between the reference velocities and the computed velocities measured using optimal imaging parameters was less than 1%. In addition, we performed tMR-O velocimetry and echocardiography on 10 healthy volunteers, for four sections of the heart (apical, midventricular, and basal sections aligned with the short-axis, and a four-chamber section aligned with the long-axis), and obtained radial and longitudinal myocardial velocities in these sections. We compared the myocardial velocities obtained using tMR-O velocimetry with those obtained using echocardiography. Our results showed good agreement between tMR-O velocimetry and echocardiography in the radial myocardial velocities in three short-axial sections and longitudinal myocardial velocities on the midventricular portion of the four-chamber section in the long-axis. In the study conducted on the rotating phantom, tMR-O velocimetry showed high accuracy; moreover, in the healthy volunteers, the myocardial velocities obtained using tMR-O velocimetry were relatively similar to those obtained using echocardiography. In conclusion, tMR-O velocimetry is a potentially feasible method for analyzing myocardial motion in the human heart.

Magnetic resonance imaging of myocardial strain: A review of current approaches

Contraction of the heart is central to its purpose of pumping blood around the body. While simple global function measures (such as the ejection fraction) are most commonly used in the clinical assessment of cardiac function, MRI also provides a range of approaches for quantitatively characterizing regional cardiac function, including the local deformation (or strain) within the heart wall. While they have been around for some years, these methods are still undergoing further technical development, and they have had relatively little clinical evaluation. However, they can provide potentially useful new ways to assess cardiac function, which may be able to contribute to better classification and treatment of heart disease. This article provides some basic background on the physical and physiological factors that determine the motion of the heart, in health and disease and then reviews some of the ways that MRI methods are being developed to image and quantify strain within the myocardium.

LEVEL OF EVIDENCE

4 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2017;46:1263-1280.

A graph theoretic approach for computing 3D+time biventricular cardiac strain from tagged MRI data

Tagged magnetic resonance imaging (tMRI) is a well-established method for evaluating regional mechanical function of the heart. Many techniques have been developed to compute 2D or 3D cardiac deformation and strain from tMRI images. In this paper, we present a new method for measuring 3D plus time biventricular myocardial strain from tMRI data. The method is composed of two parts. First, we use a Gabor filter bank to extract tag points along tag lines. Second, each tag point is classified to one of a set of indexed reference tag lines using a point classification with graph cuts (PCGC) algorithm and a motion compensation technique. 3D biventricular deformation and strain is computed at each image time frame from the classified tag points using a previously published finite difference method. The strain computation is fully automatic after myocardial contours are defined near end-diastole and end-systole. An in-vivo dataset composed of 30 human imaging studies with a range of pathologies was used for validation. Strains computed with the PCGC method with no manual corrections were compared to strains computed from both manually placed tag points and a manually-corrected unwrapped phase method. A typical cardiac imaging study with 10 short-axis slices and 6 long-axis slices required 30 min for contouring followed by 44 min of automated processing. The results demonstrate that the proposed method can reconstruct accurate 3D plus time cardiac strain maps with minimal user intervention.

Myocardial motion estimation of tagged cardiac magnetic resonance images using tag motion constraints and multi-level b-splines interpolation

Myocardial motion estimation of tagged cardiac magnetic resonance (TCMR) images is of great significance in clinical diagnosis and the treatment of heart disease. Currently, the harmonic phase analysis method (HARP) and the local sine-wave modeling method (SinMod) have been proven as two state-of-the-art motion estimation methods for TCMR images, since they can directly obtain the inter-frame motion displacement vector field (MDVF) with high accuracy and fast speed. By comparison, SinMod has better performance over HARP in terms of displacement detection, noise and artifacts reduction. However, the SinMod method has some drawbacks: 1) it is unable to estimate local displacements larger than half of the tag spacing; 2) it has observable errors in tracking of tag motion; and 3) the estimated MDVF usually has large local errors. To overcome these problems, we present a novel motion estimation method in this study. The proposed method tracks the motion of tags and then estimates the dense MDVF by using the interpolation. In this new method, a parameter estimation procedure for global motion is applied to match tag intersections between different frames, ensuring specific kinds of large displacements being correctly estimated. In addition, a strategy of tag motion constraints is applied to eliminate most of errors produced by inter-frame tracking of tags and the multi-level b-splines approximation algorithm is utilized, so as to enhance the local continuity and accuracy of the final MDVF. In the estimation of the motion displacement, our proposed method can obtain a more accurate MDVF compared with the SinMod method and our method can overcome the drawbacks of the SinMod method. However, the motion estimation accuracy of our method depends on the accuracy of tag lines detection and our method has a higher time complexity.

A validation of two-dimensional in vivo regional strain computed from displacement encoding with stimulated echoes (DENSE), in reference to tagged magnetic resonance imaging and studies in repeatability

Fast cine displacement encoding with stimulated echoes (DENSE) has comparative advantages over tagged MRI (TMRI) including higher spatial resolution and faster post-processing. This study computed regional radial and circumferential myocardial strains with DENSE displacements and validated it in reference to TMRI, according to American Heart Association (AHA) guidelines for standardized segmentation of regions in the left ventricle (LV). This study was therefore novel in examining agreement between the modalities in 16 AHA recommended LV segments. DENSE displacements were obtained with spatiotemporal phase unwrapping and TMRI displacements obtained with a conventional tag-finding algorithm. A validation study with a rotating phantom established similar shear strain between modalities prior to in vivo studies. A novel meshfree nearest node finite element method (NNFEM) was used for rapid computation of Lagrange strain in both phantom and in vivo studies in both modalities. Also novel was conducting in vivo repeatability studies for observing recurring strain patterns in DENSE and increase confidence in it. Comprehensive regional strain agreements via Bland-Altman analysis between the modalities were obtained. Results from the phantom study showed similar radial-circumferential shear strains from the two modalities. Mean differences in regional in vivo circumferential strains were -0.01 ± 0.09 (95% limits of agreement) from comparing the modalities and -0.01 ± 0.07 from repeatability studies. Differences and means from comparison and repeatability studies were uncorrelated (p > 0.05) indicating no increases in differences with increased strain magnitudes. Bland-Altman analysis and similarities in regional strain distribution within the myocardium showed good agreements between DENSE and TMRI and show their interchangeability. NNFEM was also established as a common framework for computing strain in both modalities.

Cardiovascular magnetic resonance: deeper insights through bioengineering

Heart disease is the main cause of morbidity and mortality worldwide, with coronary artery disease, diabetes, and obesity being major contributing factors. Cardiovascular magnetic resonance (CMR) can provide a wealth of quantitative information on the performance of the heart, without risk to the patient. Quantitative analyses of these data can substantially augment the diagnostic quality of CMR examinations and can lead to more effective characterization of disease and quantification of treatment benefit. This review provides an overview of the current state of the art in CMR with particular regard to the quantification of motion, both microscopic and macroscopic, and the application of bioengineering analysis for the evaluation of cardiac mechanics. We discuss the current clinical practice and the likely advances in the next 5-10 years, as well as the ways in which clinical examinations can be augmented by bioengineering analysis of strain, compliance, and stress.

Image-based three-dimensional finite element modeling approach for upper airway mechanics

The goal of the project is to investigate new computational tools for understanding normal upper airway mechanics and the pathophysiology of obstructive sleep apnea (OSA), a disorder characterized by repetitive pharyngeal collapse and occlusion during sleep. To relate the airway mechanical parameters to anatomy, and explore the roles of primary muscle properties, airway shape and structure, a new computational system for modeling of rat upper airway passive mechanics based on magnetic resonance imaging (MRI) was developed. The model is capable of computing stress and strain distributions under arbitrary loading, and dynamic animation of three-dimensional airway motion.

MR assessment of regional myocardial mechanics

Regional myocardial function can be measured by several MR techniques including tissue tagging, phase velocity mapping, and more recently, displacement encoding with stimulated echoes (DENSE) and strain encoding (SENC). Each of these techniques was developed separately and has undergone significant change since its original implementation. As a result, in the current literature, the common features and the differences between the techniques and what they measure are often unclear and confusing. This review article delivers an extensively referenced introductory text which clarifies the current methodology from the starting point of the Bloch equations. By doing this in a consistent way for each method, the similarities and differences between them are highlighted. In addition, their capabilities and limitations are discussed, together with their relative advantages and disadvantages. While the focus is on sequence design and development, the principal parameters measured by each technique are also summarized, together with brief results, with the reader being directed to the extensive literature on data processing and clinical applications for more detail.

Cardiac motion and deformation recovery from MRI: a review

Magnetic resonance imaging (MRI) is a highly advanced and sophisticated imaging modality for cardiac motion tracking and analysis, capable of providing 3D analysis of global and regional cardiac function with great accuracy and reproducibility. In the past few years, numerous efforts have been devoted to cardiac motion recovery and deformation analysis from MR image sequences. Many approaches have been proposed for tracking cardiac motion and for computing deformation parameters and mechanical properties of the heart from a variety of cardiac MR imaging techniques. In this paper, an updated and critical review of cardiac motion tracking methods including major references and those proposed in the past ten years is provided. The MR imaging and analysis techniques surveyed are based on cine MRI, tagged MRI, phase contrast MRI, DENSE, and SENC. This paper can serve as a tutorial for new researchers entering the field.

Identifying regional cardiac abnormalities from myocardial strains using nontracking-based strain estimation and spatio-temporal tensor analysis

Myocardial strain is a critical indicator of many cardiac diseases and dysfunctions. The goal of this paper is to extract and use the myocardial strain pattern from tagged magnetic resonance imaging (MRI) to identify and localize regional abnormal cardiac function in human subjects. In order to extract the myocardial strains from the tagged images, we developed a novel nontracking-based strain estimation method for tagged MRI. This method is based on the direct extraction of tag deformation, and therefore avoids some limitations of conventional displacement or tracking-based strain estimators. Based on the extracted spatio-temporal strain patterns, we have also developed a novel tensor-based classification framework that better conserves the spatio-temporal structure of the myocardial strain pattern than conventional vector-based classification algorithms. In addition, the tensor-based projection function keeps more of the information of the original feature space, so that abnormal tensors in the subspace can be back-projected to reveal the regional cardiac abnormality in a more physically meaningful way. We have tested our novel methods on 41 human image sequences, and achieved a classification rate of 87.80%. The regional abnormalities recovered from our algorithm agree well with the patient's pathology and clinical image interpretation, and provide a promising avenue for regional cardiac function analysis.

Reversible jump MCMC methods for fully automatic motion analysis in tagged MRI

Tagged magnetic resonance imaging (tMRI) is a well-known noninvasive method for studying regional heart dynamics. It offers great potential for quantitative analysis of a variety of kine(ma)tic parameters, but its clinical use has so far been limited, in part due to the lack of robustness and accuracy of existing tag tracking algorithms in dealing with low (and intrinsically time-varying) image quality. In this paper, we evaluate the performance of four frequently used concepts found in the literature (optical flow, harmonic phase (HARP) magnetic resonance imaging, active contour fitting, and non-rigid image registration) for cardiac motion analysis in 2D tMRI image sequences, using both synthetic image data (with ground truth) and real data from preclinical (small animal) and clinical (human) studies. In addition we propose a new probabilistic method for tag tracking that serves as a complementary step to existing methods. The new method is based on a Bayesian estimation framework, implemented by means of reversible jump Markov chain Monte Carlo (MCMC) methods, and combines information about the heart dynamics, the imaging process, and tag appearance. The experimental results demonstrate that the new method improves the performance of even the best of the four previous methods. Yielding higher consistency, accuracy, and intrinsic tag reliability assessment, the proposed method allows for improved analysis of cardiac motion.

Myocardial tagging by cardiovascular magnetic resonance: evolution of techniques--pulse sequences, analysis algorithms, and applications

Cardiovascular magnetic resonance (CMR) tagging has been established as an essential technique for measuring regional myocardial function. It allows quantification of local intramyocardial motion measures, e.g. strain and strain rate. The invention of CMR tagging came in the late eighties, where the technique allowed for the first time for visualizing transmural myocardial movement without having to implant physical markers. This new idea opened the door for a series of developments and improvements that continue up to the present time. Different tagging techniques are currently available that are more extensive, improved, and sophisticated than they were twenty years ago. Each of these techniques has different versions for improved resolution, signal-to-noise ratio (SNR), scan time, anatomical coverage, three-dimensional capability, and image quality. The tagging techniques covered in this article can be broadly divided into two main categories: 1) Basic techniques, which include magnetization saturation, spatial modulation of magnetization (SPAMM), delay alternating with nutations for tailored excitation (DANTE), and complementary SPAMM (CSPAMM); and 2) Advanced techniques, which include harmonic phase (HARP), displacement encoding with stimulated echoes (DENSE), and strain encoding (SENC). Although most of these techniques were developed by separate groups and evolved from different backgrounds, they are in fact closely related to each other, and they can be interpreted from more than one perspective. Some of these techniques even followed parallel paths of developments, as illustrated in the article. As each technique has its own advantages, some efforts have been made to combine different techniques together for improved image quality or composite information acquisition. In this review, different developments in pulse sequences and related image processing techniques are described along with the necessities that led to their invention, which makes this article easy to read and the covered techniques easy to follow. Major studies that applied CMR tagging for studying myocardial mechanics are also summarized. Finally, the current article includes a plethora of ideas and techniques with over 300 references that motivate the reader to think about the future of CMR tagging.

Comparison of different grid of tags detection methods in tagged cardiac MR imaging

PURPOSE

Non-invasive imaging assessment of cardiac function is important in cardiovascular disease diagnosis, especially for evaluation of local cardiac motion. Tagged cardiac MRI has been developed for this purpose, but evaluation of the results requires quantification and automation.

METHODS

Two methods utilizing active contour modeling for wall motion extraction based on tagged cardiac MRI scans were evaluated based on properties of tracking methods in the image domain and frequency domain. Three criteria were used: accuracy, inter-subject and intra-subject sensitivity. The tracking results were evaluated by a medical expert. The evaluation methodology and its possible generalization to other diagnostic methods were considered.

RESULTS

Image domain and frequency domain analysis of tagged cardiac MRI data sets were evaluated demonstrating that the image domain method provides better results. The image domain method method is much more resistant to changes in the data, this time, due to a different subject being scanned. The frequency domain approach is not suitable for clinical applications, as the global error is significantly increased (more than 20%).

CONCLUSION

The image domain method was found most effective, and it can generate a set of clearly identified parameters. The evaluation approach can be an interesting alternative to classical psychovisual studies which are time-consuming and often fastidious for clinicians.

Motion estimation of tagged cardiac magnetic resonance images using variational techniques

This work presents a new method for motion estimation of tagged cardiac magnetic resonance sequences based on variational techniques. The variational method has been improved by adding a new term in the optical flow equation that incorporates tracking points with high stability of phase. Results were obtained through simulated and real data, and were validated by manual tracking and with respect to a reference state-of-the-art method: harmonic phase imaging (HARP). The error, measured in pixels per frame, obtained with the proposed variational method is one order of magnitude smaller than the one achieved by the reference method, and it requires a lower computational cost.

Deformation analysis of 3D tagged cardiac images using an optical flow method

BACKGROUND

This study proposes and validates a method of measuring 3D strain in myocardium using a 3D Cardiovascular Magnetic Resonance (CMR) tissue-tagging sequence and a 3D optical flow method (OFM).

METHODS

Initially, a 3D tag MR sequence was developed and the parameters of the sequence and 3D OFM were optimized using phantom images with simulated deformation. This method then was validated in-vivo and utilized to quantify normal sheep left ventricular functions.

RESULTS

Optimizing imaging and OFM parameters in the phantom study produced sub-pixel root-mean square error (RMS) between the estimated and known displacements in the x (RMSx = 0.62 pixels (0.43 mm)), y (RMSy = 0.64 pixels (0.45 mm)) and z (RMSz = 0.68 pixels (1 mm)) direction, respectively. In-vivo validation demonstrated excellent correlation between the displacement measured by manually tracking tag intersections and that generated by 3D OFM (R >or= 0.98). Technique performance was maintained even with 20% Gaussian noise added to the phantom images. Furthermore, 3D tracking of 3D cardiac motions resulted in a 51% decrease in in-plane tracking error as compared to 2D tracking. The in-vivo function studies showed that maximum wall thickening was greatest in the lateral wall, and increased from both apex and base towards the mid-ventricular region. Regional deformation patterns are in agreement with previous studies on LV function.

CONCLUSION

A novel method was developed to measure 3D LV wall deformation rapidly with high in-plane and through-plane resolution from one 3D cine acquisition.

Modeling upper airway collapse by a finite element model with regional tissue properties

This study presents a new computational system for modeling the upper airway in rats that combines tagged magnetic resonance imaging (MRI) with tissue material properties to predict three-dimensional (3D) airway motion. The model is capable of predicting airway wall and tissue deformation under airway pressure loading up to airway collapse. The model demonstrates that oropharynx collapse pressure depends primarily on ventral wall (tongue muscle) elastic modulus and airway architecture. An iterative approach that involves substituting alternative possible tissue elastic moduli was used to improve model precision. The proposed 3D model accounts for stress-strain relationships in the complex upper airway that should present new opportunities for understanding pathogenesis of airway collapse, improving diagnosis and developing treatments.

Segmentation of myocardial boundaries in tagged cardiac MRI using active contours: a gradient-based approach integrating texture analysis

The noninvasive assessment of cardiac function is of first importance for the diagnosis of cardiovascular diseases. Among all medical scanners only a few enables radiologists to evaluate the local cardiac motion. Tagged cardiac MRI is one of them. This protocol generates on Short-Axis (SA) sequences a dark grid which is deformed in accordance with the cardiac motion. Tracking the grid allows specialists a local estimation of cardiac geometrical parameters within myocardium. The work described in this paper aims to automate the myocardial contours detection in order to optimize the detection and the tracking of the grid of tags within myocardium. The method we have developed for endocardial and epicardial contours detection is based on the use of texture analysis and active contours models. Texture analysis allows us to define energy maps more efficient than those usually used in active contours methods where attractor is often based on gradient and which were useless in our case of study, for quality of tagged cardiac MRI is very poor.

Ventricular restraint prevents infarct expansion and improves borderzone function after myocardial infarction: a study using magnetic resonance imaging, three-dimensional surface modeling, and myocardial tagging

BACKGROUND

Infarct expansion is associated with impaired borderzone function, adverse remodeling, and poor long-term prognosis. We hypothesized that left ventricular restraint early after myocardial infarction limits infarct expansion, preserves borderzone function, and reduces remodeling.

METHODS

We used an ovine model as well as high spatial and temporal resolution cardiac magnetic resonance imaging to quantify total and infarcted left ventricular epicardial surface area at baseline and 1 week and 12 weeks after anterior wall infarction in 10 animals. Five animals were randomly assigned to treatment with left ventricular restraint (Acorn cardiac support device) 1 week after infarction. Five animals were untreated controls. Total left ventricular surface area was measured by importing the end-diastolic magnetic resonance imaging-derived epicardial contours into custom software, which creates a three-dimensional surface from the two-dimensional magnetic resonance imaging contours. Infarct area was calculated from magnetic resonance imaging-detectable titanium markers placed at the infarct border. Borderzone radial and circumferential strains during systole were also assessed using myocardial tagging techniques as a measure of contractile function.

RESULTS

The infarct area 1 week after infarction was 1,177 +/- 386 mm(2) in the control group and 1,124 +/- 427 mm(2) in the cardiac support device group. After 12 weeks, infarct area was 3,666 +/- 1,013 mm(2) in the control group and 1,227 +/- 301 mm(2) in the cardiac support device group. Borderzone systolic radial strain decreased from 12.6% +/- 0.77% to 3.6% +/- 0.3% after infarction in the control group and 13.7% +/- 0.87% to 4.7% +/- 0.3% in the cardiac support device group. At 12 weeks after infarction, radial strain was 3.4% +/- 0.5% in the control group and 6.7% +/- 0.4% in the cardiac support device group.

CONCLUSIONS

Early postinfarction left ventricular restraint limits infarct expansion and improves borderzone contractile function.

Application of an optical flow method to inspiratory and expiratory lung MDCT to assess regional air trapping: a feasibility study

OBJECTIVE

We describe the application of an optical flow method to inspiratory and expiratory high-resolution volumetric lung MDCT for the assessment of regional air trapping.

CONCLUSION

Qualitative and quantitative assessment of regional air trapping is feasible using an optical flow method to align volumetric MDCT data sets.

Tracking myocardial motion from cine DENSE images using spatiotemporal phase unwrapping and temporal fitting

Displacement encoding with stimulated echoes (DENSE) encodes myocardial tissue displacement into the phase of the MR image. Cine DENSE allows for rapid quantification of myocardial displacement at multiple cardiac phases through the majority of the cardiac cycle. For practical sensitivities to motion, relatively high displacement encoding frequencies are used and phase wrapping typically occurs. In order to obtain absolute measures of displacement, a two-dimensional (2-D) quality-guided phase unwrapping algorithm was adapted to unwrap both spatially and temporally. Both a fully automated algorithm and a faster semi-automated algorithm are proposed. A method for computing the 2-D trajectories of discrete points in the myocardium as they move through the cardiac cycle is introduced. The error in individual displacement measurements is reduced by fitting a time series to sequential displacement measurements along each trajectory. This improvement is in turn reflected in strain maps, which are derived directly from the trajectories. These methods were validated both in vivo and on a rotating phantom. Further measurements were made to optimize the displacement encoding frequency and to estimate the baseline strain noise both on the phantom and in vivo. The fully automated phase unwrapping algorithm was successful for 767 out of 800 images (95.9%), and the semi-automated algorithm was successful for 786 out of 800 images (98.3%). The accuracy of the tracking algorithm for typical cardiac displacements on a rotating phantom is 0.24 +/- 0.15 mm. The optimal displacement encoding frequency is in the region of 0.1 cycles/mm, and, for 2 scans of 17-s duration, the strain noise after temporal fitting was estimated to be 2.5 +/- 3.0% at end-diastole, 3.1 +/- 3.1% at end-systole, and 5.3 +/- 5.0% in mid-diastole. The improvement in intra-myocardial strain measurements due to temporal fitting is apparent in strain histograms, and also in identifying regions of dysfunctional myocardium in studies of patients with infarcts.

Nonrigid image registration with subdivision lattices: application to cardiac MR image analysis

In this paper we present a new methodology for cardiac motion tracking in tagged MRI using nonrigid image registration based on subdivision surfaces and subdivision lattices. We use two sets of registrations to do the motion tracking. First, a set of surface registrations is used to create and initially align the subdivision model of the left ventricle with short-axis and long-axis MR images. Second, a series of volumetric registrations are used to perform the motion tracking and to reconstruct the 4D cardiac motion field from the tagged MR images. The motion of a point in the myocardium over time is calculated by registering the images taken during systole to the set of reference images taken at end-diastole. Registration is achieved by optimizing the positions of the vertices in the base lattice so that the mutual information of the images being registered is maximized. The presented method is validated using a cardiac motion simulator and we also present strain measurements obtained from a group of normal volunteers.

Advances in MRI tagging techniques for determining regional myocardial strain

MRI of the heart with magnetization tagging provides a potentially useful new way to assess cardiac mechanical function, through revealing the local motion of otherwise indistinguishable portions of the heart wall. Although still an evolving area, tagged cardiac MRI is already able to provide novel quantitative information on cardiac function. Exploiting this potential requires developing tailored methods for both imaging and image analysis. In this article, we review some of the progress that has been made in developing imaging methods for tagged cardiac MRI.

Cardiac support device modifies left ventricular geometry and myocardial structure after myocardial infarction

BACKGROUND

Whether mechanical restraint of the left ventricle (LV) can influence remodeling after myocardial infarction (MI) remains poorly understood. This study surgically placed a cardiac support device (CSD) over the entire LV and examined LV and myocyte geometry and function after MI.

METHODS AND RESULTS

Post-MI sheep (35 to 45 kg; MI size, 23+/-2%) were randomized to placement of the CorCap CSD (Acorn Cardiovascular, Inc) (MI+CSD; n=6) or remained untreated (MI only; n=5). Uninstrumented sheep (n=10) served as controls. At 3 months after MI, LV end-diastolic volume (by MRI) was increased in the MI only group compared with controls (98+/-8 versus 43+/-4 mL; P<0.05). In the MI+CSD group, LV end-diastolic volume was lower than MI only values (56+/-7 mL; P<0.05) but remained higher than controls (P<0.05). Isolated LV myocyte shortening velocity was reduced by 35% from control values (P<0.05) in both MI groups. LV myocyte beta-adrenergic response was reduced with MI but normalized in the MI+CSD group. LV myocyte length increased in the MI group and was reduced in the MI+CSD group. Relative collagen content was increased and matrix metalloproteinase-9 was decreased within the MI border region of the CSD group.

CONCLUSIONS

A CSD beneficially modified LV and myocyte remodeling after MI through both cellular and extracellular mechanisms. These findings provide evidence that nonpharmacological strategies can interrupt adverse LV remodeling after MI.

Magnetic resonance imaging-based spirometry for regional assessment of pulmonary function

In this work MRI-based spirometry is presented as a method for noninvasively assessing pulmonary mechanical function on a regional basis. A SPAMM tagging sequence was modified to allow continuous dynamic imaging of the lungs during respiration. A motion-tracking algorithm was developed to track material regions from time-resolved grid-tagged images. Experiments were performed to image the lungs during quiet breathing and volumetric strain was calculated from the measured displacement maps. Regional volume calculations, derived from volumetric strain, were integrated over the entire lung and compared to segmented volume calculations with good agreement. Results from this work demonstrate that MRI spirometry has the potential to become a clinically useful tool for measuring regional ventilation and assessing pulmonary diseases that regionally affect the mechanical function of the lung.

Use of an optical flow method for the analysis of serial CT lung images

RATIONALE AND OBJECTIVES

Serial CT lung studies are difficult to compare due to misregistration between image sets. An optical flow method (OFM) was adapted for use on CT lung images to register images and visualize changes between studies. Three applications were investigated: lung nodule assessment; evaluation of pulmonary enhancement; and functional changes due to air trapping.

MATERIALS AND METHODS

From an initial clinical study, a follow-up study was created by digitally manipulating the images to simulate patient positioning errors and nodule growth. Nodule growth was measured from the temporal subtraction of registered images. In application to the assessment of pulmonary enhancement, pre and postcontrast images from a patient with acute pulmonary embolism (PE) were registered. A map of the perfused blood volume was computed from the ratio of aligned lung volumes. Functional changes in the lung were demonstrated using images from a patient with air trapping. End-inspiratory and end-expiratory volumes were aligned and displacement fields estimated using the OFM. Principal strains were computed from the displacement fields.

RESULTS

All image volumes were aligned with at least 0.95 correlation. OFM estimates of displacement showed excellent agreement with the prescribed displacements with 0.33 pixel RMS error. Nodule growth was evident in the presence of significant positioning errors. In the PE case, enhancement ratios indicated a hypoperfused area consistent with an occlusive hypodense filling defect. For the air trapping case, a strain map showed functional changes along the interface of the air trap.

CONCLUSIONS

The OFM can facilitate the detection and quantification of changes between serial CT lung studies.

Tracking of Left Ventricular Long Axis From Real-Time Three-Dimensional Echocardiography Using Optical Flow Techniques

Two-dimensional echocardiography (2DE) is routinely used in clinical practice to measure left ventricular (LV) mass, dimensions, and function. The reliability of these measurements is highly dependent on the ability to obtain nonforeshortened long axis (LA) images of the left ventricle from transthoracic apical acoustic windows. Real time three-dimensional echocardiography (RT3DE) is a novel imaging technique that allows the acquisition of dynamic pyramidal data structures encompassing the entire ventricle and could potentially overcome the effects of LA foreshortening. Accordingly, the aim of this paper was to develop a nearly automated method based on optical flow techniques for the measurement of the left ventricular (LV) LA throughout the cardiac cycle from RT3DE data. The LV LA measurements obtained with the automated technique has been compared with LA measurements derived from manual selection of the LA from a volumetric display of RT3DE data. High correlation (r = .99, SEE = 1.8%, y = .94x + 5.3), no significant bias (-0.18 mm), and narrow limits of agreement (SD: 1.91 mm) were found. The comparison between the LA length derived from 2DE and RT3DE data showed significant underestimation of the 2DE based measurements. In conclusion, this study proves that RT3DE data overcome the effects of foreshortening and indicates that the method we propose allows fast and accurate quantification of LA length throughout the cardiac cycle.

A non-rigid registration approach for quantifying myocardial contraction in tagged MRI using generalized information measures

We address the problem of quantitatively assessing myocardial function from tagged MRI sequences. We develop a two-step method comprising (i) a motion estimation step using a novel variational non-rigid registration technique based on generalized information measures, and (ii) a measurement step, yielding local and segmental deformation parameters over the whole myocardium. Experiments on healthy and pathological data demonstrate that this method delivers, within a reasonable computation time and in a fully unsupervised way, reliable measurements for normal subjects and quantitative pathology-specific information. Beyond cardiac MRI, this work redefines the foundations of variational non-rigid registration for information-theoretic similarity criteria with potential interest in multimodal medical imaging.

Tagged magnetic resonance imaging of the heart: a survey

Magnetic resonance imaging (MRI) of the heart with magnetization tagging provides a potentially useful new way to assess cardiac mechanical function, through revealing the local motion of otherwise indistinguishable portions of the heart wall. While still an evolving area, tagged cardiac MRI is already able to provide novel quantitative information on cardiac function. Exploiting this potential requires developing tailored methods for both imaging and image analysis. In this paper, we review some of the progress that has been made in developing such methods for tagged cardiac MRI, as well as some of the ways these methods have been applied to the study of cardiac function.

Non-rigid image registration: theory and practice

Image registration is an important enabling technology in medical image analysis. The current emphasis is on development and validation of application-specific non-rigid techniques, but there is already a plethora of techniques and terminology in use. In this paper we discuss the current state of the art of non-rigid registration to put on-going research in context and to highlight current and future clinical applications that might benefit from this technology. The philosophy and motivation underlying non-rigid registration is discussed and a guide to common terminology is presented. The core components of registration systems are described and outstanding issues of validity and validation are confronted.

Pharyngeal airway wall mechanics using tagged magnetic resonance imaging during medial hypoglossal nerve stimulation in rats

To better understand pharyngeal airway mechanics as it relates to the pathogenesis and treatment of obstructive sleep apnoea, we have developed a novel application of magnetic resonance imaging (MRI) with non-invasive tissue tagging to measure pharyngeal wall tissue motion during active dilatation of the airway. Eleven anaesthetized Sprague-Dawley rats were surgically prepared with platinum electrodes for bilateral stimulation of the medial branch of the hypoglossus nerve that supplies motor output to the protrudor and intrinsic tongue muscles. Images of the pharyngeal airway were acquired before and during stimulation using a gated multislice, spoiled gradient recalled (SPGR) imaging protocol in a 4.7 T magnet. The tag pulses, applied before stimulation, created a grid pattern of magnetically imbedded dark lines that revealed tissue motion in images acquired during stimulation. Stimulation significantly increased cross-sectional area, and anteroposterior and lateral dimensions in the oropharyngeal and velopharyngeal airways when results were averaged across the rostral, mid- and caudal pharynx (P < 0.001). Customized software for tissue motion-tracking and finite element-analysis showed that changes in airway size were associated with ventral displacement of tissues in the ventral pharyngeal wall in the rostral, mid- and caudal pharyngeal regions (P < 0.0032) and ventral displacement of the lateral walls in the mid- and caudal regions (P < 0.0001). In addition, principal maximum stretch was significantly increased in the lateral walls (P < 0.023) in a ventral-lateral direction in the mid- and caudal pharyngeal regions and principal maximum compression (perpendicular to stretch) was significantly increased in the ventral walls in all regions (P < 0.0001). Stimulation did not cause lateral displacement of the lateral pharyngeal walls at any level. The results reveal that the increase in pharyngeal airway size resulting from stimulation of the medial branch of the hypoglossal nerve is predominantly due to ventral displacement of the ventral and lateral pharyngeal walls.

Construction of a statistical model for cardiac motion analysis using nonrigid image registration

In this paper we present a new technique for tracking the movement of the myocardium using a statistical model derived from the motion fields in the hearts of several healthy volunteers. To build the statistical model we tracked the motion of the myocardium in 17 volunteers using a nonrigid registration technique based on free-form deformations and mapped the motion fields obtained into a common reference coordinate system. A principal component analysis (PCA) was then performed on the motion fields to extract the major modes of variation in the fields between the successive time frames. The modes of variation obtained were then used to parametrize the free-form deformations and build our statistical model. The results of using our model to track the motion of the heart in normal volunteers are also presented.

Alignment of CT lung volumes with an optical flow method

RATIONALE AND OBJECTIVES

This study was performed to evaluate an optical flow method for registering serial computed tomographic (CT) images of lung volumes to assist physicians in visualizing and assessing changes between CT scans.

MATERIALS AND METHODS

The optical flow method is a coarse-to-fine model-based motion estimation technique for estimating first a global parametric transformation and then local deformations. Five serial pairs of CT images of lung volumes that were misaligned because of patient positioning, respiration, and/or different fields of view were used to test the method.

RESULTS

Lung volumes depicted on the serial paired images initially were correlated at only 28%-68% because of misalignment. With use of the optical flow method, the serial images were aligned to at least 95% correlation.

CONCLUSION

The optical flow method enables a direct comparison of serial CT images of lung volumes for the assessment of nodules or functional changes in the lung.

Biomechanical dynamics of the heart with MRI

Magnetic resonance imaging (MRI) provides a noninvasive way to evaluate the biomechanical dynamics of the heart. MRI can provide spatially registered tomographic images of the heart in different phases of the cardiac cycle, which can be used to assess global cardiac function and regional endocardial surface motion. In addition, MRI can provide detailed information on the patterns of motion within the heart wall, permitting calculation of the evolution of regional strain and related motion variables within the wall. These show consistent patterns of spatial and temporal variation in normal subjects, which are affected by alterations of function due to disease. Although still an evolving technique, MRI shows promise as a new method for research and clinical evaluation of cardiac dynamics.

Virtual tagging: numerical considerations and phantom validation

This paper presents a virtual tagging framework for measuring, as well as visualising, myocardial deformation using magnetic resonance (MR) velocity imaging. Tagging grids are allocated artificially according to the deformation gradient with varying shapes and densities. The control points are then deformed such that the difference between the induced deformation velocity and that of actually measured MR data is minimum. A full three-dimensional implementation of the technique combined with the mass conservation constraint is provided. Numerical considerations of applying the proposed framework and different optimization strategies have been investigated with both simulated and phantom experiments. The accuracy of the technique in terms of following material deformation is compared with that of conventional tagging technique.