Validation of dynamic heart models obtained using non-linear registration for virtual reality training, planning, and guidance of minimally invasive cardiac surgeries

Minimally-invasive cardiac surgery: a bibliometric analysis of impact and force to identify key and facilitating advanced training

Background

The number of citations an article receives is a marker of its scientific influence within a particular specialty. This bibliometric analysis intended to recognise the top 100 cited articles in minimally-invasive cardiac surgery, to determine the fundamental subject areas that have borne considerable influence upon clinical practice and academic knowledge whilst also considering bibliometric scope. This is increasingly relevant in a continually advancing specialty and one where minimally-invasive cardiac procedures have the potential for huge benefits to patient outcomes.

Methods

The Web of Science (Clarivate Analytics) data citation index database was searched with the following terms: [Minimal* AND Invasive* AND Card* AND Surg*]. Results were limited to full text English language manuscripts and ranked by citation number. Further analysis of the top 100 cited articles was carried out according to subject, author, publication year, journal, institution and country of origin.

Results

A total of 4716 eligible manuscripts were retrieved. Of the top 100 papers, the median (range) citation number was 101 (51–414). The most cited paper by Lichtenstein et al. (Circulation 114(6):591–596, 2006) published in Circulation with 414 citations focused on transapical transcatheter aortic valve implantation as a viable alternative to aortic valve replacement with cardiopulmonary bypass in selected patients with aortic stenosis. The Annals of Thoracic Surgery published the most papers and received the most citations (n = 35; 3036 citations). The United States of America had the most publications and citations (n = 52; 5303 citations), followed by Germany (n = 27; 2598 citations). Harvard Medical School, Boston, Massachusetts, published the most papers of all institutions. Minimally-invasive cardiac surgery pertaining to valve surgery (n = 42) and coronary artery bypass surgery (n = 30) were the two most frequent topics by a large margin.

Conclusions

This work establishes a comprehensive and informative analysis of the most influential publications in minimally-invasive cardiac surgery and outlines what constitutes a citable article. Undertaking a quantitative evaluation of the top 100 papers aids in recognising the contributions of key authors and institutions as well as guiding future efforts in this field to continually improve the quality of care offered to complex cardiac patients.

CT in Transcatheter-delivered Treatment of Valvular Heart Disease

Minimally invasive strategies to treat valvular heart disease have emerged over the past 2 decades. The use of transcatheter aortic valve replacement in the treatment of severe aortic stenosis, for example, has recently expanded from high- to low-risk patients and became an alternative treatment for those with prohibitive surgical risk. With the increase in transcatheter strategies, multimodality imaging, including echocardiography, CT, fluoroscopy, and cardiac MRI, are used. Strategies for preprocedural imaging strategies vary depending on the targeted valve. Herein, an overview of preprocedural imaging strategies and their postprocessing approaches is provided, with a focus on CT. Transcatheter aortic valve replacement is reviewed, as well as less established minimally invasive treatments of the mitral and tricuspid valves. In addition, device-specific details and the goals of CT imaging are discussed. Future imaging developments, such as peri-procedural fusion imaging, machine learning for image processing, and mixed reality applications, are presented.

Clinical Value of Virtual Reality versus 3D Printing in Congenital Heart Disease

Both three-dimensional (3D) printing and virtual reality (VR) are reported as being superior to the current visualization techniques in conveying more comprehensive visualization of congenital heart disease (CHD). However, little is known in terms of their clinical value in diagnostic assessment, medical education, and preoperative planning of CHD. This cross-sectional study aims to address these by involving 35 medical practitioners to subjectively evaluate VR visualization of four selected CHD cases in comparison with the corresponding 3D printed heart models (3DPHM). Six questionnaires were excluded due to incomplete sections, hence a total of 29 records were included for the analysis. The results showed both VR and 3D printed heart models were comparable in terms of the degree of realism. VR was perceived as more useful in medical education and preoperative planning compared to 3D printed heart models, although there was no significant difference in the ratings (p = 0.54 and 0.35, respectively). Twenty-one participants (72%) indicated both the VR and 3DPHM provided additional benefits compared to the conventional medical imaging visualizations. This study concludes the similar clinical value of both VR and 3DPHM in CHD, although further research is needed to involve more cardiac specialists for their views on the usefulness of these tools.

Advanced Medical Use of Three-Dimensional Imaging in Congenital Heart Disease: Augmented Reality, Mixed Reality, Virtual Reality, and Three-Dimensional Printing

Three-dimensional (3D) imaging and image reconstruction play a prominent role in the diagnosis, treatment planning, and post-therapeutic monitoring of patients with congenital heart disease. More interactive and realistic medical experiences take advantage of advanced visualization techniques like augmented, mixed, and virtual reality. Further, 3D printing is now used in medicine. All these technologies improve the understanding of the complex morphologies of congenital heart disease. In this review article, we describe the technical advantages and disadvantages of various advanced visualization techniques and their medical applications in the field of congenital heart disease. In addition, unresolved issues and future perspectives of these evolving techniques are described.

A method for optimizing planning target volume margins for patients receiving lung stereotactic body radiotherapy

Lung stereotactic-body radiotherapy (SBRT) places additional requirements on targeting accuracy over standard approaches. In treatment planning, a tumour volume is geometrically expanded and the resulting planning target volume (PTV) is covered with the prescribed dose. This ensures full dose delivery despite various uncertainties encountered during treatment. We developed a retrospective technique for optimizing the PTV expansion for a patient population. The method relies on deformable image registration (DIR) of the planning CT to a treatment cone-beam CT (CBCT). The resulting transformation is used to map the planned target onto the treatment geometry, allowing the computation of the achieved target/PTV overlap. Basic validation of the method was performed using an anthropomorphic respiratory motion phantom. A self-validation technique was also implemented to allow estimation of the DIR error for the data being analyzed. Our workflow was used to retrospectively optimize PTV margin for 25 patients treated over 93 fractions. Targets for these patients were contoured on 4D CT images. SBRT delivery followed CBCT acquisition and a couch correction. A post-treatment CBCT was also acquired in some cases. Our basic validation demonstrated that the DIR-based technique is capable of transforming target volumes from planning CTs to treatment CBCTs with sub-mm accuracy. Our clinical analysis showed that the minimum percentages of target volumes covered for 3, 4, and 5 mm PTV margins were 92.1, 97.6, and 99.2, respectively. Analyzing data acquired before and just after treatment demonstrated that margins exceeding 5 mm did not significantly improve coverage. Finally, a 5 mm PTV margin achieved  ⩾95% target volume coverage with  ⩾95% probability. Our technique is accurate, automated, self-validating, and incorporates complex ITV shapes/deformations to allow PTV margin optimization. The analysis of clinical data indicates a 5 mm PTV margin is optimal for our process. This approach is generalizable to other disease sites and treatment strategies.

Simulating Developmental Cardiac Morphology in Virtual Reality Using a Deformable Image Registration Approach

While virtual reality (VR) has potential in enhancing cardiovascular diagnosis and treatment, prerequisite labor-intensive image segmentation remains an obstacle for seamlessly simulating 4-dimensional (4-D, 3-D + time) imaging data in an immersive, physiological VR environment. We applied deformable image registration (DIR) in conjunction with 3-D reconstruction and VR implementation to recapitulate developmental cardiac contractile function from light-sheet fluorescence microscopy (LSFM). This method addressed inconsistencies that would arise from independent segmentations of time-dependent data, thereby enabling the creation of a VR environment that fluently simulates cardiac morphological changes. By analyzing myocardial deformation at high spatiotemporal resolution, we interfaced quantitative computations with 4-D VR. We demonstrated that our LSFM-captured images, followed by DIR, yielded average dice similarity coefficients of 0.92 ± 0.05 (n = 510) and 0.93 ± 0.06 (n = 240) when compared to ground truth images obtained from Otsu thresholding and manual segmentation, respectively. The resulting VR environment simulates a wide-angle zoomed-in view of motion in live embryonic zebrafish hearts, in which the cardiac chambers are undergoing structural deformation throughout the cardiac cycle. Thus, this technique allows for an interactive micro-scale VR visualization of developmental cardiac morphology to enable high resolution simulation for both basic and clinical science.

Mathematical Modeling and Virtual Reality Simulation of Surgical Tool Interactions With Soft Tissue: A Review and Prospective

This is an informed assessment of the state of the art and an extensive inventory of modeling approaches and methods for soft tissue/medical cutting tool interaction and of the associated medical processes and phenomena. Modeling and simulation through numerical, theoretical, computational, experimental, and other methods was discussed in comprehensive review sections each of which is concluded with a plausible prospective discussion biased toward the development of so-called virtual reality (VR) simulator environments. The finalized prospective section reflects on the future demands in the area of soft tissue cutting modeling and simulation mostly from a conceptual angle with emphasis on VR development requirements including real-time VR simulator response, cost-effective “close-to-reality” VR implementations, and other demands. The review sections that serve as the basis for the suggested prospective needs are categorized based on: (1) Major VR simulator applications including virtual surgery education, training, operation planning, intraoperative simulation, image-guided surgery, etc. and VR simulator types, e.g., generic, patient-specific and surgery-specific and (2) Available numerical, theoretical, and computational methods in terms of robustness, time effectiveness, computational cost, error control, and accuracy of modeling of certain types of virtual surgical interventions and their experimental validation, geared toward ethically driven artificial “phantom” tissue-based approaches. Digital data processing methods used in modeling of various feedback modalities in VR environments are also discussed.

Role of virtual reality in congenital heart disease

OBJECTIVE

New platforms for patient imaging present opportunities for improved surgical planning in complex congenital heart disease (CHD). Virtual reality (VR) allows for interactive manipulation of high-resolution representations of patient-specific imaging data, as a supplement to traditional 2D visualizations and 3D printed heart models.

DESIGN

We present the novel use of VR for the presurgical planning of cardiac surgery in two infants with complex CHD to demonstrate interactive real-time views of complex intra and extracardiac anatomy.

RESULTS

The use of VR for cardiac presurgical planning is feasible using existing imaging data. The software was evaluated by both pediatric cardiac surgeons and pediatric cardiologists, and felt to be reliable and operated with a very short learning curve.

CONCLUSIONS

VR with controller-based interactive capability allows for interactive viewing of 3D models with complex intra and extracardiac anatomy. This serves as a useful complement to traditional preoperative planning methods in terms of its potential for group based collaborative discussion, user defined illustrative views, cost-effectiveness, and facility of use.

Integrate imaging approach for minimally invasive and robotic procedures

Over the past two decades, robotic and minimally invasive cardiac surgery has been continuously refined and is currently an alternative to traditional open-heart surgery for some patients. The parallel evolution of imaging modalities has made robotic surgery safer and more efficient. Here, we review the pre- and post-operative use of computed tomography (CT) in minimally invasive and robotic cardiac procedures.

Ultrasound guidance for beating heart mitral valve repair augmented by synthetic dynamic CT

Minimally invasive valvular intervention commonly requires intra-procedural navigation to provide spatial and temporal information of relevant cardiac structures and device components. Recently intra-procedural trans-esophageal echocardiography (TEE) has been exploited for this purpose due to its accessibility, low cost, ease of use, and real-time imaging capacity. However, the position and orientation of tissue targets relative to surgical tools can be challenging to perceive, particularly using 2D imaging planes. In this paper, we propose the use of CT images to provide a high-quality 3D context to enhance ultrasound images through image registration, providing an augmented guidance system with minimal impact on standard clinical workflow. We also describe an approach to generate synthetic 4D CT images through non-rigid registration of available ultrasound. This can be employed to avoid a requirement for higher radiation. Synthetic CT images were validated through direct comparison of synthetic and real multi-phase CT images. Validation of CT and ultrasound image registration was performed for both dynamic and synthetic CT image datasets. Our results demonstrated that the synthetically generated dynamic CT images provide similar anatomical representation for relevant cardiac anatomy relative to real dynamic CT images, and similar high registration accuracy that can be achieved for intra-procedural TEE to this versus real dynamic CT images.

Left ventricular strain and its pattern estimated from cine CMR and validation with DENSE

Measurement of local strain provides insight into the biomechanical significance of viable myocardium. We attempted to estimate myocardial strain from cine cardiovascular magnetic resonance (CMR) images by using a b-spline deformable image registration method. Three healthy volunteers and 41 patients with either recent or chronic myocardial infarction (MI) were studied at 1.5 Tesla with both cine and DENSE CMR. Regional circumferential and radial left ventricular strains were estimated from cine and DENSE acquisitions. In all healthy volunteers, there was no difference for peak circumferential strain (- 0.18 ± 0.04 versus - 0.18 ± 0.03, p = 0.76) between cine and DENSE CMR, however peak radial strain was overestimated from cine (0.84 ± 0.37 versus 0.49 ± 0.2, p < 0.01). In the patient study, the peak strain patterns predicted by cine were similar to the patterns from DENSE, including the strain evolution related to recovery time and strain patterns related to MI scar extent. Furthermore, cine-derived strain disclosed different strain patterns in MI and non-MI regions, and regions with transmural and non-transmural MI as DENSE. Although there were large variations with radial strain measurements from cine CMR images, useful circumferential strain information can be obtained from routine clinical CMR imaging. Cine strain analysis has potential to improve the diagnostic yield from routine CMR imaging in clinical practice.

Nonrigid point registration for 2D curves and 3D surfaces and its various applications

A nonrigid B-spline-based point-matching (BPM) method is proposed to match dense surface points. The method solves both the point correspondence and nonrigid transformation without features extraction. The registration method integrates a motion model, which combines a global transformation and a B-spline-based local deformation, into a robust point-matching framework. The point correspondence and deformable transformation are estimated simultaneously by fuzzy correspondence and by a deterministic annealing technique. Prior information about global translation, rotation and scaling is incorporated into the optimization. A local B-spline motion model decreases the degrees of freedom for optimization and thus enables the registration of a larger number of feature points. The performance of the BPM method has been demonstrated and validated using synthesized 2D and 3D data, mouse MRI and micro-CT images. The proposed BPM method can be used to register feature point sets, 2D curves, 3D surfaces and various image data.

Predicting target vessel location on robot-assisted coronary artery bypass graft using CT to ultrasound registration

PURPOSE

Although robot-assisted coronary artery bypass grafting (RA-CABG) has gained more acceptance worldwide, its success still depends on the surgeon's experience and expertise, and the conversion rate to full sternotomy is in the order of 15%-25%. One of the reasons for conversion is poor pre-operative planning, which is based solely on pre-operative computed tomography (CT) images. In this paper, the authors propose a technique to estimate the global peri-operative displacement of the heart and to predict the intra-operative target vessel location, validated via both an in vitro and a clinical study.

METHODS

As the peri-operative heart migration during RA-CABG has never been reported in the literatures, a simple in vitro validation study was conducted using a heart phantom. To mimic the clinical workflow, a pre-operative CT as well as peri-operative ultrasound images at three different stages in the procedure (Stage(0)-following intubation; Stage(1)-following lung deflation; and Stage(2)-following thoracic insufflation) were acquired during the experiment. Following image acquisition, a rigid-body registration using iterative closest point algorithm with the robust estimator was employed to map the pre-operative stage to each of the peri-operative ones, to estimate the heart migration and predict the peri-operative target vessel location. Moreover, a clinical validation of this technique was conducted using offline patient data, where a Monte Carlo simulation was used to overcome the limitations arising due to the invisibility of the target vessel in the peri-operative ultrasound images.

RESULTS

For the in vitro study, the computed target registration error (TRE) at Stage(0), Stage(1), and Stage(2) was 2.1, 3.3, and 2.6 mm, respectively. According to the offline clinical validation study, the maximum TRE at the left anterior descending (LAD) coronary artery was 4.1 mm at Stage(0), 5.1 mm at Stage(1), and 3.4 mm at Stage(2).

CONCLUSIONS

The authors proposed a method to measure and validate peri-operative shifts of the heart during RA-CABG. In vitro and clinical validation studies were conducted and yielded a TRE in the order of 5 mm for all cases. As the desired clinical accuracy imposed by this procedure is on the order of one intercostal space (10-15 mm), our technique suits the clinical requirements. The authors therefore believe this technique has the potential to improve the pre-operative planning by updating peri-operative migration patterns of the heart and, consequently, will lead to reduced conversion to conventional open thoracic procedures.

On mixed reality environments for minimally invasive therapy guidance: systems architecture, successes and challenges in their implementation from laboratory to clinic

Mixed reality environments for medical applications have been explored and developed over the past three decades in an effort to enhance the clinician's view of anatomy and facilitate the performance of minimally invasive procedures. These environments must faithfully represent the real surgical field and require seamless integration of pre- and intra-operative imaging, surgical instrument tracking, and display technology into a common framework centered around and registered to the patient. However, in spite of their reported benefits, few mixed reality environments have been successfully translated into clinical use. Several challenges that contribute to the difficulty in integrating such environments into clinical practice are presented here and discussed in terms of both technical and clinical limitations. This article should raise awareness among both developers and end-users toward facilitating a greater application of such environments in the surgical practice of the future.

A pilot study on magnetic navigation for transcatheter aortic valve implantation using dynamic aortic model and US image guidance

PURPOSE

In this paper, we propose a pilot study for transcatheter aortic valve implantation guided by an augmented magnetic tracking system (MTS) with a dynamic aortic model and intra-operative ultrasound (US) images.

METHODS

The dynamic 3D aortic model is constructed from the preoperative 4D computed tomography, which is animated according to the real-time electrocardiograph (ECG) input of patient. Before the procedure, the US probe calibration is performed to map the US image coordinate to the tracked device coordinate. A temporal alignment is performed to synchronize the ECG signals, the intra-operative US image and the tracking information. Thereafter, with the assistance of synchronized ECG signals, the spatial registration is performed by using a feature-based registration. Then the augmented MTS guides the surgeon to confidently position and deploy the transcatheter aortic valve prosthesis to the target.

RESULTS

The approach was validated by US probe calibration evaluation and animal study. The US calibration accuracy achieved [Formula: see text], whereas in the animal study on three porcine subjects, fiducial, target, deployment distance and tilting errors reached [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], respectively.

CONCLUSION

Our pilot study has revealed that the proposed approach is feasible and accurate for delivery and deployment of transcatheter aortic valve prosthesis.

Accuracy considerations in image-guided cardiac interventions: experience and lessons learned

MOTIVATION

Medical imaging and its application in interventional guidance has revolutionized the development of minimally invasive surgical procedures leading to reduced patient trauma, fewer risks, and shorter recovery times. However, a frequently posed question with regard to an image guidance system is "how accurate is it?" On one hand, the accuracy challenge can be posed in terms of the tolerable clinical error associated with the procedure; on the other hand, accuracy is bound by the limitations of the system's components, including modeling, patient registration, and surgical instrument tracking, all of which ultimately impact the overall targeting capabilities of the system.

METHODS

While these processes are not unique to any interventional specialty, this paper discusses them in the context of two different cardiac image guidance platforms: a model-enhanced ultrasound platform for intracardiac interventions and a prototype system for advanced visualization in image-guided cardiac ablation therapy.

RESULTS

Pre-operative modeling techniques involving manual, semi-automatic and registration-based segmentation are discussed. The performance and limitations of clinically feasible approaches for patient registration evaluated both in the laboratory and in the operating room are presented. Our experience with two different magnetic tracking systems for instrument and ultrasound transducer localization is reported. Ultimately, the overall accuracy of the systems is discussed based on both in vitro and preliminary in vivo experience.

CONCLUSION

While clinical accuracy is specific to a particular patient and procedure and vastly dependent on the surgeon's experience, the system's engineering limitations are critical to determine whether the clinical requirements can be met.

Automatic image guidance for prostate IMRT using low dose CBCT

PURPOSE

Varian's On-Board Imager is a linac-integrated cone-beam CT (CBCT) system used at the authors' institution to acquire images prior to delivering each fraction of prostate intensity modulated radiotherapy. The images are used to determine a couch shift that realigns the tumor with the position obtained in the planning CT. However, this manual image-guided radiotherapy (IGRT) technique is operator dependent, time consuming, offers limited degrees of freedom, and requires significant imaging dose over the course of treatment. To overcome these problems, the authors propose two fully automatic IGRT techniques that require significantly less imaging dose.

METHODS

Dose is reduced by lowering the x-ray tube mA s during CBCT acquisition at the cost of increasing image noise. In "forward" IGRT, the CBCT image is automatically registered to the planning CT to obtain the necessary couch shift. The "reverse" technique offers additional degrees of freedom as it involves nonrigid registration of the planning CT to the CBCT. Both techniques were evaluated using images of an anthropomorphic phantom with simulated motion and by retrospectively analyzing data from ten prostate cancer patients.

RESULTS

IGRT error for the phantom data at 100% relative imaging dose was 8.2 +/- 3.7, 3.5 +/- 1.2,, and 2.1 +/- 0.6 mm for setup only, forward, and reverse techniques, respectively. For patient images acquired at 100% relative imaging dose, the errors were 5.4 +/- 1.7, 5.0 +/- 1.6, 5.0 +/- 2.0, and 4.0 +/- 1.6 mm for setup only, manual forward (performed clinically), automatic forward, and reverse IGRT, respectively. Furthermore, imaging dose could be reduced to 20% without a significant loss in image guidance accuracy.

CONCLUSIONS

The presented image guidance methods are accurate while requiring only 20% of the standard imaging dose. The combination of low dose, automation, and accuracy enables frequent corrections during treatment, possibly leading to reduced margins and improved treatment outcomes.

Three-dimensional echocardiographic virtual endoscopy for the diagnosis of congenital heart disease in children

Virtual endoscopy (VE) is a new post-processing method that uses volumetric data sets to simulate the tracks of a "conventional" flexible endoscope. However, almost all studies of this method have involved virtual visualizations of the cardiovascular structures applied to computed tomography (CT) and magnetic resonance (MR) datasets. This paper introduces a novel visualization method called the "three-dimensional echocardiographic intracardiac endoscopic simulation system (3DE IESS)", which uses 3D echocardiographic images in a virtual reality (VR) environment to diagnose congenital heart disease. The aim of this study was to analyze the feasibility of VE in the evaluation of congenital heart disease in children and its accuracy compared with 2DE. Three experienced pediatric cardiologists blinded to the patients' diagnoses separately reviewed 40 two-dimensional echocardiographic (2DE) datasets and 40 corresponding VE datasets and judged whether abnormal intracardiac anatomy was present in terms of a five-point scale (1 = definitely absent; 2 = probably absent; 3 = cannot be determined; 4 = probably present; and 5 = definitely present). Compared with clinical diagnosis, the diagnostic accuracy of VE was 98.7% for ASD, 92.4% for VSD, 92.6% for TOF, and 94% for DORV, respectively. Diagnostic accuracy of VE was significantly higher than that of 2DE for TOF and DORV except for ASD and VSD. The receiver operating characteristic (ROC) curve for VE was closer to the optimal performance point than was the ROC curve for 2DE. The area under the ROC curve was 0.96 for VE and 0.93 for 2DE. Kappa values (range, 0.73-0.79) for VE and 2DE indicated substantial agreement. 3D echocardiographic VE can enhance our understanding of intracardiac structures and facilitate the evaluation of congenital heart disease.

How accurate is accurate enough? A brief overview on accuracy considerations in image-guided cardiac interventions

Image-guided interventions have revolutionized the development of minimally invasive surgical procedures, leading to reduced patient trauma, fewer risks and shorter recovery times. However, one of the most frequently posed question with regards to an image guidance system is how accurate it is. In this work we provide a brief overview on accuracy considerations from our perspective on cardiac image-guided procedures: what are the clinically-imposed accuracy constraints, how do these measure against the limitations of the image-guidance system, and how can surgeons directly benefit from real-time accuracy feedback to ensure optimal navigation at all times during the intervention?

Generation of myocardial wall surface meshes from segmented MRI

This paper presents a novel method for the generation of myocardial wall surface meshes from segmented 3D MR images, which typically have strongly anisotropic voxels. The method maps a premeshed sphere to the surface of the segmented object. The mapping is defined by the gradient field of the solution of the Laplace equation between the sphere and the surface of the object. The same algorithm is independently used to generate the surface meshes of the epicardium and endocardium of the four cardiac chambers. The generated meshes are smooth despite the strong voxel anisotropy, which is not the case for the marching cubes and related methods. While the proposed method generates more regular mesh triangles than the marching cubes and allows for a complete control of the number of triangles, the generated meshes are still close to the ones obtained by the marching cubes. The method was tested on 3D short-axis cardiac MR images with strongly anisotropic voxels in the long-axis direction. For the five tested subjects, the average in-slice distance between the meshes generated by the proposed method and by the marching cubes was 0.4 mm.

Mapping of cardiac electrophysiology onto a dynamic patient-specific heart model

Electrophysiological cardiac data mapping is an essential tool for the study of cardiac rhythm disorders, such as atrial fibrillation. Over the past decade, various advanced cardiac mapping systems have been developed to create detailed cardiac maps and assist physicians in diagnosis and therapy guidance. While these systems have increased the ability to study and treat cardiac arrhythmias, inherent limitations exist. The objective of this paper is to describe and evaluate a system that extends current approaches to cardiac mapping, to create a dynamic cardiac map, using patient-specific cardiac models. This paper details novel approaches to collecting a stream of electrophysiological cardiac data, registering the data with patient-specific dynamic cardiac models, and displaying the data directly on the dynamic model surface, giving a more accurate and comprehensive visualization environment when compared to current systems. To validate the system, a series of laboratory and in vivo experiments were conducted. In the laboratory studies, the system was used to test the user's ability to accurately locate a landmark in physical space, as well as their ability to accurately navigate to a virtual location. In the in vivo studies the overall system performance was compared to an existing electrophysiological recording system, where right atrial cardiac maps were created during sinus and paced cardiac rhythms. The results showed that the new dynamic cardiac mapping system was able to maintain high accuracy in locating physical and virtual landmarks, while being able to create a dynamic cardiac map displayed on a dynamic cardiac surface model.

Off-Pump Positioning of a Conventional Aortic Valve Prosthesis through the Left Ventricular Apex with the Universal Cardiac Introducer under Sole Ultrasound Guidance, in the Pig

Objective

To test an alternative to catheter and open-heart techniques, by documenting the feasibility of implanting an unmodified mechanical aortic valve (AoV) in the off pump, beating heart using the universal cardiac introducer (UCI) attached to the left ventricular (LV) apex.

Methods

In six pigs, the LV apex was exposed by a median sternotomy. The UCI was attached to the apex. A 12-mm punching tool (punch), introduced through the UCI, was used to create a cylindrical opening through the apex. Then, the AoV, secured to a holder, was introduced into the LV, using transesophageal echocardiographic, guided through the apical LV opening, navigated into the LV outflow tract, and positioned within the aortic annulus. Trans-esophageal echocardiographic guidance was useful for navigation and positioning by superimposing the aortic annulus and prosthetic ring while Doppler imaging verified preserved prosthetic function and absence of perivalvular leaks. The valve function and hemodynamics were observed before termination for macroscopic evaluation.

Results

The punch produced a clean opening without fragmentation or myocardial embolization. During advancement of the mechanical AoV, there were no arrhythmias, mitral valve dysfunctions, evidence of myocardial ischemia, or hemodynamic instability. The AoVs were well seated over the annulus, without obstructing the coronaries or contact with the conduction system. The ring of AoVs was well circumscribed by the aortic annulus.

Conclusions

This study documented the feasibility of positioning a mechanical AoV on the closed, beating heart. These results should encourage the development of adjunct technologies to deliver current tissue or mechanical AoV with minimal side effects.

A dual propagation contours technique for semi-automated assessment of systolic and diastolic cardiac function by CMR

BACKGROUND

Although cardiovascular magnetic resonance (CMR) is frequently performed to measure accurate LV volumes and ejection fractions, LV volume-time curves (VTC) derived ejection and filling rates are not routinely calculated due to lack of robust LV segmentation techniques. VTC derived peak filling rates can be used to accurately assess LV diastolic function, an important clinical parameter. We developed a novel geometry-independent dual-contour propagation technique, making use of LV endocardial contours manually drawn at end systole and end diastole, to compute VTC and measured LV ejection and filling rates in hypertensive patients and normal volunteers.

METHODS

39 normal volunteers and 49 hypertensive patients underwent CMR. LV contours were manually drawn on all time frames in 18 normal volunteers. The dual-contour propagation algorithm was used to propagate contours throughout the cardiac cycle. The results were compared to those obtained with single-contour propagation (using either end-diastolic or end-systolic contours) and commercially available software. We then used the dual-contour propagation technique to measure peak ejection rate (PER) and peak early diastolic and late diastolic filling rates (ePFR and aPFR) in all normal volunteers and hypertensive patients.

RESULTS

Compared to single-contour propagation methods and the commercial method, VTC by dual-contour propagation showed significantly better agreement with manually-derived VTC. Ejection and filling rates by dual-contour propagation agreed with manual (dual-contour - manual PER: -0.12 +/- 0.08; ePFR: -0.07 +/- 0.07; aPFR: 0.06 +/- 0.03 EDV/s, all P = NS). However, the time for the manual method was approximately 4 hours per study versus approximately 7 minutes for dual-contour propagation. LV systolic function measured by LVEF and PER did not differ between normal volunteers and hypertensive patients. However, ePFR was lower in hypertensive patients vs. normal volunteers, while aPFR was higher, indicative of altered diastolic filling rates in hypertensive patients.

CONCLUSION

Dual-propagated contours can accurately measure both systolic and diastolic volumetric indices that can be applied in a routine clinical CMR environment. With dual-contour propagation, the user interaction that is routinely performed to measure LVEF is leveraged to obtain additional clinically relevant parameters.

Patient specific 4D coronary models from ECG-gated CTA data for intra-operative dynamic alignment of CTA with X-ray images

We present an approach to derive patient specific coronary models from ECG-gated CTA data and their application for the alignment of CTA with mono-plane X-ray imaging during interventional cardiology. A 4D (3D+t) deformation model of the coronary arteries is derived by (i) extraction of a 3D coronary model at an appropriate cardiac phase and (ii) non-rigid registration of the CTA images at different ECG phases to obtain a deformation model. The resulting 4D coronary model is aligned with the X-ray data using a novel 2D+t/3D+t registration approach. Model consistency and accuracy is evaluated using manually annotated coronary centerlines at systole and diastole as reference. Improvement of registration robustness by using the 2D+t/3D+t registration is successfully demonstrated by comparison of the actual X-ray cardiac phase with the automatically determined best matching phase in the 4D coronary model.

Virtual reality-enhanced ultrasound guidance for atrial ablation: in vitro epicardial study

In an effort to reduce morbidity of cardiac interventions, minimizing invasiveness inevitably leads to limited visual access to the surgical targets. To address these limitations, we provide the surgeons with a robust visualization environment that integrates interventional ultrasound imaging augmented with pre-operative anatomical models and virtual surgical instruments within a virtual reality environment. Here we present an in vitro study on a cardiac phantom that mimics an ablation therapy procedure, which allows us to assess the feasibility of our surgical system in comparison to traditional intra-operative ultrasound imaging. Following surgical target identification via an electro-anatomical model, the "ablation procedure" is performed blindly. A 2.8 mm RMS targeting error is achieved using our novel surgical system. This level of accuracy is adequate from both a clinical and engineering perspective, under the inherent procedure requirements and limitations of the system.

Inside the beating heart: an in vivo feasibility study on fusing pre- and intra-operative imaging for minimally invasive therapy

OBJECTIVE

An interventional system for minimally invasive cardiac surgery was developed for therapy delivery inside the beating heart, in absence of direct vision.

METHOD

A system was developed to provide a virtual reality (VR) environment that integrates pre-operative imaging, real-time intra-operative guidance using 2D trans-esophageal ultrasound, and models of the surgical tools tracked using a magnetic tracking system. Detailed 3D dynamic cardiac models were synthesized from high-resolution pre-operative MR data and registered within the intra-operative imaging environment. The feature-based registration technique was employed to fuse pre- and intra-operative data during in vivo intracardiac procedures on porcine subjects.

RESULTS

This method was found to be suitable for in vivo applications as it relies on easily identifiable landmarks, and hence, it ensures satisfactory alignment of pre- and intra-operative anatomy in the region of interest (4.8 mm RMS alignment accuracy) within the VR environment. Our initial experience in translating this work to guide intracardiac interventions, such as mitral valve implantation and atrial septal defect repair demonstrated feasibility of the methods.

CONCLUSION

Surgical guidance in the absence of direct vision and with no exposure to ionizing radiation was achieved, so our virtual environment constitutes a feasible candidate for performing various off-pump intracardiac interventions.

Computational modeling for bedside application

Advances in computer power, novel diagnostic and therapeutic medical technologies, and an increasing knowledge of pathophysiology from gene to organ systems make it increasingly feasible to apply multiscale patient-specific modeling based on proven disease mechanisms. Such models may guide and predict the response to therapy in many areas of medicine. This is an exciting and relatively new approach, for which efficient methods and computational tools are of the utmost importance. Investigators have designed patient-specific models in almost all areas of human physiology. Not only will these models be useful in clinical settings to predict and optimize the outcome from surgery and non-interventional therapy, but they will also provide pathophysiologic insights from the cellular level to the organ system level. Models, therefore, will provide insight as to why specific interventions succeed or fail.

Subject-specific models for image-guided cardiac surgery

Three-dimensional visualization for planning and guidance is still not routinely available for minimally invasive cardiac surgery (MICS). This can be addressed by providing the surgeon with subject-specific geometric models derived from 3D preoperative images for planning of port locations or to rehearse the procedure. For guidance purposes, these models can also be registered to the subject using intraoperative images. In this paper, we present a method for extracting subject-specific heart geometry from preoperative MR images. The main obstacle we face is the low quality of clinical data in terms of resolution, signal-to-noise ratio, and presence of artefacts. Instead of using these images directly, we approach the problem in three steps: (1) generate a high quality template model, (2) register the template with the preoperative data, and (3) animate the result over the cardiac cycle. Validation of this approach showed that dynamic subject-specific models can be generated with a mean error of 3.6+/-1.1 mm from low resolution target images (6 mm slices). Thus, the models are sufficiently accurate for MICS training and procedure planning. In terms of guidance, we also demonstrate how the resulting models may be adapted to the operating room using intraoperative ultrasound imaging.

Dynamic 3-D virtual fixtures for minimally invasive beating heart procedures

Two-dimensional or 3-D visual guidance is often used for minimally invasive cardiac surgery and diagnosis. This visual guidance suffers from several drawbacks such as limited field of view, loss of signal from time to time, and in some cases, difficulty of interpretation. These limitations become more evident in beating-heart procedures when the surgeon has to perform a surgical procedure in the presence of heart motion. In this paper, we propose dynamic 3-D virtual fixtures (DVFs) to augment the visual guidance system with haptic feedback, to provide the surgeon with more helpful guidance by constraining the surgeon's hand motions thereby protecting sensitive structures. DVFs can be generated from preoperative dynamic magnetic resonance (MR) or computed tomograph (CT) images and then mapped to the patient during surgery. We have validated the feasibility of the proposed method on several simulated surgical tasks using a volunteer's cardiac image dataset. Validation results show that the integration of visual and haptic guidance can permit a user to perform surgical tasks more easily and with reduced error rate. We believe this is the first work presented in the field of virtual fixtures that explicitly considers heart motion.

Towards a biomechanics-based technique for assessing myocardial contractility: an inverse problem approach

This work presents the initial development and implementation of a novel 3D biomechanics-based approach to measure the mechanical activity of myocardial tissue, as a potential non-invasive tool to assess myocardial function. This technique quantifies the myocardial contraction forces developed within the ventricular myofibers in response to electro-physiological stimuli. We provide a 3D finite element formulation of a contraction force reconstruction algorithm, along with its implementation using magnetic resonance (MR) data. Our algorithm is based on an inverse problem solution governed by the fundamental continuum mechanics principle of conservation of linear momentum, under a first-order approximation of elastic and isotropic material conditions. We implemented our technique using a subject-specific ventricle model obtained by extracting the left ventricular anatomical features from a set of high-resolution cardiac MR images acquired throughout the cardiac cycle using prospective electrocardiographic gating. Cardiac motion information was extracted by non-rigid registration of the mid-diastole reference image to the remaining images of a 4D dataset. Using our technique, we reconstructed dynamic maps that show the contraction force distribution superimposed onto the deformed ventricle model at each acquired frame in the cardiac cycle. Our next objective will consist of validating this technique by showing the correlation between the presence of low contraction force patterns and poor myocardial functionality.

Towards a biomechanical-based method for assessing myocardial tissue viability

This work presents the first steps towards the development and implementation of a novel 3D biomechanical-based method for assessing the viability of myocardial tissue, with particular interest for its application in myocardial infarction (MI) diagnosis. This assessment technique quantifies the myocardial contraction forces developed within the ventricular myofibrils in response to the electrophysiological stimulus. In this manuscript we provide a 3D finite element (FE) formulation of a contraction force reconstruction algorithm based on an inverse problem solution of linear elasticity, along with its implementation using clinical data. This algorithm has been applied to patient-specific models obtained by extracting anatomical features from high-resolution, high-contrast magnetic resonance (MR) cardiac images. The input consists of motion information extracted by nonrigid registration of the mid-diastole reference image to the remaining images of the 4D data set, acquired using ECG-gating throughout the cardiac cycle. The result consists of a display-map of the contraction force distribution superimposed on the anatomical ventricle model, which allows the clinician to identify regions of low contractility in the myocardium.

FEM-based evaluation of deformable image registration for radiation therapy

This paper presents a new concept to automatically detect the neighborhood in an image where deformable registration is mis-performing. Specifically, the displacement vector field (DVF) from a deformable image registration is substituted into a finite-element-based elastic framework to calculate unbalanced energy in each element. The value of the derived energy indicates the quality of the DVF in its neighborhood. The new voxel-based evaluation approach is compared with three other validation criteria: landmark measurement, a finite element approach and visual comparison, for deformable registrations performed with the optical-flow-based 'demons' algorithm as well as thin-plate spline interpolation. This analysis was performed on three pairs of prostate CT images. The results of the analysis show that the four criteria give mutually comparable quantitative assessments on the six registration instances. As an objective concept, the unbalanced energy presents no requirement on boundary constraints in its calculation, different from traditional mechanical modeling. This method is automatic, and at voxel level suitable to evaluate deformable registration in a clinical setting.

Dose reduction for cardiac CT using a registration-based approach

Two reasons for the recent rise in radiation exposure from CT are increases in its clinical applicability and the desire to maintain high SNR while acquiring smaller voxels. To address this emerging dose problem, several strategies for reducing patient exposure have already been proposed. One method employed in cardiac imaging is ECG-driven modulation of the tube current between 100% at one time point in the cardiac cycle and a reduced fraction at the remaining phases. In this paper, we describe how images obtained during such acquisition can be used to reconstruct 4D data of consistent high quality throughout the cardiac cycle. In our approach, we assume that the middiastole (MD) phase is imaged with full dose. The MD image is then independently registered to lower dose images (lower SNR) at other frames, resulting in a set of transformations. Finally, the transformations are used to warp the MD frame through the cardiac cycle to generate the full 4D image. In addition, the transformations may be interpolated to increase the temporal sampling or to generate images at arbitrary time points. Our approach was validated using various data obtained with simulated and scanner-implemented dose modulation. We determined that as little as 10% of the total dose was required to reproduce full quality images with a 1 mm spatial error and an error in intensity values on the order of the image noise. Thus, our technique offers considerable dose reductions compared to standard imaging protocols, with minimal effects on the quality of the final data.

Autoalignment of intervertebral disks

PURPOSE

To develop an automated planning method to obtain double oblique slices for clinically relevant diagnoses of spine-related pathologies.

MATERIALS AND METHODS

Thirty healthy volunteers, 18 patients with cervical spine (c-spine) pathologies, and 15 patients with lumbar spine (l-spine) pathologies were included in this study. The planning method requires no interaction and is calculated online on the MR scanner after two sagittal acquisitions (a MR myelogram and standard T2-weighted (T2W) acquisition). The planning of the subsequent double oblique slice packets is calculated automatically. The results from the volunteers were evaluated visually by an operating technician. The results obtained in a clinical trial from interactive planning by an operating technician and from automated planning were compared.

RESULTS

Visual assessment of the planned slices in the T2W images of all subjects confirmed the accuracy and robustness of the method for both applications. The differences in positions and orientations between interactively and automatically planned transverse series were within the range of interindividual variability.

CONCLUSION

he new approach can be used to automatically plan double oblique MR images for examination of spine-related pathologies with high reliability and robustness. The major advantage is that simultaneous planning in the transversal-coronal and transversal-sagittal orientations can be performed without any additional measurement. Another advantage is that standardized localization of the nerve roots in the center of the image can be obtained.

Towards subject-specific models of the dynamic heart for image-guided mitral valve surgery

Surgeons need a robust interventional system capable of providing reliable, real-time information regarding the position and orientation of the surgical targets and tools to compensate for the lack of direct vision and to enhance manipulation of intracardiac targets during minimally-invasive, off-pump cardiac interventions. In this paper, we describe a novel method for creating dynamic, pre-operative, subject-specific cardiac models containing the surgical targets and surrounding anatomy, and how they are used to augment the intra-operative virtual environment for guidance of valvular interventions. The accuracy of these pre-operative models was established by comparing the target registration error between the mitral valve annulus characterized in the pre-operative images and their equivalent structures manually extracted from 3D US data. On average, the mitral valve annulus was extracted with a 3.1 mm error across all cardiac phases. In addition, we also propose a method for registering the pre-operative models into the intra-operative virtual environment.

Education and research using experimental pigs in a medical school

Medium-sized animals such as miniature pigs are considered to be important for education and training in medical schools to master the skills required in surgical treatment. Much still remains to be done to establish total management for animal experiments using pigs. Improvement of the effective utilization of pigs is also required from the economical and ethical points of view. We have been providing a support system at a facility for experimental animals in a medical school for 3 years, and herein we introduce our personal experiments as an instructional lecture. Before starting surgical training using live pigs, sufficient education concerning animal ethics and dry laboratory training was completed. Four kinds of miniature pigs have been used as experimental animals; porcine rearing pens have been improved and a postoperative care system has been implemented. Moreover, staff at the center offer a preoperative service of anesthesia for surgical education, training, and research. Chronic experiments have increased to represent 35% and 48% of experiments using pigs in 2003 and 2004, respectively. Experimental pigs have undergone secondary use after being killed to reduce the number of animals used in experiments. Sharing and reuse have allowed effective use of miniature pig tissues and cells for research, and have reduced the number of animals used. We recommend that researchers consider use of our total systems because they can improve the quality of medical education and research and facilitate effective use of tissues and cells by sharing and reuse among different departments.

En bloc exclusion of the pulmonary vein region in the pig using off pump, beating, intra-cardiac surgery: a pilot study of minimally invasive surgery for atrial fibrillation

BACKGROUND

Off-pump, closed, beating heart, minimally-invasive surgery in patients with lone atrial fibrillation (AF) must be effective to become the preferred alternative to catheter ablation. Because of the inherent anatomical limitations of the epicardial access, we explored the feasibility of an intracardiac approach.

METHODS

We report an acute study of en bloc, cryo-exclusion of the pulmonary vein region in 7 pigs. The left atrial appendage (LAA) was approached via a left thoracotomy. Electrodes were attached to the posterior wall of the left atrium (LPA) and right atrial appendage (RAA) for pacing and electrophysiological monitoring. A modified Surgifrost probe was introduced via the LAA and positioned using transesophageal (TEE) and intracardiac (ICE) echocardiographic guidance to generate encircling cryolesions (3 minutes, -105 degrees C) of the pulmonary vein region.

RESULTS

A complete two-way block was achieved in 6 pigs and an incomplete block in 1. The excluded segment had very slow idiosyncratic rhythm or was electrically silent. In all pigs before isolation, sustained AF was inducible with the most rapid rhythms and fractionated electrograms recorded from the LA. While sustained AF was induced before exclusion, it was not after isolation in either the exclude or non-excluded segments. All tissue samples taken along the encircling cryolesions had transmural cryolesions on pathological examination.

CONCLUSIONS

We conclude that off-pump, closed heart, beating, intracardiac AF surgery is feasible, reliable and can duplicate the accuracy and precision of the open-heart approach. However, further developments are needed to make this novel approach an alternative to current approaches for catheter ablation.

High-Performance Medical Image Registration Using New Optimization Techniques

Optimization of a similarity metric is an essential component in intensity-based medical image registration. The increasing availability of parallel computers makes parallelizing some registration tasks an attractive option to increase speed. In this paper, two new deterministic, derivative-free, and intrinsically parallel optimization methods are adapted for image registration. DIviding RECTangles (DIRECT) is a global technique for linearly bounded problems, and multidirectional search (MDS) is a recent local method. The performance of DIRECT, MDS, and hybrid methods using a parallel implementation of Powell's method for local refinement, are compared. Experimental results demonstrate that DIRECT and MDS are robust, accurate, and substantially reduce computation time in parallel implementations.