Venkat Ramanan

Intrinsic Contrast for Characterization of Acute Radiofrequency Ablation Lesions

Background—Both intrinsic contrast (T1 and T2 relaxation and the equilibrium magnetization) and contrast agent (gadolinium)–enhanced MRI are used to visualize and evaluate acute radiofrequency ablation lesions. However, current methods are imprecise in delineating lesion extent shortly after the ablation. Methods and Results—Fifteen lesions were created in the endocardium of 13 pigs. A multicontrast inversion recovery steady state free precession imaging method was used to delineate the acute ablation lesions, exploiting T1-weighted contrast. T2 and Mo* maps were also created from fast spin echo data in a subset of pigs (n=5) to help characterize the change in intrinsic contrast in the lesions. Gross pathology was used as reference for the lesion size comparison, and the lesion structures were confirmed with histological data. In addition, a colorimetric iron assay was used to measure ferric and ferrous iron content in the lesions and the healthy myocardium in a subset of pigs (n=2). The lesion sizes measured in inversion recovery steady state free precession images were highly correlated with the extent of lesion core identified in gross pathology. Magnetic resonance relaxometry showed that the radiofrequency ablation procedure changes the intrinsic T1 value in the lesion core and the intrinsic T2 in the edematous region. Furthermore, the T1 shortening appeared to be correlated with the presence of ferric iron, which may have been associated with metmyoglobin and methemoglobin in the lesions. Conclusions—The study suggests that T1 contrast may be able to separate necrotic cores from the surrounding edematous rims in acute radiofrequency ablation lesions.

High-Resolution 3-D T

The substrate of potentially lethal cardiac arrhythmias often resides in the gray zone (GZ), a mixture of viable myocytes and collagen strands found between healthy myocardium and infarct core (IC). The specific aims of this paper are to demonstrate correspondence between regions delineated in T1* (apparent T1) maps and tissue characteristics seen in histopathology and to determine the MR imaging resolution needed to adequately identify GZ-associated substrate in chronic infarct. For this, a novel 3-D multicontrast late enhancement (MCLE) MR method was used to image ex vivo swine hearts with chronic infarction, at high resolution (0.6 × 0.6 × 1.25 mm). Pixel-wise classified tissue maps were calculated using steady-state and T1 * images as input to a fuzzy-clustering algorithm. Quantitative histology based on collagen stains was performed in n = 10 selected slabs and showed very good correlations between histologically-determined areas of heterogeneous and dense fibrosis, and the corresponding GZ (R2 = 0.96) and IC (R2 = 0.97) in tissue classified maps. Furthermore, in n = 24 slabs, we performed volumetric measurements of GZ and IC, at the original and decreased image resolutions. Our results demonstrated that the IC volume remained relatively unchanged across all resolutions, whereas the GZ volume progressively increased with diminished image resolution, with changes reaching significance at 1 × 1 × 5 mm resolution (p <; 0.05) but not at 1 × 1 × 2.5 mm, suggesting that this resolution may be sufficient to adequately identify the GZ from MCLE images, enabling an effective MR probing of remodeled myocardium in late infarct. Future work will focus on translating these findings to optimizing the current in vivo MCLE imaging of the GZ.

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BackgroundIdentification of viable slow conduction zones manifested by abnormal local potentials is integral to catheter ablation of ventricular tachycardia (VT) sites. The relationship between contrast patterns in cardiovascular magnetic resonance (CMR) and local electrical mapping is not well characterized. The purpose of this study was to identify regions of isolated, late and fractionated diastolic potentials in sinus rhythm and controlled-paced rhythm in post-infarct animals relative to regions detected by late gadolinium enhancement CMR (LGE-CMR).MethodsUsing a real-time MR-guided electrophysiology system, electrogram (EGM) recordings were used to generate endocardial electroanatomical maps in 6 animals. LGE-CMR was also performed and tissue classification (dense infarct, gray zone and healthy myocardium) was then correlated to locations of abnormal potentials.ResultsFor abnormal potentials in sinus rhythm, relative occurrence was equivalent 24%, 27% and 22% in dense scar, gray zone and healthy tissue respectively (p = NS); in paced rhythm, the relative occurrence of abnormal potentials was found to be different with 30%, 42% and 21% in dense scar, gray zone and healthy myocardium respectively (p = 0.001). For location of potentials, in the paced case, the relative frequency of abnormal EGMs was 19.9%, 65.4% and 14.7% in the entry, central pathway and exit respectively (p = 0.05), putative regions being defined by activation times.ConclusionsOur data suggests that gray zone quantified by LGE-CMR exhibits abnormal potentials more frequently than in healthy tissue or dense infarct when right ventricular apex pacing is used.

-Mapping and Quantitative Image Analysis of GRAY ZONE in Chronic Fibrosis

The introduction of electroanatomic mapping (EAM) has improved the understanding of the substrate of ventricular tachycardia. EAM systems are used to delineate scar regions responsible for the arrhythmia by creating voltage or activation time maps. Previous studies have identified the benefits of creating MR-guided voltage maps; however, in some cases voltage maps may not identify regions of slow propagation that can cause the reentrant tachycardia. In this study, we obtained local activation time maps and analyzed propagation properties by performing MR-guided mapping of the porcine left ventricle while pacing from the right ventricle. Anatomical and myocardial late gadolinium enhancement images were used for catheter navigation and identification of scar regions. Our MR-guided mapping procedure showed qualitative correspondence to conventional clinical EAM systems in healthy pigs and demonstrated altered propagation in endocardial infarct models.

Distribution of abnormal potentials in chronic myocardial infarction using a real time magnetic resonance guided electrophysiology system

Catheter ablation of ventricular tachycardia (VT) is preceded by characterization of the myocardial substrate via electroanatomical voltage mapping (EAVM). The purpose of this study was to characterize the relationship between chronic myocardial fibrotic scar detected by multicontrast late enhancement (MCLE) MRI and by EAVM obtained using an MR-guided electrophysiology system, with a final aim to better understand how these measures may improve identification of potentially arrhythmogenic substrates. Real-time MR-guided EAVM was performed in six chronically infarcted animals in a 1.5T MR system. The MCLE images were analyzed to identify the location and extent of the fibrotic infarct. Voltage maps of the left ventricle (LV) were created with an average of 231 ± 35 points per LV. Correlation analysis was conducted between bipolar voltage and three MR parameters (infarct transmurality, tissue categorization into healthy and scar classes, and normalized relaxation rate R1*). In general, tissue regions classified as scar by normalized R1* values were well correlated with locations with low bipolar voltage values. Moreover, our results demonstrate that MRI information (transmurality, tissue classification, and relaxation rate) can accurately predict areas of myocardial fibrosis identified with bipolar voltage mapping, as demonstrated by ROC analysis. MCLE can help overcome limitations of bipolar voltage mapping including long durations and lower spatial discrimination and may help identify the sites within scars, which are commonly believed to trigger arrhythmic events in postinfarction patients.

The Feasibility of Endocardial Propagation Mapping Using Magnetic Resonance Guidance in a Swine Model, and Comparison With Standard Electroanatomic Mapping

Background MR visualization of RF lesions is an application of growing interest with the potential for translation to clinical ablation procedures. In particular, intrinsic-contrast MRI avoids the dynamic contrast produced in typical Gd-based MRI, and may differentiate the reversible and irreversible thermal injury thought to be caused by RF ablation. This distinction is important for assessing the permanence of ablation to eradicate the substrate of ventricular tachycardia in structural heart disease. In this study we investigate the potential of intrinsic-contrast MRI to visualize the features of thermal injury and evolution of RF lesions that may occur immediately after ablation.

Postinfarction Ventricular Tachycardia Substrate Characterization: A Comparison Between Late Enhancement Magnetic Resonance Imaging and Voltage Mapping Using an MR-Guided Electrophysiology System

Background Following acute myocardial infarction (AMI), microvascular integrity and function may be compromised as a result of microvascular obstruction (MVO) and vasodilator dysfunction [1,2]. It has been observed that both infarct and remote myocardial territories may exhibit impaired myocardial blood flow (MBF) patterns associated with abnormal vasodilator response [3]. Arterial spin labeled (ASL) CMR is a novel non-contrast technique that can quantitatively measure MBF [4-6]. The aim of this study was to investigate the feasibility of ASL-CMR in assessing MBF in a porcine model of AMI.

Intrinsic MRI visualizes RF lesions within minutes after MR-guided ablation

Background Recently, there is an increased interest in using MRI to guide electrophysiology (EP) procedures as an alternative to X-ray fluoroscopy guidance, due to its excellent soft tissue contrast and lack of radiation. However, there exist tradeoffs between different MRI guidance schemes. Realtime 2D MR sequences are able to capture heart motion during an interventional setting, while sacrificing imaging quality, whereas high-resolution prior 3D roadmaps are static and do not reflect the respiratory motion of the heart. In this work, we explore the feasibility of deriving a motion model from these two complementary datasets, and evaluate its potential for improving the targeting accuracy of MRI-guided EP procedures.

Non-contrast myocardial perfusion assessment in porcine acute myocardial infarction using arterial spin labeled CMR

Goal: The purpose of this study is to improve the accuracy of interventional catheter guidance during intracardiac procedures. Specifically, the use of preprocedural magnetic resonance roadmap images for interventional guidance has limited anatomical accuracy due to intraprocedural respiratory motion of the heart. Therefore, we propose to build a novel respiratory motion model to compensate for this motion-induced error during magnetic resonance imaging (MRI)-guided procedures. Methods: We acquire 2-D real-time free-breathing images to characterize the respiratory motion, and build a smooth motion model via registration of 3-D prior roadmap images to the real-time images within a novel principal axes frame of reference. The model is subsequently used to correct the interventional catheter positions with respect to the anatomy of the heart. Results: We demonstrate that the proposed modeling framework can lead to smoother motion models, and potentially lead to more accurate motion estimates. Specifically, MRI-guided intracardiac ablations were performed in six preclinical animal experiments. Then, from retrospective analysis, the proposed motion modeling technique showed the potential to achieve a 27% improvement in ablation targeting accuracy. Conclusion: The feasibility of a respiratory motion model-based correction framework has been successfully demonstrated. Significance: The improvement in ablation accuracy may lead to significant improvements in success rate and patient outcomes for MRI-guided intracardiac procedures.

Respiratory motion model based correction for improving the targeting accuracy of MRI-guided intracardiac electrophysiology procedures

Methods 15 lesions were created in the endocardium of 13 pigs using approved animal protocols. NGE IR-SSFP and T2-w black-blood (double IR-FSE) images were acquired in <60min after ablation. Then, Gd-DTPA (Magnevist, 0.2 mmol/kg) was injected and LGE images were acquired repeatedly over one hour. Gross pathology was used as the reference for lesion size measurements. Two regions were measured in this reference: the pale “inner” lesion core and the “outer” lesion border including the dark rim on pathology (see Results).