Left atrial imaging and registration of fibrosis with conduction voltages using LGE-MRI and electroanatomical mapping

Highlights

• Developed a pipeline to register EAMs and LGE-MRI in patients with atrial fibrillation.

• Addition of a non-rigid ICP (NICP) algorithm resulted in reduced registration error.

• Hyperenhancement on LGE-MRI shown to overlap with low bipolar voltage on EAMs.

• Successful registration may assist in selection of candidate targets for ablation.

Abstract

Background and purpose

Abnormal electrical conduction and excitability associated with fibrosis in the left atrium (LA) may serve as a substrate for atrial fibrillation (AF). Electroanatomical voltage mapping systems (EAMs) have become a dominant facilitator to treat AF with catheter ablation assisted by additional diagnostic imaging modalities. Importantly, AF has been associated with structural changes to the extracellular matrix of the myocardium, including increased collagen deposition—a process known as fibrosis. Late gadolinium enhancement-magnetic resonance imaging (LGE-MRI) may aid in guiding AF cardiac ablation therapy by determination of location of fibrosis in the LA. To locate fibrosis for cardiac ablation, however, accurate registration between EAMs and LGE-MRI data is crucial. The purpose of this work was to develop a method for registering EAMs with late gadolinium enhancement-magnetic resonance (LGE-MR) images of fibrosis.

Methods

Twenty patients with persistent AF, who underwent magnetic resonance imaging scanning and EAMs prior to first-time catheter ablation, participated in the study. In our registration pipeline, LGE-MR images were registered to the left atrial surface on EAMs using manual alignment followed by iterative closest point (ICP), and non-rigid ICP (NICP) algorithm.

Results and conclusions

The results demonstrate that NICP provided a substantial reduction in registration error when compared to the use of affine ICP alone. Regions of fibrosis on LGE-MR images identified using the signal threshold to reference mean threshold demonstrated the most regional overlap with low bipolar voltage points on EAMs. Successful co-registration of LGE-MR images to EAMs may assist electro-physiologists in selecting candidate targets for ablation and ultimately, reduce the rate of AF recurrence for patients.

Introduction

Atrial Fibrillation (AF) is a heart rhythm disorder of the left atrium (LA) that is associated with an increased risk of stroke and death. A 2010 study estimated that 2.7 to 6.1 million people in the United States suffer from AF, and this number is expected to increase to 12.1 million in 2030 [1]. AF can be classified as paroxysmal, persistent, and longstanding persistent (also known as ‘permanent’ AF) [2]. While catheter ablation is highly successful for most patients with paroxysmal AF, it is much less effective for patients with persistent and longstanding persistent AF [3]. Despite advancements in catheter ablation technology, its success rate remains as low as 45–50% after treatment [4]. These considerable treatment challenges have motivated a number of experimental studies, including some that explored the correlation between the extent of fibrillation burden and the degree of fibrosis in the LA [5,6]. Importantly, AF has been associated with structural changes to the extracellular matrix of the myocardium, including collagen deposition—a process known as fibrosis. For example, Verma [7] showed that patients with evidence of fibrosis in the LA at the time of ablation resulted in nearly half the success rate of those without (43 vs. 81%).

Electroanatomical voltage mapping systems (EAMs) can identify the presence, location, and extent of fibrosis in the LA by detection of abnormally conductive or ‘low voltage’ tissue regions [8]. These systems enable operators to position the catheters without fluoroscopic guidance [9]. Areas of abnormal myocardium (‘low bipolar voltage regions’) from fibrotic tissue in the LA are considered as potential targets for ablation [10]. Catheter ablation is an invasive procedure in which radio frequency (RF) energy is delivered through a catheter to deliberately destroy the small regions of heart tissue that are responsible for creating abnormal electrical signals. For example, pulmonary vein (PV) isolation is a common procedure, where the tissue bordering the pulmonary veins (PVs) is targeted for ablation in order to prevent abnormal wave propagation to the rest of myocardium. When electroanatomical voltage mapping (EAM) data is fused with other imaging sources, such as magnetic resonance (MR) Imaging or Computed Tomography (CT), it may provide crucial information for guiding catheter ablation by locating fibrosis.

Together with EAMs, pre-procedural MR can provide a detailed depiction of the LA and the PVs [11]. Late gadolinium enhancement (LGE)-MR is also playing an increasingly important role in the depiction of myocardial fibrosis [12] and can provide potentially relevant information prior to ablation. In late gadolinium enhancement-magnetic resonance imaging (LGE-MRI), images are acquired 10–20 min following a bolus injection of a gadolinium (Gd) based contrast agent, which allows the contrast to distribute to the extravascular-extracellular spaces, and accumulate, in regions of fibrosis. As the use of LGE-MRI increases in pre-ablation planning, there is an urgent need for registration techniques that will enable us to compare the spatial extent of low bipolar voltage areas on EAMs with hyper-enhanced regions on late gadolinium enhancement-magnetic resonance (LGE-MR) images. Several studies have demonstrated that fibrosis regions of the LA detected using LGE-MRI prior to ablation can assist in predicting ablation outcome [[13], [14], [15]]. Leonardi [14] reported that an accurate morphological three-dimensional (3D) left atrial model produced by magnetic resonance imaging (MRI) can improve the cardiac ablation strategy. Romero [15] described the clinical impact of incorporating LGE-MRI data to EAMs, specifically that regions of LGE-MR enhancement may assist in visualizing the fibrosis border zone in the LA.

Although others have reported significant correlations between EAMs and LGE-MRI with respect to the total burden of fibrosis in the LA [16,17], the spatial correlation between regions of low bipolar voltage on EAMs and regions of hyperenhancement on LGE-MRI depends on the accurate alignment of data acquired by these two disparate image modalities (i.e., registration). To our knowledge, registration methods for integration of LGE-MRI and EAMs of the LA have not been adequately described in the literature [18,19]. Therefore, we propose a computational registration and evaluation strategy, which was validated using clinical data acquired from a prospective study of patients with persistent AF.

The primary objective of this work is to develop and validate a registration method for aligning the left atrial surface determined from LGE-MRI to the left atrial surface depicted by EAMs. After implementation of this registration strategy, the secondary objective is to evaluate the spatial correlation between fibrosis depicted in LGE-MRI determined by several segmentation algorithms and regions of low bipolar voltage on EAMs.