High-resolution CINE MRI allows estimation of 3D regional atrial strains
Authors: Varela M , Anjari M , Correia T , Zakeri R , Alskaf E , Chiribiri A , Lee J 
Published: November 2020

Abstract

Introduction
It is increasingly evident that atrial function is an important marker of cardiovascular health. Impaired global left atrial strain has been associated with risk of thromboembolic events, atrial fibrillation and heart failure. When performed at high spatial resolution, CINE MRI allows the estimation of regional atrial strains, which may facilitate earlier identification of atrial disease and improved (non-contrast) characterisation of atrial fibrosis. Nevertheless, to date, high resolution regional atrial strains has not been assessed using CINE MRI.

Purpose
We introduce a novel rapid 2.2-mm isotropic atrial CINE MRI protocol used to image healthy subjects and patients with cardiovascular disease (CVD). We additionally present a dedicated image analysis pipeline to estimate regional 3D atrial strains from these images.

Methods
We imaged 10 healthy subjects (5 female, 24–36 years old) and 6 patients referred for cardiac MRI due to known or suspected CVD (2 female, 25–80 years old). All subjects were scanned in a 1.5T Philips Ingenia MRI scanner in a single breath-hold (&lt;25 s), using a short-axis 3D bSSFP protocol (flip angle: 60°, TE/TR: 1.6/3.3 ms) with retrospective cardiac gating, SENSE = 2.3 (along both phase encode directions), typical FOV: 400 x 270 x 70 mm3, isotropic acquisition resolution of 2.2 mm3. Images were reconstructed to 20 cardiac phases with 55% view sharing.
The left atrium (LA) was manually segmented in atrial diastole. We tracked the position of evenly spaced points along the LA contour across all phases of the cardiac cycle using the Medical Image Tracking Toolbox. This was used to create a series of deforming smooth triangular meshes, from which Lagrange strain tensors were estimated.

Results
Figs a-c show 3 orthogonal views of the proposed high-resolution atrial CINE MRI scans for a representative CVD patient, with the LA segmentation overlaid in red. Representative LA principal strain directions (as arrows) with the colour indicating the amount of strain observed along this direction are shown in Fig d for active atrial contraction (posterior view). The calculated strain directions varied smoothly in space and time, as expected, and were largest in amplitude in the regions closest to the mitral valve.
Overall, principal strains were larger in healthy subjects (AC strains: 0.12±0.06) than in the CVD cohort (AC strains: 0.04±0.01). This difference was statistically significant during AC (p-value: 0.02), but not during atrial diastole (p-value: 0.06).

Conclusions
We present a novel high-resolution CINE-MRI protocol for estimating regional atrial strains in 3D, with pilot data from 10 healthy subjects and 6 cardiovascular patients. Future studies will compare regions of abnormal atrial strain with fibrosis identified in late gadolinium enhanced MRI to assess whether regional strains can provide a better characterisation of atrial tissue and improved stratification of patients at risk.
Figure 1

Funding Acknowledgement
Type of funding source: Foundation. Main funding source(s): British Heart Foundation, EPSRC/Wellcome Trust Centre for Medical Engineering