Cardiac diffusion-weighted and tensor imaging: A consensus statement from the special interest group of the Society for Cardiovascular Magnetic Resonance

Abstract

Thanks to recent developments in cardiovascular magnetic resonance (CMR), cardiac diffusion-weighted magnetic resonance is fast emerging in a range of clinical applications. Cardiac diffusion-weighted imaging (cDWI) and diffusion tensor imaging (cDTI) now enable investigators and clinicians to assess and quantify the tridimensional microstructure of the heart. Free-contrast DWI is uniquely sensitized to the presence and displacement of water molecules within the myocardial tissue, including the intracellular, extracellular, and intravascular spaces. CMR can determine changes in microstructure by quantifying: a) mean diffusivity (MD)-measuring the magnitude of diffusion; b) fractional anisotropy (FA)-specifying the directionality of diffusion; c) helix angle (HA) and transverse angle (TA)-indicating the orientation of the cardiomyocytes; d) absolute sheetlet angle (E2A) and E2A mobility-measuring the alignment and systolic-diastolic mobility of the sheetlets, respectively. This document provides recommendations for both clinical and research cDWI and cDTI, based on published evidence when available and expert consensus when not. It introduces the cardiac microstructure focusing on the cardiomyocytes and their role in cardiac physiology and pathophysiology. It highlights methods, observations, and recommendations in terminology, acquisition schemes, postprocessing pipelines, data analysis, and interpretation of the different biomarkers. Despite the ongoing challenges discussed in the document and the need for ongoing technical improvements, it is clear that cDTI is indeed feasible, can be accurately and reproducibly performed and, most importantly, can provide unique insights into myocardial pathophysiology.

Keywords: Cardiac diffusion imaging; Cardiac diffusion tensor imaging; Cardiovascular magnetic resonance; Consensus; Myocardial structure; Recommendations.

Fig. 1

Diffusion (cDWI) CMR terminology. Low and high diffusion-weighted images are acquired, then used to reconstruct ADC or cDTI maps. Primary, secondary, and tertiary eigenvalues and eigenvectors are calculated from the diffusion tensor and used to derive commonly used metrics of diffusivity and microstructure in the heart. CMR cardiovascular magnetic resonance, cDTI cardiac diffusion tensor imaging, cDWI cardiac diffusion-weighted imaging

Fig. 2

Cardiac diffusion-weighted sequences. The two most common cardiac diffusion-weighted imaging (cDWI) pulse sequences both use ECG triggering, diffusion-weighting gradients (dark blue), and single-shot echoplanar imaging (EPI) readouts. (A) Stimulated echo acquisition mode (STEAM) cDWI uses smaller diffusion-encoding gradients and longer diffusion mixing times (TM), which requires two heartbeats per image. (B) Motion-compensated (MC) spin echo (SE) uses larger diffusion-encoding gradients applied within a single heartbeat. ECG electrocardiogram, TR repetition time, TE echo time, TD trigger delay

Fig. 3

Suggested cDTI postprocessing workflow. Summary of the steps needed for the accurate postprocessing of cDTI images. cDTI cardiac diffusion tensor imaging, MD mean diffusivity, FA fractional anisotropy

Fig. 4

Coordinate system used for derivation of helix, transverse and sheetlet angles in the heart. (A) Segmentation of the myocardium. (B) Direction of primary eigenvector in each voxel, depicted using a conventional Cartesian coordinate system. (C-E) The angles of the primary and secondary eigenvectors with the local cardiac coordinate system can be used to derive the helix, transverse, and sheetlet angles. (F) Schematic of myocardial microstructure. (G-I) Coordinate system based on the radial (R), circumferential (C), and longitudinal (L) vectors in the heart. (G) The helix angle is calculated by projecting the primary eigenvector (brown) onto the epicardial tangent (L-C) plane of the LV. The helix angle is defined as the angle that this projection makes with the plane bounded by the local radial and circumferential vectors (short-axis plane). (H) Likewise, the transverse angle is defined by projecting the primary eigenvector onto the R-C (short-axis) plane and measuring the angle that this projection makes with the plane defined by the local circumferential and longitudinal vectors (epicardial tangent plane). (I) The absolute sheetlet angle (E2A) is defined as the angle between the projection of e2 into the radial-cross-myocyte plane and the cross-myocyte direction, where the cross-myocyte direction is perpendicular to e1proj within the local L-C (epicardial tangent) plane, and e1proj is e1 projected on the local L-C (epicardial tangent) plane. cDWI cardiac diffusion-weighted imaging

Fig. 5

Impact of image curation on the quality of cDTI parameter maps. Top: cDTI data (cropped to the heart only): left: entire dataset used for tensor fitting; right: images corrupted with signal loss removed manually (red transparent mask) before tensor fitting. Bottom: respective tensor parameter map estimations showing: fractional anisotropy (FA) (unitless), mean diffusivity (MD) (×10⁻³ mm²/s), helix angle (HA) (˚), and absolute sheetlet angle (E2A) (˚). Data with outlier rejection has a lower MD, a higher FA, more transmurally-structured HA, and more uniform E2A, compared to data without outlier rejection. The images shown are STEAM breath-hold acquisitions in a healthy subject in a 3T Siemens (Skyra) scanner. cDTI cardiac diffusion tensor imaging, STEAM stimulated echo acquisition mode