Imaging left-ventricular mechanical activation in heart failure patients using cine DENSE MRI: Validation and implications for cardiac resynchronization therapy

Abstract

Purpose: To image late mechanical activation and identify effective left-ventricular (LV) pacing sites for cardiac resynchronization therapy (CRT). There is variability in defining mechanical activation time, with some studies using the time to peak strain (TPS) and some using the time to the onset of circumferential shortening (TOS). We developed improved methods for imaging mechanical activation and evaluated them in heart failure (HF) patients undergoing CRT.

Materials and methods: We applied active contours to cine displacement encoding with stimulated echoes (DENSE) strain images to detect TOS. Six healthy volunteers underwent magnetic resonance imaging (MRI) at 1.5T, and 50 patients underwent pre-CRT MRI (strain, scar, volumes) and echocardiography, assessment of the electrical activation time (Q-LV) at the LV pacing site, and echocardiography assessment of LV reverse remodeling 6 months after CRT. TPS at the LV pacing site was also measured by DENSE.

Results: The latest TOS was greater in HF patients vs. healthy subjects (112 ± 28 msec vs. 61 ± 7 msec, P < 0.01). The correlation between TOS and Q-LV was strong (r > 0.75; P < 0.001) and better than between TPS and Q-LV (r < 0.62; P ≥ 0.006). Twenty-three of 50 patients had the latest activating segment in a region other than the mid-ventricular lateral wall, the most common site for the CRT LV lead. Using a multivariable model, TOS/QRS was significantly associated with LV reverse remodeling even after adjustment for overall dyssynchrony and scar (P < 0.05), whereas TPS was not (P = 0.49).

Conclusion: Late activation by cine DENSE TOS analysis is associated with improved LV reverse remodeling with CRT and deserves further study as a tool to achieve optimal LV lead placement in CRT.

Level of evidence: 1 Technical Efficacy: Stage 1 J. MAGN. RESON. IMAGING 2017;46:887-896.

Keywords: DENSE; cardiac resynchronization therapy; mechanical activation; strain imaging.

© 2017 International Society for Magnetic Resonance in Medicine.

Figures

Figure 1. Spatiotemporal patterns of mechanical activation

Five time points distributed across the cardiac cycle show typical spatiotemporal patterns of circumferential shortening in a healthy volunteer (A–E) and a patient with HF-LBBB (F – J). (A –E) illustrate a synchronous contraction for all LV segments. In contrast, (F – J) illustrate dyssynchrony, as contraction and stretch occur simultaneously. The white arrows in (F, G) show a region of late mechanical activation in the lateral wall during early systole.

Figure 2. Analysis of late mechanical activation

SVD-denoised strain matrices, Ecc-time curves and DENSE mechanical activation time maps are shown for a healthy volunteer (A – C) and a HF/CRT patient (D – E). Panels (A–C) correspond to the healthy volunteer and demonstrate synchrony of contraction, detection of the activation times using the active contour, and early mechanical activation throughout the entire slice. In contrast, panels (D–E) show a region of pre-stretch and late activation, detection of the late-activating region by the active contour, and an activation time map depicting late activation of the lateral wall.

Figure 3. Electromechanical correlations

Correlation plots are shown for: (A) TOS vs. Q-LV for patients with the LV lead location in a region without scar, (B) TOS vs. Q-LV with the LV lead placed in scar, (C) TPS vs. Q-LV for LV lead implant sites without scar, and (D) TPS vs. Q-LV LV lead implant sites with scar.

Figure 4. Electromechanical Bland-Altman analysis

Bland-Altman plots are shown to assess the agreement between TOS/QRS and QLV/QRS for the cases of (A) patients with the LV lead location in a region without scar, and (B) cases with the LV lead placed in scar. In both cases, fairly small biases are observed, with a greater delay in mechanical activation relative the electrical activation when the LV lead was placed in a region with scar.

Figure 5. Heterogeneity of the location of the latest mechanically activated segment

(A) An example patient is shown with the latest mechanical activation in the basal anterior segment of the LV. (B) An example patient is shown with latest mechanical activation occurred in the apical inferolateral segment. (C) The distribution of the location of the latest activated segments is shown for all patients.

Figure 6. Frequency of LV lead implantation in late-activation sites with the standard CRT procedure

The time to the onset of shortening (TOS) has been normalized to the QRS duration to specify activation time as a percentage of the QRS width. In more than half the patients undergoing CRT, the LV lead was placed in a region with TOS less than 60% of the QRS duration.