Imaging three-dimensional myocardial mechanics using navigator-gated volumetric spiral cine DENSE MRI

Abstract

A navigator-gated 3D spiral cine displacement encoding with stimulated echoes (DENSE) pulse sequence for imaging 3D myocardial mechanics was developed. In addition, previously described 2D postprocessing algorithms including phase unwrapping, tissue tracking, and strain tensor calculation for the left ventricle (LV) were extended to 3D. These 3D methods were evaluated in five healthy volunteers, using 2D cine DENSE and historical 3D myocardial tagging as reference standards. With an average scan time of 20.5 ± 5.7 min, 3D data sets with a matrix size of 128 × 128 × 22, voxel size of 2.8 × 2.8 × 5.0 mm(3), and temporal resolution of 32 msec were obtained with displacement encoding in three orthogonal directions. Mean values for end-systolic mid-ventricular mid-wall radial, circumferential, and longitudinal strain were 0.33 ± 0.10, -0.17 ± 0.02, and -0.16 ± 0.02, respectively. Transmural strain gradients were detected in the radial and circumferential directions, reflecting high spatial resolution. Good agreement by linear correlation and Bland-Altman analysis was achieved when comparing normal strains measured by 2D and 3D cine DENSE. Also, the 3D strains, twist, and torsion results obtained by 3D cine DENSE were in good agreement with historical values measured by 3D myocardial tagging.

Fig. 1

Timing diagram for the 3D cine DENSE pulse sequence. A navigator echo is played out before the ECG trigger so as not to interfere with displacement encoding prior to or imaging during the onset of myocardial contraction (marked as the NAV block in the diagram). Immediately after detection of the R wave, a spectrally-selective fat suppression pulse is applied (marked as the FS block in the diagram), which is followed by the displacement-encoding module. A segmented data acquisition, which includes displacement-unencoding gradients and a 3D stack-of-spirals k-space trajectory, is used to sample the displacement-encoded longitudinal magnetization at multiple cardiac phases. In this diagram, the displacement-encoding gradients are applied in the frequency-encoding direction (the gradient waveform shown in the solid line on the G_FE axis). However, more generally, displacement encoding can be applied in any direction by also applying the gradients shown in dashed lines on the G_PE and/or G_SS axes, respectively. Applying gradients in multiple directions is employed for balanced multi-point displacement encoding. In practice, to minimize the echo time, the displacement-unencoding gradients are combined with spatial encoding gradients, but are shown separately in this diagram for clarity.

Fig. 2

Example magnitude- (a,e) and phase-reconstructed 3D spiral cine DENSE images (b–d, f–h) of the heart at end-systole. The 3D volume was oriented along the principal axes of the LV. Online image reconstruction depicted short-axis planes of the LV, as shown in the magnitude- (a) and phase-reconstructed (b–d) images in the upper row. The lower row contains corresponding data reformatted offline in a long-axis four-chamber view. The images in (b) and (f) were encoded for displacement in the horizontal direction of the short-axis plane, in (c) and (g) were encoded for displacement in the vertical direction of the short-axis plane, and in (d) and (h) were encoded for displacement in the longitudinal direction.

Fig. 3

Example 3D voxel-wise displacement and strain displays of the LV at end-diastole (a,d,g), mid-systole (b,e,h) and end-systole (c,f,i) are shown. The location of each dot represents the 3D position of an element of myocardium, and the color of each dot represents the strain value. Radial strain (E_rr) is shown in the top row (a–c), circumferential strain (E_cc) is shown in the middle row (d–f), and longitudinal strain (E_ll) is shown in the bottom row (g–i). The E_rr values are mostly positive, which represents local tissue stretching in the radial direction. The E_cc and E_ll values are mostly negative, representing local tissue shortening in the circumferential and longitudinal directions, respectively. Multiphase data are displayed as corresponding 3D displacement rendering movies in online supplemental data.

Fig. 4

Mean strain-time curves for the normal strains E_rr, E_cc, and E_ll, and for the shear strains E_rc, E_rl and E_cl for the mid-ventricular slice for all 5 normal volunteers. Data are shown separately for subendocardial, mid-wall and subepicardial layers, demonstrating the measurement of transmural strain gradients for some strains. Data are shown as mean ± standard deviation.

Fig. 5

Ellipsoid visualization of 3D end-systolic LV function for one volunteer. In short-axis (a) and long-axis (b) views reconstructed from full volumetric 3D data sets, displaced (relative to their end-diastolic positions) ellipsoids represent both motion and 3D strain. For 3D strain, the lengths and orientations of the principal axes of the ellipsoids are determined by the lengths and orientations of the principal strains. Also, the ellipsoids are color coded according to E_cc. The direction of the first principal strain generally points toward the center of the LV. Transmural gradients of strain are evident, with greater radial lengthening and circumferential shortening occurring in the subendocardium vs the subepicardium. Multiphase data are displayed as corresponding ellipsoid movies in online supplemental data.

Fig. 6

(a) Twist angle as a function of time at the apex, mid-level and base of the LV. Systolic rotation of the apex is clockwise when viewed from the base, whereas the base rotates first clockwise and then counterclockwise. LV torsion as a function of time is shown in (b). Data are from 5 normal volunteers. Data are shown as mean ± standard deviation.

Fig. 7

Bull’s eye plots at end systole for (a) E_rc, (b) E_rl, and (c) twist angle (in degrees). Data are from 5 normal volunteers. Sectors are gray-scale coded according to the mean values of strains or twist angle, and are marked with mean ± standard deviation. Regional variation was observed for these parameters. Specifically, E_rc increased from base to apex, E_rl was greater in the postero-lateral wall compared to the antero-septum, and twist angles were higher at the apex compared to the base.

Fig. 8

Scatter plots (a–c) and Bland-Altman plots (d–f) show correlations and agreement for normal strains measured with both 3D and 2D cine DENSE at multiple cardiac phases and in different LV segments (16 segments for E_rr and E_cc, and 12 segments for E_ll). (a–c): Scatter plots and linear regressions for E_rr, E_cc and E_ll, respectively. (d–f): Bland-Altman plots for E_rr, E_cc and E_ll, respectively. Data are from 5 normal volunteers.