In vivo, high-frequency three-dimensional cardiac MR elastography: Feasibility in normal volunteers

Abstract

Purpose: Noninvasive stiffness imaging techniques (elastography) can image myocardial tissue biomechanics in vivo. For cardiac MR elastography (MRE) techniques, the optimal vibration frequency for in vivo experiments is unknown. Furthermore, the accuracy of cardiac MRE has never been evaluated in a geometrically accurate phantom. Therefore, the purpose of this study was to determine the necessary driving frequency to obtain accurate three-dimensional (3D) cardiac MRE stiffness estimates in a geometrically accurate diastolic cardiac phantom and to determine the optimal vibration frequency that can be introduced in healthy volunteers.

Methods: The 3D cardiac MRE was performed on eight healthy volunteers using 80 Hz, 100 Hz, 140 Hz, 180 Hz, and 220 Hz vibration frequencies. These frequencies were tested in a geometrically accurate diastolic heart phantom and compared with dynamic mechanical analysis (DMA).

Results: The 3D Cardiac MRE was shown to be feasible in volunteers at frequencies as high as 180 Hz. MRE and DMA agreed within 5% at frequencies greater than 180 Hz in the cardiac phantom. However, octahedral shear strain signal to noise ratios and myocardial coverage was shown to be highest at a frequency of 140 Hz across all subjects.

Conclusion: This study motivates future evaluation of high-frequency 3D MRE in patient populations. Magn Reson Med 77:351-360, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: cardiac MRE; cardiac elastography; myocardial stiffness.

Fig. 1

Photograph of MRE heart phantom setup.

Fig. 2

A: Photograph of the cardiac MRE passive driver. B: An example of the cardiac MRE passive driver positioning on volunteers. During cardiac MRE exams the passive driver was placed in direct contact with the volunteer's skin just to the left of the sternum and superior to the xiphoid process.

Fig. 3

A comparison of the frequency response of the custom-built cardiac MRE passive driver and the conventional liver passive driver under the same experimental conditions.

Fig. 4

ECG-gated cardiac MRE spin echo EPI timing diagram for 140-Hz vibrations. The time starts from the ECG-trigger at the R-wave peak. The axis of the motion-encoding gradients was changed to sequentially acquire motion in all three directions and is shown as a dotted wave form. The user defined time delay (Δt) was set to minimum (∼25ms) for this experiment. RF, radiofrequency pulses; G(x,y,z), gradient wave forms in each Cartesian direction; M, vibrational motion; SSP, spatial-spectral RF pulse; MEG, flow-compensated motion-encoding gradients; ET, EPI echo train.

Fig. 5

Z-direction curl wave images (top row), corresponding |G*| elastograms (middle row), and pixels with OSS-SNR > 1.6 (bottom row), obtained in the cardiac phantom. Vibrations were applied to the apex of the heart and are propagating upward.

Fig. 6

A plot of the mean (open squares) and median (open triangles) MRE magnitude of the complex shear modulus measurements compared with DMA analysis (dashes). The error bars represent the standard deviation of the mean for both the DMA and MRE measurements.

Fig. 7

Long-axis magnitude images of a volunteer's heart with no evidence of flow artifacts and bright (relatively slow moving) blood in early systole.

Fig. 8

Z-direction curl wave images (top row), corresponding elastograms (middle row), and the elastogram pixels with OSS > 1.6 (bottom row), from a single volunteer over the complete 80-220Hz frequency range.

Fig. 9

A: Voxels that remain after different OSS-SNR thresholds for “motion” (triangles) and “no motion data” (squares). The intersection of the two dashed lines indicate the 1.6 OSS-SNR threshold that corresponds to only 5% of the voxels in the “no motion” data to remain. All data from the “no motion” scans fall within the shaded gray region of plot. B: The absolute difference between the magnitude of the complex shear modulus of the “motion” and “no motion” data over the entire frequency range (left vertical axis with black line and open squares). The ratio of the complex shear modulus of the “motion” data over the “no motion” data as a function of frequency (right vertical axis with red line and circles).

Fig. 10

Box and whisker plot of mean OSS-SNR (A) and the number of total voxels with OSS-SNR > 1.6 (B) across all subjects at each vibration frequency. The “X” corresponds to the maximum and minimum values. The error bars (whiskers) represent the standard deviation of the mean across all subjects. The dashed line in A represents the OSS-SNR threshold used in this study. The mean OSS-SNR from the “no motion” scan has been denoted by the red circles and the solid line.

Fig. 11

Box and whisker plot of mean shear modulus across all subjects in voxels with OSS-SNR>1.6. The “X” corresponds to the maximum and minimum values. The error bars (whiskers) represent the standard deviation of the mean across all subjects.