Diffusion Tractography of the Entire Left Ventricle by Using Free-breathing Accelerated Simultaneous Multisection Imaging

Abstract

Purpose To develop a clinically feasible whole-heart free-breathing diffusion-tensor (DT) magnetic resonance (MR) imaging approach with an imaging time of approximately 15 minutes to enable three-dimensional (3D) tractography. Materials and Methods The study was compliant with HIPAA and the institutional review board and required written consent from the participants. DT imaging was performed in seven healthy volunteers and three patients with pulmonary hypertension by using a stimulated echo sequence. Twelve contiguous short-axis sections and six four-chamber sections that covered the entire left ventricle were acquired by using simultaneous multisection (SMS) excitation with a blipped-controlled aliasing in parallel imaging readout. Rate 2 and rate 3 SMS excitation was defined as two and three times accelerated in the section axis, respectively. Breath-hold and free-breathing images with and without SMS acceleration were acquired. Diffusion-encoding directions were acquired sequentially, spatiotemporally registered, and retrospectively selected by using an entropy-based approach. Myofiber helix angle, mean diffusivity, fractional anisotropy, and 3D tractograms were analyzed by using paired t tests and analysis of variance. Results No significant differences (P > .63) were seen between breath-hold rate 3 SMS and free-breathing rate 2 SMS excitation in transmural myofiber helix angle, mean diffusivity (mean ± standard deviation, [0.89 ± 0.09] × 10^-3 mm²/sec vs [0.9 ± 0.09] × 10^-3 mm²/sec), or fractional anisotropy (0.43 ± 0.05 vs 0.42 ± 0.06). Three-dimensional tractograms of the left ventricle with no SMS and rate 2 and rate 3 SMS excitation were qualitatively similar. Conclusion Free-breathing DT imaging of the entire human heart can be performed in approximately 15 minutes without section gaps by using SMS excitation with a blipped-controlled aliasing in parallel imaging readout, followed by spatiotemporal registration and entropy-based retrospective image selection. This method may lead to clinical translation of whole-heart DT imaging, enabling broad application in patients with cardiac disease. ^© RSNA, 2016 Online supplemental material is available for this article.

Figure 1a:

Tractography of the entire human heart in vivo. (a) Dual-gated STE sequence. Three 90° excitation pulses (RF) are applied over two successive heartbeats. The excitation (RF-1), refocusing (RF-3), and diffusion dephase and rephase occur at the same time in the R-R intervals, thereby exploiting the periodicity of heart motion to rewind motion-related dephasing. Additionally, the long diffusion time (including mixing time), allows a sufficient b value to be produced on clinical imagers without the need for an excessively long echo time (TE). (b) Tractograms of the heart in a healthy male subject in his mid-30s, acquired with STE. Twelve contiguous short-axis sections were acquired with no SMS excitation during multiple breath holds. Tractograms of the whole heart were generated and color-coded by using myofiber helix angle (HA). A magnified view of fiber tracts intersecting a region of interest in the LV lateral wall exhibits the progression in HA from endocardium to epicardium. ECG = electrocardiogram, EPI = echo-planar imaging, Gd = diffusion-encoding gradient.

Figure 1b:

Tractography of the entire human heart in vivo. (a) Dual-gated STE sequence. Three 90° excitation pulses (RF) are applied over two successive heartbeats. The excitation (RF-1), refocusing (RF-3), and diffusion dephase and rephase occur at the same time in the R-R intervals, thereby exploiting the periodicity of heart motion to rewind motion-related dephasing. Additionally, the long diffusion time (including mixing time), allows a sufficient b value to be produced on clinical imagers without the need for an excessively long echo time (TE). (b) Tractograms of the heart in a healthy male subject in his mid-30s, acquired with STE. Twelve contiguous short-axis sections were acquired with no SMS excitation during multiple breath holds. Tractograms of the whole heart were generated and color-coded by using myofiber helix angle (HA). A magnified view of fiber tracts intersecting a region of interest in the LV lateral wall exhibits the progression in HA from endocardium to epicardium. ECG = electrocardiogram, EPI = echo-planar imaging, Gd = diffusion-encoding gradient.

Figure 2:

Tractography of the LV performed by using SMS excitation with a blipped-controlled aliasing in parallel imaging readout in a healthy male subject in his mid-30s. Fiber tracts of the entire LV and tracts within a region of interest in the lateral wall were obtained from acquisitions with no SMS excitation, rate 2 SMS excitation, and rate 3 SMS excitation. The tractograms were consistent across all acquisitions. However, the number of breath holds was reduced from 96 with no SMS excitation to 48 and 32 with rate 2 and rate 3 SMS excitation, respectively.