Christopher Nguyen1, Zhaoyang Fan1, Yibin Xie1, Jianing Pang1, Xiaoming Bi2, Peter Speier3, Jon Kobashigawa4, and Debiao Li1,5
1Biomedical Imaging Research Institute, Cedars-Sinai Medical Center, Los Angeles, CA, United States, 2MR R&D, Siemens Healthcare, Los Angeles, CA, United States, 3Siemens Healthcare GmbH, Erlangen, Germany, 4Heart Institute, Cedars-Sinai Medical Center, Los Angeles, CA, United States, 5Bioengineering, University of California Los Angeles, Los Angeles, CA, United States
Synopsis
Optimized second order motion compensated (M2) diffusion tensor prepared cardiac magnetic resonance (DT-CMR) was applied in healthy volunteers and heart failure patients at 3T. The pulse sequence design focused on B1 robustness at high main field. In healthy volunteers, the proposed M2 DT-CMR was compared to zero order (M0) and first order (M1) motion compensations. In addition, heart rate dependency of the proposed M2 DT-CMR was explored with contextual comparison to M0 and M1. M2 DT-CMR was the only technique capable of application in heart failure patients without bulk motion artifacts. Introduction
Currently,
there are only two main methods to perform diffusion tensor CMR (DT-CMR) that
either rely on the subject exhibiting stable, periodic RR cycle (stimulated echo [1]) or utilize specialized
research scanners that have ultra-high gradient strengths (spin-echo [2]).
Recent work has demonstrated that gradient moment nulling (GMN) of the second order
is capable of yielding robust diffusion weighted images (DWI) [3]. To extend
this work, we present a novel DT-CMR sequence prototype that utilizes a M2 GMN gradient
scheme that is robust to imperfect B1 refocusing at high main fields (≥3T). We
compare this with no GMN compensation (M0) and first order GMN compensation
(M1). Patients with advanced heart failure (HF) were also scanned to test its
ability in a clinical setting.
Methods
Twenty healthy subjects and
five HF patients were recruited and consented under Institutional Review Board.
All subjects were scanned on a 3T Siemens (MAGNETOM Verio, Siemens Healthcare GmbH, Erlangen) with the following protocol: standard
morphological localizers and prototype sequences implementing 3 DTI scans (b30 + 6 directions b = 300
s/mm^2, free breathing prospective navigator gating, bSSFP readout, 2.7x2.7x8mm3,
flip angle = 90°, single-shot + MoCo) utilizing M0 (TEprep = 35ms), M1 (TEprep
= 46ms), and M2 (TEprep = 67ms). Acquisition was carried out during the
quiescent period of diastole. Gradient amplitudes were set to 60.8 mT/m (two 43
mT/m max gradients simultaneously on).
M2 was achieved with a dual
tripolar pulse that is completely balanced before and after the refocusing
pulse. This allows for motion compensation to be robust against imperfect B1
refocusing since only nulling of M2 depends on robust RF refocusing (M0 and M2
are nulled via gradients alone). The single refocusing pulse was a composite-adiabatic
pulse consisting of two hard pulses straddling a single BIR-4 pulse in the
fashion of a MLEV configuration (90x-180y-90x). In addition, a single crusher
gradient was played out preceding the refocusing pulse and then again unwound
during the bSSFP readout [8]. This crusher gradient eliminates signal not
dictated by the diffusion preparation, which may arise due to imperfect B1 and
T1 recovery. Numerical simulation was carried out to demonstrate how much
motion robustness could depend on imperfect B1 refocusing. Quiescent (velocity: 1.5cm/s, acceleration: 10mm^2/s) and peak (velocity: 15 cm/s, acceleration: 100 mm^2/s) phases were simulated between two M2 encodings (quadra-bipolar and proposed dual-tripolar).
DTI
reconstruction utilized custom software developed in Python using the DIPY
library [6] to generate mean diffusivity (MD), fractional anisotropy (FA), and helix
angle (HA) maps. Success rates defined by >90% of the myocardium unaffected
by motion was reported. Paired t-tests were utilized to statistically test for
significance (p<0.05).
Results
For mildly low heart rates
(HR) (< 75 beats-per-min) in volunteers, M2 was shown to have significantly
(p < 0.05) higher success rates (93%) than M1 (62%) and M0 (28%). For higher
HR, M2 was still significantly (p < 0.05) higher success rates (57%) than M1
(23%) and M0 ( 7%), but much notably lower success than at lower HR. Among the
scans with minimal motion artifacts, MD and FA were significantly (p<0.05) lower
for M2 (1.4±0.2 μm^2/ms, 0.3±0.2) than M0 (4.8±1.3 μm^2/ms, 0.8±0.6) and M1 (1.8±0.2
μm^2/ms, 0.3±0.2) with M2 values being consistent with previous literature
[1,2].
Quiescent periods in
patients were significantly (p < 0.01) shorter than in healthy volunteers
(56±5ms vs 120±30ms, respectively). Despite the higher heart rates (85±8 BPM) in HF patients, M2 alone
was only capable of yielding motion-artifact free MD, FA, and HA maps.
Discussion
The proposed B1 resistant M2 dual-tripolar pulse combined with composite adiabatic refocusing and crusher gradient dephasing scheme yielded more robust MD, FA, and HA maps in volunteers compared with M0 and M. Moreover, M0, M1, and M2 all depended on heart rate experiencing more motion robustness at lower heart rates in healthy volunteers. This is most likely due to the shorter quiescent periods and more severe motion exhibited in volunteers with higher heart rates. In patients, the high heart rate did not impede on the performance of the proposed M2 despite the short quiescent periods. However, these patients also exhibited poor ejection fraction (<30%) possibly reflecting an overall decreased and less complex bulk motion in which M2 may be sufficient in compensating. This needs to be further investigated alongside with strain, velocity, and acceleration myocardial mapping.
Conclusion
The proposed M2 was shown to be more motion robust than M1 and M0 compensation despite the shorter motion sensitivity periods. The proposed DT-CMR was the only method able to provide motion-free DT-CMR images in HF patients.
Acknowledgements
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