Quick Fire Session
SCMR 22nd Annual Scientific Sessions
LIBRE1 is a new water excitation technique that reduces lipid signal and improves vessel sharpness in free-breathing self-navigated coronary MR angiography (cMRA) at 3T using GRE2. This study aimed to test the hypothesis that LIBRE can also successfully suppress fat signal in cMRA at 1.5T using bSSFP instead of GRE.
The LIBRE pulse consists of two rectangular pulses with variable off-resonance radio frequency (RF) (fRF), subpulse duration (τ), and RF excitation angle (α).
Bloch equation simulations were performed to characterize LIBRE in bSSFP. The steady-state transverse magnetization as a function of tissue frequency and α was simulated for three tissues and three non-spatially-selective RF excitation pulses: 1) non-fat-suppressing single rectangular pulse (SP); 2) spectrally selective 1-1 water excitation pulse (WE); 3) LIBRE. Parameters: tissue frequency=-500Hz - 500Hz; α=10º - 250°; SP: τ=0.3ms,TR=2.9ms; WE: τ=0.3ms,TR=5.3ms; LIBRE: τ=0.7ms,fRF=1220Hz,TR=4.2ms. Relaxation times (T1/T2 [ms]) for the simulation: blood 1439/244; fat 154/44; myocardium 989/33.
To validate the findings of these simulations, LIBRE RF excitation pulses were integrated into a 3D radial3 bSSFP sequence on a 1.5T Aera (Siemens Healthcare). Experiments were performed on a phantom having compartments mimicking blood, fat, and myocardium (Fig 2A).
Finally, LIBRE was used for free-breathing non-contrast-enhanced whole-heart cMRA in 6 healthy adult volunteers. Results from three different prototype self-navigated 3D radial bSSFP sequences4 with T2-preparation5 using the following excitation pulses were compared: 1) SP with fat saturation (FS)6; 2) WE; 3) LIBRE. Parameters: isotropic voxel size=1.15mm; ~12000 radial readouts; receiver bandwidth=685Hz/pixel; FS: α=90°,TR=3.3ms; WE: α=90°,TR=5.7ms; LIBRE: τ=0.7ms,fRF=1160Hz,α=170°,TR=4.6ms.
Signal-to-noise (SNR) and contrast-to-noise (CNR) for blood, fat, and myocardium were measured in vitro and in vivo. Right coronary artery vessel sharpness (VS) was determined using SoapBubble7.
In vitro results were consistent with the numerical simulations (Fig 1,2). LIBRE had lower blood and myocardium SNR than SP and WE (Fig 1B,1D,2B,2D). However, LIBRE exhibited much lower fat SNR (Fig 1C,2C) and a lower dependence on α (Fig 1,2).
Preliminary cMRA experiments with LIBRE show robust epicardial fat suppression (arrows, Fig 3A). LIBRE had significantly lower fat SNR (Fig 3B) than FS, and significantly lower blood SNR and blood-myocardium CNR (Fig 3B,C) than FS and WE. There were no significant differences in VS (Fig 3D).
LIBRE in a bSSFP sequence at 1.5T attenuates fat signal over a broad range of RF excitation angles and in vivo results confirm its highly effective epicardial fat suppression in cMRA. While blood signal remains to be optimized, LIBRE may be well-suited for integration in a free-running 5D cardiac MRI sequence8.