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Reduced chemical shift-induced phase errors at 3T using novel PC-MRI encoding gradients
Journal of Cardiovascular Magnetic Resonance volume 15, Article number: W35 (2013)
Cardiovascular MRI benefits from improved SNR-efficiency at ≥3T [1,]2, but is subject to other sources of error, which require careful consideration when transitioning from primary use of 1.5T scanners. For example, chemical shift-induced PC-MRI errors  are increased at 3T compared to 1.5T. Chemical shift causes the complex perivascular fat signal to chemically shift into the vessel lumen and superposes with the complex blood (water) signal, thereby corrupting the phase (velocity) estimate. Chemical shift errors can be minimized by increasing the bandwidth (reduces the magnitude of the shifted fat signal), and by using an in-phase TE (TEIN, ensures fat and water are in-phase). Shorter TEs improve SNR, therefore it is advantageous that the minimum TEIN (TEIN,MIN) at 3T is 2.46ms, which is substantially shorter than TEIN,MIN=4.92ms at 1.5T, but such short TEs cannot be attained with conventional flow-compensated/flow-encoded (FCFE) velocity encoding strategies. The objective was to design a velocity encoding strategy void of conventional FCFE gradients that instead achieves through-plane velocity sensitivity using the slice select gradient, which yields a non-zero first gradient moment (M1) for the first PC-MRI TR: M1,1=X. The slice-select refocusing gradient (SSRG) lobe is time-shifted for the second TR to produce M1,2=X+Y, such that ΔM1=Y=π•γ-1•VENC-1. We hypothesize that the proposed SSRG velocity encoding scheme, will permit the use of TEIN,MIN for chemical shift insensitive PC-MRI measures at 3T that are both faster and have improved SNR.
PC-MRI measurements were acquired in volunteers (N=10) on a Siemens 3T scanner with SSRG: TE/TR=2.46/4.46ms (TEIN,MIN), 192×132 matrix, 1.6mm2×6mm resolution, 30° flip angle, 814Hz/pixel (high bandwidth, HBW), 4 views-per-segment, 35.7ms temporal resolution, and VENC=200cm/s. 2D through-plane velocity encoding was acquired in the ascending aorta (aAo), main pulmonary artery (PA), and right and left pulmonary arteries (RPA and LPA). For comparison FCFE PC-MRI was acquired with the following changes: TE/TR=3.08/6.04ms (TEMID), 401Hz/px (low bandwidth, LBW), and 48.3ms temporal resolution. Eddy current background phase errors were corrected . Intra-subject flow agreement (flow difference between vessels) was compared for SSRG and FCFE for all vessel pairs (aAo vs. PA, aAo vs. RPA+LPA, and PA vs. RPA+LPA).
Our 3T optimized SSRG PC-MRI sequence minimizes chemical shift-induced phase errors and improves intra-subject flow agreement compared to FCFE.
This work is supported in part by NIH/NHLBI K99-R00 HL-087614 and Siemens Medical Solutions.
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Middione, M., Ennis, D. Reduced chemical shift-induced phase errors at 3T using novel PC-MRI encoding gradients. J Cardiovasc Magn Reson 15, W35 (2013). https://doi.org/10.1186/1532-429X-15-S1-W35
- Temporal Resolution
- Velocity Encode
- Gradient Moment
- Vessel Pair
- Slice Select Gradient