Purpose To develop and validate a T1‐corrected chemical‐shift encoded MRI (CSE‐MRI) method to improve noise performance and reduce bias for quantification of tissue proton density fat‐fraction (PDFF). Methods A variable flip angle (VFA)‐CSE‐MRI method using joint‐fit reconstruction was developed and implemented. In computer simulations and phantom experiments, sources of bias measured using VFA‐CSE‐MRI were investigated. The effect of tissue T1 on bias using low flip angle (LFA)‐CSE‐MRI was also evaluated. The noise performance of VFA‐CSE‐MRI was compared to LFA‐CSE‐MRI for liver fat quantification. Finally, a prospective pilot study in patients undergoing gadoxetic acid‐enhanced MRI of the liver to evaluate the ability of the proposed method to quantify liver PDFF before and after contrast. Results VFA‐CSE‐MRI was accurate and insensitive to transmit B1 inhomogeneities in phantom experiments and computer simulations. With high flip angles, phase errors because of RF spoiling required modification of the CSE signal model. For relaxation parameters commonly observed in liver, the joint‐fit reconstruction improved the noise performance marginally, compared to LFA‐CSE‐MRI, but eliminated T1‐related bias. A total of 25 patients were successfully recruited and analyzed for the pilot study. Strong correlation and good agreement between PDFF measured with VFA‐CSE‐MRI and LFA‐CSE‐MRI (pre‐contrast) was observed before (R2 = 0.97; slope = 0.88, 0.81–0.94 95% confidence interval [CI]; intercept = 1.34, −0.77–1.92 95% CI) and after (R2 = 0.93; slope = 0.88, 0.78–0.98 95% CI; intercept = 1.90, 1.01–2.79 95% CI) contrast. Conclusion Joint‐fit VFA‐CSE‐MRI is feasible for T1‐corrected PDFF quantification in liver, is insensitive to B1 inhomogeneities, and can eliminate T1 bias, but with only marginal SNR advantage for T1 values observed in the liver.

Purpose: Transverse relaxation time (T 2 ) mapping with MRI has a plethora of clinical and research applications. Current T 2 mapping techniques are based primarily on spin-echo (SE) relaxometry strategies that rely on the signal magnitude, and often suffer from lengthy acquisition times. In this work, we propose a phase-based T 2 mapping technique where T 2 information is encoded into the signal phase of rapid gradient echo (GRE) acquisitions. Theory: Bloch equation simulations demonstrate that the phase of GRE acquisitions obtained with a very small inter-repetition RF phase increment has a strong monotonic dependence on T 2 , resulting from coherent transverse magnetization. This T 2 -dependent phase behavior forms the basis of the proposed T 2 mapping technique.To isolate T 2 -dependent phase from background phase, at least 2 data sets with different RF phase increments are acquired. The proposed method can also be combined with chemical shift encoded MRI to separate water and fat signals. Methods: The feasibility of the proposed technique was validated in a phantom experiment. In vivo feasibility was demonstrated in the brain, knee, abdomen, and pelvis. Comparisons were made with SE-based T 2 mapping, spectroscopy, and T 2 values from the literature. Results:The proposed method produced accurate T 2 maps compared with SE-based T 2 mapping in the phantom. Good qualitative agreement was observed in vivo between the proposed method and the reference. T 2 measured in various anatomies agreed well with values reported in the literature. Conclusion: A phase-based T 2 mapping technique was developed and its feasibility demonstrated in phantoms and in vivo. K E Y W O R D S gradient echo, magnetic resonance imaging, phase, quantitative imaging biomarker, relaxometry, RF spoiling, T 2 mapping 610 | WANG et Al.How to cite this article: Wang X, Hernando D, Reeder SB. Phase-based T 2 mapping with gradient echo imaging. Magn Reson Med. 2020;84:609-619.

Purpose Chemical shift encoded (CSE)‐MRI enables quantification of proton‐density fat fraction (PDFF) as a biomarker of liver fat content. However, conventional 3D Cartesian CSE‐MRI methods require breath‐holding. A motion‐robust 2D Cartesian sequential method addresses this limitation but suffers from low SNR. In this work, a novel free breathing 2D Cartesian sequential CSE‐MRI method using a variable flip angle approach with centric phase encoding (VFA‐centric) is developed to achieve fat quantification with low T1 bias, high SNR, and minimal blurring. Methods Numerical simulation was performed for variable flip angle schedule design and preliminary evaluation of VFA‐centric method, along with several alternative flip angle designs. Phantom, adults (n = 8), and children (n = 27) were imaged at 3T. Multi‐echo images were acquired and PDFF maps were estimated. PDFF standard deviation was used as a surrogate for SNR. Results In both simulation and phantom experiments, the VFA‐centric method enabled higher SNR imaging with minimal T1 bias and blurring artifacts. High correlation (slope = 1.00, intercept = 0.04, R2 = 0.998) was observed in vivo between the proposed VFA‐centric method obtained PDFF and reference PDFF (free breathing low‐flip angle 2D sequential acquisition). Further, the proposed VFA‐centric method (PDFF standard deviation = 1.5%) had a better SNR performance than the reference acquisition (PDFF standard deviation = 3.3%) with P < .001. Conclusions The proposed free breathing 2D Cartesian sequential CSE‐MRI method with variable flip angle approach and centric‐ordered phase encoding achieved motion robustness, low T1 bias, high SNR compared to previous 2D sequential methods, and low blurring in liver fat quantification.