T₁‐corrected quantitative chemical shift‐encoded MRI

Abstract

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.

Addressing concomitant gradient phase errors in time‐interleaved chemical shift‐encoded MRI fat fraction and R2* mapping with a pass‐specific phase fitting method

“…10,26 This signal model uses a constrained common phase for fat and water as proposed by Bydder et al 24 A constrained phase assumption accurately reflects the physics of SGRE acquisitions so long as the flip angle used for RF excitation is maintained relatively low (i.e., ≤ ∼ 5 • ). 27 From here, the presence of CGs can be modeled in the SGRE acquisition for a localized voxel as follows:…”

“…In this work, we use the following spoiled gradient echo (SGRE) CSE‐MRI signal model 24,25 for N echoes acquired at TEs as a reference and starting point:where is the complex signal amplitude () and constant phase (), is PDFF, is the B 0 field map (Hz), and C F is the sum of complex exponentials reflecting a multi‐peak spectral model of fat, typically 6 peaks 10,26 . This signal model uses a constrained common phase for fat and water as proposed by Bydder et al 24 A constrained phase assumption accurately reflects the physics of SGRE acquisitions so long as the flip angle used for RF excitation is maintained relatively low (i.e., ) 27 . From here, the presence of CGs can be modeled in the SGRE acquisition for a localized voxel as follows:where …”

“…Note that although this work uses monopolar echo readouts for the multi-pass CSE-MRI acquisitions, the proposed fitting can be equally applied to bipolar readout acquisitions. Furthermore, for applications where a constrained phase model is no longer valid 27,28 (e.g., applications requiring higher flip angles), the proposed signal model can be adjusted to include independent constant phase terms for water and fat signal components.…”

Longitudinal relaxation in fat‐water mixtures and its dependence on fat content at 3 T

“…The longitudinal relaxation time ( T 1 ) of fat and water are of interest for accurate fat‐water separation and fat quantification, for example, in the context of fatty liver disease 1,2 . One approach for fat quantification has included a multiresonance model of the fat spectrum, the T 1 of fat, and the T 1 of water as parameters to reduce bias 3–5 . That approach features a single T 1 for fat and thus cannot account for the different T 1 s of resonances in the fat spectrum 6 .…”

Editorial for “Bias, Repeatability and Reproducibility of Liver T1 Mapping With Variable Flip Angles”

“…The variable flip angle (VFA) method is based on rapid spoiled gradient echo (SGRE) 4 acquisitions that enable fast and three‐dimensional whole liver T1 mapping within 1–2 breath‐holds. VFA methods are feasible to incorporate with chemical shift approaches, 5 which reduces the confounding effect of fat deposition. Multiecho chemical shift approaches can also measure R2* simultaneously.…”