Nonalcoholic fatty liver disease: diagnostic and fat-grading accuracy of low-flip-angle multiecho gradient-recalled-echo MR imaging at 1.5 T

Abstract

Purpose: To assess the accuracy of four fat quantification methods at low-flip-angle multiecho gradient-recalled-echo (GRE) magnetic resonance (MR) imaging in nonalcoholic fatty liver disease (NAFLD) by using MR spectroscopy as the reference standard.

Materials and methods: In this institutional review board-approved, HIPAA-compliant prospective study, 110 subjects (29 with biopsy-confirmed NAFLD, 50 overweight and at risk for NAFLD, and 31 healthy volunteers) (mean age, 32.6 years +/- 15.6 [standard deviation]; range, 8-66 years) gave informed consent and underwent MR spectroscopy and GRE MR imaging of the liver. Spectroscopy involved a long repetition time (to suppress T1 effects) and multiple echo times (to estimate T2 effects); the reference fat fraction (FF) was calculated from T2-corrected fat and water spectral peak areas. Imaging involved a low flip angle (to suppress T1 effects) and multiple echo times (to estimate T2* effects); imaging FF was calculated by using four analysis methods of progressive complexity: dual echo, triple echo, multiecho, and multiinterference. All methods except dual echo corrected for T2* effects. The multiinterference method corrected for multiple spectral interference effects of fat. For each method, the accuracy for diagnosis of fatty liver, as defined with a spectroscopic threshold, was assessed by estimating sensitivity and specificity; fat-grading accuracy was assessed by comparing imaging and spectroscopic FF values by using linear regression.

Results: Dual-echo, triple-echo, multiecho, and multiinterference methods had a sensitivity of 0.817, 0.967, 0.950, and 0.983 and a specificity of 1.000, 0.880, 1.000, and 0.880, respectively. On the basis of regression slope and intercept, the multiinterference (slope, 0.98; intercept, 0.91%) method had high fat-grading accuracy without statistically significant error (P > .05). Dual-echo (slope, 0.98; intercept, -2.90%), triple-echo (slope, 0.94; intercept, 1.42%), and multiecho (slope, 0.85; intercept, -0.15%) methods had statistically significant error (P < .05).

Conclusion: Relaxation- and interference-corrected fat quantification at low-flip-angle multiecho GRE MR imaging provides high diagnostic and fat-grading accuracy in NAFLD.

Figure 1:

Scatterplots show reproducibility of MR imaging FF estimation by using spectroscopy (n = 36) and four image analysis methods (n = 38). FF of the initial measurement is plotted against that of the repeat measurement. Spectroscopy and all four image analysis methods have high reproducibility. * = P < .001.

Figure 2:

Scatterplots of MR spectroscopic versus imaging FFs calculated by using dual-echo, triple-echo, multiecho, and multiinterference methods. Red line represents the best fit through the data points whose spectroscopic FF is more than 6.25%, and gray line represents the null hypothesis (intercept = 0, slope = 1). The regression intercept and slope and their 95% confidence intervals are shown. * = Significant difference from intercept 0 or slope 1 according to two-tailed t test at α = .05. All methods except for the multiinterference method have varying degrees of FF estimation error (ie, intercept significantly different from 0 and/or slope significantly different from 1.)

Figure 3:

A–D, Estimated FF maps by using the four image analysis methods, E, multiecho MR spectra, and F–H, accompanying T2* maps in 18-year-old man with biopsy-confirmed NAFLD. The ROIs (circles) on imaging and spectroscopic voxel have been colocalized. The spectroscopic FF was 24.2% in E. Imaging FFs were 21.8% in A, 24.9% in B, 20.1% in C, and 24.9% in D. The estimated T2* values were 19.2 msec in F, 28.8 msec in G, and 24.7 msec in H. The triple-echo and multiinterference methods show higher quantification accuracy than the dual-echo and multiecho methods. TE = echo time.