Comparison of Gradient Echo MRI Water-Fat separation and single voxel 1H MRS for liver fat fraction measurements in a dietary intervention study at 3T

Stephen Bawden^{1,2}, Carolyn Chee^{3}, Caroline Hoad^{1}, Guruprasad Aithal^{2}, Ian Macdonald^{3}, Richard Bowtell^{1}, and Penny Gowland^{1}

This study compared hepatic fat fraction measured using MRS and gradient echo MRI at 3T. Fitting algorithms using a single fat peak and multiple fat peaks were compared with MRS data at TE=20ms and also T2-corrected MRS. Individual differences between T2 values of water and fat calculated from MRS were also used to estimate the difference between R2*-water and R2*-fat and included in the multi peak fitting per subject. The results showed a good correlation between multi-peak and MRS data (R^{2} = 0.9), but applying T2-correction to MRS increased the scatter (R^{2} = 0.67) and systematic error (gradient = 1.34). Using the new R2* corrected fitting algorithm resulted in similar scatter (R^{2} = 0.66) but improved systematic error (gradient = 1.09). The results from this study indicate the dual-R2* fitting is important at 3T and further developments should be made to optimize these methods.

20 overweight (BMI>25 kg/m2) healthy males were scanned before and after a two-week hyper-energetic (25% excess) diet, using a 3T Philips Achieva system with an XL torso coil.

*MRS*.
Water suppressed and water unsuppressed MRS data were acquired
from a 20x20x20mm^{3} voxel
using STEAM at TE=20,30,40 and 60ms. Spectra were phase corrected and water
and fat peak areas were measured. The fat-to-water ratio (R) at TE = 20ms was used
to calculate FF%_{MRSTE20}. T2 values were determined
from the multiple echo data and a correction factor was applied to provide a T2-corrected FF%_{MRS}.

*MRI*.
3D multi gradient echo images (FOV=400x400x200mm^{3}, voxel=1.3mm) were
acquired at 4 echo-times (TE_{1}=1.2ms,ΔTE=0.9ms), flip angle
of 10^{0} and TR= 5.3ms [6]. The complex signal was fitted to 3 signal
models shown in Figure 1 – a single peak model (SP), a multi peak model (MP) and
newly proposed T2-corrected multi peak model (T2MP). Data was fitted using iterative
least squares in Matlab [7] with noise bias resolved using magnitude
discrimination [6]. The resulting values of FF% (Table 1) were averaged over the MRS ROI. The
T2MP model used individual differences in 1/T2 between fat and water from MRS to estimate ΔR2* assuming that T2', which depends on
field inhomogeneities, is equal for fat and water:

Equation 1: ΔR2* = 1/T2*_{F }-1/T2*_{W} = 1/T2_{F} + 1/T2' - (1/T2_{W} + 1/T2') = 1/T2_{F} - 1/T2_{W}

The inter-subject mean±SD measured from MRS was 38 ± 18 ms for fat and
24 ± 3 ms for water, and ΔR2* = 1.1 ± 2.0 x 10^{-2} ms^{-1}.
Figure 2 a and b show that FF%_{SP} and FF%_{MP} both correlated well with
MRS, with FF%_{MP} giving less systematic
difference (gradient ~1, intercept ~0, Fig 2b). When the MRS data was T2
corrected the correlation between FF%_{MP} and FF%_{MRS} reduced and the gradient of the correlation
increased by approximately 33% (Fig 2c). The correlation between FF%_{T2MP} and
T2-corrected MRS was 0.66 (Fig 2d) with less systematic
difference (gradient ~ 1, y-intercept ~ 0).

Figure 3 shows that the change in average
hepatic fat fraction between visit 1 and visit 2 increased significantly as measured
using T2-corrected MRS (P ≤ 0.001). FF%_{MP} showed a smaller and non-significant increase
(P = 0.7), whilst FF%_{T2MP} agreed between with FF%_{MRS} , but the change between
visits was still non-significant (P = 0.1).

Table
showing the fitting equations for MRI water-fat fitting

Correlation between
a) Single peak MRI model and MRS at TE = 20ms; b) Multi peak MRI model and MRS
at TE = 20 ms; c) Multi peak MRI model and T2 Corrected MRS; d) Multi peak MRI
model with correction and T2-corrected MRS. Dotted
line indicates line of identity.

Liver fat fractions
measured before (visit 1) and after (visit 2) a hyper-energetic (25% excess)
diet from same ROI calculated using a multi peak MRI model (MP),
T2 corrected MRS, and a T2 corrected MRI model (T2MP) as labelled. * P ≤ 0.001
change from visit 1

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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