Robert Flintham1, Peter Eddowes2, Scott Semple3, Natasha McDonald4, Jonathan Fallowfield4, Tim Kendall5, Stefan Hübscher6, Philip Newsome2, Gideon Hirschfield2, and Nigel Paul Davies1,7
1Medical Physics, University Hospitals Birmingham NHS Foundation Trust, Birmingham, United Kingdom, 2Centre for Liver Research, NIHR Biomedical Research Unit, University of Birmingham, Birmingham, United Kingdom, 3Clinical Research Imaging Centre, University of Edinburgh, Edinburgh, United Kingdom, 4MRC Centre for Inflammation Research, University of Edinburgh, Edinburgh, United Kingdom, 5MRC Human Genetics Unit, University of Edinburgh, Edinburgh, United Kingdom, 6Pathology, University Hospitals Birmingham NHS Foundation Trust, Birmingham, United Kingdom, 7Institute of Cancer and Genomics, University of Birmingham, Birmingham, United Kingdom
Synopsis
MRS is proven to accurately measure liver fat fraction (FF), but
its potential to differentiate steatohepatitis from simple steatosis in non-alcohol
related fatty liver disease (NAFLD) is unexplored. MRS was acquired in 60
patients with suspected NAFLD across two centres prior to biopsy. Automated analysis
was developed using TARQUIN to estimate FF, lipid chain length (CL) and number of
double-bonds per chain (nDB) revealing strong correlations between FF, nDB, CL and
steatosis grade. nDB also negatively correlated with hepatocyte ballooning
assessed by histopathology. Further investigation of the relationship between MRS-derived
lipid composition measurements and disease severity in NAFLD is warranted.Background
Non-alcoholic fatty liver disease
(NAFLD) is the most common liver disease world-wide and prevalence is
increasing due to the dual epidemics of obesity and type-two diabetes. Hepatic steatosis is a defining feature of
NAFLD but accurate quantification is reliant on liver biopsy with its inherent
risk of serious complication. There is a need to provide accurate, non-invasive
techniques for the assessment of hepatic steatosis for use in clinical practice
and as an endpoint in interventional trials. Magnetic resonance spectroscopy
(MRS) has been shown to accurately assess hepatic steatosis
1. However,
it is not yet routinely used in clinical practice. The reasons for this include
a lack of accessible tools and/or expertise in many centres for the appropriate
acquisition and analysis of the data. Existing studies in the literature have
been restricted to expert single-centres and have largely employed manual or
semi-automated analysis methods to measure liver fat
2. In this study
we aim to quantify and characterise liver fat in NAFLD patients in comparison
with histology using a novel automated approach to the analysis of liver MRS employing open-source TARQUIN
3 software.
Methods
Unselected,
sequential patients having a standard-of-care liver biopsy for the diagnosis or
staging of NAFLD were scanned at two centres (Queen Elizabeth Hospital
Birmingham and Clinical
Research Imaging Centre, Edinburgh). Table 1 shows the patient cohort
characteristics. Histology was assessed by expert pathologists and diagnosis of
simple steatosis (SS) or non-alcoholic steatohepatitis (NASH) was by Brunt
criteria4.
MRS was
performed at 3T as part of a
comprehensive quantitative liver MRI protocol, using PRESS (TR 3s, TE 30ms) without
water suppression (WREF) and STEAM (TR 3s, TE 20ms) with water-suppression (WS) and
without water-suppression in a series of three 15 second breath-holds following automated high-order shimming. For STEAM, 5 measurements were
acquired for individual processing while the PRESS was acquired as a single
measurement with 5 averages. High-resolution
localisers in three planes were used to carefully place the voxel (20x20x20 mm)
in the right lobe of the liver avoiding large vessels and ducts.
Spectroscopy data were automatically corrected
for frequency and phase offsets and fitted using TARQUIN3. A
customised basis-set was used to model the time-domain signal of fatty liver with
components as shown in table 2. Residual water was not removed from the WS spectra. In all fits, the water peak was modelled with
two components at 4.65 ppm. Quality control was performed by visual inspection
to exclude spectra with unresolved fat peaks, severe baseline artefacts or bad
fits.
Simple
fat fraction (FF) was calculated without T2 correction using STEAM and PRESS
WREF fits. Mean chain length (CL) and number of double-bonds per chain (nDB) were estimated
from the STEAM WS fits using a method similar to Hamilton et al2.
Spearman’s correlation analysis was performed to investigate the relationship
between i) PRESS and STEAM, ii) different fat spectral components and iii) FF,
CL, nDB and histological markers of disease severity.
Results and Discussion
Good
quality spectra were acquired at two centres in a short scan time (< 2 min
including shimming) allowing accurate FF measurements in all but 1 case and lipid
profile analysis in all but 3 cases. Figure 1 shows an example of a complete
dataset for a patient with grade 2 steatosis showing good quality spectra with separation
of fat peaks and accurate fits as demonstrated by the small residuals. Close
agreement was found between FF calculated from PRESS and STEAM acquisitions,
with PRESS giving a slight over-estimation compared with STEAM (Figure 2) as
expected due to T2 effects
5. Highly significant correlations were found
between FF and histological grade of steatosis (r = 0.81, 0.77 for STEAM, PRESS
respectively, both P<0.001). Highly significant negative correlations were
found between FF and CL (r = -0.57, P<0.001) and between FF and nDB (r =
-0.44, P=0.002). In
addition, the lipid profile showed negative correlation between nDB and
hepatocyte ballooning, independent of FF (r= -0.31, P=0.03). Figure 3 illustrates this potential dependence of lipid profile on disease severity by comparing
normalised STEAM WS spectra for example cases of NASH and SS with
similar FF of 34% and 42% respectively.
Conclusions
We
demonstrate that MRS is a powerful non-invasive tool to quantify hepatic
steatosis and that automated analysis using TARQUIN is
possible, providing strong correlations with histology. Assessment of fatty
acid saturation and chain length compared with histological markers of disease
severity has not been previously reported using MRS in human liver. This study paves
the way for further investigation into the relationship between fatty acid
composition and clinically relevant markers of disease severity in NAFLD.
Acknowledgements
We would like to thank the imaging staff who facilitated data
acquisition at the Queen Elizabeth Hospital Birmingham and the Clinical
Research Imaging Centre, Edinburgh. We also acknowledge helpful correspondence
with Martin Wilson in the setting-up of the basis-set and options used for the
TARQUIN analysis.
This research was funded by the National Institute for Health Research
(NIHR)’s
Birmingham Liver Biomedical Research Unit programme. The views expressed are
those of the author(s) and not necessarily those of the
NHS, the NIHR or the Department of Health.
References
1. Georgoff
P, Thomasson D, Louie A, et al. Hydrogen-1 MR Spectroscopy for Measurement and
Diagnosis of Hepatic Steatosis. Am J Roentgenol. 2012; 199: 2–7.
2. Hamilton
G, Yokoo T, Bydder M, et al. In vivo characterization of the liver fat ¹H MR spectrum. NMR Biomed.
2011; 24: 784–790.
3. Wilson
M, Reynolds G, Kauppinen R, et al. A constrained least-squares approach to the automated
quantitation of in-vivo 1H MRS data. Magn Reson Med. 2011; 65: 1-12. (http://tarquin.sourceforge.net/)
4. Brunt
EM, Janney CG, Di Bisceglie AM, et al. Nonalcoholic steatohepatitis: a proposal
for grading and staging the histological lesions. Am J Gastroenterol. 1999;94:2467-74.
5. Hamilton
G, Middleton M, Bydder M, et al. Effect of PRESS and STEAM sequences on magnetic resonance spectroscopic liver fat quantification. J Magn
Reson Imaging. 2009; 30: 145–152.