Sevgi Emin1, Jonas Svensson1,2, Martin Englund3, and Pernilla Peterson1,2
1Medical Radiation Physics, Department of Translational Medicine, Lund University, Malmo, Sweden, 2Medical Imaging and Physiology, Skane University Hospital, Lund, Sweden, 3Orthopaedics, Department of Clinical Sciences Lund, Lund University, Lund, Sweden
Synopsis
Chemical
shift-encoded magnetic resonance imaging may be used to estimate the fatty acid
composition (FAC) of bone marrow adipose tissue (BMAT), and the method may
benefit from the improved spectral resolution at ultra-high field strength. In
this work, the in vivo feasibility of FAC quantification in BMAT at 7 T was
investigated, and simulations were used to determine the optimal image
acquisition parameters and reconstruction model. The study showed that using a
short inter-echo time, FAC quantification in BMAT was feasible and robust at 7 T,
and that the noise performance might be improved using a constrained
reconstruction model.
Introduction
The bone
marrow adipose tissue (BMAT) has unique properties compared to white adipose
tissue (WAT), and its fatty acid composition (FAC) has been linked to e.g. bone
frailty1,2 and osteoarthritis3,4. Chemical
shift-encoded magnetic resonance imaging (CSE-MRI) may be used to estimate FAC
in terms of the number of double bonds (ndb), number of methylene-interrupted
double bonds and chain length5-8, which can be used to calculate
the fractions of saturated (SFA), monounsaturated and polyunsaturated fatty
acids6.
A strong
correlation has been demonstrated to gas chromatography in WAT9,
but the shorter T2* of BMAT may make FAC estimation more challenging in this
tissue. Therefore, the method’s performance in BMAT can be different compared
to WAT and require different optimal image acquisition parameters and
reconstruction model. Furthermore, CSE-MRI of BMAT may benefit from the
improved spectral resolution at ultra-high field strength but has previously
only been attempted at 3 T10,11.
The aim of
this pilot study is to investigate the feasibility of FAC quantification in
BMAT at 7 T in the knee of healthy volunteers. Both simulations and an in vivo
experiment were used to compare two reconstruction models and find optimal
image acquisition parameters.Method
FAC-quantification
For both in vivo and simulated data, two FAC reconstruction models were
compared estimating 1: both ndb and number of methylene-interrupted double bonds (free model), or 2: ndb only
(constrained model), using an in-house iterative least-squares approach (Matlab
R2020a)6,9. Both models calculated the chain length
using the relation cl=16.32+0.38ndb, and the constrained model also
assumed number of methylene-interrupted double bonds nmidb=-0.71+0.45ndb9. In addition, fat, water, R2* and off-resonance frequency (B0)
were estimated.
Simulations
To
investigate the noise performance and robustness to model inaccuracies, signals
were simulated representing BMAT (T2*=5 ms) and WAT (T2*=30 ms) using a fat
fraction of 92 %, number of echoes nTE=10, and inter-echo time ΔTE=0.6 ms at 7 T.
The noise
performance was investigated as the maximum effective number of signals averaged
(NSA), calculated from the Cramér-Rao bound12,13. NSA describes how
efficiently a model uses the signal from several images with a theoretical
maximum of NSA=nTE.
The robustness
to model inaccuracies was estimated as the relative error in ndb when
shifting the fat frequency spectrum by 0.05 ppm in reconstruction.
In vivo
feasibility
After
ethical review board approval and informed consent, the knee of 7 healthy
volunteers was imaged in a QED knee coil (1TX/28RX) in a Philips Achieva 7T AS
system. Three volunteers were excluded from WAT analysis due to insufficient
WAT amount.
Two
multi-echo gradient echo sequences with interleaved TEs and bipolar acquisition
were acquired resulting in an effective ΔTE=0.6 ms, with
nTE=16, TE1=1.2 ms, TR=30 ms, bandwidth=1378 Hz/px, flip angle=8°, and voxel size=0.6x0.6x3 mm3.
Correction
of phase errors and a B0 first guess were obtained online using mDixon
Quant (Philips), and FAC parameters were estimated using various nTEs.
Statistics
Three sagittal
image slices were selected for each subject to define four regions-of-interest
in the distal femur: a medial and a lateral slice for definition of medial and
lateral condyles, respectively, and a central slice for definition of the
femoral shaft and posterior subcutaneous WAT. The average ndb and SFA= 1−(ndb+nmidb)/3 (for fat fraction >80%) within each
region-of-interest was estimated for all tested reconstructions. The median
(range) of subjects were presented. Results
NSA was
lower and a shorter ΔTE was more crucial in BMAT compared
to WAT (Figure 1). However, BMAT was also less affected by model inaccuracies
and values tended to stabilize at values of small relative errors as seen both
in simulations (Figure 2) and in vivo (Figure 3). The constrained model
resulted in more robust and noise efficient results compared to the free model.
Using
suitable ΔTE and nTE from Figures 1-3, in vivo
feasibility in BMAT at 7 T was demonstrated in Figure 4. The calculated in vivo
SFA values were consistent between models and tended to be lower in BMAT
compared to WAT (Figure 5). The constrained model showed lower inter-subject
variability as it was suggested by simulations in Figures 1-2.Discussion
7 T
feasibility in WAT has been demonstrated before14 but may be
especially beneficial for BMAT imaging. However, the necessary short ΔTE in this tissue at 7 T, may require interleaved or bipolar
acquisitions, or both.
The lower
noise performance of BMAT FAC imaging may be partially helped using a
constrained approach. However, this choice is limited to differentiation of SFA
only, as estimation of monounsaturated and polyunsaturated fatty acids requires
free estimation of both ndb and the number of methylene-interrupted double
bonds9.
Compared to
literature gas chromatography results, SFA values agrees well in subcutaneous
WAT15 but BMAT values in the femoral head are slightly lower16.
This discrepancy might be explained by regional FAC differences in BMAT10,11.
Further work with a larger number of subjects is planned for comparison between
BMAT regions.Conclusion
Using a
short ΔTE, robust FAC-quantification of BMAT is feasible and may benefit from
an ultra-high field strength. The constrained model may provide better noise
performance, but is limited to SFA quantification, only.Acknowledgements
No acknowledgement found.References
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