Synopsis
Sarcopenia describes muscle degeneration. In
particular with increasing age, muscle tissue is replaced by fatty
infiltrations. We developed an MRI sequence protocol (T1w TSE, PDw
SPACE, PDw TSE Dixon, q-Dixon, and HISTO) for quantifying
this degradation and applied it twice to 54 patients suffering from sarcopenia. Between
both measurements three months of whole body electromyostimulation (EMS) training were performed. Initial results
show that image data can be used for muscle segmentation and determination of
muscle volumes, fat fractions, and fat distribution within the muscles.
Muscle fat fractions correlate with muscle strength. In spectroscopy accurate voxel
repositioning is challenging.PURPOSE
Sarcopenia is a syndrome characterized by loss of skeletal muscle mass
and function, mainly caused by replacement of muscle tissue with intramuscular fat,
which restricts muscle function
1. The purpose of this study was to
develop an MRI pulse sequence protocol suitable for quantifying muscle volume, intramuscular
fat, and its distribution within the muscle.
METHODS
54 females (> 70 yrs) with sarcopenia, randomized in control (n1
= 19) and training (n2 = 35) groups, were examined twice using a 3T
system (MAGNETOM Skyrafit, Siemens, Erlangen, Germany). Between both
measurements, whole body electromyostimulation (miha bodytec, Gersthofen,
Germany) training (20 minutes, 85 Hz, 350 ms, 6 s EMS - 4 s rest, once weekly) was
carried out over a period of three months.
MRI acquisition was performed at the thigh (18-channel
BodyFlex surface coil) and included the following sequences: T1w
TSE, PDw SPACE, PDw TSE Dixon, q-Dixon (multi-echo GRE VIBE Dixon), and HISTO (multi-echo
T2-corrected single voxel spectroscopy, at musculus semitendinosus).
The total acquisition time (incl. localizer) was 16:17 min. Detailed sequence
parameters are given in Figure 1.
Multiple contrasts were acquired using the q-Dixon
sequence to quantify fat accurately2,3. T1 bias was addressed by
using a low flip angle of 4 degree and T2* decay
considered as a degree of freedom in the parameter extraction4,5.
Fat and water fractions were calculated as parametric maps.
Spectroscopic fat quantification with T2-correction
was done by extrapolating fat and water integrals for TE = 0 using an
exponential fit of signal peaks acquired at five successive TEs6.
The long TR of 3000 ms was chosen to avoid T1 bias4,7.
In addition, isokinetic maximum leg extension
and flexion strength (0.6 m/s) were measured for all patients using a leg press
(Con-Trex LP physiomed, Schnaittach, Germany) at baseline and follow-up exams.
RESULTS
All 54 patients were examined successfully twice. Figures 2 and 3 show
results of one randomly chosen training group patient for T1w TSE and
PDw SPACE sequences and PDw TSE Dixon and q-Dixon sequences, respectively. Figure 4 shows a q-Dixon fat fraction
image for three-month follow-up exam after manual muscle segmentation in
comparison with an anatomical thigh muscle model8. Fat fractions were
calculated for each muscle.
MRS (Figure 5) showed a change of fat content
in m. semitendinosus over all controls of +2.86% ± 15.59% (MV ± SD) and of
+9.53% ± 16.92% in the training group.
For the
patient exemplarily shown in Figures 2-5
maximum leg extension strengths were 1088 N and 1180 N and flexion strength 366
and 266 N at baseline and follow-up exams, respectively. For all patients average
extension strength was 1261 N ± 393 N (MV ±
SD) and flexion strength 479 N ± 203 N.
DISCUSSION
The sequences and derived MR images are suited to analyze and quantify
different parameters. T1w TSE and PDw SPACE images show high spatial
and contrast resolution beneficial for morphological analysis. PDw SPACE images
have the additional advantage of a smaller slice thickness.
Muscle-fat-boundaries always appear hypointense
on opposed-phase Dixon images (Figures 3c/f), as in these areas fat and water concentrations
are similar. Hence, they can be used for segmentation of individual muscles2.
Q-Dixon’s fat and water fraction images give additional quantitative
information on tissue composition.
Muscles stressed through extension are vastus
lateralis, rectus femoris, vastus intermedius, and vastus medialis (quadriceps
femoris) while the largest part of flexion (37%) is carried by semimembranosus9.
The high fat content (25.86%) in the semimembranosus of the shown patient (Figure 4) may explain the low flexion
strength compared to the group average. However, this finding has to be proven for
the whole patient collective before a definitive conclusion can be derived. Priorly,
a robust and automated segmentation algorithm has to be developed.
Spectroscopy
is considered as reference standard for fat quantification in e.g. liver MRI10.
Though, since muscle is flexibly formable, voxel repositioning in longitudinal
studies is limited due to inhomogeneous fat/muscle distribution within large
spectroscopic voxel volumes. While a comparable follow-up voxel position could
be selected in the patient shown in Figures
5a/b, this was not feasible in Figures
5c/d. This challenge is also
reflected in the high SD of the change in fat content. Unlike spectroscopy, imaging
sequences are easier to perform and reproduce and changes in muscle parameters
are representative for the whole muscle rather than for a single voxel.
CONCLUSION
Current results of our examination show that MRI is capable to measure
parameters quantifying muscle degradation in sarcopenic disease, while the use
of spectroscopy involves challenges with regard to voxel repositioning and
assessment of the entire muscle volume.
Acknowledgements
The study is supported by the Bavarian Research Foundation (FORMOsA
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