Alfredo Liubomir Lopez Kolkovsky1,2, Beatrice Matot1,2, Eric Giacomini1, Martin Meyerspeer3, Benjamin Marty1,2, and Harmen Reyngoudt1,2
1NMR Laboratory, Institute of Myology, Paris, France, 2NMR Laboratory, CEA\DRF\IBFJ\MIRCen, Paris, France, 3High Field MR Center, CMPBME, Medical University of Vienna, Vienna, Austria
Synopsis
We extended an interleaved
1H/31P multi-voxel sequence for simultaneous
pH monitoring based on carnosine (pH1H, PRESS) and Pi (pH31P,
semi-LASER) with a co-localized dynamic water reference (PRESS). Simultaneous pH
evaluations were done at 3 T in the soleus and gastrocnemius muscles during
a straight-leg plantar flexion paradigm. A pH1H bias of ~0.05 units
was removed by using the dynamic water reference. pH1H and pH31P
time courses (1-minute temporal resolution) were comparable. Pi and Carnosine (C2-H)
resonances broadened during exercise in the gastrocnemius. This method opens the way for new applications in
functional studies targeting individual muscles at 3 T.
Introduction
31P MRS allows
evaluating energy metabolism and dynamic changes of pH in the muscle during
physical effort. 1H MRS can provide cytoplasmic pH information by measuring
carnosine, a dipeptide playing a role in pH buffering1. Carnosine-based pH(pH1H)
determination is an interesting and complementary method to the 31P
MRS evaluation of pH using Pi(pH31P), for instance, in dystrophic
myopathies presenting “leaky” membranes2 or during exercise3. Here, we have
extended a previous interleaved 1H/31P multi-voxel
pulse sequence for simultaneous pH1H and pH31P measurements
in muscle4, by adding the acquisition of a dynamic water
reference(waterref) co-localized to the water-suppressed(WS) carnosine
MRS voxel. We evaluated the impact the that dynamic waterref has on pH1H
calculation during an exercise paradigm and compared the pH1H values
with their corresponding pH31P pairs.Methods
Experimental Setup.
6 men (42±14 y.o.) participated in this study. Experiments were done at 3 T (Siemens
Prisma). The RF coil (RAPID
Biomedical) combined a 1H birdcage transmitter (20-cm inner
diameter), a 1H 18Rx phased-array receiver and a 31P
1Tx/3Rx semi-cylindrical transceiver (11-cm resonator length). The subject’s calf
was placed facing the 31P coil and as close as possible to the center
of the 1H transmit coil.
Exercise paradigm.
Straight-leg plantar flexions were performed every 3 seconds for 13 min
on a pneumatic ergometer, gradually increasing the resistance from ~15% to ~30%
of maximum voluntary contraction. MRS lasted 25 min, starting 2 before exercise
onset.
Interleaved NMR.
An interleaved multi-nuclear sequence was
implemented, performing a WET5 water
suppression (80 Hz saturation bandwidth) and acquiring sequentially a WS 1H MR spectrum
(PRESS, TE=17.9 ms, 2048 points, 8 kHz bandwidth, carrier frequency at 8 ppm),
a 31P MR spectrum (semi-LASER, TE=21.54 ms, 2048 points, 8 kHz
bandwidth, centered at the PCr resonance frequency) and a Waterref 1H
MR spectrum (PRESS, TE=17.9 ms, 2048 points, 8 kHz bandwidth, carrier
frequency at 4.7 ppm) in the same voxel
every 3 s (figure 1A). The voxel localization alternated every 3 s between the
gastrocnemius (GAS) and the soleus (SOL) muscles (figure 1B).
Data Analysis.
In-house
Matlab routines were used for automatic phasing. Individual spectra were then inspected
and manual zero and first-order phase correction was performed if needed. Afterwards,
spectra were averaged over 1-minute intervals and exported to jMRUI. Spectra
were zero-filled (4096 points) and 31P MR spectra were
apodized (8 Hz, Gaussian filter) before AMARES quantification. For each voxel, the
carnosine and waterref 1H MR spectra were fitted simultaneously.
pH1H was calculated from the chemical shift of the carnosine C2-H (δCarn-C2)
resonance and corrected for frequency offsets using the water resonance of the respective
waterref spectra according to6: $$pH_{1H}=6.81+log_{10}(\frac{8.57-δ_{Carn-C2}}{δ_{Carn-C2}-7.65})$$. For
comparison, pH1H was also calculated using the residual water from
the WS spectra. The chemical shift difference between Pi and PCr (δPi) was
used to calculate pH31P according to7: $$pH_{31P}=6.75+log_{10}(\frac{δ_{Pi}-3.27}{5.69-δ_{Pi}})$$.Results & Discussion
Figure 2 shows the
time series of WS 1H and 31P spectra from both
voxels from a single subject. In 3 subjects, a broadening of the carnosine C2-H
resonance was observed during exercise in GAS. As shown in figure 3, PCr signal
amplitude showed a clear drop of at least 50% in GAS accompanied by a mirroring
increase in Pi. In contrast, mild to no changes were observed in SOL, as
expected during straight-leg plantar flexions from a study exploring 4 knee
angles8. A high variability
was observed for carnosine although the average signal amplitude remained
relatively stable in both muscles during the paradigm.
pH1H
values presented a systematic increase of ~0.05 units when using the water residue
of the WS spectra instead of waterref (figure 4),
demonstrating the interest of a dynamic water reference.
The pH1H
and pH31P time courses presented similar trends (figure 5, top). In
GAS, a pH increase in the first minute of exercise was followed by a steady pH
decrease during the following 6 minutes of exercise. pH recovery began in the
second or third minute of recovery. In SOL, a pH1H increase
was observed only during the second half of the exercise period.
Furthermore, when pooling
the data of all subjects, a stronger linear correlation between the two pH
measuring methods was found (figure 5C-D) in GAS(R²=0.55) than in SOL(R²=0.08). Bland-Altman analysis(figure 5E-F) revealed similar limits of
agreement in both muscles but a larger bias in GAS than SOL and the negative mean bias
found indicate that in this study pH31P values were
lower than their pH1H pairs.
Splitting or broadening of the carnosine(C2-H) resonance
has been reported after performing a 1-hour street run followed by a toe-hopping
exercise3.
Here, we found both a widening of the carnosine(C2-H) and Pi resonance in GAS
during exercise. Nevertheless, care should be taken when interpreting these
results as our spectra were averaged a minute. Future studies could include acquisitions
performed in GAS only to double the current temporal resolution. Conclusion
This study showed that adding a dynamic water
reference improved pH estimation by 1H MRS and the results were
comparable to pH values measured simultaneously on the same voxel with 31P
MRS. This method opens the way for new applications of multi-nuclear
interleaving in functional studies targeting individual muscles at 3 T. Acknowledgements
No acknowledgement found.References
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