3029

Two-pulse phase-modulated (TPPM) 1H decoupling for detection of the C1-glycogen peak in 13C MRS using a 3T clinical scanner.
Hideto Kuribayashi1 and Toshiro Inubushi2
1Siemens Healthcare K.K., Tokyo, Japan, 2Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Japan

Synopsis

Keywords: Non-Proton, Non-Proton, Spectroscopy, carbon-13

Motivation: To develop a safe MR method to non-invasively measure skeletal muscle glycogen levels in humans.

Goal(s): To reduce SAR without degrading the spectral quality of the 1H-decoupled C1-glycogen peak in 13C MRS.

Approach: To introduce the two-pulse phase-modulated (TPPM) 1H decoupling scheme, which is widely used for 13C NMR of organic solids, into the 13C MRS pulse sequence for a clinical scanner.

Results: The 1H-decoupled C1-glycogen peak could be obtained from a solution phantom using a clinical 3T scanner while reducing SAR with shortening the duration of the TPPM 1H decoupling during 13C FID acquisition.

Impact: 13C MRS with the TPPM 1H decoupling for measuring human skeletal muscle glycogen levels may be more advantageous at high magnetic fields due to the lower SAR, leading to higher SNR or shorter scan times.

INTRODUCTION

Glycogen has attracted a great deal of interest in relation with the energy metabolism for physical fatigue in skeletal muscle. A non-invasive 13C MRS method has been utilized to detect the C1-glycogen peak without spectral background 1,2. The detection at high fields is advantageous to enhance signal intensity and the weak peaks coupled with adjacent 1H spins should be detected as a single peak using 1H decoupling to increase SNR or to reduce scan times.

1H decoupling is achieved with RF irradiation at 1H resonance frequency during 13C FID acquisition. The irradiation leads to high SAR and limits the application at high fields. WALTZ-16 decoupling scheme3 is widely used in in vivo 13C MRS1. Since the pulse widths are on the order of milliseconds in clinical MR scanners, the duration of a WALTZ-16 scheme is a few tens of milliseconds, which may be too long to detect the short-T2* in vivo C1-glycogen peak.

In 13C NMR of organic solids, which requires high 1H decoupling power, the two-pulse phase-modulated (TPPM) 1H decoupling was proposed4 and has been exploited as a more efficient method with high SNR and low RF power than the continuous wave (CW) decoupling 4,5. The TPPM decoupling scheme simply repeats a pair of 180o pulses while changing the positive and negative signs of the RF phase modulation angle (Fig. 1). The pulse pair could be completed within a few milliseconds even in clinical applications, potentially reducing the required decoupling duration and power. Thus, it would be valuable if the TPPM decoupling could be applied to in vivo 13C MRS. In this study, the TPPM decoupling was introduced into the 13C MRS pulse sequence for a clinical scanner to demonstrate 1H decoupling of the C1-glycogen peak from a solution phantom.

METHODS

MRS experiments were carried out using a 3T clinical scanner (Skyra, Siemens Healthineers, Erlangen, Germany) and two aqueous solution phantoms containing [1-13C]glucose and oyster glycogen. The molar concentration of the C1-carbons in the glycogen phantom was ~150 mM. RF coils were built for human calf muscle 13C measurement with 1H imaging and decoupling capability (Fig. 2) based on Serés Roig et al6.

13C MRS pulse sequences with CW, TPPM and WALTZ-16 decoupling schemes were prepared as research prototypes. The same 1H decoupling amplitude was used in those sequences and was kept constant during 13C FID acquisition for the glucose phantom. For the glycogen phantom, the duration of 13C FID acquisition was divided into two phases as shown in Fig. 1. The reason for setting the second phase is to reduce SAR without detecting noise spikes when the decoupling power is switched off. Other identical 13C MRS parameters: the duration of an excitation rectangular RF pulse = 0.4 ms, acquisition bandwidth = 3 kHz, acquisition time = 85 ms and sampling points = 256. The SAR was simulated on the scanner as an adult male with a 170-cm height and a 70-kg weight. The signal amplitude of the highest C1-glucose peak was measured in the absolute-value display mode on the operational software for the scanner and was calculated as the average of three measurements.

RESULTS

As shown in the results below, the TPPM decoupling could be performed using the clinical 3T scanner. The higher TPPM decoupling effect was achieved with the smaller RF phase modulation angle (Fig. 3). Comparing spectral quality of the 1H-decoupled C1-glucose peaks, sidebands were observed only in the spectrum with WALTZ-16 (Fig. 4a). The decoupling bandwidth with TPPM was slightly wider than that with CW and was narrower than that with WALTZ-16 (Fig. 4b-c). In the 13C spectra of glycogen, the C1-glycogen peak could be 1H-decoupled with the TPPM scheme even if the duration of the first decoupling phase was shortened to 8 ms (Fig. 5b). This reduced the simulated SAR to less than 40% comparing to the SAR with full decoupling during 13C FID acquisition.

DISCUSSION

The effectiveness of the TPPM decoupling with the small RF phase modulation angles is consistent with the optimum condition in 13C NMR of organic solids5. The simple TPPM decoupling scheme may contribute to that the sidebands were not observed and may be more applicable to in vivo applications with RF excitations using loop coils than WALTZ-16 which requires 270o and 360o flip angles. Even with the narrow decoupling bandwidth, the TPPM decoupling may be sufficient to detect the C1-glycogen peak from human skeletal muscle.

CONCLUSION

The TPPM scheme is applicable for detection of the 1H-decoupled C1-glycogen peak in 13C MRS using clinical 3T scanners.

Acknowledgements

No acknowledgement found.

References

1. Goluch S et al. Proton-decoupled carbon magnetic resonance spectroscopy in human calf muscles at 7 T using a multi-channel radiofrequency coil. Sci Rep 2018;8:6211.

2. Shiose K et al. Muscle glycogen assessment and relationship with body hydration status: A narrative review. Nutrients 2022;15:155.

3. Shaka AJ et al. Evaluation of a new broadband decoupling sequence: WALTZ-16. J Magn Reson 1983;53:313-340.

4. Bennett AE et al. Heteronuclear decoupling in rotating solids. J Chem Phys 1995;103:6951-6958.

5. Ernst M. Heteronuclear spin decoupling in solid-state NMR under magic-angle sample spinning. J Magn Reson 2003;162:1-34.

6. Serés Roig E et al. A double-quadrature radiofrequency coil design for proton-decoupled carbon-13 magnetic resonance spectroscopy in humans at 7T. Magn Reson Med 2015;73:894-900.

Figures

Figure 1. 13C MRS pulse sequence with the TPPM 1H decoupling. The TPPM decoupling scheme is a repetition of a pair of 180o-flip angle pulses while changing the positive and negative signs of the RF phase modulation angle φ. For the glycogen phantom, the decoupling power decreased linearly from the midpoint to the end of 13C FID acquisition (Phase II). For the glucose phantom, the decoupling power was kept constant during 13C FID acquisition, thus the Phase II was not applied.

Figure 2. RF coils for human calf muscle 13C measurement with 1H imaging and decoupling capability. The coils (13C: a 75-mm loop, 1H: a pair of 95-mm loops) are connected to the MR system via an interface box containing low-pass/high-pass filters, preamplifiers and transmit receive switches (Takashima Co. Ltd., Tokyo, Japan). The coil housing materials, acrylic resin and wood, were confirmed to give no background 13C signals around the C1-glycogen peak.

Figure 3. 13C MRS results from a [1-13C]glucose phantom with the TPPM 1H decoupling. Spectra were acquired with different RF phase modulation angles. (a) The intensities are the averages among three measurements in the absolute-value display mode and the corresponding standard deviations are smaller than the size of the points on the graph. (b) The representative spectra shown as the phase-sensitive display mode. The spectral width is 1.5 kHz (= 48.4 ppm).

Figure 4. 13C MRS results from a [1-13C]glucose phantom with various 1H decoupling schemes. The representative 13C spectra (a) with the 1H decoupling offset = 4.93 ppm, which is at the midpoint between H1-α- and H1-β-glucose peaks. The C1-glucose peak intensities (b) and the representative 13C spectra (c) with different 1H decoupling offsets with respect to 4.93 ppm. The RF phase modulation angle φ for the TPPM decoupling = 30o. The spectra are shown as the phase-sensitive display mode.

Figure 5. 13C spectral results from an oyster glycogen phantom. (a) Spectra with (top) and without (bottom) TPPM 1H decoupling. (b) Spectra with different Phase I durations (Figure 1) of TPPM (top) and WALTZ-16 (W16, bottom) 1H decoupling. relSAR is calculated by dividing the simulated SAR by the SAR of Phase I = 100% with the same decoupling scheme. φ = 30o for TPPM. TR = 200 ms, flip angle for 13C excitation = 90o, 256 averages. Line broadenings = 20 and 5 Hz for a and b, respectively.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
3029
DOI: https://doi.org/10.58530/2024/3029