Synopsis

The potential of using low-field MRI scanners as a platform to enable low-cost 31P studies outside of the conventional hospital environment was examined. All experiments were conducted on an ONI 1T scanner with a custom broadband multichannel receiver. A transmit-only birdcage and receive-only four-element array were designed and built to enhance the SNR and improve linewidth. Preliminary phantom and in vivo results show the proposed design improved the linewidth from 0.71 ppm to 0.33 ppm, SNR was increased by approximately 2 times, and inorganic phosphate and phosphocreatine exchange were observed during the volunteer’s exercise.

Introduction

Methods

An ONI extremity 1T magnet is installed in the TAMU MRSL. The scanner was designed to provide 1H extremity images, so it has no spectroscopy capabilities. Because we expect the SNR of the 31P MRS will be low, we use a non-localized spectroscopy sequence. A custom broadband NMR spectrometer [1] is set up to conduct the MRS. It contains one 500W power amplifier for RF pulse, a 4-channel ADC for multichannel acquisition, and 3 gradient amplifiers to drive the gradient coils built into the magnet. The challenge to 31P MRS is also with respect to shim. A standard deviation of 0.9 ppm ∆B0 is measured from a 3.5 cm radius 9.6 cm long cylindrical region which approximates the volume of interest for skeletal. To improve linewidth and SNR, a four-channel receive array was used with a 31P birdcage transmit-only coil. Two loops and two figure 8 coils are geometrically decoupled to form a four-element receive array, shown in Fig 1. In addition to improving SNR, the 4-element receive coil array effectively divides the region of interest into two smaller volumes so that each channel experiences a narrower linewidth. In post-processing, the signal from all 4-channel signal is combined, resulting in an enhanced linewidth as well as SNR.

Before each scan, a full concentration phosphoric acid phantom is first used to measure the linewidth and conduct the power calibration for a 90-degree tip angle. A physiological concentration phosphorus phantom was used to simulate in-vivo conditions. In vivo MRS was taken with a healthy female volunteer at rest, during local exercise of the foot, and post-exercise recovery. According to the IRB approved protocol (2016-0748F), the volunteer was asked to perform an isometric plantar flexion for 6 minutes with 15-lb-weight force applied. 90 scans with a 4 s repetition time were acquired before, during and post exercise. The first non-steady state scan is taken out.

The data from all averages from each channel was saved for post-processing. Each channel was 0-order phase corrected to the PCr peak. To compensate for the differences in the static magnetic field at the location of each element, The whole spectrum was shifted to center the PCr peak at 0 ppm in order to align all four channels’ spectral peaks. The four-channel data were combined based on SNR weight. The first order correction and baseline correction are applied afterward if necessary.

Result and Discussion

A linewidth of 0.33 ppm was measured from the phosphoric acid phantom with the array. Using a large loop coil of the same overall size, a linewidth of 0.71 ppm was measured. In the 4-average physiological concentration 31P phantom result, array combined SNR is measured to be 33.6, which is approximately 2.1 times better than the average of the individual channel’s SNR. Table 1 shows the detail measurement of each channel and large coil as comparison.

Fig.2 shows eighteen-minute-protocol spectra and the first minute of spectra of a volunteer’s skeletal muscle after the exercise began. The ratio of Pi/PCr peaks’ height changed during the exercise, and the ATP-peak height stayed the same. In the first minute of the exercise, the Pi-peak height gradually increased and then became stable. The PCr-peak SNR was measured to be 21.1. The initial results shown here confirm the feasibility of conducting dynamic 31P MRS using a low-cost 1T extremity magnet. Only 4-8 averages were required to obtain 31P spectra comparable to other published results using a 1.5T/3T magnet.

This platform shows the potential for monitoring 31P in vivo metabolism with meaningful time resolution. Future work includes increasing the number of array elements, optimizing the sequence, and replacing the current 31P birdcage with a dual-tuned 31P/1H birdcage to enable NOE.

Acknowledgements

References

[1] Stephen Ogier , John C Bosshard , and Steven M Wright, A Broadband Spectrometer for Simultaneous Multinuclear Magnetic Resonance Imaging and Spectroscopy, 2016 ISMRM

[2] Sullivan, M. J., Saltin, B. E. N. G. T., Negro-Vilar, R. O. S. A., Duscha, B. D., & Charles, H. C. (1994). Skeletal muscle pH assessed by biochemical and 31P-MRS methods during exercise and recovery in men. Journal of Applied Physiology, 77(5), 2194-2200.

[3] Jung W-I, Staubert A, Widmaier S, et al. Phosphorus J-coupling constants of ATP in human brain. Magn Reson Med 1997; 37:802-4.