Introduction

Conventional treatment of liver metastasis often results in palliative care as the effectiveness of the therapy can only be monitored after several months. Metabolite imaging with 31P MRSI is therefore an upcoming field in MRI as it can provide insight in tumor stage and therapy effectiveness, allowing for improved personalized treatment. However, current applications in 31P MRSI are limited by the use of surface transceivers consequently causing small field of views. In addition, the very high RF field inhomogeneity results in an uneven flip angle distribution. Adiabatic pulses to improve FA distribution are SAR demanding subsequently lowering the SNR. To overcome these limitations, body coils for uniform 31P transmit at 7T have been proposed1,2. In this work, we demonstrate the use of the 31P body coil with a 16 channel receiver array merged to eight fractional dipole antennas to combine metabolic imaging with conventional MRI.

Methods

Phosphorus MRSI was performed using an in-house designed detunable birdcage body coil integrated into a 7T MRI system (Philips Healthcare, Best, Netherlands). A 16-channel body array receiver tuned to phosphor at 7T was used for acquisition. To allow for simultaneous proton imaging, eight fractionated dipole antennas were merged with the phosphor receive array and driven by the multi-transmit system (Philips Healthcare, Best, Netherlands)³. A male volunteer was positioned in the MR bore with the receive array wrapped around his torso and slightly skewed to the right-side of the patient to properly cover the liver (Figure 1). Image based B0 and B1 shimming was performed prior to calibration of the phosphor B1+ field, which was done by a flip angle sweep with scaling factors ranging from 250 to 750. Phosphor spectra were acquired using a 3D CSI sequence with an isotropic resolution 24mm, a matrix size of 8x8x6 and 512 samples. Other imaging parameters were BW, 8000Hz; TR, 2000ms; TE, 0.5409; NSA, 20; scan time, 20 minutes. Proton images were acquired using a multi-slice FFE with a total duration of 23 seconds. Other imaging parameters were TR, 10ms; TE, 4.93ms; resolution, 0.78 x 0.78 x 10mm³; field-of-view, 500 x 500 x 30mm³ and a flip angle of 15 degree. All data was processed in Matlab 2016b and data of the receivers was combined using WSVD⁴.

Results

The flip angle series enables calibration of the RF power using 17KW as peak power (Figure 2). Coil combination was possible with the WSVD method. The integrated body coil in combination with the receive array for 31P MRSI showed high SNR (Figure 3) throughout the body. B0 shimming and partial volume effects were suboptimal over such large FOV as shown by the broad lines in the spectra and leakage of phosphocreatine (PCr) through all voxels (Figure 4). As the antennas are inherently decoupled from the 31P coils, good 1H MRI could be obtained (Figure 3, center and background).

Discussion

Patient comfort during scanning is maintained by merging the 31P receiver array with the 1H transceiver array. Moreover, optimal 1H and 31P MRI can be obtained in one scan session without replacing RF coils. While we have shown a uniform transmit, high sensitive receiving merged with a 1H imaging setup, care must be taken in optimizing scan protocols for motion corrections and synchronized B0 shimming.

Conclusion

Localized phosphorus MRSI by simultaneous proton imaging was successfully performed within a large field of view at 7 Tesla. Future clinical studies will benefit from the additional metabolite information gathered during imaging.

Acknowledgements

No acknowledgement found.