In-vivo Simultaneous Brain 18F FDG PET Imaging and 13C MR Spectroscopy: Initial Experience

Colm McGinnity¹, Radhouene Neji², Jane MacKewn¹, James Stirling¹, Sami Jeljeli¹, Titus Lanz³, Christian Geppert⁴, Mark Oehmigen⁵, Harald Quick⁵, Gary Cook¹, and Alexander Hammers¹

¹King's College London, London, United Kingdom, ²Siemens Healthcare, Frimley, United Kingdom, ³Rapid Biomedical, Wuerzburg, Germany, ⁴Siemens Healthcare, Erlangen, Germany, ⁵University Duisburg-Essen, Essen, Germany

Synopsis

We investigate the feasibility of in-vivo simultaneous brain 18F FDG PET imaging and 13C MR spectroscopy using a dedicated dual-tuned 1H-13C coil with low PET attenuation. Simultaneous in-vivo 18F FDG PET imaging and 13C unlocalized natural abundance MR spectroscopy were performed on three patients. Initial results show the technical feasibility of the simultaneous in-vivo measurement with good spectral and PET image quality.

Introduction

Simultaneous PET-MR has introduced the possibility of combining in a single scan session the anatomical, functional and multi-parametric information that is provided by MR with the quantitative and sensitive PET measurement [1]. It also paves the way to go beyond proton imaging and spectroscopy and to explore combined PET imaging and multinuclear spectroscopy, in particular 13C MR spectroscopy, which has been reported to provide valuable information for the study of brain metabolism [2, 3].

The purpose of this work is to assess the technical feasibility of in-vivo simultaneous PET imaging and 13C MR brain spectroscopy. The underlying challenges are manifold: such an application requires the absence of interference of the PET electronics with the RF signal acquired at the specific frequency, so that artefacts might contaminate the obtained spectra. It also requires that the RF transmission at the 13C frequency does not impact the PET component of the system and additionally that the used head coil has low PET attenuation.

Methods

One volunteer and three oncology patients without reported neurological disorders had a PET-CT scan and were subsequently scanned on a 3T PET-MR scanner (Biograph mMR, Siemens Healthcare, Erlangen, Germany). Localizer, transverse T2 TSE images and 13C spectroscopy brain measurements were performed using a 1H-13C double-resonant transmit-receive birdcage coil (Rapid Biomedical, Wuerzburg, Germany) which was specifically designed to minimize PET attenuation [Fig.1]. The PET acquisition and the 13C spectroscopy measurement were run simultaneously.

Calculation of attenuation correction information: MR-based attenuation maps were estimated based on a 3D radial UTE sequence with the following parameters: 192 slices, TR = 11.94ms, TE1 = 70us, TE2 = 2.46ms, isotropic 1.6mm voxel, resulting in an acquisition time of 100s. The UTE images provide a three-compartment attenuation model (air, bone, soft tissue) [4]. The hardware attenuation map did not consider the attenuation resulting from the 13C-1H coil.

13C Spectroscopy: A natural abundance unlocalized 13C spectrum of the whole brain was acquired using a free-induction decay sequence with the following parameters: 100 us rectangular excitation pulse with a 90 deg flip angle, 100 us delay between excitation and acquisition, TR = 1500ms, 206 averages, receiver bandwidth = 5000Hz, two-step phase cycling, resolution = 2048 points, resulting in an acquisition time of approximately 5 minutes. Dual-echo GRE shimming was used in order to optimize B0 homogeneity and the obtained peak line-width.

F-18-FDG PET: The three patients had a six-hour fasting period and underwent a blood glucose test. They were injected with 18F FDG, with an uptake time of 2 hours and an approximate activity at the start of the PET-MR measurement of 170 MBq. The PET measurement was run in list-mode for 7 min, and the PET reconstruction was performed using the 3D OSEM algorithm (6 iterations, 21 subsets). A 4mm Gaussian post-reconstruction filter was applied.

Results

The measurement is not localized and no lipid suppression is applied. However, one can still observe that the acquired 13C spectra showed very good signal-to-noise ratio and peak line-width (relatively to the region of interest, i.e. whole brain)[Fig.2, 3].

Moreover, despite the fact that no CT-based attenuation map of the coil was included in the hardware attenuation map, a good PET image quality was obtained, as depicted in [Fig.4, 5], where a transverse slice of the PET image for one patient and its overlay on the transverse T2 image are shown.

Conclusion

We have presented first results showing the feasibility of simultaneous brain PET imaging and 13C unlocalized MR spectroscopy. Next steps will consist of including a CT-based attenuation map of the coil, developing localized 13C spectroscopy sequences to avoid lipid signals and exploring potential clinical applications where combining the information of 1H MR, 13C spectroscopy and PET imaging might lead to a better understanding of brain disorders.