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Abstract:
Introduction
In-vivo proton magnetic resonance spectroscopic imaging (1H MRSI) at ultra-high magnetic field may benefit from increased signal-to-noise ratio (SNR) and improved spectral resolution [1]. This can be useful when assessing the metabolism of human brain tumors, where the detection of 2-hydroxyglutarate (2HG) with MRSI at the field strength of 3 T has been reported recently [2]. This metabolite is associated with a mutation in isocitrate dehydrogenase (IDH) which occurs frequently in grade II and III gliomas and may prove to be a diagnostic and prognostic biomarker [2]. The aim of the study was to verify the usefulness of 1H MRSI at a field strength of 9.4 T for assessing the metabolism of human brain tumors.
Methods
Spectra were collected with a 9.4 T whole body MR scanner (Siemens, Erlangen, Germany) using a custom built 16 transmit/ 31 channel receive coil [3]. Data were acquired with a stimulated echo acquisition mode (STEAM) sequence with the following parameters: echo time: 20 ms, repetition time: 2000 ms, spectral bandwidth: 4 kHz, voxel size: 10 mm isotropic. The 3.1 kHz bandwidth of the excitation pulses allowed to reduce the scale of the chemical shift displacement to 39. Spectra were evaluated with LCModel software [4]. A group of 10 patients (6 with grade III and 4 with grade II brain tumors) participated in this study. All patient measurements were approved by a local ethics board.
Results
Direct comparison of the healthy (1a) and tumor (1b) spectra seen in Fig. 1 demonstrates a typical pattern with increased signals of choline (Cho), lactate (Lac) and inositol (Ins), and decreased N-acetyl aspartate (NAA). Additionally, the tumor spectrum (1b) shows further changes in the signals of glutamine (Gln), taurine (Tau) and glutamate (Glu). An example of the results obtained with LCModel can be seen in Fig. 2, where again healthy (2a) and tumor (2b) spectra are compared. A detailed analysis of the metabolite levels not only supports the previous observations but also confirms the presence of 2HG (for this particular patient).
Conclusions
We showed that, due to improved SNR and spectral resolution, MRSI at ultra-high field offers a better insight in the pathophysiology of human brain tumors. Additionally, in the patients with IDH mutation (verified by histopathology) it was possible to detect 2HG (with Cramer-Rao lower bounds below 15). In summary, due to increased sensitivity in detecting a larger number of metabolites, MRSI at ultra-high fields has a great potential for clinical applications.
