Fast and efficient free induction decay MRSI at 9.4 T: assessment of neuronal activation-related changes in the human brain biochemistry

The aim was to design a MRSI-FID sequence for ultra-high field applications with high acquisition speed and sampling efficiency. The sequence allows acquisition of a 32×32 voxel matrix within approximately 2 min, down to 30 sec using parallel imaging. We have examined the suitability of this approach for assessing biochemical changes in the human visual cortex during a visual stimulus. Obtained results were in accordance with other functional MRS studies and indicate that the developed sequence is suitable for rapid monitoring of stimulus evoked changes in human brain biochemistry at a very high spatial resolution

Tranceive Phased Array with High Transmit Performance for Human Brain Application at 9.4 T

Surface loop phased arrays have been shown to improve transmit performance and B1 homogeneity for head imaging up to 9.4T. However, transmit arrays enlarged to fit receive arrays often cannot satisfy the requirements in B1 and bandwidth for ultra-high field spectroscopic imaging. We have developed and constructed a tight fit 400MHz 8-channel transceiver array. The array improved transmit efficiency and homogeneity in the axial slab through the phantom’s center when used in CP mode. B1+ averaged over the central axial slice measured 55.6 nT/V, which corresponds to 12.4 uT per 1 kW of RF power delivered directly to the array

Magnetresonanztomografie bei ultrahohen Feldstärken - Forschungsbericht 2014 Max-Planck-Institut für biologische Kybernetik

Um die räumliche und zeitliche Auflösung der Magnetresonanztomografie zu verbessern, werden immer höhere Magnetfeldstärken verwendet: So wird das zu detektierende Signal verstärkt. Hierfür müssen technologische Herausforderungen, wie zum Beispiel die Entwicklung neuartiger Radiofrequenzspulen, bewältigt werden. Erste klinische Studien mit selbstentwickelten Spulen wurden bereits an Tumoren im menschlichen Gehirn bei einer Feldstärke von 9,4 Tesla durchgeführt. Verglichen wurden die Spektren von gesundem und tumorösem Gewebe. Die Ergebnisse zeigen großes Potenzial für medizinische Anwendungen

Free induction decay proton magnetic resonance spectroscopic imaging in the healthy human brain at 9.4 Tesla: initial results

Introduction Recent studies at ultra-high field [1,2] show that FID based MR spectroscopic imaging (FID-MRSI) avoids in-plane chemical shift displacement and minimizes T2 related signal losses. Yet, FID-MRSI spectra suffer from lipid contaminations (arising from subcutaneous fatty tissue) and phase distortions, caused by the FID truncation (due to the acquisition delay necessary for the excitation pulse and gradients). Our aim was to examine the feasibility of FID-MRSI of the healthy human brain at 9.4T. In the presented approach the missing FID points were reconstructed with an autoregressive model [3] and the lipid contaminations were minimized by improving the point spread function which was achieved by increasing the spatial resolution and Hamming filter as in [2]. Total acquisition time of FID-MRSI sequence was shortened by optimizing the water suppression (water suppression enhanced through T1 effects-WET) [4] scheme. Methods Spectra were collected at a 9.4T MR scanner (Siemens, Erlangen, Germany) using a custom built coil [5]. Water suppressed spectra (with original and optimized WET) were acquired from the brain of two volunteers. In-vivo measurements were approved by the local ethical committee. Acquisition parameters: TR: 340 and 240ms (original and optimized WET, respectively), acquisition delay: 2.3ms, 64×64 voxels, acquisition duration: 128ms, spectral bandwidth: 4kHz, nominal voxel size 3.1×3.1×10mm. These acquisition parameters resulted in a total acquisition time of 21 (original) and 15 min (optimized WET). Results The remaining water signal after suppression is depicted in Fig.1. It can be seen that in the case of optimized WET (c), not only the residual water signal is smaller, but also the suppression is more homogenous against B1+ variations (a). Acquisition at high in-plane resolution combined with Hamming filtering minimized contaminations with lipid signals. This was even the case for voxels closely located to the skull (Fig. 2, graphs 14), where the spectral range of interest between 2 and 4 ppm was unaffected. Conclusions We showed that FID-MRSI at a field strength of 9.4T is feasible. It was possible to reconstruct the missing FID points, so that the phase distortions present in the spectra could be reduced. Optimization of the water suppression scheme allowed significant reduction of total acquisition time. In conclusion, FID-MRSI at 9.4T is highly promising, as it addresses the most critical problems of chemical shift displacement and signal losses due to fast T2 relaxation

A novel approach to assess an extended biochemical profile of the human brain by the means of fast and efficient in-vivo proton Magnetic Resonance Spectroscopic Imaging at 9.4 Tesla

Introduction: Proton Magnetic Resonance Spectroscopic Imaging based on Free Induction Decay acquisition (MRSI-FID) is highly promising at ultra-high magnetic field (>3 T) as it avoids in-plane chemical shift displacement and allows short echo time [1]. However, the necessity to use fat and water saturation results in an excessive amount of time needed for signal preparation. The aim of this project was to improve the time efficiency of the MRSI-FID sequence to obtain high spatial resolution spectra within reasonable time. Methods and results: In-vivo measurements were performed at 9.4 T whole body MR scanner (Siemens, Erlangen, Germany) equipped with a custom-build head coil consisting of 16 transmit and 31 receive channels [2]. Study was conducted with the approval of the local ethics board. The developed MRSI-FID sequence was preceded by non-localized fat saturation gauss radio-frequency (RF) pulse, allowing reduction of the fat contaminations by approximately 40. The flip angles (FA) of three water suppression pulses were numerically optimized for static (B0) and transmit (B1 +) magnetic field inhomogeneities. The use of an asymmetric RF excitation pulse shortened the acquisition delay to 1.6 ms. The slice-selection and refocusing gradient shape was optimized to minimize the influence of frequency sidebands; whose presence hinder spectral quantification [3]. Specifically, the gradient frequency spectrum was numerically optimized to minimize mechanical resonances (at 550 and 1100 Hz). Additionally, acquisition duration, repetition time (TR) and excitation FA were set to achieve optimal signal-to-noise ratio (SNR) [4]. Sequence optimization resulted in total acquisition time (TA) of 2 min 8 sec for a 32×32 MRSI matrix (voxel size: 6×6×10 mm3, 2 weighted averages, TR 138 ms), and 8 min 43 sec for a 64×64 matrix (voxel size 3×3×10 mm3 and otherwise identical parameters). Further shortening of the TA was possible with the use of generalized autocalibrating partially parallel acquisition (GRAPPA) [5]. Together with further optimization of acquisition parameters it enabled shortening the TA to 38 sec for low-resolution and to 2 min 27 sec for high-resolution MRSI. High temporal resolution allowed examination of stimulus evoked changes in human brain biochemistry. Functional experiments were conducted in human visual cortex, using a flickering (7Hz) radial checkerboard as a stimulus. A clear correlation between the changes in GABA/tCr and Glu/tCr concentration ratios and the stimulation periods were observed. The average differences between the stimulus on- and off-set were ~13 and ~11, for GABA/tCr and Glu/tCr, respectively. Discussion: The proposed MRSI-FID sequence enables fast and reliable acquisition of proton spectra at 9.4T. The high SNR makes it possible to reduce the acquisition time even further by utilizing parallel imaging techniques and high temporal resolution enabled the assessment of functional related changes in metabolite concentrations during visual stimulation. Our observations of functional MRSI are in accordance with the literature [6-8]. However, strong contaminations with lipid signal currently still hinders the analysis of the spectra from the regions close to the scalp. Nevertheless, further studies, with a large number of participants will be necessary to elucidate the observed changes in concentrations of Glu and GABA in the regions associated with positive BOLD response

Fast and efficient free induction decay proton MRSI in the human brain at 9.4 T

Purpose/Introduction: Free induction decay (FID) based MRSI has been shown to be highly promising at ultra-high magnetic field [1, 2]. It avoids in-plane chemical shift displacement and allows short echo time. However, the necessity to use fat and water saturation results in an excessive amount of time needed for signal preparation. Thus, our aim was to improve the time efficiency of the MRSI-FID sequence to obtain spectra with high spatial resolution within reasonable time. Subjects and Methods: Spectra were acquired at a 9.4 T whole body scanner (Siemens, Erlangen,Germany) from the superior part of the brain of a healthy volunteer (male, 34 years old) with approval by the local ethics committee. A 16 channel transmit, 31 channel receive coil [3] was used for signal transmission/reception. B0 field inhomogeneities were minimized by second order image based shimming. The sequence was preceded by a non-localized fat saturation gauss pulse, allowing reduction of the fat contaminations by approximately 40 . The flip angles of three water saturation pulseswere optimized for aT1 range of 800–2800 ms and B1 + inhomogeneities of±50 using aBloch simulation. Exchanging the sinc excitation pulse with an asymmetric pulse shortened the acquisition delay (TE) to 1 ms. The optimized sequence is presented in Fig. 1. The following parameters were used for in vivo data acquisition: TR = 150 ms, TE = 1 ms, spectral bandwidth = 5000 Hz, acquisition duration = 108 ms, nominal voxel size 6 9 6910 mm (32 9 32 voxels), density weighting with 2 averages and acquisition timeof 2 min 19 s. Coil combinationwas performed using the adaptive combine method [4]. Fig. 1 Time diagram of the optimized MRSI-FID sequence. Abbreviations used: fat saturation pulse, FatSat; water suppression pulse, WET; excitation pulse, Excit Results: Spectra obtained with the proposed sequence are of reasonable quality (Fig. 2). Although the fat signal is still present, it does not hamper the spectral range of interest (between 4.3 and 1.8 ppm), even in the voxels which are close to scalp (black and green). Differences in the intensity of the presented spectra could be caused by B1-field. Results: Spectra obtained with the proposed sequence are of reasonable quality (Fig. 2). Although the fat signal is still present, it does not hamper the spectral range of interest (between 4.3 and 1.8 ppm), even in the voxels which are close to scalp (black and green). Differences in the intensity of the presented spectra could be caused by B1-field inhomogeneities. Discussion/Conclusion: The presented sequence allows fast and efficient acquisition of MRSI spectra. It allows to acquire a 32 9 32 spectral matrix within less than 2.5 min. Further acceleration could be done using parallel imaging techniques. Relatively large fat residuals indicate the necessity of further optimization of the fat saturation. Additional effort is also needed for reducing the influence of B0 and B1 +-variations on the measured spectra

High-Resolution Free Induction Decay Proton MRSI in the Human Brain at 9.4 T

The FID-MRSI technique is a promising tool to be used for spectroscopic imaging at ultra-high magnetic field as it enables short TE and avoids in-plane chemical shift displacement. However, due to the acquisition delay, the first points of the acquired FID signals are missing, giving rise to phase problems which may hamper quantitative analysis. Our aim was to examine the feasibility of high-resolution FID-MRSI of the healthy human brain at the field strength of 9.4 T. The missing FID points were reconstructed with an autoregressive model so that the phase problems present in the acquired spectra could be minimized

In-vivo proton MR spectroscopic imaging of the human brain gliomas at 9.4 Tesla: evaluation of metabolite coordinates

Recently it was demonstrated that the advantages of ultra-high field MR spectroscopic imaging (MRSI), namely the better signal-to-noise ratio and the improved spectral resolution, can be useful in clinical applications. Studies conducted at lower field strengths (below 3T) have shown that an evaluation with the Orthonormal Discriminant Vector method (ODV) enables differentiation between low (WHO grade II and III) and high grade (WHO grade IV) human brain tumors. The aim of this study was to verify the usefulness of the ODV method in assessing human brain tumor spectra measured with MRSI at a field strength of 9.4T

Assessment of human brain tumors with proton magnetic resonance spectroscopic imaging at 9.4 Tesla

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