Magnetic resonance spectroscopic imaging of downfield proton resonances in the human brain at 3 T.

PURPOSE: To develop an MRSI technique capable of mapping downfield proton resonances in the human brain. METHODS: A spectral-spatial excitation and frequency-selective refocusing scheme, in combination with 2D phase encoding, was developed for mapping of downfield resonances without any perturbation of the water magnetization. An alternative scheme using spectral-spatial refocusing was also investigated for simultaneous detection of both downfield and upfield resonances. The method was tested in 5 healthy human volunteers. RESULTS: Downfield metabolite maps with a nominal spatial resolution of 1.5 cm3 were recorded at 3 T in a scan time of 12 minutes. Cramer-Rao lower bounds for nine different downfield peaks were 20% or less over a single supraventricular slice. Downfield spectral profiles were similar to those in the literature recorded previously using single-voxel localization methods. The same approach was also used for upfield MRSI, and simultaneous upfield and downfield acquisitions. CONCLUSION: The developed MRSI pulse sequence was shown to be an efficient way of rapidly mapping downfield resonances in the human brain at 3 T, maximizing sensitivity through the relaxation enhancement effect. Because the MRSI approach is efficient in terms of data collection and can be readily implemented at short TE, somewhat higher spatial resolution can be achieved than has been reported in previous single-voxel downfield MRS studies. With this approach, nine downfield resonances could be mapped in a single slice for the first time using MRSI at 3 T.

High-resolution metabolic mapping of gliomas via patch-based super-resolution magnetic resonance spectroscopic imaging at 7T.

OBJECTIVES: To demonstrate the feasibility of 7 T magnetic resonance spectroscopic imaging (MRSI), combined with patch-based super-resolution (PBSR) reconstruction, for high-resolution multi-metabolite mapping of gliomas. MATERIALS AND METHODS: Ten patients with WHO grade II, III and IV gliomas (6/4, male/female; 45 ±â€¯9 years old) were prospectively measured between 2014 and 2018 on a 7 T whole-body MR imager after routine 3 T magnetic resonance imaging (MRI) and positron emission tomography (PET). Free induction decay MRSI with a 64 × 64-matrix and a nominal voxel size of 3.4 × 3.4 × 8 mm³ was acquired in six minutes, along with standard T1/T2-weighted MRI. Metabolic maps were obtained via spectral LCmodel processing and reconstructed to 0.9 × 0.9 × 8 mm³ resolutions via PBSR. RESULTS: Metabolite maps obtained from combined 7 T MRSI and PBSR resolved the density of metabolic activity in the gliomas in unprecedented detail. Particularly in the more heterogeneous cases (e.g. post resection), metabolite maps enabled the identification of complex metabolic activities, which were in topographic agreement with PET enhancement. CONCLUSIONS: PBSR-MRSI combines the benefits of ultra-high-field MR systems, cutting-edge MRSI, and advanced postprocessing to allow millimetric resolution molecular imaging of glioma tissue beyond standard methods. An ideal example is the accurate imaging of glutamine, which is a prime target of modern therapeutic approaches, made possible due to the higher spectral resolution of 7 T systems.

Mapping an Extended Neurochemical Profile at 3 and 7 T Using Accelerated High-Resolution Proton Magnetic Resonance Spectroscopic Imaging.

OBJECTIVES: The aim of this study was to compare high-resolution free induction decay magnetic resonance spectroscopic imaging (FID-MRSI) at 3 T and 7 T in the brain of healthy subjects and to showcase the clinical potential of accelerated FID-MRSI at 7 T in 2 brain tumor cases. MATERIALS AND METHODS: In this institutional review board-approved study, 10 healthy volunteers (8 men/2 women; age: 31 ± 6 years) were measured at 3 T and 7 T (Trio and 7T-Magnetom; Siemens Healthcare, Germany) and 2 patients (a 38-year-old man and a 37-year-old man), 1 with an anaplastic oligoastrocytoma (grade III) and 1 with a low-grade glioma (oligodendroglioma), were measured at 7 T.Free induction decay MR spectroscopic imaging with 3.4 × 3.4 mm in-plane resolution was acquired in 30 minutes/6 minutes (nonaccelerated/accelerated) at both field strengths. In addition, single-slice or multi-slice FID-MRSI at 7 T was measured in the 2 tumor patients at 7 T within 6 minutes/13.3 minutes. Signal-to-noise ratio, Cramer-Rao lower bounds, and parallel imaging efficiency were assessed. High-resolution maps were created for 9 different brain metabolites. RESULTS: At 7 T, 7 of 9 metabolites were reliably mapped over the whole slice but only 3 at 3 T. Parallel imaging efficiency was significantly improved at 7 T. Signal-to-noise ratios were +75%/+66% (P < 0.05) for N-acetylaspartate and +97%/+74%(P < 0.05) for glutamine + glutamate [Glx], and full-widths at half maximum were +112%/+109%(P < 0.05) higher at 7 T than at 3 T (nonaccelerated/accelerated) for N-acetylaspartate. Cramer-Rao lower bounds were more than double at 3 T (P < 0.05). CONCLUSIONS: At 7 T, FID-MRSI allowed the assessment of an extended neurochemical profile and yielded better metabolic maps in only approximately 6 minutes at 7 T than in approximately 30 minutes at 3 T. We found several potentially therapy-relevant neurochemical alterations in brain tumors that highlighted the potential of fast clinical FID-MRSI at 7 T.

In vivo (31)P magnetic resonance spectroscopy of the human liver at 7 T: an initial experience.

Phosphorus ((31) P) MRS is a powerful tool for the non-invasive investigation of human liver metabolism. Four in vivo (31) P localization approaches (single voxel image selected in vivo spectroscopy (3D-ISIS), slab selective 1D-ISIS, 2D chemical shift imaging (CSI), and 3D-CSI) with different voxel volumes and acquisition times were demonstrated in nine healthy volunteers. Localization techniques provided comparable signal-to-noise ratios normalized for voxel volume and acquisition time differences, Cramer-Rao lower bounds (8.7 ± 3.3%1D-ISIS , 7.6 ± 2.5%3D-ISIS , 8.6 ± 4.2%2D-CSI , 10.3 ± 2.7%3D-CSI ), and linewidths (50 ± 24 Hz1D-ISIS , 34 ± 10 Hz3D-ISIS , 33 ± 10 Hz2D-CSI , 34 ± 11 Hz3D-CSI ). Longitudinal (T1 ) relaxation times of human liver metabolites at 7 T were assessed by 1D-ISIS inversion recovery in the same volunteers (n = 9). T1 relaxation times of hepatic (31) P metabolites at 7 T were the following: phosphorylethanolamine - 4.41 ± 1.55 s; phosphorylcholine - 3.74 ± 1.31 s; inorganic phosphate - 0.70 ± 0.33 s; glycerol 3-phosphorylethanolamine - 6.19 ± 0.91 s; glycerol 3-phosphorylcholine - 5.94 ± 0.73 s; γ-adenosine triphosphate (ATP) - 0.50 ± 0.08 s; α-ATP - 0.46 ± 0.07 s; ß-ATP - 0.56 ± 0.07 s. The improved spectral resolution at 7 T enabled separation of resonances in the phosphomonoester and phosphodiester spectral region as well as nicotinamide adenine dinucleotide and uridine diphosphoglucose signals. An additional resonance at 2.06 ppm previously assigned to phosphoenolpyruvate or phosphatidylcholine is also detectable. These are the first (31) P metabolite relaxation time measurements at 7 T in human liver, and they will help in the exploration of new, exciting questions in metabolic research with 7 T MR.

One-dimensional image-selected in vivo spectroscopy localized phosphorus saturation transfer at 7T.

PURPOSE: To evaluate the feasibility of a one-dimensional image-selected in vivo spectroscopy (1D-ISIS) saturation transfer (ST) sequence at 7T for localized in vivo measurements of energy metabolism in different tissues in clinically reasonable examination times. METHODS: The performance of a gradient offset independent adiabacity-based 1D-ISIS localization was tested on phantom and the localized ST sequence was compared with the nonlocalized version in vivo. We performed localized measurements of basal metabolism of human liver and different muscle groups of the calf. Localized ST experiments took 15-25 minutes. RESULTS: The selectivity of the 1D-ISIS sequence was 81.63% and the outer volume suppression was 97.57%. The ST parameters acquired with the 1D-ISIS sequence and with the nonlocalized acquisition in the muscle were not statistically different. The forward rate constants for phosphocreatine (PCr)-adenosine triphosphate (ATP) and inorganic phosphate (Pi)-ATP exchange reactions were measured in the soleus (kCK = 0.30 ± 0.06 s(-1) and kATP = 0.11 ± 0.02 s(-1) , respectively) and in the medial gastrocnemius (kCK = 0.27 ± 0.06 s(-1) and kATP = 0.09 ± 0.03s(-1) , respectively) in 15 minutes per muscle group. The corresponding fluxes were FCK = 6.26 ± 1.28 µmol/g/s, FATP = 0.22 ± 0.05 µmol/g/s and FCK = 6.29 ± 1.66 µmol/g/s, FATP = 0.21 ± 0.07 µmol/g/s, for soleus and gastrocnemius, respectively. The hepatic ATP synthesis measurement was feasible in 24 minutes. CONCLUSION: The fast assessment of PCr-ATP and Pi-ATP exchange rates at 7T makes the 1D-ISIS ST sequence a promising tool for examining local resting-state metabolism in clinically acceptable measurement times.