MR spectroscopy of the human brain with enhanced signal intensity at ultrashort echo times on a clinical platform at 3T and 7T

Abstract

Recently, the spin-echo full-intensity acquired localized (SPE-CIAL) spectroscopy technique was proposed to unite the advantages of short TEs on the order of milliseconds (ms) with full sensitivity and applied to in vivo rat brain. In the present study, SPECIAL was adapted and optimized for use on a clinical platform at 3T and 7T by combining interleaved water suppression (WS) and outer volume saturation (OVS), optimized sequence timing, and improved shimming using FASTMAP. High-quality single voxel spectra of human brain were acquired at TEs below or equal to 6 ms on a clinical 3T and 7T system for six volunteers. Narrow linewidths (6.6 ؎ 0.6 Hz at 3T and 12.1 ؎ 1.0 Hz at 7T for water) and the high signal-to-noise ratio ( Stimulated-echo acquisition mode (STEAM) (1) or pointresolved spectroscopy (PRESS) (2) are the major methods used for localized proton magnetic resonance spectroscopy (MRS). As STEAM suffers a two-fold signal loss compared to PRESS, but allows shorter echo times (TEs), it is highly desirable to combine the short TE achievable with STEAM with the full signal intensity provided by PRESS. In general, short echo times below 20 ms offer the benefits of reduced signal decay due to T 2 relaxation and J-evolution of coupled spin systems. This in turn allows a more precise quantification of metabolites including those with short T 2 relaxation times. In addition, signal contributions from macromolecules that generally exhibit short T 2 s can be analyzed in short TE spectra to provide insight into specific diseases (3). Short echo times are more easily achievable with STEAM because of the lower peak radiofrequency (RF) pulse power requirements for 90°com-pared to 180°pulses. This is especially important for studies in humans, where the available peak RF power is limited. The STEAM technique was extensively used in previous studies to acquire human brain spectra at field strengths of 1.5T, 3T, 4T, and 7T using TE ϭ 5 ms (4) and 6 ms (5), 6.8 ms (6), 4 ms (7), and 6 ms (8), respectively.On the other hand, it is advantageous to acquire the signal of the full magnetization using spin-echo (SE)-based methods, such as PRESS. For the aforementioned reasons, only few studies report results acquired at short TE. Reducing crusher gradients and using short RF pulses in one approach, or utilizing a dedicated transmit/receive head coil at 3T in pediatric patients in another approach, enabled PRESS acquisitions of in vivo human single-voxel spectroscopy (SVS) data at TE Ϸ 10 ms (9,10). Using the proton echo-planar spectroscopic imaging technique, a TE of 15 ms was reported in MR spectroscopic imaging (MRSI) at 3T and 4T (11).The advantages of short TEs on the order of milliseconds (ms) were recently combined with full signal intensity achieved with SE-based scans using the SE full intensity acquired localized (SPECIAL) spectroscopy technique and applied to in vivo rat brain (12). The sequence benefits from RF coils producing strong B 1 fields and combines interleaved water suppression (WS) and outer volume saturation...

Longitudinal neurochemical modifications in the aging mouse brain measured in vivo by 1H magnetic resonance spectroscopy

“…Then, analysis with appropriate algorithms provides measurements of several compounds such as creatine, N-acetylaspartate (NAA), myo-inositol, choline-containing compounds, and glutamate plus glutamine (the so called "Glx"). In a clinical 3.0 T system, Mekle et al (2009) achieved spectral resolution sufficient to quantify a neurochemical profile composed 14 metabolites. The same neurochemical profile was determined with only 3 minutes of scan time at 7.0 T Mekle et al, 2009;Tkác et al, 2009) and at 9.4 T (Deelchand et al, 2010).…”

“…In a clinical 3.0 T system, Mekle et al (2009) achieved spectral resolution sufficient to quantify a neurochemical profile composed 14 metabolites. The same neurochemical profile was determined with only 3 minutes of scan time at 7.0 T Mekle et al, 2009;Tkác et al, 2009) and at 9.4 T (Deelchand et al, 2010). This means that, at high field, the variety of neurochemicals detected in the human brain increasingly resemble those in rodents, that is, 20 metabolites.…”

1H‐[13C] NMR spectroscopy of the rat brain during infusion of [2‐13C] acetate at 14.1 T

“…The SPECIAL localization sequence and BIR-4 pulses as a part of the FAST(EST)MAP are currently used on our clinical 3-and 7-T scanners (40). It should be straightforward to include a 13 C p pulse into BISEP.…”

Eddy current effects on a clinical 7T-68 cm bore scanner

“…Acquiring the in vivo spectra with a different coil (surface coil) than used for preemphasis adjustment (volume coil) did not lead to reduced spec- Fig. 3 Spectrum acquired at an echo-time of 6 ms from the occipital lobe of a healthy volunteer, using the SPECIAL sequence [6]. The average linewidth of the metabolite signals was about 8 Hz.…”

“…Furthermore, in vivo acquisitions were performed with an echo-time of 6 ms on a volume of 2 × 2 × 2 cm 3 situated in the occipital lobe of a human subject [6] (TR = 4 s, bandwidth = 4 kHz, 64 scans). All spectroscopy acquisitions used the SPECIAL sequence [1] preceded by 1st and 2nd order shimming with FASTMAP [7].…”