Non‐water‐suppressed proton MR spectroscopy improves spectral quality in the human spinal cord

Magnetic resonance spectroscopy enables insight into the chemical composition of spinal cord tissue. However, spinal cord magnetic resonance spectroscopy has rarely been applied in clinical work due to technical challenges, including strong susceptibility changes in the region and the small cord diameter, which distort the lineshape and limit the attainable signal to noise ratio. Hence, extensive signal averaging is required, which increases the likelihood of static magnetic field changes caused by subject motion (respiration, swallowing), cord motion, and scanner‐induced frequency drift. To avoid incoherent signal averaging, it would be ideal to perform frequency alignment of individual free induction decays before averaging. Unfortunately, this is not possible due to the low signal to noise ratio of the metabolite peaks. In this article, frequency alignment of individual free induction decays is demonstrated to improve spectral quality by using the high signal to noise ratio water peak from non‐water‐suppressed proton magnetic resonance spectroscopy via the metabolite cycling technique. Electrocardiography (ECG)‐triggered point resolved spectroscopy (PRESS) localization was used for data acquisition with metabolite cycling or water suppression for comparison. A significant improvement in the signal to noise ratio and decrease of the Cramér Rao lower bounds of all metabolites is attained by using metabolite cycling together with frequency alignment, as compared to water‐suppressed spectra, in 13 healthy volunteers. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Multislice 1H MRSI of the human brain at 7 T using dynamic B0 and B1 shimming

Proton MR spectroscopic imaging of the human brain at ultra-high field (≥7 T) is challenging due to increased radio frequency power deposition, increased magnetic field B(0) inhomogeneity, and increased radio frequency magnetic field inhomogeneity. In addition, especially for multislice sequences, these effects directly inhibit the potential gains of higher magnetic field and can even cause a reduction in data quality. However, recent developments in dynamic B(0) magnetic field shimming and dynamic multitransmit radio frequency control allow for new acquisition strategies. Therefore, in this work, slice-by-slice B(0) and B(1) shimming was developed to optimize both B(0) magnetic field homogeneity and nutation angle over a large portion of the brain. Together with a low-power water and lipid suppression sequence and pulse-acquire spectroscopic imaging, a multislice MR spectroscopic imaging sequence is shown to be feasible at 7 T. This now allows for multislice metabolic imaging of the human brain with high sensitivity and high chemical shift resolution at ultra-high field.     

B1 and T1 insensitive water and lipid suppression using optimized multiple frequency‐selective preparation pulses for whole‐brain 1H spectroscopic imaging at 3T

A new method for the simultaneous suppression of water and lipid resonances using a series of dual‐band frequency‐selective radiofrequency (RF) pulses with associated dephasing gradients is presented. By optimizing the nutation angles of the individual pulses, the water and lipid suppression is made insensitive to a range of both T₁‐relaxation times and B₁ inhomogeneities. The method consists only of preparatory RF pulses and thus can be combined with a wide variety of MRSI schemes including both long and short TE studies. Simulations yield suppression factors, in the presence of ±20% B₁ inhomogeneity, on the order of 100 for lipid peaks with three different T₁s (300 ms, 310 ms, and 360 ms), and water peaks with T₁s ranging from 0.8 s to 4 s. Excellent in vivo study performance is demonstrated using a 3 Tesla volumetric proton spectroscopic imaging (¹H‐MRSI) sequence for measuring the primary brain metabolites peaks of choline (Cho), creatine (Cr), and N‐acetyl aspartate (NAA). Magn Reson Med 61:462–466, 2009. © 2009 Wiley‐Liss, Inc.     

Magnetization exchange observed in human skeletal muscle by non‐water‐suppressed proton magnetic resonance spectroscopy

Many metabolites in the proton magnetic resonance spectrum undergo magnetization exchange with water, such as those in the downfield region (6.0–8.5 ppm) and the upfield peaks of creatine, which can be measured to reveal additional information about the molecular environment. In addition, these resonances are attenuated by conventional water suppression techniques complicating detection and quantification. To characterize these metabolites in human skeletal muscle in vivo at 3 T, metabolite cycled non‐water‐suppressed spectroscopy was used to conduct a water inversion transfer experiment in both the soleus and tibialis anterior muscles. Resulting median exchange‐independent T₁ times for the creatine methylene resonances were 1.26 and 1.15 s, and for the methyl resonances were 1.57 and 1.74 s, for soleus and tibialis anterior muscles, respectively. Magnetization transfer rates from water to the creatine methylene resonances were 0.56 and 0.28 s^?1, and for the methyl resonances were 0.39 and 0.30 s^?1, with the soleus exhibiting faster transfer rates for both resonances, allowing speculation about possible influences of either muscle fibre orientation or muscle composition on the magnetization transfer process. These water magnetization transfer rates observed without water suppression are in good agreement with earlier reports that used either postexcitation water suppression in rats, or short CHESS sequences in human brain and skeletal muscle. Magn Reson Med, 70:916–924, 2013. © 2012 Wiley Periodicals, Inc.     

Interleaved narrow-band PRESS sequence with adiabatic spatial-spectral refocusing pulses for 1H MRSI at 7T.

Proton magnetic resonance spectroscopic imaging ((1)H MRSI) is a useful technique for measuring metabolite levels in vivo, with Choline (Cho), Creatine (Cre), and N-Acetyl-Aspartate (NAA) being the most prominent MRS-detectable brain biochemicals. (1)H MRSI at very high fields, such as 7T, offers the advantages of higher SNR and improved spectral resolution. However, major technical challenges associated with high-field systems, such as increased B(1) and B(0) inhomogeneity as well as chemical shift localization (CSL) error, degrade the performance of conventional (1)H MRSI sequences. To address these problems, we have developed a Position Resolved Spectroscopy (PRESS) sequence with adiabatic spatial-spectral (SPSP) refocusing pulses, to acquire multiple narrow spectral bands in an interleaved fashion. The adiabatic SPSP pulses provide magnetization profiles that are largely invariant over the 40% B(1) variation measured across the brain at 7T. Additionally, there is negligible CSL error since the transmit frequency is separately adjusted for each spectral band. in vivo (1)H MRSI data were obtained from the brain of a normal volunteer using a standard PRESS sequence and the interleaved narrow-band PRESS sequence with adiabatic refocusing pulses. In comparison with conventional PRESS, this new approach generated high-quality spectra from an appreciably larger region of interest and achieved higher overall SNR.     

Macromolecule‐suppressed GABA‐edited magnetic resonance spectroscopy at 3T

Edited magnetic resonance spectroscopy makes possible noninvasive studies of the role of the inhibitory neurotransmitter GABA in the healthy brain and in disease processes. A major limitation of the methodology is coediting of macromolecular signals. Although it has previously been shown that macromolecular signal can be suppressed using a symmetrical editing scheme, this approach is rarely applied at field strength of 3T as insufficiently selective pulses result in loss of GABA signal (in addition to the intended suppression of macromolecular signal). In this article, the authors show that increasing the echo time to 80 ms lets more selective editing pulses be used, allowing for symmetric editing‐based suppression of coedited macromolecular signal without loss of GABA signal. The method is applied to acquire macromolecule‐suppressed GABA‐edited spectra in 10 healthy participants. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

SELOVS: brain MRSI localization based on highly selective T1- and B1- insensitive outer-volume suppression at 3T.

In vivo, high-field MR spectroscopic imaging (MRSI) profits from signal-to-noise ratio (SNR) gain and increased spectral resolution. However, bandwidth limitations of slice-selective excitation and refocusing pulses lead to strong chemical-shift displacement at high field strength when using conventional MRSI localization based on PRESS. Consequential metabolic information, particularly of border regions such as cortical brain tissue, is distorted. In addition, lipid contamination remains a major confound. To address these problems it is proposed to abandon PRESS selection and rely on a novel scheme of highly selective T(1)- and B(1)-insensitive outer-volume suppression in combination with slice-selective spin-echo acquisition for brain MRSI. Multiple cycles of overlapping suppression slabs are applied with flip angles optimized to account for tissue-dependent T(1) relaxation times and band crossings. Broadband frequency modulated saturation pulses with polynomial phase-response are utilized in order to minimize chemical-shift displacement. Efficacy of the outer-volume suppression sequence was simulated and evaluated in vitro and in vivo. Brain MRSI localization at 3T was significantly improved and reliable suppression of short-range lipid contamination enabled, leading to substantial enhancement of spectral quality, particularly in cortical tissue. Hence, the new method holds potential to expand the applicability of high-field MRSI to the entire brain.