Correction of frequency and phase variations induced by eddy currents in localized spectroscopy with multiple echo times

Proton magnetic resonance spectroscopy thermometry: Impact of separately acquired full water or partially suppressed water data on quantification and measurement error

In proton magnetic resonance spectroscopy (¹ H MRS) thermometry, separately acquired full water and partially suppressed water are commonly used for measuring temperature. This paper compares these two approaches. Single-voxel ¹ H MRS data were collected on a 3-T GE scanner from 26 human subjects. Every subject underwent five continuous MRS sessions, each separated by a 2-min phase. Each MRS session lasted 13 min and consisted of two free induction decays (FIDs) without water suppression (with full water [FW or w]) and 64 FIDs with partial water suppression (with partially suppressed water [PW or w']). Frequency differences between the two FWs, the first two PWs, the second FW and the first PW (FW₂ , PW₁ ), or between averaged water ( wav' ) and N-acetylaspartate (NAA), were measured. Intrasubject and intersubject variations of the frequency differences were used as a metric for the error in temperature measurement. The intrasubject variations of frequency differences between FW₂ and PW₁fw2-fw1' , calculated from the five MRS sessions for each subject, were larger than those between the two FWs or between the first two PWs (p = 1.54 x 10^-4 and p = 1.72 x 10^-4 , respectively). The mean values of intrasubject variations of fw2-fw1' for all subjects were 4.7 and 4.5 times those of fw2-fw1 and fw2'-fw1' , respectively. The intrasubject variations of the temperatures based on frequency differences, fw2-fNAA or ( fw1'-fNAA ), were about 2.5 times greater than those based on averaged water and NAA frequencies (fwav'-fNAA ). The mean temperature measured from (fwav'-fNAA ) (n = 26) was 0.29°C lower than that measured from fw2-fNAA and was 0.83°C higher than that from ( fw1'-fNAA ). It was concluded that the use of separately acquired unsuppressed or partially suppressed water signals may result in large errors in frequency and, consequently, temperature measurement.

A pilot in vivo proton magnetic resonance spectroscopy study of amino acid neurotransmitter response to ketamine treatment of major depressive disorder

The N-methyl-D-aspartate receptor antagonist ketamine can improve major depressive disorder (MDD) within hours. To evaluate the putative role of glutamatergic and GABAergic systems in ketamine's antidepressant action, medial prefrontal cortical (mPFC) levels of glutamate+glutamine (Glx) and γ-aminobutyric acid (GABA) were measured before, during, and after ketamine administration using proton magnetic resonance spectroscopy. Ketamine (0.5 mg kg(-1) intravenously) was administered to 11 depressed patients with MDD. Glx and GABA mPFC responses were measured as ratios relative to unsuppressed voxel tissue water (W) successfully in 8/11 patients. Ten of 11 patients remitted (50% reduction in 24-item Hamilton Depression Rating Scale and total score ⩽10) within 230 min of commencing ketamine. mPFC Glx/W and GABA/W peaked at 37.8%±7.5% and 38.0%±9.1% above baseline in ~26 min. Mean areas under the curve for Glx/W (P=0.025) and GABA/W (P=0.005) increased and correlated (r=0.796; P=0.018). Clinical improvement correlated with 90-min norketamine concentration (df=6, r=-0.78, P=0.023), but no other measures.

An investigation of amino-acid neurotransmitters as potential predictors of clinical improvement to ketamine in depression

Amino-acid neurotransmitter system dysfunction plays a major role in the pathophysiology of major depressive disorder (MDD). We used proton magnetic resonance spectroscopy (¹H-MRS) to investigate whether prefrontal levels of amino-acid neurotransmitters predict antidepressant response to a single intravenous infusion of the N-methyl-D-aspartate (NMDA) antagonist ketamine in MDD patients. Fourteen drug-free patients with MDD were scanned 1-3 d before receiving a single intravenous infusion of ketamine (0.5 mg/kg). We measured gamma aminobutyric acid (GABA), glutamate, and Glx/glutamate ratio (a surrogate marker of glutamine) in the ventromedial prefrontal cortex (VM-PFC) and the dorsomedial/dorsal anterolateral prefrontal cortex (DM/DA-PFC). Correlation analyses were conducted to determine whether pretreatment GABA, glutamate, or Glx/glutamate ratio predicted change in depressive and anxiety symptoms 230 min after ketamine administration. Pretreatment GABA or glutamate did not correlate with improved depressive symptoms in either of the two regions of interest (p>0.1); pretreatment Glx/glutamate ratio in the DM/DA-PFC was negatively correlated with improvement in depressive symptoms [r s(11)=-0.57, p<0.05]. Pretreatment glutamate levels in the VM-PFC were positively correlated with improvement in anxiety symptoms [r s(11)=0.57, p<0.05]. The findings suggest an association between lower Glx/glutamate ratio and greater improvement in response to ketamine treatment. Because glutamine is mainly contained in glia, the decreased Glx/glutamate ratio observed in this study may reflect the reduction in glial cells found in the same regions in post-mortem studies of individuals with MDD, and suggests that the presence of this neuropathological construct may be associated with antidepressant responsiveness to ketamine.

Soft constraints in nonlinear spectral fitting with regularized lineshape deconvolution

This article presents a novel method for incorporating a priori knowledge into regularized nonlinear spectral fitting as soft constraints. Regularization was recently introduced to lineshape deconvolution as a method for correcting spectral distortions. Here, the deconvoluted lineshape was described by a new type of lineshape model and applied to spectral fitting. The nonlinear spectral fitting was carried out in two steps that were subject to hard constraints and soft constraints, respectively. The hard constraints step provided a starting point and, therefore, only the changes of the relevant variables were constrained in the soft constraints step and incorporated into the linear substeps of the Levenberg-Marquardt algorithm. The method was demonstrated using localized averaged echo time point resolved spectroscopy proton spectroscopy of human brains.

Higher levels of glutamate in the associative-striatum of subjects with prodromal symptoms of schizophrenia and patients with first-episode psychosis

The glutamatergic and dopaminergic systems are thought to be involved in the pathophysiology of schizophrenia. Their interaction has been widely documented and may have a role in the neurobiological basis of the disease. The aim of this study was to compare, using proton magnetic resonance spectroscopy ((1)H-MRS), glutamate levels in the precommissural dorsal-caudate (a dopamine-rich region) and the cerebellar cortex (negligible for dopamine) in the following: (1) 18 antipsychotic-naïve subjects with prodromal symptoms and considered to be at ultra high-risk for schizophrenia (UHR), (2) 18 antipsychotic-naïve first- episode psychosis patients (FEP), and (3) 40 age- and sex- matched healthy controls. All subjects underwent a (1)H-MRS study using a 3Tesla scanner. Glutamate levels were quantified and corrected for the proportion of cerebrospinal fluid and percentage of gray matter in the voxel. The UHR and FEP groups showed higher levels of glutamate than controls, without differences between UHR and FEP. In the cerebellum, no differences were seen between the three groups. The higher glutamate level in the precommissural dorsal-caudate and not in the cerebellum of UHR and FEP suggests that a high glutamate level (a) precedes the onset of schizophrenia, and (b) is present in a dopamine-rich region previously implicated in the pathophysiology of schizophrenia.

Quantitative measurement of N-acetyl-aspartyl-glutamate at 3 T using TE-averaged PRESS spectroscopy and regularized lineshape deconvolution

This article introduces regularized lineshape deconvolution in conjunction with TE-averaged PRESS spectroscopy to measure N-acetyl-aspartyl-glutamate (NAAG). Averaging different echo times suppressed the signals of multiplets from strongly coupled spin systems near 2 ppm; thus, minimizing the interfering signals to detect the acetyl proton signal of NAAG. Signal distortion was corrected by lineshape deconvolution, and Tikhonov regularization was introduced to reduce noise amplification arising from deconvolution; as a result, spectral resolution was enhanced without significantly sacrificing signal-to-noise ratio (SNR). This new approach was used to measure NAAG in the two regions of interest of healthy volunteers, dominated by gray matter and white matter, respectively. The acetyl proton signal of NAAG was directly quantified by fitting the deconvoluted spectra to a Voigt-lineshape spectral model function, yielding the NAAG-N-acetyl-aspartate (NAA) ratios of 0.11±0.02 for the gray matter voxels (n=8) and 0.18±0.02 for the white matter voxels (n=12).

Difference spectroscopy using PRESS asymmetry: application to glutamate, glutamine, and myo-inositol

A simple, clinically viable technique utilizing PRESS and strong coupling properties is presented for discrimination of coupled brain metabolites. The method relies on signal variation due to alteration of inter-echo timings (PRESS asymmetry) while maintaining a constant total echo time. Spin response of singlets and weakly coupled spins is unchanged due to PRESS asymmetry, allowing difference spectroscopy to detect unobstructed strongly coupled resonances. No changes to the standard PRESS sequence are required except variation of inter-echo timings. The procedure is illustrated for the separate detection of glutamate from glutamine and the detection of myo-inositol in simulation, phantom, and in vivo experiments at 4.7 T. The subtraction yields calculated from the simulation were 53% for glutamate and 75% for myo-inositol, and a resultant contribution of 96% glutamate to the total glutamate/glutamine multiplet in the 2.04-2.14 ppm range. To extend the treatment to other field strengths and metabolites, an analytical approximation based on a strongly coupled AB system was used to model individual spin groups. Subtraction spectroscopy yields for different combinations of coupling parameters were calculated for the detection of various strongly coupled metabolites at common clinical field strengths. The approximation also predicts adequate glutamate/glutamine discrimination at 3.0 T using the difference spectroscopy method.

Lower glutamate levels in rostral anterior cingulate of chronic cocaine users - A (1)H-MRS study using TE-averaged PRESS at 3 T with an optimized quantification strategy

Previous studies have shown significantly lower metabolism and functional activity in the anterior cingulate cortex (ACC) of human cocaine addicts. The present study examined whether this ACC hypoactivity is associated with altered glutamate (Glu), the primary excitatory neurotransmitter in the central nervous system (CNS), which has been recently implicated in drug addiction. Participants comprised 14 chronic cocaine addicts and 14 matched healthy volunteers who were examined using (1)H magnetic resonance spectroscopy at 3 T. A new quantification strategy for echo time (TE)-averaged point-resolved spectroscopy (PRESS) was applied to disentangle relaxation effects from J-evolution of coupled spin systems such as Glu. The concentrations of Glu as well as N-acetyl aspartate (NAA), total creatine (tCr), choline-containing compounds (tCho), and myo-inositol (Ins) were estimated from both groups. Glu/tCr was significantly lower in chronic cocaine users compared to control subjects and was significantly correlated with years of cocaine use. Glu/tCr was also positively correlated with NAA/tCr. NAA/tCr significantly decreased with age but was not significantly different between the two groups. These findings suggest a metabolic/neurotransmitter dysregulation associated with cocaine addiction and support a possible therapeutic intervention strategy aimed at normalizing the Glu transmission and function in the treatment of cocaine addiction.