Proton magnetization transfer of metabolites in human brain

Detection of electrostatic molecular binding using the water proton signal

PURPOSE

Saturation transfer MRI has previously been used to probe molecular binding interactions with signal enhancement via the water signal. Here, we detail the relayed nuclear overhauser effect (rNOE) based mechanisms of this signal enhancement, develop a strategy of quantifying molecular binding affinity, i.e., the dissociation constant ( K D $$ {K}_D $$ ), and apply the method to detect electrostatic binding of several charged small biomolecules. Another goal was to estimate the detection limit for transient receptor-substrate binding.

THEORY AND METHODS

The signal enhancement mechanism was quantitatively described by a three-step magnetization transfer model, and numerical simulations were performed to verify this theory. The binding equilibria of arginine, choline, and acetyl-choline to anionic resin were studied as a function of ligand concentration, pH, and salt content. Equilibrium dissociation constants ( K D $$ {K}_D $$ ) were determined by fitting the multiple concentration data.

RESULTS

The numerical simulations indicate that the signal enhancement is sufficient to detect the molecular binding of sub-millimolar (∼100 μM) concentration ligands to low micromolar levels of molecular targets. The measured rNOE signals from arginine, choline, and acetyl-choline binding experiments show that several magnetization transfer pathways (intra-ligand rNOEs and intermolecular rNOEs) can contribute. The rNOEs that arise from molecular ionic binding were influenced by pH and salt concentration. The molecular binding strengths in terms of K D $$ {K}_{\mathrm{D}} $$ ranged from 70-160 mM for the three cations studied.

CONCLUSION

The capability to use MRI to detect the transient binding of small substrates paves a pathway towards the detection of micromolar level receptor-substrate binding in vivo.

Water removal in MR spectroscopic imaging with L2 regularization

PURPOSE

An L2-regularization based postprocessing method is proposed and tested for removal of residual or unsuppressed water signals in proton MR spectroscopic imaging (MRSI) data recorded from the human brain at 3T.

METHODS

Water signals are removed by implementation of the L2 regularization using a synthesized water-basis matrix that is orthogonal to metabolite signals of interest in the spectral dimension. Simulated spectra with variable water amplitude and in vivo brain MRSI datasets were used to demonstrate the proposed method. Results were compared with two commonly-used postprocessing methods for removing water signals.

RESULTS

The L2 method yielded metabolite signals that were close to true values for the simulated spectra. Residual/unsuppressed water signals in human brain short- and long-echo time MRSI datasets were efficiently removed by the proposed method allowing good quality metabolite maps to be reconstructed with minimized contamination from water signals. Significant differences of the creatine signal were observed between brain long-echo time MRSI without and with water saturation, attributable to the previously described magnetization transfer effect.

CONCLUSIONS

With usage of a synthesized water matrix generated based on reasonable prior knowledge about water and metabolite resonances, the L2 method is shown to be an effective way to remove water signals from MRSI of the human brain.

CEST, ASL, and magnetization transfer contrast: How similar pulse sequences detect different phenomena

Chemical exchange saturation transfer (CEST), arterial spin labeling (ASL), and magnetization transfer contrast (MTC) methods generate different contrasts for MRI. However, they share many similarities in terms of pulse sequences and mechanistic principles. They all use RF pulse preparation schemes to label the longitudinal magnetization of certain proton pools and follow the delivery and transfer of this magnetic label to a water proton pool in a tissue region of interest, where it accumulates and can be detected using any imaging sequence. Due to the versatility of MRI, differences in spectral, spatial or motional selectivity of these schemes can be exploited to achieve pool specificity, such as for arterial water protons in ASL, protons on solute molecules in CEST, and protons on semi-solid cell structures in MTC. Timing of these sequences can be used to optimize for the rate of a particular delivery and/or exchange transfer process, for instance, between different tissue compartments (ASL) or between tissue molecules (CEST/MTC). In this review, magnetic labeling strategies for ASL and the corresponding CEST and MTC pulse sequences are compared, including continuous labeling, single-pulse labeling, and multi-pulse labeling. Insight into the similarities and differences among these techniques is important not only to comprehend the mechanisms and confounds of the contrasts they generate, but also to stimulate the development of new MRI techniques to improve these contrasts or to reduce their interference. This, in turn, should benefit many possible applications in the fields of physiological and molecular imaging and spectroscopy.

Detection of dynamic substrate binding using MRI

Magnetic Resonance Imaging (MRI) is rarely used for molecular binding studies and never without synthetic metallic labels. We designed an MRI approach that can specifically detect the binding of natural substrates (i.e. no chemical labels). To accomplish such detection of substrate-target interaction only, we exploit (i) the narrow resonance of aliphatic protons in free substrate for selective radio-frequency (RF) labeling and, (ii) the process of immobilisation upon binding to a solid-like target for fast magnetic transfer of this label over protons in the target backbone. This cascade of events is ultimately detected with MRI using magnetic interaction between target and water protons. We prove this principle using caffeine as a substrate in vitro and then apply it in vivo in the mouse brain. The combined effects of continuous labeling (label pumping), dynamic reversible binding, and water detection was found to enhance the detection sensitivity by about two to three orders of magnitude.

T2 measurement of J-coupled metabolites in the human brain at 3T

Proton T(2) relaxation times of metabolites in the human brain were measured using point resolved spectroscopy at 3T in vivo. Four echo times (54, 112, 246 and 374 ms) were selected from numerical and phantom analyses for effective detection of the glutamate multiplet at ~ 2.35 ppm. In vivo data were obtained from medial and left occipital cortices of five healthy volunteers. The cortices contained predominantly gray and white matter, respectively. Spectra were analyzed with LCModel software using volume-localized calculated spectra of brain metabolites. The estimate of the signal strength vs. TE was fitted to a monoexponential function for estimation of apparent T(2) (T(2)(†)). T(2)(†) was estimated to be similar between the brain regions for creatine, choline, glutamate and myo-inositol, but significantly different for N-acetylaspartate singlet and multiplet. T(2)(†)s of glutamate and myo-inositol were measured as 181 ± 16 and 197 ± 14 ms (mean ± SD, N = 5) for medial occipital cortices, and 180 ± 12 and 196 ± 17 ms for left occipital cortices, respectively.

Fat-free MRI based on magnetization exchange

An MRI technique is proposed for complete fat signal elimination. This approach exploits the fact that water rapidly exchanges magnetization with protons in protein and membrane phospholipid of tissue and cells but does not exchange magnetization with triglyceride or fat protons in the tissue. Saturation of the proton signal from protein and membrane phospholipid thus results in partial saturation of the water proton signal, allowing acquisition of an image including a portion of the water signal and the full fat signal. Subtraction of this image from the standard image, containing both water and fat signals, results in an image in which all fat signal is cancelled. This fat-free image is sensitive to magnetization transfer and to water density and relaxation time, providing the possibility of additional contrast. Unlike most fat suppression techniques, this method is not compromised by the static or radiofrequency field heterogeneity and is equally efficient for all fat resonances independent of their chemical shift frequency.

Magnetization transfer effect on human brain metabolites and macromolecules

A pulse sequence was implemented to observe the magnetization transfer (MT) effect on metabolites, water, and macromolecules in human frontal lobes in vivo at 1.5 Tesla. Signals were compared following the application of three hard pulses of 0.745 muT amplitude, applied at frequency offsets of either 2500 Hz or 30 kHz, preceding a conventional point-resolved spectroscopy (PRESS)-localized acquisition with an echo time (TE) of 30 ms and repetition time (TR) of 3 s. This gave an MT effect on water in vivo of 46%, while direct saturation by the MT pulses at 2.5 kHz offset was confirmed to be under 4% for all metabolites. We observed significant MT saturation in vivo for N-acetylated compounds, choline (Cho), myo-inositol, and lactate (Lac); a trend of an effect on glutamate + glutamine (Glx); and the typically observed effect on creatine (Cr). No significant MT effect was seen on the macromolecule signal, which was observed using metabolite nulling.

Localized two-dimensional1H magnetic resonance exchange spectroscopy: A preliminary evaluation in human muscle

A localized two-dimensional (2D) (1)H MR chemical exchange spectroscopic (L-EXSY) sequence has been implemented on a whole-body 1.5-T MRI/MRS scanner. The second spectroscopic encoding to monitor the chemical exchange was an integral part of the single-volume localization using three slice-selective 90 degrees radiofrequency (RF) pulses, thereby eliminating the need for any additional RF pulses, off-resonance/continuous wave saturation, or selective inversion, which are essential in the one-dimensional (1)H MR exchange spectroscopy. Even though the TM-crusher dephased single- and higher-order multiple-quantum coherences, the zero-quantum coherences were indistinguishable from the longitudinal magnetization leading to J-coupled 2D cross peaks similar to COSY. With TM of 300 ms, two different exchange cross peaks were recorded in human calf muscle: a first peak, between the mobile tissue water and total creatine pools, and a second peak, possibly between the olefinic and magnetically equivalent poly methylene protons of unsaturated lipids. Our preliminary results demonstrate that the intermolecular and intramolecular chemical exchange mechanisms can be monitored noninvasively in human calf muscle using 2D L-EXSY.

Structural neural networks subserving oculomotor function in first-episode schizophrenia

BACKGROUND

Smooth pursuit and antisaccade abnormalities are well documented in schizophrenia, but their neuropathological correlates remain unclear.

METHODS

In this study, we used statistical parametric mapping to investigate the relationship between oculomotor abnormalities and brain structure in a sample of first-episode schizophrenia patients (n = 27). In addition to conventional volumetric magnetic resonance imaging, we also used magnetization transfer ratio, a technique that allows more precise tissue characterization.

RESULTS

We found that smooth pursuit abnormalities were associated with reduced magnetization transfer ratio in several regions, predominantly in the right prefrontal cortex. Antisaccade errors correlated with gray matter volume in the right medial superior frontal cortex as measured by conventional magnetic resonance imaging but not with magnetization transfer ratio.

CONCLUSIONS

These preliminary results demonstrate that specific structural abnormalities are associated with abnormal eye movements in schizophrenia.

1H metabolite relaxation times at 3.0 tesla: Measurements of T1 and T2 values in normal brain and determination of regional differences in transverse relaxation

PURPOSE

To measure 1H relaxation times of cerebral metabolites at 3 T and to investigate regional variations within the brain.

MATERIALS AND METHODS

Investigations were performed on a 3.0-T clinical whole-body magnetic resonance (MR) system. T2 relaxation times of N-acetyl aspartate (NAA), total creatine (tCr), and choline compounds (Cho) were measured in six brain regions of 42 healthy subjects. T1 relaxation times of these metabolites and of myo-inositol (Ins) were determined in occipital white matter (WM), the frontal lobe, and the motor cortex of 10 subjects.

RESULTS

T2 values of all metabolites were markedly reduced with respect to 1.5 T in all investigated regions. T2 of NAA was significantly (P < 0.001) shorter in the motor cortex (247 +/- 13 msec) than in occipital WM (301 +/- 18 msec). T2 of the tCr methyl resonance showed a corresponding yet less pronounced decrease (162 +/- 16 msec vs. 178 +/- 9 msec, P = 0.021). Even lower T2 values for all metabolites were measured in the basal ganglia. Metabolite T1 relaxation times at 3.0 T were not significantly different from the values at 1.5 T.

CONCLUSION

Transverse relaxation times of the investigated cerebral metabolites exhibit an inverse proportionality to magnetic field strength, and especially T2 of NAA shows distinct regional variations at 3 T. These can be attributed to differences in relative WM/gray matter (GM) contents and to local paramagnetism.

Magnetization transfer effect on the creatine methyl resonance studied by CW off-resonance irradiation in human skeletal muscle on a clinical MR system

Magnetization transfer (MT) between the mobile (MR-visible) spin pool and immobile (MR-invisible) spin pool of creatine (Cr) was studied on a clinical 1.5 T MR scanner in human skeletal muscle using continuous wave (CW) pre-irradiation as the saturation method for the immobile pool. For this purpose, only slight modifications to the MR system were made. A specially designed electronic circuit was used to couple a CW amplifier to the RF channel of the scanner. The CW pulse power (gammaB(2)/2pi) and pulse length were determined to be approximately 550 Hz and 3 s, respectively, for optimal signal attenuation of the Cr methyl signal. The bound Cr fraction in human gastrocnemius muscle was determined to be 0.4-1.3% using a two-pool exchange model function to describe the MT effect.

Indirect imaging of ethanol via magnetization transfer at high and low magnetic fields

Ethanol (EtOH) is believed to exert its neurochemical effects through interactions with brain cellular components, which causes a fraction of brain EtOH to have a lower molecular mobility. This facilitates magnetization transfer to other molecules similarly associated with macromolecules, such as water. It was hypothesized that this effect can be used in vivo to image EtOH indirectly via the much stronger brain tissue water resonance. EtOH-containing bovine serum albumin samples were used to demonstrate magnetic coupling between EtOH and water at 7 T and 1.5 T. Spectroscopy and imaging experiments demonstrated that EtOH signal saturation yielded greater water signal reduction than inversion and that this reduction scaled with EtOH concentration in the BSA samples. In human brain at physiologically relevant brain EtOH concentrations, water signal reductions were measurable when saturating the EtOH resonance. Strengths and limitations of indirectly imaging brain EtOH are discussed.

Proton magnetization transfer effect in rat liver lactate

Off-resonance lactate magnetization transfer (MT) experiments were performed on the in situ rat liver under perfused and ischemic conditions. A significant MT effect for lactate methyl protons was observed. The effect was larger for the ischemic condition than for the perfused condition, and was largest in the blood-filled ischemic livers. The size of the motionally restricted lactate pool, determined using a two-pool model fit, was estimated to be about 1% in perfused livers and about 1.8-2.5% after more than 1 hr of onset of ischemia, suggesting that lactate in liver is almost fully NMR-visible. The MT data for both the perfused and the ischemic condition appeared to be better approximated when assuming a superLorentzian lineshape for the immobile pool rather than a Gaussian lineshape. Finally, the experiments demonstrated a coupling between the lactate methyl and water protons, which may be mediated by macromolecules.

Magnetization transfer of fluoxetine in the human brain using fluorine magnetic resonance spectroscopy

Fluorine magnetic resonance spectroscopy ((19)F MRS) measurements of fluoxetine and metabolite concentration in the human brain underestimate true drug levels because of a bound, MRS-"invisible" pool of drug molecules. Magnetization transfer (MT) spectroscopy may be a useful technique for characterizing this bound pool of fluoxetine in the brain. Six subjects on consistent daily doses of fluoxetine underwent (19)F MT spectroscopy on a 1.5-T scanner using a train of three preparation pulses at -3000 Hz off resonance with 0.5 W of peak power deposition in tissue. One subject was scanned at multiple time points after initiation of drug therapy. Magnetization transfer signal contrast was quantified using VARPRO-based time domain fitting software. Magnetization transfer signal contrast was quantifiable with mean MT signal depression of 12.5% (SD = 5.0, n = 6). An inverse relationship between brain concentration and the MT signal contrast of fluoxetine was found (r = -.82, Spearman coefficient =.007). This study is the first in vivo application of (19)F MT spectroscopy and the first to demonstrate a quantifiable MT effect for a psychotropic medication in the human brain. Findings suggest that fluoxetine is substantially bound in the brain and that individual differences, inversely related to brain concentration, can be detected in the magnitude of MT contrast.

Magnetization transfer MRS

This review deals with magnetization transfer (MT) effects observed in in vivo NMR spectroscopy. The basic experimental methods of MT experiments, the underlying kinetic mechanisms as well as the evaluation of measured data by fits to two- or three-pool models are described. Experimental results of both (31)P and (1)H in vivo MRS are reviewed showing the potential of MT experiments to characterize kinetic equilibrium reactions. This includes reactions where all involved components are MR visible, as well as situations where one indirectly measures pools of bound spins which cannot directly be observed in vivo. In particular, MT effects are described which have been observed in in vivo (1)H NMR spectra measured on the animal or human brain or on skeletal muscle. Possible mechanisms for the strong MT effects observed for the signals of creatine/phosphocreatine, lactate, alcohol and other metabolites are discussed. It is also emphasized that MT effects caused by water suppression techniques may lead to systematic errors in the quantification of in vivo (1)H NMR spectra.

Magnetization transfer in MRI: a review

This review describes magnetization transfer (MT) contrast in magnetic resonance imaging. A qualitative description of how MT works is provided along with experimental evidence that leads to a quantitative model for MT in tissues. The implementation of MT saturation in imaging sequences and the interpretation of the MT-induced signal change in terms of exchange processes and direct effects are presented. Finally, highlights of clinical uses of MT are outlined and future directions for investigation proposed.

On the importance of exchangeable NH protons in creatine for the magnetic coupling of creatine methyl protons in skeletal muscle

The methyl protons of creatine in skeletal muscle exhibit a strong off-resonance magnetization transfer effect. The mechanism of this process is unknown. We previously hypothesized that the exchangeable amide/amino protons of creatine might be involved. To test this the characteristics of the creatine magnetization transfer effect were investigated in excised rat hindleg skeletal muscle that was equilibrated in either H2O or D2O solutions containing creatine. The efficiency of off-resonance magnetization transfer to the protons of mobile creatine in excised muscle was similar to that previously reported in intact muscle in vivo. Equilibrating the isolated muscle in D2O solution had no effect on the magnetic coupling to the immobile protons. It is concluded that exchangeable protons play a negligible role in the magnetic coupling of creatine methyl protons in muscle.

Magnetic coupling between water and creatine protons in human brain and skeletal muscle, as measured using inversion transfer (1)H-MRS

Using the inversion transfer technique, the possible magnetic coupling between water protons and the protons of low-molecular weight metabolites was investigated in human brain and skeletal muscle at 1.5 T. The localized (1)H-MR spectra were recorded at different times after selective inversion of the water resonance. Water inversion led to a significant transient reduction in the signal intensity of the methyl protons of creatine/phosphocreatine, in both tissues. This is indicative of magnetic coupling between the protons of water and those of creatine/phosphocreatine. Neither the choline and N-acetylaspartate protons in brain nor the protons of the trimethylammonium pool in skeletal muscle showed a significant magnetic coupling to mobile water.