Magnetization transfer in lamellar liquid crystals

Testing quantitative magnetization transfer models with membrane lipids

PURPOSE

Quantitative magnetization transfer (qMT) models aim to quantify the contributions of lipids and macromolecules to the MRI signal. Hence, a model system that relates qMT parameters and their molecular sources may improve the interpretation of the qMT parameters. Here we used membrane lipid phantoms as a meaningful tool to study qMT models. By controlling the fraction and type of membrane lipids, we could test the accuracy, reliability, and interpretability of different qMT models.

METHODS

We formulated liposomes with various lipid types and water-to-lipids fractions and measured their signals with spoiled gradient-echo MT. We fitted three known qMT models and estimated six parameters for every model. We tested the accuracy and reproducibility of the models and compared the dependency among the qMT parameters. We compared the samples' qMT parameters with their water-to-lipid fractions and with a simple MTnorm (= MTon/MToff) calculation.

RESULTS

We found that the three qMT models fit the membrane lipids signals well. We also found that the estimated qMT parameters are highly interdependent. Interestingly, the estimated qMT parameters are a function of the membrane lipid type and also highly related to the water-to-lipid fraction. Finally, we find that most of the lipid sample's information can be captured using the common and easy to estimate MTnorm analysis.

CONCLUSION

qMT parameters are sensitive to both the water-to-lipid fraction and to the lipid type. Estimating the water-to-lipid fraction can improve the characterization of membrane lipids' contributions to qMT parameters. Similar characterizations can be obtained using the MTnorm analysis.

Bloch simulator-driven deep recurrent neural network for magnetization transfer contrast MR fingerprinting and CEST imaging

PURPOSE

To develop a unified deep-learning framework by combining an ultrafast Bloch simulator and a semisolid macromolecular magnetization transfer contrast (MTC) MR fingerprinting (MRF) reconstruction for estimation of MTC effects.

METHODS

The Bloch simulator and MRF reconstruction architectures were designed with recurrent neural networks and convolutional neural networks, evaluated with numerical phantoms with known ground truths and cross-linked bovine serum albumin phantoms, and demonstrated in the brain of healthy volunteers at 3 T. In addition, the inherent magnetization-transfer ratio asymmetry effect was evaluated in MTC-MRF, CEST, and relayed nuclear Overhauser enhancement imaging. A test-retest study was performed to evaluate the repeatability of MTC parameters, CEST, and relayed nuclear Overhauser enhancement signals estimated by the unified deep-learning framework.

RESULTS

Compared with a conventional Bloch simulation, the deep Bloch simulator for generation of the MTC-MRF dictionary or a training data set reduced the computation time by 181-fold, without compromising MRF profile accuracy. The recurrent neural network-based MRF reconstruction outperformed existing methods in terms of reconstruction accuracy and noise robustness. Using the proposed MTC-MRF framework for tissue-parameter quantification, the test-retest study showed a high degree of repeatability in which the coefficients of variance were less than 7% for all tissue parameters.

CONCLUSION

Bloch simulator-driven, deep-learning MTC-MRF can provide robust and repeatable multiple-tissue parameter quantification in a clinically feasible scan time on a 3T scanner.

Understanding aqueous and non-aqueous proton T1 relaxation in brain

A full picture of longitudinal relaxation in complex heterogeneous environments like white matter brain tissue remains elusive. In tissue, successive approximations, from the solvation layer model to the two pool model, have highlighted how longitudinal magnetization evolution depends on both inter-compartmental exchange and spin-lattice relaxation. In white matter, however, these models fail to capture the behaviour of the two distinct aqueous pools, myelin water and intra/extra-cellular water. A challenge with testing more comprehensive multi-pool models lies in directly observing all pools, both aqueous and non-aqueous. In this work, we advance these efforts by integrating three main experimental and analytical elements: direct observation of the longitudinal relaxation of both the aqueous and the non-aqueous protons in white matter, a wide range of different initial conditions, and application of an analysis pipeline which includes lineshape, CPMG, and fitting of a four pool model. An eigenvector interpretation of the four pool model highlights how longitudinal relaxation in white matter depends on initial conditions. We find that a single set of model parameters is able to describe the entire range of relaxation behaviour observed in all the separable aqueous and non-aqueous pools in experiments involving six different initial conditions. Understanding of the nature and connectedness of the tissue components is crucial in the design and interpretation of many MRI measurements, especially those based on magnetization transfer and longitudinal relaxation. In particular, the dependency of relaxation behaviour on initial conditions is likely the basis for understanding method-dependent discrepancies in in vivo T₁.

Magnetization transfer in liposome and proteoliposome samples that mimic the protein and lipid composition of myelin

Although magnetization transfer (MT) has been widely used in brain MRI, for example in brain inflammation and multiple sclerosis, the detailed molecular origin of MT effects and the role that proteins play in MT remain unclear. In this work, a proteoliposome model system was used to mimic the myelin environment and to examine the roles of protein, cholesterol, brain cerebrosides, and sphingomyelin embedded in the liposome matrix. Exchange parameters were determined using a double-quantum filter experiment. The goal was to determine the relative contributions to exchange and MT of cerebrosides, sphingomyelin, cholesterol, and proteins in 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayers. The main finding was that cerebrosides produced the strongest exchange effects, and that these were even more pronounced than those found for proteins. Sphingomyelin (which also has exchangeable groups at the head of the fatty acid chains, albeit closer to the lipid acyl chains) and cholesterol showed only minimal transfer. Overall, the extracted exchange rates appeared much smaller than commonly assumed for -OH and -NH groups.

Probing chemical exchange using quantitative spin-lock R1ρ asymmetry imaging with adiabatic RF pulses

PURPOSE

CEST is commonly used to probe the effects of chemical exchange. Although R_1ρ asymmetry quantification has also been described as a promising option for detecting the effects of chemical exchanges, the existing acquisition approaches are highly susceptible to B₁ RF and B₀ field inhomogeneities. To address this problem, we report a new R_1ρ asymmetry imaging approach, AC-iTIP, which is based on the previously reported techniques of irradiation with toggling inversion preparation (iTIP) and adiabatic continuous wave constant amplitude spin-lock RF pulses (ACCSL). We also derived the optimal spin-lock RF pulse B₁ amplitude that yielded the greatest R_1ρ asymmetry.

METHODS

Bloch-McConnell simulations were used to verify the analytical formula derived for the optimal spin-lock RF pulse B₁ amplitude. The performance of the AC-iTIP approach was compared to that of the iTIP approach based on hard RF pulses and the R_1ρ -spectrum acquired using adiabatic RF pulses with the conventional fitting method. Comparisons were performed using Bloch-McConnell simulations, phantom, and in vivo experiments at 3.0T.

RESULTS

The analytical prediction of the optimal B₁ was validated. Compared to the other 2 approaches, the AC-iTIP approach was more robust under the influences of B₁ RF and B₀ field inhomogeneities. A linear relationship was observed between the measured R_1ρ asymmetry and the metabolite concentration.

CONCLUSION

The AC-iTIP approach could probe the chemical exchange effect more robustly than the existing R_1ρ asymmetry acquisition approaches. Therefore, AC-iTIP is a promising technique for metabolite imaging based on the chemical exchange effect.

Protein aggregation linked to Alzheimer's disease revealed by saturation transfer MRI

The goal of this study was to develop a molecular biomarker for the detection of protein aggregation involved in Alzheimer's disease (AD) by exploiting the features of the water saturation transfer spectrum (Z-spectrum), the CEST signal of which is sensitive to the molecular configuration of proteins. A radial-sampling steady-state sequence based ultrashort echo time (UTE) readout was implemented to image the Z-spectrum in the mouse brain, especially the contributions from mobile proteins at the frequency offsets for the composite protein amide proton (+3.6 ppm) and aliphatic proton (-3.6 ppm) signals. Using a relatively weak radiofrequency (RF) saturation amplitude, contributions due to strong magnetization transfer contrast (MTC) from solid-like macromolecules and direct water saturation (DS) were minimized. For practical measure of the changes in the mobile protein configuration, we defined a saturation transfer difference (ΔST) by subtracting the Z-spectral signals at ±3.6 ppm from a control signal at 8 ppm. Phantom studies of glutamate solution, protein (egg white) and hair conditioner show the capability of the proposed scheme to minimize the contributions from amine protons, DS, and MTC, respectively. The ST signal at ±3.6 ppm of the cross-linked bovine serum albumin (BSA) solutions demonstrated that the ΔST signal can be used to monitor the aggregation process of the mobile proteins. High-resolution ΔST images of AD mouse brains at ±3.6 ppm of mouse brains showed significantly reduced ΔST (-3.6) signal compared to the age-matched wild-type (WT) mice. Thus, this signal has potential to serve as a molecular biomarker for monitoring protein aggregation in AD.

Physical and numerical phantoms for the validation of brain microstructural MRI: A cookbook

Phantoms, both numerical (software) and physical (hardware), can serve as a gold standard for the validation of MRI methods probing the brain microstructure. This review aims to provide guidelines on how to build, implement, or choose the right phantom for a particular application, along with an overview of the current state-of-the-art of phantoms dedicated to study brain microstructure with MRI. For physical phantoms, we discuss the essential requirements and relevant characteristics of both the (NMR visible) liquid and (NMR invisible) phantom materials that induce relevant microstructural features detectable via MRI, based on diffusion, intra-voxel incoherent motion, magnetization transfer or magnetic susceptibility weighted contrast. In particular, for diffusion MRI, many useful phantoms have been proposed, ranging from simple liquids to advanced biomimetic phantoms consisting of hollow or plain microfibers and capillaries. For numerical phantoms, the focus is on Monte Carlo simulations of random walk, for which the basic principles, along with useful criteria to check and potential pitfalls are reviewed, in addition to a literature overview highlighting recent advances. While many phantoms exist already, the current review aims to stimulate further research in the field and to address remaining needs.

Separating fast and slow exchange transfer and magnetization transfer using off-resonance variable-delay multiple-pulse (VDMP) MRI

PURPOSE

To develop a method that can separate and quantify the fast (>1 kHz) and slow exchange transfer and magnetization transfer components in Z-spectra.

METHODS

Z-spectra were recorded as a function of mixing time using a train of selective pulses providing variable-delay multipulse build-up curves. Fast and slow transfer components in the Z-spectra were separated and quantified on a voxel-by-voxel basis by fitting the mixing time-dependent CEST signal using a 3-pool model.

RESULTS

Phantom studies of glutamate solution, bovine serum albumin solution, and hair conditioner showed the capability of the proposed method to separate fast and slow transfer components. In vivo mouse brain studies showed a strong contrast between white matter and gray matter in the slow-transferring map, corresponding to an asymmetric component of the conventional semisolid magnetization transfer contrast. In addition, a fast-transferring proton map was found that was homogeneous across the brain and attributed to the total contributions of the fast-exchanging protons from proteins, metabolites, and a symmetric magnetization transfer contrast component.

CONCLUSIONS

This new method provides a simple way to extract fast and slow transfer components from the Z-spectrum, leading to novel MRI contrasts, and providing insight into the different magnetization transfer contrast contributions.

Investigation of the contribution of total creatine to the CEST Z-spectrum of brain using a knockout mouse model

The current study aims to assign and estimate the total creatine (tCr) signal contribution to the Z-spectrum in mouse brain at 11.7 T. Creatine (Cr), phosphocreatine (PCr) and protein phantoms were used to confirm the presence of a guanidinium resonance at this field strength. Wild-type (WT) and knockout mice with guanidinoacetate N-methyltransferase deficiency (GAMT-/-), which have low Cr and PCr concentrations in the brain, were used to assign the tCr contribution to the Z-spectrum. To estimate the total guanidinium concentrations, two pools for the Z-spectrum around 2 ppm were assumed: (i) a Lorentzian function representing the guanidinium chemical exchange saturation transfer (CEST) at 1.95 ppm in the 11.7-T Z-spectrum; and (ii) a background signal that can be fitted by a polynomial function. Comparison between the WT and GAMT-/- mice provided strong evidence for three types of contribution to the peak in the Z-spectrum at 1.95 ppm, namely proteins, Cr and PCr, the latter fitted as tCr. A ratio of 20 ± 7% (protein) and 80 ± 7% tCr was found in brain at 2 μT and 2 s saturation. Based on phantom experiments, the tCr peak was estimated to consist of about 83 ± 5% Cr and 17 ± 5% PCr. Maps for tCr of mouse brain were generated based on the peak at 1.95 ppm after concentration calibration with in vivo magnetic resonance spectroscopy.

Spectral characteristics of semisolid protons in human brain white matter at 7 T

PURPOSE

To inform the quantification of MRI magnetization transfer contrast at high field by measuring the spectral characteristics of 1 H protons in semisolids in human brain at 7 T, while avoiding prohibitive radiofrequency (RF) tissue heating and confounding effects from chemical exchange.

METHODS

Saturation-recovery type experiments were performed using brief, frequency-specific RF pulses that saturate semisolid proton magnetization. Analysis of the subsequent recovery of water proton magnetization with a two-pool model of exchange allowed the study of spectral characteristics of semisolid protons.

RESULTS

We show that in white matter, the semisolid proton spectrum can be approximated with a symmetric, super-Lorentzian line at -2.58 ± 0.05 ppm from the water resonance and an average transverse relaxation time constant (T2 ) of 9.6 ± 0.6 μs.

CONCLUSIONS

These results are consistent with studies at lower field that have indicated a major contribution from methylene protons to magnetization transfer contrast, and will facilitate the design and quantification of magnetization transfer studies at 7 T. Magn Reson Med 78:1950-1958, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Magnetization transfer in a partly deuterated lyotropic liquid crystal by single- and dual-frequency RF irradiations

The mechanism of magnetization transfer (MT) in a lyotropic liquid crystal made of sodium dodecyl sulfate, decanol, and water molecules is investigated by using deuterated molecules and single- and dual-frequency RF irradiations. The resulting Z-spectra suggest that the decanol molecules are mainly responsible for the MT effects in this system, through proton exchange to water. This is further confirmed by monitoring the relaxation of dipolar order, which allows one to estimate the transfer rate of magnetization from decanol to water. The potential benefits of using dual-frequency RF irradiation for inducing MT effects are explored through numerical solutions to a MT model based on Provotorov's partial saturation theory.

Optimization of inhomogeneous magnetization transfer (ihMT) MRI contrast for preclinical studies using dipolar relaxation time (T1D ) filtering

A pulsed inhomogeneous magnetization transfer (ihMT)-prepared fast imaging sequence was implemented at 11.75 T for preclinical studies on mouse central nervous system. A strategy based on filtering the ihMT signal originating from short dipolar relaxation time (T_1D ) components is proposed. It involves increasing the repetition time of consecutive radiofrequency (RF) pulses of the dual saturation and allows improved signal specificity for long T_1D myelinated structures. Furthermore, frequency offset, power and timing saturation parameters were adjusted to optimize the ihMT sensitivity. The optimization of the ihMT sensitivity, whilst preserving the strong specificity for the long T_1D component of myelinated tissues, allowed measurements of ihMT ratios on the order of 4-5% in white matter (WM), 2.5% in gray matter (GM) and 1-1.3% in muscle. This led to high relative ihMT contrasts between myelinated tissues and others (~3-4 between WM and muscle, and ≥2 between GM and muscle). Conversely, higher ihMT ratios (~6-7% in WM) could be obtained using minimal T_1D filtering achieved with short saturation pulse repetition time or cosine-modulated pulses for the dual-frequency saturation. This study represents a first stage in the process of validating ihMT as a myelin biomarker by providing optimized ihMT preclinical sequences, directly transposable and applicable to other preclinical magnetic fields and scanners. Finally, ihMT ratios measured in various central nervous system areas are provided for future reference.

The physical mechanism of "inhomogeneous" magnetization transfer MRI

Inhomogeneous MT (ihMT) is a new magnetic resonance imaging technique that shows promise for myelin selectivity. Materials with a high proportion of lipids, such as white matter tissue, show a reduced intensity in magnetic resonance images acquired with selective prepulses at positive and negative offsets simultaneously compared to images with a single positive or negative offset prepulse of the same power. This effect was initially explained on the basis of hole-burning in inhomogeneously broadened lines of the lipid proton spin system. Our results contradict this explanation. ihMT in lipids can be understood with a simple spin-1 model of a coupled methylene proton pair. More generally, Provotorov theory can be used to consider the evolution of dipolar order in the non-aqueous spins during the prepulses. We show that the flip-angle dependence of the proton spectrum of a model lipid system (Prolipid-161) following dipolar order generation is in quantitative agreement with the model. In addition, we directly observe dipolar order and ihMT signals in the non-aqueous components of Prolipid-161 and homogeneously-broadened systems (hair, wood, and tendon) following ihMT prepulses. The observation of ihMT signals in tendon suggests that the technique may not be as specific to myelin as previously thought. Our work shows that ihMT occurs because of dipolar couplings alone, not from a specific type of spectral line broadening as its name suggests.

In vivo measurement of a new source of contrast, the dipolar relaxation time, T1D , using a modified inhomogeneous magnetization transfer (ihMT) sequence

PURPOSE

This paper describes a technique that can be used in vivo to measure the dipolar relaxation time, T_1D , of macromolecular protons contributing to magnetization transfer (MT) in tissues and to produce quantitative T_1D maps.

THEORY AND METHODS

The technique builds upon the inhomogeneous MT (ihMT) technique that is particularly sensitive to tissue components with long T_1D . A standard ihMT experiment was altered to introduce a variable time for switching between positive and negative offset frequencies for RF saturation. A model for the dependence of ihMT was developed and used to fit data acquired in vivo.

RESULTS

Application of the method to images from brains of healthy volunteers produced values of T_1D  = (5.9 ± 1.2) ms in gray matter and T_1D  = (6.2 ± 0.4) ms in white matter regions and provided maps of the T_1D parameter.

CONCLUSION

The model and experiments described provide access to a new relaxation characteristic of tissue with potentially unique diagnostic information. Magn Reson Med 78:1362-1372, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Rapid measurement of brain macromolecular proton fraction with transient saturation transfer MRI

PURPOSE

To develop an efficient MRI approach to estimate the nonwater proton fraction (f) in human brain.

METHODS

We implement a brief, efficient magnetization transfer (MT) pulse that selectively saturates the magnetization of the (semi-) solid protons, and monitor the transfer of this saturation to the water protons as a function of delay after saturation.

RESULTS

Analysis of the transient MT effect with two-pool model allowed robust extraction of f at both 3 and 7 T. This required estimating the longitudinal relaxation rate constant (R_1,MP and R_1,WP ) for both proton pools, which was achieved with the assumption of uniform R_1,MP and R_1,WP across brain tissues. Resulting values of f were approximately 50% higher than reported previously, which is partly attributed to MT-pulse efficiency and R_1,MP being higher than assumed previously.

CONCLUSION

Experiments performed on human brain in vivo at 3 and 7 T demonstrate the ability of the method to robustly determine f in a scan time of approximately 5 min. Magn Reson Med 77:2174-2185, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

On-resonance variable delay multipulse scheme for imaging of fast-exchanging protons and semisolid macromolecules

PURPOSE

To develop an on-resonance variable delay multipulse (VDMP) scheme to image magnetization transfer contrast (MTC) and the chemical exchange saturation transfer (CEST) contrast of total fast-exchanging protons (TFP) with exchange rate above approximately 1 kHz.

METHODS

A train of high power binomial pulses was applied at the water resonance. The interpulse delay, called mixing time, was varied to observe its effect on the water signal reduction, allowing separation and quantification of MTC and CEST contributions as a result of their different proton transfer rates. The fast-exchanging protons in CEST and MTC are labeled together with the short T₂ components in MTC and separated out using a variable mixing time.

RESULTS

Phantom studies of selected metabolite solutions (glucose, glutamate, creatine, myo-inositol), bovine serum albumin (BSA), and hair conditioner show the capability of on-resonance VDMP to separate out exchangeable protons with exchange rates above 1 kHz. Quantitative MTC and TFP maps were acquired on healthy mouse brains using this method, showing strong gray/white matter contrast for the slowly transferring MTC protons, whereas the TFP map was more uniform across the brain but somewhat higher in gray matter.

CONCLUSIONS

The new method provides a simple way of imaging fast-exchanging protons and MTC components with a slow transfer rate. Magn Reson Med 77:730-739, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

A combined analytical solution for chemical exchange saturation transfer and semi-solid magnetization transfer

Off-resonant RF irradiation in tissue indirectly lowers the water signal by saturation transfer processes: on the one hand, there are selective chemical exchange saturation transfer (CEST) effects originating from exchanging endogenous protons resonating a few parts per million from water; on the other hand, there is the broad semi-solid magnetization transfer (MT) originating from immobile protons associated with the tissue matrix with kilohertz linewidths. Recently it was shown that endogenous CEST contrasts can be strongly affected by the MT background, so corrections are needed to derive accurate estimates of CEST effects. Herein we show that a full analytical solution of the underlying Bloch-McConnell equations for both MT and CEST provides insights into their interaction and suggests a simple means to isolate their effects. The presented analytical solution, based on the eigenspace solution of the Bloch-McConnell equations, extends previous treatments by allowing arbitrary lineshapes for the semi-solid MT effects and simultaneously describing multiple CEST pools in the presence of a large MT pool for arbitrary irradiation. The structure of the model indicates that semi-solid MT and CEST effects basically add up inversely in determining the steady-state Z-spectrum, as previously shown for direct saturation and CEST effects. Implications for existing previous CEST analyses in the presence of a semi-solid MT are studied and discussed. It turns out that, to accurately quantify CEST contrast, a good reference Z-value, the observed longitudinal relaxation rate of water, and the semi-solid MT pool size fraction must all be known.