Lipid bilayer and water proton magnetization transfer: effect of cholesterol

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.

The relayed nuclear Overhauser effect in magnetization transfer and chemical exchange saturation transfer MRI

Magnetic resonance (MR) is a powerful technique for noninvasively probing molecular species in vivo but suffers from low signal sensitivity. Saturation transfer (ST) MRI approaches, including chemical exchange saturation transfer (CEST) and conventional magnetization transfer contrast (MTC), allow imaging of low-concentration molecular components with enhanced sensitivity using indirect detection via the abundant water proton pool. Several recent studies have shown the utility of chemical exchange relayed nuclear Overhauser effect (rNOE) contrast originating from nonexchangeable carbon-bound protons in mobile macromolecules in solution. In this review, we describe the mechanisms leading to the occurrence of rNOE-based signals in the water saturation spectrum (Z-spectrum), including those from mobile and immobile molecular sources and from molecular binding. While it is becoming clear that MTC is mainly an rNOE-based signal, we continue to use the classical MTC nomenclature to separate it from the rNOE signals of mobile macromolecules, which we will refer to as rNOEs. Some emerging applications of the use of rNOEs for probing macromolecular solution components such as proteins and carbohydrates in vivo or studying the binding of small substrates are discussed.

High-resolution magnetization-transfer imaging of post-mortem marmoset brain: Comparisons with relaxometry and histology

Cell membranes and macromolecules or paramagnetic compounds interact with water proton spins, which modulates magnetic resonance imaging (MRI) contrast providing information on tissue composition. For a further investigation, quantitative magnetization transfer (qMT) parameters (at 3T), including the ratio of the macromolecular and water proton pools, F, and the exchange-rate constant as well as the (observed) longitudinal and the effective transverse relaxation rates (at 3T and 7T), R₁^obs and R₂^*, respectively, were measured at high spatial resolution (200 µm) in a slice of fixed marmoset brain and compared to histology results obtained with Gallyas' myelin stain and Perls' iron stain. R₁^obs and R₂^* were linearly correlated with the iron content for the entire slice, whereas distinct differences were obtained between gray and white matter for correlations of relaxometry and qMT parameters with myelin content. The combined results suggest that the macromolecular pool interacting with water consists of myelin and (less efficient) non-myelin contributions. Despite strong correlation of F and R₁^obs, none of these parameters was uniquely specific to myelination. Due to additional sensitivity to iron stores, R₁^obs and R₂^* were more sensitive for depicting microstructural differences between cortical layers than F.

Obesity-Related Neuroinflammation: Magnetic Resonance and Microscopy Imaging of the Brain

Obesity is a major risk factor of Alzheimer's disease and related dementias. The principal feature of dementia is a loss of neurons and brain atrophy. The mechanistic links between obesity and the neurodegenerative processes of dementias are not fully understood, but recent research suggests that obesity-related systemic inflammation and subsequent neuroinflammation may be involved. Adipose tissues release multiple proinflammatory molecules (fatty acids and cytokines) that impact blood and vessel cells, inducing low-grade systemic inflammation that can transition to tissues, including the brain. Inflammation in the brain-neuroinflammation-is one of key elements of the pathobiology of neurodegenerative disorders; it is characterized by the activation of microglia, the resident immune cells in the brain, and by the structural and functional changes of other cells forming the brain parenchyma, including neurons. Such cellular changes have been shown in animal models with direct methods, such as confocal microscopy. In humans, cellular changes are less tangible, as only indirect methods such as magnetic resonance (MR) imaging are usually used. In these studies, obesity and low-grade systemic inflammation have been associated with lower volumes of the cerebral gray matter, cortex, and hippocampus, as well as altered tissue MR properties (suggesting microstructural variations in cellular and molecular composition). How these structural variations in the human brain observed using MR imaging relate to the cellular variations in the animal brain seen with microscopy is not well understood. This review describes the current understanding of neuroinflammation in the context of obesity-induced systemic inflammation, and it highlights need for the bridge between animal microscopy and human MR imaging studies.

Brain development in children with developmental delay using amide proton transfer-weighted imaging and magnetization transfer imaging

IMPORTANCE

The process of brain development in children with developmental delay is not well known. Amide proton transfer-weighted (APTw) imaging is a novel molecular magnetic resonance imaging (MRI) technique that can noninvasively detect cytosolic endogenous mobile proteins and peptides involved in the myelination process, and may be useful for providing insights into brain development.

OBJECTIVE

To assess the contribution of amide proton transfer-weighted (APTw) imaging and magnetization transfer (MT) imaging to the evaluation of children with developmental delay (DD).

METHODS

Fifty-one patients with DD were recruited to this study. The patients were divided into two groups according to the state of myelination assessed on conventional magnetic resonance imaging (MRI). Thirty patients (10 girls, 20 boys; age range: 1-8 months; median age: 4 months) in group A showed delayed myelination on MRI, while 21 patients (3 girls, 18 boys; age range: 12-36months; median age: 25months) in group B showed normal myelination on MRI. Fifty-one age- and sex-matched children with normal developmental quotient (DQ) and normal MRI appearance were recruited as normal controls. Three-slice APTw/MT axial imaging was performed at the level of the centrum semiovale, the basal ganglia and the pons. Quantitative data of the MT ratio (MTR) and APTw were analyzed for multiple brain regions. Independent-sample t-tests were used to compare differences in APTw and MTR signals between the two DD groups and normal controls. Analysis of Covariance was conducted to correct the statistical results. The level of statistical significance was set to P < 0.05.

RESULTS

For group A, the MTR values were lower in all regions (P = 0.004-0.033) compared with the normal controls, while the APTw values were higher in the pons, middle cerebellar peduncle, corpus callosum, frontal white matter, occipital white matter and centrum semiovale (P = 0.004-0.040 ). For Group B, the MTR values were slightly reduced, and the APTw values were slightly increased compared with the normal controls, but the differences were not statistically significant (P > 0.05).

INTERPRETATION

For DD patients showing signs of delayed myelination on MRI, MTR and APTw imaging can help to diagnose myelination delay by quantifying semi-solid macromolecules and cytosolic endogenous mobile proteins and peptides at a molecular level, providing a new method for comprehensive evaluation of DD. For DD patients with normal myelination on MRI, the clinical values of MTR and APTw imaging remain to be explored.

Role of the cholesterol hydroxyl group in the chemical exchange saturation transfer signal at -1.6 ppm

Chemical exchange saturation transfer (CEST) can provide metabolite-weighted images in the clinical setting; therefore, understanding the origin of each CEST signal is essential to revealing the changes in diseases at the molecular level, which would provide further insight for diagnoses and treatments. The CEST signal at -1.6 ppm is attributed to the choline methyl group of phosphatidylcholines. The methyl groups have no exchangeable protons, so the corresponding CEST signals must result from the relayed nuclear Overhauser effect (rNOE); however, the detailed mechanism remains unclear. Cholesterol is a major component of biological membranes, and its content is closely related to the dynamics and phases of these lipids. However, cholesterol has a hydroxyl group, which could participate in proton exchange to complete the rNOE process. In this study, we used liposomes containing cholesterol and its analogs (5α-cholestane and progesterone), which presumably have similar capabilities of influencing lipid bilayers, and found that the steroid hydroxyl group is the key to inducing the rNOE at -1.6 ppm. Our results suggest that the origin of the rNOE at -1.6 ppm likely requires an intermolecular NOE between the proton of the choline methyl group and that of the cholesterol hydroxyl group, and a chemical exchange between the cholesterol hydroxyl group and bulk water. However, the phenomenon in which the rNOE at -1.6 ppm appears when the cholesterol concentration is high seems to contradict the in vivo results, suggesting a more complicated mechanism associated with the rNOE at -1.6 ppm in biological membranes.

A phantom system for assessing the effects of membrane lipids on water proton relaxation

Quantitative MRI (qMRI) is a method for the non-invasive study of brain-structure-associated changes expressed with measurable units. The qMRI-derived parameters have been shown to reflect brain tissue composition such as myelin content. Nevertheless, it remains a major challenge to identify and quantify the contributions of specific molecular components to the MRI signal. Here, we describe a phantom system that can be used to evaluate the contribution of membrane lipids to qMRI-derived parameters. We used a hydration-dehydration dry film technique to formulate liposomes that can be used as a model of the bilayer lipid membrane. The liposomes were comprised of the most abundant types of lipid found in the human brain. We then applied clinically available qMRI techniques with adjusted bias corrections in order to test the ability of the phantom system to estimate multiple qMRI parameters such as proton density (PD), T₁ , T₂ , T₂ * and magnetization transfer. In addition, we accurately measured the phantom sample water fraction (normalized PD). A similar protocol was also applied to the human brain in vivo. The phantom system allows for a reliable estimation of qMRI parameters for phantoms composed of various lipid types using a clinical MRI scanner. We also found a comparable reproducibility between the phantom and in vivo human brain qMRI estimations. To conclude, we have successfully created a biologically relevant liposome phantom system whose lipid composition can be fully controlled. Our lipid system and analysis can be used to measure the contributions to qMRI parameters of membrane lipids found in the human brain under scanning conditions that are relevant to in vivo human brain scans. Such a model system can be used to test the contributions of lipidomic changes in normal and pathological brain states.

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.

Magnetic resonance imaging of brain cell water

In the central nervous system of vertebrates, cell bodies of neurons are often assembled as nuclei or cellular layers that play specific roles as functional units. The purpose of this work was to selectively highlight such cell assemblies by magnetic resonance imaging using signals from water protons that are associated with intracellular paramagnetic ions, while saturating lipid-associated water protons as well as extracellular free water protons. Given the significant correlation between image signal intensity and water proton density, the high signal intensities observed for such cell assemblies must be attributed to their abundant paramagnetic-ion-associated water protons. In the hippocampal formation, the technique visualized cell assemblies that were so far not depicted in human in vivo. In the brainstem, the technique delineated noradrenergic neuron groups such as the locus coeruleus in human and mice in vivo. Their reduced magnetization-transfer ratios together with their prolonged relaxation times compared to other gray matter indicate that the source of their high signal intensity is not the presence of T₁-shortening molecules, e.g., neuromelanin, but their high water content. Given the general absence of neuromelanin in noradrenergic neurons of rodents, their high signal intensity in mice in vivo further supports this view.

Amid proton transfer (APT) and magnetization transfer (MT) MRI contrasts provide complimentary assessment of brain tumors similarly to proton magnetic resonance spectroscopy imaging (MRSI)

OBJECTIVES

Using MRSI as comparison, we aimed to explore the difference between amide proton transfer (APT) MRI and conventional semi-solid magnetization transfer ratio (MTR) MRI, and to investigate if molecular APT and structural MTR can provide complimentary information in assessing brain tumors.

METHODS

Seventeen brain tumor patients and 17 age- and gender-matched volunteers were included and scanned with anatomical MRI, APT and MT-weighted MRI, and MRSI. Multi-voxel choline (Cho) and N-acetylaspartic acid (NAA) signals were quantified from MRSI and compared with MTR and MTRasym(3.5ppm) contrasts averaged from corresponding voxels. Correlations between contrasts were explored voxel-by-voxel by pooling values from all voxels into Pearson's correlation analysis. Differences in correlation coefficients were tested with the Z-test (set at p<0.05).

RESULTS

APT and MT provide good contrast and quantitative parameters in tumor imaging, as do the metabolite (Cho and NAA) maps. MTRasym(3.5ppm) significantly correlated with MTR (R=-0.61, p<0.0001), Cho (R=0.568, p<0.0001) and NAA (R=-0.619, p<0.0001) in tumors, and MTR also significantly correlated with Cho (R=-0.346, p<0.0001) and NAA (R=0.624, p<0.0001). In healthy volunteers, MTRasym(3.5ppm) was non-significantly correlated with MTR (R=-0.049, p=0.239), Cho (R=0.030, p=0.478) and NAA (R=-0.083, p=0.046). Significant correlations were found among MTR with Cho (R=0.199, p<0.0001) and NAA (R=0.263, p<0.0001) in the group of healthy volunteers with lower correlation R values than those in tumor patients.

CONCLUSIONS

APT and MT could provide independent and supplementary information for the comprehensive assessment of molecular and structural changes due to brain tumor cancerogenesis.

KEY POINTS

• MTR asym(3.5ppm) positively correlated with Cho while negatively with NAA in tumors. • MTR positively correlated with NAA while negatively with Cho in tumors. • Combining APT/MT provides molecular and structural information similarly to MRSI.

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.

Multi-parametric quantitative MRI reveals three different white matter subtypes

INTRODUCTION

Magnetic resonance imaging (MRI) shows slight spatial variations in brain white matter (WM). We used quantitative multi-parametric MRI to evaluate in what respect these inhomogeneities could correspond to WM subtypes with specific characteristics and spatial distribution.

MATERIALS AND METHODS

Twenty-six controls (12 women, 38 ±9 Y) took part in a 60-min session on a 3T scanner measuring 7 parameters: R1 and R2, diffusion tensor imaging which allowed to measure Axial and Radial Diffusivity (AD, RD), magnetization transfer imaging which enabled to compute the Macromolecular Proton Fraction (MPF), and a susceptibility-weighted sequence which permitted to quantify R2* and magnetic susceptibility (χm). Spatial independent component analysis was used to identify WM subtypes with specific combination of quantitative parameters values.

RESULTS

Three subtypes could be identified. t-WM (track) mostly mapped on well-formed projection and commissural tracts and came with high AD values (all p < 10(-18)). The two other subtypes were located in subcortical WM and overlapped with association fibers: f-WM (frontal) was mostly anterior in the frontal lobe whereas c-WM (central) was underneath the central cortex. f-WM and c-WM had higher MPF values, indicating a higher myelin content (all p < 1.7 10(-6)). This was compatible with their larger χm and R2, as iron is essentially stored in oligodendrocytes (all p < 0.01). Although R1 essentially showed the same, its higher value in t-WM relative to c-WM might be related to its higher cholesterol concentration.

CONCLUSIONS

Thus, f- and c-WMs were less structured, but more myelinated and probably more metabolically active regarding to their iron content than WM related to fasciculi (t-WM). As known WM bundles passed though different WM subtypes, myelination might not be uniform along the axons but rather follow a spatially consistent regional variability. Future studies might examine the reproducibility of this decomposition and how development and pathology differently affect each subtype.

Clinical implications of myelin regeneration in the central nervous system

INTRODUCTION

Amongst strategies to repair the brain, myelin repair offers genuine cause for optimism. Myelin, which sheaths most axons in the central nervous system (CNS), is vital for normal neurological function, as demonstrated by the functional deficits that accrue when it is absent in a range of debilitating myelin diseases. Following demyelination, post-mortem and imaging studies have shown that extensive regeneration of myelin is possible in the human brain. Over recent decades preclinical research has given us a strong understanding of the biology of myelin regeneration, opening up several exciting therapeutic opportunities that are on the cusp of clinical translation. Areas covered: This review discusses diseases that compromise the function of myelin, the endogenous capacity of the CNS to regenerate myelin, and why this sometimes fails. We then outline the extensive progress that has been made towards therapies that promote the regeneration of myelin. Expert commentary: Finally, a commentary on the first examples of these therapies to reach human patients and the evidence base that supports them, giving our opinion on where attention should be focused going forward is provided.

Advances in noninvasive myelin imaging

Myelin is important for the normal development and healthy function of the nervous system. Recent developments in MRI acquisition and tissue modeling aim to provide a better characterization and more specific markers for myelin. This allows for specific monitoring of myelination longitudinally and noninvasively in the healthy brain as well as assessment of treatment and intervention efficacy. Here, we offer a nontechnical review of MRI techniques developed to specifically monitor myelin such as magnetization transfer (MT) and myelin water imaging (MWI). We further summarize recent studies that employ these methods to measure myelin in relation to development and aging, learning and experience, and neuropathology and psychiatric disorders. © 2017 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 78: 136–151, 2018

MR imaging of a novel NOE-mediated magnetization transfer with water in rat brain at 9.4 T

PURPOSE

To detect, map, and quantify a novel nuclear Overhauser enhancement (NOE)-mediated magnetization transfer (MT) with water at approximately -1.6 ppm [NOE(-1.6)] in rat brain using MRI.

METHODS

Continuous wave MT sequences with a variety of radiofrequency irradiation powers were optimized to achieve the maximum contrast of this NOE(-1.6) effect at 9.4 T. The distribution of effect magnitudes, resonance frequency offsets, and line widths in healthy rat brains and the differences of the effect between tumors and contralateral normal brains were imaged and quantified using a multi-Lorentzian fitting method. MR measurements on reconstituted model phospholipids as well as two cell lines (HEK293 and 9L) were also performed to investigate the possible molecular origin of this NOE.

RESULTS

Our results suggest that the NOE(-1.6) effect can be detected reliably in rat brain. Pixel-wise fittings demonstrated the regional variations of the effect. Measurements in a rodent tumor model showed that the amplitude of NOE(-1.6) in brain tumor was significantly diminished compared with that in normal brain tissue. Measurements of reconstituted phospholipids suggest that this effect may originate from choline phospholipids.

CONCLUSION

NOE(-1.6) could be used as a new biomarker for the detection of brain tumor. Magn Reson Med 78:588-597, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Molecular, dynamic, and structural origin of inhomogeneous magnetization transfer in lipid membranes

PURPOSE

To elucidate the dynamic, structural, and molecular properties that create inhomogeneous magnetization transfer (ihMT) contrast.

METHODS

Amphiphilic lipids, lamellar phospholipids with cholesterol, and bovine spinal cord (BSC) specimens were examined along with nonlipid systems. Magnetization transfer (MT), enhanced MT (eMT, obtained with double-sided radiofrequency saturation), ihMT (MT - eMT), and dipolar relaxation, T_1D , were measured at 2.0 and 11.7 T.

RESULTS

The amplitude of ihMT ratio (ihMTR) is positively correlated with T_1D values. Both ihMTR and T_1D increase with increasing temperature in BSC white matter and in phospholipids and decrease with temperature in other lipids. Changes in ihMTR with temperature arise primarily from alterations in MT rather than eMT. Spectral width of MT, eMT, and ihMT increases with increasing carbon chain length.

CONCLUSIONS

Concerted motions of phospholipids in white matter decrease proton spin diffusion leading to increased proton T_1D times and increased ihMT amplitudes, consistent with decoupling of Zeeman and dipolar spin reservoirs. Molecular specificity and dynamic sensitivity of ihMT contrast make it a suitable candidate for probing myelin membrane disorders. Magn Reson Med 77:1318-1328, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

In Vivo Brain MR Imaging at Subnanoliter Resolution: Contrast and Histology

This article provides an overview of in vivo magnetic resonance (MR) imaging contrasts obtained for mammalian brain in relation to histological knowledge. Emphasis is paid to the (1) significance of high spatial resolution for the optimization of T1, T2, and magnetization transfer contrast, (2) use of exogenous extra- and intracellular contrast agents for validating endogenous contrast sources, and (3) histological structures and biochemical compounds underlying these contrasts and (4) their relevance to neuroradiology. Comparisons between MR imaging at subnanoliter resolution and histological data indicate that (a) myelin sheaths, (b) nerve cells, and (c) the neuropil are most responsible for observed MR imaging contrasts, while (a) diamagnetic macromolecules, (b) intracellular paramagnetic ions, and (c) extracellular free water, respectively, emerge as the dominant factors. Enhanced relaxation rates due to paramagnetic ions, such as iron and manganese, have been observed for oligodendrocytes, astrocytes, microglia, and blood cells in the brain as well as for nerve cells. Taken together, a plethora of observations suggests that the delineation of specific structures in high-resolution MR imaging of mammalian brain and the absence of corresponding contrasts in MR imaging of the human brain do not necessarily indicate differences between species but may be explained by partial volume effects. Second, paramagnetic ions are required in active cells in vivo which may reduce the magnetization transfer ratio in the brain through accelerated T1 recovery. Third, reductions of the magnetization transfer ratio may be more sensitive to a particular pathological condition, such as astrocytosis, microglial activation, inflammation, and demyelination, than changes in relaxation. This is because the simultaneous occurrence of increased paramagnetic ions (i.e., shorter relaxation times) and increased free water (i.e., longer relaxation times) may cancel T1 or T2 effects, whereas both processes reduce the magnetization transfer ratio.

Grey and White Matter Magnetisation Transfer Ratio Measurements in the Lumbosacral Enlargement: A Pilot In Vivo Study at 3T

Magnetisation transfer (MT) imaging of the central nervous system has provided further insight into the pathophysiology of neurological disease. However, the use of this method to study the lower spinal cord has been technically challenging, despite the important role of this region, not only for motor control of the lower limbs, but also for the neural control of lower urinary tract, sexual and bowel functions. In this study, the feasibility of obtaining reliable grey matter (GM) and white matter (WM) magnetisation transfer ratio (MTR) measurements within the lumbosacral enlargement (LSE) was investigated in ten healthy volunteers using a clinical 3T MRI system. The mean cross-sectional area of the LSE (LSE-CSA) and the mean GM area (LSE-GM-CSA) were first obtained by means of image segmentation and tissue-specific (i.e. WM and GM) MTR measurements within the LSE were subsequently obtained. The reproducibility of the segmentation method and MTR measurements was assessed from repeated measurements and their % coefficient of variation (%COV). Mean (± SD) LSE-CSA across 10 healthy subjects was 59.3 (± 8.4) mm2 and LSE-GM-CSA was 17.0 (± 3.1) mm2. The mean intra- and inter-rater % COV for measuring the LSE-CSA were 0.8% and 2.3%, respectively and for the LSE-GM-CSA were 3.8% and 5.4%, respectively. Mean (± SD) WM-MTR was 43.2 (± 4.4) and GM-MTR was 40.9 (± 4.3). The mean scan-rescan % COV for measuring WM-MTR was 4.6% and for GM-MTR was 3.8%. Using a paired t-test, a statistically significant difference was identified between WM-MTR and GM-MTR in the LSE (p<0.0001). This pilot study has shown that it is possible to obtain reliable tissue-specific MTR measurements within the LSE using a clinical MR system at 3T. The MTR acquisition and analysis protocol presented in this study can be used in future investigations of intrinsic spinal cord diseases that affect the LSE.

Oxidation-responsive Eu(2+/3+)-liposomal contrast agent for dual-mode magnetic resonance imaging

An oxidation-responsive contrast agent for magnetic resonance imaging was synthesized using Eu(2+) and liposomes. Positive contrast enhancement was observed with Eu(2+), and chemical exchange saturation transfer was observed before and after oxidation of Eu(2+). Orthogonal detection modes render the concentration of Eu inconsequential to molecular information provided through imaging.

Magnetization transfer in lamellar liquid crystals

PURPOSE

This study examines the relationship between quantitative magnetization transfer (qMT) parameters and the molecular composition of a model lamellar liquid crystal (LLC) system composed of 1-decyl alcohol (decanol), sodium dodecyl sulfate (SDS), and water.

METHODS

Samples were made within a stable lamellar mesophase to provide different ratios of total semisolid protons (SDS + decanol) to water protons. Data were collected as a function of radiofrequency power, frequency offset, and temperature. qMT parameters were estimated by fitting a standard model to the data. Fitting results of four different semisolid line shapes were compared.

RESULTS

A super-Lorentzian line shape for the semisolid component provided the best fit. The estimated amount of semisolids was proportional to the ratio of decanol-to-water protons. Other qMT parameters exhibited nonlinear dependence on sample composition. Magnetization transfer ratio (MTR) was a linear function of the semisolid fraction over a limited range of decanol concentration.

CONCLUSION

In LLC samples, MT between semisolid and water originates from intramolecular nOe among decanol aliphatic chain protons followed by proton exchange between decanol hydroxyl and water. Exchange kinetics is influenced by SDS, although SDS protons do not participate in MT. These studies provide clinically relevant range of semisolid fraction proportional to detected MTR.

Magnetic resonance functional nano-hydroxyapatite incorporated poly(caprolactone) composite scaffolds for in situ monitoring of bone tissue regeneration by MRI

In this study, we have reported the incorporation of a multi-modal contrast agent based on hydroxyapatite nanocrystals, within a poly(caprolactone)(PCL) nanofibrous scaffold by electrospinning. The multifunctional hydroxyapatite nanoparticles (MF-nHAp) showed simultaneous contrast enhancement for three major molecular imaging techniques. In this article, the magnetic resonance (MR) contrast enhancement ability of the MF-nHAp was exploited for the purpose of potentially monitoring as well as for influencing tissue regeneration. These MF-nHAp containing PCL scaffolds were engineered in order to enhance the osteogenic potential as well as its MR functionality for their application in bone tissue engineering. The nano-composite scaffolds along with pristine PCL were evaluated physico-chemically and biologically in vitro, in the presence of human mesenchymal stem cells (hMSCs). The incorporation of 30-40 nm sized MF-nHAp within the nanofibers showed a substantial increase in scaffold strength, protein adsorption, proliferation, and osteogenic differentiation of hMSCs along with enhanced MR functionality. This preliminary study was performed to eventually exploit the MR contrast imaging capability of MF-nHAp in nanofibrous scaffolds for real-time imaging of the changes in the tissue engineered construct.

High-resolution imaging of magnetisation transfer and nuclear Overhauser effect in the human visual cortex at 7 T

The aim of this study was to optimise a pulse sequence for high-resolution imaging sensitive to the effects of conventional macromolecular magnetisation transfer (MT(m)) and nuclear Overhauser enhancement (NOE), and to use it to investigate variations in these parameters across the cerebral cortex. A high-spatial-resolution magnetisation transfer-prepared turbo field echo (MT-TFE) sequence was designed to have high sensitivity to MT(m) and NOE effects, whilst being robust to B0 and B1 inhomogeneities, and producing a good point spread function across the cortex. This was achieved by optimising the saturation and imaging components of the sequence using simulations based on the Bloch equations, including exchange and an image simulator. This was used to study variations in these parameters across the cortex. Using the sequence designed to be sensitive to NOE and MT(m), a variation in signals corresponding to a variation in MT(m) and NOE across the cortex, consistent with a reduction in myelination from the white matter surface to the pial surface of the cortex, was observed. In regions in which the stria was visible on T2*-weighted images, it could also be detected in signals sensitive to MT(m) and NOE. There was greater variation in signals sensitive to NOE, suggesting that the NOE signal is more sensitive to myelination. A sequence has been designed to image variations in MT(m) and NOE at high spatial resolution and has been used to investigate variations in contrast in these parameters across the cortex.

Translational dynamics of water at the phospholipid interface

The residual water-proton magnetic relaxation dispersion profile obtained from suspensions of phospholipid vesicles in deuterium oxide was found to be a logarithmic function of the proton Larmor frequency at high magnetic field strengths, and independent of Larmor frequency at low magnetic field strengths. The residual proton relaxation is caused by dipole-dipole coupling between the residual water proton in otherwise deuterated water and the phospholipid protons. The logarithmic dependence on magnetic field strength is the signature of water-proton diffusive exploration on the interface that is approximately two-dimensionally constrained. Application of relaxation theory for two-dimensional diffusion to the spin-lattice relaxation data yields a translational correlation time of approximately 70 ps for water diffusing in the interface of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) vesicles.

Quantitative MRI in the very preterm brain: assessing tissue organization and myelination using magnetization transfer, diffusion tensor and T₁ imaging

Magnetization transfer ratio (MTR), diffusion tensor imaging (DTI) parameters and T(1) relaxometry values were used to create parametric maps characterizing the tissue microstructure of the neonatal brain in infants born very premature (24-32 gestational weeks) and scanned at preterm and term equivalent age. Group-wise image registration was used to determine anatomical correspondence between individual scans and the pooled parametric data at the preterm and term ages. These parametric maps showed distinct contrasts whose interrelations varied across brain regions and between the preterm and term period. Discrete patterns of regional variation were observed for the different quantitative parameters, providing evidence that MRI is sensitive to multiple independent aspects of brain maturation. MTR values showed a marked change in the pattern of regional variation at term equivalent age compared to the preterm period such that the ordinal ranking of regions by signal contrast changed. This was unlike all other parameters where the regional ranking was preserved at the two time points. Interpreting the data in terms of myelination and structural organization, we report on the concordance with available histological data and demonstrate the value of quantitative MRI for tracking brain maturation over the neonatal period.

Relaxation time mapping in multiple sclerosis

Several relaxation mapping techniques have been proposed to quantitatively assess disease-related brain tissue changes in multiple sclerosis. Newer developments also account for the distribution of hydrogen protons in different tissue compartments, and therefore provide markers for myelin and macromolecular content. This article will cover the broad spectrum of the pulse sequences and analysis techniques related to this topic that are currently available. Various technical and practical limitations linked with specific approaches will be discussed. These include acquisition time, accuracy and precision, radiofrequency absorption and limited coverage of the brain. Finally, the application of these techniques in the context of multiple sclerosis will be reviewed.

Fast bound pool fraction imaging of the in vivo rat brain: association with myelin content and validation in the C6 glioma model

Cross-relaxation imaging (CRI) is a quantitative magnetic resonance technique that measures the kinetic parameters of magnetization transfer between protons bound to water and protons bound to macromolecules. In this study, in vivo, four-parameter CRI of normal rat brains (N=5) at 3.0 T was first directly compared to histology. The bound pool fraction, f, was strongly associated with myelin density (Pearson's r=0.99, p<0.001). The correlation persisted in separate analyses of gray matter (GM; r=0.89, p=0.046) and white matter (WM; r=0.97, p=0.029). Subsequently, a new time-efficient approach for solely capturing the whole-brain parametric map of f was proposed, validated with histology, and used to estimate myelin density. Since the described approach for the rapid acquisition of f applied constraints to other CRI parameters, a theoretical analysis of error was performed. Estimates of f in normal and pathologic tissue were expected to have <10% error. A comparison of values for f obtained from the traditional four-parameter fit of CRI data versus the proposed rapid acquisition of f was within this expected margin for in vivo rat brain gliomas (N=4; mean±SE; 3.9±0.2% vs. 4.0±0.2%, respectively). In both whole-brain f maps and myelin density maps, replacement of normal GM and WM by proliferating and invading tumor cells could be readily identified. The rapid, whole-brain acquisition of the bound pool fraction may provide a reliable method for detection of glioma invasion in both GM and WM during animal and human imaging.

Potential of magnetization transfer MRI for target volume definition in patients with non-small-cell lung cancer

PURPOSE

To develop a magnetization transfer (MT) module in conjunction with a single-shot MRI readout technique and to investigate the MT phenomenon in non-small-cell lung cancer (NSCLC) as an adjunct for radiation therapy planning.

MATERIALS AND METHODS

A total of 10 patients with inoperable NSCLC were investigated using a 1.5T MR scanner. MT ratio (MTR) maps of several slices throughout the tumor were assessed. Each MTR-map was acquired within a short breathhold. Fluorodeoxyglucose positron emission tomography (FDG-PET) investigations were performed in addition to the MRI protocol. A total of 60 structures appearing conspicuous in FDG-PET were compared with structures appearing conspicuous in corresponding MTR maps. Quantification of similarity between both modalities was performed using similarity index calculation.

RESULTS

MTR-maps showed different contrast than FDG-PET images. However, structures that appeared conspicuous in FDG-PET images, either by a marked signal enhancement or signal decrease, were found to be similarly present in MTR maps. A mean similarity index of 0.65 was calculated. MTR values of suspected atelectasis were on average lower than MTR values of tumor tissue.

CONCLUSION

The proposed MT-MRI technique provides a high MT efficiency, while being robust and fast enough for breathhold acquisition. The results obtained encourage for further exploration of MT-MRI as an adjunct for radiotherapy planning in NSCLC.

Freezing point depression of water in phospholipid membranes: a solid-state NMR study

Lipid-water interaction plays an important role in the properties of lipid bilayers, cryoprotectants, and membrane-associated peptides and proteins. The temperature at which water bound to lipid bilayers freezes is lower than that of free water. Here, we report a solid-state NMR investigation on the freezing point depression of water in phospholipid bilayers in the presence and absence of cholesterol. Deuterium NMR spectra at different temperatures ranging from -75 to + 10 degrees C were obtained from fully (2)H2O-hydrated POPC (1-palmitoyl-2-oleoylphosphatidylcholine) multilamellar vesicles (MLVs), prepared with and without cholesterol, to determine the freezing temperature of water and the effect of cholesterol on the freezing temperature of water in POPC bilayers. Our 2H NMR experiments reveal the motional behavior of unfrozen water molecules in POPC bilayers even at temperatures significantly below 0 degrees C and show that the presence of cholesterol further lowered the freezing temperature of water in POPC bilayers. These results suggest that in the presence of cholesterol the fluidity and dynamics of lipid bilayers can be retained even at very low temperatures as exist in the liquid crystalline phase of the lipid. Therefore, bilayer samples prepared with a cryoprotectant like cholesterol should enable the performance of multidimensional solid-state NMR experiments to investigate the structure, dynamics, and topology of membrane proteins at a very low temperature with enhanced sample stability and possibly a better sensitivity. Phosphorus-31 NMR data suggest that lipid bilayers can be aligned at low temperatures, while 15N NMR experiments demonstrate that such aligned samples can be used to enhance the signal-to-noise ratio of is 15N chemical shift spectra of a 37-residue human antimicrobial peptide, LL-37.

MR imaging of brain maturation

Magnetic resonance imaging (MRI) is the imaging tool of choice to evaluate brain maturation and especially brain myelination. Magnetic resonance imaging also provides functional insight through diffusion images and proton spectroscopy. In this review the MRI techniques are analyzed for both pre- and postnatal periods. The origin of MR signal changes is also detailed in order to understand normal myelination evolution and the consequences on brain maturation of the different pathologies encountered prior and after birth. Because MRI is "blind" in terms of signal on conventional sequences after 2 years of age, a particular attention is given to diffusion images and proton spectroscopy of the developing brain.

Measuring injury and repair of myelin and neurons in multiple sclerosis

Neuroprotection in MS needs to be considered in the context of several pathological processes: limitation of acute inflammatory injury to myelin and axons, remyelination, survival of demyelinated axons, and limitation of more diffuse, nonlesional pathology that affects myelin and axons. Advanced MRI techniques are capable of reporting on all of these different pathological features of MS and will be an important aspect of the assessment of neuroprotection strategies in MS, when these become available.

Assessment of cortical maturation with prenatal MRI. Part I: Normal cortical maturation

Cortical maturation, especially gyral formation, follows a temporospatial schedule and is a good marker of fetal maturation. Although ultrasonography is still the imaging method of choice to evaluate fetal anatomy, MRI has an increasingly important role in the detection of brain abnormalities, especially of cortical development. Knowledge of MRI techniques in utero with the advantages and disadvantages of some sequences is necessary, in order to try to optimize the different magnetic resonance sequences to be able to make an early diagnosis. The different steps of cortical maturation known from histology represent the background necessary for the understanding of maturation in order to be then able to evaluate brain maturation through neuroimaging. Illustrations of the normal cortical maturation are given for each step accessible to MRI for both the cerebral hemispheres and the posterior fossa.

1H magnetization transfer in hydrated gluten and flour: effects of wheat aging

The interaction of water with flour or gluten in hydrated samples was investigated by proton magnetization transfer measurements. Flour and gluten from both durum and bread wheat seeds, either unaged or artificially aged over different periods of time, were investigated. Measurements were performed at several radio frequency power levels and frequency offsets, and the data were quantitatively modeled by two interacting pools, a liquid (water) and a solid (macromolecules) one. A super-Lorentzian line shape well described the magnetization of the solid pool. Magnetization transfer was found to be more efficient for flour with respect to gluten samples, in agreement with their hydrophilic/hydrophobic behavior. The aging treatment of seeds resulted in a minor degree of interaction between macromolecules and water.

MR properties of excised neural tissue following experimentally induced inflammation

Changes in the MR parameters of inflamed neural tissue were measured in vitro. Tumor necrosis factor-alpha (TNF-alpha) was injected into rat sciatic nerves to induce inflammation with negligible axonal loss and demyelination. The MR parameters, such as T1/T2 relaxation and magnetization transfer (MT), were measured 2 days after TNF-alpha injection and were found to be substantially different from those of normal nerves. The average T1/T2 relaxation times increased, whereas the MT ratio (MTR) and the quantitative MT parameter M0B (which describes the semisolid pool of protons) decreased. The MR parameters correlated very well with the extracellular volume fraction (EM) of neural tissue evaluated by quantitative histopathology. The multicomponent T2 relaxation was shown to provide the best quantitative assessment of changes in neural tissue microstructure, and allowed us to distinguish between the processes of inflammation and demyelination. In comparison, the MT measurements were less successful due to competing contributions of demyelination and pH-sensitive changes in the MT effect.

Contribution of mitochondria to cardiac muscle water/macromolecule proton magnetization transfer

The contribution of mitochondria to water-macromolecule proton magnetization transfer (MT) was evaluated in porcine heart tissue. An examination of isolated mitochondria in suspension, at the same concentration as found in heart tissue, revealed MT effects very similar in magnitude and bandwidth to those in intact heart tissue. Disruption of the gross structure of the mitochondria by freeze-thawing or with detergent resulted in only approximately 25% decreases in MT, which suggests that the structure of the mitochondria is not critical for these effects. The current data indicate that mitochondria macromolecules contribute significantly to MT in the intact heart.

Magnetization transfer contrast imaging in Niemann pick type C mouse liver

PURPOSE

To investigate livers of mice afflicted with Niemann Pick type C (NP-C) disease using magnetization transfer contrast (MTC) imaging and to test the hypothesis that the MT ratio reproducibly changes during disease progression.

BACKGROUND

NP-C is a heritable defect of lipid metabolism that results in the accumulation of unesterified cholesterol and gangliosides in virtually all cells. Symptoms predominate in brain and liver, which have high endogenous rates of lipid turnover. It is fatal to children, usually early in the second decade of life. Previous work has shown that the efficiency of magnetization transfer (MT) can be affected by cholesterol and collagen in tissues. The MT ratio (MTR) was calculated and compared during growth and therapy of diseased and control mice.

RESULTS

Significant differences in the MTR were observed between livers of diseased and control mice. These ratios were consistent with collagen deposition associated with fibrosis, and not the accumulation of unesterified cholesterol in this organ.

CONCLUSION

These results indicate that MTC imaging may have clinical potential for monitoring progression and therapy in NP-C disease.

Magnetization transfer changes of grey and white matter in Parkinson's disease

Since the attempt to evidence structural brain damage in Parkinson's disease (PD) by conventional magnetic resonance imaging (MRI) is usually disappointing, we have investigated whether the magnetization transfer ratio (MTR) can reflect changes in grey and white matter of PD patients. MTR was quantified in 44 regions of interest (ROIs) in both grey and white matter of 11 non-demented PD patients, ranging from 2 to 4 on the Hoehn and Yahr Scale, and eight age-matched healthy subjects. MTR differences between patients and controls were found in the supratentorial white matter and in the brainstem. In particular, lower MTR values were found in the paraventricular white matter of PD patients (p<0.05) while no differences were observed in corpus callosum, frontal, parietal, occipital lobes or centrum semiovalis. Lower MTR values were found in substantia nigra (p<0.001), red nucleus (p<0.05) and pons (p<0.05) of the patient group. No differences were discovered in basal ganglia and thalamus. These findings suggest that MTR measurements in the paraventricular white matter and brainstem may help to recognize a marker for probable PD.

Functional analysis of human parotid gland in vivo using the (1)H MRS MT effect

Magnetization transfer (MT) was measured in the parotid gland in vivo by (1)H MR spectroscopy in 10 adult volunteers. A comparison was made of stimulated (excess saliva) and resting parotid gland (SPG and RPG, respectively). Following irradiation at an MT pulse of 150 Hz downfield from the water proton signal, signal reductions in SPG and RPG were 83.8 +/- 4.7 and 91.4 +/- 5.7%, respectively. The larger reduction for SPG indicates that an increase in the amount of water in gland cells for the production of more parotid saliva may lead to greater affinity between the protons adjacent to macromolecules and free water which contributes to the MT effect. Activity in the parotid gland correlates with the effect. This method is useful for diagnosing disorders of parotid gland secretion.

Magnetization transfer imaging of the canine brain: a review

Magnetization transfer imaging is a modality capable of examining the non-water components of brain tissue by examining the effects they have on water protons. It may be used qualitatively to increase the visibility of lesions seen during magnetic resonance angiography and following the administration of an intravenous paramagnetic contrast medium. Quantitatively, it can be used to examine the effect of pathology on magnetization transfer contrast, to provide a measurement of myelination, as well as to quantify disease progression in trauma, neoplasia, neurodegeneration and other disorders of the brain. This paper reviews the theory of magnetization transfer imaging, its applications, and provides an example of its use in examining the canine brain.

The magnetization transfer characteristics of human breast tissues: an in vitro NMR study

A series of freshly excised human breast tissues was analysed using a nuclear magnetic resonance spectrometer and then subjected to routine histopathology examination. Tissues comprised normal parenchymal, adipose, fibrocystic, fibroadenoma and malignant types. An inversion-recovery sequence performed both with and without magnetization transfer allowed T1, T1s, M0 and Ms values to be obtained. From this information, the magnetization transfer rate constant, K, was calculated for each tissue sample. These data show that T1s provided greater discrimination between neoplasic and normal tissues than did T1. However, neither T1s nor K values provided a means of discriminating between benign and malignant disease.

A multicenter measurement of magnetization transfer ratio in normal white matter

To assess the importance of intercenter variations when measuring magnetization transfer ratio (MTR) in the brain, six European centers measured MTR in normal white matter. MTR ranged from 9 to 51 percent units (25 sequences). The effective flip angle of the saturating pulse divided by the pulse repetition time (ENRsat degrees/msec) was a good predictor of MTR (MTR = 3.25 ENRsat).

Protein mediated magnetic coupling between lactate and water protons

The magnetic coupling between methyl lactate protons and water protons in samples of cross-linked bovine serum albumin (BSA) is studied. Cross-relaxation spectroscopy shows efficient magnetization transfer from immobilized BSA to both water and methyl lactate protons. Transient and steady-state NOE experiments reveal a negative intermolecular NOE between methyl lactate and water protons. Lactate is indirectly detected by selectively saturating the methyl lactate protons and measuring the decrease in water proton magnetization. Indirect detection of methyl lactate protons is an order of magnitude more sensitive than direct detection in these model systems. Lactate was indirectly imaged, via the water proton resonance, with 1.1-microliter voxels in 2 min. Immobilized BSA reduces the intermolecular correlation time between water and lactate protons into the spin-diffusion limit where the NOE is negative. Possible molecular mechanisms for this coupling and applications to in vivo spectroscopy are discussed.

Magnetization transfer characteristics in atherosclerotic plaque components assessed by adapted binomial preparation pulses

Increasing the contrast between atheromatous plaque components is a major issue in cardiovascular MRI research. It would allow one to identify unstable plaque by differentiating the lipid core associated with vulnerability, from the fibrous cap, considered as a factor of stability. T2 and diffusion-weighted imaging have already provided satisfying results. Magnetization transfer (MT) between restricted protons Hr and free-water protons Hf could achieve a different contrast related to collagen and lipoprotein macromolecules present in the fibrous cap and lipid core, respectively. The purpose of this work was to evaluate in vitro the MT effect produced by adapted T2-selective 1-3-3-1 binomial pulses on isolated samples of atheromatous arteries at 3 T. A method based on simulation was used in order to improve the MT specificity: it is shown that 50% 1-3-3-1 pulses (the percentage indicating the level of Hr saturation) allow an estimation of T2r, the Hr T2. Using this technique, magnetization transfer was observed for the first time in atherosclerotic plaque components, an effect more pronounced for the fibrous cap and media than for the lipid core and adventitia. The T2r estimation gave values ranging from 20 to 25 micros for the four samples. This preliminary study provides a basis for establishing an MT imaging sequence of atheromatous arteries, by using 50% 1-3-3-1 pulses calibrated for saturating protons with a 20 micros T2. This MT protocol should be further compared to T2 and diffusion-weighted imaging.