Differentiation and characterization of healthy versus pathological tau using chemical exchange saturation transfer

Neurofibrillary tangles of tau constitute one of the key biological hallmarks of Alzheimer's disease. Currently, the assessment of regional tau accumulation requires intravenous administration of radioactive tracers for PET imaging. A noninvasive MRI-based solution would have significant clinical implications. Herein, we utilized an MRI technique known as chemical exchange saturation transfer (CEST) to determine the imaging signature of tau in both its monomeric and pathologic fibrillated conformations. Three sets of purified recombinant full-length (4R) tau protein were prepared for collection of CEST spectra using a 9.4 T NMR spectrometer at varying temperatures (25, 37, and 42 °C) and RF intensities (0.7, 1.0, 1.5, and 2.2 μT). Monomeric and fibrillated tau were readily distinguished based on their Z-spectrum profiles. Fibrillated tau demonstrated a less prominent peak at 3.5 ppm with additional peaks near 0.5 and 1.5 ppm. No significant differences were identified between fibrillated tau prepared using heparin versus seed-competent tau. In conclusion, monomeric and fibrillated tau can be readily detected and distinguished based on their CEST-derived Z-spectra, pointing to the potential utility of CEST-MRI as a noninvasive biomarker of regional pathologic tau accumulation in the brain. Further testing and validation in vitro and in vivo will be necessary before this can be applied clinically.

CEST-MRI for body oncologic imaging: are we there yet?

Chemical exchange saturation transfer (CEST) MRI has gained recognition as a valuable addition to the molecular imaging and quantitative biomarker arsenal, especially for characterization of brain tumors. There is also increasing interest in the use of CEST-MRI for applications beyond the brain. However, its translation to body oncology applications lags behind those in neuro-oncology. The slower migration of CEST-MRI to non-neurologic applications reflects the technical challenges inherent to imaging of the torso. In this review, we discuss the application of CEST-MRI to oncologic conditions of the breast and torso (i.e., body imaging), emphasizing the challenges and potential solutions to address them. While data are still limited, reported studies suggest that CEST signal is associated with important histology markers such as tumor grade, receptor status, and proliferation index, some of which are often associated with prognosis and response to therapy. However, further technical development is still needed to make CEST a reliable clinical application for body imaging and establish its role as a predictive and prognostic biomarker.

Inhomogeneous magnetization transfer imaging: Concepts and directions for further development

Off-resonance radio frequency irradiation can induce the ordering of proton spins in the dipolar fields of their neighbors, in molecules with restricted mobility. This dipolar order decays with a characteristic relaxation time, T_1D , that is very different from the T₁ and T₂ relaxation of the nuclear alignment with the main magnetic field. Inhomogeneous magnetization transfer (ihMT) imaging is a refinement of magnetization transfer (MT) imaging that isolates the MT signal dependence on dipolar order relaxation times within motion-constrained molecules. Because T_1D relaxation is a unique contrast mechanism, ihMT may enable improved characterization of tissue. Initial work has stressed the high correlation between ihMT signal and myelin density. Dipolar order relaxation appears to be much longer in membrane lipids than other molecules. Recent work has shown, however, that ihMT acquisitions may also be adjusted to emphasize different ranges of T_1D . These newer approaches may be sensitive to other microstructural components of tissue. Here, we review the concepts and history of ihMT and outline the requirements for further development to realize its full potential.

Shimon Vega in the eyes of his students and postdocs

Professor Shimon Vega (1943-2021) of the Weizmann Institute of Science passed away on the 16-th of November. Shimon Vega established theoretical frameworks to develop and explain solid-state nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP) techniques and methodologies. His departure left a profound mark on his many students, postdocs, and colleagues. Shortly after his passing, we all assembled spontaneously for an international online meeting to share our reflections and memories of our experiences in Shimon's lab and how they affected us deeply during that period of timeand throughout our scientific careers. These thoughts and feelings were put here into writing.

The use of variable delay multipulse chemical exchange saturation transfer for separately assessing different CEST pools in the human brain at 7T

PURPOSE

Current challenges of in vivo CEST imaging include overlapping signals from different pools. The overlap arises from closely resonating pools and/or the broad magnetization transfer contrast (MTC) from macromolecules. This study aimed to evaluate the feasibility of variable delay multipulse (VDMP) CEST to separately assess solute pools with different chemical exchange rates in the human brain in vivo, while mitigating the MTC.

METHODS

VDMP saturation buildup curves were simulated for amines, amides, and relayed nuclear Overhauser effect. VDMP data were acquired from glutamate and bovine serum albumin phantoms, and from six healthy volunteers at 7T. For the in vivo data, MTC removal was performed via a three-pool Lorentzian fitting. Different B₁ amplitudes and mixing times were used to evaluate CEST pools with different exchange rates.

RESULTS

The results show the importance of removing MTC when applying VDMP in vivo and the influence of B₁ for distinguishing different pools. Finally, the optimal B₁ and mixing times to effectively saturate slow- and fast-exchanging components are also reported. Slow-exchanging amides and rNOE components could be distinguished when using B₁ = 1 μT and t_mix = 10 ms and 40 ms, respectively. Fast-exchanging components reached the highest saturation when using a B₁ = 2.8 μT and t_mix = 0 ms.

CONCLUSION

VDMP is a powerful CEST-editing tool, exploiting chemical exchange-rate differences. After MTC removal, it allows separate assessment of slow- and fast-exchanging solute pools in in vivo human brain.

Combining inhomogeneous magnetization transfer and multipoint Dixon acquisition: Potential utility and evaluation

PURPOSE

The recently introduced inhomogeneous magnetization transfer (ihMT) method has predominantly been applied for imaging the central nervous system. Future applications of ihMT, such as in peripheral nerves and muscles, will involve imaging in the vicinity of adipose tissues. This work aims to systematically investigate the partial volume effect of fat on the ihMT signal and to propose an efficient fat-separation method that does not interfere with ihMT measurements.

METHODS

First, the influence of fat on ihMT signal was studied using simulations. Next, the ihMT sequence was combined with a multi-echo Dixon acquisition for fat separation. The sequence was tested in 9 healthy volunteers using a 3T human scanner. The ihMT ratio (ihMTR) values were calculated in regions of interest in the brain and the spinal cord using standard acquisition (no fat saturation), water-only, in-phase, and out-of-phase reconstructions. The values obtained were compared with a standard fat suppression method, spectral presaturation with inversion recovery.

RESULTS

Simulations showed variations in the ihMTR values in the presence of fat, depending on the TEs used. The IhMTR values in the brain and spinal cord derived from the water-only ihMT multi-echo Dixon images were in good agreement with values from the unsuppressed sequence. The ihMT-spectral presaturation with inversion recovery combination resulted in 24%-35% lower ihMTR values compared with the standard non-fat-suppressed acquisition.

CONCLUSION

The presence of fat within a voxel affects the ihMTR calculations. The IhMT multi-echo Dixon method does not compromise the observable ihMT effect and can potentially be used to remove fat influence in ihMT.

IMPROVING THE TRAINING PROCESS OF 16–17-YEAR-OLD BIATHLETES BY MEANS OF HYPOXIC-HYPERCAPNIC EXPOSURES AND CONTROL OF CHANGES IN THE PREMORBID CONDITION OF THE CARDIOVASCULAR SYSTEM IN THE BASIC STAGE OF SPORTS PREPARATION

Aim. The article deals with theoretical and experimental substantiation of the effectiveness of basic and ergogenic (hypoxic-hypercapnic exposures) aids in the preparation of 16–17-year-old biathletes. Materials and methods. The study involved two groups of 16–17-year-old biathletes. All athletes underwent an in-depth medical examination at the beginning of the experiment. Throughout the study, their functional status was assessed by the premorbid index of the cardio-vascular system obtained with the CardioSoft diagnostic system (USA). Results. Hypoxic-hypercapnic exposures in sports training along with the general training program aimed at deve­loping local-regional muscle endurance is a promising approach that can ensure a high level of functional abilities in athletes and improve their sports results. Conclusion. The results of the study contribute to the improvement of the training system for 16-17-year-old biathletes at the basic stage of sports preparation. The study proves that the innovative method proposed is effective for controlling the premorbid state of the cardiovascular system in athletes.

Imaging molecules

Molecular imaging using MRI is gaining momentum. While sensitivity of MR is limited compared to other molecular imaging modalities, the molecular specificity is high in comparison. Moreover, MRI offers contrast based on multitude of processes and scales, from intramolecular relaxation pathways to water diffusion. Living tissue offers abundance of potential molecular targets of interest in biology and medicine. In this short perspective we focus on some direct and indirect methods to visualize endogenous molecules. We briefly discuss Spectroscopic Imaging (MRSI), Chemical Exchange Saturation Transfer (CEST) and Magnetization Transfer Contrast (MTC). Imaging molecules with MRI is part of the larger universe of imaging methods. Moreover, it is part of ever increasing pool of data combining imaging with other modalities, biology and patient outcomes.

Accelerating chemical exchange saturation transfer MRI with parallel blind compressed sensing

PURPOSE

Chemical exchange saturation transfer is a novel and promising MRI contrast method, but it can be time-consuming. Common parallel imaging methods, like SENSE, can lead to reduced quality of CEST. Here, parallel blind compressed sensing (PBCS), combining blind compressed sensing (BCS) and parallel imaging, is evaluated for the acceleration of CEST in brain and breast.

METHODS

The CEST data were collected in phantoms, brain (N = 3), and breast (N = 2). Retrospective Cartesian undersampling was implemented and the reconstruction results of PBCS-CEST were compared with BCS-CEST and k-t sparse-SENSE CEST. The normalized RMSE and the high-frequency error norm were used for quantitative comparison.

RESULTS

In phantom and in vivo brain experiments, the acceleration factor of R = 10 (24 k-space lines) was achieved and in breast R = 5 (30 k-space lines), without compromising the quality of the PBCS-reconstructed magnetization transfer rate asymmetry maps and Z-spectra. Parallel BCS provides better reconstruction quality when compared with BCS, k-t sparse-SENSE, and SENSE methods using the same number of samples. Parallel BCS overperforms BCS, indicating that the inclusion of coil sensitivity improves the reconstruction of the CEST data.

CONCLUSION

The PBCS method accelerates CEST without compromising its quality. Compressed sensing in combination with parallel imaging can provide a valuable alternative to parallel imaging alone for accelerating CEST experiments.

Z-spectrum appearance and interpretation in the presence of fat: Influence of acquisition parameters

PURPOSE

Chemical exchange saturation transfer (CEST) MRI is increasingly evolving from brain to body applications. One of the known problems in the body imaging is the presence of strong lipid signals. Although their influence on the CEST effect is acknowledged, there was no study that focuses on the interplay among echo time, fat fraction, and Z-spectrum. This study strives to address these points, with the emphasis on the application in the breast.

METHODS

Z-spectra were simulated in phase and out of phase of the main fat peak at -3.4 ppm, with the fat fraction varying from 0 to 100%. The magnetization transfer ratio asymmetry in two ranges, centering at the exchanging pool and at 3.5 ppm approximately opposite the nonexchanging fat pool, were calculated and were plotted against fat fraction. The results were verified in phantoms and in vivo.

RESULTS

The results demonstrate the combined influence of fat fraction and echo time on the Z-spectrum for gradient echo based CEST acquisitions. The influence is straightforward in the in-phase images, but it is more complicated in the out-of-phase images, potentially leading to erroneous CEST contrast.

CONCLUSIONS

This study provides a basis for understanding the origin and appearance of lipid artifacts in CEST imaging, and lays the foundation for their efficient removal. Magn Reson Med 79:2731-2737, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Microstructural correlates of 3D steady-state inhomogeneous magnetization transfer (ihMT) in the human brain white matter assessed by myelin water imaging and diffusion tensor imaging

PURPOSE

To compare the recently introduced inhomogeneous magnetization transfer (ihMT) technique with more established MRI techniques including myelin water imaging (MWI) and diffusion tensor imaging (DTI), and to evaluate the microstructural attributes correlating with this new contrast method in the human brain white matter.

METHODS

Eight adult healthy volunteers underwent T₁ -weighted, ihMT, MWI, and DTI imaging on a 3T human scanner. The ihMT ratio (ihMTR), myelin water fraction (MWF), fractional anisotropy (FA), radial diffusivity (RD), axial diffusivity (AD), and mean diffusivity (MD) values were calculated from different white matter tracts. The angle ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>θ</mml:mi></mml:math> ) between the directions of the principal eigenvector, as measured by DTI, and the main magnetic field was calculated for all voxels from various fiber tracts. The ihMTR was correlated with MWF and DTI metrics.

RESULTS

A strong correlation was found between ihMTR and MWF (ρ = 0.77, P < 0.0001). This was followed by moderate to weak correlations between ihMTR and DTI metrics: RD (ρ = -0.30, P < 0.0001), FA (ρ = 0.20, P < 0.0001), MD (ρ = -0.19, P < 0.0001), AD (ρ = 0.02, P < 0.0001). A strong correlation was found between ihMTR and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>θ</mml:mi></mml:math> (ρ = -0.541, P < 0.0001).

CONCLUSION

The strong correlation with myelin water imaging and its low coefficient of variation suggest that ihMT has the potential to become a new structural imaging marker of myelin. The substantial orientational dependence of ihMT should be taken into account when evaluating and quantitatively interpreting ihMT results.

Balanced Steady-State Free Precession (bSSFP) from an effective field perspective: Application to the detection of chemical exchange (bSSFPX)

Chemical exchange saturation transfer (CEST) is a novel contrast mechanism and it is gaining increasing popularity as many promising applications have been proposed and investigated. Fast and quantitative CEST imaging techniques are further needed in order to increase the applicability of CEST for clinical use as well as to derive quantitative physiological and biological information. Steady-state methods for fast CEST imaging have been reported recently. Here, we observe that an extreme case of these methods is a balanced steady-state free precession (bSSFP) sequence. The bSSFP in itself is sensitive to the exchange processes; hence, no additional saturation or preparation is needed for CEST-like data acquisition. The bSSFP experiment can be regarded as observation during saturation, without separate saturation and acquisition modules as used in standard CEST and similar experiments. One of the differences from standard CEST methods is that the bSSFP spectrum is an XY-spectrum not a Z-spectrum. As the first proof-of-principle step, we have implemented the steady-state bSSFP sequence for chemical exchange detection (bSSFPX) and verified its feasibility in phantom studies. These studies have shown that bSSFPX can achieve exchange-mediated contrast comparable to the standard CEST experiment. Therefore, the bSSFPX method has a potential for fast and quantitative CEST data acquisition.

UCEPR: Ultrafast localized CEST-spectroscopy with PRESS in phantoms and in vivo

PURPOSE

Chemical exchange saturation transfer (CEST) is a contrast mechanism enhancing low-concentration molecules through saturation transfer from their exchangeable protons to bulk water. Often many scans are acquired to form a Z-spectrum, making the CEST method time-consuming. Here, an ultrafast localized CEST-spectroscopy with PRESS (UCEPR) is proposed to obtain the entire Z-spectrum of a voxel using only two scans, significantly accelerating CEST.

THEORY AND METHODS

The approach combines ultrafast nonlocalized CEST spectroscopy with localization using PRESS. A field gradient is applied concurrently with the saturation pulse producing simultaneous saturation of all Z-spectrum frequencies that are also spatially encoded. A readout gradient during data acquisition resolves the spatial dependence of the CEST responses into frequency. UCEPR was tested on a 3T scanner both in phantoms and in vivo.

RESULTS

In phantoms, a fast Z-spectroscopy acquisition of multiple pH-variant iopamidol samples was achieved with four- to seven-fold acceleration as compared to the conventional CEST methods. In vivo, amide proton transfer (APT) in white matter of healthy human brain was measured rapidly in 48 s and with high frequency resolution (≤ 0.2 ppm).

CONCLUSION

Compared with conventional CEST methods, UCEPR has the advantage of rapidly acquiring high-resolution Z-spectra. Potential in vivo applications include ultrafast localized Z-spectroscopy, quantitative, or dynamic CEST studies.

Selective spectroscopic imaging of hyperpolarized pyruvate and its metabolites using a single-echo variable phase advance method in balanced SSFP

PURPOSE

In balanced steady state free precession (bSSFP), the signal intensity has a well-known dependence on the off-resonance frequency, or, equivalently, the phase advance between successive radiofrequency (RF) pulses. The signal profile can be used to resolve the contributions from the spectrally separated metabolites. This work describes a method based on use of a variable RF phase advance to acquire spatial and spectral data in a time-efficient manner for hyperpolarized 13C MRI.

THEORY AND METHODS

The technique relies on the frequency response from a bSSFP acquisition to acquire relatively rapid, high-resolution images that may be reconstructed to separate contributions from different metabolites. The ability to produce images from spectrally separated metabolites was demonstrated in vitro, as well as in vivo following administration of hyperpolarized 1-13C pyruvate in mice with xenograft tumors.

RESULTS

In vivo images of pyruvate, alanine, pyruvate hydrate, and lactate were reconstructed from four images acquired in 2 s with an in-plane resolution of 1.25 × 1.25 mm(2) and 5 mm slice thickness.

CONCLUSION

The phase advance method allowed acquisition of spectroscopically selective images with high spatial and temporal resolution. This method provides an alternative approach to hyperpolarized 13C spectroscopic MRI that can be combined with other techniques such as multiecho or fluctuating equilibrium bSSFP. Magn Reson Med 76:1102-1115, 2016. © 2015 Wiley Periodicals, Inc.

pH imaging of mouse kidneys in vivo using a frequency-dependent paraCEST agent

PURPOSE

This study explored the feasibility of using a pH responsive paramagnetic chemical exchange saturation transfer (paraCEST) agent to image the pH gradient in kidneys of healthy mice.

METHODS

CEST signals were acquired on an Agilent 9.4 Tesla small animal MRI system using a steady-state gradient echo pulse sequence after a bolus injection of agent. The magnetic field inhomogeneity across each kidney was corrected using the WASSR method and pH maps were calculated by measuring the frequency of water exchange signal arising from the agent.

RESULTS

Dynamic CEST studies demonstrated that the agent was readily detectable in kidneys only between 4 to 12 min postinjection. The CEST images showed a higher signal intensity in the pelvis and calyx regions and lower signal intensity in the medulla and cortex regions. The pH maps reflected tissue pH values spanning from 6.0 to 7.5 in kidneys of healthy mice.

CONCLUSION

This study demonstrated that pH maps of the kidney can be imaged in vivo by measuring the pH-dependent chemical shift of a single water exchange CEST peak without prior knowledge of the agent concentration in vivo. The results demonstrate the potential of using a simple frequency-dependent paraCEST agent for mapping tissue pH in vivo. Magn Reson Med 75:2432-2441, 2016. © 2015 Wiley Periodicals, Inc.

Structural investigation of cell wall polysaccharides of Lactobacillus delbrueckii subsp. bulgaricus 17

Lactobacilli are valuable strains for commercial (functional) food fermentations. Their cell surface-associated polysaccharides (sPSs) possess important functional properties, such as acting as receptors for bacteriophages (bacterial viruses), influencing autolytic characteristics and providing protection against antimicrobial peptides. The current report provides an elaborate molecular description of several surface carbohydrates of Lactobacillus delbrueckii subsp. bulgaricus strain 17. The cell surface of this strain was shown to contain short chain poly(glycerophosphate) teichoic acids and at least two different sPSs, designated here as sPS1 and sPS2, whose chemical structures were examined by 2D nuclear magnetic resonance spectroscopy and methylation analysis. Neutral branched sPS1, extracted with n-butanol, was shown to be composed of hexasaccharide repeating units (-[α-d-Glcp-(1-3)-]-4-β-l-Rhap2OAc-4-β-d-Glcp-[α-d-Galp-(1-3)]-4-α-Rhap-3-α-d-Galp-), while the major component of the TCA-extracted sPS2 was demonstrated to be a linear d-galactan with the repeating unit structure being (-[Gro-3P-(1-6)-]-3-β-Galf-3-α-Galp-2-β-Galf-6-β-Galf-3-β-Galp-).

CEST: from basic principles to applications, challenges and opportunities

Chemical Exchange Saturation Transfer (CEST) offers a new type of contrast for MRI that is molecule specific. In this approach, a slowly exchanging NMR active nucleus, typically a proton, possessing a chemical shift distinct from water is selectively saturated and the saturated spin is transferred to the bulk water via chemical exchange. Many molecules can act as CEST agents, both naturally occurring endogenous molecules and new types of exogenous agents. A large variety of molecules have been demonstrated as potential agents, including small diamagnetic molecules, complexes of paramagnetic ions, endogenous macromolecules, dendrimers and liposomes. In this review we described the basic principles of the CEST experiment, with emphasis on the similarity to earlier saturation transfer experiments described in the literature. Interest in quantitative CEST has also resulted in the development of new exchange-sensitive detection schemes. Some emerging clinical applications of CEST are described and the challenges and opportunities associated with translation of these methods to the clinical environment are discussed.

Keyhole chemical exchange saturation transfer

The keyhole technique, which involves the acquisition of dynamic data at low resolution in combination with a high-resolution reference, is developed for the purposes of chemical exchange saturation transfer (CEST) imaging, i.e., Keyhole CEST. Low-resolution data are acquired with saturation applied at different frequencies for Z-spectra, along with a high-resolution reference image taken without saturation. Three methods for high-resolution reconstruction of Keyhole CEST are evaluated using the values from quantitative high-resolution CEST maps. In addition, Keyhole CEST is applied for collection of data used for B(0) correction. The keyhole approach is evaluated for CEST contrast generation using exchanging protons in hydroxyl groups. First, the techniques are evaluated in vitro using samples of dextrose and chondroitin sulfate. Next, the work is extended in vivo to explore its applicability for gagCEST. Comparable quantitative gagCEST values are found using Keyhole CEST, provided the structure or region of interest is not limited by the low-resolution dataset.

pCEST: Positive contrast using Chemical Exchange Saturation Transfer

Chemical Exchange Saturation Transfer (CEST) contrast utilizes selective pre-saturation of a small pool of exchanging protons and subsequent detection of the decrease in bulk water signal. The CEST contrast is negative and requires detection of small signal change in the presence of a strong background signal. Here we develop a Positive CEST (pCEST) detection scheme utilizing the analogous nature of the CEST and off-resonance T(1)(ρ) experiments and exploring increased apparent relaxation rates in the presence of the selective pre-saturation. pCEST leads to the positive contrast, i.e., increased signal intensity as the result of the presence of the agent and RF pre-saturation. Simultaneously substantial background suppression is achieved. The contrast can be switched "ON" and "OFF", similar to the original CEST.

Structure of the core part of the lipopolysaccharide from Proteus mirabilis genomic strain HI4320

The structure of the core part of the lipopolysaccharide from Proteus mirabilis genomic strain HI4320 was studied. Core oligosaccharide was isolated by mild acid hydrolysis of the lipopolysaccharide and analyzed by NMR spectroscopy and mass spectrometry as well as chemical methods. The structure of the oligosaccharide was established.

Amide proton transfer imaging with improved robustness to magnetic field inhomogeneity and magnetization transfer asymmetry using saturation with frequency alternating RF irradiation

Amide proton transfer (APT) imaging has shown promise as an indicator of tissue pH and as a marker for brain tumors. Sources of error in APT measurements include direct water saturation, and magnetization transfer (MT) from membranes and macromolecules. These are typically suppressed by postprocessing asymmetry analysis. However, this approach is strongly dependent on B(0) homogeneity and can introduce additional errors due to intrinsic MT asymmetry, aliphatic proton features opposite the amide peak and radiation damping-induced asymmetry. Although several methods exist to correct for B(0) inhomogeneity, they tremendously increase scan times and do not address errors induced by asymmetry of the z-spectrum. In this article, a novel saturation scheme-saturation with frequency alternating RF irradiation (SAFARI)-is proposed in combination with a new magnetization transfer ratio (MTR) parameter designed to generate APT images insensitive to direct water saturation and MT, even in the presence of B(0) inhomogeneity. The feasibility of the SAFARI technique is demonstrated in phantoms and in the human brain. Experimental results show that SAFARI successfully removes direct water saturation and MT contamination from APT images. It is insensitive to B(0) offsets up to 180 Hz without using additional B(0) correction, thereby dramatically reducing scanning time.

Perfusion imaging with a freely diffusible hyperpolarized contrast agent

Contrast agents that can diffuse freely into or within tissue have numerous attractive features for perfusion imaging. Here we present preliminary data illustrating the suitability of hyperpolarized (13)C labeled 2-methylpropan-2-ol (also known as dimethylethanol, tertiary butyl alcohol and tert-butanol) as a freely diffusible contrast agent for magnetic resonance perfusion imaging. Dynamic (13)C images acquired in rat brain with a balanced steady-state free precession sequence following administration of hyperpolarized 2-methylpropan-2-ol show that this agent can be imaged with 2-4 s temporal resolution, 2 mm slice thickness, and 700 μm in-plane resolution while retaining adequate signal-to-noise ratio. (13)C relaxation measurements on 2-methylpropan-2-ol in blood at 9.4 T yield T(1) = 46 ± 4s and T(2) = 0.55 ± 0.03 s. In the rat brain at 4.7 T, analysis of the temporal dynamics of the balanced steady-state free precession image intensity in tissue and venous blood indicate that 2-methylpropan-2-ol has a T(2) of roughly 2-4s and a T(1) of 43 ± 24 s. In addition, the images indicate that 2-methylpropan-2-ol is freely diffusible in brain and hence has a long residence time in tissue; this in turn makes it possible to image the agent continuously for tens of seconds. These characteristics show that 2-methylpropan-2-ol is a promising agent for robust and quantitative perfusion imaging in the brain and body.

CEST and PARACEST MR contrast agents

In this review we describe the status of development for a new class of magnetic resonance (MR) contrast agents, based on chemical exchange saturation transfer (CEST). The mathematics and physics relevant to the description of the CEST effect in MR are presented in an appendix published in the online version only. We discuss the issues arising when translating in vitro results obtained with CEST agents to using these MR agents in in vivo model studies and in humans. Examples are given on how these agents are imaged in vivo. We summarize the status of development of these CEST agents, and speculate about the next steps that may be taken towards the demonstration of CEST MR imaging in clinical applications.

A concentration-independent method to measure exchange rates in PARACEST agents

The efficiency of chemical exchange dependent saturation transfer (CEST) agents is largely determined by their water or proton exchange kinetics, yet methods to measure such exchange rates are variable and many are not applicable to in vivo measurements. In this work, the water exchange kinetics of two prototype paramagnetic agents (PARACEST) are compared by using data from classic NMR line-width measurements, by fitting CEST spectra to the Bloch equations modified for chemical exchange, and by a method where CEST intensity is measured as a function of applied amplitude of radiofrequency field. A relationship is derived that provides the water exchange rate from the X-intercept of a plot of steady-state CEST intensity divided by reduction in signal caused by CEST irradiation versus 1/omega(1)(2), referred to here as an omega plot. Furthermore, it is shown that this relationship is independent of agent concentration. Exchange rates derived from omega plots using either high-resolution CEST NMR data or CEST data obtained by imaging agree favorably with exchange rates measured by the more commonly used Bloch fitting and line-width methods. Thus, this new method potentially allows access to a direct measure of exchange rates in vivo, where the agent concentration is typically unknown.

Hyperpolarized long-lived states in solution NMR: three-spin case study in low field

Recent work has shown that singlet states in two-spin systems can possess lifetimes exceeding the T(1) relaxation time, provided that the system is kept under conditions that minimize the effects of the chemical shift Hamiltonian (for instance under low magnetic field or RF irradiation). Similar observations have been made in hyperpolarized states of multi-spin systems prepared via parahydrogen-induced polarization (PHIP). However, lifetime prolongation mechanisms in multi-spin systems are still under investigation. Here we present experimental observations of a long-lived state in a three-spin system prepared by PHIP and stored at low field. The observed lifetime of the long-lived state is 144s, about twice as long as the longest T(1) measured in the system at high field. The results are analyzed using a recently proposed theory of lifetime prolongation in multi-spin systems in low field. It is shown that quantum mechanical selection rules governing intramolecular dipolar relaxation in low field account for the enhanced lifetime and spectral features of this state.

Long-lived states in solution NMR: theoretical examples in three- and four-spin systems

Long-lived spin states have been observed in a variety of systems. Although the dynamics underlying the long lifetimes of these states are well understood in the case of two-spin systems, the corresponding dynamics in systems containing more spins appear to be more complex. Recently it has been shown that a selection rule for transitions mediated by intramolecular dipolar relaxation may play a role in determining the lifetimes of long-lived states in systems containing arbitrary numbers of spins. Here we present a theory of long-lived states in systems containing three and four spins and demonstrate how it can be used to identify states that have little or no intramolecular dipolar relaxation.

MRI detection of paramagnetic chemical exchange effects in mice kidneys in vivo

In this report, the On resonance PARamagnetic CHemical Exchange Effects (OPARACHEE) method was implemented in vivo using WALTZ-16* as a preparation pulse with a standard spin echo sequence to detect the accumulation and clearance of the TmDOTA-4AmC(-) in mouse kidney. The performance of the technique in vivo is described in terms of the magnitude of the contrast effect versus the bolus agent concentration and signal-to-noise ratio (SNR) levels. The lowest injected concentration of TmDOTA-4AmC(-), 200 microL of a 2-mM stock solution (corresponds to approximately 0.2 mM agent in plasma), reduced the total water signal in the kidney papilla by 45% 3 min after the a bolus injection. The results show that the OPARACHEE methodology employing low-amplitude RF trains can detect paramagnetic exchanging agents in vivo.

Long-lived states in solution NMR: selection rules for intramolecular dipolar relaxation in low magnetic fields

Recent work has shown that singlet states of two-spin systems in low magnetic fields can have lifetimes up to an order of magnitude longer than the usual spin-lattice relaxation time. This result may enable new applications of NMR, and in particular hyperpolarized NMR via parahydrogen-induced polarization, to the study of slow processes that take place over previously inaccessible timescales. At present it is unclear whether similar results apply to multi-spin systems, or if these long lifetimes are a peculiarity of the two-spin case. Moderately long-lived states have been observed in systems containing more than two spins, although the mechanisms that prolong their lifetimes are not well understood. Here we present formalism for the study of relaxation in multi-spin systems in low magnetic fields. This approach is used to derive a family of quantum-mechanical selection rules governing intramolecular dipolar relaxation at low field that may account for the extended lifetimes observed in multi-spin systems.

Structure and serological analysis of the Hafnia alvei 481-L O-specific polysaccharide containing phosphate in the backbone chain

The lipopolysaccharide was extracted from cells of Hafnia alvei 481-L bacterial strain and, after mild acid hydrolysis, the O-specific polysaccharide was isolated and characterised. On the basis of chemical analyses and NMR spectroscopic studies of the polysaccharide and oligosaccharides obtained after Smith degradation, or hydrogen fluoride treatment, it was found that the repeating unit of the O-specific polysaccharide is a phosphorylated hexasaccharide: [see text]. The biological repeating unit of the H. alvei 481-L O-antigen has galactose phosphate at the nonreducing terminus. Serological tests indicate that this strain represents an individual serotype in the H. alvei genus.

High-resolution anatomic, diffusion tensor, and magnetization transfer magnetic resonance imaging of the optic chiasm at 3T

PURPOSE

To evaluate techniques for anatomical and physiological imaging of the intracranial optic nerve (ON), optic chiasm (OC), and optic tract (OT) at 3T with the aim of visualizing axonal damage in multiple sclerosis (MS).

MATERIALS AND METHODS

Imaging was performed on a 3T scanner employing a custom-designed head coil that consisted of a coil array with four coils (30 x 30 cm(2)). Oblique fast spin echo (FSE) images, magnetization transfer (MT)-enhanced 3D gradient-echo (GRE) time-of-flight (TOF) images, and line scan diffusion images (LSDI) were obtained. Full diffusion tensor (DT) analysis was performed, and apparent diffusion coefficient (ADC), fractional anisotropy (FA), and fiber direction maps were obtained.

RESULTS

FSE anatomic images were obtained with an in-plane resolution of 0.39 x 0.52 mm(2). The in-plane resolution of the MT and LSDI images was 0.78 x 0.78 mm(2). The OC, intracranial ON, and OT can be seen on these images. The dominant fiber orientations in the OC, ON, and OT, as derived from the DT images, are displayed.

CONCLUSION

This study shows that by using 3T and a custom-designed, four-channel head coil, it is possible to acquire high-resolution anatomical and physiological images of the OC, ON, and OT. The pilot results presented here pave the way for imaging the anterior visual pathway in patients with MS.

On-resonance low B1 pulses for imaging of the effects of PARACEST agents

Application of the exchange-sensitive, low-power RF pulses positioned on the bulk water resonance for imaging of the effects of PARACEST agents is proposed as an alternative to the standard CW off-resonance irradiation. Specifically, we applied a low-power WALTZ-16 RF train, with the 90 degrees pulse unit replaced by a pulse of the fixed length (WALTZ-16*). Using this sequence, the bulk water signal was found to be sensitive to exchange lifetimes with PARACEST complex bound protons, the transverse relaxation time of bulk water, and longitudinal relaxation time of bound protons. In this report, the concept of using WALTZ-16* to "activate" a PARACEST effect is introduced and some of the salient features of this technique with respect to experimental conditions and performance levels are discussed. Computational predictions are verified and explored by comparison with experimental spectroscopic and imaging data. It is shown that WALTZ-16* can be used to detect PARACEST agents with an RF intensity as low as 200 Hz for concentrations as low as a few tens of microM for lanthanide chelates having appropriate water-exchange rates (Tm,Dy).

High resolution heteronuclear correlation NMR spectroscopy between quadrupolar nuclei and protons in the solid state

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t1 under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t1 period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t2 time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and 23Na-1H 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons.

Improvement of homonuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance utilising radiofrequency imperfections

The often annoying imperfections in the phases and pulses of typical radiofrequency multiple-pulse irradiation schemes for homonuclear dipolar decoupling are revisited and analysed here. The analysis is with respect to one such multiple-pulse sequence, namely, the windowed phase-modulated Lee-Goldburg sequence. The error terms in the Hamiltonian due to pulse imperfections may lead to effective rotation of the spins around the z-axis giving rise to image free and high-resolution 1H spectra. Certain precautions to be taken with regard to scale factor estimation are also detailed. The analysis also points out the range of off-set values where the best homonuclear dipolar decoupling performance of a particular pulse scheme may be obtained.

Structural and genetic characterization of glycosylation of type a flagellin in Pseudomonas aeruginosa

Type a flagellins from two strains of Pseudomonas aeruginosa, strains PAK and JJ692, were found to be glycosylated with unique glycan structures. In both cases, two sites of O-linked glycosylation were identified on each monomer, and these sites were localized to the central, surface-exposed domain of the monomer in the assembled filament. The PAK flagellin was modified with a heterogeneous glycan comprising up to 11 monosaccharide units that were O linked through a rhamnose residue to the protein backbone. The flagellin of JJ692 was less complex and had a single rhamnose substitution at each site. The role of the glycosylation island gene cluster in the production of each of these glycosyl moieties was investigated. These studies revealed that the orfA and orfN genes were required for attachment of the heterologous glycan and the proximal rhamnose residue, respectively.

Selective polarization inversion of protons in rotating solids

The selective inversion of lines under phase modulated Lee-Goldburg (PMLG) decoupling in MAS proton spectroscopy is demonstrated. Short pulses inserted between consecutive PMLG irradiation intervals selectively invert the polarization of an on-resonance line while sustaining a high resolution proton evolution. The pulse scheme is combined with windowed-PMLG detection to obtain a one-dimensional high resolution spectrum with one of the proton lines inverted. Initial preparation of the protons in selectively inverted states can be used to follow the flow of polarization during spin diffusion. Examples of proton-proton spin exchange in alanine and histidine are demonstrated. Selective inversion is also used in conjunction with proton carbon LG-cross-polarization to achieve carbon spectra with lines characterized by different polarization states.

Structure of the Lipopolysaccharide Core Region of the Bacteria of the Genus Proteus

The lipopolysaccharide (LPS) core structure was studied in seven rough strains of Proteus and 26 smooth strains belonging to various Proteus O-serogroups. All LPSs share a common heptasaccharide fragment, which includes two Kdo, three Hep, one Glc, and one GalA residue. Core structures differ between strains and within each strain in the presence of a variety of additional monosaccharides and non-sugar substituents. In many strains, the LPS includes a cyclic acetal of GalNAc in the open-chain form, which builds up a new type of linkage between monosaccharides. The covalent linkage of aliphatic polyamines, e.g. putrescine and spermidine, to the LPS was confirmed for the first time and the location of the amines at the carboxyl group of a GalA residue established. Analyses revealed peculiar features of the core structure, which are characteristic of P. mirabilis on one hand and P. vulgaris and P. penneri on the other hand.

Structural analysis of the core region of the lipopolysaccharides from eight serotypes of Klebsiella pneumoniae

The core regions of the lipopolysaccharides (LPS) from Klebsiella pneumoniae serotypes O1, O2a, O2a,c, O3, O4, O5, O8, and O12 were analysed using NMR spectroscopy, ESI-MS spectroscopy, and chemical methods. All the LPSs had similar core structures, as shown below, differing only in the number and position of beta-D-galacturonic acid substituents: [carbohydrate structure: see text] where P is H or alpha-Hep, J, K is H or beta-GalA. LPS from all serotypes contained varying proportions of structures having additional or missing phosphate substituents. The core from serotype O1 contained a minor amount of a previously described variant with alpha-DD-Hep-(1-->2)-alpha-DD-Hep-(1-->6)-alpha-GlcN-(1--> replacing the alpha-Hep-(1-->4)-alpha-Kdo-(2-->6)-alpha-GlcN-(1--> component.

Structural analysis of the carbohydrate components of the outer membrane of the lipopolysaccharide-lacking cellulolytic ruminal bacterium Fibrobacter succinogenes S85

The polysaccharides from the outer membrane of the Gram-negative ruminal bacterium Fibrobacter succinogenes were isolated by phenol/water extraction and separated by size-exclusion chromatography in the presence of deoxycholate detergent into a lower-molecular-mass fraction designated 'glycolipid' and a high-molecular-mass 'capsular polysaccharide' fraction. Both fractions lacked typical lipopolysaccharide components including 2-keto-3-deoxyoctulosonic acid and 3-hydroxy fatty acids. Carbohydrate components of these fractions were represented by two polysaccharides and one oligosaccharide (possibly glycolipid) with the following structures: : : where HEAEP is N-(2-hydroxyethyl)-2-aminoethylphosphonic acid, found for the first time in natural compounds. The polysaccharides contained pentadecanoic acid and anteisopentadecanoic acid, possibly present as the acyl components. All constituent monosaccharides except L-rhamnose had a D-configuration. In addition to having a structural role in the outer membrane, these polysaccharides may provide protection for this lipopolysaccharide-less bacterium in the highly competitive ruminal environment, as phosphonic acids covalently linked to membrane polymers have in the past been attributed the function of stabilizing membranes in the presence of phosphatases and lipases.

The structure of the lipopolysaccharide O-antigen produced by Flavobacterium psychrophilum (259-93)

Flavobacterium psychrophilum, a Gram-negative bacterium, is the etiological agent of rainbow trout fry syndrome and bacterial cold water disease, septicemic infections in reared salmonids. In humans Flavobacterium spp. have been associated with neonatal meningitis and septicemia, catheter-associated bacteremia, and pneumonia. Recently, several F. psychrophilum surface molecules, including lipopolysaccharide (LPS), have been implicated in its pathogenesis and identified as potential vaccine and diagnostic candidate macromolecules. Studies on the LPS produced by the bacterium are reported herein. The structure of the antigenic O-polysaccharide contained in the LPS of F. psychrophilum was deduced by the application of analytical NMR spectroscopy, mass spectrometry, glycose and methylation analysis, and partial hydrolysis degradations, and was found to be an unbranched polymer of trisaccharide repeating units composed of L-rhamnose (L-Rhap), 2-acetamido-2-deoxy-L-fucose (L-FucpNAc) and 2-acetamido-4-((3S,5S)-3,5-dihydroxyhexanamido)-2,4-dideoxy-D-quinovose (D-Quip2NAc4NR, 2-N-acetyl-4-N-((3S,5S)-3,5-dihydroxyhexanoyl)-D-bacillosamine) (1 : 1 : 1) and having the structure: -->4)-alpha-L-FucpNAc-(1-->3)-alpha-D-Quip2NAc4NR-(1-->2)- alpha-L-Rhap-(1--> where R is (3S,5S)-CH3CH(OH)CH2CH(OH)CH2CO-.

The structure of the core region of the lipopolysaccharide from Klebsiella pneumoniae O3. 3-deoxy-alpha-D-manno-octulosonic acid (alpha-Kdo) residue in the outer part of the core, a common structural element of Klebsiella pneumoniae O1, O2, O3, O4, O5, O8, and O12 lipopolysaccharides

The structure of lipid A-core region of the lipopolysaccharide (LPS) from Klebsiella pneumoniae serotype O3 was determined using NMR, MS and chemical analysis of the oligosaccharides, obtained by mild acid hydrolysis, alkaline deacylation, and deamination of the LPS: [carbohydrate structure see text] where P is H or alpha-Hep; J is H or beta-GalA; R is H or P (in the deacylated oligosaccharides). Screening of the LPS from K. pneumoniae O1, O2, O4, O5, O8, and O12 using deamination showed that they also contain alpha-Hep-(1-->4)-alpha-Kdo-(2-->6)-GlcN and alpha-Kdo-(2-->6)-GlcN fragments.

The structure of the carbohydrate backbone of the core-lipid A region of the lipopolysaccharide from Proteus penneri strain 40: new Proteus strains containing open-chain acetal-linked N-acetylgalactosamine in the core part of the LPS

Analysis of the core part of the LPS from several strains of Proteus revealed that P. penneri strains 2, 11, 19, 107, and P. vulgaris serotypes 04 and 08 have the same structure with a new type of linkage between monosaccharidesan open-chain acetal--that was previously determined for P. vulgaris OX2 and P. penneri 17. The LPS from P. penneri strain 40 contains the same structure substituted with one additional monosaccharide: [molecular structure: see text] where (1S)-GalaNAc1 is a residue of N-acetyl-D-galactosamine in the open-chain form. It is connected as a cyclic acetal to positions 4 and 6 of the galactosamine residue having a free amino group. All other sugars are in the pyranose form.

The structure of the nonreducing terminal groups in the O-specific polysaccharides from two strains of Bordetella bronchiseptica

The structures of the polysaccharide chains of the LPS from Bordetella bronchiseptica strains 110H and Bp512 were analysed by NMR spectroscopy and mass spectrometry. The polysaccharides consist of alpha-(1-4)-linked 2,3-diacetamido-2,3-dideoxy-L-galacturonic acid repeating units. Polysaccharides from both strains have 2,3, 4-triamino-2,3,4-trideoxy-alpha-galacturonamide derivatives at their nonreducing ends, a monosaccharide identified for the first time in nature. The polymers from the two strains differ in the nature of the acylation of the amino groups of this monosaccharide. In the strain 110H, the residue is formylated at positions 3 and 4, and has N-formyl-L-alanyl or L-alanyl substituents at N-2. In the strain Bp512, the amino group at position 2 is acetylated, at position 3 it is formylated, and the amino group at position 4 bears a 2-methoxypropionyl substituent. The distribution of the acyl groups was determined from long range 1H-13C correlation (HMBC) NMR spectra. Measurement of the spectra under different pH conditions showed that carboxyl groups of the inner uronic acid residues of the polymeric chain are free, and that carboxyl groups of the terminal residues are amidated. These conclusions were confirmed by the results of mass spectrometric analysis.