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  • 1
    Keywords: ENERGIES ; SPECTRA ; Germany ; human ; IN-VIVO ; VIVO ; SYSTEM ; SYSTEMS ; TISSUE ; ACCURACY ; FREQUENCY ; FIELD ; H-1 ; INVIVO ; SPECTROSCOPY ; FORM ; DIFFERENCE ; ENERGY ; NMR-SPECTROSCOPY ; MUSCLE ; SKELETAL-MUSCLE ; ORIENTATION ; H-1 NMR ; carnosine,in vivo H-1 NMR,human calf muscle,second-order spectra,Breit-Rabi formula ; HUMAN SKELETAL-MUSCLE ; INTRACELLULAR PH ; LACTATE ; ORIGIN ; POSTMORTEM ; PROTON MR SPECTROSCOPY
    Abstract: Spin systems with residual dipolar couplings such as creatine, taurine, and lactate in skeletal muscle tissue exhibit first-order spectra in in vivo H-1 NMR spectroscopy at 1.5 T because the coupled protons are represented by (nearly) symmetrized eigenfunctions. The imidazole ring protons (H2, H4) of carnosine are suspected to form also a coupled system. The ring's stiffness could enable a connectivity between these anisochronous protons with the consequence of second-order spectra at low field strength. Our purpose was to study whether this deviation from the Paschen-Back condition can be used to detect the H2-H4 coupling in localized 1D H-1 NMR spectra obtained at 1.5 T (64 MHz) from the human calf in a conventional whole-body scanner. As for the hydrogen hyperfine interaction, a Breit-Rabi equation was derived to describe the transition from Zeeman to Paschen-Back regime for two dipolar-coupled protons. The ratio of the measurable coupling strength (Sk) and the difference in resonance frequencies of the coupled spins (Deltaomega) induces quantum-state mixing of various degree upon definition of an appropriate eigenbase of the coupled spin system. The corresponding Clebsch-Gordan coefficients manifest in characteristic energy corrections in the Breit-Rabi formula. These additional terms were used to define an asymmetry parameter of the line positions as a function of Sk and Deltaomega. The observed frequency shifts of the resonances were found to be consistent with this parameter within the accuracy achievable in in vivo NMR spectroscopy. Thus it was possible to identify the origin of satellite peaks of H2, H4 and to describe this so far not investigated type of residual dipolar coupling in vivo. (C) 2003 Elsevier Inc. All rights reserved
    Type of Publication: Journal article published
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  • 2
    Keywords: DYNAMICS ; RELAXATION ; RAT-LIVER ; NMR-SPECTROSCOPY ; human brain ; CARTILAGE ; inversion-recovery ; B-0 inhomogeneity ; Coherence ; Multiple-quantum filter ; NA-23 ; SHIFT-REAGENT ; Sodium MRI ; SPIN-3/2
    Abstract: Triple-quantum (TQ) filtered sodium MR imaging has been proposed for separation of sodium signal arising from different physiological compartments. in a three-pulse sequence without refocussing pulse, the TQ signal is strongly sensitive to inhomogeneities of the B-0 field. We examine the dependence of the TQ signal intensity on the sequence parameters and propose a modified phase-cycling scheme to improve image quality. A new method for correction of B-0 inhomogeneity artefacts in TQ filtered sodium imaging is presented which requires only two acquisitions to obtain a correction as far as the B-0 inhomogeneity and the Pulse widths are not too large. The method was verified in phantom experiments. (C) 2009 Elsevier Inc. All rights reserved
    Type of Publication: Journal article published
    PubMed ID: 20004122
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  • 3
    Keywords: SYSTEM ; MRI ; WATER ; RATES ; PH ; human brain ; POWER ; RESONANCE ; PARACEST AGENTS ; PROTON-EXCHANGE PROCESSES ; 3-Pool model ; Amide proton transfer ; Bloch-McConnell equations ; Chemical exchange saturation transfer ; Magnetization transfer ; Proton transfer rate ; SATURATION-TRANSFER ; Spillover ; z-Spectra modeling
    Abstract: Chemical exchange saturation transfer (CEST) processes in aqueous systems are quantified by evaluation of z-spectra, which are obtained by acquisition of the water proton signal after selective RF presaturation at different frequencies. When saturation experiments are performed in vivo, three effects are contributing: CEST, direct water saturation (spillover), and magnetization transfer (MT) mediated by protons bound to macromolecules and bulk water molecules. To analyze the combined saturation a new analytical model is introduced which is based on the weak-saturation-pulse (WSP) approximation. The model combines three single WSP approaches to a general model function. Simulations demonstrated the benefits and constraints of the model, in particular the capability of the model to reproduce the ideal proton transfer rate (PTR) and the conventional MT rate for moderate spillover effects (up to 50% direct saturation at CEST-resonant irradiation). The method offers access to PTR from z-spectra data without further knowledge of the system, but requires precise measurements with dense saturation frequency sampling of z-spectra. PTR is related to physical parameters such as concentration, transfer rates and thereby pH or temperature of tissue, using either exogenous contrast agents (PARACEST, DIACEST) or endogenous agents such as amide protons and -OH protons of small metabolites.
    Type of Publication: Journal article published
    PubMed ID: 21641247
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  • 4
    Keywords: CELLS ; CONTRAST ; MRI ; MAGNETIC-RESONANCE ; LOCALIZATION ; RECONSTRUCTION ; NANOPARTICLES ; PASSIVE TRACKING ; REMOTE-CONTROL ; DEFLECTION
    Abstract: Interventional devices with ferromagnetic components can be manipulated remotely using forces induced by the MRI gradients. To deflect the tip of an endovascular catheter, large ferromagnetic spheres of 2mm diameter are required to exert sufficiently high magnetic forces; however, tracking of these devices is difficult due to the large image artifacts. In this study, a new dual-echo technique is proposed to improve the stability of localizing and tracking medical devices with ferromagnetic components. MR tracking methods with selective off-resonant excitation and phase compensation with a rephasing gradient can detect ferromagnetic spheres up to a diameter of 1mm only. In this work, a dual-echo technique is used with two rephasing gradients to stabilize the off-set localization. With rephasing being applied in orthogonal directions, an SNR of 5 was achieved in the signal projections. Compared to a single-echo acquisition the dual-echo method reduces the position error in a phantom from 8mm to 1.6mm. In an in vivo study a tracking precision of 4mm was measured without steering gradients at an image update rate of 2 images per second. Steering experiments were successfully performed with a prototype catheter with ferromagnetic sphere in an aorta phantom and in the vena cava of a pig.
    Type of Publication: Journal article published
    PubMed ID: 23892103
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  • 5
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    Keywords: DOMAIN ; MRI ; SPECTROSCOPY ; RECONSTRUCTION ; EXCHANGE SATURATION-TRANSFER ; SPARSITY
    Abstract: We describe a new MR imaging method for the rapid characterization or screening of chemical exchange saturation transfer (CEST) contrast agents. It is based on encoding the chemical shift dimension with an additional gradient as proposed in previous ultrafast CEST spectroscopy approaches, but extends these with imaging capabilities. This allows us to investigate multiple compounds simultaneously with an arbitrary sample tube arrangement. The technique requires a fast multislice readout to ensure the saturation is not lost during data acquisition due to T1 relaxation. We therefore employ radial subsampling, acquiring only 10 projections per CEST image with a 128x128 matrix. To recover the images, we use a heuristic reconstruction algorithm that incorporates low rank and limited object support as prior knowledge. This way, we are able to acquire a spectral CEST data set consisting of 15 saturation offsets more than 16 times faster than compared with conventional CEST imaging.
    Type of Publication: Journal article published
    PubMed ID: 24721681
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  • 6
    Abstract: The hyperpolarization of nuclear spins promises great advances in chemical analysis and medical diagnosis by substantially increasing the sensitivity of nuclear magnetic resonance (NMR). Current methods to produce a hyperpolarized sample, however, are arduous, time-consuming or costly and require elaborate equipment. Recently, a much simpler approach was introduced that holds the potential, if harnessed appropriately, to revolutionize the production of hyperpolarized spins. It was reported that high levels of hyperpolarization in nuclear spins can be created by irradiation with a laser beam carrying orbital angular momentum (twisted light). Aside from these initial reports however, no further experimental verification has been presented. In addition, this effect has so far evaded a critical theoretical examination. In this contribution, we present the first independent attempt to reproduce the effect. We exposed a sample of immersion oil or a fluorocarbon liquid that was placed within a low-field NMR spectrometer to Laguerre-Gaussian and Bessel laser beams at a wavelength of 514.5nm and various topological charges. We acquired (1)H and (19)F NMR free induction decay data, either during or alternating with the irradiation that was parallel to B0. We observed an irregular increase in NMR signal in experiments where the sample was exposed to beams with higher values of the topological charge. However, at no time did the effect reach statistical significance of 95%. Given the measured sensitivity of our setup, we estimate that a possible effect did not exceed a hyperpolarization (at 5mT) of 0.14-6%, depending on the assumed hyperpolarized volume. It should be noted though, that there were some differences between our setup and the previous implementation of the experiment, which may have inhibited the full incidence of this effect. To approach a theoretical description of this effect, we considered the interaction of an electron with a plane wave, which is known to be able to induce electronic (e.g. in rubidium) and subsequent nuclear hyperpolarization. Compared to the plane wave, the additional transitions caused by a twisted wave are of the order of 10(-3) less. This suggests that the twist of the laser is unlikely to be responsible for the hyperpolarization of nuclear spins, unless a new mechanism of momentum transfer is identified.
    Type of Publication: Journal article published
    PubMed ID: 27179228
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  • 7
    Abstract: Diffusion pore imaging (DPI) has recently been proposed as a means to acquire images of the average pore shape in an image voxel or region of interest. The highly asymmetric gradient scheme of its sequence makes it substantially demanding in terms of the hardware of the NMR system. The aim of this work is to show the feasibility of DPI on a preclinical 9.4T animal scanner. Using water-filled capillaries with an inner radius of 10mum, four different variants of the DPI sequence were compared in 1D and 2D measurements. The pulse sequences applied cover the basic implementation using one long and one temporally narrow gradient pulse, a CPMG-like variant with multiple refocusing RF pulses as well as two variants splitting up the long gradient and distributing it on either side of the refocusing pulse. Substantial differences between the methods were found in terms of signal-to-noise ratio, contrast, blurring, deviations from the expected results and sensitivity to gradient imperfections. Each of the tested sequences was found to produce characteristic gradient mismatches dependent on the absolute value, direction and sign of the applied q-value. Read gradients were applied to compensate these mismatches translating them into time shifts, which enabled 1D DPI yielding capillary radius estimations within the tolerances specified by the manufacturer. For a successful DPI application in 2D, a novel gradient amplitude adaption scheme was implemented to correct for the occurring time shifts. Using this adaption, higher conformity to the expected pore shape, reduced blurring and enhanced contrast were achieved. Images of the phantom's pore shape could be acquired with a nominal resolution of 2.2mum.
    Type of Publication: Journal article published
    PubMed ID: 28351813
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  • 8
    Keywords: IN-VIVO ; MRI ; ALZHEIMERS-DISEASE ; NMR-SPECTROSCOPY ; ISCHEMIA ; human brain ; NA+ ; TISSUE SODIUM CONCENTRATION ; TRANSVERSE RELAXATION ; QUANTUM
    Abstract: A new method is proposed for acquiring 3D biexponential-weighted sodium images with two instead of three RF pulses to allow for shorter repetition time at high magnetic fields (B-o 〉= 7T) and reduced SAR. The second pulse converts single- into triple-quantum coherences in regions containing sodium ions which are restricted in mobility. Since only single-quantum coherences can be detected, an image acquired after the second pulse is intrinsically single-quantum-filtered and can be used to generate a biexponential-weighted sodium image by a weighted subtraction with the spin-density-weighted image acquired between the pulses. The proposed sequence generates biexponential-weighted sodium images of in vivo human brain with 140% higher SNR than triple-quantum-filtered sodium images and 4% higher SNR than a biexponential-weighted sequence with three RF pulses at equal acquisition time and with 1/3 lower SAR. As SAR is reduced, accordingly repetition time can be spared to obtain even higher SNR-time efficiency. In comparison to a difference image generated from two images of a double-readout sequence, the proposed two-pulse sequence yields about 14% higher SNR. Our new two-pulse biexponential-weighted sequence allows for acquisition of full 3D data sets of the human brain in vivo with a nominal resolution of (5 mm)(3) in about 10 min.
    Type of Publication: Journal article published
    PubMed ID: 24530955
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  • 9
    Keywords: IN-VIVO ; TUMORS ; SUPPRESSION ; SPECTROSCOPY ; Coherence ; SEL-MQC ; SINGLE-SCAN ; RESONANCES
    Abstract: Selective detection of lactate signals in in vivo MR spectroscopy with spectral editing techniques is necessary in situations where strong lipid or signals from other molecules overlap the desired lactate resonance in the spectrum. Several pulse sequences have been proposed for this task. The double-quantum filter SSel-MQC provides very good lipid and water signal suppression in a single scan. As a major drawback, it suffers from significant signal loss due to incomplete refocussing in situations where long evolution periods are required. Here we present a refocused version of the SSel-MQC technique that uses only one additional refocussing pulse and regains the full refocused lactate signal at the end of the sequence.
    Type of Publication: Journal article published
    PubMed ID: 25909643
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  • 10
    Keywords: IN-VIVO ; RESTRICTED DIFFUSION ; SPIN-ECHO ; LAPLACIAN EIGENFUNCTIONS ; POROUS-MEDIA ; SELF-DIFFUSION ; MAGNETIC-FIELD GRADIENT ; NUCLEAR MAGNETIZATION ; SURFACE RELAXATION ; CPMG SEQUENCE
    Abstract: The time-dependent apparent diffusion coefficient as measured by pulsed gradient NMR can be used to estimate parameters of porous structures including the surface-to-volume ratio and the mean curvature of pores. In this work, the short-time diffusion limit and in particular the influence of the temporal profile of diffusion gradients on the expansion as proposed by Mitra et al. (1993) is investigated. It is shown that flow-compensated waveforms, i.e. those whose first moment is zero, are blind to the term linear in observation time, which is the term that is proportional to mean curvature and surface permeability. A gradient waveform that smoothly interpolates between flow-compensated and bipolar waveform is proposed and the degree of flow-compensation is used as a second experimental dimension. This two-dimensional ansatz is shown to yield an improved precision when characterizing the confining domain. This technique is demonstrated with simulations and in experiments performed with cylindrical capillaries of 100mum radius.
    Type of Publication: Journal article published
    PubMed ID: 26254733
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