Biomedical subjects

Wolfgang Dreher

Publications and source records attributed to Wolfgang Dreher.

16 recordsLinked to original sources

PubMed2007

Fast 3D 1H spectroscopic imaging at 3 Tesla using spectroscopic missing-pulse SSFP with 3D spatial preselection.

Three-dimensional (3D) (1)H MR spectroscopic imaging (SI) allows metabolic changes in human tissue to be identified. In clinical practice, fast acquisition techniques are required to achieve an adequate spatial resolution within acceptable total measurement times. In this study a novel fast pulse sequence for 3D (1)H SI based on the condition of steady-state free precession (SSFP), termed "spectroscopic missing-pulse SSFP" (spMP-SSFP), is proposed. It combines 3D spatial preselection with the acquisition of full spin echoes (SEs), and thus makes subsequent phase correction of spectra redundant. The sequence was applied to a phantom and healthy human brains in vivo at 3 Tesla. Metabolic images are acquired with a spatial resolution of 1.8 cm(3) within a total measurement time of about 6 min. With a lower signal-to-noise ratio (SNR) per unit measurement time compared to previous spectroscopic SSFP implementations, 3D spatial preselection can now be realized with spMP-SSFP. Since the method does not require separate techniques for water and lipid suppression, and employs a simple data-processing approach, spMP-SSFP is a robust, fast SI method that requires only minimal user interaction.

Brain↗

PubMed2006

Fast 1H spectroscopic imaging using steady state free precession and spectral-spatial RF pulses.

Recently, new methods for fast (1)H spectroscopic imaging based on the condition of steady state free precession (SSFP) were introduced to achieve a high signal-to-noise ratio at short minimum measurement times. In this work, a major improvement is presented to overcome a crucial drawback in some of the former sequences: the lack of spatial selectivity. Good spectral selectivity at very high sampling efficiency can be achieved by using spectral-spatial RF pulses, and combined with localised shimming. Results are shown from both phantom experiments and in vivo studies on the rat brain acquired at 4.7 T.

Animals↗

PubMed2006

Effects of zero-filling and apodization on spectral integrals in discrete Fourier-transform spectroscopy of noisy data.

The influence of noise on the standard deviation of spectral integrals is examined. Calculations assuming discrete Fourier-transform data are compared with Monte-Carlo simulations. The effects of zero-filling and apodization are examined for free-induction-decay (FID) signals and for symmetric spin-echo signals in one and two dimensions, with particular attention to features not previously presented in the literature. Findings suggest that for mild apodization, the known sensitivity enhancement due to zero-filling in either the real or the imaginary part signal [E. Bartholdi, R.R. Ernst, Fourier spectroscopy and the causality principle, J. Magn. Reson., 11 (1973) 9-19] is maintained; however, for stronger apodization filters, this enhancement can be obliterated completely. It is shown that results obtained by analysis of one-dimensional signals can be readily applied to multi-dimensional data. Furthermore, zero-filling has a negligible effect for symmetric spin-echo signals with implications for signal averaging in magnetic resonance imaging and spectroscopic imaging.

Journal Article↗

PubMed2006

Fast 3D echo planar SSFP-based 1H spectroscopic imaging: demonstration on the rat brain in vivo.

A fast proton spectroscopic imaging pulse sequence based on the condition of steady-state free precession is presented. High 3D spatial and temporal resolution is achieved using simultaneous detection of both one spatial and one spectral dimension, with a time-dependent gradient cycle known from echo planar imaging. Additionally, in order to increase the spectral width of the measurement, an interleaved acquisition scheme is shown either for systems with limited gradient switching capabilities or applications with a wide chemical shift range. The pulse sequence is implemented on a standard 4.7-T nuclear magnetic resonance animal imaging system. Measurements with a total measurement time of less than 2.5 min and a nominal voxel size of 6.75 microl using a total of 64 x 32 x 16 voxels are performed on phantoms and healthy rat brain in vivo allowing the rapid detection of signals from both uncoupled and J-coupled spin systems with high signal-to-noise ratio.

Animals↗

PubMed2006

Toward quantitative short-echo-time in vivo proton MR spectroscopy without water suppression.

A methodological development for quantitative short-echo-time (TE) in vivo proton MR spectroscopy (MRS) without water suppression (WS) is described that integrates experimental and software approaches. Experimental approaches were used to eliminate frequency modulation sidebands and first-order phase errors. The dominant water signal was modeled and extracted by the matrix pencil method (MPM) and was used as an internal reference for absolute metabolite quantification. Spectral fitting was performed by combining the baseline characterization by a wavelet transform (WT)-based technique and time-domain (TD) parametric spectral analysis using full prior knowledge of the metabolite model spectra. The model spectra were obtained by spectral simulation instead of in vitro measurements. The performance of the methodology was evaluated by Monte Carlo (MC) studies, phantom measurements, and in vivo measurements on rat brains. More than 10 metabolites were quantified from spectra measured at TE = 20 ms on a 4.7 T system.

Algorithms↗

PubMed2005

Structure elucidation of phase II metabolites by tandem mass spectrometry: an overview.

The present paper provides a summary of the collision-induced dissociation of protonated and deprotonated phase II metabolites of drugs and pesticides. This overview is based on published literature and unpublished data from the authors. In particular, glutathione conjugates and their biotransformation products are discussed in detail. In addition, the fragmentation of the major classes of conjugates, i.e. glucuronides, glucosides, malonylglucosides, sulfates, acetates, methyl and glycine conjugates, is reported. Collision-induced dissociation, as studied by tandem mass spectrometry, allows the rapid identification of the type of conjugate, whereas the exact conjugation site can in general be determined only by additional NMR experiments.

Mass Spectrometry↗

PubMed2005

New method for the simultaneous detection of metabolites and water in localized in vivo 1H nuclear magnetic resonance spectroscopy.

A new two-scan method for localized 1H in vivo NMR spectroscopy (MRS) without water suppression (WS) is described. In one of the scans, two chemical shift selective 180 degrees pulses are applied prior to a standard localization sequence to invert all metabolite signals upfield and downfield from water, which remains unaffected. The difference spectrum records the metabolites whereas water and accompanying gradient induced artifacts are widely suppressed. The method was implemented on a 4.7-T system using point resolved spectroscopy with a short echo time of 18 ms. Phantom measurements proved the feasibility of absolute quantification using water as an internal reference. Measurements on healthy rat brain yielded comparable spectrum quality as measurements with water presaturation. The method does not require additional adjustments or sophisticated data postprocessing and scales favorably with increasing B(0) field. Therefore, the method should be useful for 1H MRS without WS. Although the two-step method doubles the minimum total measurement time, it may also be of interest for spectroscopic imaging (SI) without WS, in particular if fast SI techniques are applied.

Algorithms↗

PubMed2004

Experimental method to eliminate frequency modulation sidebands in localized in vivo 1H MR spectra acquired without water suppression.

An experimental method is described to eliminate frequency modulation sidebands in localized in vivo proton MR spectra acquired without water suppression. Based on the fact that the phases of the sideband artifact signals are coherent with the phases of the gradient pulses, the proposed method measures and combines signals with sequences using opposite gradients to cancel the sidebands. Factors effecting the results of the cancellation are investigated. Optimized cancellation is achieved by trimming the refocusing gradients, optimizing the spoiler gradients, or replacing the spoiler gradients with a 16-step RF phase cycling. The performance of the method is demonstrated by phantom and in vivo experiments on the healthy rat brain.

Animals↗

PubMed2004

[Evidence-based medicine and public health law: statutory health insurance].

Beyond all differences in terminology and legal principles between the laws governing private health insurance, the governmental financial support for civil, servants and statutory health insurance the fundamental issues to be solved by the courts in case of litigation are quite similar. But only a part of these refer to the quality of medical services, which is the main concern of Evidence-based Medicine (EbM); EbM, though, is not able to contribute towards answering the equally important question of how to distinguish between "treatment" and "(health-relevant) lifestyle". The respective definitions that have been developed in the particular fields of law are only seemingly divergent from each other and basically unsuitable to aid the physician in his clinical decision-making because the common blanket clauses of public health law are regularly interpreted as rules for the exclusion of certain claims and not as a confirmatory paraphrase of what is clinically necessary. If on the other hand medical quality is what lies at the core of litigation, reference to EbM may become necessary. In fact, it is already common practice in the statutory health insurance system that decision-making processes in the Federal Committee being responsible for quality assurance (Bundesausschuss) are based on EbM principles and that in exceptional cases only the courts have to medically review the Federal Committee's decisions.

Evidence-Based Medicine↗

PubMed2003

PRESS-based proton single-voxel spectroscopy and spectroscopic imaging with very short echo times using asymmetric RF pulses.

Modified point-resolved spectroscopy (PRESS) sequences for single voxel spectroscopy (MRS) and spectroscopic imaging (SI) with very short echo time ( T(E)) are described using asymmetric radio-frequency (RF) pulses as well as an optimized design and timing of the PRESS sequence. The proposed sequences were implemented on a standard 4.7 T imaging system yielding a T(E) of 6.0 ms only. Simulations and experimental data measured on phantoms and the rat brain in vivo are presented for MRS and SI showing a high signal-to-noise ratio and hardly any phase distortions caused by J-coupling.

Algorithms↗

PubMed2003

Fast U-FLARE-based correlation-peak imaging with complete effective homonuclear decoupling.

A new fast correlation-peak imaging technique is presented that combines 2D correlation spectroscopy with an ultrafast low-angle rapid acquisition with relaxation enhancement (U-FLARE) imaging module. Constant time chemical shift (CS) encoding is used in both time dimensions to achieve effective homonuclear decoupling in both frequency dimensions. The intervals between excitation and mixing (t(c1)) and between mixing and start of the MRI module (t(c2)) can be optimized to maximize the coherence transfer for a particular metabolite. Experiments were performed with evolution times t(c1) and t(c2) of 100 ms and 50 ms, respectively, which were determined by simulation of the spectroscopic part of the sequence for the spin systems of myo-inositol (Ins) and taurine (Tau). The use of a circularly reduced CS-encoding scheme shortened the minimum total measurement time to 35 min. The sequence was implemented on a 4.7 T imaging system and tested on a spherical phantom filled with a solution of Ins. The in vivo application of this method on healthy rat brain demonstrates its improved spectral resolution, as cross-peak signals from both Ins and Tau can be separated clearly.

Animals↗

PubMed2003

Fast proton spectroscopic imaging using steady-state free precession methods.

Various pulse sequences for fast proton spectroscopic imaging (SI) using the steady-state free precession (SSFP) condition are proposed. The sequences use either only the FID-like signal S(1), only the echo-like signal S(2), or both signals in separate but adjacent acquisition windows. As in SSFP imaging, S(1) and S(2) are separated by spoiler gradients. RF excitation is performed by slice-selective or chemical shift-selective pulses. The signals are detected in absence of a B(0) gradient. Spatial localization is achieved by phase-encoding gradients which are applied prior to and rewound after each signal acquisition. Measurements with 2D or 3D spatial resolution were performed at 4.7 T on phantoms and healthy rat brain in vivo allowing the detection of uncoupled and J-coupled spins. The main advantages of SSFP based SI are the short minimum total measurement time (T(min)) and the high signal-to-noise ratio per unit measurement time (SNR(t)). The methods are of particular interest at higher magnetic field strength B(0), as TR can be reduced with increasing B(0) leading to a reduced T(min) and an increased SNR(t). Drawbacks consist of the limited spectral resolution, particularly at lower B(0), and the dependence of the signal intensities on T(1) and T(2). Further improvements are discussed including optimized data processing and signal detection under oscillating B(0) gradients leading to a further reduction in T(min).

Animals↗

PubMed2003

Diffusion in compartmental systems. I. A comparison of an analytical model with simulations.

This article examines the way in which microscopic tissue parameters affect the signal attenuation of diffusion-weighted MR experiments. The influence of transmembrane water flux on the signal decay is emphasized using the Kärger equations, which are modified with respect to the cellular boundary restrictions for intra- and extracellular diffusion. This analytical approach is extensively compared to Monte-Carlo simulations for a tissue model consisting of two compartments. It is shown that diffusion-weighted MR methods provide a unique tool for estimation of the intracellular exchange time. Restrictions of applicability to in vivo data are examined. It is shown that the intracellular exchange time strongly depends on the size of a cell, leading to an apparent diffusion time dependence for in vivo data. Hence, an analytical model of a two-compartment system with an averaged exchange time is inadequate for the interpretation of signal curves measured in vivo over large ranges of b-values. Furthermore, differences of multiexponential signal curves, as obtained by different methods of diffusion weighting, can be explained by the influence of transmembrane water flux.

Algorithms↗

PubMed2003

Diffusion in compartmental systems. II. Diffusion-weighted measurements of rat brain tissue in vivo and postmortem at very large b-values.

Diffusion-weighted single-voxel (1)H spectroscopic measurements were performed on rat brain tissue in vivo and postmortem. Diffusion weighting was achieved by varying the diffusion time from 23 ms to 1.18 sec via the mixing time in a stimulated echo sequence. A series of constant gradient (cg-) experiments of eight effective gradient strengths q(2)(q(2) = gamma(2)delta(2)g(2)) from 24.2 x 10(3) to 490.2 x 10(3) mm(-2) was performed, resulting in a maximum attenuation factor of b = 580,000 s/mm(2). A fit of three exponential terms was found to be appropriate to represent the attenuation signal over the whole b-range. The behavior of the slowest decaying component can be fully understood in terms of a long time limit of a modified Kärger formalism for a two-compartment system. This allowed estimation of the transmembrane water exchange rate: the intracellular exchange time was determined to be 622 +/- 29 ms and 578 +/- 20 ms in vivo and postmortem, respectively.

Animals↗

PubMed2002

Fast proton spectroscopic imaging with high signal-to-noise ratio: spectroscopic RARE.

A new fast spectroscopic imaging (SI) method is presented which is based on spatial localization by the fast MRI method of rapid acquisition with relaxation enhancement (RARE) and encoding of the chemical shift information by shifting the position of a refocusing 180 pulse in a series of measurements. This method is termed spectroscopic RARE. In contrast to spectroscopic ultrafast low-angle RARE (U-FLARE), the formation of two echo families (odd and even) is suppressed by using a train of 180 RF pulses with an internal four-step phase cycle. By this means a high signal-to-noise ratio (SNR) per unit measurement time is obtained, because the separation of odd and even echoes, as well as dummy echoes to stabilize the echo amplitudes, is not needed anymore. The method is of particular interest for detecting signals of coupled spins, as effective homonuclear decoupling can be achieved by use of constant evolution time chemical shift encoding. The pulse sequence was implemented on a 4.7 T imaging system, tested on phantoms, and applied to the healthy rat brain in vivo. Spectroscopic RARE is particularly useful if T2* double less-than sign T2, which is typically fulfilled for in vivo proton SI measurements at high magnetic field strength.

Animals↗

PubMedDate not supplied

Long-term influence of perinatal asphyxia on the social behavior in aging rats.

BACKGROUND: Various groups have been addressing the question of whether perinatal asphyxia (PA) affects the behavior of young animals, but no information is available on long-term effects of PA on the behavior in aged rats, although it has been postulated that PA may lead to neurological and psychiatric deficits in adult life. OBJECTIVE: We, therefore, decided to study the effects of PA on social and anxiety-related behaviors in 2-year-old rats, using a noninvasive animal model resembling the clinical situation. METHODS: For the behavioral studies, the open-field test, the elevated plus-maze test, and a social interaction test in pairs were performed. Magnetic resonance imaging of the brain was selected to rule out neuropathological changes due to the aging process per se, as well as asphyxia-induced pathologies in the brain areas known to play an important role in the modulation of behavior. RESULTS: The social interaction test revealed a statistically significant increase in the number of social grooming episodes and the time spent running alone, whereas the numbers of social sniffing and fighting episodes and the time spent running together were decreased in the asphyxiated group. The elevated plus- maze test revealed a higher presence of entries into the closed arm. Furthermore, sniffing and self-grooming episodes were significantly increased in the asphyxiated group. CONCLUSIONS: We found a significantly decreased social aggressiveness and an increased social contact behavior as well as increased anxiety levels in the asphyxiated animals. The present findings may provide important information on the long-term behavioral sequelae of PA in the aged individual.

Aggression↗