Biomedical subjects

Harald E Möller

Publications and source records attributed to Harald E Möller.

14 recordsLinked to original sources

PubMed2005

Quantifying venous flow dynamics by flow-dephased and flow-rephased functional magnetic resonance imaging.

By combining flow-dephased and flow-rephased diffusion weighting with blood oxygenation level dependent functional magnetic resonance imaging, it is possible to study flow dynamics in the venous network of the human brain. Thereby, ballistic flow, which conserves direction and velocity during echo time, is separated from diffusive flow with many changes in direction and velocity. By using this technique with very low diffusion/flow weighting, the mean velocity of ballistic flow was quantified in this study. The result of 10.9+/-3.2 cm/s strongly indicates that large venous vessels are the source of ballistic flow.

Blood Flow Velocity↗

PubMed2005

Towards quantification of blood-flow changes during cognitive task activation using perfusion-based fMRI.

Multi-slice perfusion-based functional magnetic resonance imaging (p-fMRI) is demonstrated with a color-word Stroop task as an established cognitive paradigm. Continuous arterial spin labeling (CASL) of the blood in the left common carotid artery was applied for all repetitions of the functional run in a quasi-continuous fashion, i.e., it was interrupted only during image acquisition. For comparison, blood oxygen level dependent (BOLD) contrast was detected using conventional gradient-recalled echo (GE) echo planar imaging (EPI). Positive activations in BOLD imaging appeared in p-fMRI as negative signal changes corresponding to an enhanced transport of inverted water spins into the region of interest, i.e., increased cerebral blood flow (CBF). Regional differences between the localization of activations and the sensitivity of p-fMRI and BOLD-fMRI were observed as, for example, in the inferior frontal sulcus and in the intraparietal sulcus. Quantification of CBF changes during cognitive task activation was performed on a multi-subject basis and yielded CBF increases of the order of 20-30%.

Adult↗

PubMed2005

Serial proton spectroscopy in a case of adult-onset subacute sclerosing panencephalitis.

A case of subacute sclerosing panencephalitis in a 27-year-old man was serially evaluated with proton magnetic resonance spectroscopy. Metabolic abnormalities included decreased N-acetylaspartate and elevated choline and myo-inositol in a lesion visible on magnetic resonance imaging and in normal-appearing white matter. Lactate appeared increased within the lesion. Metabolic impairment was persistent after intrathecal interferon-alpha treatment. Spectroscopy pointing to ongoing inflammation, gliosis, and possible membrane turnover was more sensitive than imaging in detecting widespread pathology within the white matter.

Adult↗

PubMed2005

Is there a change in water proton density associated with functional magnetic resonance imaging?

In a recent series of studies (see, for example, Stroman et al. Magn Reson Imag 2001; 19:827-831), an increase of water proton density has been suggested to correlate with neuronal activity. Owing to the significant implications of such a mechanism for other functional experiments, the functional signal changes in humans at very short echo times were re-examined by spin-echo EPI at 3 T. The results do not confirm the previous hypothesis of a significant increase in extravascular proton density at TE = 0. Instead, an alternative explanation of the effect is offered: The use of a low threshold to identify activated voxels may generate an artificial offset in functional contrast due to the inclusion of false-positives in the analysis.

Adult↗

PubMed2005

Functional magnetic resonance imaging with intermolecular double-quantum coherences at 3 T.

Functional magnetic resonance imaging (fMRI) based on the selection of intermolecular double-quantum coherences (iDQC) was performed with a standard birdcage coil at 3 T in a group of normal human volunteers. Suppression of spurious signal contributions from unwanted coherence-transfer pathways was achieved by combining a two-step phase cycle and a long repetition time of 5 s. A gradient-recalled echo iDQC sequence (echo time, T(E) = 80 ms) yielded robust activation with a visual paradigm. Maximum z-scores were about half of those observed with conventional blood-oxygen level dependent fMRI, whereas the functional signal change increased by more than a factor of 5. No activation was obtained with a spin-echo iDQC sequence (T(E) = 160 ms), in which dephasing accumulated during the evolution period was fully rephased by an appropriate delay time. It is hypothesized that substantial inherent diffusion weighting of the iDQC technique efficiently suppresses intravascular contributions to the functional contrast. A consistent quantitative explanation of the observed amount of signal change currently remains speculative.

Adult↗

PubMed2005

Continuous arterial spin labeling at the human common carotid artery: the influence of transit times.

In evaluating the sensitivity of arterial spin labeling (CASL) and for quantification of perfusion, knowledge of the transit time from the labeling plane to the imaging slice is crucial. The purpose of the current study was to obtain estimates of transit times relevant under the specific experimental conditions of CASL in human subjects using a separate local labeling coil at the neck. Specifically, the post-label delay (PLD), i.e. the time between the end of the labeling period and the image acquisition, was varied either with or without additional application of crusher gradients to suppress intravascular signal contributions. The overall sensitivity change for varying the PLD between 1000 and 1700 ms was low. A tissue transit time from the neck to an axial supraventricular section through Broca's knee was obtained by fitting the PLD dependence to a two-compartment model. Averaging over subjects yielded 1930 +/- 110 ms for the tissue transit time, and 73 +/- 5 ml min(-1) 100 g(-1) for the cerebral blood flow. Small areas that exhibited a very high signal change upon labeling were indicative of regional variation in cerebral blood flow related to vascular anatomy.

Adult↗

PubMed2005

A comparison of signal instability in 2D and 3D EPI resting-state fMRI.

Spatiotemporally structured noise, such as physiological noise, is a potential source of artifacts in functional magnetic resonance imaging (fMRI) and is the main limiting factor for the detection of small blood oxygen level-dependent (BOLD) signal variations. fMRI was employed to detect low-frequency BOLD signal fluctuations, which are thought to be related to spontaneous neuronal activity in the resting human brain. The sensitivity to noise, that is, signal variations of non-BOLD origin, was investigated for two- (2D) and three-dimensional (3D) imaging techniques. Incomplete relaxation between subsequent scans increases the level of temporally and spatially correlated signal variations originating from physiological and/or systemic noise. Although inflow effects are suspected to be reduced in 3D echo-planar imaging (EPI) compared with multi-slice 2D EPI, the noise level was higher in the 3D technique. The noise level in 3D fMRI experiments was significantly increased by instabilities of the transverse steady-state magnetization as the repetition time was of the order of T(2). By implementing radiofrequency spoiling, temporal signal fluctuations and erroneous inter-regional correlation in connectivity maps were diminished to a level present in data sets acquired with 2D EPI.

Adult↗

PubMed2004

Magnetic resonance spectroscopy in patients with MELAS.

Localized magnetic resonance spectroscopy (MRS) yields sensitive metabolic markers to provide insight into the pathophysiology of mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) in vivo. Findings in full MELAS syndrome at 1H MRS of the brain typically include severely elevated lactate and reduced N-acetylaspartate, glutamate, myo-inositol, and total creatine concentrations in stroke-like lesions. Similar but less extreme alterations are also common in gray matter (GM) regions that appear normal at magnetic resonance imaging. Phosphorus spectroscopy of peripheral muscle permits investigation of the bioenergetic status. A decline of the phosphorylation potential indicates a low energy reserve at rest. Phosphocreatine resynthesis during post-exercise recovery is delayed pointing to reduced mitochondrial capacity. As MRS is inherently non-invasive, follow-up studies can be performed to assess treatment response quantitatively.

Adenosine Triphosphate↗

PubMed2004

Efficiency of flow-driven adiabatic spin inversion under realistic experimental conditions: a computer simulation.

Continuous arterial spin labeling (CASL) using adiabatic inversion is a widely used approach for perfusion imaging. For the quantification of perfusion, a reliable determination of the labeling efficiency is required. A numerical method for predicting the labeling efficiency in CASL experiments under various experimental conditions, including spin relaxation, is demonstrated. The approach is especially useful in the case of labeling at the carotid artery with a surface coil, as consideration of the experimental or theoretical profile of the B(1) field is straightforward. Other effects that are also accounted for include deviations from a constant labeling gradient, and variations in the blood flow velocity due to the cardiac cycle. Assuming relevant experimental and physiological conditions, maximum inversion efficiencies of about 85% can be obtained.

Blood Flow Velocity↗

PubMed2004

Quantifying the intra- and extravascular contributions to spin-echo fMRI at 3 T.

Functional MRI (fMRI) by means of spin-echo (SE) techniques provides an interesting alternative to gradient-echo methods because the contrast is based primarily on dynamic averaging associated with the blood oxygenation level-dependent (BOLD) effect. In this article the contributions from different brain compartments to BOLD signal changes in SE echo planar imaging (EPI) are investigated. To gain a better understanding of the underlying mechanisms that cause the fMRI contrast, two experiments are presented: First, the intravascular contribution is decomposed into two fractions with different regimes of flow by means of diffusion-weighting gradient schemes which are either flow-compensated, or will maximally dephase moving spins. Second, contributions from the intra- and extravascular space are selectively suppressed by combining flow-weighting with additional refocusing pulses. The results indicate two qualitatively different components of flowing blood which contribute to the BOLD contrast and a nearly equal share in functional signal from the intra- and extravascular compartments at TE approximately 80 ms and 3 T. Combining these results, there is evidence that at least one-half of the functional signal originates from the parenchyma in SE fMRI at 3 T. The authors suggest the use of flow-compensated diffusion weighting for SE fMRI to improve the sensitivity to the parenchyma.

Blood Flow Velocity↗

PubMed2003

Functional perfusion imaging using continuous arterial spin labeling with separate labeling and imaging coils at 3 T.

Functional perfusion imaging with a separate labeling coil located above the common carotid artery was demonstrated in human volunteers at 3 T. A helmet resonator and a spin-echo echo-planar imaging (EPI) sequence were used for imaging, and a circular surface coil of 6 cm i.d. was employed for labeling. The subjects performed a finger-tapping task. Signal differences between the condition of finger tapping and the resting state were between -0.5% and -1.1 % among the subjects. The imaging protocol included a long post-label delay (PLD) to reduce transit time effects. Labeling was applied for all repetitions of the functional run to reduce the sampling interval.

Adult↗

PubMed2003

Brain imaging and proton magnetic resonance spectroscopy in patients with phenylketonuria.

Magnetic resonance imaging studies in patients with phenylketonuria (PKU) revealed white matter alterations that correlated to most recent blood phenylalanine (Phe) concentrations as well as to brain Phe concentrations measured by magnetic resonance spectroscopy. The clinical significance of these changes is unknown. Magnetic resonance imaging data thus have no impact on therapeutic recommendations for adolescents and adults with PKU. Kinetic investigations of patients by magnetic resonance spectroscopy showed differences in brain Phe concentrations despite similar blood Phe levels. These were influenced by interindividual variations of blood-brain barrier Phe transport constants and by variations of the individual brain Phe consumption rate. Blood-brain barrier Phe transport characteristics as well as brain Phe consumption rates thus seem to be causative factors for the individual outcome in PKU.

Blood-Brain Barrier↗

PubMed2002

MRI of the lungs using hyperpolarized noble gases.

The nuclear spin polarization of the noble gas isotopes (3)He and (129)Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments.

Animals↗

PubMed2002

Application of NMR spectroscopy to monitoring MELAS treatment: a case report.

1H magnetic resonance spectroscopy (MRS) of the brain and (31)P MRS and saturation transfer of resting skeletal muscle were used to investigate intracellular metabolites and fluxes through the creatine kinase (CK) reaction in a patient with the syndrome of mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS). Acute cortical lesions were characterized by severely elevated lactate levels and reduced concentrations of N-acetylaspartyl compounds, glutamate, and myo-inositol. Similar but less extreme alterations were also observed in gray matter regions that appeared normal on magnetic resonance images. Investigation of the gastrocnemius muscle at rest demonstrated a reduced phosphocreatine level, elevated concentrations of inorganic phosphate and free adenosine 5'-diphosphate, and an abnormally low phosphorylation potential. Besides a moderately increased muscular phosphocreatine concentration, none of the metabolic disturbances detected on MRS improved with oral creatine supplementation. Forward and reverse fluxes through the CK reaction did not significantly change upon creatine treatment. Follow-up MRS investigations may thus provide objective markers of treatment response in vivo without the hazards or inconvenience of biopsy.

Adolescent↗