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K D Merboldt

Publications and source records attributed to K D Merboldt.

34 records · Page 2Linked to original sources
PubMed1989

Localized proton NMR spectroscopy in different regions of the human brain in vivo. Relaxation times and concentrations of cerebral metabolites.

High-resolution proton NMR spectra of normal human brain in vivo have been obtained from selected 27- and 64-ml volumes-of-interest (VOI) localized in the insular area, the occipital area, the thalamus, and the cerebellum of normal volunteers. Localization was achieved by stimulated echo (STEAM) sequences using a conventional 1.5-T whole-body MRI system (Siemens Magnetom). The proton NMR spectra show resonances from lipids, lactate, acetate, N-acetylaspartate (NAA), gamma-aminobutyrate, glutamine, glutamate, aspartate, creatine and phosphocreatine, choline-containing compounds, taurine, and inositols. While T1 relaxation times of most of these metabolites were about 1100-1700 ms without significant regional differences, their T2 relaxation times varied between 100 and 500 ms. The longest T2 values of about (500 +/- 50) ms were observed for the methyl protons of NAA in the white matter of the occipital lobe compared to (320 +/- 30) ms in the other parts of the brain. No significant regional T2 differences were found for choline and creatine methyl resonances. The relative concentrations of NAA in gray and white matter were found to be 35% higher than those in the thalamus and cerebellum. Assuming a concentration of 10 mM for total creatine the resulting NAA concentrations of 13-18 mM are by a factor of 2-3 higher than previously reported using analytical techniques. Cerebral lactate reached a maximum concentration of about 1.0 mM.

Brain↗
PubMed1989

The influence of flow and motion in MRI of diffusion using a modified CE-FAST sequence.

Severe motion and flow artifacts are a problem in MRI of diffusion in vivo due to the application of strong magnetic field gradients. Here it is shown that image artifacts can be removed by using a modified fast-scan MRI sequence (CE-FAST) in conjunction with averaging of diffusion-weighted images. In phantom studies slow (coherent) flow (less than 1 mm s-1) in the presence of strong diffusion gradients is shown to cause signal losses in diffusion-weighted images that depend on the relative orientations of the flow direction and the diffusion gradient. On the other hand, pulsatile motions of macroscopic dimensions (e.g., 1 mm, 1 Hz, in-plane) lead to smearing and ghosting of signal intensities along the phase-encoding direction of the images. In both phantoms and rabbit brains in vivo motion artifacts were found to be reducible by averaging 8-16 images. Unfortunately, the resulting image contrast no longer represents a "true" diffusion contrast but is affected by additional signal losses due to motion averaging. All experiments were performed on a 40-cm-bore 2.35-T Bruker Medspec system.

Animals↗
PubMed1989

Localized NMR spectroscopy in vivo. Progress and problems.

Metabolites in brain and muscle of normal human volunteers have been studied by localized 1H and 31P NMR spectroscopy in vivo. Localization was achieved by means of stimulated echo (STEAM) sequences for both water-suppressed 1H NMR (TE = 20 ms) and 31P NMR (TE = 3 ms). Volumes-of-interest and measuring times selected for brain spectroscopy were 8 mL and 6.5 min for 1H NMR and 125 mL and 13 min for 31P NMR, respectively. General problems relating to spatial localization, spectral resolution, and quantitation of in vivo NMR data are discussed with respect to the nucleus and organs under investigation. They are correlated to studies of tissue extracts obtained at field strengths of 2.35 T (Bruker Biospec) and 7.0 T (Bruker MSL 300). Human studies were performed at 2.0 T on a whole-body research system (Siemens Magnetom).

Brain↗
PubMed1989

Noninvasive differentiation of tumors with use of localized H-1 MR spectroscopy in vivo: initial experience in patients with cerebral tumors.

A recently developed method for image-selected localized hydrogen-1 magnetic resonance (MR) spectroscopy was assessed in the differential diagnosis of nine primary and secondary cerebral tumors, including four gliomas, two meningiomas, one neurilemoma, one arachnoid cyst, and one metastasis of breast cancer. Well-resolved H-1 MR spectra of these tumors were obtained in vivo with a conventional 1.5-T whole-body MR imaging system. All tumor spectra were remarkably different from spectra from normal brain tissue. Spectra obtained from different tumors exhibited reproducible differences, while histologically similar tumors yielded characteristic spectra with only minor differences. The observed spectral alterations reflect variations in concentrations and relaxation times of the H-1 MR sensitive pool of free (mobile) metabolites within the tissues. In most cases, the concentrations of N-acetyl-aspartate and creatine/phosphocreatine are reduced below detectability, whereas choline-containing compounds are generally enhanced. The spectral differences between the tumors are mainly due to the differing concentrations of lipids, lactic acid, and carbohydrates. Localized H-1 MR spectroscopy may become an important clinical tool for the differentiation of tumors as well as for therapeutic control.

Adult↗
PubMed1988

Direct FLASH MR imaging of magnetic field inhomogeneities by gradient compensation.

MR images based on gradient echoes are sensitive to artifacts caused by inhomogeneities of the static magnetic field. This paper describes the effects of local gradients in rapid FLASH MR images and presents a way of directly imaging affected areas. The idea is to compensate for signal losses due to mutual cancellation of dephased magnetizations by deliberate "misadjustments" of the refocusing part of the slice selection gradient. In contrast to conventional field imaging techniques no three-dimensional data acquisition or subsequent Fourier analysis is required to obtain images at a particular gradient strength. Conventional as well as inhomogeneity compensated FLASH images have been obtained on phantoms and human heads using a 2.35-T 40-cm magnet and a 1.5-T whole-body system, respectively.

Head↗
PubMed1988

Rapid line scan NMR angiography.

This paper describes a new technique for NMR angiography based on rapid line scan projection imaging and presaturation of stationary spins. The resulting line scan angiograms are free from both misregistration artifacts common to subtraction methods and motion artifacts encountered in Fourier imaging. Moreover, line scan angiograms may be recorded within seconds and offer arbitrary fields of view as well as gradient zooming without problems due to aliasing. Three-dimensional information is obtained by rotation of the read gradient axis generating multiple view angles. Experimental line scan angiograms of the forearms of healthy volunteers have been recorded using a Bruker 2.35-T 40-cm magnet.

Angiography↗
PubMed1987

Flow suppression in rapid FLASH NMR images.

Rapid FLASH (fast low angle shot) NMR images are very sensitive to flow phenomena. In particular, a steady reflow of unsaturated spins from outside the imaging plane results in high image intensities which depend on the flip angle, the repetition time, and the flow velocity. Here we describe a technique that suppresses these signals by saturating remote spins prior to entering the imaging plane, e.g., by intercalating the FLASH sequence with slice-selective 1-2-1 pulse packages. Experiments have been carried out on phantoms and human extremities. Flow artifacts in the phase-encoding direction of the images are strongly reduced. Vascular structures may be easily delineated using difference images obtained with and without flow suppression.

Arm↗
PubMed1987

ECG-triggered arterial FLASH-MR flow measurement using an external standard.

In ECG-triggered FLASH-MR images, the inflow of unsaturated spins into the imaging plane results in the reproducible delineation of time variant flow in the arterial system. With the additional acquisition of an external reference image upstream the arterial vessel under investigation, the quantification of flow is possible with the FLASH-MR sequence in one measurement. The method allows the rapid measurement of arterial flow at least in great vessels.

Aorta, Abdominal↗
PubMed1986

Multipurpose NMR imaging using stimulated echoes.

STEAM (stimulated-echo acquisition mode) imaging techniques recently introduced by the authors are demonstrated to provide a versatile tool for improving the parametric specificity in NMR imaging. Stimulated echoes can be excited by a sequence of at least three rf pulses with flip angles of 90 degrees or less. The main characteristics of the STEAM method are based on the great functional flexibility of an imaging sequence comprising three rf pulses unequal to 180 degrees and three intervals prior to acquisition of the data. Major advantages are the easy access to contiguous multiplanar images, to CHESS (chemical-shift-selective) images, and to T1 information. Moreover, the rf power deposition is considerably reduced as compared to spin-echo NMR imaging sequences. Here first in vivo results on human extremities are presented including contiguous multislice images, multiple CHESS images, and spin-lattice relaxation time images calculated from a series of simultaneously recorded T1-weighted STEAM images.

Biophysical Phenomena↗
PubMedDate not supplied

Dynamic MR imaging of human brain oxygenation during rest and photic stimulation.

Dynamic FLASH (fast low-angle shot) magnetic resonance (MR) imaging was used to monitor changes in brain oxygenation in the human visual cortex during photic stimulation. The approach exploits the sensitivity of the gradient-echo signal to susceptibility changes induced by varying concentrations of paramagnetic deoxyhemoglobin in the cerebral blood pool. After the onset of binocular photic stimulation (10 Hz, red light, checker-board), there was a distinct increase in the MR signal in the calcarine cortex within 6-9 seconds, indicating a decrease in the total deoxyhemoglobin concentration. After the stimulation was switched off, the MR signal returned to a basal value within a similar period of time. Assuming enhanced blood flow and only a minor increase in oxygen consumption (production of deoxyhemoglobin) during physiologic activation, the results reflect an enhanced supply of diamagnetic oxyhemoglobin and an increase in the partial oxygen pressure in the capillary and venous blood pools. In addition, a decrease in the basal MR signal in the calcarine cortex was observed during the first 60-90 seconds of persistent activation, which may be understood as an autoregulatory adaptation to increased overall brain activity associated with information processing due to continuous perception of visual stimuli.

Adult↗
PubMedDate not supplied

On the interpretation of proton NMR spectra from brain tumours in vivo and in vitro.

Localized proton NMR spectroscopy in vivo allows focal studies of cerebral metabolites in both man and laboratory animals from image-defined regions as small as 1 mL or 64 microL, respectively. Although brain tumours lead to remarkable spectral alterations relative to normal brain, a number of problems may compromise the interpretation of the results. Potential complications arise from the chosen experimental conditions (method, TE, size and location of volume of interest), from regional metabolic heterogeneity in and around tumours, from differences between human tumours and animal models, and from discrepancies between in vivo and in vitro findings. Strategies and pitfalls are illustrated with use of selected examples from primary brain tumours, a rat tumour model and perchloric acid extracts of resected specimens.

Adult↗
PubMedDate not supplied

Localized proton NMR spectroscopy of brain tumors using short-echo time STEAM sequences.

Recent progress in localized proton NMR spectroscopy has been utilized to improve the spatial resolution and the metabolic specificity in a study of 19 patients with intracranial tumors. Selected examples demonstrate that short echo time stimulated echo acquisition mode sequences are able (a) to account for macroscopic tissue heterogeneity by reducing the volume of interest to 2-8 ml and (b) to facilitate a reasonable characterization of tumor metabolism by increasing the number of accessible metabolites. Proton NMR spectra were acquired within measuring times of 6.5 min on a 2.0 T whole-body system using the imaging headcoil.

Adult↗
PubMedDate not supplied

Proton NMR spectroscopy of cerebral metabolic alterations in infantile peroxisomal disorders.

Noninvasive studies of cerebral metabolism were performed with use of localized proton MR spectroscopy (MRS) in both healthy controls (n = 4, age 6 weeks to 2 years) and infants (n = 4, age 3-15 months) who had impaired peroxisomal functions classified as variants of Zellweger syndrome. All patients revealed a marked decrease of N-acetylaspartate in white and gray matter, thalamus, and cerebellum, indicating impairment of normal neuronal development as well as neuronal loss. In two cases an increase of cerebral glutamine and a decrease of the cytosolic polyol myo-inositol in gray matter and striatum reflected the impact of a concomitant effect on hepatic function. Two cases 3 and 6 months of age exhibited a notable elevation of mobile lipids and/or cholesterol in white matter. These patients with severe disease died within 4 weeks after the MRS examination. While an increase of free fatty acids generally associated with a lysosomal storage disease was not consistently observed by proton MRS of brain, this technique provides a convenient and safe tool for the direct assessment of neuropathologic aspects of Zellweger syndrome such as neuronal degeneration, demyelination, and consequences of compromised liver function.

Adrenoleukodystrophy↗
PubMedDate not supplied

Signal strength in subsecond FLASH magnetic resonance imaging: the dynamic approach to steady state.

Subsecond fast low-angle shot (FLASH) magnetic resonance imaging (MRI) allows single shot studies of the human heart within measuring times of about 100-300 ms depending on the data matrix. In contrast to conventional FLASH MRI subsecond applications acquire data during the approach to steady state. A detailed analysis of the saturation behavior of the signal is given for the ideal case of a rectangular slice profile. In a second step, realistic slice profiles assuming Gaussian-shaped excitation pulses were taken into account by means of a numerical solution of the Bloch equations. It turns out that the signal strength and the resulting image intensity is considerably higher than may be expected from steady-state considerations. Correspondingly optimized flip angles depend on the number of phase-encoding steps. Assuming long T1 relaxation times as, for example, encountered in muscle and brain tissue and repetition times of 5 ms or less, optimum flip angles are 12 degrees-16 degrees. The use of even higher flip angles (greater than or equal to 20 degrees) causes heavily distorted slice profiles and a dynamic increase of the effective slice thickness. Flip angles of the order of the Ernst angle (6 degrees) correspond to steady-state conditions and lead to considerable signal losses. The theoretical results are confirmed by subsecond FLASH MRI studies of the human heart using a 2.0 T whole-body system (Siemens Magnetom).

Biophysical Phenomena↗