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Biomedical subjects

J Frahm

Publications and source records attributed to J Frahm.

At least 163 records · Page 9Linked to original sources
PubMed1986

Multiple chemical shift selective (CHESS) MR imaging using stimulated echoes.

Recently, stimulated echo acquisition mode (STEAM) magnetic resonance (MR) imaging has been demonstrated as a new tool for multiparametric MR imaging studies. Applications of the chemical shift selective (CHESS) STEAM technique using a 2.0-T whole-body MR imaging system are reported in which a series of contiguous cross-sectional images of the head and the pelvis were acquired. Because selective excitation of the desired component is employed rather than elimination of the unwanted component, this method yields an improved degree of spectral resolution which is dependent only on the homogeneity of the static magnetic field. For routine medical applications, no sophisticated adjustment of the CHESS pulse is needed, as reported in previous methods.

Adipose Tissue↗
PubMed1985

Chemical-shift-selective magnetic-resonance imaging of avascular necrosis of the femoral head.

Chemical-shift-selective (CHESS) nuclear-magnetic-resonance (NMR) imaging is a new method for separating "water" and "fat" (CH2) resonances in proton NMR imaging. Its major advantage over conventional composite NMR imaging is the contrast enhancement, which in particular allows the detection of small water-containing structures, such as vessels or joint surfaces, normally obscured by fat signals. First clinical findings in the femoral head with avascular necrosis are reported.

Adipose Tissue↗
PubMed1985

Chemical shift selective MR imaging using a whole-body magnet.

We have applied a new method for separating water and fat resonances in proton magnetic resonance (MR) imaging to human studies using a whole-body MR imaging system at 2.0 T. Chemical shift selective (CHESS) MR imaging provides either a water or fat image in a single experimental run within the same time needed for a conventional composite image. Although the technique requires a spectral resolution of about 1 ppm over the entire imaging region, first images of the human head and hip indicated that CHESS MR imaging is extremely promising for use in clinical investigations. Moreover, CHESS MR imaging can be combined arbitrarily with other imaging modalities and is easy to implement in any high-field MR imaging system.

Adipose Tissue↗
PubMedDate not supplied

A comparative FLASH and EPI study of repetitive and sustained visual activation.

Functional responses to either brief repetitive or sustained activation of the human visual cortex (movie presentation) were monitored using both fast low angle shot and echo planar imaging sequences. To allow for proper comparisons, native image contrasts were equally sensitized to changes in cerebral blood oxygenation with other experimental conditions matched as much as possible. Putative influences of receiver bandwidth and absolute voxel size were specifically addressed. In all cases resulting correlation maps and regional signal intensity time courses showed excellent spatial and temporal congruence, respectively. In particular, for a 6 min protocol of sustained activation, both FLASH and EPI yielded an initial signal increase (oxygenation overshoot), a subsequent signal decrease during ongoing stimulation, and a marked signal drop (oxygenation undershoot) after the end of stimulation. These findings exclude technical differences between FLASH and EPI as the source of previous contradictory observations more likely to be explained by differences in stimulus design.

Adult↗
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 experimental gliomas in rat brain in vivo.

Experimental brain tumors produced in rats (n = 10) by stereotactic implantation of cells from the F98 anaplastic glioma clone into the right caudate nucleus were studied in vivo using localized proton NMR and in vitro using high-resolution proton NMR, bioluminescent imaging of lactate, ATP and glucose distributions, and fluorescent imaging of regional pH. In vivo spectra from normal brain contralateral to the tumor regions showed resonances assignable to N-acetyl aspartate (NAA), creatines, choline-containing compounds, myo-inositol, glutamate and glucose in a pattern similar to those obtained from normal anaesthetized rats. In vivo tumor spectra were characterized by the almost complete absence of NAA, a substantial reduction of total creatine and glucose, and an increase of cholines. Based on the in vitro spectra the increase of the myo-inositol signal observed in vivo was mainly attributed to glycine. Histological examination as well as bioluminescent and fluorescent imaging indicated two stages of tumor development, i.e., solid vital tumors and tumors with necrosis. However, there was no consistent relationship between proton NMR observations and tumor development.

Animals↗
PubMedDate not supplied

Identification of Scyllo-inositol in proton NMR spectra of human brain in vivo.

Scyllo-inositol has been identified in proton NMR spectra of mammalian brain in vitro and in vivo. In contrast to myo-inositol this isomer comprises six equivalent CH protons that yield a singlet resonance at a chemical shift of 3.35 ppm. 1-D and 2-D J-resolved proton NMR studies (7.0 T) of perchloric acid extracts of brain tissues revealed different amounts of scyllo-inositol in man, sheep, cow and rat. Absolute quantification of localized short-echo time proton NMR spectra (2.0 T) of human brain in vivo resulted in scyllo-inositol concentrations of 0.35 +/- 0.06 mM for white matter (n = 25), 0.43 +/- 0.11 mM for grey matter (n = 23) and 0.57 +/- 0.14 mM for cerebellum (n = 10). Evidence for a tight metabolic link to myo-inositol was supported by a simultaneous variation of myo- and scyllo-inositol in patients with various brain diseases.

Animals↗
PubMedDate not supplied

Gender-specific alterations of cerebral metabolites with aging and cortisol treatment.

Excess availability of the adrenocortical glucocorticoid hormone cortisol has been correlated with structural brain changes and a decline of cognitive functions during aging. Pertinent studies need to consider gender as a potential confound because of sexual dimorphism in the regulation of hypothalamus-pituitary-adrenal axis activity. In vivo localized proton magnetic resonance spectroscopy of male and female tree shrews revealed similar concentrations of cerebral metabolites in young adult animals but gender-specific alterations with aging as well as in response to cortisol treatment. In comparison with adult tree shrews, aged males had reduced concentrations of N-acetylaspartate (-33%; P<0.01) and total creatine (-34%; P< 0.01). These findings are in line with the occurrence of neuronal loss. In contrast, aged females exhibited increased concentrations of choline-containing compounds (+27%; P<0.05) which--together with a tendency for increased creatine (+24%) and myo-inositol (+14%)--is indicative of glial proliferation. After chronic administration of cortisol (4 mg/day for 28 days), male but not female tree shrews showed a specific reduction of the choline-containing compounds (-29%; P< 0.05). The observed sex differences with age are likely to result from differences in the regulation of stress-related hormones which is further supported by the gender-specific responses to cortisol.

Aging↗
PubMedDate not supplied

Alzheimer disease: absolute quantification of cerebral metabolites in vivo using localized proton magnetic resonance spectroscopy.

In vivo magnetic resonance spectroscopy of brain metabolites such as N-acetylaspartate and myo-inositol has been proposed for the diagnosis of Alzheimer disease. Thirty patients with probable Alzheimer disease as well as 22 elderly controls underwent quantitative proton magnetic resonance spectroscopy of parietal gray and white matter with use of a short-echo time localization technique (echo time, 20 ms; repetition times, 6,000 and 3,000 ms, 2.0 Tesla) providing access to the regional concentrations of N-acetylaspartate, creatine, choline-containing compounds, myoinositol, glutamate, glutamine, and lactate. No statistically significant alterations of the metabolites were found in patients relative to controls. There were also no differences between patients with early and late onset of the disease and with respect to the presence of APOE-epsilon4 phenotype. A general trend for slightly decreased levels of N-acetylaspartate and creatine was not observed for their respective concentration ratios. In summary, the spectroscopic findings were in accord with known Alzheimer disease neuropathology, i.e., mild gliosis in white matter as well as mildly enhanced cortical atrophy in comparison to elderly controls. However, cortical atrophy with little or no N-acetylaspartate changes provided no evidence for a major decrease of neuronal density or loss of viable neurons. The data do not support the utility of proton magnetic resonance spectroscopy as an early diagnostic tool for Alzheimer disease.

Age of Onset↗
PubMedDate not supplied

Rapid three-dimensional MR imaging using the FLASH technique.

Fast low-angle shot (FLASH) imaging is a new technique for rapid magnetic resonance (MR) imaging that reduces acquisition times to seconds while retaining spatial resolution. This article deals with a three-dimensional (3D) variant of the FLASH method that allows the recording of a 3D-data set of 128 X 128 X 128 pixels within an acquisition time of only 4 min. The method is demonstrated using a 2.35 T 40 cm bore MR system. Experiments are carried out on rabbit head and human extremities. Depending on the field of view, the isotropic resolution is 1 mm or even less leading to cross-sectional images with a 1 mm slice thickness. In principle, FLASH imaging techniques are applicable to any MR system without the need of major hardware modifications. However, high-speed computers, large storage capacity, and rapid image display routines greatly facilitate an advantageous use of the 3D-FLASH variant.

Animals↗