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J Frahm

Publications and source records attributed to J Frahm.

At least 37 records · Page 2Linked to original sources
PubMed2001

Functional MRI with reduced susceptibility artifact: high-resolution mapping of episodic memory encoding.

Visual episodic memory encoding was investigated using echoplanar magnetic resonance imaging at 2.0 x 2.0 mm2 resolution and 1.0 mm section thickness, which allows for functional mapping of hippocampal, parahippocampal, and ventral occipital regions with reduced magnetic susceptibility artifact. The memory task was based on 54 image pairs each consisting of a complex visual scene and the face of one of six different photographers. A second group of subjects viewed the same set of images without memory instruction as well as a reversing checkerboard. Apart from visual activation in occipital cortical areas, episodic memory encoding revealed consistent activation in the parahippocampal gyrus but not in the hippocampus proper. This finding was most prominently evidenced in sagittal maps covering the right hippocampal formation. Mean activated volumes were 432 +/- 293 microl and 259 +/- 179 microl for intentional memory encoding and non-instructed viewing, respectively. In contrast, the checkerboard paradigm elicited pure visual activation without parahippocampal involvement.

Adult↗
PubMed2001

Regional modulation of BOLD MRI responses to human sensorimotor activation by transcranial direct current stimulation.

Blood oxygenation level dependent (BOLD) MRI was used to monitor modulations of human sensorimotor activity by prior transcranial direct current stimulation (tDCS). Activation maps for a right hand sequential finger opposition task were obtained for six subjects before as well as 0-5 min and 15-20 min after a 5-min period of 1 mA cathodal and, in a separate session, anodal tDCS of the left-hemispheric motor cortex. Cathodal tDCS resulted in a global decrease of the mean number of activated pixels by 38% (P < 0.01) 0-5 min after stimulation, which reduced to 28% (P < 0.05) 15-20 min after stimulation. A region-of-interest analysis revealed a 57% decrease of activated pixels (P < 0.001) in the supplementary motor area, but no change in the hand area of the primary motor cortex. Anodal tDCS yielded a nonsignificant 5% increase of activated pixels with no regional differences. These findings support the view that reduced neuroaxonal excitability after cathodal tDCS causes reduced brain activity. However, rather than affecting the primary sensorimotor input of an active task, the process appears to dampen those responses that rely on cortico-cortical connections and related processing. Magn Reson Med 45:196-201, 2001.

Adult↗
PubMed2001

Modulation of cerebral blood oxygenation by indomethacin: MRI at rest and functional brain activation.

The modulation of blood oxygenation level-dependent (BOLD) cerebral MRI contrast by the vasoconstrictive drug indomethacin (i.v. 0.2 mg/kg b.w.) was investigated in 10 healthy young adults without and with functional challenge (repetitive and sustained visual activation). For comparison, isotonic saline (placebo, 20 mL) and acetylsalicylate (i.v. 500 mg) were investigated as well, each in separate sessions using identical protocols. After indomethacin, dynamic T2*-weighted echo-planar MRI at 2.0 T revealed a rapid decrease in MRI signal intensity by 2.1%-2.6% in different gray matter regions (P < or = 0.001 compared to placebo), which was not observed for acetylsalicylate and the placebo condition. Regional signal differences were not significant within gray matter, but all gray matter regions differed significantly from the signal decrease of only 1.2% +/- 0.7% observed in white matter (P = 0.001). For the experimental parameters used, a 1% MRI signal decrease in response to indomethacin was estimated to correlate with a decrease of the cerebral blood flow by about 12 ml/100 g/minute, and an increase of the oxygen extraction fraction by about 15%. Responses to visual activation were not affected by saline or acetylsalicylate, and yielded 5.0%-5.5% BOLD MRI signal increases both before and after drug application. In contrast, indomethacin reduced the initial response strength to 82%-85% of that obtained without the drug. The steady-state response during sustained activation reached only 47% of the corresponding pre-drug level (P < 0.01). During repetitive activation the BOLD contrast was reduced to 66% of that observed for control conditions (P < 0.001). In conclusion, indomethacin attenuates the vasodilatory force at functional brain activation, indicating different mechanisms governing neurovascular coupling.

Adult↗
PubMed2001

Mapping of retinal projections in the living rat using high-resolution 3D gradient-echo MRI with Mn2+-induced contrast.

This study describes the neuroaxonal tracing of the visual pathway in the living rat using high-resolution T1-weighted 3D gradient-echo MRI (195 x 195 x 125 microm3) at 8, 24, 48, and 72 h after intraocular Mn2+ injection (0.1 microl of 1 M aqueous MnCl2). Best results were obtained at 24 h postinjection, revealing a continuous pattern of anterograde labeling from the retina, optic nerve, and chiasm to the contralateral optic tract, the dorsal and ventral lateral geniculate nucleus, the superior colliculus and its brachium, the olivary pretectal nucleus, the nucleus of the optic tract, and the suprachiasmatic nucleus. These results underline the feasibility of repeated MRI tract tracing in living animals after a single injection of Mn2+. The approach is expected to advance studies of neuroaxonal function in behaving animals with special emphasis on applications in developmental neurobiology.

Animals↗
PubMed2001

Functional MRI of the human amygdala?

In view of an increasing number of publications that deal with functional mapping of the human amygdala using blood oxygenation-level-dependent (BOLD) magnetic resonance imaging, we reevaluated the underlying image quality of T2*-weighted echoplanar imaging (EPI) and fast low angle shot (FLASH) sequences at 2.0-T with regard to susceptibility-induced signal losses and geometric distortions. Apart from the timing of the gradient echoes, the degree of susceptibility influences is controlled by the image voxel size. Whereas published amygdala studies report voxel sizes ranging from 22 to 125 microl, the present results suggest that reliable imaging of the amygdala with BOLD sensitivity requires voxel sizes of 4 to 8 microl or less. Preferentially, acquisitions should be performed with a coronal section orientation. Although high-resolution BOLD MRI is at the expense of temporal resolution and volume coverage, it seems to provide the only solution to this physical problem.

Amygdala↗
PubMed2001

Simultaneous EEG and functional MRI of epileptic activity: a case report.

OBJECTIVES: Attempts to localize the source of epileptic activity by linking electroencephalographic (EEG) abnormalities to blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) signal alterations are hampered mainly by EEG distortions during MRI, subject motion, and unknown hemodynamic response characteristics. METHODS: Using T2*-weighted echo-planar imaging at 2.0 T (2 s temporal resolution, 2 x 2 x 4 mm(3) spatial resolution), this work demonstrates strategies to alleviate some of these problems while studying a patient who had ideopathic generalized epilepsy with poly-spike and slow-wave complexes. RESULTS: Continuous EEG recordings during dynamic MRI (500 ms scanning, 1500 ms delay) and post-examination derivation of an EEG reference function for MRI analysis revealed positive BOLD MRI responses with temporal characteristics similar to those obtained for functional challenges. CONCLUSIONS: The ability to map focal epileptic activity and/or associated cognitive processing provides new potential for both epilepsy research and clinical patient management.

Adult↗
PubMed2000

Direct mapping of ocular dominance columns in human primary visual cortex.

Functional magnetic resonance imaging at 2.0T was employed to identify columnar structures in human visual cortex. Sagittal sections (4 mm thickness) covering the calcarine cortex were acquired with use of a multiecho low flip angle gradient-echo sequence at 4.0 s temporal resolution and 0.25 x 0.25 mm2 spatial resolution. Extending earlier attempts based on a differential paradigm contrasting left vs right eye stimulation, this work presents the first direct mapping of human ocular dominance columns by measuring separate activation maps with left and right eye stimulation. The resulting individual maps reveal patterns of ocular dominance as spots or bands of altered activity in calcarine cortex. Their superposition shows only little spatial overlap of eye-specific encoding which strongly supports the genuineness of these functional units.

Adult↗
PubMed2000

Diffusion tensor mapping of the human brain using single-shot line scan imaging.

A recently developed single-shot line scan imaging technique for diffusion measurements (Finsterbusch and Frahm, Magn Reson Med 1999;42:772-778) was extended to full diffusion tensor mapping of the human brain. Because the sequence acquires stimulated echoes from individual columns of magnetization ("lines"), the approach is affected neither by spatial aliasing when studying inner volumes nor by resonance offset effects or T2* dephasing as in diffusion-weighted echoplanar imaging. Experiments on healthy subjects were performed at 2.0 T using 31 single-shot images (5b values, 6 orientations, 520 msec each) at 1.5 x 1.5 mm2resolution (interpolated) and 6.0 mm section thickness. Apart from calculated images with isotropic diffusion weighting, the results include maps of the six independent diffusion tensor components, the apparent diffusion coefficient, the relative anisotropy, and the main diffusion direction. The achievable signal-to-noise ratio and resolution allow the identification of differently oriented nerve fibers in the brain stem. J. Magn. Reson. Imaging 2000;12:388-394.

Anisotropy↗
PubMed2000

Rapid isotropic diffusion mapping without susceptibility artifacts: whole brain studies using diffusion-weighted single-shot STEAM MR imaging.

A subsecond magnetic resonance imaging (MRI) technique for isotropic diffusion mapping is described which, in contrast to echo-planar imaging (EPI), is insensitive to resonance offsets, i.e., tissue susceptibility differences, magnetic field inhomogeneities, and chemical shifts. It combines a diffusion-weighted (DW) spin-echo preparation period and a high-speed stimulated echo acquisition mode (STEAM) MRI sequence and yields single-shot images within measuring times of 559 msec (80 echoes). Here, diffusion encoding involved one scan without DW, three DW scans with b = 490 sec mm(-2), and three DW scans with b = 1000 sec mm(-2) (orthogonal gradient orientations). An automated on-line evaluation resulted in isotropic DW images as well as ADC maps (trace of the diffusion tensor). Experiments at 2.0 T covered the brain of healthy subjects in 20 contiguous sections of 6 mm thickness and 2.0 x 2.0 mm(2) in-plane resolution within a total measuring time of 78 sec. High-resolution studies at 1.0 x 1.0 mm(2) (interpolated from 2.0 x 1.0 mm(2) acquisitions) were obtained within 5 min 13 sec using four averages. In comparison with EPI, DW single-shot STEAM MRI exhibits only about half the SNR, but completely avoids regional signal losses, high intensity artifacts, and geometric distortions.

Adult↗
PubMed2000

Reducing inhomogeneity artifacts in functional MRI of human brain activation-thin sections vs gradient compensation.

We evaluated two methods for correcting inhomogeneity-induced signal losses in magnetic resonance gradient-echo imaging that either use gradient compensation or simply acquire thin sections. The strategies were tested in the human brain in terms of achievable quality of T2*-weighted images at the level of the hippocampus and of functional activation maps of the visual cortex. Experiments were performed at 2.0 T and based on single-shot echo-planar imaging at 2. 0 x 2.0 mm(2) resolution, 4 mm section thickness, and 2.0 s temporal resolution. Gradient compensation involved a sequential 16-step variation of the refocusing lobe of the slice-selection gradient (TR/TE = 125/53 ms, flip angle 15 degrees ), whereas thin sections divided the 4-mm target plane into either four 1-mm or eight 0.5-mm interleaved multislice acquisitions (TR/TE = 2000/54 ms, flip angle 70 degrees ). Both approaches were capable of alleviating the inhomogeneity problem for structures in the base of the brain. When compared to standard 4-mm EPI, functional mapping in the visual cortex was partially compromised because of a lower signal-to-noise ratio of inhomogeneity-corrected images by either method. Relative to each other, consistently better results were obtained with the use of contiguous thin sections, in particular for a thickness of 1 mm. Multislice acquisitions of thin sections require minimal technical adjustments.

Adult↗
PubMed2000

Gradient-echo line scan imaging using 2D-selective RF excitation.

A gradient-echo line scan imaging technique was developed which employs two-dimensional spatially selective radiofrequency (2DRF) pulses for consecutively exciting individual columns of transverse magnetization, i.e., image lines. Although a variety of trajectories are possible for 2DRF excitation, the current implementation involved a blipped-planar trajectory in conjunction with additional saturation RF pulses to suppress side excitations above and below the desired image section, i.e., along the blip direction of the 2DRF pulse. Human brain imaging at 2.0 T (Siemens Vision, Erlangen, Germany) resulted in measuring times of 5.2 s for a 5-mm section at 1.0 x 1.0 mm in-plane resolution. Functional neuroimaging of the motor cortex at 1.2 s temporal resolution and 0.78 x 1.56 mm in-plane resolution exploited the capability of imaging inner volumes (here a 25-mm strip) without signal aliasing.

Algorithms↗
PubMed2000

Artifacts caused by transcranial magnetic stimulation coils and EEG electrodes in T(2)*-weighted echo-planar imaging.

We investigated the effects of transcranial magnetic stimulation (TMS) coils and electroencephalographic (EEG) electrodes on T(2)*-weighted echo-planar images (EPI) at 2.0 T (gradient-echo EPI, mean TE = 53 ms, 2x2x4 mm(3)). In comparison with anatomic gradient-echo images (3D FLASH, TE = 4 ms, 1x1x1 mm(3)), T(2)*-weighted EPI acquisitions of a water-filled spherical phantom revealed severe signal losses and geometric distortions in the vicinity of TMS coils. Even remote effects were observed for image orientations perpendicular to the coil plane. EEG electrodes and the fixation gel caused milder localized distortions. In humans, complications were avoided by the large distance between the TMS coil and the cortical surface and when using an EPI orientation parallel to the plane of the coil. It is concluded that T(2)*-weighted EPI studies of human brain function may be performed without distortions caused by TMS coils and EEG electrodes.

Artifacts↗
PubMed2000

Quantitative proton magnetic resonance spectroscopy of focal brain lesions.

The diagnostic value of single-voxel proton magnetic resonance spectroscopy (2 T, stimulated echo acquisition mode, TR = 6,000 ms, TE = 20 ms, 4-5 mL volumes-of-interest) was assessed for a differentiation of focal brain lesions of unknown etiology in 17 patients 1-14 years of age. Absolute metabolite concentrations were compared with age-matched control subjects and an individual control region. Most of the brain tumors were characterized by strongly reduced total N-acetylaspartyl compounds and marked increases of myo-inositol and choline-containing compounds, consistent with a lack of neuroaxonal tissue and a proliferation of glial cells. Lactate was elevated in only four patients. When using this pattern for a metabolic discrimination of brain tumors from other focal lesions, proton spectroscopy correctly identified 14 of 17 abnormalities, as confirmed by histologic examination after neurosurgical intervention. One false-positive tumor diagnosis was a severe reactive gliosis mimicking a typical tumor spectrum. Two inconclusive cases comprised an astrocytoma with moderately elevated myo-inositol but reduced choline-containing compounds and a patient with an abscess leading to a marked reduction of all metabolites but strong contributions from mobile lipids. In summary, quantitative proton spectroscopy has considerable clinical value for preoperative characterization of focal brain lesions.

Adolescent↗
PubMed2000

Noninvasive functional imaging of human brain using light.

Analysis of photon transit time for low-power light passing into the head, and through both skull and brain, of human subjects allowed for tomographic imaging of cerebral hemoglobin oxygenation based on photon diffusion theory. In healthy adults, imaging of changes in hemoglobin saturation during hand movement revealed focal, contralateral increases in motor cortex oxygenation with spatial agreement to activation maps determined by functional magnetic resonance imaging; in ill neonates, imaging of hemoglobin saturation revealed focal regions of low oxygenation after acute stroke, with spatial overlap to injury location determined by computed tomography scan. Because such slow optical changes occur over seconds and co-localize with magnetic resonance imaging vascular signals whereas fast activation-related optical changes occur over milliseconds and co-localize with EEG electrical signals, optical methods offer a single modality for exploring the spatio-temporal relationship between electrical and vascular responses in the brain in vivo, as well as for mapping cortical activation and oxygenation at the bedside in real-time for clinical monitoring.

Adult↗
PubMed1999

Neither short-term nor long-term administration of oral choline alters metabolite concentrations in human brain.

BACKGROUND: This study reexamined conflicting proton magnetic resonance spectroscopy (MRS) reports of increased or unaffected choline-containing compounds (Cho) in human brain in response to a single dose of 50 mg/kg choline bitartrate. METHODS: The present work was based on a well-established strategy for quantitative proton MRS (2.0 T, STEAM localization sequence, TR/TE/TM = 6000/20/10 ms, LCModel automated spectral evaluation) that allows the determination of cerebral metabolite concentrations rather than T1-weighted resonance intensity ratios. Moreover, the investigations were extended to a possible long-term effect of oral choline by monitoring the continuous ingestion of 2 x 16 g of lecithin per day for 4 weeks. Six young healthy volunteers participated in each study and metabolite concentrations were determined in standardized locations in gray matter, white matter, cerebellum, and thalamus. RESULTS: Neither for short-term nor for long-term administration of choline do the data reveal statistically significant deviations from the basal concentrations of Cho, total N-acetyl-containing compounds (neuronal markers), total creatine, and myo-inositol (glial marker) in any of the investigated brain regions. CONCLUSIONS: Previous reports of increased Cho are not confirmed.

Administration, Oral↗
PubMed1999

Volumetric MRI measurements of the tree shrew hippocampus.

Protocols suitable for repeated magnetic resonance imaging (MRI) studies of the tree shrew's brain were established. This included the development of (i) a technique for prolonged inhalation anesthesia by endotracheal intubation; (ii) a reproducible fixation of the animal's head in a stereotaxic frame and finally (iii) the set-up of the hardware (rf coil) and software (MRI sequences) of the MRI system. The endotracheal intubation as well as the repeated and prolonged anesthesia showed no complications. The in vivo measurements of the tree shrew's hippocampal formation revealed a high reproducibility. Right and left hippocampal volume was determined as 85.2 mm3 +/- 8% and 87.4 mm3 +/- 10%, respectively. The utility of MRI in delineating alterations in brain anatomy was demonstrated in three animals receiving cortisol via the drinking water (5 mg/animal/day). After a 4-week treatment, in two of the three tree shrews a reduction in hippocampal volume was observed. Thus, the MRI protocols used here allow for repeated and non-invasive measurements of changes in hippocampal anatomy within the same animal and to monitor the temporal dynamics of structural alterations within this brain structure.

Animals↗