Armin de Greiff

Diffusive sensitivity to muscle architecture: a magnetic resonance diffusion tensor imaging study of the human calf

The aim of this study was to examine the diffusive properties of adjacent muscles at rest, and to determine the relationship between diffusive and architectural properties, which are task-specific to muscles. The principle, second, and third eigenvalues, trace of the diffusion tensor, and two anisotropic parameters, ellipsoid eccentricity (e) and fractional anisotropy (FA), of various muscles in the human calf were calculated by diffusion tensor imaging (DTI). Linear correlations of the calculated parameters to the muscle physiological cross-sectional area (PCSA), which is proportional to maximum muscle force, were performed to ascertain any linear relation between muscle architecture and diffusivity. Images of the left calf were acquired from six healthy male volunteers. Seven muscles were investigated in this study. These comprised the soleus, lateral gastrocnemius, medial gastrocnemius, posterior tibialis, anterior tibialis, extensor digitorum longus, and peroneus longus. All data were presented as the mean and standard error of the mean (SEM). In general, differences in diffusive parameter values occurred primarily between functionally different muscles. A strong correlation was also found between PCSA and the third eigenvalue, e, and FA. A mathematical derivation revealed a linear relationship between PCSA and the third eigenvalue as a result of their dependence on the average radius of all fibers within a single muscle. These findings demonstrated the ability of DTI to differentiate between functionally different muscles in the same region of the body on the basis of their diffusive properties.

Morphometric changes of sensorimotor structures in focal dystonia

Idiopathic cervical dystonia (CD) and benign essential blepharospasm (BEB) are the most common forms of focal dystonia. Previous autopsy and imaging studies suggested that these disorders are not accompanied by structural brain abnormalities. However, recent brain voxel‐based morphometry (VBM) studies of these conditions suggest that there actually may be changes in gray matter. The objective of this stdy was to detect possible gray matter abnormalities in patients with CD and BEB using VBM and to compare the results between the two conditions and with age‐ and gender‐matched controls. High‐resolution MRI was employed to evaluate healthy controls and individuals with BEB and CD. Eleven BEB, 9 CD, and 14 healthy control subjects were imaged. VBM revealed alterations of gray matter structures involved in sensorimotor processing in the individuals with focal dystonia. In CD subjects there was increased gray matter in the thalamus, caudate head bilaterally, superior temporal lobe, and left cerebellum, while gray matter was decreased in the putamen bilaterally. BEB subjects had increased gray matter in the caudate head and cerebellum bilaterally as well as decrease in the putamen and thalamus bilaterally. These findings strongly underline the recent notion that idiopathic focal dystonias might have a detectable structural correlate. They also demonstrate structural similarities of the investigated focal dystonias, possibly reflecting a shared common pathophysiological origin.

Brain diffusion after single seizures.

Summary: u2002Purpose: Diffusion‐weighted magnetic resonance imaging (DWI) after focal status epilepticus has demonstrated focal alterations of the apparent diffusion coefficient (ADC) in the epileptogenic zone. We hypothesized that localized dynamic alterations of brain diffusion during the immediate postictal state will be detectable by serial DWI and correlate with the epileptogenic zone.

Movement preparation in self-initiated versus externally triggered movements: an event-related fMRI-study

In the present study, we used fMRI to investigate whether event-related preparatory processes of self-initiated and externally triggered movements differ. Twenty subjects were examined with 1000 T2*-weighted images in two consecutive sessions. During the first session subjects performed self-initiated abductions of the right index finger. For the second session subjects were instructed to perform the movements in response to visual cues. Number and timing of movements were matched between conditions. For statistical inference on multisubject level, random effects analyses were performed. Significantly enhanced activity during self-initiated compared to externally triggered movements was found within the left SMA, the left pre- and sensorimotor cortex, the right putamen, the left anterior cingulate gyrus, and the left inferior parietal lobe. The significantly increased activity during self-initiated in comparison to externally triggered movements might represent differential demands of the two conditions on the neuronal motor net during movement preparation, reflecting utilization of precise knowledge when to move in self-initiated movements. Our results emphasize a possible role of the primary motor cortex for movement preparation as observed in electrophysiological studies, but do not support a specific involvement of the dorsolateral prefrontal cortex as suggested by former block design studies.

Brain Representation of Hemifield Stimulation in Poststroke Visual Field Defects

Background and Purpose— Plasticity in extended, parallel, or reciprocal operating networks is well recognized. Changes in neuronal activity after lesions to distinct localized structures, such as the primary visual cortex, are less well characterized. We investigated the cortical reorganization in patients with poststroke visual field defects using blood oxygen level–dependent functional MRI. Methods— Brain activation was measured in 7 patients with a single occipital cortical lesion and partially recovered hemianopia and in 7 age-matched control subjects. Differences in activation between rest and visual hemifield stimulation were assessed with statistical parametric mapping (SPM’99). Results— In normal subjects, significant activation was found in the contralateral primary visual cortex and bilaterally in the extrastriate cortex. During hemifield stimulation of the unaffected side of stroke patients, a similar pattern was found compared with that seen in control subjects. During stimulation of the hemianopic side, bilateral activation was seen within the extrastriate cortex, stronger in the ipsilateral hemisphere. The primary visual cortex was not significantly activated in either hemisphere during stimulation of the hemianopic side. Conclusions— Visual field defects after stroke are associated with bilateral activation of the extrastriate visual cortex. This pattern of activation indicates altered neuronal activity in the visual system. Further investigation is necessary to determine the relationship between functional reorganization and recovery of lost visual function after poststroke hemianopia.

How Much Myocardial Damage Is Necessary to Enable Detection of Focal Late Gadolinium Enhancement at Cardiac MR Imaging

PURPOSEnTo assess the visibility of small myocardial lesions at magnetic resonance (MR) imaging and to estimate how much myocardial damage is necessary to enable detection of late gadolinium enhancement (LGE) in vivo.nnnMATERIALS AND METHODSnThe study was approved by the local bioethics committee. Coronary microembolization was performed by injecting 300,000 microspheres into the distal portion of the left anterior descending artery in 18 anesthetized minipigs to create multifocal areas of myocardial damage. In vivo MR imaging was performed a mean of 6 hours after microembolization by using an inversion-recovery spoiled gradient-echo sequence (repetition time msec/echo time msec, 8/4; inversion time, 240-320 msec; flip angle, 20 degrees; spatial resolution, 1.3 x 1.7 x 5.0 mm(3)) after injection of 0.2 mmol gadopentetate dimeglumine per kilogram of body weight. High-spatial-resolution imaging of the explanted heart was performed by using the same sequence with a higher spatial resolution (0.5 x 0.5 x 2.0 mm(3)). Imaging results were verified with histologic examination. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of in vivo and ex vivo images were calculated, and a t test was used to analyze observed differences.nnnRESULTSnMultifocal myocardial damage was successfully induced in all animals. Areas of LGE with low SNR (mean, 36.3 +/- 29.4 [standard deviation]) and CNR (23.7 +/- 19.8) were observed in vivo in 12 (67%) of 18 animals, whereas ex vivo imaging revealed spotted to streaky areas of LGE with higher SNR (91.4 +/- 27.8, P < .0001) and CNR (72.1 +/- 25.4, P < .0001) among normal-appearing myocardium in all cases (100%). Focal myocardial lesions exceeding 5% of myocardium per slice at histologic examination were detected in vivo with a sensitivity of 83%.nnnCONCLUSIONnFocal myocardial damage exceeding 5% of myocardium within the region of interest seems to be necessary for detection of LGE in vivo in an experimental model of coronary microembolization.

Brain Perfusion Following Single Seizures

Summary:u2002 Purpose: The aim of this study was to assess the regional relative interictal and postictal perfusion changes in temporal and parietal lobe epilepsy.

Presurgical Evaluation of Epilepsy by Brain Diffusion : MR-detected Effects of Flumazenil on the Epileptogenic Focus

Summary: u2002Purpose: After focal status epilepticus, focal alterations of the apparent diffusion coefficient (ADC) have been demonstrated in the epileptogenic zone by using diffusion‐weighted magnetic resonance (MR)imaging (DWI). Effects of flumazenil on an epileptogenic focus have been demonstrated by EEG recordings, but not by functional MRI. We hypothesized that dynamic spatiotemporal alterations of brain diffusion of the epileptogenic focus after application of flumazenil will be detectable by DWI and correlate with the epileptogenic zone.

Sparse imaging and continuous event-related fMRI in the visual domain: A systematic comparison

Continuous image acquisition as used in most functional magnetic resonance imaging (fMRI) designs may conflict with specific experimental settings due to attendant, noisy gradient switching. In sparse fMRI, single images are recorded with a delay that allows the registration of the predicted peak of an evoked hemodynamic response (HDR). The aim of this study was to assess validity and sensitivity of single‐trial sparse imaging within the visual domain. Thirteen subjects were scanned twice. Either continuous or sparse image acquisition was applied while participants viewed single trains of flashlights. Sparse fMRI results were compared to continuous event‐related fMRI results on single‐ and multisubject level regarding spatial extent, overlap, and intensity of activation. In continuously recorded data, the variability of the HDR peak latency was examined because this measure determined the timing of sparse image acquisition. In sparse fMRI, the sensitivity was analyzed considering different numbers of averaged trials. Sparse imaging detected the core activity revealed using continuous fMRI. The intensity of signal changes detected by continuous or sparse fMRI was comparable. The HDR peak latency was stable across sessions, but intersubject and regional variability might have affected the power of sparse fMRI. In sparse imaging, adding trials resulted in extension of activation and improvement in statistical power. The comparison with established continuous fMRI confirms the validity of sparse imaging. Conventional event‐related data acquisition and analysis provided more comprehensive results. However, only sparse fMRI offers the opportunity to apply stimuli and record further biosignals free of scanner‐related artifacts during intervals without image acquisition. Hum. Brain Mapping 24:130–143, 2005.

Cortical activation in hemianopia after stroke

Changes in neuronal activity of the visual cortex have been described in patients with hemianopia. The anatomical areas that are involved in neuroplastic changes have not been studied in a larger group of stroke patients with a homogenous structural pathology of the visual cortex. Brain activation was measured in 13 patients with a single ischemic lesion of the striate cortex and partially recovered hemianopia and in 13 age-matched control subjects using blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI). Differences in activation between rest and visual hemifield stimulation were assessed with statistical parametric mapping using group and multi-group studies. In normal subjects, the most significant activation was found in the contralateral primary visual cortex (area 17) and bilaterally in the extrastriate cortex (areas 18 and 19). In patients, these areas were also activated when the intact hemifield was stimulated. During stimulation of the hemianopic side, bilateral activation was seen within the extrastriate cortex, stronger in the ipsilateral (contralesional) hemisphere. Stimulation of the hemianopic visual field is associated with ipsilateral activation of the extrastriate visual cortex. This pattern of activation suggests extensive neuronal plasticity within the visual cortex after postgeniculate ischemic lesions and may have implications for therapeutic interventions.