Biomedical subjects

Stefan Posse

Publications and source records attributed to Stefan Posse.

10 recordsLinked to original sources

PubMed2006

Reproducibility of activation in Broca's area during covert generation of single words at high field: a single trial FMRI study at 4 T.

Although functional magnetic resonance imaging (FMRI) has arguably become the most ubiquitously used imaging modality, questions remain about the reproducibility of the observed patterns of activation and the acquisition time required to achieve statistically significant and reproducible maps. In the current study, we investigated the reliability of activation in Broca's area, on both a voxel-wise and region of interest level, in response to the covert generation of a single word at 4 T. We also assessed the effects of different parametric (P < 0.01; P < 0.005; P < 0.001) and spatial thresholds (25%, 50% and 75%) on the reproducibility of activation within our region of interest and other randomly selected areas of the brain. We report that the inter-trial consistency of activation within Broca's area for a single trial design using multi-echo EPI is roughly equivalent to previous studies that averaged across a much larger number of trials. However, reliability estimates varied dramatically (approximately 55%) depending on the different parametric and spatial criteria thresholds that were applied to the data. These results show that increased sensitivity at high field strength can be used to reduce the time needed to localize functional activation patterns, which is beneficial for clinical studies such as pre-surgical mapping. Additional benefits of single trial designs, such as the ability to immediately assess for extraneous cognitive processes, are also discussed.

Adult↗

PubMed2006

fMRI pattern classification using neuroanatomically constrained boosting.

Pattern classification in functional MRI (fMRI) is a novel methodology to automatically identify differences in distributed neural substrates resulting from cognitive tasks. Reliable pattern classification is challenging due to the high dimensionality of fMRI data, the small number of available data sets, interindividual differences, and dependence on the acquisition methodology. Thus, most previous fMRI classification methods were applied in individual subjects. In this study, we developed a novel approach to improve multiclass classification across groups of subjects, field strengths, and fMRI methods. Spatially normalized activation maps were segmented into functional areas using a neuroanatomical atlas and each map was classified separately using local classifiers. A single multiclass output was applied using a weighted aggregation of the classifier's outputs. An Adaboost technique was applied, modified to find the optimal aggregation of a set of spatially distributed classifiers. This Adaboost combined the region-specific classifiers to achieve improved classification accuracy with respect to conventional techniques. Multiclass classification accuracy was assessed in an fMRI group study with interleaved motor, visual, auditory, and cognitive task design. Data were acquired across 18 subjects at different field strengths (1.5 T, 4 T), with different pulse sequence parameters (voxel size and readout bandwidth). Misclassification rates of the boosted classifier were between 3.5% and 10%, whereas for the single classifier, these were between 15% and 23%, suggesting that the boosted classifier provides a better generalization ability together with better robustness. The high computational speed of boosting classification makes it attractive for real-time fMRI to facilitate online interpretation of dynamically changing activation patterns.

Attention↗

PubMed2006

Use of tissue water as a concentration reference for proton spectroscopic imaging.

A strategy for using tissue water as a concentration standard in (1)H magnetic resonance spectroscopic imaging studies on the brain is presented, and the potential errors that may arise when the method is used are examined. The sensitivity of the method to errors in estimates of the different water compartment relaxation times is shown to be small at short echo times (TEs). Using data from healthy human subjects, it is shown that different image segmentation approaches that are commonly used to account for partial volume effects (SPM2, FSL's FAST, and K-means) lead to different estimates of metabolite levels, particularly in gray matter (GM), owing primarily to variability in the estimates of the cerebrospinal fluid (CSF) fraction. While consistency does not necessarily validate a method, a multispectral segmentation approach using FAST yielded the lowest intersubject variability in the estimates of GM metabolites. The mean GM and white matter (WM) levels of N-acetyl groups (NAc, primarily N-acetylaspartate), choline (Ch), and creatine (Cr) obtained in these subjects using the described method with FAST multispectral segmentation are reported: GM [NAc] = 17.16 +/- 1.19 mM; WM [NAc] = 14.26 +/- 1.38 mM; GM [Ch] = 3.27 +/- 0.47 mM; WM [Ch] = 2.65 +/- 0.25 mM; GM [Cr] = 13.98 +/- 1.20 mM; and WM [Cr] = 7.10 +/- 0.67 mM.

Adult↗

PubMed2006

Signal-to-noise ratio and spectral linewidth improvements between 1.5 and 7 Tesla in proton echo-planar spectroscopic imaging.

This study characterizes gains in sensitivity and spectral resolution of proton echo-planar spectroscopic imaging (PEPSI) with increasing magnetic field strength (B(0)). Signal-to-noise ratio (SNR) per unit volume and unit time, and intrinsic linewidth (LW) of N-acetyl-aspartate (NAA), creatine (Cr), and choline (Cho) were measured with PEPSI at 1.5, 3, 4, and 7 Tesla on scanners that shared a similar software and hardware platform, using circularly polarized (CP) and eight-channel phased-array (PA) head coils. Data were corrected for relaxation effects and processed with a time-domain matched filter (MF) adapted to each B(0). The SNR and LW measured with PEPSI were very similar to those measured with conventional point-resolved spectroscopy (PRESS) SI. Measurements with the CP coil demonstrated a nearly linear SNR gain with respect to B(0) in central brain regions. For the PA coil, the SNR-B(0) relationship was less than linear, but there was a substantial SNR increase in comparison to the CP coil. The LW in units of ppm decreased with B(0), resulting in improved spectral resolution. These studies using PEPSI demonstrated linear gains in SNR with respect to B(0), consistent with theoretical expectations, and a decrease in ppm LW with increasing B(0).

Artifacts↗

PubMed2004

Neural correlates of internally-generated disgust via autobiographical recall: a functional magnetic resonance imaging investigation.

Converging lines of evidence suggest the involvement of the insula and basal ganglia in the processing of disgust, an important primary emotion that guides the avoidance of potential physical contamination and disease. Prior human lesion and functional brain imaging studies have employed exteroceptive sensory stimuli such as facial expressions of disgust, and disgust-eliciting pictures. Thus, the neural substrates underlying the internal experience of disgust remain unknown. The present fMRI study examined the neural correlates of self-induced disgust aided by the recall and re-experience of personally salient life events. Subjects were scanned while they recalled and re-experienced either a recent situation that evoked intense disgust or a time-matched, equally vivid neutral/non-emotional event. Relative to the emotionally neutral condition, self-induced disgust was associated with activation of the insula, hippocampus, anterior and posterior cingulate cortex, basal ganglia, thalamus, and primary visual cortex. These findings suggest that areas previously associated with the perception of disgust (e.g., insula, basal ganglia) are also involved interoceptive experience of disgust.

Adult↗

PubMed2004

Real-time fMRI of cortico-limbic brain activity during emotional processing.

The ability to detect dynamic changes in brain activity during affective processing within individual subjects in real-time can advance our understanding of the neural mechanisms of emotion, psychiatric illness, and therapeutic intervention. We investigated whether activity in limbic and paralimbic regions elicited by blocks of aversive (AV) and neutral (NEU) pictures can be detected by real-time fMRI. Real-time analysis of signal change during each block revealed that activations in insula and medial frontal cortex were more frequent during AV than NEU epochs. Single subject and group analysis off-line with conventional statistical parametric mapping methods matched the results obtained in real-time. Detecting cortico-limbic brain activation during perception and experience of emotionally salient visual stimuli with real-time fMRI technology is feasible.

Adult↗

PubMed2004

Activation of cerebellar hemispheres in spatial memorization of saccadic eye movements: an fMRI study.

What mechanisms allow us to direct a precise saccade to a remembered target position in space? The cerebellum has been proposed to be involved not only in motor and oculomotor control, but also in perceptual and cognitive functions. We used functional MRI (Echoplanar imaging at 1.5 T) to investigate the role of the cerebellum in the control of externally triggered and internally generated saccadic eye movements of high and low memory impact, in six healthy volunteers. Memory-guided saccades to remembered locations of 3 targets (triple-step saccades) in contrast to either central fixation or to visually guided saccades activated the cerebellar hemispheres predominantly within lobuli VI-crus I. The same areas were activated when an analogous visuospatial working memory task was contrasted to the triple-step saccades. Visually guided saccades activated the posterior vermis and the triple-step saccades, contrasted to the working memory task, activated predominantly the posterior vermis and paravermal regions. Our data confirm the primary involvement of the posterior vermis for visually-triggered saccadic eye movements and present novel evidence for a role of the cerebellar hemispheres in the mnemonic and visuospatial control of memory-guided saccades.

Adult↗

PubMed2003

Proton echo-planar spectroscopic imaging with highly effective outer volume suppression using combined presaturation and spatially selective echo dephasing.

A highly effective outer volume suppression (OVS) technique, termed spatially selective echo dephasing (SSED), which employs gradient dephasing of spatially selective spin echoes, is introduced. SSED, which is relatively insensitive to T(1) dispersion among lipid signals and B(1) inhomogeneity, was integrated with very high spatial resolution 2D proton echo-planar spectroscopic imaging (PEPSI) to assess residual lipid bleeding into cortical regions in the human brain. The method was optimized to minimize signal refocusing of secondary spin-echoes in areas of overlapping suppression slices. A comparison of spatial presaturation with single or double SSED, and with combined presaturation and SSED shows that the latter method has superior performance with spatially uniform lipid suppression factors in excess of 70. Metabolite mapping (choline, creatine, and NAA) with a 64 x 64 spatial matrix and 0.3 cm(3) voxels in close proximity to peripheral lipid regions was demonstrated at 1.5 T with a scan time of 32 min using the standard head coil.

Brain↗

PubMed2003

Single-shot T(2)* mapping with 3D compensation of local susceptibility gradients in multiple regions.

Macroscopic magnetic field inhomogeneities severely limit sensitivity of blood oxygenation level-dependent (BOLD) functional MRI (fMRI) in frontal and central brain regions close to brain stem. A single-shot multiecho echo-planar imaging method (TurboPEPSI) was developed that combines quantitative T(2)* mapping with gradient compensation of local susceptibility inhomogeneities in multiple volumes of interest (VOIs). Gradient compensation was optimized in individual subjects based on magnetic field mapping and applied at selected echo times, interleaved with acquisition of uncompensated echoes. Intrinsic T(2)* values from uncompensated echoes were obtained in real-time simultaneously with effective T(2)* values from gradient compensated echoes. It is demonstrated that up to three VOIs can be compensated in a single excitation, in addition to collecting uncompensated data, using 8-echo acquisition on a clinical 1.5 Tesla scanner. A theory was developed to optimize the sequence of uncompensated and compensated echoes to achieve maximum BOLD sensitivity. Gradient compensation increased effective T(2)* values in left and right amygdala on average by 18.8 +/- 7.5 ms, while maintaining sensitivity in uncompensated brain areas. In orbitofrontal cortex effective T(2)* values increased by 22.2 +/- 5.3 ms. A CO(2) challenge paradigm was used to demonstrate that this gradient compensation method significantly enhances BOLD signal changes in amygdala as compared to conventional echo-planar imaging (EPI) and uncompensated TurboPEPSI.

Artifacts↗

PubMed2003

Real-time fMRI of temporolimbic regions detects amygdala activation during single-trial self-induced sadness.

Temporolimbic circuits play a crucial role in the regulation of human emotion. A highly sensitive single-shot multiecho functional magnetic resonance imaging (fMRI) technique with gradient compensation of local magnetic field inhomogeneities and real-time data analysis were used to measure increases in amygdala activation during single 60-s trials of self-induced sadness. Six healthy male and female subjects performed a validated mood induction paradigm with randomized presentation of sad or neutral faces in 10 trials per scan. Subjects reported the intensity of experienced sadness after each trial. Immediate feedback of amygdala activation was given to the subjects during the ongoing scan to reinforce mood induction. Correspondence between increased intensity of predominantly left sided amygdala activation and self-rating of sadness was found in 78% of 120 sad trials, in contrast to only 14% of neutral trials. Amygdala activation was reproducible during repeated scanning sessions and displayed the strongest correlation with self-rating among all regions. These results suggest that amygdala activation may be closely associated with self-induced sadness. This novel real-time fMRI technology is applicable to a wide range of neuroscience studies, particularly those of the limbic system, and to neuropsychiatric conditions, such as depression, in which pathology of the amygdala has been implicated.

Adult↗