Jörg Stadler

The impact of physiological noise correction on fMRI at 7 T

Cognitive neuroimaging studies typically require fast whole brain image acquisition with maximal sensitivity to small BOLD signal changes. To increase the sensitivity, higher field strengths are often employed, since they provide an increased image signal-to-noise ratio (SNR). However, as image SNR increases, the relative contribution of physiological noise to the total time series noise will be greater compared to that from thermal noise. At 7 T, we studied how the physiological noise contribution can be best reduced for EPI time series acquired at three different spatial resolutions (1.1 mm × 1.1 mm × 1.8 mm, 2 mm × 2 mm × 2 mm and 3 mm × 3 mm × 3 mm). Applying optimal physiological noise correction methods improved temporal SNR (tSNR) and increased the numbers of significantly activated voxels in fMRI visual activation studies for all sets of acquisition parameters. The most dramatic results were achieved for the lowest spatial resolution, an acquisition parameter combination commonly used in cognitive neuroimaging which requires high functional sensitivity and temporal resolution (i.e. 3 mm isotropic resolution and whole brain image repetition time of 2 s). For this data, physiological noise models based on cardio-respiratory information improved tSNR by approximately 25% in the visual cortex and 35% sub-cortically. When the time series were additionally corrected for the residual effects of head motion after retrospective realignment, the tSNR was increased by around 58% in the visual cortex and 71% sub-cortically, exceeding tSNR ~ 140. In conclusion, optimal physiological noise correction at 7 T increases tSNR significantly, resulting in the highest tSNR per unit time published so far. This tSNR improvement translates into a significant increase in BOLD sensitivity, facilitating the study of even subtle BOLD responses.

Susceptibility weighted imaging at ultra high magnetic field strengths: Theoretical considerations and experimental results

We present numerical simulations and experimental results for susceptibility weighted imaging (SWI) at 7 T. Magnitude, phase, and SWI contrast were simulated for different voxel geometries and imaging parameters, resulting in an echo time of 14 msec for optimum contrast between veins and surrounding tissue. Slice thickness of twice the in‐plane voxel size or more resulted in optimum vessel visibility. Phantom and in vivo data are in very good agreement with the simulations and the delineation of vessels at 7 T was superior compared to lower field strengths. The phase of the complex data reveals anatomical details that are complementary to the corresponding magnitude images. Susceptibility weighted imaging at very high field strengths is a promising technique because of its high sensitivity to tissue susceptibility, its low specific absorption rate, and the phases negligible sensitivity to B1 inhomogeneities. Magn Reson Med 60:1155–1168, 2008.

High Field fMRI Reveals Thalamocortical Integration of Segregated Cognitive and Emotional Processing in Mediodorsal and Intralaminar Thalamic Nuclei

Thalamocortical loops, connecting functionally segregated, higher order cortical regions, and basal ganglia, have been proposed not only for well described motor and sensory regions, but also for limbic and prefrontal areas relevant for affective and cognitive processes. These functions are, however, more specific to humans, rendering most invasive neuroanatomical approaches impossible and interspecies translations difficult. In contrast, non-invasive imaging of functional neuroanatomy using fMRI allows for the development of elaborate task paradigms capable of testing the specific functionalities proposed for these circuits. Until recently, spatial resolution largely limited the anatomical definition of functional clusters at the level of distinct thalamic nuclei. Since their anatomical distinction seems crucial not only for the segregation of cognitive and limbic loops but also for the detection of their functional interaction during cognitive–emotional integration, we applied high resolution fMRI on 7 Tesla. Using an event-related design, we could isolate thalamic effects for preceding attention as well as experience of erotic stimuli. We could demonstrate specific thalamic effects of general emotional arousal in mediodorsal nucleus and effects specific to preceding attention and expectancy in intralaminar centromedian/parafascicular complex. These thalamic effects were paralleled by specific coactivations in the head of caudate nucleus as well as segregated portions of rostral or caudal cingulate cortex and anterior insula supporting distinct thalamo–striato–cortical loops. In addition to predescribed effects of sexual arousal in hypothalamus and ventral striatum, high resolution fMRI could extent this network to paraventricular thalamus encompassing laterodorsal and parataenial nuclei. We could lend evidence to segregated subcortical loops which integrate cognitive and emotional aspects of basic human behavior such as sexual processing.

High-resolution mechanical imaging of the human brain by three-dimensional multifrequency magnetic resonance elastography at 7T.

Magnetic resonance elastography (MRE) is capable of measuring the viscoelastic properties of brain tissue in vivo. However, MRE is still limited in providing high-resolution maps of mechanical constants. We therefore introduce 3D multifrequency MRE (3DMMRE) at 7T magnetic field strength combined with enhanced multifrequency dual elasto-visco (MDEV) inversion in order to achieve high-resolution elastographic maps of in vivo brain tissue with 1mm(3) resolution. As demonstrated by phantom data, the new MDEV-inversion method provides two high resolution parameter maps of the magnitude (|G*|) and the phase angle (ϕ) of the complex shear modulus. MDEV inversion applied to cerebral 7T-3DMMRE data of five healthy volunteers revealed structures of brain tissue in greater anatomical details than previous work. The viscoelastic properties of cortical gray matter (GM) and white matter (WM) could be differentiated by significantly lower values of |G*| and ϕ in GM (21% [P<0.01]; 8%, [P<0.01], respectively) suggesting that GM is significantly softer and less viscous than WM. In conclusion, 3DMMRE at ultrahigh magnetic fields and MDEV inversion open a new window into characterizing the mechanical structure of in vivo brain tissue and may aid the detection of various neurological disorders based on their effects to mechanical tissue properties.

A high-resolution 7-Tesla fMRI dataset from complex natural stimulation with an audio movie.

Here we present a high-resolution functional magnetic resonance (fMRI) dataset – 20 participants recorded at high field strength (7 Tesla) during prolonged stimulation with an auditory feature film (“Forrest Gump”). In addition, a comprehensive set of auxiliary data (T1w, T2w, DTI, susceptibility-weighted image, angiography) as well as measurements to assess technical and physiological noise components have been acquired. An initial analysis confirms that these data can be used to study common and idiosyncratic brain response patterns to complex auditory stimulation. Among the potential uses of this dataset are the study of auditory attention and cognition, language and music perception, and social perception. The auxiliary measurements enable a large variety of additional analysis strategies that relate functional response patterns to structural properties of the brain. Alongside the acquired data, we provide source code and detailed information on all employed procedures – from stimulus creation to data analysis. In order to facilitate replicative and derived works, only free and open-source software was utilized.

Retinotopic mapping of the human visual cortex at a magnetic field strength of 7 T

OBJECTIVE fMRI-based retinotopic mapping data obtained at a magnetic field strength of 7T are evaluated and compared to 3T acquisitions. METHODS With established techniques retinotopic mapping data were obtained in four subjects for 25 slices parallel to the calcarine sulcus at 7 and 3T for three voxel sizes (2.5(3), 1.4(3), and 1.1(3)mm(3)) and in two subjects for 49 slices at 7T for 2.5(3)mm(3) voxels. The data were projected to the flattened representation of T1 weighted images acquired at 3T. RESULTS The obtained retinotopic maps allowed for the identification of visual areas in the occipito-parietal cortex. The mean coherence increased with magnetic field strength and with voxel size. At 7T, the occipital cortex could be sampled with high sensitivity in a short single session at high resolution. Alternatively, at lower resolution simultaneous mapping of a great expanse of occipito-parietal cortex was possible. CONCLUSION Retinotopic mapping at 7T aids a detailed description of the visual areas. Here, recent findings of multiple stimulus-driven retinotopic maps along the intraparietal sulcus are supported. SIGNIFICANCE Retinotopic mapping at 7T opens the possibility to detail our understanding of the cortical visual field representations in general and of their plasticity in visual system pathologies.

Investigations on the effect of caffeine on cerebral venous vessel contrast by using susceptibility-weighted imaging (SWI) at 1.5, 3 and 7 T.

Caffeine lowers the blood oxygenation level-dependent (BOLD) signal by acting as an adenosine antagonist, thus decreasing the cerebral blood flow (CBF). The aims of this study were to demonstrate the sensitivity of susceptibility-weighted imaging (SWI) to caffeine-induced changes in CBF and to investigate the time course and magnitude of signal change in caffeine-habituated and -abstinent volunteers. High-resolution susceptibility-weighted images were acquired with both groups at 1.5 T using a fully velocity compensated 3D gradient echo sequence. Following a native scan, subjects were given a tablet containing 200 mg of caffeine. Scans were repeated for about 1 h and the acquired 3D data sets were co-registered to each other. BOLD signal changes of several venous vessels were analyzed in dedicated ROIs. Maps of relative signal change clearly visualized the caffeine-induced signal response of veins. Only very weak signal changes of about -2+/-1% were found in both, grey and white matter and -1+/-2% in the ventricles. Maximum signal decrease of veins occurred 40-50 min after caffeine ingestion. The signal decrease was -16.5+/-6.5% and -22.7+/-8.3% for the caffeine users group and abstainers, respectively. The signal difference of both groups was statistically significant (Students t-test, t=2.16, p=0.021). Data acquired at 1.5, 3 and 7 T with echo times scaled to the respective field strength display very similar temporal signal behavior.

Examining the McGurk illusion using high-field 7 Tesla functional MRI

In natural communication speech perception is profoundly influenced by observable mouth movements. The additional visual information can greatly facilitate intelligibility but incongruent visual information may also lead to novel percepts that neither match the auditory nor the visual information as evidenced by the McGurk effect. Recent models of audiovisual (AV) speech perception accentuate the role of speech motor areas and the integrative brain sites in the vicinity of the superior temporal sulcus (STS) for speech perception. In this event-related 7 Tesla fMRI study we used three naturally spoken syllable pairs with matching AV information and one syllable pair designed to elicit the McGurk illusion. The data analysis focused on brain sites involved in processing and fusing of AV speech and engaged in the analysis of auditory and visual differences within AV presented speech. Successful fusion of AV speech is related to activity within the STS of both hemispheres. Our data supports and extends the audio-visual-motor model of speech perception by dissociating areas involved in perceptual fusion from areas more generally related to the processing of AV incongruence.

In vivo magnetic resonance elastography of human brain at 7 T and 1.5 T

To investigate the feasibility of quantitative in vivo ultrahigh field magnetic resonance elastography (MRE) of the human brain in a broad range of low‐frequency mechanical vibrations.

Localizing the human primary auditory cortex in vivo using structural MRI.

Currently there are no routine methods to delineate the primary auditory cortex (PAC) of humans in vivo. Due to the large differences in the location of the PAC between subjects, labels derived from post-mortem brains may be inaccurate when applied to different samples of in vivo brains. Recent magnetic resonance (MR) imaging studies suggested that MR-tissue properties can be used to define the location of the PAC region in vivo. The basis for such an approach is that the PAC region is more strongly myelinated than the secondary areas. We developed a fully automatic method to identify the PAC in conventional anatomical data using a combination of two complementary MR contrasts, i.e., T1 and T2, at 3T with 0.7mm isotropic resolution. Our algorithm maps the anatomical MR data to reconstructed cortical surfaces and uses a classification approach to create an artificial contrast that is highly sensitive to the effects of an increased myelination of the cortex. Consistent with the location of the PAC defined in post-mortem brains, we found a compact region on the medial two thirds of Heschls gyrus in both hemispheres of all 39 subjects. With further improvements in signal-to-noise ratio of the anatomical data and manual correction of segmentation errors, the results suggest that the primary auditory cortex can be defined in the living brain of single subjects.