Thomas R. Knösche

Deterministic and Probabilistic Tractography Based on Complex Fibre Orientation Distributions

We propose an integral concept for tractography to describe crossing and splitting fibre bundles based on the fibre orientation distribution function (ODF) estimated from high angular resolution diffusion imaging (HARDI). We show that in order to perform accurate probabilistic tractography, one needs to use a fibre ODF estimation and not the diffusion ODF. We use a new fibre ODF estimation obtained from a sharpening deconvolution transform (SDT) of the diffusion ODF reconstructed from q-ball imaging (QBI). This SDT provides new insight into the relationship between the HARDI signal, the diffusion ODF, and the fibre ODF. We demonstrate that the SDT agrees with classical spherical deconvolution and improves the angular resolution of QBI. Another important contribution of this paper is the development of new deterministic and new probabilistic tractography algorithms using the full multidirectional information obtained through use of the fibre ODF. An extensive comparison study is performed on human brain datasets comparing our new deterministic and probabilistic tracking algorithms in complex fibre crossing regions. Finally, as an application of our new probabilistic tracking, we quantify the reconstruction of transcallosal fibres intersecting with the corona radiata and the superior longitudinal fasciculus in a group of eight subjects. Most current diffusion tensor imaging (DTI)-based methods neglect these fibres, which might lead to incorrect interpretations of brain functions.

Involuntary Motor Activity in Pianists Evoked by Music Perception

Pianists often report that pure listening to a well-trained piece of music can involuntarily trigger the respective finger movements. We designed a magnetoencephalography (MEG) experiment to compare the motor activation in pianists and nonpianists while listening to piano pieces. For pianists, we found a statistically significant increase of activity above the region of the contralateral motor cortex. Brain surface current density (BSCD) reconstructions revealed a spatial dissociation of this activity between notes preferably played by the thumb and the little finger according to the motor homunculus. Hence, we could demonstrate that pianists, when listening to well-trained piano music, exhibit involuntary motor activity involving the contralateral primary motor cortex (M1).

Tangential derivative mapping of axial MEG applied to event-related desynchronization research.

OBJECTIVES A problem with the topographic mapping of MEG data recorded with axial gradiometers is that field extrema are measured at sensors located at either side of a neuronal generator instead of at sensors directly above the source. This is problematic for the computation of event-related desynchronization (ERD) on MEG data, since ERD relies on a correspondence between the signal maximum and the location of the neuronal generator. METHODS We present a new method based on computing spatial derivatives of the MEG data. The limitations of this method were investigated by means of forward simulations, and the method was applied to a 150-channel MEG dataset. RESULTS The simulations showed that the method has some limitations. (1) Fewer channels reduce accuracy and amplitude. (2) It is less suitable for deep or very extended sources. (3) Multiple sources can only be distinguished if they are not too close to each other. Applying the method in the calculation of ERD on experimental data led to a considerable improvement of the ERD maps. CONCLUSIONS The proposed method offers a significant advantage over raw MEG signals, both for the topographic mapping of MEG and for the analysis of rhythmic MEG activity by means of ERD.

Diffusion imaging in humans at 7T using readout-segmented EPI and GRAPPA.

Anatomical MRI studies at 7T have demonstrated the ability to provide high‐quality images of human tissue in vivo. However, diffusion‐weighted imaging at 7T is limited by the increased level of artifact associated with standard, single‐shot, echo‐planar imaging, even when parallel imaging techniques such as generalized autocalibrating partially parallel acquisitions (GRAPPA) are used to reduce the effective echo spacing. Readout‐segmented echo‐planar imaging in conjunction with parallel imaging has the potential to reduce these artifacts by allowing a further reduction in effective echo spacing during the echo‐planar imaging readout. This study demonstrates that this approach does indeed provide a substantial improvement in image quality by reducing image blurring and susceptibility‐based distortions, as well as by allowing the acquisition of diffusion‐weighted images with a high spatial resolution. A preliminary application of the technique to high‐resolution diffusion tensor imaging provided a high level of neuroanatomical detail, which should prove valuable in a wide range of applications. Magn Reson Med 64:9–14, 2010.

Modeling of the human skull in EEG source analysis

We used computer simulations to investigate finite element models of the layered structure of the human skull in EEG source analysis. Local models, where each skull location was modeled differently, and global models, where the skull was assumed to be homogeneous, were compared to a reference model, in which spongy and compact bone were explicitly accounted for. In both cases, isotropic and anisotropic conductivity assumptions were taken into account. We considered sources in the entire brain and determined errors both in the forward calculation and the reconstructed dipole position. Our results show that accounting for the local variations over the skull surface is important, whereas assuming isotropic or anisotropic skull conductivity has little influence. Moreover, we showed that, if using an isotropic and homogeneous skull model, the ratio between skin/brain and skull conductivities should be considerably lower than the commonly used 80:1. For skull modeling, we recommend (1) Local models: if compact and spongy bone can be identified with sufficient accuracy (e.g., from MRI) and their conductivities can be assumed to be known (e.g., from measurements), one should model these explicitly by assigning each voxel to one of the two conductivities, (2) Global models: if the conditions of (1) are not met, one should model the skull as either homogeneous and isotropic, but with considerably higher skull conductivity than the usual 0.0042 S/m, or as homogeneous and anisotropic, but with higher radial skull conductivity than the usual 0.0042 S/m and a considerably lower radial:tangential conductivity anisotropy than the usual 1:10. Hum Brain Mapp, 2010.

K-space and q-space: Combining ultra-high spatial and angular resolution in diffusion imaging using ZOOPPA at 7T

There is ongoing debate whether using a higher spatial resolution (sampling k-space) or a higher angular resolution (sampling q-space angles) is the better way to improve diffusion MRI (dMRI) based tractography results in living humans. In both cases, the limiting factor is the signal-to-noise ratio (SNR), due to the restricted acquisition time. One possible way to increase the spatial resolution without sacrificing either SNR or angular resolution is to move to a higher magnetic field strength. Nevertheless, dMRI has not been the preferred application for ultra-high field strength (7 T). This is because single-shot echo-planar imaging (EPI) has been the method of choice for human in vivo dMRI. EPI faces several challenges related to the use of a high resolution at high field strength, for example, distortions and image blurring. These problems can easily compromise the expected SNR gain with field strength. In the current study, we introduce an adapted EPI sequence in conjunction with a combination of ZOOmed imaging and Partially Parallel Acquisition (ZOOPPA). We demonstrate that the method can produce high quality diffusion-weighted images with high spatial and angular resolution at 7 T. We provide examples of in vivo human dMRI with isotropic resolutions of 1 mm and 800 μm. These data sets are particularly suitable for resolving complex and subtle fiber architectures, including fiber crossings in the white matter, anisotropy in the cortex and fibers entering the cortex.

Perception of phrase structure in music

Neither music nor spoken language form uniform auditory streams, rather, they are structured into phrases. For the perception of such structures, the detection of phrase boundaries is crucial. We discovered electroencephalography (EEG) and magnetoencephalography (MEG) correlates for the perception of phrase boundaries in music. In EEG, this process was marked by a positive wave approximately between 500 and 600 ms after the offset of a phrase boundary with a centroparietal maximum. In MEG, we found major activity in an even broader time window (400–700 ms). Source localization revealed that likely candidates for the generation of the observed effects are structures in the limbic system, including anterior and posterior cingulate as well as posterior mediotemporal cortex. The timing and topography of the EEG effect bear some resemblance to a positive shift (closure positive shift, CPS) found for prosodic phrase boundaries during speech perception in an earlier study, suggesting that the underlying processes might be related. Because the brain structures, which possibly underlie the observed effects, are known to be involved in memory and attention processes, we suggest that the CPS may not reflect the detection of the phrase boundary as such, but those memory and attention related processes that are necessary to guide the attention focus from one phrase to the next, thereby closing the former and opening up the next phrase. Hum Brain Mapp 24:259–273, 2005.

A guideline for head volume conductor modeling in EEG and MEG

For accurate EEG/MEG source analysis it is necessary to model the head volume conductor as realistic as possible. This includes the distinction of the different conductive compartments in the human head. In this study, we investigated the influence of modeling/not modeling the conductive compartments skull spongiosa, skull compacta, cerebrospinal fluid (CSF), gray matter, and white matter and of the inclusion of white matter anisotropy on the EEG/MEG forward solution. Therefore, we created a highly realistic 6-compartment head model with white matter anisotropy and used a state-of-the-art finite element approach. Starting from a 3-compartment scenario (skin, skull, and brain), we subsequently refined our head model by distinguishing one further of the above-mentioned compartments. For each of the generated five head models, we measured the effect on the signal topography and signal magnitude both in relation to a highly resolved reference model and to the model generated in the previous refinement step. We evaluated the results of these simulations using a variety of visualization methods, allowing us to gain a general overview of effect strength, of the most important source parameters triggering these effects, and of the most affected brain regions. Thereby, starting from the 3-compartment approach, we identified the most important additional refinement steps in head volume conductor modeling. We were able to show that the inclusion of the highly conductive CSF compartment, whose conductivity value is well known, has the strongest influence on both signal topography and magnitude in both modalities. We found the effect of gray/white matter distinction to be nearly as big as that of the CSF inclusion, and for both of these steps we identified a clear pattern in the spatial distribution of effects. In comparison to these two steps, the introduction of white matter anisotropy led to a clearly weaker, but still strong, effect. Finally, the distinction between skull spongiosa and compacta caused the weakest effects in both modalities when using an optimized conductivity value for the homogenized compartment. We conclude that it is highly recommendable to include the CSF and distinguish between gray and white matter in head volume conductor modeling. Especially for the MEG, the modeling of skull spongiosa and compacta might be neglected due to the weak effects; the simplification of not modeling white matter anisotropy is admissible considering the complexity and current limitations of the underlying modeling approach.

Anatomical and functional parcellation of the human lateral premotor cortex

The lateral premotor cortex (PM) of the macaque monkey is an anatomically multifaceted area, which serves multiple sensorimotor and cognitive functions. While evidence for the functional organization of human premotor cortex accumulates, much less is known about the underlying anatomical properties of this brain region. We used diffusion tractography and functional magnetic resonance imaging (fMRI) to investigate whether the precentral gyrus in humans can be segregated on the basis of anatomical connectivity and of functional activation in a set of cognitive and motor tasks. Tractographic data suggested a distinction between ventral and dorsal premotor cortex, and furthermore inferior and superior subparcellation of both. Functional MRI data corroborated these four areas, showing that anatomical parcellation predicts the distribution of functional activation and vice versa (preliminary evidence). These results may encourage the application of combining diffusion tractography and fMRI in vivo in order to shed light on the correspondence of brain function and anatomy.

The perception of musical phrase structure: A cross-cultural ERP study

Electroencephalography (EEG) was used in a cross-cultural music study investigating phrase boundary perception. Chinese and German musicians performed a cultural categorization task under Chinese and Western music listening conditions. Western music was the major subject for both groups of musicians, while Chinese music was familiar to Chinese subjects only. By manipulating the presence of pauses between two phrases in the biphrasal melodies, EEG correlates for the perception of phrase boundaries were found in both groups under both music listening conditions. Between 450 and 600 ms, the music CPS (closure positive shift), which had been found in earlier studies with a false tone detection task, was replicated for the more global categorization task and for all combinations of subject group and musical style. At short latencies (100 and 450 ms post phrase boundary offset), EEG correlates varied as a function of musical styles and subject group. Both bottom-up (style properties of the music) and top-down (acculturation of the subjects) information interacted during this early processing stage.