@phdthesis{Hock2024, author = {Hock, Michael}, title = {Methods for Homogenization of Spatio-Temporal B\(_0\) Magnetic Field Variations in Cardiac MRI at Ultra-High Field Strength}, doi = {10.25972/OPUS-34821}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-348213}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2024}, abstract = {Cardiovascular disease is one of the leading causes of death worldwide and, so far, echocardiography, nuclear cardiology, and catheterization are the gold standard techniques used for its detection. Cardiac magnetic resonance (CMR) can replace the invasive imaging modalities and provide a "one-stop shop" characterization of the cardiovascular system by measuring myocardial tissue structure, function and perfusion of the heart, as well as anatomy of and flow in the coronary arteries. In contrast to standard clinical magnetic resonance imaging (MRI) scanners, which are often operated at a field strength of 1.5 or 3 Tesla (T), a higher resolution and subsequent cardiac parameter quantification could potentially be achieved at ultra-high field, i.e., 7 T and above. Unique insights into the pathophysiology of the heart are expected from ultra-high field MRI, which offers enhanced image quality in combination with novel contrast mechanisms, but suffers from spatio-temporal B0 magnetic field variations. Due to the resulting spatial misregistration and intra-voxel dephasing, these B0-field inhomogeneities generate a variety of undesired image artifacts, e.g., artificial image deformation. The resulting macroscopic field gradients lead to signal loss, because the effective transverse relaxation time T2* is shortened. This affects the accuracy of T2* measurements, which are essential for myocardial tissue characterization. When steady state free precession-based pulse sequences are employed for image acquisition, certain off-resonance frequencies cause signal voids. These banding artifacts complicate the proper marking of the myocardium and, subsequently, systematic errors in cardiac function measurements are inevitable. Clinical MR scanners are equipped with basic shim systems to correct for occurring B0-field inhomogeneities and resulting image artifacts, however, these are not sufficient for the advanced measurement techniques employed for ultra-high field MRI of the heart. Therefore, this work focused on the development of advanced B0 shimming strategies for CMR imaging applications to correct the spatio-temporal B0 field variations present in the human heart at 7 T. A novel cardiac phase-specific shimming (CPSS) technique was set up, which featured a triggered B0 map acquisition, anatomy-matched selection of the shim-region-of-interest (SROI), and calibration-based B0 field modeling. The influence of technical limitations on the overall spherical harmonics (SH) shim was analyzed. Moreover, benefits as well as pitfalls of dynamic shimming were debated in this study. An advanced B0 shimming strategy was set up and applied in vivo, which was the first implementation of a heart-specific shimming approach in human UHF MRI at the time. The spatial B0-field patterns which were measured in the heart throughout this study contained localized spots of strong inhomogeneities. They fluctuated over the cardiac cycle in both size and strength, and were ideally addressed using anatomy-matched SROIs. Creating a correcting magnetic field with one shim coil, however, generated eddy currents in the surrounding conducting structures and a resulting additional, unintended magnetic field. Taking these shim-to-shim interactions into account via calibration, it was demonstrated for the first time that the non-standard 3rd-order SH terms enhanced B0-field homogeneity in the human heart. However, they were attended by challenges for the shim system hardware employed in the presented work, which was indicated by the currents required to generate the optimal 3rd-order SH terms exceeding the dynamic range of the corresponding shim coils. To facilitate dynamic shimming updated over the cardiac cycle for cine imaging, the benefit of adjusting the oscillating CPSS currents was found to be vital. The first in vivo application of the novel advanced B0 shimming strategy mostly matched the simulations. The presented technical developments are a basic requirement to quantitative and functional CMR imaging of the human heart at 7 T. They pave the way for numerous clinical studies about cardiac diseases, and continuative research on dedicated cardiac B0 shimming, e.g., adapted passive shimming and multi-coil technologies.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{Kleineisel2024, author = {Kleineisel, Jonas}, title = {Variational networks in magnetic resonance imaging - Application to spiral cardiac MRI and investigations on image quality}, doi = {10.25972/OPUS-34737}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-347370}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2024}, abstract = {Acceleration is a central aim of clinical and technical research in magnetic resonance imaging (MRI) today, with the potential to increase robustness, accessibility and patient comfort, reduce cost, and enable entirely new kinds of examinations. A key component in this endeavor is image reconstruction, as most modern approaches build on advanced signal and image processing. Here, deep learning (DL)-based methods have recently shown considerable potential, with numerous publications demonstrating benefits for MRI reconstruction. However, these methods often come at the cost of an increased risk for subtle yet critical errors. Therefore, the aim of this thesis is to advance DL-based MRI reconstruction, while ensuring high quality and fidelity with measured data. A network architecture specifically suited for this purpose is the variational network (VN). To investigate the benefits these can bring to non-Cartesian cardiac imaging, the first part presents an application of VNs, which were specifically adapted to the reconstruction of accelerated spiral acquisitions. The proposed method is compared to a segmented exam, a U-Net and a compressed sensing (CS) model using qualitative and quantitative measures. While the U-Net performed poorly, the VN as well as the CS reconstruction showed good output quality. In functional cardiac imaging, the proposed real-time method with VN reconstruction substantially accelerates examinations over the gold-standard, from over 10 to just 1 minute. Clinical parameters agreed on average. Generally in MRI reconstruction, the assessment of image quality is complex, in particular for modern non-linear methods. Therefore, advanced techniques for precise evaluation of quality were subsequently demonstrated. With two distinct methods, resolution and amplification or suppression of noise are quantified locally in each pixel of a reconstruction. Using these, local maps of resolution and noise in parallel imaging (GRAPPA), CS, U-Net and VN reconstructions were determined for MR images of the brain. In the tested images, GRAPPA delivers uniform and ideal resolution, but amplifies noise noticeably. The other methods adapt their behavior to image structure, where different levels of local blurring were observed at edges compared to homogeneous areas, and noise was suppressed except at edges. Overall, VNs were found to combine a number of advantageous properties, including a good trade-off between resolution and noise, fast reconstruction times, and high overall image quality and fidelity of the produced output. Therefore, this network architecture seems highly promising for MRI reconstruction.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{Portmann2023, author = {Portmann, Johannes}, title = {Accelerated inversion recovery MRI of the myocardium using spiral acquisition}, doi = {10.25972/OPUS-30282}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-302822}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {This work deals with the acceleration of cardiovascular MRI for the assessment of functional information in steady-state contrast and for viability assessment during the inversion recovery of the magnetization. Two approaches are introduced and discussed in detail. MOCO-MAP uses an exponential model to recover dynamic image data, IR-CRISPI, with its low-rank plus sparse reconstruction, is related to compressed sensing. MOCO-MAP is a successor to model-based acceleration of parametermapping (MAP) for the application in the myocardial region. To this end, it was augmented with a motion correction (MOCO) step to allow exponential fitting the signal of a still object in temporal direction. Iteratively, this introduction of prior physical knowledge together with the enforcement of consistency with the measured data can be used to reconstruct an image series from distinctly shorter sampling time than the standard exam (< 3 s opposed to about 10 s). Results show feasibility of the method as well as detectability of delayed enhancement in the myocardium, but also significant discrepancies when imaging cardiac function and artifacts caused already by minor inaccuracy of the motion correction. IR-CRISPI was developed from CRISPI, which is a real-time protocol specifically designed for functional evaluation of image data in steady-state contrast. With a reconstruction based on the separate calculation of low-rank and sparse part, it employs a softer constraint than the strict exponential model, which was possible due to sufficient temporal sampling density via spiral acquisition. The low-rank plus sparse reconstruction is fit for the use on dynamic and on inversion recovery data. Thus, motion correction is rendered unnecessary with it. IR-CRISPI was equipped with noise suppression via spatial wavelet filtering. A study comprising 10 patients with cardiac disease show medical applicability. A comparison with performed traditional reference exams offer insight into diagnostic benefits. Especially regarding patients with difficulty to hold their breath, the real-time manner of the IR-CRISPI acquisition provides a valuable alternative and an increase in robustness. In conclusion, especially with IR-CRISPI in free breathing, a major acceleration of the cardiovascular MR exam could be realized. In an acquisition of less than 100 s, it not only includes the information of two traditional protocols (cine and LGE), which take up more than 9.6 min, but also allows adjustment of TI in retrospect and yields lower artifact level with similar image quality.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{GraetzgebDittmann2022, author = {Graetz [geb. Dittmann], Jonas}, title = {X-Ray Dark-Field Tensor Tomography : a Hitchhiker's Guide to Tomographic Reconstruction and Talbot Imaging}, doi = {10.25972/OPUS-28143}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-281437}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {X-ray dark-field imaging allows to resolve the conflict between the demand for centimeter scaled fields of view and the spatial resolution required for the characterization of fibrous materials structured on the micrometer scale. It draws on the ability of X-ray Talbot interferometers to provide full field images of a sample's ultra small angle scattering properties, bridging a gap of multiple orders of magnitude between the imaging resolution and the contrasted structure scale. The correspondence between shape anisotropy and oriented scattering thereby allows to infer orientations within a sample's microstructure below the imaging resolution. First demonstrations have shown the general feasibility of doing so in a tomographic fashion, based on various heuristic signal models and reconstruction approaches. Here, both a verified model of the signal anisotropy and a reconstruction technique practicable for general imaging geometries and large tensor valued volumes is developed based on in-depth reviews of dark-field imaging and tomographic reconstruction techniques. To this end, a wide interdisciplinary field of imaging and reconstruction methodologies is revisited. To begin with, a novel introduction to the mathematical description of perspective projections provides essential insights into the relations between the tangible real space properties of cone beam imaging geometries and their technically relevant description in terms of homogeneous coordinates and projection matrices. Based on these fundamentals, a novel auto-calibration approach is developed, facilitating the practical determination of perspective imaging geometries with minimal experimental constraints. A corresponding generalized formulation of the widely employed Feldkamp algorithm is given, allowing fast and flexible volume reconstructions from arbitrary tomographic imaging geometries. Iterative reconstruction techniques are likewise introduced for general projection geometries, with a particular focus on the efficient evaluation of the forward problem associated with tomographic imaging. A highly performant 3D generalization of Joseph's classic linearly interpolating ray casting algorithm is developed to this end and compared to typical alternatives. With regard to the anisotropic imaging modality required for tensor tomography, X-ray dark-field contrast is extensively reviewed. Previous literature is brought into a joint context and nomenclature and supplemented by original work completing a consistent picture of the theory of dark-field origination. Key results are explicitly validated by experimental data with a special focus on tomography as well as the properties of anisotropic fibrous scatterers. In order to address the pronounced susceptibility of interferometric images to subtle mechanical imprecisions, an efficient optimization based evaluation strategy for the raw data provided by Talbot interferometers is developed. Finally, the fitness of linear tensor models with respect to the derived anisotropy properties of dark-field contrast is evaluated, and an iterative scheme for the reconstruction of tensor valued volumes from projection images is proposed. The derived methods are efficiently implemented and applied to fiber reinforced plastic samples, imaged at the ID19 imaging beamline of the European Synchrotron Radiation Facility. The results represent unprecedented demonstrations of X-ray dark-field tensor tomography at a field of view of 3-4cm, revealing local fiber orientations of both complex shaped and low-contrast samples at a spatial resolution of 0.1mm in 3D. The results are confirmed by an independent micro CT based fiber analysis.}, subject = {Dreidimensionale Rekonstruktion}, language = {en} } @phdthesis{Eirich2022, author = {Eirich, Philipp}, title = {Accelerated non-Cartesian cardiovascular MR Imaging at 3T and 7T}, doi = {10.25972/OPUS-25397}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-253974}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2022}, abstract = {In this work, accelerated non-Cartesian Magnetic Resonance Imaging (MRI) methods were established and applied to cardiovascular imaging (CMR) at different magnetic field strengths (3T and 7T). To enable rapid data acquisition, highly efficient spiral k-space trajectories were created. In addition, hybrid sampling patterns such as the twisting radial lines (TWIRL) k-space trajectory were studied. Imperfections of the dynamic gradient system of a MR scanner result in k-space sampling errors. Ultimately, these errors can lead to image artifacts in non-Cartesian acquisitions. Among other reasons such as an increased reconstruction complexity, they cause the lack of spiral sequences in clinical routine compared to standard Cartesian imaging. Therefore, the Gradient System Transfer Functions (GSTFs) of both scanners were determined and used for k-space trajectory correction in post-correction as well as in terms of a pre-emphasis. The GSTF pre-emphasis was implemented as a fully automatic procedure, which enabled a precise correction of arbitrary gradient waveforms for double-oblique slice orientations. Consequently, artifacts due to trajectory errors could be mitigated, which resulted in high image quality in non-Cartesian MRI. Additionally, the GSTF correction was validated by measuring pre-emphasized spiral gradient outputs, which showed high agreement with the theoretical gradient waveforms. Furthermore, it could be demonstrated that the performance of the GSTF correction is superior to a simple delay compensation approach. The developed pulse sequences were applied to gated as well as real-time CMR. Special focus lied on the implementation of a spiral imaging protocol to resolve the beating heart of animals and humans in real time and free breathing. In order to achieve real-time CMR with high spatiotemporal resolution, k-space undersampling was performed. For this reason, efficient sampling strategies were developed with the aim to facilitate compressed sensing (CS) during image reconstruction. The applied CS approach successfully removed aliasing artifacts and yielded high-resolution cardiac image series. Image reconstruction was performed offline in all cases such that the images were not available immediately after acquisition at the scanner. Spiral real-time CMR could be performed in free breathing, which led to an acquisition time of less than 1 minute for a whole short-axis stack. At 3T, the results were compared to the gold standard of electrocardiogram-gated Cartesian CMR in breath hold, which revealed similar values for important cardiovascular functional and volumetric parameters. This paves the way to an application of the developed framework in clinical routine of CMR. In addition, the spiral real-time protocol was transferred to swallowing and speech imaging at 3T, and first images were presented. The results were of high quality and confirm the straightforward utilization of the spiral sequence in other fields of MRI. In general, the GSTF correction yielded high-quality images at both field strengths, 3T and 7T. Off-resonance related blurring was mitigated by applying non-Cartesian readout gradients of short duration. At 7T, however, B1-inhomogeneity led to image artifacts in some cases. All in all, this work demonstrated great advances in accelerating the MRI process by combining efficient, undersampled non-Cartesian k-space coverage with CS reconstruction. Trajectory correction using the GSTF can be implemented at any scanner model and enables non-Cartesian imaging with high image quality. Especially MRI of dynamic processes greatly benefits from the presented rapid imaging approaches.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{Ullherr2021, author = {Ullherr, Maximilian}, title = {Optimization of Image Quality in High-Resolution X-Ray Imaging}, doi = {10.25972/OPUS-23117}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-231171}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {The SNR spectra model and measurement method developed in this work yield reliable application-specific optima for image quality. This optimization can either be used to understand image quality, find out how to build a good imaging device or to (automatically) optimize the parameters of an existing setup. SNR spectra are here defined as a fraction of power spectra instead of a product of device properties. In combination with the newly developed measurement method for this definition, a close correspondence be- tween theory and measurement is achieved. Prior approaches suffer from a focus on theoretical definitions without fully considering if the defined quantities can be measured correctly. Additionally, discrepancies between assumptions and reality are common. The new approach is more reliable and complete, but also more difficult to evaluate and interpret. The signal power spectrum in the numerator of this fraction allows to model the image quality of different contrast mechanisms that are used in high-resolution x-ray imaging. Superposition equations derived for signal and noise enable understanding how polychromaticity (or superposition in general) affects the image quality. For the concept of detection energy weighting, a quantitative model for how it affects im- age quality was found. It was shown that—depending on sample properties—not detecting x-ray photons can increase image quality. For optimal computational energy weighting, more general formula for the optimal weight was found. In addition to the signal strength, it includes noise and modulation transfer. The novel method for measuring SNR spectra makes it possible to experimentally optimize image quality for different contrast mechanisms. This method uses one simple measurement to obtain a measure for im- age quality for a specific experimental setup. Comparable measurement methods typically require at least three more complex measurements, where the combination may then give a false result. SNR spectra measurements can be used to: • Test theoretical predictions about image quality optima. • Optimize image quality for a specific application. • Find new mechanisms to improve image quality. The last item reveals an important limitation of x- ray imaging in general: The achievable image quality is limited by the amount of x-ray photons interacting with the sample, not by the amount incident per detector area (see section 3.6). If the rest of the imaging geometry is fixed, moving the detector only changes the field of view, not the image quality. A practical consequence is that moving the sample closer to the x-ray source increases image quality quadratically. The results of a SNR spectra measurement represent the image quality only on a relative scale, but very reliable. This relative scale is sufficient for an optimization problem. Physical effects are often already clearly identifiable by the shape of the functional relationship between input parameter and measurement result. SNR spectra as a quantity are not well suited for standardization, but instead allow a reliable optimization. Not satisfying the requirements of standardization allows to use methods which have other advantages. In this case, the SNR spectra method describes the image quality for a specific application. Consequently, additional physical effects can be taken into account. Additionally, the measurement method can be used to automate the setting of optimal machine parameters. The newly proposed image quality measure detection effectiveness is better suited for standardization or setup comparison. This quantity is very similar to measures from other publications (e.g. CNR(u)), when interpreted monochromatically. Polychromatic effects can only be modeled fully by the DE(u). The measurement processes of both are different and the DE(u) is fundamentally more reliable. Information technology and digital data processing make it possible to determine SNR spectra from a mea- sured image series. This measurement process was designed from the ground up to use these technical capabilities. Often, information technology is only used to make processes easier and more exact. Here, the whole measurement method would be infeasible without it. As this example shows, using the capabilities of digital data processing much more extensively opens many new possibilities. Information technology can be used to extract information from measured data in ways that analog data processing simply cannot. The original purpose of the SNR spectra optimization theory and methods was to optimize high resolution x-ray imaging only. During the course of this work, it has become clear that some of the results of this work affect x-ray imaging in general. In the future, these results could be applied to MI and NDT x-ray imaging. Future work on the same topic will also need to consider the relationship between SNR spectra or DE(u) and sufficient image quality.This question is about the minimal image quality required for a specific measurement task.}, subject = {Bildqualit{\"a}t}, language = {en} } @phdthesis{PonceGarcia2018, author = {Ponce Garcia, Irene Paola}, title = {Strategies for optimizing dynamic MRI}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-162622}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {In Magnetic Resonance Imaging (MRI), acquisition of dynamic data may be highly complex due to rapid changes occurred in the object to be imaged. For clinical diagnostic, dynamic MR images require both high spatial and temporal resolution. The speed in the acquisition is a crucial factor to capture optimally dynamics of the objects to obtain accurate diagnosis. In the 90's, partially parallel MRI (pMRI) has been introduced to shorten scan times reducing the amount of acquired data. These approaches use multi-receiver coil arrays to acquire independently and simultaneously the data. Reduction in the amount of acquired data results in images with aliasing artifacts. Dedicated methods as such Sensitivity Encoding (SENSE) and Generalized Autocalibrating Partially Parallel Acquisition (GRAPPA) were the basis of a series of algorithms in pMRI. Nevertheless, pMRI methods require extra spatial or temporal information in order to optimally reconstruct the data. This information is typically obtained by an extra scan or embedded in the accelerated acquisition applying a variable density acquisition scheme. In this work, we were able to reduce or totally eliminate the acquisition of the training data for kt-SENSE and kt-PCA algorithms obtaining accurate reconstructions with high temporal fidelity. For dynamic data acquired in an interleaved fashion, the temporal average of accelerated data can generate an artifact-free image used to estimate the coil sensitivity maps avoiding the need of extra acquisitions. However, this temporal average contains errors from aliased components, which may lead to signal nulls along the spectra of reconstructions when methods like kt-SENSE are applied. The use of a GRAPPA filter applied to the temporal average reduces these errors and subsequently may reduce the null components in the reconstructed data. In this thesis the effect of using temporal averages from radial data was investigated. Non-periodic artifacts performed by undersampling radial data allow a more accurate estimation of the true temporal average and thereby avoiding undesirable temporal filtering in the reconstructed images. kt-SENSE exploits not only spatial coil sensitivity variations but also makes use of spatio-temporal correlations in order to separate the aliased signals. Spatio-temporal correlations in kt-SENSE are learnt using a training data set, which consists of several central k-space lines acquired in a separate scan. The scan of these extra lines results in longer acquisition times even for low resolution images. It was demonstrate that limited spatial resolution of training data set may lead to temporal filtering effects (or temporal blurring) in the reconstructed data. In this thesis, the auto-calibration for kt-SENSE was proposed and its feasibility was tested in order to completely eliminate the acquisition of training data. The application of a prior TSENSE reconstruction produces the training data set for the kt-SENSE algorithm. These training data have full spatial resolution. Furthermore, it was demonstrated that the proposed auto-calibrating method reduces significantly temporal filtering in the reconstructed images compared to conventional kt-SENSE reconstructions employing low resolution training images. However, the performance of auto-calibrating kt-SENSE is affected by the Signal-to-Noise Ratio (SNR) of the first pass reconstructions that propagates to the final reconstructions. Another dedicated method used in dynamic MRI applications is kt-PCA, that was first proposed for the reconstruction of MR cardiac data. In this thesis, kt-PCA was employed for the generation of spatially resolved M0, T1 and T2 maps from a single accelerated IRTrueFISP or IR-Snapshot FLASH measurement. In contrast to cardiac dynamic data, MR relaxometry experiments exhibit signal at all temporal frequencies, which makes their reconstruction more challenging. However, since relaxometry measurements can be represented by only few parameters, the use of few principal components (PC) in the kt-PCA algorithm can significantly simplify the reconstruction. Furthermore, it was found that due to high redundancy in relaxometry data, PCA can efficiently extract the required information from just a single line of training data. It has been demonstrated in this thesis that auto-calibrating kt-SENSE is able to obtain high temporal fidelity dynamic cardiac reconstructions from moderate accelerated data avoiding the extra acquisition of training data. Additionally, kt-PCA has been proved to be a suitable method for the reconstruction of highly accelerated MR relaxometry data. Furthermore, a single central training line is necessary to obtain accurate reconstructions. Both reconstruction methods are promising for the optimization of training data acquisition and seem to be feasible for several clinical applications.}, subject = {Kernspintomografie}, language = {en} } @phdthesis{TranGia2014, author = {Tran-Gia, Johannes}, title = {Model-Based Reconstruction Methods for MR Relaxometry}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-109774}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2014}, abstract = {In this work, a model-based acceleration of parameter mapping (MAP) for the determination of the tissue parameter T1 using magnetic resonance imaging (MRI) is introduced. The iterative reconstruction uses prior knowledge about the relaxation behavior of the longitudinal magnetization after a suitable magnetization preparation to generate a series of fully sampled k-spaces from a strongly undersampled acquisition. A Fourier transform results in a spatially resolved time course of the longitudinal relaxation process, or equivalently, a spatially resolved map of the longitudinal relaxation time T1. In its fastest implementation, the MAP algorithm enables the reconstruction of a T1 map from a radial gradient echo dataset acquired within only a few seconds after magnetization preparation, while the acquisition time of conventional T1 mapping techniques typically lies in the range of a few minutes. After validation of the MAP algorithm for two different types of magnetization preparation (saturation recovery \& inversion recovery), the developed algorithm was applied in different areas of preclinical and clinical MRI and possible advantages and disadvantages were evaluated.}, subject = {Kernspintomographie}, language = {en} } @phdthesis{Heinzel2012, author = {Heinzel, Sebastian}, title = {Multimodal neuroimaging of prefrontal cortex (dys)function: EEG, fNIRS, fNIRS-fMRI and Imaging Genetics approaches}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-75710}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {The present cumulative dissertation comprises three neuroimaging studies using different techniques, functional tasks and experimental variables of diverse nature to investigate human prefrontal cortex (PFC) (dys)function as well as methodological aspects of functional near-infrared spectroscopy (fNIRS). (1) Both dopamine (DA) availability ("inverted U-model") and excitatory versus inhibitory DA receptor stimulation ("dual-state theory") have been linked to PFC processing and cognitive control function. Electroencephalography (EEG) was recorded during a Go/NoGo response inhibition task in 114 healthy controls and 181 adult patients with attention-deficit/hyperactivity disorder (ADHD). As a neural measure of prefrontal cognitive response control the anteriorization of the P300 centroid in NoGo- relative to Go-trials (NoGo anteriorization, NGA) was investigated for the impact of genetic polymorphisms modulating catechol-O-methyltransferase efficiency (COMT, Val158Met) in degrading prefrontal DA and inhibitory DA receptor D4 sensitivity (DRD4, 48bp VNTR). Single genes and ADHD diagnosis showed no significant impact on the NGA or behavioral measures. However, a significant COMT×DRD4 interaction was revealed as subjects with relatively increased D4-receptor function (DRD4: no 7R-alleles) displayed an "inverted U"-relationship between the NGA and increasing COMT-dependent DA levels, whereas subjects with decreased D4-sensitivity (7R) showed a U-relationship. This interaction was supported by 7R-allele dose-effects and also reflected by an impact on task behavior, i.e. intraindividual reaction time variability. Combining previous theories of PFC DA function, neural stability at intermediate DA levels may be accompanied by the risk of overly decreased neural flexibility if inhibitory DA receptor function is additionally decreased. The findings of COMT×DRD4 epistasis might help to disentangle the genetic basis of dopaminergic mechanisms underlying prefrontal (dys)function. (2) While progressive neurocognitive impairments are associated with aging and Alzheimer's disease (AD), cortical reorganization might delay difficulties in effortful word retrieval, which is one of the earliest cognitive signs of AD. Therefore, cortical hemodynamic responses were measured with fNIRS during phonological and semantic verbal fluency, and investigated in 325 non-demented, healthy subjects (age: 51-82 years). The predictive value of age, sex, verbal fluency performance and years of education for the cortical hemodynamics was assessed using multiple regression analyses. Age predicted bilaterally reduced inferior frontal junction (IFJ) and increased middle frontal and supramarginal gyri activity in both task conditions. Years of education as well as sex (IFJ activation in females > males) partly predicted opposite effects on activation compared to age, while task performance was not a significant predictor. All predictors showed small effect sizes (-.24 < β < .22). Middle frontal and supramarginal gyri activity may compensate for an aging-related decrease in IFJ recruitment during verbal fluency. The findings of aging-related (compensatory) cortical reorganization of verbal fluency processing might, in combination with other (risk) factors and using longitudinal observations, help to identify neurodegenerative processes of Alzheimer's disease, while individuals are still cognitively healthy. (3) Individual anatomical or systemic physiological sources of variance may hamper the interpretation of fNIRS signals as neural correlates of cortical functions and their association with individual personality traits. Using simultaneous fNIRS and functional magnetic resonance imaging (fMRI) of hemodynamic responses elicited by an intertemporal choice task in 20 healthy subjects, variability in crossmodal correlations and divergence in associations of the activation with trait "sensitivity to reward" (SR) was investigated. Moreover, an impact of interindividual anatomy and scalp fMRI signal fluctuations on fNIRS signals and activation-trait associations was studied. Both methods consistently detected activation within right inferior/middle frontal gyrus, while fNIRS-fMRI correlations showed wide variability between subjects. Up to 41\% of fNIRS channel activation variance was explained by gray matter volume (simulated to be) traversed by near-infrared light, and up to 20\% by scalp-cortex distance. Extracranial fMRI and fNIRS time series showed significant temporal correlations at the temple. Trait SR was negatively correlated with fMRI but not fNIRS activation elicited by immediate rewards of choice within right inferior/middle frontal gyrus. Higher trait SR increased the correlation between extracranial fMRI signal fluctuations and fNIRS signals, suggesting that task-evoked systemic arousal-effects might be trait-dependent. Task-related fNIRS signals might be impacted by regionally and individually weighted sources of anatomical and systemic physiological error variance. Traitactivation correlations might be affected or biased by systemic physiological arousal-effects, which should be accounted for in future fNIRS studies of interindividual differences.}, subject = {Pr{\"a}frontaler Kortex}, language = {en} } @phdthesis{Haddad2011, author = {Haddad, Dana}, title = {Design of oncolytic viruses for the imaging and treatment of cancer: The vaccinia construct GLV-1h153 carrying the human sodium iodide symporter}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-56441}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Therapien mittels replikations-kompetenter onkolytischer Viren zeigten bereits vielversprechende Erfolge in klinischen Studien zur Bek{\"a}mpfung verschiedener Krebserkrankungen. Die Viren sind in der Lage, sich pr{\"a}ferentiell und selektiv in Krebszellen zu vermehren, wodurch das Tumorgewebe durch Zelllyse zerst{\"o}rt, das gesunde Gewebe jedoch nicht gesch{\"a}digt wird. Biopsien sind zurzeit der Gold-Standard zur {\"U}berwachung onkolytischer Virus Therapien. In der pr{\"a}klinischen und fr{\"u}hen klinischen Phasen ist dies auch durchf{\"u}hrbar, doch f{\"u}r weitere Studien am Menschen werden Methoden ben{\"o}tigt, die eine nicht-invasive {\"U}berwachung der Therapie erm{\"o}glichen. Das Nachverfolgen der Viren k{\"o}nnte Klinikern die M{\"o}glichkeit geben, die Verteilung der Viren im K{\"o}rper nachzuverfolgen, die Effizienz und therapeutische Effekte zu korrelieren bzw. die m{\"o}gliche virale Toxizit{\"a}t zu {\"u}berwachen. Im Fokus dieser Arbeit stand die Konstruktion und das Austesten des VACV Stamms GLV-1h153, welches das Gen f{\"u}r den humanen Natrium-Iodid-Symporter (hNIS) kodiert, das als Reportergen f{\"u}r nicht-invasive bildgebende Nachverfolgung der Viren diente. Demzufolge diente das hier vorgestellte Projekt der Entwicklung von Bildgebungsverfahren, die in der onkolytischen Virustherapie eingesetzt werden k{\"o}nnen. Weiterhin sollte als weitere Strategie zur Krebsbek{\"a}mpfung die M{\"o}glichkeit untersucht werden, mit Unterst{\"u}tzung der Viren eine gezielte Radiotherapie durchzuf{\"u}hren. Bei hNIS handelt es sich um ein intrinsisches Membranprotein welches den aktiven Transport und die Anreicherung von Iodid in Schilddr{\"u}senzellen und einigen anderen Geweben vermittelt. Zudem wird das Gen, neben einigen anderen humanen Genen, bereits in pr{\"a}klinischen Studien als Reportergen verwendet und wurde in klinischen Studien bereits zur Darstellung von Viren in Prostata-Krebspatienten benutzt. Der Transfer des hNIS-kodierenden Gens mittels viraler Vektoren k{\"o}nnte es erm{\"o}glichen, dass infizierte Tumorzellen Tr{\"a}ger-freie Radionuklidproben wie z.B. Iodid-124 (124I), Iodid-131 (131I), und 99m-Technecium Pertechtenate (99mTcO4), anreichern, welche schon lange f{\"u}r die Verwendung am Menschen zugelassen sind. Weitere Vorteile bei der Verwendung von hNIS als Reportergen humanen Ursprungs sind zum einen seine minimale Immunogenit{\"a}t und zum anderen die intrazellul{\"a}re Signalamplifikation durch die Transportfunktion des Systems. Der Stamm GLV1h153 wurde in der Pankreas-Adenokarzinom Zelllinie PANC-1 getestet. GLV-1h153 konnte diese Zellen infizieren, sich in ihnen replizieren und sie in Zellkultur schließlich ebenso effizient abt{\"o}ten wie GLV-1h68. Zudem wurde eine Dosis-abh{\"a}ngige Expression von hNIS in infizierten Zellen nachgewiesen. Immunfluoreszenzanalysen best{\"a}tigten den erfolgreichen Transport des Proteins an die Zellmembran bevor die Zelllyse stattfand, was die Zeit- und Dosis-abh{\"a}ngigen Aufnahme von 131I verst{\"a}rkte. In vivo war GLV-1h153, ebenso wie GLV-1h68, sicher und f{\"u}hrte zu einer effektiven Regression der Pankreasxenograft Tumoren. Die Infektion des Tumors wurde weiterhin durch optische Bildgebung und histologische Untersuchungen best{\"a}tigt. GLV-1h153 erm{\"o}glichte weiterhin die Bildgebung von Viren in Tumoren mittels 124I-abh{\"a}ngiger Positronen-Emissions-Tomographie (PET) sowie 99m-Technecium Pertechnat-abh{\"a}ngiger (99mTcO4) Gamma Szintigraphie. Die Darstellung konnte sowohl mit intratumoral, wie auch mit intraven{\"o}s applizierten Viren erfolgen, war quantitativ, und die Radiotracer konnten bis zu 24 bzw. sogar 48 h nach deren Injektion nachgewiesen werden. Die quantitative Analyse der Radionuklidaufnahme aus PET-Bildgebungsdaten korrelierte mit den Daten der Bioverteilungsdaten aus isolierten Gewebn. Autoradiographische Untersuchungen von GLV-1h153 infizierten Tumoren zeigten, dass das Vorhandensein von Viren (visualisiert durch die viral vermittelte GFP Expression), lebendes Gewebe und ausreichender Blutfluss ben{\"o}tigt werden, um die Aufnahme des Radiotracers in den Tumor zu erh{\"o}hen. Dosimetrische Analysen infizierter Tumoren zeigten das Potential f{\"u}r eine systemisch applizierte Radiotherapie des Tumors auf. So f{\"u}hrte eine Kombination aus GLV-1h153 mit 131I-Behandlung zu geringf{\"u}gig besseren therapeutischen Erfolgen, als eine alleinige Therapie mit GLV-1h153. Zusammengefasst, ist GLV-1h153 demnach ein vielversprechender Kandidat zur Behandlung von Bauchspeicheldr{\"u}senkrebs und zur nichtinvasiven Bildgebung der viralen Therapie. Die Ergebnisse untermauern die Notwendigkeit weiterer Untersuchungen und Entwicklungen in der Langzeitverfolgung viraler Therapien sowie synergistischer Effekte einer Radioiod-Kombinationstherapie mit dieser neuen therapeutischen und bildgebenden Substanzklasse.}, subject = {Onkolyse}, language = {en} } @phdthesis{Zeller2011, author = {Zeller, Julia}, title = {Funktionelle Aktivierungen im Verlauf eines Jahres bei Patienten mit Alzheimer-Erkrankung und gesunden Kontrollen}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-67408}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Hintergrund: Die Alzheimer-Erkrankung ist die h{\"a}ufigste neurodegenerative Erkrankung. Da es zurzeit f{\"u}r sie noch keine Heilung gibt, richtet sich das Hauptaugenmerk auf eine m{\"o}glichst fr{\"u}he Diagnose und die Behandlung mit krankheitsverz{\"o}gernden Medikamenten. Vor allem die funktionelle Bildgebung gilt im Bereich der Fr{\"u}hdiagnose als vielversprechend. Neben dem Ged{\"a}chtnis werden die visuell-r{\"a}umliche Informationsverarbeitung, exekutive Funktionen und Aufmerksamkeitsprozesse untersucht. Hierbei zeigen sich zentralnerv{\"o}se Aktivierungsauff{\"a}lligkeiten in kortikalen Zielregionen etwa im pr{\"a}frontalen und im parietalen Kortex. Verlaufsuntersuchungen konzentrieren sich vor allem darauf aus der Gehirnaktivierung Vorhersagen {\"u}ber kognitive Ver{\"a}nderungen bei {\"a}lteren Personen mit und ohne Ged{\"a}chtnisst{\"o}rung treffen zu k{\"o}nnen. Nur wenige Studien erfassen dabei jedoch die Gehirnaktivierung zu mehreren Messzeitpunkten. Gerade f{\"u}r große Stichproben und wiederholte Messungen k{\"o}nnte die funktionelle Nahinfrarotspektroskopie (fNIRS) sich als Alternative zur Magnetresonanztomographie anbieten. Ziel: Ziel der Studie war es, mit fNIRS und ereigniskorrelierten Potentialen funktionelle Unterschiede zwischen Alzheimer-Patienten und gleichaltrigen Kontrollen in mehreren Funktionsbereichen darzustellen und ihre Ver{\"a}nderung {\"u}ber den Zeitraum eines Jahres zu verfolgen. Zum ersten Mal sollte im Rahmen einer prospektiven Untersuchung mit fNIRS gepr{\"u}ft werden ob kortikale Aktivierungen zur Vorhersage von neuropsychologischen Testwerten genutzt werden k{\"o}nnen. Zus{\"a}tzlich stellte sich die Frage, ob fNIRS f{\"u}r Verlaufsuntersuchungen an {\"a}lteren Stichproben geeignet ist. Methoden: Im Rahmen der vorliegenden Arbeit wurden zum ersten Messzeitpunkt (T1) 73 Patienten und 71 Kontrollen mit vier Paradigmen in den drei Funktionsbereichen visuell-r{\"a}umliche Informationsverarbeitung, exekutive Funktionen und zentralnerv{\"o}se Filtermechanismen mit fNIRS und ereigniskorrelierten Potentialen gemessen. Die Probanden durchliefen eine Line Orientation Aufgabe, zwei Versionen einer Wortfl{\"u}ssigkeitsaufgabe (phonologisch und semantisch) und das P50-Doppelklickparadigma. Zielparameter waren dabei die aufgabenbezogene Aktivierung im parietalen Kortex, im dorsolateralen Pr{\"a}frontalkortex (DLPFC) und das sensorische Gating, gemessen durch die P50-Supression nach wiederholter Reizdarbietung. Zus{\"a}tzlich wurden zwei typische Tests zur Demenzdiagnostik (MMST und DemTect) erhoben. Die zweite Messung (T2) fand nach 12 Monaten statt und lief identisch zur ersten Untersuchung ab. Zu T2 konnten 14 Patienten und 51 Kontrollen erneut rekrutiert werden. Ergebnisse: Zu T1 konnte mit fNIRS ein Aktivierungsdefizit f{\"u}r Patienten im DLPFC w{\"a}hrend der phonologischen Wortfl{\"u}ssigkeitsaufgabe und im rechten Parietalkortex w{\"a}hrend der Line Orientation Aufgabe festgestellt werden. F{\"u}r die semantische Wortfl{\"u}ssigkeitsaufgabe und das sensorische Gating zeigten sich keine zentralnerv{\"o}sen Unterschiede. {\"U}ber das Jahr hinweg nahm die aufgabenbezogene Aktivierung der Patienten im linken DLPFC f{\"u}r beide Versionen der Wortfl{\"u}ssigkeitsaufgabe deutlich ab, w{\"a}hrend gleichaltrige Kontrollpersonen keine kortikalen Ver{\"a}nderungen zeigten. Zu T2 war das sensorische Gating der Patienten außerdem deutlich schlechter im Vergleich zu gesunden Kontrollen. Die Ver{\"a}nderungen der Oxygenierung w{\"a}hrend der Wortfl{\"u}ssigkeitsaufgabe konnten f{\"u}r gesunde Kontrollen Verschlechterungen im MMST und im DemTect vorhersagen. Vor allem ein Verlust der Lateralisierung ging mit einem Abfall in den kognitiven Tests einher. Schlussfolgerung: Spezifische Defizite in der kortikalen Aktivierung konnten bei Alzheimer-Patienten mit fNIRS beobachtet und genauer beschrieben werden. Auch die Ver{\"a}nderung im Verlauf eines Jahres ließ sich mit dieser Methode verfolgen. F{\"u}r L{\"a}ngsschnittuntersuchungen, die sich mit der kortikalen Aktivierung als Pr{\"a}diktor f{\"u}r dementielle Entwicklungen besch{\"a}ftigen, bietet sich fNIRS somit als praktische Alternative zur fMRT an, zumal die gemessenen Ver{\"a}nderungen in der Oxygenierung auch prognostischen Wert f{\"u}r {\"a}ltere Kontrollpersonen besaßen. Vor allem die funktionelle Lateralisierung in frontalen Kortexbereichen scheint als Pr{\"a}diktor kognitiver Leistungen im Alter von Bedeutung zu sein.}, subject = {Alzheimer-Krankheit}, language = {de} } @phdthesis{Oechsner2011, author = {Oechsner, Markus}, title = {Morphologische und funktionelle 1H-Magnetresonanztomographie der menschlichen Lunge bei 0.2 und 1.5 Tesla}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-66942}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {Das Ziel dieser Arbeit war es, Methoden und Techniken f{\"u}r die morphologische und funktionelle Bildgebung der menschlichen Lunge mittels Kernspintomographie bei Feldst{\"a}rken von 0,2 Tesla und 1,5 Tesla zu entwickeln und zu optimieren. Bei 0,2 Tesla wurde mittels der gemessenen Relaxationszeiten T1 und T2* eine 2D und eine 3D FLASH Sequenz zur Untersuchung der Lungenmorphologie optimiert. Sauerstoffgest{\"u}tzte Messungen der Relaxationszeiten T1 und T2* sowie eine SpinLabeling Sequenz liefern funktionelle Informationen {\"u}ber den Sauerstofftransfer und die Perfusion der Lungen. Bei 1,5 Tesla wurde die Lungenperfusion mittels MR-Kontrastmittel mit einer 2D und einer 3D Sequenz unter Verwendung der Pr{\"a}bolus Technik quantifiziert. Zudem wurden zwei MR-Navigationstechniken entwickelt, die es erm{\"o}glichen Lungenuntersuchungen unter freier Atmung durchzuf{\"u}hren und aus den Daten artefaktfreie Bilder zu rekonstruieren. Diese Techniken k{\"o}nnen in verschiedenste Sequenzen f{\"u}r die Lungenbildgebung implementiert werden, ohne dass die Messzeit dadurch signifikant verl{\"a}ngert wird.}, subject = {NMR-Bildgebung}, language = {de} } @phdthesis{Dieler2011, author = {Dieler, Alica Christina}, title = {Investigation of variables influencing cognitive inhibition: from the behavioral to the molecular level}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-65955}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2011}, abstract = {The present work investigated the neural mechanisms underlying cognitive inhibition/thought suppression in Anderson's and Green's Think/No-Think paradigm (TNT), as well as different variables influencing these mechanisms at the cognitive, the neurophysiological, the electrophysiological and the molecular level. Neurophysiological data collected with fNIRS and fMRI have added up to the existing evidence of a fronto-hippocampal network interacting during the inhibition of unwanted thoughts. Some evidence has been presented suggesting that by means of external stimulation of the right dlPFC through iTBS thought suppression might be improved, providing further evidence for an implication of this region in the TNT. A combination of fNIRS with ERP has delivered evidence of a dissociation of early condition-independent attentional and later suppression-specific processes within the dlPFC, both contributing to suppression performance. Due to inconsistencies in the previous literature it was considered how stimulus valence would influence thought suppression by manipulating the emotional content of the to-be-suppressed stimuli. Findings of the current work regarding the ability to suppress negative word or picture stimuli have, however, been inconclusive as well. It has been hypothesized that performance in the TNT might depend on the combination of valence conditions included in the paradigm. Alternatively, it has been suggested that inconsistent findings regarding the suppression of negative stimuli or suppression at all might be due to certain personality traits and/or genetic variables, found in the present work to contribute to thought inhibition in the TNT. Rumination has been shown to be a valid predictor of thought suppression performance. Increased ruminative tendencies led to worse suppression performance which, in the present work, has been linked to less effective recruitment of the dlPFC and in turn less effective down-regulation of hippocampal activity during suppression trials. Trait anxiety has also been shown to interrupt thought suppression despite higher, however, inefficient recruitment of the dlPFC. Complementing the findings regarding ruminative tendencies and decreased thought inhibition a functional polymorphism in the KCNJ6 gene, encompassing a G-to-A transition, has been shown to disrupt thought suppression despite increased activation of the dlPFC. Through the investigation of thought suppression at different levels, the current work adds further evidence to the idea that the TNT reflects an executive control mechanism, which is sensitive to alterations in stimulus valence to some extent, neurophysiological functioning as indicated by its sensitivity to iTBS, functional modulations at the molecular level and personality traits, such as rumination and trait anxiety.}, subject = {Kognitiver Prozess}, language = {en} } @phdthesis{Hofmann2008, author = {Hofmann, Stefanie}, title = {Diagnostik vor organerhaltender (nephron sparing) Nierentumorchirurgie in W{\"u}rzburg - Ergebnisse von 1997 bis 2002}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-36980}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Bei fehlender Chemotherapie- und Strahlensensibilit{\"a}t stellt die Operation derzeit die einzige kurative Therapie des Nierenzellkarzinoms (NZK) dar. Dabei konnte die organerhaltende Nierentumorexstirpation sich in den letzten Jahren auch bei elektiven Indikationsstellungen zunehmend etablieren und gilt bei kleinen organbegrenzten NZK inzwischen als Methode der Wahl, auch wenn {\"u}ber Effektivit{\"a}t und Sicherheit hinsichtlich Tumorkontrolle und Patienten{\"u}berleben weiterhin diskutiert wird. Die radikale Tumornephrektomie bleibt den fortgeschrittenen Tumorstadien vorbehalten. Retrospektiv zeigt sich, dass nur dann gleichwertige onkologische Ergebnisse bei der organerhaltenden Nierentumorchirurgie im Vergleich zur radikalen Nephrektomie erzielt werden k{\"o}nnen, wenn pr{\"a}operativ die Selektion eines geeigneten Patientenkollektivs erfolgt. Welche Parameter hierbei vorwiegend entscheidend waren, wurde anhand unseres Patientengutes (auch im Vergleich mit aktuellen Literaturdaten) ausgewertet. Gerade bei elektiven F{\"a}llen ist nicht zuletzt durch das Fehlen objektiver Kriterien die Grenze zwischen radikaler Tumornephrektomie und organerhaltender Nierentumorexstirpation fliessend. Oftmals ist hier im Rahmen der pr{\"a}operativen Analyse eine multifaktorielle Einsch{\"a}tzung des einzelnen Patienten erforderlich. Besonders im Fokus stand bei der Auswertung die pr{\"a}operative bildgebende Diagnostik, der bei der Indikationsstellung zugunsten einer radikalen Nephrektomie respektive einer nierenerhaltenden Tumorentfernung eine entscheidende Rolle zukommt. Ziel dieser Dissertation war es zu beurteilen, ob die pr{\"a}operative Diagnostik als sichere Grundlage bei der Entscheidung der Operationstechnik gesehen werden kann und welche Nachteile aus onkologischer als auch nephrologischer Sicht jeweils bei organerhaltender Nierentumorexstirpation und radikaler Tumornephrektomie resultieren.}, subject = {Nephrektomie}, language = {de} } @phdthesis{ReissZimmermann2008, author = {Reiß-Zimmermann, Martin}, title = {Entwicklung und Erprobung eines standardisierten Auswerteverfahrens f{\"u}r die Bestimmung der Myokardperfusion mit der MRT}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-34560}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Mit dem hier vorgestellten Untersuchungs- und Auswertealgorithmus konnte die Myokardperfusion weitgehend automatisiert semiquantitativ und quantitativ bestimmt werden. Daf{\"u}r wurden zun{\"a}chst die Bilddaten segmentiert und in Signalintensit{\"a}ts-Zeit-Kurven transferiert. Durch eine Basislinien- und Kontaminationskorrektur wurden Artefakte minimiert. Mit Anwendung des Parallel-Bildgebungs-Verfahrens Auto-SENSE konnte zus{\"a}tzlich eine Erh{\"o}hung der Schichtanzahl erreicht werden. Durch die angewendete Basislinienkorrektur konnten Inhomogenit{\"a}ten verringert werden, welche durch Verwendung einer Oberfl{\"a}chenspule methodenbedingt auftreten. Partialvolumeneffekte, die durch die Morphologie des Herzens insbesondere basis- und spitzennah auftraten, f{\"u}hrten durch eine Mischung aus KM-Anflutung im Myokard und Kontamination aus dem Ventrikellumen zu einer Beeinflussung der Perfusionsergebnisse. Durch die Verwendung der vorgestellten Kontaminationskorrektur konnten diese Artefakte erheblich minimiert werden. Die so errechneten Perfusionswerte korrelierten gut mit den in der Literatur angegebenen Daten, welche sowohl in tierexperimentellen als auch Probanden- und auch Patientenstudien mit unterschiedlichen Modalit{\"a}ten ermittelt wurden. Eine regionale Heterogenit{\"a}t konnte nicht signifikant nachgewiesen werden. Molekular-physiologische Untersuchungen legen zwar nahe, dass es diese Heterogenit{\"a}t gibt, die regionale Verteilung der Perfusion wird jedoch kontrovers und noch keinesfalls abschließend in der Literatur diskutiert. Durch Anwendung von Auto-SENSE konnte mit einer Erh{\"o}hung der Schichtanzahl bei gleichbleibender Schichtdicke das gesamte linksventrikul{\"a}re Myokard untersucht werden. Trotz verringertem SNR waren die Ergebnisse vergleichbar mit der konventionellen Turbo-FLASH-Technik. Ob das Potential der Parallelbildgebung f{\"u}r eine Abdeckung des gesamten Herzens oder f{\"u}r eine h{\"o}here Aufl{\"o}sung von 3-4 Schichten pro Untersuchungen genutzt werden soll, ist in der aktuellen Literatur noch Gegenstand der Diskussion. Die hochaufgel{\"o}sten Untersuchungen scheinen jedoch derzeit vorteilhafter aufgrund geringerer Partialvolumeneffekte sowie der besseren Beurteilbarkeit einer subendokardialen Zone und eines transmuralen Perfusionsgradienten. Die MR-Perfusionsbildgebung ist ein aktives und rasch wachsendes Gebiet innerhalb der kardialen Bildgebung mit großem Entwicklungspotential. Durch Einbindung in ein umfassendes Herz-MR-Untersuchungsprotokoll (z.B. Morphologie, Kinetik, evtl. MR-Koronarangiographie) ist in einem Untersuchungsgang eine umfassende Diagnostik bei Patienten mit Verdacht auf KHK m{\"o}glich.}, subject = {NMR-Tomographie}, language = {de} } @phdthesis{Schreppel2008, author = {Schreppel, Theresa}, title = {Der Einfluss von Aufmerksamkeit und Interferenzkontrolle auf die Verarbeitung visueller Stimuli}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-32496}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Gegenstand der vorliegenden Arbeit war die Frage, inwieweit die neuronale Verarbeitung visueller Stimuli durch Prozesse der Aufmerksamkeit und des Arbeitsged{\"a}chtnisses moduliert werden kann. Dar{\"u}ber hinaus wurde untersucht, welche „top down" Prozesse diese Modulation steuern. Dabei wurden zwei konkurrierende Ansichten als m{\"o}gliche Erkl{\"a}rungsmodelle zugrunde gelegt und {\"u}berpr{\"u}ft. Zum einen w{\"a}re es m{\"o}glich, dass selektive Aufmerksamkeit zwei qualitativ unterschiedliche Mechanismen beinhaltet. Demnach w{\"u}rde neben dem Fokussieren auf relevante Informationen auch ein aktiver Prozess der Inhibition der Verarbeitung irrelevanter Stimuli existieren. Zum anderen ist es aber auch denkbar, dass aufgrund begrenzter Verarbeitungsressourcen das Fokussieren auf relevante Reize automatisch mit dem Nichtbeachten irrelevanter Stimuli einhergeht und nur ein Mechanismus existiert. In einem ersten Experiment wurde vorab die Alertness als ein grundlegender Prozess der Aufmerksamkeit mit der Nah-Infrarot Spektroskopie (NIRS) untersucht. Mittels eines zweigestuften Studiendesigns wurden in einem ersten Schritt f{\"u}r die Alertness relevante Regionen {\"u}ber fronto-temporalen Hirnarealen definiert. Als relevant erwiesen sich Areale des mittleren und superioren temporalen Kortex der rechten Hemisph{\"a}re und der ventrale Teil des inferioren frontalen Kortex der linken Hemisph{\"a}re. In einer zweiten Datenerhebung konnte f{\"u}r diese Regionen eine signifikant h{\"o}here Aktivierung w{\"a}hrend der Alertnessbedingung im Vergleich zu einer visuellen und motorischen Kontrollbedingung gefunden werden. Mit dem zweiten Experiment sollten bestehende, mit dem Elektroenzephalogramm (EEG) erhobene, Befunde zur Modulation der neuronalen Verarbeitung visueller Stimuli repliziert werden. Dies geschah mithilfe eines neu entwickelten Untersuchungsparadigmas, einer modifizierten n-back Aufgabe. Wie erwartet fand sich eine erh{\"o}hte Verarbeitung aufgaben-relevanter Reize im Vergleich zu einer perzeptuellen Kontrollbedingung. Die Verarbeitung irrelevanter Reize wurde allerdings nicht unterdr{\"u}ckt. Explorativ fand sich ein entsprechendes Korrelat der Aufmerksamkeitslenkung {\"u}ber frontalen Elektroden. In einem dritten Experiment wurde das modifizierte n-back Paradigma an die Anforderungen einer NIRS Messung angepasst, um frontale Effekte der Aufmerksamkeitslenkung direkter erfassen zu k{\"o}nnen als mit dem EEG. Wie erwartet fand sich bez{\"u}glich des Beachtens wie auch des Ignorierens von Stimuli eine Beteiligung frontaler Strukturen. Auf beachtete Stimuli folgte eine bilaterale Aktivierung des dorsolateralen pr{\"a}frontalen Kortex (DL-PFK) und eine Aktivierung des linken inferioren frontalen Kortex bis hin zum pr{\"a}- und postzentralen Kortex. Das Ignorieren visueller Stimuli f{\"u}hrte zu einer weitl{\"a}ufigen Aktivierung des rechten pr{\"a}frontalen Kortex (PFK). Eine Beteiligung des linken inferioren frontalen Gyrus an der Interferenzkontrolle konnte nicht wie erwartet nachgewiesen werden. Der Vergleich der beiden Aktivierungsmuster ergab keine signifikanten Unterschiede. Die zugrunde liegenden Prozesse des Arbeitsged{\"a}chtnisses und der Interferenzkontrolle f{\"u}hrten also zu einer Aktivierung stark {\"u}berlappender Hirnregionen. Nachdem die Ergebnisse der Experimente 2 und 3 keinerlei Hinweise auf einen aktiven Prozess der Interferenzinhibition nachweisen konnten, wurde im Experiment 4 die bisher genutzte 1-back Aufgabe durch eine schwierigere 2-back Aufgabe ersetzt. Aufgrund der erh{\"o}hten Auslastung des Arbeitsged{\"a}chtnisses sollte eine st{\"a}rkere Anstrengung und damit eine verst{\"a}rkte frontale Aktivierung bei der Interferenzinhibition auftreten. Diese Hypothese wurde mit einer frontalen NIRS Messung {\"u}berpr{\"u}ft (Experiment 4a). Wie erwartet f{\"u}hrte die erh{\"o}hte Auslastung des Arbeitsged{\"a}chtnisses zu einer verst{\"a}rkten Aktivierung des PFK bez{\"u}glich beachteter Reize. Hinsichtlich ignorierter Reize fand sich allerdings keine frontale Beteiligung. Parallel erhobene EEG Daten zeigten keinen Unterschied zwischen der Verarbeitung beachteter und ignorierter Gesichter. Die Verarbeitung passiv betrachteter Gesichter war im Gegensatz zu beachteten und ignorierten Gesichtern vermindert. Im zweiten Teil der Studie (Experiment 4b) wurden erstmals die okzipitalen Effekte der Aufmerksamkeitslenkung mit der NIRS erfasst. Im Einklang mit den Ergebnissen der ersten EEG Studie (Experiment 2) fand sich zwar eine verst{\"a}rkte Verarbeitung beachteter, aber keine verminderte Verarbeitung ignorierter Reize. Zusammengenommen sprechen die fehlende aktive Inhibition von Distraktorreizen im okzipitalen Kortex und die vergleichbaren neuronalen Korrelate von Prozessen des Arbeitsged{\"a}chtnisses und der Interferenzinhibition im frontalen Kortex f{\"u}r die Hypothese einer Aufteilung von begrenzten Verarbeitungsressourcen zugunsten beachteter Reize.}, subject = {Arbeitsged{\"a}chtnis}, language = {de} } @phdthesis{Seiberlich2008, author = {Seiberlich, Nicole}, title = {Advances in Non-Cartesian Parallel Magnetic Resonance Imaging using the GRAPPA Operator}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-28321}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2008}, abstract = {Magnetic Resonance Imaging (MRI) is an imaging modality which provides anatomical or functional images of the human body with variable contrasts in an arbitrarily positioned slice without the need for ionizing radiation. In MRI, data are not acquired directly, but in the reciprocal image space (otherwise known as k-space) through the application of spatially variable magnetic field gradients. The k-space is made up of a grid of data points which are generally acquired in a line-by-line fashion (Cartesian imaging). After the acquisition, the k-space data are transformed into the image domain using the Fast Fourier Transformation (FFT). However, the acquisition of data is not limited to the rectilinear Cartesian sampling scheme described above. Non-Cartesian acquisitions, where the data are collected along exotic trajectories, such as radial and spiral, have been shown to be beneficial in a number of applications. However, despite their additional properties and potential advantages, working with non-Cartesian data can be complicated. The primary difficulty is that non-Cartesian trajectories are made up of points which do not fall on a Cartesian grid, and a simple and fast FFT algorithm cannot be employed to reconstruct images from non-Cartesian data. In order to create an image, the non-Cartesian data are generally resampled on a Cartesian grid, an operation known as gridding, before the FFT is performed. Another challenge for non-Cartesian imaging is the combination of unusual trajectories with parallel imaging. This thesis has presented several new non-Cartesian parallel imaging methods which simplify both gridding and the reconstruction of images from undersampled data. In Chapter 4, a novel approach which uses the concepts of parallel imaging to grid data sampled along a non-Cartesian trajectory called GRAPPA Operator Gridding (GROG) is described. GROG shifts any acquired k-space data point to its nearest Cartesian location, thereby converting non-Cartesian to Cartesian data. The only requirements for GROG are a multi-channel acquisition and a calibration dataset for the determination of the GROG weights. Chapter 5 discusses an extension of GRAPPA Operator Gridding, namely Self-Calibrating GRAPPA Operator Gridding (SC-GROG). SC-GROG is a method by which non-Cartesian data can be gridded using spatial information from a multi-channel coil array without the need for an additional calibration dataset, as required in standard GROG. Although GROG can be used to grid undersampled datasets, it is important to note that this method uses parallel imaging only for gridding, and not to reconstruct artifact-free images from undersampled data. Chapter 6 introduces a simple, novel method for performing modified Cartesian GRAPPA reconstructions on undersampled non-Cartesian k-space data gridded using GROG to arrive at a non-aliased image. Because the undersampled non-Cartesian data cannot be reconstructed using a single GRAPPA kernel, several Cartesian patterns are selected for the reconstruction. Finally, Chapter 7 discusses a novel method of using GROG to mimic the bunched phase encoding acquisition (BPE) scheme. In MRI, it is generally assumed that an artifact-free image can be reconstructed only from sampled points which fulfill the Nyquist criterion. However, the BPE reconstruction is based on the Generalized Sampling Theorem of Papoulis, which states that a continuous signal can be reconstructed from sampled points as long as the points are on average sampled at the Nyquist frequency. A novel method of generating the "bunched" data using GRAPPA Operator Gridding (GROG), which shifts datapoints by small distances in k-space using the GRAPPA Operator instead of employing zig-zag shaped gradients, is presented in this chapter. With the conjugate gradient reconstruction method, these additional "bunched" points can then be used to reconstruct an artifact-free image from undersampled data. This method is referred to as GROG-facilitated Bunched Phase Encoding, or GROG-BPE.}, subject = {NMR-Tomographie}, language = {en} } @phdthesis{Pracht2007, author = {Pracht, Eberhard}, title = {Entwicklung und Optimierung von Bildgebungssequenzen f{\"u}r die 1H-Magnetresonanztomographie der Lunge}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26398}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2007}, abstract = {No abstract available}, subject = {NMR-Tomographie}, language = {de} }