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Multi-directional diffusion weighted imaging: Implementation, verification and clinical application

A water diffusion in a human brain can be used to analyze fibers within the white matter. The basis of the method is a measurement of a MR signal attenuation due to the water particles motion in a presence of a magnetic field gradient. The signal loss is related to the free path that a particle can travel. The obstacles keep the signal from the water spins coherent. Therefore, the attenuation of MR signal is stronger in the directions of the axons. The information about the directional structure of the white matter can be extracted from the data with various methods. Those approaches can be divided into two groups: a model-based and direct measurements. Both groups have their own advantages and disadvantages. The model-based approach like for example a diffusion tensor imaging (DTI) or a spherical harmonics decovnolution (SHD) can shorten the acquisition time and give high resolution images. However, the additional information from the acquisition have to be provided by some prior knowledge (response function in SHD) or assumptions (diffusion function can be described by a tensor). The results from those methods are biased by the first choice of the input functions. At the other hand, a direct measurement is free from the mentioned above defects, but it requires more measurement time. The additional information has to be gathered in order to directly image the whole diffusion process. In clinical studies the acquisition time is important, because it is difficult to scan the patients longer than 90 minutes. The aims of the thesis were to implement, verify and show in clinical conditions usefulness and advantages of a high angular resolution diffusion imaging in the diffusion measurements and the fiber tracing. All measurements were performed on a 3T MR scanner approved for the clinical routines. The computer software layer was implemented during the work on this thesis. Within this thesis a measurement sequence for multiple diffusion directions and processing routines were developed. Furthermore, it was presented how the implementation can be optimized. All results were obtained using only own software: a standard DTI tractography, a probabilistic segmentation, a q-ball fibre tracking. All implemented routines allow to measure and use a wide range of diffusion methods and adjust all the parameters according to a specific clinical task. It was presented that all implementation of algorithms were validated with a digital or an artificial phantom as well as in in-vivo experiments. The validation addressed the second aim of the thesis. It was shown, that the proposed MR sequence modifications measure properly the diffusion properties of the human brain. All the data processing software shows a perfect agreement between the results and the assumed geometries and the tests. This gives a confidence in the results obtained in-vivo from the human brain. It was demonstrated that an acquisition of a high angular resolution data is possible with typical parameters for the standard DTI measurements. The retrieved information about the water self-diffusion process allowed to reconstruct the optic radiation. This was possible with the most important feature of the q-ball measurements and the tracking: a crossing fibre reconstruction. The long optic radiation were successfully reconstructed and presented. Those fibres go through whole brain volume from the anterior to the posterior passing multiple crossing regions on their way. It was presented that the high angular resolution diffusion imaging can reconstruct the crossing fibers in an in-vivo experiment. A fiber crossing between the corpus callosum and the motor tract were shown as an example. Better quality of the directional data leads to the better fibre reconstruction. Additional information about the second diffusion direction not only makes the estimation more precise, but also enables the tracking algorithms to access regions of the brain usually not traceable. The high angular resolution diffusion imaging is a step forward for white matters studies. Two major approaches compete between each other. At the one side, there is a data reconstruction without using any assumptions like q-ball and DSI. At the other side there are methods of a signal decomposition using the prior-knowledge (PAS-MRI, SHD). This work is part of this discussion showing capabilities both approaches and comparing the results obtained by both approaches. The presented material proves that the HARDI methods are acceptable for the clinical application in the human brain. Moreover, presented results show that the HARDI methods have the same reliability of the diffusion direction finding as the standard DTI, but the HARDI methods provide more sophisticated results. Multiple diffusion directions per voxel solves many of the problems found in DTI. The HARDI methods should replace in future the DTI measurements as the fiber crossing regions should be properly resolved for the fiber tracking. Multidirektionale diffusionsgewichtete MR-Bildgebung: Implementierung, Verifizierung und klinische Anwendung