abstract: Magnetic Resonance Imaging using spiral trajectories has many advantages in speed, efficiency in data-acquistion and robustness to motion and flow related artifacts. The increase in sampling speed, however, requires high performance of the gradient system. Hardware inaccuracies from system delays and eddy currents can cause spatial and temporal distortions in the encoding gradient waveforms. This causes sampling discrepancies between the actual and the ideal k-space trajectory. Reconstruction assuming an ideal trajectory can result in shading and blurring artifacts in spiral images. Current methods to estimate such hardware errors require many modifications to the pulse sequence, phantom measurements or specialized hardware. This work presents a new method to estimate time-varying system delays for spiral-based trajectories. It requires a minor modification of a conventional stack-of-spirals sequence and analyzes data collected on three orthogonal cylinders. The method is fast, robust to off-resonance effects, requires no phantom measurements or specialized hardware and estimate variable system delays for the three gradient channels over the data-sampling period. The initial results are presented for acquired phantom and in-vivo data, which show a substantial reduction in the artifacts and improvement in the image quality. / Dissertation/Thesis / M.S. Bioengineering 2013

A novel non-invasive imaging method of unique k-space trajectory named “3D center-out EPI with cylindrical encoding” was developed and implemented for fast imaging of the human brain. The method based on a variant of 3D hybrid EPI combines advantages of the Cartesian and the radial encoding to achieve ultra-short echo time independent of spatial resolution and reasonably short echo train length yielding a quality image of high signal-to-noise ratio. Unlike rectilinear sampling, the method offers not only less motion and flow artifacts but enables also the undersampling capability. As a result, the method improves temporal resolution by shortening the measurement time. Nonetheless, artifacts induced from long-term drifts of the magnetic field as well as geometrical distortions caused by B0 inhomogeneity were removed with the average phase of the k-space center lines and an additional field map scan. Compared to other cylindrical k-space trajectories based on echo-planar imaging, which lead to progressively increasing echo time upon increasing the spatial resolution, the proposed method offers more benefits. As a significant application, imaging readout of the novel technique was applied to true 3D cine imaging which was later used in the combination of pseudo-continuous arterial spin labeling module in order to track a short arterial spin labeling (ASL) bolus of well-defined length along the fast passage through the large vessel compartment of the brain. Parametric maps of ASL signal change, estimated time-to-peak and ASL bolus width were extracted in order to characterize the macrovascular compartments of the brain-feeding arteries. Consequently, bolus dispersion within a single arterial branch was also assessed.

A novel non-invasive imaging method of unique k-space trajectory named “3D center-out EPI with cylindrical encoding” was developed and implemented for fast imaging of the human brain. The method based on a variant of 3D hybrid EPI combines advantages of the Cartesian and the radial encoding to achieve ultra-short echo time independent of spatial resolution and reasonably short echo train length yielding a quality image of high signal-to-noise ratio. Unlike rectilinear sampling, the method offers not only less motion and flow artifacts but enables also the undersampling capability. As a result, the method improves temporal resolution by shortening the measurement time. Nonetheless, artifacts induced from long-term drifts of the magnetic field as well as geometrical distortions caused by B0 inhomogeneity were removed with the average phase of the k-space center lines and an additional field map scan. Compared to other cylindrical k-space trajectories based on echo-planar imaging, which lead to progressively increasing echo time upon increasing the spatial resolution, the proposed method offers more benefits. As a significant application, imaging readout of the novel technique was applied to true 3D cine imaging which was later used in the combination of pseudo-continuous arterial spin labeling module in order to track a short arterial spin labeling (ASL) bolus of well-defined length along the fast passage through the large vessel compartment of the brain. Parametric maps of ASL signal change, estimated time-to-peak and ASL bolus width were extracted in order to characterize the macrovascular compartments of the brain-feeding arteries. Consequently, bolus dispersion within a single arterial branch was also assessed.