K-space undersampling strategies for functional and cardiac MRI: achieving rapid acquisition while maintaining image quality

Northeastern University Department of Electrical and Computer Engineering Doctor of Philosophy in Electrical Engineering K-space undersampling strategies for functional and cardiac MRI: Achieving rapid acquisition while maintaining image quality by Onur Afacan Scan time is the limiting factor in many magnetic resonance imaging applications. One approach to reduce the scan time is undersampling k-space below the Nyquist sampling rate. In this work, for three different MRI applications, we investigate the effects of existing and novel undersampling schemes on both the acquistion time and the resulting image quality. In the first application, a new accelerated multi-shot 3D EPI sequence is proposed to increase the temporal resolution of complex cognitive fMRI studies. A careful combination of two modern acceleration techniques, UNFOLD and GRAPPA, is proposed for use in the secondary phase encoding direction to reduce the scan time effectively with minimal loss in image quality. The sequence was tested using two different fMRI paradigms. The accelerated sequence was able to provide more information on timing of the complex sequence of interrelated brain functions. In the second application, fast spin echo sequences for fMRI in functional brain mapping studies of small animals were investigated. Partial Fourier acquisition was implemented to reduce the number of phase encoding lines in order to reduce the blurring resulting from T2 decay. The sequence was tested on a 7T animal scanner using CO2 challenge and forepaw stimulation experiments. With the proposed sequence it was possible to generate images with good functional sensitivity as well as high quality anatomical information. In the third application, different undersampling strategies and reconstruction methods for variable density spiral k-space trajectories were compared. Comparison metrics included signal to noise ratio, root mean square error and resolution. We also present the use of a new analysis tool, the singular value spectrum of the Fourier basis cross-correlation matrix, to analyze the information content and SNR efficiency of undersampled spiral trajectories.