Reduction of Artifacts Arising From Non-Ideal Gradients in Fast Magnetic Resonance Imaging
thesisposted on 15.04.2014, 00:00 by Novena A. Rangwala
This project focuses on the development and validation of techniques to reduce two image artifacts arising from gradient system imperfections in fast magnetic resonance imaging (MRI). These artifacts can severely compromise image quality and adversely affect clinical diagnoses. The first artifact is a wrap-around artifact called the cusp artifact in a pulse sequence called fast spin echo (FSE). This artifact arises from gradient non-linearity away from the magnet isocenter, and appears as a line or ‘feather’ on non-axial FSE images of the spine or knee. To reduce this artifact, an FSE pulse sequence was modified to slightly tilt the slice selected by the radiofrequency (RF) excitation pulse away from the slice selected by the RF refocusing pulses. As a result, the signals from the artifact-prone region are reduced, while the signals from within the field of view are largely retained. This technique reduced the artifact intensity by 50~90% on phantom and volunteer images, and can be implemented on most MRI scanners without hardware modification, complicated calibration, sophisticated image reconstruction, or patient-handling alteration. The second artifact is the Nyquist ghost, specifically as seen on images acquired using a class of pulse sequences called echo planar imaging (EPI)-based PROPELLER (EPI-PROPELLER). An EPI-PROPELLER image is acquired through multiple acquisitions, called 'blades', where every blade is acquired by using a distinct combination of the physical gradients. The Nyquist ghost, arising from three types of phase errors, occurs due to eddy currents in the gradients combined with anisotropy between gradient axes, and is modulated by the acquisition gradients, necessitating correction on a per-blade basis in EPI-PROPELLER. A time-efficient correction technique was developed to simultaneously reduce the ghost in EPI-PROPELLER sequences. Phase errors calculated from two or three reference scans, acquired along orthogonal physical gradient axes, were used to predict and correct phase errors for all blades. The phase correction technique reduced the Nyquist ghost to 0.5~5.0% on phantom and volunteer images. The ability to simultaneously and time-efficiently reduce the Nyquist ghost arising from different phase errors is expected to improve the robustness of EPI-PROPELLER sequences, particularly on MRI scanners with residual eddy currents and gradient anisotropy.