| Magnetic resonance imaging (MRI) offers excellent soft tissue contrast, arbitrary image plane orientation, and 3D imaging. However, MR images often have artifacts originating from cardiac contraction, respiration, peristalsis, and voluntary movement. This thesis presents three new methods of mitigating motion artifacts.; For nearly periodic motions (respiration and cardiac contraction), artifacts have been reduced by detecting the motion and restricting image data acquisition to a portion of the motion cycle. Current cardiac MRI studies employ electrocardiography to infer the phase of cardiac motion. Unfortunately, time-varying magnetic fields that generate MR images corrupt electrocardiograms.; This thesis develops an alternative to electrocardiography for MRI: use of the MR signal itself for cardiac cycle detection and image data acquisition triggering. The method interlaces a triggering pulse sequence with an imaging sequence. The triggering sequence, executed repeatedly, measures aortic blood velocity as a cardiac phase surrogate. The acquired data is processed after each sequence iteration. When the desired cardiac phase is detected, the imaging sequence acquires data. Following implementation of the method in an interactive environment, the technique enabled coronary artery imaging superior to that with the conventional triggering method.; Decreasing image acquisition time, through improved gradient hardware and pulse sequence techniques, also reduces motion artifacts. However, the new rapid imaging techniques with diagnostically useful contrast, such as fast spin echo and spoiled gradient recalled imaging, yield suboptimal signal-to-noise ratios (SNR). Another rapid imaging method called steady-state free precession (SSFP) fully refocuses magnetization over a repetition interval to offer higher SNR, but lacks clinical utility due to a nonuniform spectral response and high lipid signal.; This thesis presents two fully refocused sequences with high SNR and potentially useful contrast. Each method modifies the spectral response of SSFP: one employs novel excitation phase cycles and the other combines data from multiple SSFP sequences. With each method, multiple images of different contrasts are simultaneously obtained; thus, lipid and water images can be rapidly acquired, a significant advancement over conventional SSFP. Additionally, with short repetition times, the sequences have image contrast similar to diagnostically-useful T2-weighting. Potential applications of these sequences include musculoskeletal, abdominal, breast, and angiographic imaging. |
