Fast magnetic resonance imaging techniques through parallel imaging

Abstract

Magnetic resonance imaging (MRI) has been extensively used for clinical examinations and neuroscience studies. However, the scan time of MRI is considerable longer than other imaging techniques such as X-ray and ultra-sound. Recently, the availability of multichannel receiver coils has enabled vast acceleration of MRI. This technique known as parallel imaging can partially replace conventional gradient based encoding by exploiting the extra information in spatial sensitivity variation of coils. The objective of this thesis is to develop advanced parallel imaging methods for two important MRI sequences – rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and echo planar imaging (EPI), which are widely used for anatomical and functional imaging, respectively. In contrast to traditional parallel imaging methods, the proposed methods can robustly maintain high signal-to-noise ratio (SNR) and low artifact levels. Firstly, a new sensitivity encoding (SENSE) reconstruction method was proposed for in-plane accelerated PROPELLER MRI. PROPELLER is known for the motion robustness, but it prolongs scan time and is restricted mainly to T2 contrast. The proposed multi-step joint-blade (MJB) SENSE utilizes the fact that PROPELLER blades contain sharable information and blade-combined images can serve as regularization references. It consists of three steps. First, conventional blade-combined images are obtained using the conventional simple single-blade (SSB) SENSE, which reconstructs each blade separately. Second, the blade-combined images are employed as regularization for blade-wise noise reduction. Last, with virtual high-frequency data resampled from the previous step, all blades are jointly reconstructed to form the final images. Compared to SSB SENSE, MJB SENSE greatly reduced the noise amplification at various acceleration factors, leading to increased image SNR in all simulation and in vivo experiments, including T1-weighted imaging with short echo trains. Moreover, MJB SENSE was fully compatible with existing motion correction algorithms. Secondly, the proposed multi-step joint-blade SENSE reconstruction was extended to simultaneous multislice (SMS) PROPELLER. Simulations and in vivo data were used to evaluate MJB SENSE for noise, artifact, and motion correction. Similar to in-plane accelerated PROPELLER, MJB SENSE considerably reduced noise amplification in SMS PROPELLER at various multiband factors. In addition, by customizing coil sensitivity maps in each blade, artifact associated with narrow blades was mostly removed. Lastly, a novel SENSE based reconstruction method was proposed for robust Nyquist ghost correction of SMS EPI. This method derives coil sensitivity and slice-dependent phase error maps from fully sampled single-band EPI with EPI parameters matched to SMS acquisition. First, the reference data are organized into positive- and negative-echo virtual channels where missing data are jointly estimated using low-rank based simultaneous autocalibrating and k-space estimation (SAKE) for coil sensitivity maps. Second, positive- and negative-echo images are SENSE reconstructed from the reference data. Their phase difference or error map is then calculated. Finally, SMS EPI is reconstructed using phase error correction SENSE (PEC-SENSE) that incorporates the phase error map into coil sensitivity maps. Phantom and in vivo experiments at 7T showed that the proposed method substantially reduced all ghost-related artifacts originating either directly from SMS EPI data or indirectly from EPI based coil sensitivity maps.