PROPELLER‐EPI with parallel imaging using a circularly symmetric phased‐array RF coil at 3.0 T: Application to high‐resolution diffusion tensor imaging

Abstract

A technique integrating multishot periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and parallel imaging is presented for diffusion echo-planar imaging (EPI) at high spatial resolution. The method combines the advantages of parallel imaging to achieve accelerated sampling along the phase-encoding direction, and PROPELLER acquisition to further decrease the echo train length (ETL) in EPI. With an eight-element circularly symmetric RF coil, a parallel acceleration factor of 4 was applied such that, when combined with PROPELLER acquisition, a reduction of geometric distortions by a factor substantially greater than 4 was achieved. Because it offers rapid data acquisition and requires only a few RF pulses, single-shot echo-planar imaging (EPI) is generally used as the signal readout for diffusion-weighted MRI (DWI). However, owing to EPI's relatively slow sampling rate along the phase-encoding direction, diffusion images generally exhibit substantial susceptibility-related geometric distortions along the phase-encoding direction, especially around regions that show inhomogeneous magnetic field distributions (1). These distortions can strongly degrade the quality of neurological investigations near the skull base, and may result in a misleading clinical diagnosis (2).Methods to reduce geometric distortion can be divided into at least two different categories. The single-shot approach involves an increase in the bandwidth along the phase-encoding direction (BW PE ) by acquiring sufficient data for image reconstruction with a reduced echo train length (ETL) within one shot. A typical example is parallel imaging employing the use of phased-array RF coils (3,4). In contrast to the single-shot approach, methods in the second category reduce the ETL by splitting the entire data acquisition into multiple shots. In multishot diffusion imaging, the phase difference induced by physiological motion in each shot can lead to severe ghosting artifacts (5), since any motion is transformed into a phase change of the spin isochromats after DW magnetization preparation. Therefore, in multishot DWI, robust correction techniques must accompany the acquisition. One technique that could achieve this aim via inherent navigator correction is the multishot periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) method (6,7).In the present study we integrated the two aforementioned approaches (parallel imaging and PROPELLER acquisition) to obtain multishot high-resolution diffusion images with substantially reduced geometric distortions. We were particularly interested in the PROPELLER imaging method because, as will be shown in a later section, it fills the k-space by using several acquisitions of parallel lines around the center k-space with rotated phase-encoding directions (7). The rotational symmetry of k-space trajectory is especially suitable for parallel imaging acceleration when a circularly symmetric phase array coil is used for signal receiving. In other words,...

Comparison of Brain MR Images at 1.5T Using BLADE and Rectilinear Techniques for Patients Who Move during Data Acquisition

“…Our total imaging time was longer for BLADE/ PROPELLER for all sequences except axial T1WI, likely due to a lower number of acquisitions employed. A number of strategies, such as a decrease in the number of blades, 10 parallel imaging, 15,16 novel field-of-view strategies, [17][18][19] and Turboprop, 20 are now being explored to reduce the imaging time. Our study was limited by the variations in imaging parameters for T1WI and T2WI for BLADE and rectilinear acquisitions.…”

Diffusion Tensor MR Imaging and Fiber Tractography: Technical Considerations

“…This new technique is called PROPELLER EPI 28 and can be further enhanced by parallel imaging. 29 Alternatively, parallel imaging can be directly incorporated into multishot EPI for improved-quality DWI without loss of SNR efficiency compared with SS-EPI; for this purpose, GRAPPA was found to produce fewer off-resonance and motion artifacts than SENSE.…”

Development and initial evaluation of 7-T q-ball imaging of the human brain

“…Recently developed methods that are particularly promising for high field DWI include PROPELLER-EPI [39] and single-shot STEAM [40], the latter of which has recently been applied to 7T DTI of the mouse brain in vivo [41]. As with ssSE-EPI, some of the drawbacks of these alternative sequences at high field might also be solved by highly parallel imaging, as has been shown for PROPELLER-EPI [42].…”