Increased speed and image quality in single‐shot fast spin echo imaging via variable refocusing flip angles

Abstract

Purpose To develop and validate clinically a single shot fast spin echo (SSFSE) sequence utilizing variable flip angle refocusing pulses to shorten acquisition times via reductions in specific absorption rate (SAR) and improve image quality Materials and Methods A variable refocusing flip angle SSFSE sequence (vrfSSFSE) was designed and implemented, with simulations and volunteer scans performed to determine suitable flip angle modulation parameters. With IRB approval/informed consent, patients referred for 3T abdominal MRI were scanned with conventional SSFSE and either half-Fourier (n=25) or full-Fourier vrfSSFSE (n=50). Two blinded radiologists semi-quantitatively scored images on a scale from −2 to 2 for contrast, noise, sharpness, artifacts, cardiac-motion related signal loss, and the ability to evaluate the pancreas and kidneys. Results vrfSSFSE demonstrated significantly increased speed (~2-fold, p<0.0001). Significant improvements in image quality parameters with full-Fourier vrfSSFSE included increased contrast, sharpness, and visualization of pancreatic and renal structures with higher bandwidth technique (mean scores 0.37, 0.83, 0.62, and 0.31, respectively, p≤0.001), and decreased image noise and improved visualization of renal structures when used with equal bandwidth technique (mean scores 0.96 and 0.35, respectively, p<0.001). Increased cardiac-motion related signal loss with full-Fourier vrfSSFSE was seen in the pancreas but not the kidney. Conclusion vrfSSFSE increases speed at 3T over conventional SSFSE via reduced SAR, and when combined with full-Fourier acquisition can improve image quality although with some increased sensitivity to cardiac-motion related signal loss.

T2 shuffling: Sharp, multicontrast, volumetric fast spin‐echo imaging

“…The signal decay that leads to blurring is partially mitigated by reducing ETLs and interleaving slices in multi-shot 2D FSE (1). Variable refocusing flip angles have also been used to reduce blurring in single-shot 2D FSE (59). We plan to explore the application of T 2 Shuffling to 2D FSE to further reduce blurring due to long ETLs as well as resolve multiple image contrasts.…”

Increased Speed and Image Quality for Pelvic Single-Shot Fast Spin-Echo Imaging with Variable Refocusing Flip Angles and Full-Fourier Acquisition

“…The vrfSSFSE pulse sequence was implemented as described previously (13). In brief, the refocusing flip angle approach developed for three-dimensional FSE imaging described by Busse et al (11) was adapted to the case of two-dimensional SSFSE imaging.…”

“…Of the refocusing flip angle parameters (a init , a min , a cent , and a last ), image quality is known to be most dependent on a min ; in addition to reducing SAR, lowering a min leads to relative prolongation of T2 decay, and the resultant stabilization of signal over the echo train should reduce blurring (13). However, lower flip angles lead to a greater dependence on longer refocusing pathways in the echo train, creating longer time periods over which phase shifts from motion can accumulate and result in signal loss (18).…”

“…However, lower flip angles lead to a greater dependence on longer refocusing pathways in the echo train, creating longer time periods over which phase shifts from motion can accumulate and result in signal loss (18). This increased sensitivity to motion-related signal loss has previously been demonstrated to be of potentially limiting effect in imaging structures in close proximity to the heart, such as the left lobe of the liver when a min is set to 90° (19) and the pancreas when a min is set to 60° (13). In this study, no increased propensity for imaging artifacts was seen for vrf SSFSE imaging with a min set to 60°, despite the free-breathing nature of the imaging acquisition, which indicates that with the refocusing flip angle parameters chosen, sufficient motion robustness is maintained for imaging structures in the pelvis.…”

Slice profile effects on nCPMG SS‐FSE

“…Both minimum echo time (TE ¼ 30.4 ms) as well as repetition time (TR ¼ 800 ms) were used in a multi-slice sequence with slice interleaving using a skip factor of 4 (i.e., in a 12 slice sequence, slice were acquired with the order of [1,5,9,2,6,10,3,7,11,4,8,12]). Images were acquired by doubling the field-of-view to better visualize the intermediate aliasing caused by any instability in the nCPMG echotrain that arises for low refocusing flip angles.…”