Center‐out EPI (COEPI): A fast single‐shot imaging technique with a short TE

Abstract

Purpose:To develop a center-out echo planar imaging (COEPI) acquisition technique to increase SNR through minimizing the TE. Methods: In single-shot COEPI, the phase-encoding starts from the center (k y = 0) toward both sides of k-space to substantially shorten the TE compared to the conventional single-shot EPI. The phase-encoding gradient waveform is partially overlapped with the frequency-encoding gradient waveform to keep the echo spacing constant during the echo train readout. A reconstruction pipeline was developed to correct for phase and off-resonance errors in COEPI. Gradient-recalled echo (GRE), spin echo (SE), and DWI COEPI were obtained in phantoms and healthy brains at 1.5 tesla (T) and 3.0T. The SNR in COEPI and single-shot partial k y EPI was compared. Results: Acquisition matrix of 128 × 80 (16 overscan lines) was obtained in both COEPI and EPI. At 1.5T/3.0T, a minimum TE of 3 ms/4 ms in GRE-COEPI, 11 ms/12 ms in SE-COEPI, and 40 ms in DWI-COEPI (3.0T only, maximum b value = 2000 s/mm 2 ) was achieved, compared to a minimum TE of 18 ms/16 ms in GRE-EPI, 37 ms/34 ms in SE-EPI, and 66 ms in DWI-EPI, respectively. Image blurring and Nyquist ghost appear in COEPI and were substantially reduced after corrections. At 1.5T/3.0T, a SNR increase of 27.7% ± 6.9%/20.7% ± 7.0% in GRE-COEPI and 37.7% ± 5.7%/28.2% ± 1.3% in SE-COEPI was observed in white matter of human brains, compared to GRE-EPI and SE-EPI, respectively. At 3.0T, a SNR increase of 41.2% ± 4.1% in DWI-COEPI was observed in white matter of 5 subjects at 5 b values (0~2000 s/mm 2 ), compared to DWI-EPI. Conclusion: The feasibility of COEPI and its SNR benefit were demonstrated in this study. K E Y W O R D Scenter-out, diffusion imaging, echo planar imaging, SNR, T 2 effect 788 | CHEN Et al.

Single‐shot pseudo‐centric EPI for magnetization‐prepared imaging

“…Recently, another single-shot center-out EPI was proposed. 19 In center-out EPI, the middle part of the k-space was filled in a centric order and only one peripheral region of the k-space was encoded linearly, and then the data were reconstructed using partial Fourier algorithm. To minimize the increase in readout duration, the PE jump gradients were overlapped with readout gradients, necessitating regridding of the acquire data into Cartesian coordinate.…”

“…This is the first study that enables the centric PE ordering in the single‐shot EPI in Cartesian coordinate. Recently, another single‐shot center‐out EPI was proposed 19 . In center‐out EPI, the middle part of the k‐space was filled in a centric order and only one peripheral region of the k‐space was encoded linearly, and then the data were reconstructed using partial Fourier algorithm.…”

Quantitative diffusion MRI of the abdomen and pelvis

“…A variety of techniques enable improved SNR in diffusion MRI of the abdomen and pelvis. These developments include novel receive coil arrays, or specialized coils (e.g., endorectal coils, or wearable coils 104 for pelvic MRI), improved gradient systems that enable shorter echo times, novel pulse sequences, 105 as well as postprocessing‐based denoising methods 106–109 …”

SNR-efficient distortion-free diffusion relaxometry imaging using ACcelerated Echo-train shifted EPTI (ACE-EPTI)

“…At high resolution, the benefit of TE reduction using echo-train shifting over the conventional acquisition is more prominent, since the long echo train before the spin-echo with large echo spacing will lead to long TE in ss-EPI. The benefit of minimizing the TE has also been studied in many previous works, such as center-out spiral (49,50), and center-out EPI (51). One challenge of using a center-out trajectory in conventional single-shot acquisition is that the fast T 2 * decay would lead to more image blurring that needs to be corrected.…”