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

To develop a center-out echo planar imaging (COEPI) acquisition technique to increase SNR through minimizing the TE.

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.

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/mm² ) 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/mm² ), compared to DWI-EPI.

The feasibility of COEPI and its SNR benefit were demonstrated in this study.