RARE two‐point Dixon with dual bandwidths

Abstract

Purpose To investigate the impact of dual readout bandwidths (dBW) in a dual echo fat/water acquisition and describe a dBW‐rapid acquisition relaxation enhanced, or turbo spin echo sequence where the concept is used to improve SNR by removing dead times between refocusing pulses and avoiding redundant Chemical‐shift encoded. Methods Cramér‐Rao bounds and Monte Carlo simulations were used to investigate a two‐point fat/water model where the difference in bandwidths is incorporated. In vivo images were acquired at 1.5 and 3 T with the dBW‐rapid acquisition relaxation enhanced, or turbo spin echo sequence. Typical bandwidth ratios were 1:2. SNR was compared with a single bandwidth sequence under identical scan parameters at 3T. Results Monte Carlo simulations and Cramér‐Rao analysis demonstrate that number of signal averages can be improved with dual bandwidths compared to conventional single bandwidth acquisitions. The dBW‐rapid acquisition relaxation enhanced, or turbo spin echo sequence can acquire images with high readout resolutions with well‐conditioned sampling. An SNR improvement of 52% was measured, in line with the theoretical gain of 54%. Conclusions The proposed dBW‐rapid acquisition relaxation enhanced, or turbo spin echo sequence is a highly SNR‐efficient two‐point rapid acquisition relaxation enhanced, or turbo spin echo sequence without dead times, and can acquire images at higher resolutions than current vendor‐supplied alternatives.

Chemical shift encoding using asymmetric readout waveforms

“…Even with a high‐performance gradient system, readout resolutions higher than about 1 mm are not achievable with conventional fTED at 3T. By only encoding two echoes with different bandwidths and partial Fourier sampling, we recently reported readout resolutions of 0.5 mm at 3T 10 …”

“…By only encoding two echoes with different bandwidths and partial Fourier sampling, we recently reported readout resolutions of 0.5 mm at 3T. 10 Multi-TR acquisitions, on the other hand, do not have a resolution restriction as only one echo is acquired during T tot , allowing much larger k-space coverage. The downside of multi-TR is the increased scan time, and dead times arising from the shifts, resulting in a more than twofold increase in scan time compared to single-TR acquisitions.…”

“…By design, the density with the quadratic and spline waveform hits this line at the k-space center T 2 -weighted images of the optic nerve were also acquired. SNR maps were generated as described previously, 10 with regions of interests placed to avoid bias from flow artifacts. The apparent SNR was measured by acquiring each phase encoding line twice, thus doubling the scan time.…”

“…6 Theoretically, it has been shown that, with optimal sampling, the scan time penalty can be completely counterbalanced by the signal contribution from additional echoes. [7][8][9][10] The theoretical SNR efficiency of 100% is appealing, but hard to achieve in practice due to requirements specific to the pulse sequence, such as CPMG conditions. This defuses the major benefit of all acquired data contributing to the final image.…”

“…Recently, we described a two-point single-TR RARE sequence where readout asymmetry is achieved by employing two different readout amplitudes (dual bandwidths). 10 While an in-plane resolution of 0.5 mm was achieved at 3T, with a high-performance gradient system, resolutions beyond that are only feasible with a multi-TR acquisition. In this work, we propose using asymmetric readout waveforms to achieve efficient sampling with chemical shift encoding without dead time or redundancy.…”