Rapid Fetal HASTE MR Imaging

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Project Summary Single-shot T2-weighted (SST2w) imaging is the most common acquisition sequence in fetal MRI. Its popularity stems from its ability to supply excellent T2 contrast, and encode information from each imaging slice in <2 seconds. Despite these advantages, the unmet need for technical improvements renders SST2w highly under- developed, and constrains the acquisition to run at a fraction of its potential speed. Such inefficient acquisition makes SST2w vulnerable to fetal motion and necessitates a large number of repeat scans, during which pregnant women have to tolerate being inside a confined environment and remain motionless in uncomfortable positions. These inefficient acquisitions fail to provide the desired image quality and suffer from severe resolution loss during the long readout window, which incurs strong signal decay and requires aggressive partial Fourier sampling. We propose optimized acquisition and reconstruction technology to speed up SST2w acquisition by 5-fold, while providing sharper images with reduced voxel blurring. To make these possible, we will begin by tailoring our radiofrequency (RF) refocusing pulses to reduce their energy deposition dramatically. This is important because SST2w acquisition plays a large number of refocusing pulses in rapid succession, which requires a long “cool-off” period to keep RF heating within safety limits. This dead time prolongs the acquisition time of standard SST2w by 3–4-fold. We will replace this train of constant flip angle refocusing pulses with a variable refocusing flip (vrf) angle scheme, which will reduce the energy deposition by 3-fold while matching the contrast and SNR of the standard acquisition. Not only this will allow us to decrease the unused sequence time to improve scan efficiency, but also reduce the energy deposition to low enough levels to permit additional simultaneous multislice (SMS) encoding. vrf will also allow for prolonging the echo time to reduce the partial Fourier factor, thereby decreasing voxel blurring by ~35%. Using low-energy multiPINS pulse design for SMS refocusing will let us incorporate 3-fold SMS acceleration. Combination of vrf and multiPINS refocusing will thus provide a 5-fold net speed up over standard SST2w, while remaining within RF heating safety levels. We will use our wave-CAIPI extreme controlled aliasing k-space trajectory to ensure high image quality when 3-fold SMS acceleration is introduced. Augmenting wave encoded image reconstruction with LORAKS phase- constraint will better compensate partial Fourier sampling and further reduce blurring. Finally, we will evaluate the ability of the developed acquisition and reconstruction technologies to improve the acquisition speed and apparent resolution of fetal SST2w imaging while closely matching the contrast and SNR of the standard acquisition by scanning 10 pregnant women at 3T.