Fetal MRI has transformed prenatal patient care, allowing for earlier and better detection of complex congenital anomalies. However, fetal MRI’s diagnostic utility is often limited by technical barriers that introduce artifacts and reduce image quality. The main determinants of fetal MRI image quality are speed of acquisition, spatial resolution and SNR. Optimization of imaging is a challenge because improvements in one aspect generally leads to a tradeoff in the others. Moreover, the recent introduction of 3T fetal MRI to achieve better SNR adds to the complexity. Motion, banding artifacts, and aliasing artifacts impact the quality of fetal acquisitions at any field strength. High SAR and artifacts from inhomogeneities in the RF field are important limitations of high field strength imaging. Fetal and maternal motion create the need for rapid image acquisition. This generally limits imaging to SST2w, bSSFP, T1w spoiled-GRE, and EPI sequences. Acquisitions can be degraded by low SNR, resulting in grainy images. This can be improved by increasing the number of coil elements or increasing the field strength. Blurry images can be a result of low resolution and can be improved by decreasing the FOV (at expense of SNR) or increasing the field strength. Most strategies to improve quality will also increase SAR, which can be controlled by decreasing the number of slices, lowering the excitation or refocusing flip angles, increasing the TR or lowering the field strength. Fast sequences can prevent some motion artifacts, other strategies include decreasing the number of slices, swapping the PE and FE directions, maternal breath hold or oral sedatives, and image reconstruction by motion correction algorithms. Aliasing artifacts can be effectively addressed by phase-oversampling (at expense of SAR in SST2w sequences) or rotating the long axis of the PE direction. Banding artifacts of bSSFP sequences can be minimized by decreasing TR (at expense of SAR) or lowering field strength. High field strength imaging can be severely limited by shading secondary to RF field inhomogeneities. Multichannel field transmit and complex field strength shaping show promising results to address this issue. In this educational exhibit we will use visual examples to discuss: 1) fetal MRI quality optimization tradeoffs, 2) technical barriers and artifacts that impact image quality, and 3) solutions to improve quality and reduce artifacts.
SPR 2020 Annual Meeting & Postgraduate Course
Kirsch John E,