Fetal MRI: A technical update with educational aspirations

Abstract

Fetal magnetic resonance imaging (MRI) examinations have become well-established procedures at many institutions and can serve as useful adjuncts to ultrasound (US) exams when diagnostic doubts remain after US. Due to fetal motion, however, fetal MRI exams are challenging and require the MR scanner to be used in a somewhat different mode than that employed for more routine clinical studies. Herein we review the techniques most commonly used, and those that are available, for fetal MRI with an emphasis on the physics of the techniques and how to deploy them to improve success rates for fetal MRI exams. By far the most common technique employed is single-shot T2-weighted imaging due to its excellent tissue contrast and relative immunity to fetal motion. Despite the significant challenges involved, however, many of the other techniques commonly employed in conventional neuro- and body MRI such as T1 and T2*-weighted imaging, diffusion and perfusion weighted imaging, as well as spectroscopic methods remain of interest for fetal MR applications. An effort to understand the strengths and limitations of these basic methods within the context of fetal MRI is made in order to optimize their use and facilitate implementation of technical improvements for the further development of fetal MR imaging, both in acquisition and post-processing strategies.

Fetal lung apparent diffusion coefficient measurement using diffusion-weighted MRI at 3 Tesla: Correlation with gestational age

“…This reduces the signal-to-noise ratio, raising the need for imaging at lower spatial resolutions to boost signal, which is undesirable. With fetal MRI increasingly performed on 3T scanners (14, 15) using multi-channel body coils, there are gains in the signal-to-noise ratio. However, this results in an increased risk for the third challenge, which are increased artifacts at higher magnetic fields.…”

“…However, this results in an increased risk for the third challenge, which are increased artifacts at higher magnetic fields. Although signal level increases with field strength, MRI artifacts, such as main magnetic field inhomogeneity (B0) and RF field inhomogeneity (B1), also increase with field strength (14,15). These artifacts are especially critical in fetal MRI as the fields of view (FOV) tend to be larger than those used in many other studies.…”

Automated template-based brain localization and extraction for fetal brain MRI reconstruction

“…Successful brain localization, extraction, and reconstruction is highly dependent on good image acquisition practice and on the robustness of the automatic image processing algorithms [1]. Good image acquisition practice addresses the need for 1) appropriate MRI sequence parameters that allow motion-robust slice acquisitions at about few hundred milliseconds instant of k -space sampling for each slice and 2) multiple repeated stacks to provide the required redundancy for brain extraction refinement, motion estimation, and reconstruction.…”

“…Fast single shot multi-slice MRI sequences are used to freeze maternal and fetal motion; but the acquisition of thick slices (about 2 to 4 mm ) is necessary to obtain acceptable Signal-to-Noise Ratio (SNR) given the short acquisition time used to avoid motion at each slice acquisition. Thick slices and interslice motion artifacts limit the accuracy of volumetric analysis for clinical diagnosis and neuroscience studies [1]. …”

Fetal MRI at 3T—ready for routine use?

“…Currently, fetal MRI is performed mainly at 1.5 T, but, recently, a growing interest in 3.0 T examinations has emerged, not only for university and research use. [2][3][4] The wish to perform fetal MR at 3.0 T is due to the increased signal-to-noise ratio (SNR) and probable decreased acquisition time, increased spatial resolution and ultimately a more precise fetal depiction. 5,6 Another point is that some institutions may have access only to a 3.0 T scanner.…”