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Key Terms
Diffusion-weighted imaging (DWI): an MR sequence that has increased signal when Brownian motion of water molecules is restricted, such as highly cellular tumors, fibrosis, hypoxic edema of the brain, and abscess with viscous content.
Fat suppression: various techniques used to make fat dark (low signal intensity) on MR images in order to highlight other tissues and, depending on the technique used, confirm the presence of fat within a structure.
Magnetic resonance (MR) imaging: a modality that uses high-field-strength magnets, radio frequencies, and a computerized analysis to produce detailed in vivo images of internal anatomy of the body.
Orthogonal views: MR views at right angles to each other, typically axial, coronal, and sagittal views of the fetus.
Signal-to-noise ratio (SNR): the amount of desired signal or useful information divided by the background level of false data or noise. High SNR results in a clear image.
Steady-state MR imaging: an MR technique that is rapid with excellent SNR and relatively motion insensitive based on a fast gradient echo sequence.
T1-weighted imaging: MR technique that is less sensitive for fetal anatomy but highlights characteristics of fat, certain stages of bleeding, liver parenchyma, and meconium in the colon at later gestations based on sequences using a short repetition time and short echo time. Simple fluid is dark (low signal intensity) on a T1-weighted image.
T2-weighted imaging: MR technique that defines fetal anatomy and dysmorphology with specific tissue characteristics based on a spin echo sequence using a long repetition time and long echo time. Simple fluid is white (high signal intensity) on a T2-weighted image.
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When first used in the mid-1980s, magnetic resonance (MR) image acquisition was slow, necessitating maternal sedation and even temporary fetal paralysis with intramuscular injection to limit motion artifact. The last few decades have brought many technological advances allowing very fast MR acquisitions. Vendor-specific protocols are widely available and have been used to effectively image the fetus. Images are acquired in 1 second or less, eliminating the need for sedation. Motion artifact, although still present, is significantly reduced. MR has better soft tissue contrast than ultrasound, can image a larger field of view allowing a global perspective of the fetus, and is not hindered by bony interfaces, maternal obesity, oligohydramnios, or an engaged fetal head. Referral for fetal MR evaluation is most frequently encountered in the setting of suspected fetal central nervous system (CNS), thoracic, genitourinary, or gastrointestinal disorders. Maternal indications include evaluation of abdominal and pelvic pain, suspected pelvic masses, or placental invasion.
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A commentary by the American College of Obstetrics and Gynecology was published summarizing the National Institutes of Health Workshop on fetal imaging technology, including present and future fetal MR applications, which are discussed in the following section.1
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A primary concern of referring physicians and their patients is the safety of both mother and ...