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Definitions
Diffusion-weighted imaging: an MR sequence that produces images of water diffusion, which reflects hypoxic edema of the brain.
MR imaging: 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 void: the appearance of darkness on an image based on low MR signal that helps characterize tissues and fluid on certain sequences.
T2-weighted imaging: MR technique that defines fetal anatomy and dysmorphology with specific tissue characteristics based on a specific spin echo sequence using a long repetition time and long echo time.
T1-weighted imaging: MR technique that is less sensitive for anatomy but highlights certain characteristics of fat, certain stages of bleeding, liver, and meconium in the colon at later gestations based on a specific spin echo sequence using a short repetition time and short echo time.
Volumetry: measurement by volume calculated by areas multiplied by slice thickness.
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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. Newer vendor-specific protocols 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 hypothetically produces superior image resolution to ultrasonography, based on its underlying technology, and is not as hindered by bony interfaces, maternal obesity, oligohydramnios, or an engaged fetal head. Referral for 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 suspected pelvic masses, placental invasion, pelvimetry, pelvic floor, and cervix.
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A current commentary of 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 below.1
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A primary concern to referring physicians and their patients is the safety of both mother and fetus. MR utilizes no ionizing radiation, but questions arise regarding fluctuating electromagnetic fields, high sound intensity levels, and fetal heart rate patterns during the procedure.
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Several animal and tissue studies have been conducted to determine the biologic effects of electromagnetic fields with many conflicting reports. An early study looked at the long-term effects of repetitive exposure to a static 1.5-tesla (T) magnetic field on human lung fibroblasts.2 Study and control group proliferation was similar, indicating no adverse effect from repetitive MR exposure. Human studies are rare. A large epidemiologic retrospective study compared spontaneous abortion rates, infertility rates, incidence of low birth weight, and premature delivery ...