Magnetic resonance technology does not use ionizing radiation, and its application is cited throughout this book. Advantages include high soft-tissue contrast, ability to characterize tissue, Of all of the major advances in obstetrics, the development of and acquisition of images in any plane-particularly axial, sagsonography for study of the fetus and mother certainly is one of ittal, and coronal. An entire section in Chapter 10 (p. 215) is the greater achievements. The technique has become virtually devoted to mechanisms that generate MR images.
The most recent update of the expert panel on MR safety of the American College of Radiology was summarized by Kanal and colleagues (20l3). The panel concluded that no harmful human efects are reported from MR imaging. Similar conclusions were reached by the Canadian Task Force on Preventive Health Care (Patenaude, 2014).
Early studies found diferences in blastocyst formation of early murine embryos exposed to MR imaging with 1.5 T (T = tesla) (Chew, 2001). When operated within standardized limits, maternal and fetal imaging can be safely performed at clinical magnet strengths-3 T and below. MR imaging can be used, regardless of trimester: (1) if the information cannot be obtained with another nonionizing modality, namely sonography, (2) if the results of the study will guide maternal or fetal management during pregnancy, and (3) if the imaging cannot be delayed until the woman is no longer pregnant. he decision to use a magnetic field strengthr> 1.5 T may be made for specific maternal indications. Early work also suggests imaging at 3 T can improve fetal assessment (Victoria, 2016). A magnetic ield strength up to 4 T appears safe in animals (Magin, 2000). Vadeyar and associates (2000) reported no demonstrable fetal heart rate pattern changes during MR imaging of gravidas. Studies evaluating children exposed in utero have shown no deleterious efects (Clements, 2000; Kok, 2004; Reeves, 2010).
Contraindications to MR imaging include internal cardiac pacemakers, neurostimulators, implanted defibrillators and infusion pumps, cochlear implants, shrapnel or other metal in biologically sensitive areas, some intracranial aneurysm clips, and any metallic foreign body in the eye. Of more than 51,000 nonpregnant patients scheduled for MR imaging, Dewey and colleagues (2007) found that only 0.4 percent had an absolute contraindication to the procedure.
Elemental gadolinium cheates are used to create paramagnetic contrast. hese cross the placenta and are found in the fetus, placenta, and nnionic fluid (Oh, 2015). In doses approximately 10 times the human
FIGURE 46-5 Multipara at 29 weeks' gestation with suspected appendicitis. A. Coronal T2-weighted magnetic resonance image demonstrates severe hydronephrosis (arrow) and perinephric fluid (white arrowhead) suggesting caliceal rupture. The normal appendix is seen laterally (black arrowhead). B. Coronal computed tomography (CT) better defines the source of obstruction as an 8-mm radiopaque stone at the ureteropelvic junction (blue arrow) and distal to the hydronephrosis (white arrow). C and D. In the same woman, more anterior coronal and sagittal CT images show the expected displacement of a noninflamed appendix yellow arrow) into the upper abdomen with advancing gestation.
FIGURE 46-6 Nullipara at 27 weeks' gestation. A. Axial T2-weighted MR image demonstrates mild fetal unilateral ventriculomegaly involving the left lateral ventricle (arrow).
B. Sagittal T2-weighted MR image demonstrates normal development of the corpus callosum (arrowheads) and vermis (arrow).
TABLE 6-9. Guidelines for Diagnostic Imaging During Pregnancy and Lactation