Magnetic Resonance Imaging
The principal advantage of MRI over ultrasonography and computed tomography is the ability to image deep soft tissue structures in a manner that is not operator dependent and does not use ionizing radiation. There are no precautions or contraindications specific to the pregnant woman. Magnetic resonance imaging is similar to ultrasonography in the diagnosis of appendicitis, but when MRI is readily available, it is preferred because of its lower rates of nonvisualization 6. Although there are theoretical concerns for the fetus, including teratogenesis, tissue heating, and acoustic damage, there exists no evidence of actual harm. With regard to teratogenesis, there are no published human studies documenting harm, and the preponderance of animal studies do not demonstrate risk 1. Tissue heating is proportional to the tissue’s proximity to the scanner and, therefore, is negligible near the uterus 1 7. Finally, available studies in humans have documented no acoustic injuries to fetuses during prenatal MRI 1. In considering available data and risk of teratogenicity, the American College of Radiology concludes that no special consideration is recommended for the first (versus any other) trimester in pregnancy 8.
Unlike CT, MRI adequately images most soft tissue structures without the use of contrast. However, there are diagnostic situations in which contrast enhancement is of benefit. Two types of MRI contrast are available: 1) gadolinium-based agents and 2) superparamagnetic iron oxide particles. Gadolinium-based agents are useful in imaging of the nervous system because they cross the blood-brain barrier when this barrier has been disrupted, such as in the presence of a tumor, abscess, or demyelination 9. Although gadolinium-based contrast can help define tissue margins and invasion in the setting of placental implantation abnormalities, noncontrast MRI still can provide useful diagnostic information regarding placental implantation and is sufficient in most cases 7.
Bạn đang xem: Guidelines for Diagnostic Imaging During Pregnancy and Lactation
Even though it can increase the specificity of MRI, the use of gadolinium-based contrast enhancement during pregnancy is controversial. Uncertainty surrounds the risk of possible fetal effects because gadolinium is water soluble and can cross the placenta into the fetal circulation and amniotic fluid. Free gadolinium is toxic and, therefore, is only administered in a chelated (bound) form. In animal studies, gadolinium agents have been found to be teratogenic at high and repeated doses 1, presumably because this allows for gadolinium to dissociate from the chelation agent. In humans, the principal concern with gadolinium-based agents is that the duration of fetal exposure is not known because the contrast present in the amniotic fluid is swallowed by the fetus and reenters the fetal circulation. The longer gadolinium-based products remain in the amniotic fluid, the greater the potential for dissociation from the chelate and, thus, the risk of causing harm to the fetus 8. The only prospective study evaluating the effect of antepartum gadolinium administration reported no adverse perinatal or neonatal outcomes among 26 pregnant women who received gadolinium in the first trimester 10. More recently, a large retrospective study evaluated the long-term safety after exposure to MRI in the first trimester of pregnancy or to gadolinium at any time during pregnancy 11. This study interrogated a universal health care data-base in the province of Ontario, Canada to identify all births of more than 20 weeks of gestation, from 2003 to 2015. Comparing first-trimester MRI (n=1,737) to no MRI (n=1,418,451), there were 19 stillbirths or deaths versus 9,844 in the unexposed cohort (adjusted relative risk [RR], 1.68; 95% CI, 0.97-2.90). The risk also was not significantly higher for congenital anomalies, neoplasm, or vision or hearing loss. However, comparing gadolinium MRI (n=397) with no MRI (n=1,418,451), the outcome of any rheumatologic, inflammatory, or infiltrative skin condition occurred in 123 versus 384,180 births (adjusted hazard ratio, 1.36; 95% CI, 1.09-1.69). Stillbirths and neonatal deaths also occurred more frequently among 7 gadolinium MRI-exposed versus 9,844 MRI unexposed pregnancies (adjusted RR, 3.70; 95% CI, 1.55-8.85). Limitations of the study assessing the effect of gadolinium during pregnancy include using a control group who did not undergo MRI (rather than patients who underwent MRI without gadolinium) and the rarity of detecting rheumatologic, inflammatory, or infiltrative skin conditions 12. Given these findings, as well as ongoing theoretical concerns and animal data, gadolinium use should be limited to situations in which the benefits clearly outweigh the possible risks 8 12.
Xem thêm : Moscas volantes
To date, there have been no animal or human fetal studies to evaluate the safety of superparamagnetic iron oxide contrast, and there is no information on its use during pregnancy or lactation. Therefore, if contrast is to be used, gadolinium is recommended.
The water solubility of gadolinium-based agents limits their excretion into breast milk. Less than 0.04% of an intravascular dose of gadolinium contrast is excreted into the breast milk within the first 24 hours. Of this amount, the infant will absorb less than 1% from his or her gastrointestinal tract. Although theoretically any unchelated gadolinium excreted into breast milk could reach the infant, there have been no reports of harm. Therefore, breastfeeding should not be interrupted after gadolinium administration 13 14.
Nguồn: https://vuihoctienghan.edu.vn
Danh mục: Info