Introduction to Risks From MRI During Pregnancy

Introduction to Risks From MRI During Pregnancy

The current guidelines of the FDA require labeling of the MRI devices to indicate that the safety of MRI with respect to the fetus "has not been established". Safety concerns arise with respect to both mother and fetus. Maternal safety concerns are the same as fora non-pregnant patient, and are addressed by pre-scan screening. Fetal concerns are twofold; first, the possibility of teratogenic effects, and second, the possibility of acoustic damage. In general, it should be noted that most studies evaluating MRI safety during pregnancy show no ill effects .Magnetic resonance (MR) is now emerging as an important complementary imaging modality in the field of fetal assessment and in the evaluation of maternal medical problems during pregnancy. Early fetal MR imaging, e.g. in the 1980s and early '90s, was limited by slow acquisition times, with fetal motion substantially degrading image quality and limiting diagnostic acumen. During the early 1990s, fetal MRI was revolutionized by the development of "ultrafast" imaging methods, effectively freezing physiologic movement and allowing quality imaging with no invasive interventions.

Key point: Most studies evaluating MRI safety during pregnancy show no ill effects.

INDICATIONS FOR AN MRI IN PREGNANCY

Major indications for fetal MRI include the confirmation of inconclusive sonographic findings and the evaluation of sonographically occult diagnoses in high-risk pregnancies.

The intrinsic safety of MRI and its ability to accurately show abdominal and pelvic disease in pregnant women make it highly useful in the evaluation of these patients. The following diseases have been correctly diagnosed using MRI in pregnancy: appendicitis, appendiceal abscess, pancreatitis, bilateral adrenal hemorrhage, pyelonephritis, hydronephrosis, intra-abdominal and rectus muscle abscess, intussusception, ulcerative colitis, Crohn's disease with diffuse peritoneal inflammation, uterine fibroid degeneration, degeneration and torsion of a submucosal uterine fibroid, simple ovarian cysts, and ovarian torsion.

Risk of Teratogensis From MRI During Pregnancy

A small number of studies have raised the possibility of teratogenic effects of MRI exposure in early pregnancy. A reduction in crown-rump length was seen in mice exposed to MRI in midgestation . Exposure to the electromagnetic fields simulating a clinical study caused eye malformations in a genetically predisposed mouse strain [2]. Several hours of exposure of chick embryos in the first 48 hours of life to a strong static magnetic field and rapid electromagnetic gradient fluctuations resulted in an excess number of dead or abnormal chick embryos, when examined at day 5 . Possible mechanisms for apparent deleterious effects include the heating effect of MR gradient changes, and direct non-thermal interaction of the electromagnetic field with biological structures. Tissue heating is greatest at the maternal body surface, and approaches negligible levels near the body center [4], making it unlikely that thermal damage to the fetus is a serious risk. A possible criticism of many of these studies is that they are not applicable to humans. However, they provide sufficient cause for concern such that a cautionary approach should be taken regarding fetal MRI in the first trimester. Accordingly, the guidelines of the National Radiological Protection Board in the United Kingdom is that "it might be prudent to exclude pregnant women during the first three months of pregnancy"]. An additional concern in the first trimester is the underlying relatively high rate of spontaneous abortion in this period. An MRI study could be coincidentally followed by a spontaneous abortion, but might give rise to parental concerns regarding causal effect. From a practical viewpoint, first trimester MRI will usually be performed for maternal rather than fetal indications, and in this context MRI is still preferable to any imaging study involving ionizing radiation .

Key point: It is good practice to avoid MRI during pregnancy, particularly for elective studies or during the first trimester, but MRI remains preferable to any studies using ionizing radiation.

Risk of Acoustic Damage From MRI During Pregnancy

A less obvious concern is the potential risk of acoustic damage to the fetus, due to the loud tapping noises generated by the coils of the MR scanner as they are subjected to rapidly oscillating electromagnetic currents, especially with EPI, which is the noisiest sequence in current clinical use. In a follow-up study of 18 patients who had undergone EPI as fetuses, 16 passed their 8 month hearing test, compared to 16.7 expected [1]. In a second study, a microphone was passed through the esophagus into the fluid filled stomach of a volunteer [2]. The aim was to simulate the acoustic environment of the gravid uterus. The sound intensity in the stomach was measured during MRI scanning across a range of radiofrequencies. The attenuation of the transmitted sound was greater than 30 dB, sufficient to reduce sound intensity from near the dangerous level of 120 dB to an acceptable level of under 90 dB. The results of these studies provide reassuring clinical and experimental evidence that there is no significant risk of acoustic injury to the fetus during prenatal MRI.

Key point: Acoustic damage from MRI during pregnancy appears to be a theoretical rather than a real concern.

RISK OF TERATOGENESIS FROM GADOLINIUM

Intravenous gadolinium is teratogenic in animal studies, albeit at high and repeated doses . While teratogenic effects have not been observed in a small number of human studies where gadolinium has been given in pregnancy [2, 3], it is clear that gadolinium should not be administered in pregnancy unless there is an absolutely essential clinical indication, particularly during the period of organogenesis. Administration of gadolinium later in pregnancy may be reasonable, although such indication would likely be for a maternal or obstetric indication rather than for evaluation of a fetal abnormality. Examples might include gadolinium enhanced imaging for a maternal brain tumor or suspected placenta accreta. Gadolinium crosses the placenta where it is presumably excreted by the fetal kidneys into the amniotic fluid. In the era of gadolinium-induced nephrogenic systemic fibrosis, this raises theoretical concerns of toxicity related to disassociation and persistence of free gadolinium. Such concerns reinforce the regulatory advice on gadolinium use in pregnancy. The 2007 ACR guidance document for safe MRI practices recommends that intravenous gadolinium should be avoided during pregnancy and should only be used if absolutely essential; furthermore, the risks and benefits of gadolinium use must be discussed with the pregnant patient and referring clinician [4]. Gadolinium is classified as a category C drug by the Food and Drug Administration and can be used if considered critical (only to be administered “if the potential benefit justifies the potential risk to the fetus”).

Key point: Intravenous gadolinium is contra-indicated in pregnancy, and should only be used if absolutely essential, and only after discussion of risks and benefits with the patient and referring clinician

HAZARDS 

There are no known biological risks associated with MR imaging in adults, but the effects of fetal exposure can't be known with certainty. According to the Safety Committee of the Society for Magnetic Resonance Imaging, MR procedures are indicated for use in pregnant women when other non-ionizing diagnostic imaging methods, such as US, are inadequate. They also recommend MRI in pregnancy when the examination will provide important information that would otherwise require exposure to ionizing radiation, such as CT. Pregnant patients need to be informed, however, that the safety of MRI during pregnancy hasn't been definitively proven, although to date there's no indication that it produces any deleterious effects.

Also, it's well known that during the first trimester the developing embryo is susceptible to injury from a variety of physical agents. Because we only have limited data, MRI in the first trimester of pregnancy should be avoided whenever possible. In other words, MR is considered acceptable after 16-18 weeks estimated gestational age. Another concern is the potential effect on a child's hearing, as the scanners produce considerable ambient noise of 80-100 dB. Initial studies suggest that this isn't a major problem, but additional large, multi-centre long-term studies are required.

USE OF CONTRAST MEDIA DURING LACTATION

The traditional and standard recommendation is that lactating women who receive intravascular iodinated contrast or gadolinium should discontinue breast-feeding for 24 hours, and the expressed milk during this period should be discarded [1]. The rationale for this recommendation appears weak, for several reasons:

• Only tiny amounts of iodinated or gadolinium-based contrast medium given to a lactating mother reach the milk. For example, a recent study of 20 lactating women found that less than 0.04% of the maternal dose of intravenous gadolinium passes into the breast milk [2].
• Only a tiny fraction of iodinated contrast or gadolinium entering the infant gut is absorbed. For example, only 1-2% of oral iodinated contrast is absorbed into the bloodstream [3].

Given these considerations, and in accordance with the results of a comprehensive review by the European Society of Urogenital Radiology, the very small potential risk associated with absorption of contrast medium may be insufficient to warrant stopping breast-feeding for 24 hours following either iodinated or gadolinium contrast agents [4]. A recent review in the New England Journal of Medicine also concluded that iodinated contrast administered to breast-feeding women posed no risk to the infant [5].

Key point: Lactating women who receive iodinated contrast or gadolinium can continue breast feeding without interruption.

CAN INTRAVENOUS CONTRAST BE GIVEN?

Gadolinium-based intravenous contrast material isn't recommended for use in pregnant patients. It's been shown to cross the placenta and appear within the fetal bladder moments after intravenous administration. Potential deleterious effects in the fetus are unknown.

THE BOTTOM LINE

MR imaging as an adjunct to prenatal US may provide valuable fetal information that could add to the prenatal evaluation and treatment of some fetal anomalies, particularly those involving the central nervous system. MRI can also be used to diagnose acute abdominal and pelvic conditions affecting the pregnant mother. At present, fetal MR imaging should be limited to fetuses with suspected anomalies -- or high-risk cases -- for additional evaluation of structures that are critical, but sub-optimally visualized with US.

With ongoing improvements in technology, MRI will continue to be a rapidly growing field in future years. Pregnant women in the second and third trimesters can be reassured that MRI poses no known risk to the fetus. Although safety has not been positively established, any hazard appears negligible and is outweighed by the potential diagnostic benefit. A more cautious approach should be taken when MRI is required during the first trimester, and you should discuss individual cases with your local radiologist.

Although most clinicians consider ultrasound to be the diagnostic procedure of choice during pregnancy, the authors of this report note that magnetic resonance (MR) techniques have revolutionized our ability to image the pregnant patient and the fetus. With the advent of ultrafast MRI scanning techniques, many of the previous problems related to slow and more cumbersome examinations have been eliminated, opening up a whole new diagnostic modality to the obstetrician.

Pelvic MR imaging during pregnancy is an excellent technique to evaluate maternal anatomy and abnormalities, such as adnexal masses, when ultrasound does not provide the needed answers. In many cases, MR scans provide images that are superior to CT scans. MR does not produce any ionizing radiation, and permits imaging in more than one plane. For more than 20 years, MR scans have been used to image the fetus, and the potential for this technique to evaluate for placenta previa and maternal and fetal anatomy has been previously described in numerous reports (Radiology 1985;154:427;AJR 1983;141:1119; Radiology 1985;157:715).

There are some limitations of MR imaging in pregnancy. Probably the major one is the long acquisition time, which may be difficult for pregnant patients to tolerate. In addition, there is a need for sedation to limit fetal motion if the unborn subject is to be evaluated. Fetal motion during the MRI scan has been one of the major problems with image quality in evaluating fetal abnormalities. With the development of fast MR imaging techniques, much of this difficulty has been overcome. This article provides a detailed discussion of some of the faster MR imaging techniques currently available.

There are no known biological risks to the mother or fetus associated with MR imaging. There have been no reported delayed adverse sequelae secondary to MR examinations, and the potential risks are considered extremely small or nonexistent. The women who work with MR scanners have not had an increase in adverse pregnancy outcomes. As of February 2002, the U.S. Food and Drug Administration has taken a cautious approach. The FDA suggests that although there have been no definitively proven adverse effects of MR imaging, theoretical teratogenic issues should be discussed with a pregnant woman prior to scanning. To be on the safe side, some conservative radiologists will avoid MR imaging in patients in the first trimester. If it is logistically feasible, most radiologists do require informed consent. Exactly what that informed consent must consist of is unclear and varies among institutions.

Gadolinium-based contrast material is generally avoided in pregnant women. Some contrast materials will cross the placenta, appear in the fetal bladder after IV administration, and be excreted into amniotic fluid where it may be subsequently swallowed. Therefore, contrast material is generally avoided in pregnancy.

Pelvimetry, an older method that utilized plain radiographs to examine anatomy prior to labor induction or breech presentations, largely has been abandoned. MR imaging offers extremely accurate measurements of the bone structures in the pelvis without any ionizing radiation, and the MRI has become the preferred method in some centers when pelvimetry is required. The numerous images and views that can be obtained with MRI provide an excellent evaluation of the birthing canal for the obstetrician.

MR imaging also has been used to address placenta abnormalities, although placenta previa is usually evaluated via ultrasound. Recently MR imaging has been used to evaluate hydronephrosis, a common physiologic change during pregnancy. MR scanning also has been used for evaluating renal colic, and kidney stones can be evaluated without the use of radiation or contrast material required for IVP. Other associated problems with pregnancy that can be diagnosed with MR imaging include abdominal pregnancy, pelvic vein thrombosis, and small bowl obstruction.

The MR scan also is quite useful in the evaluation of fetal morphology, although fetal motion has been problematic in the past. With the faster scans, the need to sedate the mother and fetus has diminished. Anatomical abnormalities are best depicted when the fetus is older than 20 weeks gestation. MR imaging is useful in evaluating ventricle size and the brain parenchyma, with clarity much superior to ultrasound. Other abnormalities that can be nicely evaluated with an MR scan include arachnoid cysts, neural tube defects, and posterior fossa abnormalities. Other non-CNS fetal anomalies that have been evaluated with MR imaging include diaphragmatic hernia, omphalocele, and polycystic kidneys.

The authors note that there are some contraindications to MR imaging during pregnancy. These are contraindications in a non-pregnant patient as well, and include ferromagentic material in the body ( such as an aneurysm clip), severe claustrophobia, and the inability for patients to lie on their backs, a particular problem during the third trimester. The authors believe that with the development of fast MR imaging techniques, the use of MRI during pregnancy will likely escalate. MRI is considered a technique with great potential and promise to benefit fetal and maternal care.

Comment: Magnetic resonance imaging provides excellent resolution of difficult-to-evaluate areas of the body, and produces images in the axial, saggital, and coronal planes. To date, there have been no harmful effects of MR during pregnancy or mutagenic effects with standard low dose (less than 2 tesla field strength) magnets. Higher strength magnets may be problematic, and are best avoided in early pregnancy, but most hospitals do not utilize such high-tech scanning yet. There are no issues of exposure to ionizing radiation with the MRI.

Most emergency physicians have never ordered an MRI on a pregnant woman, and we usually rely on ultrasonography to sort out obstetrical problems of concern in the ED. However, this paper is relatively cutting edge, and indicates that the MRI scan is safe, free of ionizing radiation, and provides excellent detail above and beyond information obtained with ultrasound. Although generally not considered, the MRI provides excellent evaluation of pelvic masses during pregnancy. The MRI has revolutionized our ability to image the pregnant patient and the fetus. This modality will likely become more commonplace in the near future. The emergency department will not likely have to use the MR on a pregnant woman, but if one is considering spinal cord compression, epidural abscess, or occult fractures, an MRI scan may be the ideal imaging choice.