Fetal MRI - General Information
INDICATIONS AND LIMITATIONS
Fetal MR is indicated when:
- An abnormality on ultrasound is not clearly defined and more information is sought in order to make a decision about therapy, delivery, or to advise a family about prognosis. Example indications include a potential anomaly in the setting of maternal obesity, oligohydramnios, or advanced gestational age.
- An abnormality is identifed on ultrasonography and the treating physician desires MR-specific information in order to make decisions about care. An example might include the calculation of MR-derived fetal lung volumes in cases of congenital diaphragmatic hernia.
- A fetus is significantly at risk for abnormality that will affect prognosis even if no finding is discovered with ultrasound, eg neurologic ischemia after laser ablation of placental anastomoses in Twin-Twin Transfusion Syndrome.
Limitations to fetal MRI include
- Gestational age < 17 weeks
- Maternal weight exceeding table recommendations (per manufacturer)
- Claustrophobia. Many women who are concerned about claustrophobia can be coaxed through an examination with patience, the consolation of a family member of friend holding their hand in the MR unit, supplemental oxygen, repositioning into decubitus position, and/or the use of diversions including eye coverings/DVD viewers.
- Assessment of many (but not all) fetal skeletal abnormalities
REQUIREMENTS AND NEAR-REQUIREMENTS
- gestational age: The ACR recommends that fetal MR be deferred until after 20 weeks. This is in part because the fetus is so small at earlier gestational ages that MR is unlikely to add much additional information to ultrasound, but it is also because the effects of energy deposition on developing structures are not completely understood. There is concern for the developing auditory apparatus in particular.
- MR environment: A 1.5 T system is standard. Some centers are experimenting with 3T units, but one would have to be reassured that energy deposition translated as heating (SAR) remains within recommended levels. Coil selection depends on the size of the patient: the higher resolutions achieved with surface coils (such as a cardiac coil) are preferred, but larger multichannel torso coils are used successfully, and sometimes no surface coil can be used because there is no room between the patient and the magnet bore. An abdominal circumference greater than 130 cm has been suggested as an indication for a larger surface coil (ref: Prayer). Surface coils of course can be repositioned during the exam to better cover the fetal part being imaged. The patient is best imaged supine, or in left lateral decubitus postion to avoid compression of the inferior vena cava.
- Ultrasound : Some centers always perform a complete fetal ultrasound prior to MRI. The advantages are that a demise is detected before MR is inititated; fetal lie is known so that coil position is optimized if only limited sequences can be obtained; the anatomic questions are well understood so that the MR can be appropriately protocoled to target the answers. For some centers, ultrasound first is not possible, and in this situation, it will be necessary to perform an ultrasound following the MR, or at least to have a report from recent ultrasonography for correlation of biometric measurements and anatomy.
RISKS AND BENEFITS
The principal theoretical risk of fetal MR relates to excess tissue heating, which has been associated with growth retardation and congenital malformations in various animal models. Similar malformations have been observed in children born to women who experienced significant hyperthermia in the first trimester of pregnancy. Numerous reports in the biological physics literature have addressed the theoretical risks of tissue heating related to exposure to the radio-frequency electromagnetic fields that are generated by MR. Guidelines for the general public do exist for maximum exposure limits for the rate at which energy is absorbed by the body when exposed to RF electromagnetic fields (as measured by the Specific Absorption Rate – SAR). Safe limits have also been suggested for maternal and fetal body temperature. Biological physicists studying the effects of MR have noted in experimental models that meeting exposure limits for the pregnant woman may not protect the fetus from overexposure, and that the safety of the fetus may be overestimated in numerous models. These reports, which rely heavily on mathematical and computational modeling for MR effects, emphasize that a paucity of data are available regarding exposure of the fetus to MR generated radio-frequency electromagnetic fields, and that further work is required.
- Pediaditis M. RF-EMF exposure of fetus and mother during magnetic resonance imaging. Phys. Med. Biol. 53 (2008) 7187–7195).
- Kikuchi S. Temperature elevation in the fetus from electromagnetic exposure during magnetic resonance imaging. Phys. Med. Biol. 55 (2010) 2411–2426
Current ACR recommendations regarding fetal MR safety emphasize that no present data conclusively document or reproducibly demonstrate harmful effects to the pregnant woman or the fetus. The ACR suggests that special considerations are unnecessary for fetal MR imaging at 1.5 T at any stage of pregnancy. The ACR cautions that most of the research on the potential harmful effects of fetal MR imaging has been done for magnetic fields of 1.5 or less, and that less information is available about the safety of fetal MR at field strengths of 3T. Click here to review the ACR/SPR Guidelines for Fetal MRI Safety.
The ACR further recommends that the theoretical risks must be carefully weighed against the potential benefits of the information that is obtained.