MRI is a non-invasive diagnostic technique that uses a strong magnetic field and radio waves to create detailed images of organs and soft tissues inside the body. Unlike X-ray or Computed Tomography (CT) scans, MRI does not use ionizing radiation. Non-contrast MRI is generally considered a low-risk imaging option for pregnant women when a diagnosis is needed. However, the procedure is typically reserved for specific situations where other imaging methods, such as ultrasound, cannot provide the necessary medical information.
Understanding the Safety Profile and Timing
The magnetic fields and radiofrequency pulses used in MRI have shown no evidence of causing harm to a developing fetus across decades of clinical use. Professional medical organizations suggest that the non-ionizing nature of the scan makes it a safe option for both the mother and the unborn child. The strongest recommendation is to adhere to the principle of “as low as reasonably achievable” (ALARA). This means the scan should only be performed when the potential diagnostic benefit clearly outweighs the theoretical risk of delaying the procedure.
Physicians exercise greater caution during the first trimester, which spans the first 12 weeks of pregnancy. This period is when organogenesis, the formation of the fetus’s organs, occurs rapidly, making it a time of heightened sensitivity to external factors. Avoiding the first trimester is a standard precautionary measure unless there is a clear and immediate need for the information.
The second and third trimesters are generally considered a more appropriate time for non-urgent diagnostic scans, as the fetus’s organs are more developed. Concerns in later stages focus on acoustic noise exposure and the theoretical risk of tissue heating from the radiofrequency energy. To mitigate these factors, MRI centers utilize specific protocols, known as low Specific Absorption Rate (SAR) settings, to limit the amount of energy deposited into the body. Limiting the total scan duration also helps manage any slight rise in maternal and fetal temperature.
Why Contrast Agents are Avoided During Pregnancy
A separate and stringent safety consideration applies to the use of Gadolinium-based contrast agents (GBCAs). These agents are injected intravenously to enhance the clarity and detail of certain tissues on the MRI images. Gadolinium is a heavy metal that is chelated, or bound, to a molecule for safe administration, but it is known to cross the placental barrier into the fetal circulation. Once in the fetus, the agent is filtered by the fetal kidneys and then accumulates in the amniotic fluid.
The concern arises because the gadolinium can be retained in the fetal tissues and recirculate within the amniotic fluid, which the fetus continually swallows. While data from human studies are limited, there is a theoretical risk of long-term effects related to the retained gadolinium. For this reason, the use of GBCAs during pregnancy is strongly discouraged and almost universally avoided.
Contrast agents might only be considered in extremely rare, life-threatening maternal situations where the diagnostic information is immediately necessary to save the mother’s life. In such cases, the immediate benefit of a precise diagnosis is deemed to outweigh the unknown, long-term risks to the fetus. If a GBCA is necessary, specific types known as macrocyclic agents are preferred due to their greater stability.
Medical Reasons for the Scan
Magnetic Resonance Imaging is typically employed during pregnancy only after a less invasive method, such as ultrasound, has been performed and yielded inconclusive or insufficient information. MRI serves as a secondary, high-resolution tool to clarify complex findings that require a detailed anatomical assessment. The indications for the scan generally fall into two categories: complex maternal conditions and specific fetal abnormalities.
Maternal Conditions
For the pregnant patient, an MRI may be required to diagnose or stage serious conditions. These include acute neurological events, such as a suspected stroke, or to assess for cancer that is newly diagnosed or monitored during the pregnancy. It is also frequently used to investigate sources of acute, non-obstetric abdominal or pelvic pain, which can occur from appendicitis or other severe internal issues.
Fetal Abnormalities
When the concern is fetal, MRI is highly valued for its ability to provide clear images through the mother’s body, particularly in cases of maternal obesity or poor fetal positioning. The scan is especially useful for detailed evaluation of the fetal central nervous system, including complex anomalies of the brain or spine. It also assesses the severity of congenital defects in the chest or abdomen. Furthermore, MRI is precise for evaluating placental issues, such as determining the depth of placental invasion into the uterine wall (placenta accreta), which significantly impacts the delivery plan.
Preparing for the Procedure
Preparing for an MRI while pregnant involves several practical steps focused on safety and maximizing comfort, especially as the pregnancy progresses. The strong magnetic field necessitates the removal of all metal objects, including jewelry, hairpins, and any clothing with metallic components. Patients must inform the technologist of any internal metal implants, such as surgical clips or certain medical devices, which may be contraindications for the scan.
Patient positioning is a significant consideration, especially from the second trimester onward. The growing uterus can compress the vena cava, a major vein that returns blood to the heart. To prevent this compression and maintain proper circulation, the patient is often positioned with a slight tilt onto her left side, sometimes using supportive foam pads and bolsters.
The MRI machine generates significant noise from the rapid switching of magnetic gradients, producing loud clicking and knocking sounds. The patient will be provided with earplugs or headphones to protect against this noise. Technologists work to ensure the patient is as comfortable and still as possible, as movement can blur the images and necessitate repeating sequences, which prolongs the scan time. Imaging protocols are optimized to be as short as possible, often lasting 30 to 60 minutes, to limit the duration of the patient’s time inside the scanner. Throughout the procedure, the technologist monitors the patient and can communicate with her via an intercom to address any concerns or discomfort.