MRI means Magnetic Resonance Imaging. The technique is not new, but being able to perform MRI on a fetus has only been possible relatively recently, due to technological advances that allow specialists to obtain images faster than ever before. Obtaining images quickly is important for fetal MRI, since the fetus moves constantly!
Ultrasound remains the predominant modality for evaluating disorders related to the fetus and pregnancy overall. Ultrasound continues to have several obvious advantages over MRI. It is safe and relatively inexpensive and is a widely available technology that allows for real-time imaging. Compared with MR imaging, US provides superior spatial resolution, permits assessment of fetal well-being, and provides quantitative assessment of fetal and placental blood flow with Doppler US.
However, US does have important limitations. First, it is uniquely operator- and interpreter-dependent. In addition, relative to MRI, US provides a small field-of-view, and the resolution of US images is restricted by penetration through soft tissues and bone. Thus, the sensitivity of US in evaluating the fetus is reduced in obese patients and in women whose pregnancies are complicated by low amniotic fluid volume. Examination of intracranial anatomy is limited by the attenuation of the US beam through the bony skull in later gestation. Finally, US quality is influenced by fetal position. A prone, breech fetus may be much more difficult to examine than a vertex supine fetus. If the head is low in the maternal pelvis, it may also be difficult to assess using US.
A fetal MRI is a specialized imaging test that may be helpful in evaluating your fetus for congenital anomalies and associated complications. You may have had an ultrasound which detected an abnormality, or perhaps genetic testing revealed an increased risk for a syndrome.
MRI visualization of the fetus is not significantly limited by maternal obesity, fetal position, or oligohydramnios, and visualization of the brain is not restricted by the ossified skull. It provides superior soft-tissue contrast resolution and the ability to distinguish individual structures such as lung, liver, kidney, bowel, and gray and white matter. MRI provides a large field-of-view, facilitating examination of fetuses with large or complex anomalies, and visualization of the lesion within the context of the entire body of the fetus.
A fetal MRI would especially be recommended 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.