Presentation
Two fetuses are presented below, one at 19w7d and one at 35w3d. Neither had any indication of placental abnormality and deliveries were normal.
Patient Data
An example of the placental-myometrial interface in a second trimester fetus. This fetal evaluation was for an intrathoracic finding on ultrasound. At no point was the placenta considered abnormal and the delivery was normal.
The normal gravid myometrium has three layers:
- T2 hypointense outer layer
- T2 hyperintense middle layer
- T2 hypointense inner layer
In some images, the interface between the placenta and the inner myometrium may be blurred, but this is not uncommon and usually artifactual. Make sure you can see placental-myometrial abnormality in two planes before suggesting placental invasion. In this case, there is a small amount of blurring of the interface on the sagittal image, but it does not hold up on the axial image.
The placenta normally has homogeneous signal intensity when <24 weeks.
The green trace outlines the placenta. The blue bracketed area appears slightly blurry, and one might think of placental invasion... however, this does not hold up on other images.
The red arrows point to the inner and outer myometrial layer. The yellow arrow points at the inner T2 hyperintense layer.
An example of the placental-myometrial interface in a third trimester fetus. This fetal evaluation was also for an intrathoracic finding on ultrasound. At no point was the placenta considered abnormal and the delivery was normal.
In this case, the interface may be harder to see because the thin myometrial layer is pressed up against the abdominal wall, but it is still normal. Often it is useful to try to trace the outer myometrial wall and work inwards, to at least feel comfortable that placenta percreta and increta can be excluded.
The placenta normally develops a more heterogeneous signal intensity when >24 weeks.
Case Discussion
MRI can be useful for additional problem-solving of placental problems after ultrasound examination, including abnormal placental implantation.
IV contrast is not needed for evaluation (in fact, it is contraindicated). Instead, six key sequences are needed:
- triplanar balanced steady state free precession sequences (bSSFSP)
- triplanar T2 single shot fast spine echo sequences (SSFSE or HASTE)
Some institutions add a T1 fat sat axial sequence as well.
The goal of these sequences is to provide the best tradeoff of spatial and contrast resolution of the placental-myometrial interface.
Usually, a fetal MRI is ordered to evaluate the fetus rather than the placenta, but the placenta signal, shape, and attachment should still be commented upon.
For more information, see placental evaluation with MRI.