MRI Appearance of Normal Placenta
The placenta appears pear shaped with uniform outer contour and maximum thickness of 2 – 4 cm,
attached to anterior or posterior uterine wall and display different appearances during gestation (1,
2).
It shows homogenous intermediate T2 signal,
particularly if imaged between 19 – 23 weeks of gestation (1).
In more advanced pregnancy,
placenta becomes more lobulated with internal T2 hypointense septa running into myometrium.
These septa are uniformly thin and evenly spaced (2).
The myometrium becomes thinner as the pregnancy progresses and shows three distinct layers; an inner and outer T2 dark bands and an intermediate signal thicker middle layer.
Subplacental and myometrial flow voids representing vascularity can be normally seen (2).
Myometrial contractions can also be typically seen as focal areas of low T2 signal transient myometrial thickening (1) (Figure 1) Table (1).
MRI Findings in Abnormal Placentation
Earlier MRI signs described in the literature including visualization of direct placental invasion into or outside the myometrium,
indistinct thin myometrium and loss of T2 dark uteroplacental interface,
were non-specific (3).
Lax et al described the three most useful features for diagnosis of abnormal placentation: Heterogenous signal of the placenta,
intraplacental T2 dark bands and focal uterine contour bulge (7).
The intraplacental T2 dark bands are best seen in T2 HASTE and True FISP sequences as thick irregular dark linear or nodular bands extending from the placenta-myometrium interface into placenta,
thought to be due to fibrin deposition (2,
6) (Figure 2 and 3).
Placental heterogeneity depends on presence of T2 dark bands and can also represent areas of hemorrhage.
When placenta is homogenous,
diagnosis of abnormal placentation is unlikely (2,
6).
Diagnostic criteria for abnormal placentation are summarized in Table (2).
Focal thinning of myometrium and loss of T2 dark uteroplacental interface are considered unreliable signs.
MRI becomes less reliable in differentiating different degrees of placental invasion particularly between accrete and increta (2,
6).
Table (3)
Use of MRI Contrast
Gadolinium crosses the placenta,
circulates in the amniotic fluid and can be swallowed by the fetus.
Since its half-life in the fetus is unknown and its safety has not been established,
gadolinium based contrast material is classified as a pregnancy class C drug (2) and its use in not recommended in many institutions.
Some authors suggested that use of gadolinium based contrast can add to the specificity of the diagnosis and help distinguish between placenta accrete and percreta as the margins between placenta and myometrium are clearly delineated in post contrast images (8,
9).