Type:
Educational Exhibit
Keywords:
Obstetrics, Intrauterine diagnosis, MR, Obstetrics (Pregnancy / birth / postnatal period), Genital / Reproductive system female
Authors:
G. GARCIA GALARRAGA1, E. Para Margüello1, E. Zabía Galíndez1, P. A. Encinas Escobar1, N. Pérez Peláez2, M. De la Puente Herraiz1, I. NAVAS FERNANDEZ-SILGADO1; 1Madrid/ES, 2Madrid, Madrid/ES
DOI:
10.26044/ecr2019/C-2254
Findings and procedure details
PATHOPHYSIOLOGY OF INVASIVE PLACENTATION:
Placental accretism is thought to be due to an absence or deficiency of the Nitabuch layer,
which is a layer of fibrin that lies between the deciduous basal and the frondosum chorion.
Benirschke and Kaufman suggest that this situation is the result of a failure in the reconstitution of the endometrium / decidua basalis,
especially after the repair of a cesarean section.
MR IMAGING PROTOCOL Fig. 3
Different protocols are used depending on the institution.
There are two essential sequences to assess placental accretism,
both of which allow us to assess the morphology of the placenta and the limits between placenta and myometrium:
- T2-weihgted single shot echo-planar fast spin echo sequences (HASTE,
SS-FSE,
SSH-TSE,
PHASE) in all three plans respect the uterus (axial,
coronal and sagittal): These sequences are fast and minimize fetal motion artifacts.
- T2-weighted fast spin echo sequences in all three plans of the uterus (axial,
coronal and sagittal): These sequences allow to better assess the heterogeneity of the placenta accreta but frequently present artefacts by fetal movement.
Other optional sequences:
- Axial fat-saturated fast spin echo T1-weighted: Useless to distinguish between placenta and myometrium,
but it allows detecting blood products in the placenta related to placental abruption.
- DWI: It can be useful to differentiate limits between placenta and myometrium.
Gadolinium is not used in most centers because:
- The use of Gadolinium has not shown to increase diagnostic sensitivity.
- The Gadolinium crosses the placental barrier and the effects on the fetus are unknown.
Recommendations:
- The patient's bladder should be moderately full to better assess the placental invasion.
- If the examination is performed in the 2nd trimester,
it can be performed with the patient in the supine position,
but if it is done in the third trimester,
it is recommended to perform the exploration in the left lateral decubitus position to avoid compression of the inferior vena cava.
- It can be used oxygen glasses to try to reduce fetal movement.
When to perform the MRI: Between weeks 24-30
- It has been shown that before week 24,
MRI greatly decreases the diagnostic sensitivity for placental accreta.
(article by Howoritz).
- Last weeks of the third trimester may be more difficult to evaluate placental-myometrial limit due to the great decrease in the thickness of the myometrium.
But the rest of the signs of placental accreta can be seen.
FINDINGS IN PLACENTAL ACRETISM Fig. 4 Fig. 5 Fig. 6 Fig. 7
MRI is able to differentiate between normal placenta and placenta accreta,
but it is very difficult to determine the degree of placental invasion. The most useful findings are: Uterine bulging,
intraplacental dark bands and heterogeneous placenta.
- Uterine bulging: Loss of the normal form of the uterus with areas of lump.
- Intraplacental dark bands : Linear or nodular hypointense areas in T2-weighted images are thicker than the normal placental septas.
Frequently they originate from the maternal side of the placenta.
This finding is highly suggestive of placenta accreta,
although it can occur isolatedly in normal placentas.
Unlike normal placental septa,
these bands have a changing thickness and originate from the uterine-myometrial interface.
- Heterogeneous placenta: In placental accretism,
it usually presents a heterogeneous signal in sequences enhanced in T2.
It is probably due to areas of intraplacental hemorrhage and artifacts. Fig. 8
Other findings:
- Round edges: Unlike normal placentas whose borders appear as acute angles,
in the placental acretism the presence of rounded edges can be observed (obtuse angles).
Fig. 10
- Focal myometrial interruption: Findings widely used in ultrasound,
however,
have little use in MRI,
since in many normal cases due to the thin thickness of the myometrium,
the limit between placenta and myometrium can not be distinguished
- Extrauterine invasion: Loss of the fatty plane with adjacent organs.
The most frequently affected are bladder and bowel.