MRI offers a great aid in diagnosis of abnormal placentation. This presentation describes the normal MRI appearance of the placenta and the MRI signs of placental adhesion disorders.

1. Dr. Hazem Abu Zeid Yousef
Ass. Prof. Radiodiagnosis.
The MRI features of placental
adhesion disorder
Pearls and pitfalls

2. Placenta accreta (PA) occurs when the chorionic villi (CV)
invade the myometrium abnormally due to defect in the
decidua basalis.
The frequency of placenta accreta has increased by more
than 10- fold in the past 30 years to approximately three
cases per 1000 deliveries. This is largely because of the
increasing number of cesarean deliveries.

3. Risk factors.
Major risk factors for PA are placenta
previa and previous cesarean section. The
risk of developing PA is 3% in women
with only placenta previa and increases to
24% in those with placenta previa and
one prior cesarean delivery. The risk
increases with the number of previous
cesarean deliveries.

4. Classification

5. Classification
• Placenta accreta
• Placenta increta
• Placenta percreta
CV in contact with myometrium, but not invading it.
CV partially invading the myometrium.
CV penetrating through the entire myometrial thickness
or beyond serosa.

6. Sonography is the primary diagnostic tool for PAS and is initially
performed at the fetal screening examination at 18–20 weeks’
gestational age. If low-lying placenta or placenta previa is seen in a
woman with previous cesarean delivery, a follow-up examination
after 32 weeks’ gestation is performed for a more definitive
evaluation. Imaging features associated with PAS include placenta
previa, lacunae, abnormal color Doppler imaging patterns, loss of
the retroplacental clear space, and reduced myometrial thickness.
An irregular bladder wall has been described with placenta
percreta. A recent meta-analysis of ultrasound diagnostic
performance reported a sensitivity of 91% and specificity of 97%.
Diagnosis

7. MRI: when and how to do it?
The FIGO consensus recommendation states that MRI is not
essential for making a prenatal diagnosis of suspected PAS
disorders but may be useful in evaluating the pelvic
extension of a placenta percreta or areas difficult to evaluate
on US.
MRI is indicated in the diagnostic workup when the
ultrasound evaluation is equivocal or for patients with high
clinical risk factors for PA. In cases where US has already
made a definitive diagnosis, MRI is often used to plan the
cesarean section delivery and peripartum hysterectomy.

8. MRI does have some distinct advantages, and there are
situations when it provides added information, which
may be valuable for surgical planning. The large FOV
gives an excellent “big picture” view of all the pelvic
anatomy, including bladder, bowel, ureters, and
surrounding vascular anatomy. There are no imaging
blind spots with all areas equally well seen, and it is
less affected by maternal obesity.

9. Advances in fast imaging techniques, have dramatically advanced
obstetrical MRI. Basic principles include single-shot T2-weighted fast
spin-echo sequences to minimize the effects of motion and optimize
contrast, with thin (3–5 mm) slice thickness for anatomic detail. T1W is
often included for the detection of blood products. The maternal
bladder should be partially full to best evaluate the uterine-placental-
bladder interface, which cannot be accurately assessed if the bladder is
empty. Conversely, over distention compresses these structures and
can erroneously cause over diagnosis of placental invasion. It may also
cause maternal discomfort, resulting in motion artifact.

10. Ideal timing of the exam is between 24- and 30-week
gestation. Examinations before 24 weeks have proven
unreliable for predicting abnormal placentation. After
30 weeks, the placenta becomes more heterogeneous
and the uterine wall thins, decreasing the specificity of
many of the described MRI features of PAS.

11. Normal placenta through pregnancy
The normal placenta is uniform in thickness, measuring 2–4 cm in
the midportion, and shows a smooth external contour. The thickness
gradually tapers toward edges that appear as smooth angles. The
maternal surface of the placenta contains placental lobules that are
surrounded by placental septa. These septa are a normal finding that
can be visualized on T2W MR images as thin bands of T2 signal.
At 24–30 weeks of gestation, the normal placenta exhibits
homogeneous intermediate signal and is usually distinct from the
myometrium, which is more heterogeneous and hyperintense.

12. The uterine myometrium also varies in appearance throughout
pregnancy. At less than 30 weeks, it shows three distinct layers
of signal intensity. The inner and outer layers of the
myometrium are seen as thin bands of decreased signal; the
middle layer is hyperintense relative to the placenta and
becomes brighter as the pregnancy progresses. The
myometrium often contains multiple flow voids representing
the normal vascularity. As the myometrium thins with
progression of gestation after 30 weeks, the layers become
less distinct.

14. Imaging Features of Invasive Placentation
Many of the MRI features of PAS are related to the
abnormal implantation, fixation, and tethering of the
placenta to the area of the scar, disrupting normal
development.

15. Intraplacental dark T2 bands
An important feature
of PAS: Thick, linear
areas of low signal
intensity primarily
involving basal plate of
placenta.

16. Uterine bulging
Focal outward
contour bulge
and loss of
the normal
pear-shaped
uterus.

17. Focal interruption of the uterine wall
Thinning or complete
penetration of the
myometrium.

19. Lumpy placental edges

20. Abnormal intraplacental vascularity

21. Focal exophytic mass
Placental tissue seen breaking
through and extending beyond
uterine serosa.

22. Tenting of the UB

23. When one reads a case, none of these signs
are viewed in isolation, and the more findings
that are present, the greater the concern.

24. Diagnostic Reporting for Treatment Planning
Currently, cesarean-hysterectomy, represents the most common
management approach for PAS. In this context, the MRI
examination is used not only to diagnose or confirm suspected PA
but also for preoperative planning. Thus, the report should
include fetal lie, placental location relative to maternal anatomy
and the uterine endocervical os, and identification of uterine
structures (fibroids or placenta) that could affect the surgeon’s
ability to mobilize the organ or the decision on the hysterotomy
location. In cases of placenta percreta, all adjacent involved
structures should be identified so that, if necessary, the relevant
surgical expertise (e.g. urology, vascular, colorectal, or plastic
surgery) can be recruited before the procedure.

25. Pitfalls in Diagnosis
Motion artifacts

26. Imaging outside this recommended time frame may affect
accuracy. Before 24 weeks, the placenta is immature and
proliferation of vessels at the placentalmyometrial interface
cannot be differentiated from signs of invasion. After 30
weeks, the placenta normally appears more heterogeneous.
The normally enlarged uterus with a thin imperceptible wall
late in pregnancy means that bulging and myometrial
disruption cannot be reliably assessed.

27. Ultrasonography-MRI concordance
Sonographic and MRI findings should be correlated
because this reliably improves image interpretation.
On occasion, the results of US and MRI assessment may
not be concordant. MRI interpretation should be
performed in accordance with recognized image
interpretation criteria and represents an independent
evaluation. In cases where the conclusions from the two
tests differ, treatment planning generally errs toward
anticipating the gravest diagnosis.

28. Thank you.