Mri of placenta accreta spectrum

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.

13. 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.

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

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

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

17. Lumpy placental edges

18. Abnormal intraplacental vascularity

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

20. Tenting of the UB

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

22. 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.

23. Pitfalls in Diagnosis Motion artifacts

24. 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.

25. 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.

26. Thank you.