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ECR 2019 / C-1504
Certificate of Merit
MRI in diagnosing different types of morbidly adherent placenta
Congress: ECR 2019
Poster No.: C-1504
Type: Educational Exhibit
Keywords: Obstetrics (Pregnancy / birth / postnatal period), Genital / Reproductive system female, Pelvis, MR, Diagnostic procedure, Obstetrics
Authors: U. S. Umer, S. Alam, A. N. Khan, S. G. Ghaus, S. Gul, S. Gul, H. Abid, A. Safi, K. Nawab; Peshawar/PK
DOI:10.26044/ecr2019/C-1504

Findings and procedure details

Magnetic Resonance Imaging in Morbidly Adherent Placenta:

 

Procedure Details:

 

The maternal pelvis is imaged in the axial, coronal, and sagittal planes with multichannel body surface coil. The field of view (FOV) is usually 38 cm but can be increased to optimize for the enlarged uterine size. T2-weighted PROPELLER i.e. periodically rotated overlapping parallel lines with enhanced reconstruction (PROP) and T2 FATSAT (FatS) images are ideally acquired in the axial, sagittal, and coronal planes with repetition/echo times of 2000-4000/98-120 milliseconds, a slice thickness of 5 mm with slice spacing of 1.5mm, Echo train length of 30-32 and a typical field of view 38 cm.  T1WI or Dual Echo should be acquired in axial plane, the role of which would be emphasized below.  No role of intravenous gadolinium in MRI of placental invasiveness. All MR images should be evaluated by atleast two qualified radiologists with experience in reading pelvic MRI to locate the site suspicious for placental adhesion. MRI report should evaluate the placenta and grade the placental adherence.

 

Brief Anatomy:

 

Knowing the normal placental anatomy is essential in understanding the imaging appearances of the invasive placentation. Placenta has two surfaces: fetal and maternal. Fetal surface is limited by the chorionic plate and amniotic membrane. The maternal surface is limited by the basal plate. In between these two surfaces lie the villi, intervillous spaces and placental septi. 

 

Placental MRI Anatomy:

 

Placenta appears as a regular homogeneous structure at 19-23 weeks. Subsequently, placenta appears slightly lobulated at 24-31 weeks due to visualization of faint sporadic septa. Universal appearance of septa and stratification of placenta into lobules are seen after 36 weeks5 (Figure.2 & 3).

 

MRI appearances of normal placentation2 should be:

 

1. Homogeneous T2-intermediate signal intensity of placenta (Fig.2).

 

2. Subtle thin, regularly spaced placental septa (Fig.3).

 

3. Placental-Myometrial interface: Placenta is usually clearly distinct from the underlying myometrium by a T2 low signal placental-myometrial interface.

 

4. Normal subplacental vascularity is seen as numerous flow voids just under placenta. Few intraplacental flow voids can also be seen, usually in the region of umbilical cord insertion..

 

5. Triple-layered sandwich appearance of myometrium.

 

6. Pear-shape of normal gravid uterus with smooth contour.

 

 

Fig. 2: Normal Placenta. Universal appearance of regularly spaced placental septa and stratification of placenta into lobules are seen after 36 weeks.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK
Fig. 3: Normal Placenta. Universal appearance of regularly spaced placental septa and stratification of placenta into lobules are seen after 36 weeks.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

        

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On MRI, Normal myometrium of a term gravid uterus should have the following features:

 

  • Myometrium thins as the pregnancy progresses.
  • Myometrium has a triple layer appearance (Fig.4), when it is not thinned.
  • The inner and outer layers are seen as thin bands of T2 low signal intensity (Figure.2). The middle layer is seen as thick intermediate to high T2 signal intensity and frequently shows multiple vascular flow voids.
  • Uterine contractions can cause transient focal T2-hypointense myometrial thickening.

     


    Uterine Contour: Gravid uterus usually shows smooth contour with a wider body and fundus than the lower segment.

 

 

Fig. 4: Trilayered appearance of myometrium seen on sagittal T2WI (arrows). Open arrows point at inner and outer T2 low signal layers with intervening intermediate signal intensity. Circled lower anterior uterine wall is showing the thin myometrium with loss of trilayered appearance and traversing signal void vessels suggesting the site of invasive placenta (percreta).
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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MR imaging signs predictive of abnormal placentation:

 

The most acceptable signs with good inter-rater reliability are listed below 6,7,8 (Figures 5, 6)  Table 2

 

 

  1. Dark intraplacental bands on T2-weighted images.

     

  2.  Heterogeneity within the placenta.

     

  3. Abnormal disorganized placental vascularity.

     

  4. Uterine bulging.

     

  5. Traversing vessels.

     

  6. Focal interruptions of the myometrial wall (high specificity for increta and percreta).

     

  7. Direct placental invasion.

     

  8. Tenting of urinary bladder (highly specific for percreta).

 

Dark intraplacental T2 bands:

 

Dark intraplacental bands represent areas of fibrin deposition within the placenta.

  • Nodular or linear areas of low signal intensity on T2-weighted images.
  • Extend from the placenta–myometrium interface.
  • These bands are thicker than the normally fine placental septa and show a random distribution.

 

Fig. 5: T2 low bands. Sagittal T2 weighted image of placenta percreta and previa. There is heterogeneity of placenta with internal T2 low bands (arrows)
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK


                              ------------------------------------------

 

Placental heterogeneity:

 

Heterogeneous signal intensity in the placenta depends primarily on the presence or absence of abnormal T2 dark bands (Fig.6), areas of hemorrhage in the placenta or increased vascularity. Homogeneous placenta can exclude abnormal placentation with high levels of confidence. 
Abnormal disorganized placental vascularity is described as hypertrophied, tortuous disorganized vessels deep within the placenta, located in some of the areas of dark bands.

 


Uterine bulge:

A focal outward contour bulge or disruption of the normal pear shape of the uterus, with the lower uterine segment being wider than the fundus, can be seen in MAP. Focal interruptions of the myometrial wall or extension through the myometrium with occasional invasion of adjacent structures can also be seen.

 

 

Fig. 6: Sagittal T2 weighted image of a gravid uterus.There is heterogeneity of placenta with internal T2 low bands, thinning of lower myometrium with bulging placenta and traversing signal void vessels (placenta percreta). The placenta is low lying and covering the internal os consistent with placenta previa. There is abnormal uterine contour bulge in lower segment. The lower uterine segment is appearing larger than the upper segment. Note the intact trilayered appearance of normal myometrium in upper uterine segment.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

                                      ------------------------------

 

 

Fig. 7: Sagittal T2 weighted image of Placenta percreta showing traversing signal void vessels (arrow) with thinning of myometrium, loss of normal trilayered appearance, bulging invasive placenta and background placental heterogeneity with internal T2 low bands.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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Direct placental invasion:

Placenta directly invading or tenting the urinary bladder is highly specific for placenta percreta. MRI is particularly useful in showing parametrial extension which is not apparent on USG.

 

 

Fig. 8: Percreta involving the anterior abdominal wall (red arrow). Green arrows showing normal abdominal wall.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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Grades of Morbidly adherent placenta

depending on depth of invasion:

 

Fig. 1: Diagram of grades of abnormal placental infiltration. (1) Placenta accreta is adherent to the myometrium. (2) Placenta increta invades the myometrium. (3) Placenta percreta extends into or beyond the uterine serosa and may invade any other organ.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

Specific MRI findings for each grade:( Table 2 )

 

Placenta Accreta:

  • Focal loss of placental - myometrial interface
  • Heterogeneity
  • T2 low bands
  • Focal uterine contour bulge

 

Fig. 9: Placenta Accreta. Sagittal T2WI shows focal invasion of placenta into the anterior lower myometrium associated with focal uterine contour bulge in the lower segment.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK


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Fig. 10: Placenta Accreta. Axial T2WI shows focal loss of myometrial-placental interface associated with T2 heterogeneity (circle).
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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Fig. 11: Coronal FIESTA image and illustration showing loss of placental-myometrial interface.The yellow line of placental interface is interrupted.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK


                                  -----------------------------------

 

Role of FIESTA in Accreta

 

 

Fig. 12: Accreta and role of FIESTA in placental-myometrial interface. Coronal T2 image showing focal heterogeneity in right lower placenta with T2 low bands. T2 FATSAT image showing more sharpness in detail of placental structure as compared to T2. FIESTA image showing focal loss of placental-myometrial interface (arrow), better visualized as compared to T2 PROP.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

 

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Placenta Increta

 

 

In placenta increta, there is placental invasion into the myometrium such that it is surrounded by myometrium. On MRI, the imaging features would include uterine serosal bulge in addition to the findings to be suspected in accreta like T2 heterogeneity, low bands, loss of placental-myometrial interface, the feature of serosal bulge seen in patients with increta.( Fig, 13, Fig. 14 , Fig.15)

 

Fig. 13: Increta: Sagittal T2 Fat suppression image showing focal heterogeneity with T2 low bands evident in anterior lower placenta.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

 

                                       ---------------------------

 

 

Fig. 15: Increta. Sagittal T2 PROP image and drawing marked on the image showing contour bulge in lower anterior uterine segment, focal interruptions in the inner low signal myometrial layer (inner red line is interrupted with yellow showing the inavading placenta) - compare the normal trilayered myometrial appearance in the posterior fundal uterine wall with anterior lower segment showing loss of placental-myometrial interface and focal heterogeneity in anterior lower placenta (T2 low bands).
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

                                  -----------------------------------

 

 

Placenta Percreta:

 

In placenta percreta, there is involvement of uterine serosa or extra-uterine involvement of adjacent organs like bladder, rectum or abdominal wall. (Fig.4, Fig. 5, Fig. 6, Fig. 7, Fig.8, Fig.16, Fig.17) 

 

Fig. 7: Sagittal T2 weighted image of Placenta percreta showing traversing signal void vessels (arrow) with thinning of myometrium, loss of normal trilayered appearance, bulging invasive placenta and background placental heterogeneity with internal T2 low bands.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

                                            -------------------- 

 

 

Fig. 16: Percreta infiltrating the anterior abdominal wall at level of previous C-section scar(red arrow). Green arrows showing normal un-involved abdominal wall muscles.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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Fig. 17: Axial T1 / Inphase Dual Echo images showing signal void traversing vessels in a morbidly adherent placenta percreta.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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Table 2: MRI findings in morbidly adherent placenta.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

 

                        ---------------------------------------------------

 

Role of Moderately full bladder during MRI scan:
( Fig. 18 and Fig.19)

MRI should be performed in atleast moderately full bladder. Fig.18 shows how scan done with empty bladder is giving false appearance of percreta directly infiltrating the bladder wall.

 

Fig. 18: Morbidly adherent placenta with heterogeneity and T2 low bands (arrow) on Sagittal T2 FATSAT image.The collapsed bladder appears to be involved by the heterogeneous placenta raising suspicion of percreta (open arrow).
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

Fig. 19: MRI Sagittal T2 images of the same patient of Fig.18 (Fig 18 showed collapsed bladder appearing to be involved by the heterogeneous placenta). These MRI images were obtained in the same patient but with moderately full bladder confirming that the bladder was not involved.To conclude, this is a case of Placenta increta evident as focal heterogeneity and T2 low bands with associated uterine surface serosal bulge at level of bladder dome and no involvement of bladder.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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No role of Intravenous gadolinium: Fig. 20  

 

There is no added benefit of giving intravenous gadolinium to assess the grade of morbidly adherent placenta.

 

Fig. 20: Contrast enhanced T1 image and Sagittal T2 image show placenta previa with focal heterogeneity in its anterior part suggesting possible accreta. Contrast enhanced T1 image shows no added benefit of contrast in improving detection of placental invasiveness.
References: Radiology Department, Rehman Medical Institute, Rehman Medical Institute - Peshawar/PK

 

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