Obstetric imaging
From imaging to clinical diagnosis
Interest in fusion ultrasound in diagnosis of placenta accreta
Fusion ultrasound is an aid in the diagnostic arsenal of placental implantation anomalies. by combining a complementary, synchronized and focussed semiology, this examination improves the study of the placenta accreta.
Technical principles
The realization of this fusion ultrasound requires an ultrasound equipped with an image fusion module. The previously performed MRI volumes are loaded into the ultrasound using a CDRom. A magnetic field generator and a magnetic sensor clipped on the abdominal or endovaginal probe allow the spacing of probe movements. The patient is lying on the ultrasound table with a sufficient bladder repletion of at least 300 cc, ideally the same bladder volume as during MRI. T2 MRI sequences are usually used, but all types of sequences can be imported. Injection of gadolinium is not necessary since intraplacental vascularization will be studied by using the Doppler mode in fusion ultrasound. The synchronization of the MRI volume and ultrasonic plans is performed using fixed markers independent of the fetal mobile: insertion of cord and cervical channel (Figures 1a and b).
Synchronizing plans. 1A. Sync images to cervical channel (arrow): identification of internal and external openings of the cervix to ultrasound and MRI.
Synchronizing plans. 1B. Synchronizing images with the cord insertion (arrow).
The ultrasound monitor displays the ultrasonic cutting plane and the same cutting plane in T2 magnetic resonance imaging. These two planes mobilize synchronously when the probe is scanned along the placental structure. The operator can modify ultrasonic settings according to technical constraints and use the Doppler mode.
Patients affected
The fusion ultrasound, in this indication, concerns all patients whose location and/or appearance of the placenta are suspect in obstetric ultrasound: scarring uterus and placenta præ via mainly, but also an atypical placental aspect and antecedent of myometrial surgery (cure of synechia, aspiration, myomectomy..). The fusion ultrasound is programmed within days of the MRI performed between 28 and 38 weeks of amenorrhea in the suspicions of accreta.
Semiology
For educational purposes, but also to help systematize the reports, we propose to group the semiology of the placenta accreta into 4 categories of signs (framed).
| Semiology of the placenta accreta in fusion ultrasound |
A = Intraplacental
B = Interface Abnormality
C = Anomalies of Vascularization Doppler
D = Overall
|
Intraplacental signs (A)
• Intraplacental ultrasonic deficiencies are irregular and numerous (> 4). These gaps are not always located in front of the healing zone. In Doppler mode, there is a turbulent flow within these gaps.
• Intraplacental black strips with T2 magnetic resonance.
• A heterogeneity of the signal strength of the placenta in T2.
Signs of the interface (B)
• Myometer thickness < 1 mm or a near disappearance of the myometer (note, this sign is not specific in scarring uterus in the 3rd quarter; indeed, this fine aspect is often present in relation to the lower scar segment, amplified at the end of pregnancy.
• Absence of the hypoechogenic line between the placenta and myometer: this hypoechogenic margin represents, in fact, the thickness of the decimal deciduous that is missing in the case of the adherent placenta.
• Loss of the triple slip. The triple leaf corresponds to a hyposignal edge (the placenta/myometer interface), a hypersignal edge (the myometer) and a hyposignal edge between the myometer and the serous bladder. In case of an accreta: loss of continuity of myometer/placenta interface, extension of T2 hypersignal placental tissue to adja cent organs, including bladder. The uterine/bladder serous interface is thinner or interrupted.
• Interruption of the hyperechogenic zone between uterine serum and bladder wall. The boundary between the bladder and the myometer is normally hyperechogenic and smooth.
Anomalies of vascularization in Doppler (C) mode
• unusual aspect of the vascularization of the subchorial, very dense, with vessels perpendicular to the chorial plate.
• Typically, the vascular network runs along the choral plate and a few vessels dive between the cotyledons in the placenta. In the case of a placental insertion anomaly, this network is anarchic. The abnormal vessels are perpendicular to the chorium plate and sometimes run full-channel into the large gaps.
Pseudo-tumor (D) global aspect
• An abnormal forward spraying of the placenta next to the lower segment compared to the usual myometric curvature.
• Pseudo-tumor aspect, very heterogeneous.
Case 1. Normal placenta. Sagittal Cup. Few gaps, few bands. Typical vascular aspect. Qualitative score: low risk. Final diagnosis: placenta non accreta.
Case 2. Placenta accreta. Right lateral axial cut. Large irregular gaps and bands. Significant backward vascularization with perpendicular vessels throwing full-channel in the gaps. Qualitative score: significant risk.
Case 3. Another placenta accreta. Sagittal Cup. Place, your irregular and numerous shortcomings, gangs. Vascularization very dense in front of the suspect area. Qualitative score: significant risk.
The score
We are working on a semiological fusion score, including already recognized signs of ultrasound and MRI. By grouping them into categories and weighting them according to their reliability, we hope to improve the sensitivity and diagnostic specificity of placental imaging. We first propose the following quantitative score:
- Association of synchronous signs A + B: significant risk.
- A or B + C or D: moderate risk.
- Neither A nor B: low risk.
A prospective study of this qualitative fusion score and a quantitative score, by the teams of Bicêtre-Créteil, will be published shortly. Intermediate results are encouraging.
Conclusion
Ultimately, fusion ultrasound is an imaging that has few technical constraints and is easy to perform. It offers a dual ultrasonic semiology and synchronized MRI, focused on areas of interest. The grouping of signs into 4 categories makes it possible to systematize the semiology of the placenta accreta more systematically. The fusion ultrasound is also a valuable help in the initial and continuing formation of the imagers.
The contribution to the positive diagnosis of placenta accreta remains to be assessed on a cohort large enough to obtain statistical reliability of diagnostic scores.
Patients whose location and/or appearance of the placenta makes an accreta suspect should respond to a specific treatment adapted.
Article written in collaboration with Mr. GAYET (Kremlin-Bicêtre CHU), Mr. V. SENNAT (Kremlin-Bicêtre CHU) and C. TOUBOUL (Creteil Intercommunal Hospital Center)
Laurence GITZ*, Jean-Marc LEVAILLANT**,***,****
* Centre de l’Avancée, Antony
** CHU Bicêtre, Le Kremlin-Bicêtre
*** Centre Hospitalier Intercommunal de Créteil
**** CEFFE, Créteil
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