Transabdominal and transvaginal ultrasonography remain the imaging modalities of choice in detecting placenta accreta. In women with a history of cesarean delivery, second-trimester sonographic evaluation of placenta accreta may be helpful. The location of the previous cesarean section should be closely examined, because the placenta has a tendency to adhere to this area.
MRI is commonly used when ultrasonography remains uncertain. Although ultrasonography and MRI provide a high degree of suspicion for placenta accreta, no imaging study can diagnose placenta accreta with absolute accuracy. This has been demonstrated in numerous studies that have compared the imaging findings of placenta accreta with the pathologic evaluation. Sonographic findings that have a high index of suspicion for placenta accreta include, but are not limited to, loss of the hypoechoic retroplacental myometrial zone; thinned, hyperechoic uterine/bladder interface; presence of placental lacunae; and focal exophytic masses within the bladder.
Color Doppler imaging also aids in the evaluation of placenta accreta. The use of color Doppler has improved the sensitivity of gray-scale ultrasonography because it depicts the local vascular anatomy within the uterus and related organs.
MRI has come to the forefront in the evaluation of placenta accreta. MRI findings that are suggestive of placenta accreta include uterine bulging, heterogeneous signal intensity within the placenta, and the presence of dark intraplacental bands on T2-weighted imaging.
Studies have compared the accuracy of MRI and ultrasonography in the diagnosis of placenta accreta. One study found that ultrasonography had a sensitivity of 77% and a specificity of 96%. In comparison, MRI had a sensitivity of 88% and a specificity of 100%. The superiority of MRI over ultrasonography, however, was not statistically significant. In this study, all patients with ultrasonographic examinations suggestive of placenta accreta underwent MRI. If placenta accreta was suspected, the patient also underwent a dynamic gadolinium-enhanced MRI series. This confirmed the presence of deep invasion.
Palacios Jaraquemada and Bruno studied 300 patients with suspected placenta accreta using gadolinium contrast in an attempt to classify the depth and topographic areas in relation to the posterior bladder wall. The MRI scans were compared with pathologic findings. The results of this study, however, were not used to further define the screening characteristics of placenta accreta, but rather to establish a modified surgical technique. The aim was to reduce complications at the time of surgery. Because gadolinium crosses the placenta and its half-life and safety profile in pregnancy have not been established, the American College of Radiology recommends its use only if the benefits outweigh the potential risks.
Kim and Narra defined the appearance of the placental myometrial interface using the half-Fourier acquisition single-shot turbo spin-echo (HASTE) sequence. The HASTE sequence has three layers: an inner low-signal-intensity layer, a middle high-signal-intensity layer, and an outer low-signal-intensity layer. In placenta accreta, focal nonvisualization of the inner layer was noted. The limitations of this study were that only five patients were included, all of whom had prior cesarean deliveries, and no comparative studies could be made.
Because a limited amount of literature describes the normal anatomy of this area with the use of fast sequences, MRI remains only an adjuvant to inconclusive ultrasonography findings or can be used to evaluate placenta accreta when adherence occurs in the posterior or fundal portions of the uterus.[1,2,3]
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Cite this: Craig Johnson, Zachary Redus, Michael Mader, et. al. A 30-Year-Old Woman With an Abnormal Fetal Ultrasound - Medscape - Apr 25, 2017.