Diagnosis of placenta accreta spectrum using ultra‐high‐frequency probe and Superb Microvascular Imaging

Placenta accreta spectrum (PAS) is an obstetric complication associated with high maternal morbidity. Several prenatal ultrasound signs of PAS have been reported, the main ones being disappearance of the normal uteroplacental interface (clear zone), excessive thinning of the underlying myometrium and vascular changes within the placenta (lacunae) and placental bed (hypervascularity)1, 2. Detection of PAS on ultrasound is dependent upon several factors, such as operator experience, gestational age at assessment, number of abnormal ultrasound findings and degree of placental invasion. Inappropriate antenatal evaluation of PAS may result in missed diagnosis increasing the risk of complications during delivery1. Prenatal diagnosis of PAS has been reported to improve the prognosis of cases associated with hemorrhagic morbidity, by allowing planned treatment in centers with surgical expertise, compared with cases diagnosed intraoperatively with PAS3, 4. However, accurate prenatal diagnosis of PAS is challenging because it is based on indirect findings in the placenta and uterus, and certain pathologies of PAS involve only thin tissue between the placenta and the uterine myometrium which can be difficult to investigate on ultrasound. Advances in ultrasound imaging technology have facilitated antenatal diagnosis of PAS through improved image quality and hemodynamic delineation of small vessels. New ultra-high-frequency probes, which utilize frequencies of up to 18–24 MHz, provide considerably enhanced spatial resolution on both grayscale and Doppler imaging with sufficient penetration. In addition, the new blood-flow imaging technique, Superb Microvascular Imaging (SMI) (Canon Medical Systems, Otawara, Japan), which employs a unique algorithm to minimize motion artifacts by eliminating signals derived from tissue movement, can provide significantly better visualization of low-velocity blood flow in small vessels compared with conventional blood-flow imaging methods such as color and power Doppler5. Here, we demonstrate a case of placenta previa complicated by placenta increta located on the anterior uterine wall with a Cesarean scar, investigated using high-frequency linear probes and SMI. Examinations were performed using an Aplio i800 (Canon Medical Systems) ultrasound machine, equipped with an 18- (PLI-1205BX) or 24-MHz (PLI-2004BX) linear transabdominal transducer (Canon Medical Systems). At 34 gestational weeks, the placenta was seen bulging into the bladder, and presence of placental lacunae was investigated using B-mode. Conventional color Doppler demonstrated enlarged bridging vessels on the bladder wall (Figure S1). Transabdominal ultrasonography, performed using an 18-MHz linear probe, demonstrated clearly absence of uterine myometrium at the site of the Cesarean scar (Figure 1a and Videoclip S1). On the SMI image, dilated villous stem vessels were seen at the same site of the placenta (Figures 2 and S2). These ultrasound findings suggested a diagnosis of placenta increta. Planned Cesarean section was performed at 35 gestational weeks and hysterectomy was carried out 3 days later. The infant was delivered from the fundus of the uterus without placental separation. A 24-MHz linear transducer was used to investigate directly the uterine serosa during Cesarean section, which revealed an obliterated myometrium (Figure 1b, Videoclip S2). Placental separation, bleeding or symptoms of infection were not observed after Cesarean section. Hysterectomy was performed successfully following uterine artery embolization to reduce blood flow to the uterus (Figure S3). SMI was able to detect the decrease in uteroplacental blood flow during the procedure. Although blood flow in the intervillous space was observed even after uterine artery embolization, blood flow in the uterine myometrium was significantly reduced postoperatively (Figure 3, Videoclip S3). We compared prenatal ultrasonographic findings with the pathologic findings after delivery. Prenatal SMI showed small vessels (tertiary stem vessels), which were uniformly small and tapered in the uterine body where there was no adhesion of the placenta (Figure 2a). On the contrary, in the lower uterine segment with PAS, enlarged blunt main and secondary stem vessels were observed, and no tertiary tapered vessels (Figure 2b). Very similar pathologic findings of dilated main stem villous vessel were obtained (Figures 4 and S4). Furthermore, placental imaging of the area around the dilated vessels in the lower uterine segment showed hypoechogenicity and absence of SMI Doppler signals, which indicated avascular or obliterated intervillous space (Figure 2b). At the same site, pathologic findings confirmed infarction of the peripheral villi and absence of the intervillous space probably due to the placental tissue adhering strongly to the myometrium. Thus, the high-frequency probe could not only demonstrate the thin myometrium but also detect the avascularity of the peripheral villous tree and congested stem vessels due to damaged peripheral villous vessels. High-frequency transducers and SMI offer significant clinical benefits for obstetricians, allowing delineation of histological structures and hemodynamics of minute vessels of the placenta. Compared with conventional ultrasound, sonohistological evaluation using this high-resolution probe has the potential to improve the accuracy of ultrasound evaluation of PAS and could facilitate its diagnosis at earlier gestations3, 6 since it can depict small lesions. Accurate prenatal diagnosis of PAS and planned treatment in centers with surgical expertise should help improve the outcome of women with PAS. We are grateful to Ms Mayumi Nieda for helpful adjustments of ultrasound equipment and valuable comments. This study was supported by JSPS KAKENHI Grant Numbers JP15K10688 and JP18K09240. Figures S1–S4 and Videoclips S1–S3 may be found in the online version of this article. Figure S1 Ultrasound images obtained using 8-MHz convex probe in woman with placenta previa complicated by placenta accreta spectrum, showing enlarged bridging vessels on bladder wall. Figure S2 Panoramic view ultrasound images obtained using B-mode and superb microvascular imaging with 24-MHz linear probe, showing absence of placenta accreta spectrum at uterine body and presence of placenta increta at lower uterine segment. Figure S3 Uterus just before hysterectomy (a) and after placental separation except at the site of previous Cesarean scar with placenta increta (arrow) during dorsal approach hysterectomy (b). Figure S4 Superb Microvascular Imaging and macroscopic findings of placenta. Videoclip S1 Transabdominal ultrasound performed using 18-MHz linear probe showing placenta accreta spectrum at site of Cesarean scar. Videoclip S2 Direct investigation of uterine serosa during Cesarean section using 24-MHz linear probe showing placenta accreta spectrum at site of previous Cesarean scar. Videoclip S3 Superb Microvascular Imaging before and after uterine artery embolization. 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