Supplementary Material for: Mirror Artifacts in Obstetric Ultrasound: Case Presentation of a Ghost Twin during the Second-Trimester Ultrasound Scan

Mirror artifacts are produced by the reflection of ultrasound waves after they propagate through a structure and encounter a strong and smooth interface capable of acting as a mirror. Ultrasound waves bounce back and forth between the mirroring interface and the reflective object and then eventually return to the transducer. The typical display of the mirror artifact consists of two similar structures separated and at similar distances from the reflective interface. We report a mirror artifact in a patient with a singleton gestation at 18 weeks. The image was interpreted as consistent with a twin gestation using transabdominal and transvaginal ultrasound. The differential diagnosis consisted of an abdominal heterotopic pregnancy. The presence of synchronized but opposite movements of both fetuses, and the blurred image of the second fetus, suggested a mirror artifact. The reflective surface was created by the interface located between a distended rectosigmoid filled with gas and the posterior uterine wall. Mirror artifacts can lead to diagnostic errors. This case illustrates how a distended rectosigmoid colon can generate an image that simulates either a twin gestation or an abdominal heterotopic pregnancy.

镜像伪影（mirror artifact）系超声波穿过某一结构后，遇到具备镜面反射能力的强回声光滑界面时发生反射所产生。超声波在该镜面界面与反射目标之间来回反射，最终返回超声换能器（transducer）。镜像伪影的典型影像学表现为存在两个相似结构，二者与反射界面的间距相近且彼此分离。本文报告1例孕18周单胎妊娠（singleton gestation）患者出现的镜像伪影：经腹超声（transabdominal ultrasound）与经阴道超声（transvaginal ultrasound）最初将该图像误诊为双胎妊娠（twin gestation），当时的鉴别诊断包括腹部异位妊娠（abdominal heterotopic pregnancy）。观察到两个“胎儿”的运动同步但方向相反，且第二“胎儿”图像模糊，提示为镜像伪影。该反射界面由充气扩张的直肠乙状结肠（rectosigmoid）与子宫后壁之间的组织界面形成。镜像伪影可导致诊断失误，本例旨在说明充气扩张的直肠乙状结肠可生成模拟双胎妊娠或腹部异位妊娠的影像学假象。