Effect of ultrasound transducer frequency on the appearance of the fetal bowel

We evaluated how ultrasound transducer frequency affected the appearance of the fetal bowel. One hundred women with singleton pregnancies, who were undergoing routine ultrasonographic examination, were assessed at a single institution. Patients with known fetal anomalies, abnormal biochemical screening results, or a history of cystic fibrosis were excluded. Images of the fetal abdomen were obtained in all patients using a single multi-Hertz transducer, with transducer frequencies set at 5 MHz and 8 MHz. Images were read separately by two radiologists, blinded to patient name and transducer frequency. Observers rated the presence or absence of echogenic bowel, defined as bowel with echogenicity greater than or equal to that of adjacent bone. Using the 8 MHz frequency, the radiologists interpreted 31% of the cases as having echogenic bowel, whereas using the 5 MHz frequency, the radiologists interpreted only 3% of the cases as having echogenic bowel (P<0.0001). A fetus was 10 times as likely to be given a diagnosis of echogenic bowel by both observers when the 8 MHz transducer was used than when the 5 MHz transducer was used by one observer (relative risk 10, 95% CI 3-11). Furthermore, using the 8 MHz frequency transducer, at least one of the radiologists interpreted echogenic bowel in 62% of the cases. We concluded that echogenic fetal bowel is a very common observation when imaging is performed with an 8 MHz transducer, and thus echogenic bowel diagnosed with an 8 MHz transducer is unlikely to reflect underlying abnormality. Identification of echogenic bowel with an 8 MHz transducer should not prompt further testing.