Sonography during early pregnancy: Dependence of threshold and discriminatory values on transvaginal transducer frequency

OBJECTIVE, Our goal was to determine if normal and abnormal pregnancies cou ld be distinguished at smaller sac sizes with a higher frequency transvagin al transducer than with a 5-MHz transducer. SUBJECTS AND MATERIALS. Thirty-nine patients with potentially abnormal preg nancies identified with a 5-MHz transvaginal transducer were immediately re imaged with a 9-5-MHz transducer. We compared our ability to visualize the yolk sac, embryo, and cardiac activity relative to mean sac diameter on ima ging at both frequencies in women with normal and abnormal pregnancies. RESULTS. Of the 39 pregnancies, 22 (56%) were normal or probably normal. Us ing the 5-MHz transducer, a yolk sac was first seen in a 6.4-mm gestational sac but was not definitively seen in 12 gestational sacs measuring 5-13 mm . Using the 9-5-MHz transducer, yolk sacs were identified in all gestationa l sacs measuring 4.6-13 mm, and live embryos were seen in five of eight sac s measuring 8.1-13 mm. The largest normal gestational sac without a live em bryo measured 11 mm. When we compared these pregnancies with 17 (44%) abnor mal pregnancies, we found that all pregnancies that had no yolk sac by the time the gestational sac measured 5.0 mm or no live embryo by 13 mm had abn ormal findings on higher frequency imaging. CONCLUSION. The ability to visualize the yolk sac and embryo in early pregn ancy is critically dependent on transvaginal transducer frequency. Threshol d values and discriminatory sizes used to distinguish normal and abnormal p regnancies are smaller on higher frequency than on lower frequency imaging and, therefore, should be redetermined for specific transducer frequencies.