Pulse Intensity: Fetal Eye vs Eye Following Birth
David A. Toms, MD FRCPC
In 1992 the regulatory acoustic intensity limits were raised about 7.5-fold for general imaging, including obstetrical ultrasound, but relatively lower limits were retained for ophthalmic ultrasound. The resulting limits for current equipment complying with the output display standard are described in an FDA document: maximum permissible MI for non-ophthalmic applications of 1.9, but maximum permissible MI for ophthalmic applications of 0.23 (9c). This leads to a bizarre inconsistency – the fetal eye is allowed to be exposed to much higher levels of ultrasound than the eye following birth. Given that an MI of 1 is common in fetal ultrasound this is of some concern, especially with reference to extended exposures of the fetal eye during first trimester screening of nuchal thickness for Down’s syndrome and also in entertainment/bonding ultrasound with lengthy visualization of the fetal face. The pretty 3D pictures one sees of fetal faces, including the eyes, were probably obtained with MI intensities above the regulatory limits for ophthalmic ultrasound.