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Pulse Intensity
fetal eye vs following birth

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.  Health Canada has followed the FDA example in its regulations (15a).  I have been unable to obtain an explanation for this inconsistency from the relevant experts at Health Canada.  My cynical guess is that the problem will be quietly solved by the regulatory authorities by eliminating the specific limits for ophthalmic ultrasound.

Here is another inconsistency:  Exhortations to observe the ALARA principle in obstetric ultrasound are found in many sources (e.g. 9d, 12, 15b, 17).  There is, however, no regulatory requirement to display onscreen MI values if the system cannot exceed an MI of 1, and no requirement at all to display an MI value below 0.4 (17).  This hampers serious attempts to minimize exposure; in this framework anyone wanting to generate exposures to the fetal eye below the ophthalmic MI limit of 0.23 may not have the onscreen information to do so with any precision. Our Toshiba Aplio does not usually display MI values below 0.4 in obstetric mode, but some other systems do. These regulations probably derive from preoccupation with known mechanisms requiring relatively high intensities such as cavitation and hemorrhage in gas containing organs.  As noted previously, both intuition and some scientific findings suggest the existence of other mechanisms.

Exposure limits are somewhat arbitrary, given the paltry amount of clinical and experimental data, but prudence would seem an important underlying principle.  In view of this, a debate about removing all upper limits (16) strikes me as being of questionable social responsibility.