One regrettable consequence of this website may be increased anxiety in pregnant women who are having sonograms.  I very much doubt that there are any fetal consequences; the purpose here is to encourage nuts-and-bolts prudence by the ultrasound community.  Sadly, this community will not do much on its own and some gentle prodding by patients/clients is the only way that I can see to improve the situation.  Is it not peculiar that medical users tend to be self-righteously critical of commercial fetal viewing facilities because of safety concerns?

Thermal Indices:
Increase in tissue temperature caused by ultrasound was the earliest mechanism considered for potential biological effects.   “The TI gives a relative indication of the potential for temperature increase at a specific point along the ultrasound beam.” (17).   One related index - TIB - assesses the particular potential for heating adjacent to fetal bones.  Reference 17 elaborates on the difficulties with using the TI and related indices.  Only the MI is required to be displayed in B-mode; for Doppler and color-flow imaging the TI is supposed to be displayed as the overall energy intensity is relatively higher.  TI may be displayed with B-mode.  TI/TIB values are low for B-mode imaging, but relatively higher for color and PW Doppler.  Looking at the video clips helps explain why, with markedly increased flow effects reflecting higher energy inputs.

The conventional wisdom is that one can draw considerable reassurance from the fact that B-mode ultrasound is unlikely to cause significant heating (4).  I find high TI/TIB values to be rather disturbing, but do not take much reassurance from relatively low values in view of considerations surrounding mechanical effects previously outlined.  I seem to recall a radiology text I once owned mentioning that a fatal whole-body dose of ionizing radiation would only raise body temperature by 0.001 C. 

Ionizing radiation comparisons:
Some years ago I spent time as a medical advisor (from Prince Edward Island!) to our then Atomic Energy Control Board, and retain an interest in risk evaluation of ionizing radiation.  The most recent overview publication is the BEIR VII document from the (US) National Research Council (23). It notes the absence of genetic effects demonstrable in humans, including an extensive follow-up of 30,000 children of exposed atomic bomb survivors.  Given findings obtained from laboratory studies, genetic risks are very low and “one would not expect to see an excess of adverse hereditary effects in a sample of about 30,000 children” (page 9).  Approximate genetic risks are projected from experimental data (page 12).  While attention has been shifting to radiation-induced cancers, minimizing gonadal exposure in diagnostic examinations remains important.  Gonadal exposure is given a significant weighting factor in calculation of whole-body equivalent doses when studies are confined to a limited part of the body.  So, with the comparatively puny data base for reassurance for fetal ultrasound follow-up, why do users tend to act as if fetal sonography is known to be harmless? The information deficiency is especially true for higher output equipment following the increase in regulatory limits in 1992.  Do we have a double standard?

Detecting the risk of breast cancer induction from ten years of annual mammography as an increase over “natural” cancer risk by patient follow-up would require cohorts of tens of millions of women (24) and decades of observation.  Risk estimates for mammography are derived from higher dose exposures, with extrapolation to the diagnostic range on a linear no-threshold assumption.  This assumption is conservative and reasonable, but not universally accepted.

ALARA in practice:
Easier said than done in terms of downward adjustment of power settings during examinations.  This is especially true when using harmonic imaging; I find that our Toshiba Aplio can only create satisfactory fetal images on harmonics.  With the real-world pressure to get the optimum study in a reasonable time, constant twiddling with the power and gain settings is probably not practical.  Dr. Abramowicz, in response to my question about whether there was downward adjustment of power settings in the observational study of which he was the senior author, replied that “examinations were usually performed with default settings” (4,21,22).

What to do?
My answer: Those who buy equipment for fetal ultrasound should seek out units and probes that will provide satisfactory images on the lowest default MI settings.  A starting point would be to keep MI values below 0.23 to ensure that fetal eye exposure is no more than the regulatory limits following birth.

Not only will this result in minimized exposure, but it will create market pressures to bring intensities down in general.  Additionally, manufacturers should be encouraged to provide an option for automated real-time adjustments within their systems so that intensities will be dropped when the signal path passes through significant amounts of non-attenuating fluid (maternal bladder, amniotic fluid) before reaching the region of interest as identified by the focal zone selected.  This should be a simple exercise for programmers.  I do not want to inject an adversarial element into the experiences of individual patients, but if they politely encourage the user community perhaps some progress can be made about equipment selection.  I have seen no evidence to suggest that any other approach will work.

Who guards the guards?
This ancient question (Quis custodiet ipsos custodes?) applies in surveillance of modern technology.  Evaluation of safety issues in obstetric/fetal ultrasound should be transferred to some organization with no vested interest and not encumbered by the work patterns of government departments.  There should be more representation from younger individuals – at present there are too many of us older folks with baggage.  We also need a lot more research, although where the money for it would come from is uncertain.  Maybe it sounds delusional, but could the Bill and Melinda Gates Foundation spare a few million for this orphan area?

Justification. 
The ALARA principle, as borrowed from radiation protection, has two pillars: justification and optimization.  So far this site has largely addressed the optimization aspect in fetal ultrasound – minimizing exposure in an examination that is considered necessary.  Some space for examining justification of fetal examinations is required.  Most recently this has been brought up by the use of 3D/4D sonography for entertainment and bonding purposes, especially with independent commercial facilities offering this service - discussed in section 8.    

Prudently applied, 3D ultrasound can actually shorten exposure duration compared to conventional 2D imaging by taking five 3D volumes of the fetus and then using them “offline” to generate images by slicing in desired planes (26). “It took a mean time of 1.1 minutes to obtain the 3D volumes…With the standard 2D technique, the structural surveys were done in a mean time of 13.9 minutes”.

Although screening ultrasound is now entrenched practice, the benefits may not be as dramatic as one might think.  A Cochrane review of nine trials showed earlier detection of multiple pregnancies and reduced rate of induction of labor for post-term pregnancy, but no differences for substantive clinical outcomes such as perinatal mortality.  When a search for fetal abnormalities was part of the examination, there were increased numbers of pregnancy terminations for fetal anomalies (27).  At the time that screening ultrasound was becoming popularized I wrote an article (28) hoping to stimulate discussion and debate, but without success.  I have to admit that increasing patient obesity now renders it harder to make reliable clinical assessments.

Detection of fetal anomalies can be therapeutically important, but screening can also cause anxiety and distress when “soft markers” for chromosomal abnormalities or anatomical findings of questionable significance are detected.  One such situation is to encounter mild fullness of the drainage of the fetal kidneys; despite knowing that this finding is almost certainly within normal limits the examiner may decide that it is medicolegally a good idea to suggest a repeat study later in the pregnancy.  This sort of problem has been elegantly summarized in the title of an article “Antenatal diagnosis of renal tract anomalies: has it increased the sum of human happiness?” (29).   

An encouraging development has been prevention of neural tube defects (spina bifida etc) with folic acid before and around conception.  Will there be further progress along these lines?

A disturbing consequence of the ability to identify fetal gender has been termination of female fetuses in some societies; it has been estimated that up to 10 million female fetuses have been aborted in India in the last 20 years (30).  It would seem that fetal ultrasound has ended more life than it has saved.  I am pro-choice with misgivings, and find this to be an illustration of Bouchier’s Columbus Principle: Any new activity will cause more trouble than you can possibly imagine.

Addendum September 2007:  The dismal state of what clinical users know about acoustic intensities, and onscreen displays of these values, has been nicely documented in an article in the March 2007 issue of the Journal of Ultrasound in Medicine.  This was a survey of users, principally obstetricians and technologists.  Only 3.8% could describe the mechanical index (MI) properly and only 20% were aware that acoustic indices are displayed on the sonographic monitor during examinations (33).  65% of respondents were female.