MI & TI & Ultrasound Bioeffects


OBGYN.net Conference CoverageFrom 9th World Congress On Ultrasound in Obstetrics & Gynecology, November, 1999 - Buenos Aires, Argentina

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Terry DuBose: "This is Terry DuBose at the 9th Congress for Ultrasound in Obstetrics and Gynecology in Buenos Aires, Argentina. I have Dr. Jacques Abramowicz with me who's been active in societies and committees dealing with bioeffects and safety in the advancement of ultrasound, and the safe use of it. I know you're speaking tomorrow on new indices for control of the equipment and trying to advance the bioeffects, or the safety. Kind of introduce yourself, Dr. Abramowicz, and tell us something about what we're doing, some current developments..."

Dr. Jacques Abramowicz: "I'm Dr. Jacques Abramowicz. I'm a professor of Obstetrics, Gynecology, Radiology, and Biomedical Engineering at the University of Rochester. I'm Director of Obstetrical and Gynecological Ultrasound there.

If you look at any of your new machines, the monitors of any machine that you've bought recently, you'll see in one of the corners two letters - MI and TI - and most people don't really pay attention to what that is. These are two indices that were introduced relatively recently in the last few years under pressure from the FDA.

MI stands for Mechanical Index, TI stands for Thermal Index, and these are the two major potential bioeffects of ultrasound. Ultrasound is a form of energy. We know that it can heat tissues under certain conditions, and we know that it can cause bubbles, if they are present in the tissue, to actually explode or implode, thereby releasing huge amounts of energy in very, very tiny places. The reason why this has appeared now on the monitors of the machines is because in 1985, the FDA had put some limitations on the energy that one can use, and the famous 94-milliwatt per centimeter square was born in 1985 for fetal ultrasound. Now machines can go much higher, but that was the limit for fetal ultrasound. Manufacturers came along and said, 'you know we really need to have higher powers because as you know, increasing frequency gives you better resolution but much less penetration.' They wanted more penetration and better resolution, and the FDA said, 'fine, we'll allow you to go higher on the condition that these indices, which are signs of what ultrasound can do, appear on screen,' and that's how the output display indices were born. Because of that, machines today for fetal applications can go as high as 720-milliwatt per centimeter square, which is eight times higher than the limits of a few years ago. Therefore, one has to be conscious of what ultrasound can do.

Now as far as we know, ultrasound has never been shown to cause any bad, harmful effects to the fetus in the human. It has been shown to cause lung hemorrhage and bowel hemorrhage in animals. It has also been shown to cause problems in growth, but never in humans. There's no epidemiological study, which is the best type of study, that has shown it is at all dangerous to humans. A lot of things have been looked at - growth, growth after birth, and cancer in childhood. None have been shown to be caused by ultrasound, the only one that seems to be - withstanding examination - is non-right-handedness. There were several studies showing that infants of mothers who had ultrasound were more often not right-handed. There's really only one study that potentially shows that, and that shows that only in boys with some statistical significance, not in girls, and certainly the significance of that is very unclear. In fact, I've been to meetings where people mention 'non-right-handedness and other central nervous system problems,' and I always raise my hand in those cases, and say, 'you know, I resent that because I'm left handed.'"

Terry DuBose:: "I am too."

Dr. Jacques Abramowicz: "The important point that I want to make is that the end-user, whether it's a physician or a sonographer, must know what TI and MI stand for. In your machines, if you do some manipulation, particularly in Doppler and particularly in color, all of a sudden you see TI 3.4, or 3.7 - what does TI stand for? TI, fortunately, is equal to... it's a unitless number, but it is equal to the number of degrees centigrade that can be obtained or how much the temperature can be elevated at the focus of the ultrasound. Therefore, if you see TI 3.7 that means that at the focus of the ultrasound, the temperature could potentially increase by 3.7 degrees centigrade. Now, if you take a normal patient with a temperature of 37, the fetus usually has a temperature of half a degree centigrade higher, particularly in the third trimester. So now you have a fetus with a temperature of 37.3, if you raise it by 3.7 degrees you're already at 40 degrees. If the mother has the flu - and most of us don't ask the mother if she has a temperature - she has a temperature of maybe 39 when you do the ultrasound, and the fetus is at 39.5. You raise the temperature by 3 degrees if you're looking at the internal cerebral artery, the medial cerebral artery, and now you're potentially raising this fetus' brain temperature to 42 or 43 degrees. Now I say 'potentially' because babies are moving, mothers are moving, even if you are a superb sonographer or sonologist, your hand is moving, and there's heat dissipation by movement of blood, etc., so the actual risk is negligible. We're speculating all the time about worst case scenarios, and these numbers were obtained in the lab with a fixed transducer or no circulation, so this certainly will not happen in humans. But we have to be aware."

Terry DuBose: "Yes, you made a point of superb sonographers and sonologists. I think we all agree that that's one of the best ways of reducing the bioeffects - to know what you're doing, to get your exam and the information quickly - and lab accreditation is one of those. Don't you think?"

Dr. Jacques Abramowicz: "Absolutely, absolutely. A good sonographer or sonologist will take less time to do the exam, will certainly respect the ALARA principle, which is very important from that standpoint and, therefore, even if there are some potential effects, I think that they are greatly reduced. The only issues in the United States, because of fetal lawsuits, is that you need to make sure when you give a patient a picture, the TI in the corner is not 3 or 4, as I have seen several times. In fact, I have seen 6 or 7 and, again, even though the risk is minimal or probably negligible, or probably absent, you don't want someone to come back to you with a picture saying, 'when you were examining my fetus, you raised the temperature by 6 or 7 degrees centigrade!' We certainly want to avoid that."

Terry DuBose: "There was a report fairly recently from Edinburgh that we spoke about earlier... could you talk about that for a moment?"

Dr. Jacques Abramowicz: "Anything that the media can grab that can potentially capture the attention of the public is always questionable, and this is a study that had been done in mice. The study has never been published in an official journal, and it has not been through peer review. In some mice under certain conditions, some cells in the bowel were found to be necrosed and to be dead. There's certainly no proof as far as we can see that this was due to ultrasound, and this has not been reproduced. Therefore, I think that until we see that in a peer review journal, there's really no reason to be alarmed by this publication."

Terry DuBose: "Great."

Dr. Jacques Abramowicz: "The MI - the Mechanical Index - probably has even less risk in the fetus, since you need to have a focus that can cavitate or resonate or expand and contract under ultrasound. And since there are no bubbles of air in the fetus - there's no air in the lungs, of course, and there's probably no air in the bowels - there's no risk as far as we know from the Mechanical Index. It still is important to have it on the screen, but if anything, the Thermal Index is probably a little more important, in the case of the fetus."

Terry DuBose: "So the MI doesn't relate to the dissolved gases as much as free gas?"

Dr. Jacques Abramowicz: "No, you must have a focus. You must have a bubble, whether it's natural or introduced, in the case of contrast media, which so far in obstetrics have not found any use. You must have a focus to resonate to get that effect."

Terry DuBose: "Do you know if the MI is that great of a concern when we use contrast media?"

Dr. Jacques Abramowicz: "It potentially is. We've actually done a very interesting study where we've looked at destruction of red blood cells in the presence of contrast media. We've used fetal red blood cells and adult red blood cells and have shown unequivocally that the fetal red blood cells are destroyed much more than adult red blood cells in the presence of contrast media under lab conditions. So there's no doubt that contrast media can cause an ultrasound effect to become enhanced, as opposed to ultrasound with no contrast media. Again, at the moment there's no obstetrical indication. I don't know of any manufacturer of contrast agents that is looking for FDA approval for the use of their agents in obstetrics."

Terry DuBose: "Yes, I'm sure that's way, way down the road."

Dr. Jacques Abramowicz: "Right."

Terry DuBose: "Though it might be beneficial if they can find a way on the maternal side, maybe of the placenta."

Dr. Jacques Abramowicz: "That's right."

Terry DuBose: "Very good, I appreciate your time."

Dr. Jacques Abramowicz: "My pleasure."

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