Mount Sinai Emergency Medicine Ultrasound

What does the MI on the sidebar of the ultrasound machine screen stand for?
The Mechanical Index is a safety metric which lets the operator know how much energy is being transmitted into the patient during sonography. Remember that sound is created by pressure waves,  so mechanical energy is transmitted into any object which receives sound. Sound waves can be quite powerful- remember we use them to disintegrate kidney stones and to clean jewelry. And not vice versa. So best to make sure that you are using the lowest power possible, or As Low As Reasonably Achievable, for diagnostic imaging.

Back to the Mechanical Index. It is defined as the peak negative pressure (PNP) of the ultrasound wave (point of maximal rarefaction) measured in milliPascals divided by the square root of the center frequency (Fc)of the ultrasound wave. Not a very complicated equation, once you know the components:

What the heck is this? Think pressure change divided by time. Lots of pressure change over short periods of time can be damaging. Dr. David Toms, who writes www.fetalultrasoundsafety.net puts this into perspective very nicely. Imagine a MI of 1 in a system using a 4 MHz probe. Pretty typical parameters. That would mean a peak negative pressure of 2 MPa. According to Dr. Toms:

The corresponding positive side of the ultrasound wave would be similar in the other direction, giving an overall pressure difference within half of a 4MHz cycle of 4 MPa, equivalent to being submerged or brought up from 400 metres (1300 feet or ¼ mile) underwater in 1/8 of a microsecond.  Although the 1/8 microsecond in which this 400 metre movement would occur makes the analogy impossible – it would be 10 times the speed of light – the point is to emphasize that pressure fluctuations within the ultrasound pulse are large, rapid and far from intuitively trivial.

The FDA has established a maximum MI of 1.9 for diagnostic imaging. Any machine capable of generating MI greater than 1.0 must display the MI onscreen. The FDA MI limit for obstetric sonography is 1.0.

How does this this affect care in the acute setting?

• Keep scan times to a minimum
• Avoid using pulsed wave Doppler or color flow through the fetus for determination of fetal heart rate
• Use M-Mode instead
• Use Tissue Harmonic Imaging (THI) only when necessary, not as a default setting

What does the TI on the sidebar of the ultrasound display stand for?

Thermal Index (TI) is a biosafety metric used to describe the potential of the ultrasound beam to raise temperature in the path of the beam. It is the ratio of the power used by the machine to the power required to raise tissue temperature by one degree Celsius.  It does not reflect an actual temperature change, and does not correlate with absolute numbers. A TI of 2 is double the output power but does NOT mean a 2-degree Celcius temperature rise.

How much temperature rise is acceptable? According to the AIUM:

For exposure durations up to 50 hours, there have been no significant, adverse biological effects observed due to temperature increases less than or equal to 2°C above normal.

The British Medical Ultrasound Society has great guidelines for the safe use of diagnostic ultrasound equipment which include this graphic:

The subxiphoid four chamber view is commonly used in cardiac assessments and the FAST exam and for many is the initial “go-to” view of the heart. Difficulty obtaining this window can frustrate novice and seasoned operators, and there are a few tips which can help optimize the view.

1. It’s called SUB-xiphoid for a reason. Don’t jam the probe up against the xiphoid process. Imaging through bone is difficult, the patient will be in pain, and the angle is too steep. Instead, place the probe a few centimeters south of the xiphoid process and work up from there.
2. Get a good view of the liver, THEN use that to get a good view of the heart. You may find that starting to the patient’s right of midline gives a better liver window, since the stomach tends to obscure the subxiphoid view as you go further left.

This video illustrates the huge difference that left vs. right can make. It was taken with the probe in a midline subxiphoid position. Starting with the probe angled towards the patient’s left, the entire screen is obscured by gas in the stomach. As the operator changes the angle towards the patient’s right, we see the liver come into view. This yields an excellent window through which the heart can be visualized.

The figure below, taken from the midpoint of the video, illustrates the point a bit more clearly. To the right of the green line (patient left), superficial stomach gas (arrow) obscures everything behind it, creating a terrible view. On the other side of the green line, liver (L) is visualized which creates a good window for viewing the heart behind it.

On April 18 the FDA released an alert regarding Other-Sonic Generic Ultrasound Transmission Gel, manufactured by Pharmaceutical Innovations Inc. The ultrasound gel was found to be contaminated with Pseudomonas aeruginosa and Klebsiella oxytoca.

According to the FDA Press Announcement,

U.S. Marshals, acting at the request of the Food and Drug Administration, have seized Other-Sonic Generic Ultrasound Transmission Gel located at Pharmaceutical Innovations Inc. in Newark, N.J., after an FDA analysis found that product samples contained dangerous bacteria. The seizure included all lots of the gel product manufactured between June 2011 and December 2011….The FDA received a report involving 16 surgical patients infected with Pseudomonas aeruginosa. The patients had transesophageal ultrasound procedures, while undergoing heart valve replacement, using Other-Sonic Generic Ultrasound Transmission Gel.

Yes, that first line said “U.S. Marshals.” The FDA does not mess around. So take a minute and check your gel! Maybe a good time to wipe the whole machine down while you are at it.

We are pleased to present our annual critical care ultrasound pre-conference course at the Clinical Decision Making in Emergency Medicine symposium in Ponte Vedra, Florida on Wednesday, June 20. Each year this intensive, hands on course features ultrasound faculty from across the country working in small groups with live models and plenty of hands-on scanning time.

The course is held at the beautiful and historic Ponte Vedra Inn and Club.

Please visit here for Registration information

Highlights of the four-hour course include:

1. Cardiac ultrasound
2. Thoracic ultrasound
3. Ultrasound for venous access
4. Assessment of the hypotensive patient

Faculty for this year’s course include:

• Bret Nelson, MD, RDMS (course director)
• Petra Duran, MD
• Joseph Wood, MD, JD, RDMS