Tag Archives: physics

What the Heck 3

The-Shadow-KnowsSo we are scanning the left thorax in a patient with shortness of breath, in an effort to assess for pleural effusion. The following video was obtained:

The operator correctly noted the presence of a pleural effusion, and a bit of lung tissue can be seen towards the left side of the screen floating in fluid. In addition, there are THREE shadows evident, each from a different source. Can you spot them?

Left pleural effusion

So let’s take these one at a time, with labels:

Left pleural effusion

Shadow A

Is the easiest one. It extends almost from the first pixel at the top of the screen down to the far field. We can’t even see the characteristic echotexture of skin or subcutaneous tissue in the near field. There’s no contact here between the transducer and skin, possibly due to:

  • the probe not touching at all
  • clothing or an EKG lead getting in the way
  • not enough gel (the novice’s answer to everything but sometimes still true)

Shadow B

The most interesting one of the bunch. Probably two major factors at work here. First, this section of diaphragm is a particularly bright reflector so it can create a shadow behind it due to the sheer amount of reflection occurring. Second, the density difference between the diaphragm and pleural effusion is creating a refraction artifact, often referred to as an edge artifact. Beams of sound which were roughly parallel as they struck this interface get bent at different angles based on whether they hit the dense diaphragm or the less dense fluid. The space in between the formerly tightly spaced beams is displayed as blackness, or the absence of returning echoes.

Shadow C

That’s a rib shadow. Did you know that ribs grow back if you remove them?


Angles for Doppler

A prior post discussed the optimal imaging angle for 2D scanning.

Quick quiz: what is that angle?
45 degrees
90 degrees
180 degrees
360 degrees

In this post we’ll illustrate the optimal imaging angle for Doppler evaluation. Let’s start with basic Doppler physics.
Where to police officers situate themselves to aim a radar gun at speeding cars?

The maximal Doppler shift will be seen at 180 degrees. In fact at the instant the car passes the officer, (90 degrees) there will be zero Doppler shift. At that instant there is no movement between the object and the listener. So they aim the gun directly at the oncoming traffic, so the direction of their beam is parallel to the direction of [traffic] flow.

The image below illustrates Doppler shift of ultrasound reflected off a red blood cell:

  1. Top: A normal ultrasound wave
  2. Middle: Doppler shift reflected off the RBC moving toward the transducer (thus increasing the frequency of the returning wave)
  3. Bottom: Doppler shift reflected off the RBC moving away from the transducer (thus decreasing the frequency of the returning wave).

Thanks to equipmentexplained.com for the image. Imaging at 180 degrees is impractical for diagnostic ultrasound, since the optimal B-mode imaging angle is 90 degrees. Therefore, most authorities recommend an imaging angle between 45-60 degrees for Doppler ultrasound imaging . If you are imaging a vascular structure at 90 degrees and getting no Doppler signal, try lowering your angle.


microbubbles form behind propeller, courtesy of U.S. Navy

Rounding out our recent trifecta of biosafety posts is a description of cavitation. Cavitation is the formation of microbubbles in liquid which has been subjected to rapid pressure changes. This can happen from a variety of causes from beating Dolphin tails, propellers, cracking your knuckles, and with ultrasound. The Mechanical Index is used to represent the risk of cavitation in tissue during ultrasound evaluation, though most authorities do not think cavitation occurs in the normal operating parameters of diagnostic ultrasound.

During rarefaction (the low pressure portion of the ultrasound pressure wave) air-filled structures expand. They then quickly contract again during the remaining phases of the sound wave. Cavitation is deliberately employed in lithotrypsy, as well as non-medical applications such as metal cleaning.

According to Wikipedia:

The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature.

So there are two major bioeffects of ultrsound: Heat and cavitation. The risks of either are vanishingly small with normal diagnostic ultrasound use. No studies have demonstrated any ill effects of diagnostic ultrasound in humans or even fetuses. But understanding these processes at least helps us recognize the issues behind bioeffect concerns.

Mechanical Index

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


Thermal Index

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:


Finding the right angle is critical to optimal imaging. In fact ‘right angle’ or perpendicular imaging is the best way to get a clear image. At 90 degrees, many more sound beams reflect back to the transducer than at more shallow angles.

In addition, the ultrasound energy is more spread out when it connects to the tissue at an angle, as seen above.
In this image of the kidney, notice the inferior aspect of the kidney (right arrow) is imaged at nearly 90 degrees. The white lines represent the plane of the kidney.

It has the sharpest border and is well-distinguished from the liver. The middle arrow represents the path of ultrasound energy hitting the the kidney off 90 degrees.  Not a bad image but doesn’t look as good as the one imaged at 90 degrees. Finally, the left arrow represents the beam hitting the kidney almost parallel. Note that the kidney-liver interface looks fuzzy and there is a great loss of detail. Most of the ultrasound energy is reflecting off the surface AWAY from the transducer- hardly any is available to reflect back towards the transducer and yield a good image.

Thus, angling the probe 90 degrees to the structure you want to image can increase resolution and improve your image quality.