Pick A Probe, But Choose Wisely!

Trying to decide which probe to use when ultrasounding your patient?  Curved or straight? High frequency or low frequency?  What's with the phased array?  This may seem like choosing the Holy Grail, but it can actually be very simple if you know what to look for.

Looking at each of the ultrasound probes there are many differences in the shape, size and use, but deep down they are all the same.  Each probe houses the piezoelectric (PZE) crystals which make diagnostic ultrasound possible.  The PZE crystals convert electrical energy to mechanical energy and then back to electrical energy.

Each PZE crystal is attached to an electrical filament.  When an electrical charge goes through the filament, it causes the PZE crystal to vibrate.  The vibration is the sound wave that is sent into the body.  The sound waves strike the tissue and are bounced back to the probe.  The returning vibrations strike the PZE crystal and cause it to vibrate again.  This returning vibration is converted to an electrical current which travels down the filament and is converted to a white dot on the screen.  The intensity (or amplitude) of the returning vibration determines the brightness of the dot on the screen.

Now lets look at the difference between probes.  We can start to differentiate probes based on their frequency:  high vs low frequencies.  High frequency probes give us a lot of detail, but because they give us so much detail,  unfortunately they cannot scan very deep.  Lower frequency probes lack the detail, but can scan farther into the body.  This is due to attenuation, the progressive weakening of a sound wave over distance.  The higher the frequency the quicker it attenuates or loses signal strength, making it useful for short distances only.

An easy way to think about the difference between high and low frequency probes is this:  Imagine you are blindfolded and only able to take three steps.  You can only look at the ground where your foot steps and you must determine the terrain based on those three steps.  Taking three large or wide steps can tell you about the overall path.

"I am walking in a misty meadow"

Taking three steps close together (heel to toe) gives you a detailed view of the path, but only in one small area.

"I am walking over freshly cut grass"

Does this make sense?  High frequency limits the amount of steps we can take from our starting point, which limits are depth of scan.  Although we would like as much detail about what we are scanning as possible, if the area of interest is beyond the depth limit of the probe, then we must sacrifice some detail in order to view the are of interest.

So why all the different probe shapes?  The shape of the probe determines the shape of the image on the screen.  The crystals within each probe are lined up in a straight line along the probe face.  They then send out their sound beams in a straight line.  Thus, a curved face probe creates a pie shaped image, whereas linear probed create a rectangular image.

Well, then you ask, what about the phased array probe?  It's got a flat linear face, yet creates a pie shaped image on the screen, how is that possible?  This is a special probe and an exception due to the fact that the crystals are lined up linearly, but that they alternate in which direction they send out their sound waves.  This alteration of angle causes a pie shaped image, despite the flat face.

So why would it matter to have a pie shaped image versus a rectangle?  The shape of the image is not important, but using the probe most suited for your imaging goal is.  The footprint or the part that touches the patient is important depending on what you are imaging.  Sector probes and phased array probes are all low frequency probes, but large sector probes allow you to see a larger area than smaller sector probes or phased array probes since there is a larger footprint touching the patient.  When scanning the abdomen, larger sector probes can give a larger view of the area in one shot.  However, if you want to get in between ribs, a smaller footprint is more beneficial.

Endocavitary probes, on the other hand, have a small probe face, but the footprint is large proportionally, maximizing its field of view.  They are high frequency probes giving us detailed images of the surrounding area.

Linear probes are also high frequency, but the flat face allows us to maximize the area in contact with the patient's skin.  You could place a central line with the endocavitary probe it you need to, but it's a little awkward!

So then, what of the reverse, how can you tell which probe made which image?  Easy!  You can always tell what the footprint shape of the probe was that created the image by looking at the top of the screen.  If the image begins flat, the probe face was flat.  If the image is curved, the probe face was curved.

I hope you've enjoyed learning some US probe basics and that this will help you to "choose wisely" the next time you need to pick your probe!

Where's the Gallbladder?

The Gallbladder can be a tricky organ to find at times.  Obesity, recent meals, and positioning can make imaging of the gallbladder tough.  Even when you think you found it, it may not be the gallbladder.  So how can you ensure that the object you are looking at is actually the gallbladder?

1.  Use the Kidney:  If you look at the right kidney in a transverse plane while in the RUQ, you should see the gallbladder also in a transverse plane adjacent to it as you fan up to the patient's head. 

When you see the GB in this plane, you can then slide around to the anterior plane, keeping the GB in view.

2.  Roll 'Em:  If you are having difficulty visualizing the GB in the anterior plane due to bowel gas, roll the patient onto their left side and the gallbladder will flip more anterior. 

This is also an important technique if you find stones in the GB.  Rolling the patient into the left lateral decubitus position will help you to assess if the stones are stuck in the neck of the GB or if they are freely movable.  If the stones is impacted that makes acute cholecysitis more likely. 
Normally, stones should move freely in the GB.

3.  Always image it from 2 planes.  Many times what looks like a transverse GB will reveal itself as another structure when visualized in the long axis plane.

•  Bowel:  A transverse image of the transverse colon can look like the GB with stones to the untrained eye.  Note that the gas in the bowel will create a "dirty shadow," which means it will create what looks like a shadow from stones.  The appearance of peristalsis can aid in determining bowel from GB.

•  IVC:  a transverse view of the inferior vena cava can mimic a transverse stone-free GB.  However, if you place your color flow doppler over the GB, it will not show flow.  Also the posterior position of the IVC should steer you away from confusing it with the GB

4.  The invisible GB:  there some conditions that can make it difficult to visualize the GB even when you are looking directly at it!

• Wall Echo Shadow (WES) sign:  can make the GB difficult to see since the majority of the GB is hidden from view within the shadow.  A WES sign will be seen when the GB is almost completely full of stones and there is just a thin rim of the anterior wall visible and then the stones.

• Post-Prandial:  the GB normally contracts after eating, making it difficult to find.  The wall will appear thickened and the internal chamber will appear small.  It's often helpful to ask the patient when the last time they ate was, so that you don't waste your time looking for a contracted GB!

• Emphysematous GB: when the GB is infected with a gas producing organism, gas will appear in the walls, which can make visualization difficult.  However, gas in soft tissue always looks the same (see the post: Visualizing the enemy for more details).  If you see the characteristic "shmear" of air in the GB wall, it is likely due to infection!

Emphysematous GB pics from USCases.info
Emphysematous Gallbladder video

Hopefully these tips will help you to visualize the GB a little easier and not be fooled by other intrabdominal structures that can mimic the gallbladder. 
For more GB images and video, visit:  USCases.info

Air on Ultrasound: Visualizing the Enemy

Hopefully by now you have heard my US mantra:  

Fluid is our friend and air is our enemy!  

So how do we identify the enemy in our midst?  There are times that visualization of gas can be useful: necrotising fasciitis, forniere's gangrene, intrabdominal free air, and emphysematous cholecystitis, to name a few.  All of these diagnoses share the same basic findings on US.
Air on US has a characteristic appearance.  Recall that as a media, it is poor at perpetuating sound in a straight line, but actually diffuses sound waves in all directions.

This is why you can hear a screaming aisle patient throughout the ED!

That scattering of sound waves creates a speckled line as the sound waves hit the tissue/air interface.  The sound beam is then scattered diffusely.  So what you see is the hyperechoic or bright line of the air interface and a gradual fade to black distally.

Notice that there is not a well demarcated shadow posterior to the area of gas as you see with something solid (gallstones).

It looks somewhat like a painter smeared the white line down the screen.  Think Bob Ross, the famous PBS painter creating clouds, "place a little white here and drag your brush down!"

So let's look at the difference between a true acoustic shadow and a shadow created by an air interface.

• Acoustic Shadow:  Sound waves hit the dense object and immediate get bounced back completely, creating a hyperechoic or white line.  Notice that immediately under or distal to the solid object there is a void of sound, which creates the characteristic shadow.

• Gas:  Sound waves hit the air interface and get scattered around.  There is a hyperechoic white line and then a smear of the white distal to the gas.  This smear "shadows" out the surrounding tissue not by creating a void of sound but by obscuring the surrounding tissue with the scattered sound waves.

Examples of gas in the soft tissue:

 Examples of emphysematous cholecystitis:
Emphysematous GB pics from USCases.info
Emphysematous Gallbladder video

All of these examples show the same characteristic pattern of gas in the soft tissues.  Hopefully now you can visualize the diffrence between acoustic shadow and the dirty shadow of gas in the tissue.  Remember: Air is Our Enemy!

Faster FAST

The FAST exam was created as a rapid screening test to detect free fluid in the abdomen, but how can you get the FAST exam done, well, fast?  It seems like it always takes 5 minutes to find the correct views in the trauma center with X-ray breathing down your neck to get in for the CXR.  So how can we get the FAST exam done faster?  Here are 5 tips to make you a faster FASTer!

1.  Find Morrison's pouch without the search:  I have found that the quickest way to identify where to place the probe for your RUQ view consistently every time just requires a quick look at the chest before you even touch the patient. Looking at the chest wall, notice how the ribs crate a horizontal line until a point where they begin to drop off at an angle.

At this point, draw an imaginary line down to the bed.  Place your probe at that point and ...viola, Morrison's Pouch!

2.  Know where LUQ view should be:  so now that you have easily located your RUQ view and Morrison's Pouch, you can use that information to find the LUQ view.  Draw an imaginary line across the patients chest from the RUQ view point to the left coronal plane.

Once you find this point, place your probe one rib space cranial of this point.  Now you should see the LUQ view.

3.  Remember that the kidney are retroperitoneal:  One of the biggest mistakes that I see is that someone will place the probe in the correct location, but will angle the probe anteriorly.  Recall the relationship between the kidney and the major vessels.

Note on this transverse view of the abdomen that the kidney are more posterior than the IVC and Aorta.  So when you angle anterior, you are aiming you sound beam at the major vessels and not the kidney/liver or kidney/spleen junction.

So if you angle your sound beam toward the bed or posterior, then the RUQ or LUQ view comes into view.

The key is angling toward the bed on both sides of the patient.

4.  Pelvic view:  just as angling the probe posteriorly is essential to locating the kidneys, the angle of the sound beam is essential to locating the bladder for the pelvic view.  I always start in the transverse plane when attempting to visualize the bladder.  In this plane the bladder is easier to identify as a square.  Place the probe just superior to the pubic bone.

 Now, angle the sound beam toward the patient's feet.

If you are unable to see the bladder, then increase your angle and the bladder will come into view. 

5.  Subxiphiod view:  When attempting this view, remember that we are scanning structure that are a lot farther from our probe than in the other views.  We are going through the liver, past the diaphragm and the getting to the heart.  So you must increase your depth of scan.

I usually increase the depth to 21cm initially and then adjust the depth to best fit the heart once its in view on the screen. 

Make sure that you have increased the depth to allow for the posterior pericardium to be visualized on the screen.



Not deep enough....



Still need to go deeper...

Finally!!!

By increasing the depth to allow the posterior pericardium in view, you can now assess both cardiac motion and if a pericardial effusion is present. 

I hope these tips can help you to increase both your speed and accuracy when performing the FAST exam!

PHYSICS IS FUN: Tennis Anyone?

So today I want to discuss an important concept in ultrasound and it involves physics.  Yes, I know, you thought that once you took the MCAT and got into medical school, you would be done with physics, but  no!  To truly understand ultrasound, you gotta know a little physics.

Lets start with how different tissue types create different images on the screen.  The sound waves sent out from the probe strike the body and return to the probe.  Depending on what they strike depends on how much or how little of the initial sound beam return.

Lets image that we have a bucket of tennis balls and we are ECC.

If we thrown ten tennis balls at the wall, how many come back to us?

All of them, correct?  This is due to the fact that the wall is dense, just like a gallstone or bone is dense.  In US, those sound waves strike this dense "wall" and cannot penetrate further, just like the tennis balls cannot go through the wall.  Since the solid object blocks the sound waves from penetrating further into the body, there is a void of sound waves posterior to the solid.  This void of sound is called Acoustic Shadowing.

This is a useful property as it can help us determine pathologic states or creates landmarks for us to follow.

So what if we throw our tennis balls at the curtain between resus beds 3 and 4, how many will come back to us?

It depends on the stiffness of the curtain, of course, but a certain amount will continue through the curtain and some will bounce back to us.  This is what occur in soft tissues.  Some sound waves bounce back to the machine and some penetrate through to hit structures underneath.  As with the curtain, the strength to which the tennis balls bounce back varies with the stiffness or density of the curtain.  Same for ultrasound: the amplitude of the returning sound wave varies depending on the density of the soft tissue it strikes.  As the sound waves go through each progressive layer of soft tissue, the amplitude becomes weaker and weaker, until finally there is not any sound left.

This is how we can see different tissue planes on ultrasound and determine the difference between different tissue types.

Now let's image that we take our bucket of tennis balls and throw them down the aisle toward the fishbowl.  How many will return to us?

None, right?  They all go until they hit the bowl or the floor.  This is what happens with liquids.  Very little of the sound beam gets diverted as it goes through the liquid, so the vast majority of the beam hit the structures posterior to the fluid collection.  This is way fluid is so great for ultrasound.  It transmits sound waves to allow us visualization of distal structures, a property called Acoustic Enhancement.

So we have talked about solids, semi-solids, liquids, but what about gases?  Let's go back to the tennis balls.  Imagine if we set up the Bair Hugger hose, shooting air out the end.  Now lets throw out tennis balls through that air current.  What would happen to the tennis balls?

They would go in all directions, right?  We couldn't predict where any of the balls would end up because the air current would disperse them in all directions!  The same is true of ultrasound.  When our sound beam strikes an air interface, the sound beam is scattered in all directions and we cannot see beyond it since the beam is now gone.

The dispersion of sound is what makes us able to hear that fishbowl patient yelling even when we're in Resus.  Although this is an important property for ED management, it is awful for US. 

Well, that's it for today.  I hope you have enjoyed this first ED US Teaching Cast and will take home an important message:  Air is our Enemy and Fluid is our Friend!