Ultrasound probes. Why are there different kinds of them and how do they work? One of the more perplexing aspects for newcomers to ultrasound is often the probes. Isn’t there an “abdominal probe” and a “cardiac probe”, so why don’t they make an “EMS probe?” Today we will talk about the ultrasound probe and hopefully answer some your questions on the journey towards prehospital ultrasound nirvana.
The ultrasound probe (technically called the transducer) is the part of the machine that both generates and receives the ultrasound waves. It does this using piezoelectric crystals, similar to what’s in a quartz watch. When electricity is applied the crystals generate sound that travels into the patient where it bounces off of their internal structures. On return the sound waves impact the crystal array creating a change in energy that is displayed as various shades of gray on the machine’s display. A lot of the image quality has to do with various timing tricks and post-processing but the true heart of the ultrasound machine is that crystal array at the tip of the probe.
The original ultrasound probes had just one crystal that needed to be manually swept over the patient to build up an image line by line over time. You can think of this as something akin to finding your way in the dark using a laser pointer. If you wanted to image something that was moving, like the heart, you had to quickly move the probe back and forth to paint the picture. As you can imagine this was cumbersome to work with so they eventually submerged the probe head in oil and hooked it up to an electric motor that swung it back and forth for you. This worked pretty well but its biggest drawback was that it was fragile. Not only was it a mechanical moving head in an oil bath, it only had one crystal, so if that one crystal went out you lost everything.
Modern probes are made of many crystals aligned in groups called arrays. The beam is electronically swept across the array to create the image in near real time so there is no need for a spinning oil filled head. The crystals themselves can still be somewhat sensitive but there are many of them so if a crystal breaks you may get a dead line on the screen but the remainders still work. The physical shape and layout of these arrays dictates the functions and capabilities of the probe. There are nearly limitless design possibilities for these different arrays so we will focus on the four most common types.
The phased array probe has a small square head and is often referred to as the “cardiac probe”. This is really a misnomer as for the most part the probe differences have more to do with their physical properties than their intended uses. The phased array probe uses a complex electronic steering mechanism to sweep the image and create a real-time display of a large area even though it has a small physical footprint. This is probably why the name cardiac probe came up because this probe is especially well suited to fit between ribs in the intercostal spaces and then fan back out to image a large area. The phased array probe is also well suited for doppler imaging which calculates the direction and intensity of flow. In EMS the phased array probe is probably the most versatile probe we have available. It is well suited for cardiac and abdominal imaging, it can even do sliding lung sign imaging for pneumothorax detection if you set the depth shallow enough and is the only probe capable of transcranial doppler ultrasound (if the rest of your equipment is capable of doing this as well). If I could only afford one probe, it would be this one.
The curved array probe has a large curved head and is often referred to as the “abdominal probe.” This again is a misnomer as it is capable of doing much more. The curved array probe is actually an older design than the phased array probe and consists of many crystals laid along a curved line. This provides for a slightly better quality image than the phased array probe. The only drawback is the large head size will not fit into the intercostal spaces between the ribs. This may not necessarily be a bad thing, but the image does look different on a curved probe than it does on a phased. Also, there are some issues with the layout not allowing for as much doppler control as the phased array probe. You can still use the curved array for sliding lung sign if you turn the depth down shallow just like the phased array. I personally like the curved probe for both cardiac and abdominal imaging and I find that while the image quality gap between phased and curved has narrowed in the newer generation of machines, in older machines the curved array provides a significantly higher image quality. If you have so much money you don’t know what to do with, then buy a phased and curved. However I think for the most part the phased array will do fine and there is no need to buy both in EMS.
The linear array has a thin head and is used for starting IV’s and doing anesthesia blocks. Due to its high frequency it excels at shallow depth, high quality imaging. Because of this it is also often used for sliding lung sign for pneumothorax. There are many physical shapes to this type of probe, straight and “hockey stick” are two of the most common. This probe is invaluable as it is the only probe capable of starting IV’s. You cannot start an IV with the curved or phased array probe, only the linear array probe. There are even some manufacturers that add special needle guides or needle tracking technology to assist you in placing or visualizing the needle under the skin. Because of the importance of ultrasound guided IV’s I feel it should be very high priority to get a linear array probe along with your phased array probe.
This is a very small curved array or convex array probe on a stick designed to image through the vagina. It is sometimes used in emergency medicine to detect early pregnancy, ectopic pregnancy and more. It has absolutely no place on an ambulance and is a lawsuit waiting to happen.
One thing all probes share in common is an orientation marker. This marker helps you determine left vs right. If you have the probe backwards you are viewing a flipped image of the patient. This is more often a problem with IV placement than anything else because if you have it backwards when you think you’re to the left of the vein, you’re actually to the right. Some people don’t bother using these markers and prefer tricks like touching one side of the probe down to see where it shows up on the screen, but I feel it’s a good practice to get into to spot the marker. The best way to keep your orientation down is to line up the orientation marker on the probe with the orientation marker on the screen.
Hopefully this is a good little primer into ultrasound probes. There are lots of them but these are the main ones to know about for emergency medicine and EMS usage. See you next time!