Transducers and Instrumentation

Published on 06/02/2015 by admin

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Last modified 22/04/2025

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Chapter 3

Transducers and Instrumentation

Christopher J. Gallagher and John C. Sciarra

Piezoelectric Effect

Understanding the piezoelectric effect takes the mystery out of “Just what the hell is that little gizmo at the end of my probe, anyway?”

To make a sound wave, you need to wiggle something.

Bang-a-gong, the metal vibrates, and the sound waves go forth. Now let’s just tie a little creature to the end of a gastroscope, and have him bang-a-gong fast enough to create 7 million cycles/second for 20 minutes straight.

No go. We need a better way to get so much wiggling. The guy banging the gong just won’t do.

Millions of times per second? Better go to electricity, that’s the only thing that can give you that many wiggles per second. But how to get electricity to wiggle something? Electrify a gong?

Piezoelectric crystals to the rescue! These are quartz or ceramic things that have a magical property. When a current is applied to them, the polarized particles align perpendicular to the face of the crystal. When the current goes off, the particles no longer align. This alternating aligning and nonaligning results in the face of the crystal bowing out, then coming back, in effect wobbling just like the gong.

(Who the hell figures this stuff out the very first time, I want to know.)

OK, groovy, so this electrical thing makes a mechanical wave. How does a piezoelectric crystal “hear”?

Well, according to the Principle of Electromechanical Turn-It-Around-ness, when a wave comes into and hits the piezoelectric crystal, it causes a mechanical deformation that then makes a current change. So,

Electricity makes a mechanical wave.

A mechanical wave makes electricity.

Then, through some kind of voodoo known only to electrical engineers and people with pocket protectors, the TEE sorts all this out and gives you an image.

Crystal Thickness and Resonance

A thin crystal resonates at a high frequency (think of a thin wine glass that goes “TING!” when you tap it). A thick crystal (think glass beer stein; better yet, get one and fill it to the top if you’re slogging through this physics junk) resonates at a low frequency. No big shocker there.

Damping

When the signal comes back to the crystal, you don’t want the crystal to wiggle too wildly. Hence, behind the piezoelectric crystal, damping material is in place. The damping material allows a short pulse length, hence improved resolution. Go back to the concept of the ringing gong. After our hero has hit the gong, he doesn’t want it ringing and ringing. He grabs the gong; that allows it to become still, and then he can hit the gong again.

Sound Beam Formation

Electricity in a short burst (typically 1 to 6 microseconds) hits the crystal and produces the short blast of ultrasound by means of the piezoelectric effect. The damping material keeps the crystal from “wiggling” too long, as mentioned above. These short bursts allow better axial (along the direction of the beam) resolution.

(As you can see, the Content Outline of the PTEeXAM chops up the individual items you need to know. In reality, this stuff all flows together in one smooth explanation in the TEE review course syllabus and in Otto’s textbook.)

Focusing

A sound beam tends to spread apart, like ripples in a pond. (I can almost envision a “TEE Haiku” coming out of this.) TEE needs a tight beam to be able to make some split-second measurements of small places, so the transducer focuses the sound beam. A mechanical lens does this.

Axial and Lateral Resolution

(Here again, we’re chopping up stuff that should run together.)

Axial Resolution

Short bursts—that is, high frequency—give you better axial (along the line) resolution. Why? You will have a lot of information bouncing back to you, so you’ll be able to tell, “Aha! This reflected signal tells me something is just right there, and this other signal tells me that something is just a little further along the line.” If this, admittedly weak, explanation doesn’t convince you, then try this line of reasoning: Imagine very infrequent signals going out. How could you tell things are close together then?

Lateral Resolution

Lateral resolution tells you that things at the same depth are side by side.

Beam width at a given depth is the most important determinant of lateral resolution. If the beam is smeared out all over the place, you can’t tell one thing from the other, but if the beam is narrow, you will be able to tell things apart.

Also important in lateral resolution is the focus of the beam. A beam of ultrasound has a near field, then the beam diverges and you have a far field. The focus is best where these two fields meet. Your best lateral resolution is right there, at the focus.

Arrays

To get the vast amount of information necessary for a “movie of the heart,” you could have one “supertransducer” sweeping back and forth. That doesn’t fly, though; instead, modern TEE relies on a bunch of transducers spread out and all looking in the same direction. Some kick-ass mathematics and computer stuff straighten all those signals out.

The most likely transducer you will use is the phased array. So if some know-it-all asks you what kind of transducer you have in your hand, say “a phased array”.

The way it works is in the diagram above. Basically, crystals that are lined up fire in sequence. Where the individual waves meet (summation front) is a point, and this forms a single line or sector. Put a whole bunch of sectors together and you have the pie-shaped image we are so familiar with.

The biggest complication is the hardest to quantify—distraction. I kid thee not, people will glue their eyes to that echo screen and ignore a blood pressure of 60 or a heart rate of 140, they get so mesmerized by the image. Especially when first learning, make sure someone is “guarding the fort” while you tiptoe through the ultrasound airwaves.

Mechanical damage to teeth or upper airway and (most dreaded of all) esophageal rupture are also complications. Patients may also complain of difficulty swallowing post TEE insertion.

Instrumentation

A quarter-million-dollar rolling TV?

More knobs than Miami Beach has sand granules?

That’s MY summary of TEE instrumentation, but the test may go into more detail than that. The scope itself is a modified gastroscope with the precious transducer at the end. Ancient probes, unearthed in Pompeii, had only one plane or two planes, but all the modern ones have the omniplane capability.

The ultrasound TV and its associated rat’s nest of knobs, video connections, and computer connections is called a platform. You cannot get Walking Dead on the TV, no matter how much you roll around the track ball, so satisfy yourself with ultrasound images.

The test may zoink you on how the knobs work. The next time you do an echo, make a point of wiggling every damn knob every which way and seeing what happens on the screen. On the test, they may, for example, pull the knobs to very high gain at a certain depth on the Depth Gain Compensation knobs and give you a streak of snow halfway down the picture and ask you, “What just changed?”.

Here’s a rundown on the knobs, taken from the (cutely named) “Knobology” Lecture at the TEE conference. (This stuff is dry as toast and easily goes into the Insta-Forget sulcus of your brain, so do what I said before: play with the knobs on your machine and know what each one does.)

Displays

(I’ll be honest, I’m not quite sure what they’re driving at here, but this is my guess.)

The image we get is displayed upside-down relative to the patient. That is, the image as we see it, with the pointy part of the pie slice at the top of the image, shows the patient as if we were looking at a prone patient from the top of the bed. The tip is the left atrium. The left side of the screen is the patient’s right side, and the right side of the screen is the patient’s left side. If the omniplane angle goes 180 degrees around, then the right/left situation is reversed. The patient’s right side is the screen’s right side, and the patient’s left side is the screen’s left side.

When the omniplane is at 90 degrees, then the patient’s inferior aspect is on the left and the anterior aspect is on the right of the screen.

B-Mode, M-Mode, and Two-dimensional Echocardiography

B-mode

The “B” stands for “brightness.” This would just show different brightness at various interfaces and isn’t of much use to us. It is an “ice-pick” view.

M-mode

If you roll a B-Mode out over time, then you can see the “ice-pick” view of the heart go on over time. You could then, for example, see valve movement over time. This is groovy, but we anesthesia types much prefer the next mode so we can see stuff go on. Cardiologists understand M-mode better than we do because they are smarter and tend to dress better than anesthesiologists.

Two-dimensional

Here, an array of views gives us the familiar picture of the whole heart moving in real time. This is the usual image you see:

Signal Processing and Related Factors

Processing changes the appearance of the displayed image. You can, for example, change the gain (too much = snow, too little = dark) to alter your signal. Changing the gray scale or dynamic helps you adjust the image to get sharper edges. No matter how you fiddle with the signal, it bears repeating that “garbage in, garbage out”. For example, if you don’t empty the patient’s stomach and a big hunk of pepperoni affixes itself to the front of your probe, then no amount of signal processing will help you out.

Questions

1. Ultrasound output is suspended by pressing the freeze button. True or false?

2. Lateral resolution is determined by:

E The amount of resolution on the sides

3. The ultrasound probe you use every day is probably:

A A linear array

B A mechanical sector array

C A phased array

D A refurbished unit

E A curvilinear array

4. The focal point of a transducer is:

A About the zone of the specular reflectors

B Between the near field and far field

C On the edge of the focal zone

D Focused on a point within the focal length

Answers

1. True. When you press the freeze the machine stops sending signals. This may help cool the probe if it is overheating.

2. D. Beam width is the major determiner of lateral resolution. Make your whole scanning sector smaller for better resolution.

3. C. Phased array.

4. B. The focal point can be moved, but it is between the near field and far field. It is in the focal zone, which begins right after the focal length.

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