A primer on Ultrasound

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Transcript

Welcome to our third lecture a primer on ultrasound. Most of you probably have some idea already about how ultrasound works, but we'll start with a short refresher, then move on to ultrasound modes. ultrasound is basically sound waves just above the upper range of human hearing, which is 20 kilohertz. Commercial ultrasound machines use frequencies ranging from 1.5 to 7.5 megahertz. These waves are generated by an area of crystals in the ultrasound probe when stimulated by electricity. These ultrasound waves can travel through any medium just like sound waves, and then bounce off structures and reflect back to the probe, where more crystals detected and convert it back to electrical signals, which are converted by the echo machine to an image in this image structures which reflect ultrasound instead of it, allowing it to pass through our white or echo dots.

These structures include solid structures like myocardium, and valve leaflets. On the other hand structures that ultrasound waves travel through and reflected show up black on the screen. Echo Lucent. These are fluids or fluid filled cavities such as the inside of the heart chambers and blood vessels. Here, as in the lung will also reflect ultrasound and show up white on the image. So that's why we have to stay away from the lungs and look at the heart through certain windows and the chest wall that we will be discussing in the next lecture.

Ultrasound has several modes that we use. There's 2d 3d mode color Doppler pulse wave Doppler continuous wave Doppler tissue Doppler. We'll discuss a few of them here and we'll start with the mode that most people are familiar with 2d ultrasound. 2d is the default mode when you first turn on the ultrasound machine. If you switch to a different mode and then want to switch back to 2d you're going to press the button labeled 2d on your control panel. On some machines that will be called B mode.

2D gives you a two dimensional section of the heart. This is a standard 2d ultrasound image which shows the heart and motion can press the freeze button to freeze the image on some machines will be labeled with an asterisk instead of the word freeze. You'll be using this button a lot to freeze the image so that you can be able to take measurements. These are two ventricles and these are the two atria. As you can see the ventricular and atrial walls as well as the valves are white or eco dense because they are like ultrasound, while the blood inside is black because ultrasound waves move through the through the now the way this image is generated is that a lot of crystals fire ultrasound beams in several directions at once forming an arc ultrasound beam. And then each crystal use the reflective waves to render a single line image of the structures ahead of it.

By putting together all of these lines, a two dimensional image is formed. In the illustration I've used just five beams for simplicity but in reality there are thousands of crystals that work simultaneously. Now that we understand this, the next ultrasound mode m mode becomes simple. Mo stands for motion mode as Just one of those lines plotted against time. So just one crystal keeps firing and draws its line. And millisecond later it fires again and draws another line next to the first one.

Now, if the heart weren't moving, just keep getting the same line over and over again like this. But since the heart is moving, what you actually get is the frame by frame rendering of the structures along this line. The button for is usually simply labeled m mode. This is an example of M mode echoed across the left ventricle. You can see the 2d image above and below is the resultant mo trace, showing the movement of the front and back walls along this line. Mo DS used to track motion with high temporal resolution and allows measuring the extent of this motion.

As we'll see further along the course. Color Doppler is easy to understand. It'll usually be a button Mark color or CFM. Short for color flow motion on control panel. When you press it, you'll get this box you can move around if you You press the left trackball button. Alright, in this case, you can resize this box, you press it again and you can reposition the box.

And what this box is it basically shows you blood flow in the heart, so red blood is flow toward the probe, and blue is flow away from the probe. Sometimes you'll see green, such as these arrows in between what we call that is aliasing. And this indicates turbulent but flow with high velocities. We'll see examples of that later in the course. pulse wave and continuous wave Doppler as a kind of like em mode for blood flow, in that they show blood flow along just one line. Let's first pulse wave.

You can reposition the cursor that's expected happens first again, the 2d image Just shown at the top, and you get this line that's unfreeze on. You get this line which you can move around. And this line has these two little dashes which are called the sample volume. You can move it up and down using the trackball and move the whole line sideways also using the track. The sample volume is where the measurement happens in the case of pulse wave Doppler. So now I'm measuring blood flow at this spot.

Now I'm measuring it this file. And depending on where I want to enter the measurement, I'm gonna place the sample volume. You can also hear the blood flow similar to what was contained. continuous wave Doppler similar to pulse wave Doppler if it's similar to pulse wave except that it measures the fastest blood flow along the whole line. So you can't pick the point to measure the blood flow. It just measures the fastest paucity among the whole line and it gives you that advantage over pulse wave speed.

Its advantage over a pulse wave is that it can measure high velocities such as valid outflow. postally can only measure low velocities, but it has the plus being able to pick exact spot. As we move on, you learn when each of them is used. This image we have now is continuously increasing across the mitral valve against time, the axis shows velocity and centimeters per second. And the this axis shows time in seconds. in milliseconds, I'm sorry.

Flow above the baseline is toward the probe and flow below it is away from the probe. I know this was kind of a juicy lecture with a lot of information to process. Take your time. Go over it one more time if you want, and I'll see you in the next lecture. The basic views

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