Transcription
Welcome back. So, by now, we've looked at different parameters that we can use in order to describe a sound wave itself. And we've looked at certain parameters that are specific to pulse-echo ultrasonography. And pulse-echo ultrasonography is the type of ultrasound that we can use to generate an image that we can display on a screen.
Now, we're going to look at how those ultrasound waves, or how those pulses, interact with matter, interact with the tissues within our patient. In order to understand these interactions, we need to understand a concept known as acoustic impedance.
Now, when we look at ultrasound interactions with matter, you can see there are multiple different types of interactions, and all of these we're going to cover in the following two talks.
Now, in this top row here, this is what's called reflection, where we have an incident ultrasound wave reflecting off a tissue boundary. Now, a tissue boundary describes the interface between two separate tissues. We can have an ultrasound wave going through muscle and then going through fat. That boundary between the muscle and the fat is what's known as a tissue boundary, and that's commonly where tissue interactions occur.
Now, at these tissue boundaries, we can get reflection, either partial reflection of that wave with some of that incident wave being transmitted through to the next tissue, complete reflection of that incident wave where all of that energy returns as an echo towards our ultrasound machine, and then we can get what's known as specular and non-specular reflection, both of which we'll look at in our next talk.
Then, if an ultrasound wave interacts with a tissue boundary at an angle and the speed of sound between these two different tissues varies, we get what's known as refraction, and we're going to spend an entire talk looking at refraction itself.
The last type of tissue interaction we get is what is known as scattering. Now, when an ultrasound wave interacts with units within that tissue that are smaller than the wavelength of that incident ultrasound wave, we get what's known as scattering. We get loss of energy of that incident ultrasound wave with small sound waves being produced in any direction, scattering out amongst the tissue.
Now, I like to think of scattering as a large wave in the sea coming towards a beach that is made of pebbles. Now, those pebbles are much smaller than the wavelength of that wave, and as that wave crashes into the bank, that wave disperses in all directions as it interacts with those pebbles, and that's what we get with scattering. The energy is dissipated out into lots of smaller little waves.
Now, I've mentioned that it's acoustic impedance that determines these interactions at tissue boundaries. So, what exactly is acoustic impedance? Well, acoustic impedance is an inherent property that is specific to a specific tissue type, and we denote acoustic impedance by the letter Z here. This capital Z, this is acoustic impedance. And acoustic impedance can be calculated by the product of both the tissue density and the speed at which sound travels through that tissue.
Now, we measure density as kilograms per meter cubed, how much weight is there within a cubic meter of tissue, and we determine speed as meters per second. So, our acoustic impedance SI units are kilograms per meter squared per second.
Now, as you can see, the tissue density as well as the speed contributes to this acoustic impedance value. And we've seen that speed is determined by the tissue's bulk modulus, its stiffness, its resistance to compression, and the tissue's density itself. And because we're multiplying density by speed here, we can see that it's a bulk modulus itself that contributes the most to acoustic impedance. So, it's the tissue stiffness or lack of stiffness, the tissue's compressibility or resistance to compressibility, that determines the acoustic impedance.
So, when we release an ultrasound pulse into a patient's tissue and that pulse comes into contact with a tissue boundary here, it's the difference in acoustic impedance between those two tissues that will determine how much of that ultrasound wave is transmitted through that tissue boundary and how much is reflected back as an echo towards our ultrasound machine.
So, here I've tabulated the various different acoustic impedance values for different soft tissues that we may come into contact with in diagnostic ultrasound imaging. Now, the acoustic impedance SI units we've seen here are kilograms per meter squared second. Now, there's another special name given to this, which is known as Rayls, and this is a value we generally use when talking about acoustic impedance.
Now, we can see the Rayl value between air and bone. There is a vast difference here, and that's the bulk modulus predominantly that accounts for this drastic change in the acoustic impedance values.
Now, if the two tissues have similar acoustic impedance values, most of that ultrasound wave at that tissue boundary will be transmitted with a very small echo coming back. And in our next talk, we're going to look at how we can calculate the transmittance value versus the reflected ultrasound value.
When we look at a tissue boundary, we've seen this diagram. Now, at the beginning of the talk, we talk about the acoustic impedance value of our first tissue and compare that to the acoustic impedance value of our second tissue here. Now, when there is a large difference in these two acoustic impedance values, we will get most of that ultrasound wave reflecting back towards our ultrasound machine. Now, it doesn't matter if the difference is coming from a low acoustic impedance to a high acoustic impedance or vice versa. As long as that difference is large, we will get a large reflection back.
The best way to think of this is think of this yellow wave as our incident ultrasound wave. This is coming through our first tissue and interacting with our second tissue here. Now, if the acoustic impedance value of our second tissue is high, it says that that tissue is stiff, it is resistant to compression. Now, imagine this as a spring here, and we were to pulse a wave through that spring, and that wave was to come into contact with a tightly packed spring that wasn't very compressible. It's much like this spring being attached to a wall, and if you were to pulse it back, you can see how most of that energy will bounce back, be reflected as a pulse echo heading back towards where that wave came from.
Now, if we were to connect two springs that had very similar acoustic impedance values, we can see how that wave that we generate would most likely carry on as a wave as it heads to that second spring. There's very little reflection or echo.
Now, if we were to have a spring that wasn't connected to anything, it was connected to a tissue that was highly compressible, had a very low acoustic impedance value, we can see that the energy reaching the end of this spring will have nothing to transfer its energy onto, and that energy will then come back as those units in the medium rare effect come back to their resting place, and that energy will then be transferred back to where that pulse came from. So, the amount of reflection, the amount of echo coming back, is determined by the difference between the two acoustic impedances.
In our next talk, we're going to be looking at reflection, seeing echoes that are generated and heading back towards our ultrasound machine, and we're going to see how we can use these acoustic impedance values to calculate that amount of echo versus the amount of transmission at a tissue boundary. So, I'll see you all in that talk. Goodbye, everybody.