Transcription
Hello everybody, and welcome back. So, up until now, we've assumed that the tissue within the slice that we're trying to image has remained stationary throughout the pulse sequence. In the next few talks, I want to focus on what happens when tissues move into and out of the slice, or through the slice, whilst we're acquiring the data and entering that data into k-space and ultimately transforming that into the MRI image that we're seeing.
Specifically, I want to focus on blood vessels and what the signal will be within blood vessels. You would have noticed when you look at MRI scans that the signal within blood vessels can fall anywhere on the spectrum from dark blood vessels (hypointense) to bright blood vessels (hyperintense). And the reason for the grayscale that you see on the image has multiple different contributing factors. We know that the sequence that we choose, the type that we choose, either spin echo or gradient echo, the parameters that we set, the TE, the TR, the slice thickness, all of those are going to affect whether the signal is bright or dark within vessels. And hopefully, when we go through these next talks, I can show you how tweaking those parameters are going to change the signal within vessels.
The second factors that play a big role in the signal within vessels is actual underlying physiology. What is the velocity of the blood traveling in the vessel? What's the orientation of that vessel or the anatomy that we're trying to scan? Which direction is the blood flowing? All of these affect the signal that's going to be in blood vessels.
Now, use this slide as a bit of a contents page for the talks coming up. Broadly speaking, we're talking about angiography here. Now, I've color-coded these as to how we're going to approach these. I've split them into things that cause darkness in vessels and things that cause brightness in vessels, but as you can see, there's lots of overlap, and this is a complex topic. We're going to start with what's known as time-of-flight effects. Those are effects as when tissue moves into or out of the slice while we're imaging. And we'll start today by looking at a time-of-flight effect known as high-velocity signal loss, and I've paired that with turbulence, which is also another high-velocity state that causes signal loss within vessels.
Then we're going to move on to the second type of time-of-flight effect known as flow-related enhancement, which causes brightness in vessels. And I've paired that with saturation bands. I'll show you how we can use saturation bands to null signal or to make hyperintense signal in blood vessels in a certain orientation while still getting brightness in blood vessels in their different orientation. We'll then move on to spin-phase effects and see how we lose as a spin travels through a slice during a pulse repetition. And we'll pair that with gradient moment nulling, which will show us how we can reverse these effects. Finally, we'll wrap off this Mr. angiography section by looking at phase-contrast Mr. angiography and then round off with contrast-enhanced M. So this is where we're going. This is a broad overview. We're going to start today with high-velocity signal loss and turbulence.
So let's take an MRI slice here of the abdomen. We can see the liver, kidneys, vertebral column, our aorta and IVC. We've got bowel gas here, we've got the tip of the spleen there, erector spinae muscles. We've got our orientation here. You can see there are regions with no signal. Now, what is actually causing signal within an MRI scan? Firstly, we have to have protons that can give us signal. So the number of protons is going to affect how much signal we get. Secondly, we need transverse magnetization. We can't measure longitudinal magnetization because of that main magnetic field that's always on. We need to flip that net magnetization vector into the transverse plane. And thirdly, we can have transverse magnetization, but those spins can be completely out of phase. And how in-phase those spins are is also going to contribute to how much signal we're getting.
So when we look at bowel gas here, we know that there is a lack of protons. That's predominantly the reason why we're not getting any signal here. But when we look at the aorta and the and the IVC or the branches of the aorta here, we see that there's low signal in this image. We've still got tissue here, there's still still blood there, and theoretically, that blood should be giving off signal if we've applied a pulse sequence that gives us an image like this.
Now, today we're going to focus specifically on the aorta and the IVC. These are due to time-of-flight effects known as high-velocity signal loss. In future talks, we're going to look at these renal arteries or the superior mesenteric artery here, where the loss of signal here, or the hypointensity here, comes from a different mechanism other than high-velocity signal loss.
So how do we go about creating this specific slice here? We've placed our patient within the scanner and we've slice-selected with our slice-selection gradient in combination with an RF pulse, a specific slice of anatomy that we want to image. Remember, our slice-selection gradient applies a gradient along the entire longitudinal axis of the patient, and only the spins that are processing at a specific frequency related to that slice-selection gradient, only the spins that match the radiofrequency frequency will process and resonate and flip into the transverse plane. That net magnetization vector in the transverse plane is only coming from this slice because of the slice-selection gradient. That's a really important point.
Now, as I've said, we've assumed up until now that the tissue within that slice is remaining stationary. Now, that's not the case with blood vessels. Let's look at what happens when blood moves into and out of a slice during a spin echo pulse sequence. So let's take a diagrammatic representation here of our patient. The gray areas represent tissue that is remaining stationary. The red area here represents blood flowing into and out of the slice, and I've represented the slice here by this orange region to match with the slice-selection gradient here.
Now, we know that the blood is traveling in the aorta. Our patient lying, this is the longitudinal plane here, our aorta here, blood is flowing in this diagram from left to right. Let's look at what happens to the blood within the slice when we start applying our pulse sequence. Blood in the slice is resonating at a frequency that matches the radiofrequency pulse. It's within the slice, so we get a flip of the net magnetization vector into the transverse plane here.
Now, time goes by until the 180-degree RF pulse, and we know that that time is half of our TE. That 180-degree RF pulse is what's going to cause this spin echo generation here, where our leaders become our laggers, they catch up, and they generate a spin echo, bringing signal back back up to the levels of T2. Now, what happens if the blood moves out of that slice prior to the next 180-degree RF pulse, or prior to the next RF pulse? That blood then moves, and we've got fresh blood entering the slice. A 180-degree RF pulse is slice-selected. It's selecting for the same slice that we initially excited those spins with resonance into the transverse plane. This blood is no longer within the slice. It's not going to experience the 180-degree RF pulse because the blood has moved out of the slice. The slice-selection gradient will mean that this blood is now processing at a faster frequency that won't match this RF pulse.
So we've flipped the net magnetization vector into the transverse plane with our 90-degree RF pulse. Time has passed, and that blood has left the slice, and we are losing signal at a rate of T2 star free induction decay. This tissue now is not going to experience the 180-degree RF pulse. The signal from this blood is going to continue to decay at T2 star. We're going to rapidly lose signal. This blood that has entered the slice is going to receive the 180-degree RF pulse. It's slice-selected. It now has the right frequency that matches up with the RF pulse. It's going to flip its magnetization vector 180 degrees.
Now, what magnetization vector does this spin have? Well, let's reverse the blood back. We see that initially, during the 90-degree RF pulse, this tissue here that's not in the slice doesn't get any transverse magnetization. It remains in the longitudinal plane. So we've got this longitudinal magnetization vector with no transverse magnetization. The blood then enters the slice and receives the 180-degree RF pulse. What's happened to that magnetization vector? It's now still parallel to the main magnetic field. There is no transverse magnetization that's never received that 90-degree RF pulse. What's giving a signal? It's the transverse magnetization, as well as how in-phase those spins are. This has got no transverse magnetization. It's not going to give any signal, and that is the basis for high-velocity signal loss in the aorta here. The blood is moving through the slice quicker than the period of time between the 90-degree and the 180-degree RF pulses.
So you can see how depending on our slice thickness, if our slice is very thick, blood won't have time to leave the slice, and we're still going to get some signal. Some of that blood is going to receive the 180-degree RF pulse and generate a spin echo. When the blood velocity is high enough to leave the slice prior to the 180-degree RF pulse, that is when we get high-velocity signal loss. Now, notice it doesn't matter which direction this blood is flowing, and that's why we get loss of signal both in the IVC and the aorta, despite blood moving in different directions in both cases. Blood is going to leave the slice, and that's what's known as a time-of-flight effect. So this is a reason for signal loss, high-velocity signal loss, time-of-flight effect.
There's another high-velocity state that causes signal loss, but it has nothing to do with this mechanism here. That's what we're going to move on to next, which is turbulence. Now, important to note before we move on, there this happens with a spin echo pulse sequence, and it's the 180-degree RF pulse that is missed. We're going to look later on at blood entering a slice in gradient echo imaging where we actually get bright blood, and that's because of this lack of 180-degree RF pulse. So remember, high-velocity signal loss generally occurs with spin echo imaging.
So let's move on to how turbulence can reduce signal within a blood vessel. This is a separate mechanism, but turbulence is still high-velocity blood. Now, when we have blood flowing through a vessel and we assume that the blood is constantly flowing and there's no branches of that vessel, we get what's known as laminar flow, where blood in the center of the vessel has a higher velocity than blood towards the peripheries of that vessel. The same thing will happen if we're running water through a hose pipe. Now, laminar flow occurs when we have a rigid tube that's not pulsatile and we have constant flow. Now, that doesn't happen necessarily in the human body because we've got the cardiac cycle and we've got elasticity within our blood vessels. Generally, physiologically, we get a flattening of this laminar flow. We still have increased velocities in the center of the vessel, but we call it plug flow because of the elasticity of the vessel and because of the cardiac cycle with the pulsatility of blood flow.
Now, if we were to image this cross-section, but we were to use an imaging technique that allowed us to get bright blood within the vessel, those are techniques we're going to look at in upcoming talks. If we had a technique that allowed us to get bright blood in the vessel, this slice would look like this in cross-section. Stationary tissues have their own contrast depending on the type of sequence that we've generated, and the blood within the vessel has bright signal. We can see the vessel wall here, which is staying still, has a different signal to the blood that's moving through, and we're going to look at mechanisms for this.
Now, what happens if we get turbulence? Now, turbulence can occur through multiple different mechanisms. The blood velocity could reach a critical velocity over which the blood no longer exhibits laminar flow, and we get turbulence. There could be stenosis, narrowing of the vessel, that is going to cause a change in velocity, and after that stenosis, we're going to get turbulence. Or it can happen naturally with a bifurcation of a blood vessel, as blood now has physically has to move into two separate vessels from a single vessel, we will also get turbulent flow.
Let's have a look at an example here. In this example, I'm going to generate some stenosis within this vessel. We have the same blood, but now encountering stenosis, and you can see we get disruption of that initial plug flow. Now, what would happen if we were to look at the vessel in cross-section here? Well, prior to the stenosis, we have the same image that we looked at when we had laminar flow, bright in the blood vessel, we've got normal flow through that vessel. At the level of stenosis, we've no longer got flow on the peripheries of the vessel and only have flow in the center of the vessel. We're going to get an image that looks like this. We get loss of signal based on the whatever is causing the narrowing, be it sclerosis or external compression or a tumor within the vessel, or whatever is causing the stenosis. We can see that we get loss of signal. Now, that may not be black signal depending on the characteristics of the tissue, it might provide its own type of signal, but you can see how the signal has changed.
Now, blood exhibits two different flow states here. The first is what's known as vortex flow, where we get slow, organized, circular flow of blood immediately after the stenosis or the bifurcation or the aneurysm, whatever's causing a disruption in blood flow. Then we get what's known as turbulent flow. Now, turbulent flow is a high-velocity flow state, but you can see it's a disorganized flow state. There is randomness to the movement of flow here. We don't have this organized flow that we saw in laminar flow. Not only is it a high-velocity flow state, but the velocities vary greatly within this region here. And it's the combination of the varying velocities as well as the varying orientations that cause rapid dephasing of the spins within this region here. That rapid dephasing is going to reduce the amount of signal that we measure. Remember, signal comes from the number of protons, the degree of transverse magnetization, as well as how in-phase those spins are. And in this region, there's rapid loss of phase. As a result, we're not going to get signal from this turbulent region here.
Now, there's not a narrowing here, the vessel is patent, but when we look at what our slice looks like, we get loss of signal, darkness within our vessel. And you can see if we were looking at these slices sequentially in an MRI stack, we would see we get tapering of signal and loss of signal. We might think that this vessel is fully occluded from whatever is causing the stenosis, where in fact, it's actually turbulence that's causing this signal loss. And that's a common issue that we come across with turbulent flow, figuring out whether the signal loss is from turbulence or whether it's from an actual obstruction or an actual narrowing within the lumen.
Now, you can see how turbulent flow, it's not exiting the slice quickly like we had in high-velocity signal loss. It's not a time-of-flight effect. It's a physical phenomenon that's causing rapid dephasing. Two separate mechanisms here that we covered today that cause signal loss or hypointensity within a vessel. In the next talk, we're going to look at how we can get hyperintense signal within a vessel, still using time-of-flight effects. Blood entering the slice is now going to give us a greater signal. It's the opposite of what we looked at in high-velocity signal loss, where blood entering the slice is not going to give us any signal because of its lack of transverse magnetization.
So I hope this has helped. Mr. angiography is something that is very logical. If you can think through the steps of the pulse sequence, more often than not, you're going to get these answers right, especially if you're answering them for an exam. And if you are studying for an exam, check out the question bank I've linked below. Many people are finding it incredibly helpful. I'm getting lots of great feedback, so if that's you, check it out in the description below. Until next time, I'll see you all. Goodbye everybody.