Transcription
Hello everybody and welcome back. I hope you haven't been scared off by the first talk, the introductory talk in this MRI module. We covered many important topics, but only very superficially, and we haven't given any one of them really enough time of day to understand the underlying physics principles that go into creating an MRI image.
Now's our turn to take a step back and look at each one of those topics in a little bit more depth, and then hopefully bring that knowledge together and ultimately get a good understanding of the components in MRI physics. So today, I want to focus on the MRI machine itself, the various different magnets that go into creating the magnetic field strengths that ultimately create our MRI signal.
Now, you can see that the MRI machine is made of multiple different layers, and each one of these layers represents a different magnet. Now, we're going to start with the main layer, the outermost layer, which is known as the main coil. Now, if you cast your mind back to high school, you'll know that running a current through a wire will generate a magnetic field around that wire. That's Ampere's law, the right-hand rule. And if we run these wires in a coil, the magnetic fields will superimpose along one another, and they will ultimately create a large single magnetic field that runs through the center of that coil. And that's exactly what's happening in the main coil in our MRI machine. We create what is known as the B naught, or the main magnetic field, that runs along the longitudinal or Z axis in the MRI machine.
Now, the strength of that main magnetic field is dependent on two factors. It's dependent on the number of coils of wire and the amount of current that is running through that wire. Now, as we increase current more and more, we will get increased resistance within that wire, and we need to use what is known as a superconductor in order to generate sufficient current that will allow us to have a strong enough magnetic field strength.
Now, in order to do this, we need to utilize the principle of superconductivity. There are certain materials that at a low enough temperature will act as a superconductor. One of such material is Niobium-Titanium alloys. That's generally what's used in MRI machines. Now, in order to keep the temperature low enough, we circulate liquid helium around these coils. The liquid helium is generally below 4 degrees Kelvin.
Now, when electrons are running through the wire, current flowing through a wire, they interact with the lattice of that wire. Now, the hotter the temperature in that wire, the more the lattice is vibrating, and the more it interferes with those flowing electrons. As we cool that temperature, the lattice vibrates less and less, and those electrons can pass with less resistance. Now, if we're looking at a non-superconductor, the lower we make the temperature, the lower the resistance, but there will always be some form of resistance, even at zero degrees Kelvin. In a superconductor, there's an abrupt drop-off. That's what's known as our critical temperature.
Now, the critical temperature is generally around 4 degrees Kelvin, and if we keep the wire temperature below 4 degrees Kelvin, we will actually, in fact, get no resistance in that wire. It allows us to pass a large current through those wires. Now, in the MRI machine, we need to constantly replace that liquid helium in order to keep that temperature below 4 degrees Kelvin. If the temperature ever gets above 4 degrees Kelvin, we will suddenly get a large amount of resistance within that wire, and we will generate heat within the wire. As we generate heat, we get more and more resistance, and this liquid helium will now become a gas, and that gas will expand. We know that gases expand to fill space, and we get a process known as quenching, where that liquid helium is then released off as gaseous helium into the room. And this is a safety feature in an MRI machine. We need to be able to release that helium off if we no longer get superconductivity within these wires. That process is known as quenching.
So, the function of the main magnetic coil is to apply this B naught magnetic field along the longitudinal axis of our patient. And we've seen that if we apply a large magnetic field along the Z axis of our patient, it will cause hydrogen atoms, free hydrogen atoms within that patient, to align with that magnetic field. And each one of those hydrogen protons will precess at a set frequency. Now, the precession frequency is determined by the atom itself. Hydrogen has a set, what's known as a gyromagnetic ratio, which we're going to look at in our next talk when we look at nuclear magnetic resonance. And it's determined by the strength of the magnetic field. The stronger the magnetic field, the faster the hydrogens will precess around their axis, along the parallel direction of that main magnetic field. So, we can see that manipulating the current within the main magnetic coil will manipulate the magnetic field strength, and that change in magnetic field strength will change the precessional frequency of the hydrogen protons within the patient.
Now, when we are looking at the diagram of the magnetic coil, you may have noticed these two structures sitting within that main coil. Now, these are what is known as shims. Now, what exactly is a shim? Well, a shim in woodworking is a triangular piece of wood that you can wedge between two objects in order to make those objects more steady. If you're at a table that's wobbly, you will place a shim under the shortest leg in order to prevent that wobble within the table. Now, shims in MRI imaging are doing the same thing. They are manipulating that main magnetic field. In an ideal world, we want a perfectly homogeneous magnetic field running along the longitudinal axis of the patient. But because of the various different components of the MRI machine itself and the way in which magnetic fields are generated, we don't actually get a perfect homogeneous magnetic field along the Z axis. Now, the shims will alter that magnetic field in order to try and make the main magnetic field as homogeneous as possible.
Now, as you can see, there are two separate types of shims. The first type of shim is what's known as a passive shim. That's a magnetic sheet or a ferromagnetic metal that is placed within the bore of the MRI machine that will passively manipulate the magnetic field. We know that ferromagnetic substances will manipulate magnetic fields that run past them. The second type of shim is known as an active shim. Now, active shims have their own electricity supply, their own current running through the coils within these shims, and we can change that current in order to manipulate the main magnetic field. Again, shims are trying to make that magnetic field as homogeneous as possible. The more homogeneous that main magnetic field, the more accurately we can localize those signals generated in MRI imaging.
Now, active shims can either be superconductive, be within our helium, or they can be resistive shims that lie within the bore of the magnet themselves. They have their own electricity supply, and we can manipulate the amount that they influence the main magnetic field.
Now, let's move on and look at the next layer of the MRI machine itself, this purple layer here, and this is what's known as the gradient coils. Now, the gradient coils do exactly that. They apply a gradient along the magnetic field. Now, as you see here, the gradient coils lie perpendicular to one another here, in both the Y plane and the X plane. They also lie along the Z plane. Now, this becomes really important when we get to spatial localization of signals within MRI.
If we take away the two cents of gradient coils and we're left with the flanking gradient coils along the Z axis here, we know that we are applying a B naught, or a main magnetic field, along the Z axis. Now, we know that these hydrogen atoms are going to precess at a frequency that is determined by the B naught magnetic field. And generally, in clinical imaging, we're talking about a magnetic field strength between 1 and 3 Tesla.
Now, the gradient coils can create their own magnetic field. We can take this gradient coil here and run current through that coil. If we run current through the coil like this, what we're going to do is create a magnetic field in the left-to-right direction here, a magnetic field that superimposes along the same direction of our B naught magnetic field. We can do the opposite in this gradient coil and run a current in the opposite direction. We are then going to create a magnetic field that runs in the opposite direction to the main B naught magnetic field. So, we've generated two separate magnetic fields from these gradient coils that are ultimately going to influence the main magnetic field that is running through the Z axis here.
If the magnetic field generated by this gradient coil is superimposed over the main magnetic field, we will get a reduction in the magnetic field at this end of the MRI scanner. If we superimpose this gradient coil over the main magnetic field, we are adding to the magnetic field strength at this end of the MRI scanner. And you can see how superimposing these gradient coil magnetic fields will manipulate our B naught magnetic field, and we get manipulation of this main magnetic field that was constant along the Z axis to become a gradient between those two gradient coils. We are now applying a differential magnetic field strength to these various protons within the field. We can see that the field strength on this end of the Z axis will be less than the field strength at this end.
Now, importantly, this is not a vector here. We are not changing the direction of that mean magnetic field. We're changing the strength of the magnetic field as we head along the Z axis. What we've created now is a gradient where the magnetic field strength is stronger at the Z axis here than it is at the Z axis here. Now, because the magnetic field strengths differ along the Z axis, now the precessional frequencies of those hydrogen protons will also differ. As the gradient gets stronger, or as the magnetic field strength is stronger, the precessional frequency gets faster. Now, that is the main function of gradient coils. They change or manipulate the magnetic field strength along the separate axes in the Cartesian plane. Not only can we change precessional frequency along the Z axis, we can do the same along the X axis and Y axis, and this is a foundation for spatial encoding of the signal that we are generating in MRI imaging.
Now, the part of the magnetic field that is unchanged is what is known as the isocenter. That's the part of the magnetic field that has the same magnetic field strength as that background B naught magnetic field that we've created. Now, when we look at the gradient coil itself, we can see that they lie perpendicular to one another, and we can use these coils here to generate gradients along both the X axis and the Y axis.
Now, let's move on to the last magnet within the MRI machine, the radio frequency coil. Now, the radio frequency coil generates a magnetic field that is perpendicular to the main magnetic field. We've seen that hydrogen protons will precess at a specific frequency that is dependent on the strength of the external magnetic field that is being applied, and we've seen how we can apply a gradient along an axis within the patient. Now, that gradient is causing these hydrogen protons to precess at different frequencies.
Now, the radio frequency coil generates an alternating magnetic field in the perpendicular or transverse axis here, the XY plane. Now, if you think of each one of these hydrogen protons, or net magnetization vectors, as being children swinging on a swing at a set frequency, they're all swinging at different frequencies. The radio frequency coil is generating magnetic pulses in the transverse plane here. They're like the dads at the swing pushing their children. Now, the dads are closing their eyes and pushing at a set frequency. That's the radio frequency pulse. Only the children that are swinging at the frequency that the dads are pushing will get more and more energy and swing further and further out. Those children that are swinging naturally at a different frequency to what the dad is pushing, they won't end up matching up with the dad and they won't gain any energy from the dad pushing. The same thing happens with the radio frequency pulse. Only the hydrogen atoms that are precessing at the exact same frequency as the radio frequency pulse will gain more and more energy.
We've seen that hydrogen atoms are precessing at a set frequency, and if the radio frequency pulse is the same as that precessional frequency, two things will happen. First, the protons will start to fan out more and more, and second, those protons will now become in phase with one another. Initially, they were out of phase. The radio frequency pulse causes them to become in phase. They become in phase and they fan more and more out at a bigger and bigger angle, known as the flip angle. Now, the longer we generate that radio frequency pulse, the bigger and bigger that flip angle becomes, and that movement out of the longitudinal plane into the transverse plane is what allows us to measure that signal. That's the main function of the radio frequency pulse. It allows us to change our net magnetization vector from the longitudinal plane to the transverse plane.
Now, in this example, if we match our radio frequency pulse with the central frequency here, we will see that only that net magnetization vector will flip over 90 degrees in this case here. And it's the radio frequency pulse that allows us to isolate specific hydrogen atoms within the patient, and this is what's known as slice selection, which we're going to look at in signal localization later.
So, to summarize, we've looked at the main coil that generates the main magnetic field along the longitudinal axis of our patient, and we can use both active and passive shims to manipulate that magnetic field to make it as homogeneous as possible. We can then apply a gradient field strength along that magnetic field, altering the strength of the magnetic field either in the Z, X, or Y axes, and that's the responsibility of the gradient coils. We've then looked at the radio frequency coils, which select specific hydrogen protons that are precessing at a set frequency and then flip those hydrogen protons into the transverse plane, allowing us to measure signal in the transverse plane and ultimately generate our MRI image.
Now, we're going to shift our attention to the hydrogen atoms themselves and look at nuclear magnetic resonance, why nuclear magnetic resonance occurs, and how we can use nuclear magnetic resonance in order to generate signal in the image. Again, if you're studying for a radiology physics exam, check out the curated question banks that I've linked in the top line in the description below. Otherwise, I'll see you all in the next talk. Goodbye, everybody.