MRI Machine - Main, Gradient and RF Coils/ Magnets | MRI Physics Course | Radiology Physics Course#2

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 add a low enough temperature will act as a superconductor one of such material is niobian 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 four 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 gas is expand to full 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 process 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 process 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 processional 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 and 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 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 now 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 process 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 call 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 processional frequencies of those hydrogen protons will also differ as the gradient gets stronger or as a magnetic field strength is stronger the processional 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 processional 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 iso sensor 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 process 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 process at different frequencies now the radio frequency coil generates an alternating magnetic field in the perpendicular or transverse axis here the X Y plane now if you think of each one of these hydrogen protons or net magnetize 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 processing at the exact same frequency as

the radio frequency poles will gain more and more energy we've seen that hydrogen atoms are processing at a set frequency and if the radio frequency pulse is the same as that processional 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 rate 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 is 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 in 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 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 processing at a set frequency and then flips those hydrogen protons into the transverse plane allowing us to measure signal in the transverse plane and ultimately generates our MRI image now we're going to shift our attention to the hydrogen atoms itself 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