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

Summary

This educational talk delves into the intricacies of MRI technology, focusing on the machine's components and their roles in creating MRI images. It explains how the main magnetic field is generated by the main coil, using superconductors like niobium titanium to maintain a strong, homogeneous field. Shims are introduced to refine field uniformity. Gradient coils are highlighted for their ability to spatially localize signals by altering magnetic field strength along different axes. The lecture also covers the radio frequency coil's function in exciting hydrogen protons, which is essential for signal detection and image formation. The talk promises further exploration into nuclear magnetic resonance in subsequent sessions.

Takeaways

• 🧲 The MRI machine is composed of multiple layers of magnets, each contributing to the creation of the magnetic field necessary for imaging.
• 🔌 The main coil generates the primary magnetic field (B naught) along the longitudinal axis (Z-axis) of the patient, which is crucial for aligning hydrogen atoms.
• 💡 The strength of the main magnetic field is determined by the number of wire coils and the current flowing through them, with superconductors like niobium-titanium alloys used to maintain high current without resistance.
• ❄️ Liquid helium is circulated around the coils to keep them at temperatures below 4 Kelvin, ensuring the superconductivity necessary for strong magnetic fields.
• 🛠 Shims, both passive (ferromagnetic metals) and active (with their own electric supply), are used to adjust the main magnetic field, making it as homogeneous as possible for accurate imaging.
• 🧲 Gradient coils apply a gradient to the magnetic field, allowing for spatial localization of signals within the MRI by manipulating the field strength along different axes.
• 🌀 The gradient coils create differential magnetic field strengths, which in turn affect the precessional frequencies of hydrogen protons, enabling spatial encoding of the MRI signal.
• 📡 The radio frequency (RF) coil generates a magnetic field perpendicular to the main field, resonating with hydrogen protons at specific frequencies to selectively excite them.
• 🔄 The RF pulse causes hydrogen protons to move from the longitudinal plane into the transverse plane, which is essential for signal measurement and image generation.
• 🔎 The MRI process relies on the principles of nuclear magnetic resonance (NMR), where hydrogen atoms' alignment and precession within the magnetic field are manipulated to produce an image.

Q & A

What is the main purpose of the main coil in an MRI machine?

The main coil in an MRI machine is responsible for generating the B naught or the main magnetic field along the longitudinal or Z axis, which is essential for aligning hydrogen atoms and creating the MRI signal.

How does the strength of the main magnetic field in an MRI machine depend on the number of coils and current?

The strength of the main magnetic field is dependent on the number of coils of wire and the amount of current running through that wire. More coils and increased current result in a stronger magnetic field.

What is the role of superconductors in MRI machines, and why are they necessary?

Superconductors are necessary in MRI machines to generate sufficient current for a strong magnetic field without resistance. Materials like niobium titanium alloys are used as they exhibit superconductivity at low temperatures, typically below 4 degrees Kelvin, allowing large currents to flow without resistance.

Why is liquid helium used in MRI machines, and what happens if the temperature rises above 4 degrees Kelvin?

Liquid helium is used to maintain the temperature of the superconducting coils below 4 degrees Kelvin, which is necessary for superconductivity. If the temperature rises above this critical point, the superconductivity is lost, leading to increased resistance and heat generation, which can cause the liquid helium to expand and potentially lead to a quenching process where the helium is released as a gas.

What are shims in MRI imaging and how do they help in creating a homogeneous magnetic field?

Shims in MRI imaging are used to manipulate the main magnetic field and make it as homogeneous as possible. They can be passive, made of ferromagnetic materials, or active, with their own electric supply, and are used to correct for imperfections in the magnetic field.

How do gradient coils contribute to spatial localization in MRI imaging?

Gradient coils apply a gradient to the magnetic field, making it stronger or weaker along different axes (X, Y, Z). This differential magnetic field strength allows for spatial encoding of the signals, which is crucial for localizing the source of the MRI signal within the body.

What is the isocenter in the context of MRI imaging?

The isocenter in MRI imaging refers to the point in the magnetic field where the magnetic field strength is equal to the background B naught magnetic field. It is the region where the magnetic field is most homogeneous and不受 gradient influence.

What is the function of the radio frequency (RF) coil in an MRI machine?

The radio frequency coil generates an alternating magnetic field perpendicular to the main magnetic field. It selects specific hydrogen protons that are precessing at a set frequency and flips them into the transverse plane, allowing the MRI machine to measure the signal and generate images.

How does the radio frequency pulse affect hydrogen protons in an MRI scan?

The radio frequency pulse matches the precessional frequency of hydrogen protons, causing them to gain energy, fan out, and come into phase. This results in a flip angle, which moves the net magnetization vector from the longitudinal plane to the transverse plane, enabling signal measurement.

What is slice selection in MRI imaging, and how does it relate to the radio frequency pulse?

Slice selection in MRI imaging is the process of isolating a specific plane within the patient to acquire image data. It is achieved using the radio frequency pulse, which is tuned to match the precessional frequency of hydrogen protons in the desired slice, allowing for the selective excitation of these protons.

Outlines

00:00

👋 Introduction to MRI Module and the Importance of Physics

The speaker welcomes the audience and reflects on the introductory MRI talk, which touched upon important topics but only superficially. The focus now is on understanding the physics behind MRI, starting with the machine itself. Specifically, the discussion centers on the different magnets within the MRI and their roles in creating the magnetic field needed for imaging. The main magnetic field (B naught) is generated by the outermost magnet, known as the main coil, using principles such as Ampère's Law and superconductivity. The importance of materials like niobian titanium alloys and the role of liquid helium to maintain superconductivity are highlighted, emphasizing the critical temperature to avoid resistance and the quenching process.

05:03

🔧 Understanding MRI Shims for Field Homogeneity

This section introduces the concept of 'shims,' devices that help stabilize the magnetic field in MRI imaging. The goal of shims is to make the magnetic field as homogeneous as possible, given that imperfections in the machine disrupt the ideal uniform field. Two types of shims are explained: passive shims, made from ferromagnetic materials that adjust the field naturally, and active shims, which are powered by electricity to further fine-tune the magnetic field. Active shims may either be superconductive or resistive, both contributing to more accurate MRI signals by improving magnetic field uniformity.

10:03

📈 Gradient Coils and Their Role in Spatial Localization

The gradient coils, located after the main and shim coils, are discussed in terms of their ability to manipulate the magnetic field strength along different planes (X, Y, and Z). The speaker explains how these coils can create a gradient in the magnetic field along the Z-axis by superimposing additional magnetic fields over the main field. This manipulation of field strength enables the MRI to vary the processional frequencies of hydrogen protons, which is crucial for spatial localization in MRI scans. By controlling gradients in the magnetic field, MRI machines can differentiate between signals from different parts of the body.

15:04

📐 The Function of Gradient Coils in Signal Encoding

This part continues the discussion on gradient coils, focusing on how they affect the processional frequency of hydrogen protons based on their position within the magnetic field. By changing the magnetic field strength across different axes, the gradient coils help encode spatial information in the MRI signal. This encoding allows for the creation of detailed images by ensuring the MRI machine can differentiate between protons based on their location in the body. The concept of the 'iso-sensor,' where the magnetic field strength remains unchanged, is also introduced as a reference point for signal encoding.

📡 Radio Frequency Coils and Their Role in Signal Measurement

This section introduces the radio frequency (RF) coils, which create an alternating magnetic field perpendicular to the main field. These coils are responsible for selecting hydrogen protons based on their processional frequency and flipping them into the transverse plane, a key step in measuring the MRI signal. The analogy of children swinging and being pushed by their fathers (representing protons and the RF pulse) is used to explain how only protons oscillating at the same frequency as the RF pulse will gain energy. This process aligns the protons, making them easier to measure and enabling MRI imaging through slice selection.

🧪 Summary and Transition to Nuclear Magnetic Resonance

The speaker recaps the different components of the MRI machine, including the main magnetic coil, shims, gradient coils, and radio frequency coils. These components work together to generate, manipulate, and measure the magnetic fields necessary for MRI imaging. Finally, attention shifts to the hydrogen atoms themselves, introducing the concept of nuclear magnetic resonance, which will be covered in the next talk. The speaker encourages viewers to check out curated question banks and ends the talk with a goodbye.

Keywords

💡MRI machine

The MRI machine, as described in the script, is a complex medical imaging device that uses magnetic fields and radio waves to generate detailed images of the internal structures of the body. It is composed of multiple layers, each representing a different magnet, and is crucial for creating the magnetic field strengths necessary for MRI signal production. The script delves into the various components of the MRI machine, such as the main coil and gradient coils, which are essential for generating the magnetic field and spatially localizing the signals within the body.

💡Main coil

The main coil is the outermost layer of the MRI machine, responsible for generating the primary magnetic field known as B naught. As per the script, this is achieved by running a current through a wire, which, according to Ampere's law, creates a magnetic field. The strength of this main magnetic field is dependent on the number of coils and the amount of current running through them. The main coil's function is critical as it aligns hydrogen atoms within the patient along the magnetic field, which is a fundamental step in MRI imaging.

💡Superconductor

Superconductors are materials that exhibit zero electrical resistance when cooled below a certain critical temperature. In the context of the MRI machine, superconductors like niobium-titanium alloys are used in the main coil to generate a strong magnetic field without significant energy loss. The script explains that liquid helium is circulated around the coils to maintain a temperature below 4 Kelvin, allowing the superconductor to function effectively and maintain the magnetic field strength necessary for MRI.

💡Shims

Shims in MRI imaging are used to manipulate the main magnetic field to make it as homogeneous as possible. The script mentions two types of shims: passive shims, which are ferromagnetic materials that passively affect the magnetic field, and active shims, which have their own electrical supply and can actively adjust the magnetic field. The goal of shims is to correct for imperfections in the magnetic field, which is essential for accurate MRI imaging as it affects the localization of signals within the body.

💡Gradient coils

Gradient coils are part of the MRI machine that apply a gradient to the magnetic field, allowing for spatial localization of signals within the body. The script explains that these coils lie perpendicular to one another and can create magnetic fields that superimpose on the main magnetic field, thereby altering its strength along different axes. This gradient is crucial for distinguishing between different regions of the body in the MRI image, as it allows for the differentiation of hydrogen protons based on their location within the magnetic field.

💡Radio frequency (RF) coil

The radio frequency coil is another key component of the MRI machine, responsible for generating an alternating magnetic field perpendicular to the main magnetic field. As detailed in the script, this coil emits radio frequency pulses that match the precession frequency of hydrogen protons within the patient. The RF pulse causes these protons to absorb energy and align in phase, a process known as excitation. This is essential for MRI as it allows for the measurement of signals from specific regions of the body, which is the basis for image formation.

💡Precession frequency

The precession frequency of hydrogen protons is determined by their interaction with the magnetic field and is a fundamental concept in MRI. The script explains that when a hydrogen atom is placed in a magnetic field, its protons precess at a frequency that is dependent on the strength of the magnetic field. This precession frequency is used by the MRI machine to selectively excite and measure signals from specific hydrogen atoms, which is crucial for image formation.

💡Flip angle

The flip angle, as mentioned in the script, refers to the angle through which the net magnetization vector of hydrogen protons is rotated from the longitudinal plane into the transverse plane by the radio frequency pulse. A 90-degree flip angle is typically used for optimal signal detection. The flip angle is a critical parameter in MRI as it affects the amount of signal that can be detected and thus influences the quality and contrast of the resulting images.

💡Nuclear magnetic resonance (NMR)

Nuclear magnetic resonance is the physical phenomenon that underlies MRI technology. The script hints at NMR as the reason why hydrogen atoms align and precess in a magnetic field, which is essential for generating the MRI signal. NMR occurs because of the magnetic properties of atomic nuclei, like hydrogen protons, which have a property called spin. When placed in a magnetic field, these spins align and precess, allowing them to absorb and emit radio frequency energy, which is detected to form MRI images.

💡Spatial localization

Spatial localization is a critical aspect of MRI technology that allows for the pinpointing of signals to specific regions within the body. The script explains that this is achieved through the use of gradient coils, which create a gradient in the magnetic field strength along different axes. By manipulating the magnetic field in this way, the MRI machine can distinguish between signals from different locations, enabling the creation of detailed images of internal body structures.

Highlights

Introduction to the physics behind MRI imaging, focusing on the MRI machine and its components.

Explanation of how the main coil in the MRI machine creates the B naught magnetic field using Ampere's law.

The role of superconductors, like niobium titanium alloys, in generating strong magnetic fields without resistance.

The necessity of liquid helium to maintain the superconducting state of MRI machine coils at below 4 degrees Kelvin.

The concept of quenching as a safety feature to release helium gas if superconductivity is lost.

The function of shims in MRI to make the main magnetic field as homogeneous as possible.

Differentiation between passive shims, which are ferromagnetic metals, and active shims, which have their own electric supply.

The gradient coils' role in applying a gradient to the magnetic field for spatial localization of signals.

How gradient coils manipulate the magnetic field strength along different axes in the Cartesian plane.

The isocenter concept, where the magnetic field strength is the same as the background B naught magnetic field.

The radio frequency coil's function to generate a magnetic field perpendicular to the main magnetic field for signal measurement.

The process of matching the radio frequency pulse with the hydrogen protons' precession frequency for energy transfer.

The importance of the flip angle in moving the net magnetization vector from the longitudinal to the transverse plane.

Slice selection as a method to isolate specific hydrogen atoms within the patient using the radio frequency pulse.

Summary of the MRI machine components and their roles in generating the MRI image.

Anticipation of the next talk focusing on nuclear magnetic resonance and its application in MRI imaging.

Transcripts

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