Lecture 1-2 Magnet, RF Coils, and Gradient Coils for MRI

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En provenance du cours de Korea Advanced Institute of Science and Technology

MRI Fundamentals

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Korea Advanced Institute of Science and Technology

MRI Fundamentals

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Welcome! In this course learners will develop expertise in basic magnetic resonance imaging (MRI) physics and principles and gain knowledge of many different data acquisition strategies in MRI. In particular, learners will get to know what is magnetic resonance phenomenon, how magnetic resonance signals are generated, how an image can be formulated using MRI, how soft tissue contrast can change with imaging parameters. Also introduced will be MR imaging sequences of spin echo, gradient echo, fast spin echo, echo planar imaging, inversion recovery, etc.

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1 Overview of MRI

Welcome! This first week you will be introduced to the course and key principles of mri. You will learn what is mri and the major components of mri.

Lecture 1-1 MRI as one of the Biomedical Imaging Modalites11:06

Lecture 1-2 Magnet, RF Coils, and Gradient Coils for MRI13:58

Rencontrer les enseignants

Sunghong Park
Associate Professor
Department of Bio and Brain Engineering

So, we will talk about three major hardware components of MRI in detail.

And in this video lecture,

we will talk about magnet first.

So, MRI magnet, so how strong are the magnets in MRI machines,

and magnetic field strengths of earth ranges from 0.25 to 0.65 gauss,

so on average about 0.5 gauss.

And one Tesla, Tesla is

the typical unit used for MRI to represent MRI magnetic field strength,

and one Tesla is 10,000 gauss.

So, one Tesla MRI scanner has 20,000 times the earth's magnet,

and human MRI scanners are typically in the order of Tesla.

And recently, FDA approved seven Tesla human MRI for clinical diagnoses.

But however, most MRI scanners for clinical diagnosis is up to three Tesla at this point.

And beyond that three Tesla is mostly for research purpose at this point.

A main function of magnet is,

that causes proton spins to rotate well,

which we represent as a precess.

So, those spins rotate at a frequency proportional to the magnetic field,

which is called resonance frequency.

So, this is the main function of magnet,

just rotate, make the proton spins to precess at a specific frequency.

So, without magnetic field,

all the proton spins in our body,

they rotate but they are not precessing.

They are just aligned along random directions.

But, when they are placed in a strong magnetic field,

they start to be rotate in itself and also they precess along a specific direction.

This is a main feature or main function of magnet.

There are two types of MRI magnet.

So, one that superconducting magnet,

and the other is a permanent magnet.

The superconducting magnet is,

for that it can achieve

high magnetic field and also it has a high maintenance costs for a magnet.

And it's most popular in hospital

because it can achieve high magnetic field and high magnetic field provides high SNR.

So, in most cases people will use superconducting magnet for MRI scanners.

And permanent magnet is also used in some,

most of the case in local hospitals.

So, because magnetic field strength is limited to about one Tesla.

But, it does not cause maintenance cost,

almost no maintenance costs for magnet,

so the maintenance cost is much low.

So, it's more useful for local hospitals.

But, the signal, image quality that we can achieve is a little

bit lower than the magnet we can use for the superconducting MRI.

And why are MRI scans so expensive?

And the reason is, it's difficult to make the magnetic field uniform.

So, MRI scanning is certainly pretty expensive.

It's more difficult with a higher magnetic field and wider bore size,

which is directly related to the price of MRI.

So, the human scanner requires wide bore size,

and also, it requires high magnetic fields,

which provide high signal.

So, which is directly related to the price of MRI and it's

also directly related to the cost of each MRI scan.

And also, helium is required for

maintaining a superconducting magnet and helium is expensive.

And these days, people try to develop a new magnet which minimize helium usage,

and also recycles the helium.

Boiled-off helium can be recycled for the usage.

And then that can minimize the cost for the helium.

But still, the maintenance cost is still expensive.

So, MRI is an integration of a lot of hardware equipment,

so that requires high maintenance costs.

And also, because of that,

MRI scans are so expensive.

We will talk about resonance part here.

So, the resonance part is about the radio frequency coil.

And for the radio frequency coil, we transmit energy.

So, we have transmission coils and also receives signal, so reception coil.

So, these two can be separated,

so we may have two separate radio frequency coils for transmission and reception.

Or sometimes, we may have

just one single RF coil which can be

used for both transmission and reception of MR signals.

So, for the transmission,

we like to have a uniform excitation profile,

and also for the reception,

we want to have a higher sensitivity.

So, for the transmission,

we typically have a body coil, RF coil,

which has the biggest RF coil in

MRI system which can be used for the transmission of MR signal.

And also and typically,

reception coil can be used for the purpose for the imaging region.

And then, we have a small RF coil that is closer to the region of interest.

But sometimes, we can use for small RF coil for both transmission and reception too.

Let's talk about the transmission part in detail.

The proton spins may have a different precession frequency.

And proton spins that have the same frequency as

the frequency in the transmission coil will receive energy and get excited.

And this is called Magnetic Resonance.

So, as shown in this figure,

so the one with the same frequency will get excited,

and then those with a different frequency will not get excited.

So, this example explains the concept of magnetic resonance.

And transmission of homogeneous energy is important.

So, the transmission is often performed with a large RF coil,

typically a body coil premounted on the scanner as I mentioned before.

And the reception part is when that transmission of RF energy is off,

the MR signals will be induced and detectable in the receiver RF coil.

So, these protons spins get excited and then,

when they return back to original position, they emit signal,

so which can be considered,

like this small magnet is rotating,

and this rotation of small magnet will induce current in this coil.

And this is the same procedure in detecting the signal in the MRI.

So, this has high sensitivity to the object is important.

So, the reception of MR is often performed with a small RF coil close to the object.

This is the imaging part.

So, the imaging part is about gradient coils,

which provide spatial information.

So, in contrast to other imaging modalities,

like CT, x-ray, SPECT,

and PET, ultrasound for optical imaging.

So, positional information over detector is not used to get spatial information in MRI.

So in case of MRI, the detectors are RF coils.

And then, these are not useful to get the spatial information for MRI.

And then, how can we get the spatial information

in MRI and that is about the gradient coils.

So, these gradient coils provide additional magnetic field,

which is just modulate magnetic field spatially and then,

that provides additional information,

and rather than just location of detectors.

So, these are the gradient coils for spatial information.

So, we have three different gradient coils along three different directions x,

y, and z direction.

So, these gradient coils generate,

modulate magnetic field along x,

or y, or z direction.

So, the main magnetic fields are along the z direction or range.

And the x and y gradient coils are to

modulate the main magnetic field strength along x and y,

but not to generate any fields along x and/or y.

So, to this x gradient coil,

y gradient coil does not generate the field along x or y direction,

but they just modulate

strengths of main magnetic field which is always along z direction.

So, that strength is different as a function of x,

or a function of y, or a function of z.

So, that's what gradient coils do for encoding the spatial information in MRI.

So, magnetic field induced by a gradient coil is a

superimposed on the field of the main MRI magnetic field.

And then, so it provides a varying magnetic field or gradient.

So, they cause precession frequency omega,

to be a function of spatial location.

So, B field is functional spatial location.

So, that means precession frequency is also a function of spatial location.

And spatially, different precession frequencies

enable us to get spatial information i.e., imaging.

So, as shown in this diagram.

So, there is main magnetic field.

And then, when there only exist gradient,

when current is applied through a gradient coil,

and gradient's magnetic field strength is added to this main magnetic field.

And then, some portion may have higher magnetic field strength than the origin,

than the other location and some of them maybe even lower than the origin location.

So, this is what gradient coils do to get the spatial information.

Let's talk about MRI safety briefly and then summarize the contents of the three.

So, is MRI safe?

Yes, it is safe and it does not cause any ionizing radiation.

But, people with pacemakers or artificial limbs or

devices that contain metal are not allowed for MRI imaging.

And I, myself have been volunteered for being scanned in MRI and for about 20 years.

And then, it has no,

so far though, I had no problem with MRI scans.

And another issue is a specific absorption rate.

So, it's a measure of the rate at which energy is absorbed

by the human body when exposed to radio frequency electromagnetic field.

So, this means the RF coil may generate heat in our body if the energy is too high,

but that can be represented in a unit of watts per kilogram.

There is an upper limit of SAR for safety,

and most clinical scanners do not allow any scan that exceeds the SAR limit.

So, MRI is safe.

So, it's already protected in the clinical scanner.

So, you don't need to worry about the MRI scan.

Here is the summary of the contents of this week.

So, MRI is noninvasive, biomedical imaging device,

with a relatively high spatial resolution,

and it provides various soft tissue contrast.

And it's an integration of many different imaging modalities in single scanner.

That's beauty of MRI.

And typically, clinical MRI had a field strength up to three Tesla,

which is 60,000 times greater than the earth's magnetic field.

And the magnet generates magnetic fields that causes protons to precess at

a frequency proportional to the strength of

the magnetic field which is called resonance frequency.

A radio frequency coil transmits and

receive and/or receives signal at the resonance frequency.

And gradient coils modulate

the resonance frequency depending on the spatial location along x,

y, and z direction.

So, these three hardware,

main magnet, and RF coils and gradient coil.

So, these are the three main hardware component of MRI.

And MRI is safe and does not cause ionizing radiation but

requires caution about the metal object in the body and also specific absorption rate.

This is the end of the lecture in the week.

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