MRI physics overview | MRI Physics Course | Radiology Physics Course #1

Summary

This introductory MRI physics module aims to build a comprehensive understanding of MRI imaging principles. It compares learning to assembling a puzzle, emphasizing the importance of gradually mastering individual concepts before integrating them for a complete picture. The script covers the basics, including the role of hydrogen atoms, the Cartesian plane for image localization, and the significance of magnetic moments and resonance frequencies. It also introduces key MRI parameters like TE and TR, which are crucial for generating image contrast, and hints at more complex topics like pulse sequences and k-space that will be explored in subsequent lectures.

Takeaways

• 🧩 The MRI physics module is designed to build a comprehensive understanding of MRI principles through multiple detailed talks.
• 🧭 The learning process is likened to solving a puzzle, where concepts are broken down and mastered before being combined for a complete picture.
• 🌐 MRI imaging uses the Cartesian plane to localize signals within the patient, with the Z-axis representing the longitudinal axis and the X-Y plane for transverse sections.
• 🌀 Nuclear magnetic resonance (NMR) is central to MRI, utilizing a large magnetic field to resonate hydrogen atoms, which are abundant and have a non-zero spin in the human body.
• 🧲 Hydrogen atoms, acting as tiny bar magnets, align and precess around their axis when subjected to an external magnetic field, creating a magnetic moment.
• 📊 The net magnetization vector, representing the combined magnetic moments of hydrogen atoms, is crucial for MRI imaging and is influenced by the external magnetic field.
• 🔄 The radio frequency (RF) pulse is applied perpendicular to the main magnetic field to move the net magnetization vector into the transverse plane, enabling signal measurement.
• 📉 The loss of transverse magnetization, or T2 decay, and the regaining of longitudinal magnetization, or T1 recovery, are key processes that occur independently and are exploited to generate image contrast.
• 🔄 The flip angle of the net magnetization vector, induced by the RF pulse, is a critical parameter in MRI imaging, affecting the signal generated and the resulting image contrast.
• 🔺 The time of echo (TE) and time of repetition (TR) are adjustable parameters that control the contrast in MRI images by manipulating the signal's sensitivity to T1 and T2 relaxation times.
• 🛠 Advanced imaging techniques, pulse sequences, and the use of k-space for data storage and image creation will be explored in further detail throughout the module.

Q & A

What is the main difference between MRI imaging and other imaging modalities like X-ray, CT, and ultrasound?

The main difference is that in MRI imaging, the signal used to generate the image comes from within the patient, specifically from the hydrogen atoms in the body, whereas in X-ray, CT, and ultrasound, the signal comes from external sources or mechanical vibrations.

What is the Cartesian plane used for in MRI imaging?

The Cartesian plane is used to localize the signal within the patient. It separates the image into three axes: the longitudinal (Z or z-axis), and the transverse (X-Y plane), which are essential for understanding the orientation of the imaging slices.

Why is the hydrogen atom used in MRI imaging?

The hydrogen atom is used because it is abundant in the human body, and it has a non-zero spin, which means it acts as a tiny bar magnet with a north and south pole, contributing to the magnetic moment used in image generation.

What is nuclear magnetic resonance and how is it relevant to MRI imaging?

Nuclear magnetic resonance is a phenomenon where certain atomic nuclei, like hydrogen, resonate in a large magnetic field. In MRI imaging, this principle is used to induce resonance in hydrogen atoms within the patient, which is essential for image formation.

How does the magnetic field influence the hydrogen atoms in the MRI scanner?

The magnetic field causes the hydrogen atoms to align with it and precess around their own axis at a frequency determined by the type of atom and the strength of the magnetic field.

What is the significance of the net magnetization vector in MRI imaging?

The net magnetization vector represents the combined magnetic moments of all the free hydrogen atoms in the body. It is crucial for MRI imaging as it is the vector that is manipulated and measured to generate the image.

What is the purpose of the radio frequency (RF) pulse in MRI imaging?

The RF pulse is applied to move the net magnetization vector perpendicular to the main magnetic field, allowing for the measurement of the signal. It also causes the protons to precess in phase, which is necessary for generating a detectable signal.

What is the flip angle in the context of MRI imaging?

The flip angle refers to the angle at which the net magnetization vector is tilted from its alignment with the main magnetic field to a position perpendicular to it by the RF pulse.

Why is it important to consider the time of echo (TE) and time of repetition (TR) in MRI imaging?

The TE and TR are parameters that control the timing of the MRI pulse sequence, affecting the contrast in the images by exploiting differences in the relaxation times (T1 and T2) of various tissues.

What is the role of k-space in MRI imaging?

K-space is a mathematical space used to encode the data for different slices in an MRI image. It is filled with data that is then used to reconstruct the image by stacking the k-spaces on top of one another.

What are the different types of pulse sequences mentioned in the script, and what is their significance?

The script mentions spin echo, inversion recovery, and gradient echo sequences. These pulse sequences are used to generate images with different contrasts and are essential for understanding various tissue properties and diagnosing conditions.

Outlines

00:00

🧠 Introduction to MRI Physics Module

This paragraph introduces the MRI physics module, emphasizing the detailed nature of the course and the goal of building a conceptual understanding of MRI physics. The instructor likens learning MRI physics to assembling a puzzle, suggesting a step-by-step approach to grasp the subject. The module's content is structured around various talks that delve into specific topics, and the instructor presents a 3D model of an MRI machine to illustrate the complexity of its components. The fundamental difference between MRI and other imaging techniques is highlighted, with MRI relying on signals generated from within the patient, necessitating precise localization techniques like the Cartesian plane. The concept of nuclear magnetic resonance (NMR) is introduced, along with the importance of hydrogen atoms in MRI imaging due to their abundance and non-zero spin, which acts as tiny bar magnets within the body.

05:02

🌀 Understanding Net Magnetization and MRI Signal Generation

The second paragraph delves into the concept of net magnetization, explaining how the magnetic moments of hydrogen atoms combine to form a net magnetization vector within the sample. It discusses the alignment of hydrogen atoms in parallel or anti-parallel directions relative to the applied magnetic field and the resulting energy states. The paragraph further explains how the net magnetization vector is influenced by the MRI machine's magnetic fields and how the radio frequency (RF) pulse is used to move this vector into the transverse plane for signal measurement. The importance of matching the RF pulse frequency to the proton precession frequency is underscored, as is the process of signal induction in the receiver coil due to the movement of the net magnetization vector in the transverse plane.

10:03

📉 Exploring T2* Decay and T1 Recovery in MRI

This paragraph explores the processes of T2* decay and T1 recovery, which are crucial for generating contrast in MRI images. T2* decay refers to the loss of phase coherence among protons in the transverse plane, leading to a reduction in the net magnetization vector and signal intensity. The rate of this decay varies across different tissues, allowing for differentiation in the image. Simultaneously, the paragraph explains T1 recovery, which is the regaining of longitudinal magnetization over time. The rate of T1 recovery also varies among tissues and is independent of T2* decay. The paragraph highlights the importance of these processes in creating contrast within MRI images and mentions the use of different echo times (TE) and repetition times (TR) to manipulate image contrast.

15:04

🔄 The Role of TE and TR in Image Contrast

The fourth paragraph discusses the technical aspects of MRI imaging, focusing on the manipulation of time of echo (TE) and time of repetition (TR) to generate image contrast. It illustrates how varying TE and TR affects the signal intensity from different tissues, using the example of CSF and fat. The paragraph explains that a short TR can emphasize differences in T1 recovery, while a long TR, combined with an appropriate TE, can highlight differences in T2 decay. The importance of these parameters in creating T1-weighted and T2-weighted images is emphasized, with T1 images showing differences in longitudinal relaxation and T2 images reflecting differences in transverse relaxation.

20:06

🛠️ Advanced MRI Techniques and Image Generation

The final paragraph provides an overview of the advanced MRI techniques and pulse sequences that will be covered in the module, such as spin echo, inversion recovery, and gradient echo sequences. It also mentions the exploration of MR spectroscopy and various angiography techniques. The paragraph outlines the process of image generation, including the storage and utilization of data in k-space, which is essential for encoding different slices in the MRI image. The instructor assures that each component will be examined in detail in subsequent talks, helping to build a comprehensive understanding of MRI imaging. The paragraph concludes with an invitation to use the provided question bank for practice and knowledge assessment.

Keywords

💡MRI

Magnetic Resonance Imaging (MRI) is a medical imaging technique that uses a strong magnetic field and radio waves to create detailed images of the body's internal structures. The script discusses how MRI works and its physics, making it the central theme of the video. MRI is differentiated from other imaging modalities like X-ray and CT, emphasizing its unique method of using signals from within the patient's body.

💡Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance is a physical phenomenon that MRI technology is based on. It involves the resonance of atomic nuclei in a magnetic field. The script explains that MRI uses a large magnetic field to induce resonance in hydrogen atoms within the patient, which have a non-zero spin and act like tiny bar magnets, contributing to image generation.

💡Hydrogen Atom

The hydrogen atom is crucial in MRI because of its abundance in the human body and its non-zero spin, which allows it to act as a tiny bar magnet with a magnetic moment. The script mentions that MRI imaging specifically uses hydrogen atoms due to these properties, making them key to the imaging process.

💡Cartesian Plane

The Cartesian plane is used in MRI to localize the signal coming from within the patient. It is divided into three axes—longitudinal (Z or z-axis), and transverse (X and Y axes). The script describes how the Cartesian plane is essential for understanding the orientation of MRI images and the localization of signals.

💡Magnetic Moment

Magnetic moment refers to the measure of the magnetic strength and orientation of an object or particle that has a magnetic nature. In the script, it is used to describe how the hydrogen atoms, with their non-zero spin, have a magnetic moment that can be influenced by an external magnetic field, which is vital for MRI imaging.

💡Precession

Precession is the change in orientation of the magnetic moment of an object in a magnetic field. The script explains that hydrogen atoms precess around their own axis when subjected to the magnetic field of the MRI scanner, and this precession occurs at a frequency determined by the strength of the magnetic field and the type of atom.

💡Radio Frequency Pulse

A radio frequency pulse is used in MRI to manipulate the net magnetization vector of the hydrogen atoms. The script describes how this pulse, when matched to the precessional frequency of the protons, can cause the protons to fan out and align more perpendicularly to the main magnetic field, which is a critical step in generating the MRI signal.

💡Flip Angle

The flip angle is the angle by which the net magnetization vector is rotated from its initial alignment with the main magnetic field. The script uses the term to explain how the radio frequency pulse can flip the net magnetization vector to different angles, such as 90 degrees, to generate a transverse magnetization for signal detection.

💡T1 and T2 Relaxation

T1 and T2 relaxation times are parameters that describe how quickly the magnetization of tissues returns to its equilibrium state after being disturbed by an RF pulse. The script explains that these relaxation times are different for different tissues and are used to generate contrast in MRI images. T1 recovery refers to the regain of longitudinal magnetization, while T2 decay refers to the loss of transverse magnetization.

💡Echo Time (TE) and Repetition Time (TR)

Echo time (TE) and repetition time (TR) are timing parameters in MRI pulse sequences that influence the contrast in the resulting images. The script explains that manipulating TE and TR can emphasize different relaxation properties of tissues. TE is the time between the RF pulse and the peak of the signal, while TR is the time between consecutive RF pulses.

💡k-Space

k-Space is a mathematical space used in MRI to represent the spatial frequencies that make up an image. The script mentions that data is stored in k-space and then used to create the final MRI image by stacking these spatial frequencies. Filling k-space is a critical step in the image formation process.

Highlights

Introduction to the MRI physics module, emphasizing the detailed exploration of MRI physics concepts.

Learning MRI physics is compared to solving a puzzle, suggesting a structured approach to understanding complex topics.

Explanation of the MRI machine's structure, highlighting the layers representing different types of magnets used for imaging.

MRI's distinction from other imaging modalities due to its reliance on signals originating from within the patient.

Introduction of the Cartesian plane for localizing signals within the patient during MRI imaging.

The concept of nuclear magnetic resonance and its application in MRI using the hydrogen atom.

Explanation of the hydrogen atom's non-zero spin and its role as a tiny bar magnet within the body.

Description of the net magnetic moment generated by the combination of magnetic moments of free hydrogen atoms.

The influence of an external magnetic field on hydrogen atoms, causing them to align and precess.

The relationship between the strength of the magnetic field and the precessional frequency of hydrogen atoms.

Discussion on the alignment of hydrogen atoms in parallel or anti-parallel to the magnetic field and their energy states.

Introduction of the net magnetization vector and its significance in MRI imaging.

The process of moving the net magnetization vector perpendicular to the main magnetic field for signal measurement.

Explanation of the radio frequency pulse and its role in altering the net magnetization vector for image generation.

The concept of flip angle in MRI and its impact on the transverse magnetization of protons.

The use of Faraday's law of induction to measure the signal generated by the movement of the net magnetization vector.

Discussion on the loss of phase coherence and its effect on the signal generated from the transverse plane.

Introduction of T2 star curves and their significance in differentiating tissue types within the body.

The independent processes of transverse magnetization loss and longitudinal magnetization regain.

The importance of measuring signal perpendicular to the main magnetic field in MRI imaging.

Overview of generating image contrast by manipulating time of echo and time of repetition (TE and TR) parameters.

Explanation of T1 and T2 imaging, and how differences in tissue relaxation times contribute to image contrast.

Introduction to pulse sequences used in MRI, including spin echo, inversion recovery, and gradient echo sequences.

Discussion on advanced imaging techniques, MR spectroscopy, and angiography in MRI imaging.

Overview of k-space and its role in encoding data for different slices in MRI imaging.

Mention of a question bank for self-testing in radiology physics exams, linked in the description.

Transcripts

hello everybody and welcome to the MRI

physics module I can't wait to share the

upcoming talks with you now this course

consists of multiple different talks and

each one dives into a fair amount of

detail regarding that specific topic and

it's my hope that by the end of this

module you'll have a good conceptual

understanding as to how exactly MRI

physics works now I think about learning

MRI physics much like building a large

puzzle if I was to pour all the puzzle

pieces out on the table and pick up one

piece it'll be very difficult for me to

accurately place that piece where it

goes on the table what we want to do is

separate the puzzle into the edge pieces

find the corners separated into various

different color groups and then work on

each one of those groups individually

before combining them to give us the

overall picture now what I want to do

today is show you the front cover of the

puzzle we're trying to build show you

where we're going throughout this course

then we can take a step back and work on

each one of these individual sections

before putting them together and

hopefully having a good clear understand

scanning of how MRI physics works now as

you'll see here is a 3D model of the MRI

machine itself and you can see it's made

of multiple different layers and each

one of these layers represents a

different type of magnet that we're

going to use to generate our image now

if we look at the machine from side on

and then open up that machine we can see

where the patient lies within the MRI

machine

now MRI is different from X-ray and CT

Imaging as well as ultrasound imaging in

the fact that the signal that we use to

generate our image is actually coming

from within the patient and because the

signal is coming from within the patient

we need a way of localizing where

exactly that signal is coming from and

what we use is what's known as the

Cartesian plane we can separate this

image into three separate axes the first

by convention is the longitudinal axis

the axis that runs from head to toe

along the patient and that's always

labeled the Z or z-axis we can then cut

the patient in transverse section an

axial plane using the X Y axis here or

the X Y plane and that's what's known as

the transverse plane so we've got the

longitudinal plane and the transverse

plane and these are really important

Concepts to take forward into the

upcoming talks now in MRI imaging we use

a concept known as nuclear magnetic

resonance we use a large magnetic field

in order to induce resonance in certain

atoms within the patient and in MRI

imaging we use the hydrogen atom to do

this now the hydrogen atom is useful one

because it's abundant within the body

there are billions of hydrogen atoms

within the human body and two the

hydrogen atom has what's known as

non-zero Spin and atoms with non-zero

spin effectively act as Tiny bar magnets

within the body they have a north and a

South Pole and as a result have what's

known as a Magnetic Moment now the

Magnetic Moment In These diagrams is

represented by this Arrow here

now the arrow can actually be used as a

vector within the MRI machine it has

both Direction and magnitude and the

combination of the magnetic moments

amongst all the free hydrogen atoms

within the body is what's used to

generate the image now in conventional

MRI imaging we only use the hydrogen

atom to create our MRI imaging so we can

think of our patient as being a

combination of multiple different

hydrogen atoms that are moving randomly

within the body moving with Brownian

motion and the amount of movement is

determined by the temperature of that

patient

now because hydrogen protons have a

magnetic moment they will be influenced

by an external magnetic field much like

a compass aligns with the magnetic field

of the Earth's core we can also pass a

large magnetic field across the patient

that magnetic field will cause two

things to happen the first is that the

hydrogen atoms will align with the

magnetic field and the second is that

they will precess around their own axis

if you think of a spinning top on a

table experiencing Gravity the spinning

top processes like this around its own

axis the same thing is happening to

these hydrogen atoms within the patient

they're along the main magnetic field of

our MRI scanner and they process at a

certain frequency now that frequency is

determined by the type of atom so here

it's hydrogen and it's determined by the

strength of the magnetic field the

Precision frequency is directly

proportional to the strength of that

magnetic field higher the magnetic field

the higher the processional frequency

now don't worry this is starting to

confuse you we're going to look at each

one of these factors in isolation in the

coming talks now as you can see the

hydrogen atoms either align parallel to

the magnetic field or anti-parallel to

the magnetic field and in fact when we

look at quantum physics later the

hydrogen atom itself exists in both of

these states but for now what's

important is that the absolute number of

hydrogen atoms that exist in the

parallel Direction exceed those of that

in the anti-parallel direction and those

in the parallel direction are in a

slightly lower energy state to those in

the anti-parallel direction now we can

combine these magnetic moments to create

a net Magnetic Moment within the sample

that we are applying this magnetic field

to and as you can see there are more

magnetic moments in the parallel

Direction than they are in the

anti-parallel direction

secondly to note although the hydrogen

atoms are presetting at the same

frequency they are out of phase from one

another the X and Y vectors on each

individual Magnetic Moment here cancel

each other out you can see there's an

equal distribution within the X and Y

plane and what we get here is what's

known as net magnetization Vector we

combine all of these magnetic moments

here and we get the net magnetization

Vector now the net magnetization Vector

is along the longitudinal axis the

z-axis on the Cartesian plane there is

no X or Y value here because those

processional frequencies are out of

phase with one another

now we mustn't think of individual

hydrogen atoms when we are looking at

MRI imaging we need to think of the net

magnetization vector and how that is

influenced by changing magnetic fields

within the MRI machine so what we can do

is replace these hydrogen atoms with the

net magnetization Vector here

now within MRI imaging what we want to

measure is this net magnetization Vector

but we can't measure it along the

parallel Direction along the

longitudinal Direction here because our

main magnetic field strength is too

strong and it will interfere with our

measurement of this net magnetization

Vector what we want to do is move that

net magnetization Vector perpendicular

to our main magnetic field that will

allow us to measure that signal and

that's exactly what we do in MRI imaging

we have our main magnetic field that is

forcing those protons into the parallel

Direction what we then do is apply a

second magnetic field known as the radio

frequency pulse now the radio frequency

pulse acts in the perpendicular plane to

that main magnetic field and the radio

frequency pulse alternates at a

frequency that is equal to the

processional frequency of the protons if

the frequency of the radio frequency

pulse matches that of the process

additional frequency of the hydrogen

atoms within the patient two things will

happen the first is that the protons

will start to Fan out and become more

perpendicular with the main magnetic

field and the second is that the

processional frequencies of those

protons will start to process in Phase

our net magnetization Vector now will

get some transverse magnetization so

magnetization in the X Y plane so as we

apply this radio frequency pulse that

net magnetization Vector will start

gaining some transverse magnetization

and the angle at which we flip that net

magnetization Vector is what's known as

the flip angle in this example we

flipped it 90 degrees here now the

protons are all processing in Phase with

one another and they now align 90

degrees to the main magnetic field

now what we can do is place a small coil

here and the movement of a magnet as

we've seen with Faraday's law of

induction the movement of a magnet can

induce a current and it's the movement

of this net magnetization Vector that

induces a current within our receiver

coil that we then use that signal to

generate our image so we can see this

now Vector precessing in the transverse

plane in the X Y plane and we can

measure a signal based on the movement

of that Vector within the transverse

plane now this Vector is only moving in

this plane because of that radio

frequency pulse and importantly that

radio frequency pulse has to match the

processional frequency of the hydrogen

atoms if you're jumping on a trampoline

and someone is jumping at exactly the

same time as you you will get double

bounce you will get extra energy and you

will jump higher and higher if other

people are jumping on that trampoline

but not at the same time they're not

getting that extra energy they're

bouncing the same only when those

frequencies match will that energy be

transferred those protons start to

process in phase and the angle of

magnetization will start changing and

that angle changes dependent on the time

of the radio frequency pulse as well as

the amplitude of that radio frequency

pulse now that we've moved that Vector

into the transverse plane and we've

generated a signal we want to stop this

radio frequency pulse now we can see the

signal that has been generated here is

based on that net magnetization Vector

precessing at the frequency of the radio

frequency pulse now we don't actually

get a signal like this because what we

actually do is apply a radio frequency

pulse and then stop that radio frequency

pulse now what actually happens is the

net magnetization vectors are all

processing at that radio frequency pulse

and when we stop the radio frequency

pulse they will start to go out of phase

again and it's that loss of phase

coherence that will cause a net

magnetization Vector in the transverse

plane to get smaller and smaller so

let's have a look at an example here

here this Arrow here represents the net

magnetization Vector in the transverse

plane

as we stop that radio frequency pulse we

will see the various net magnetization

vectors start to become out of phase

with one another the more and more out

of phase they become the less our net

magnetization Vector in the transverse

plane will be and we see that the signal

that is generated becomes less and less

now this curve that we draw down like

that is what's known as the free

induction decayed curve or the t2 star

curve now importantly each and every

tissue within the body will have

different T2 star curves or different

free induction Decay curves if we look

at Water the free induction Decay is

very slow over time and if we look at

something like bone or fat the free

induction Decay is much faster and it's

those differences in loss of transverse

magnetization that we can use to start

generating contrast within our image and

we're going to look at that more within

this talk now this process is happening

simultaneously with a separate

independent process the loss of

transverse magnetization the loss of the

vector within the X Y plane is purely

because of that loss of phase between

the separate protons within the various

different tissues and the rate at which

we lose that transverse magnetization is

what's known as free induction Decay now

at the same time we are also gaining or

regaining the longitudinal magnetization

within our sample if we have the net

magnetization Vector perpendicular to

our main magnetic field we have lost all

of the longitudinal magnetization or the

net magnetization in the z-axis

as time goes by and that radio frequency

pulse has been turned off what will

happen is that net magnetization Vector

will slowly regain longitudinal

magnetization so we can see now as as

time goes by we will regain some

longitudinal magnetization the y-axis

here is representing the amount of MZ or

longitudinal magnetization along the

z-axis of our Cartesian plane along the

longitudinal axis of the patient

now importantly as we're gaining

longitudinal magnetization here we are

not losing transverse magnetization

because of the tilt of the protons we

lose transverse magnetization because of

those protons going out of phase with

one another that free induction Decay or

T2 star the loss of transverse

magnetization happens much quicker than

this regaining of the longitudinal

magnetization

now as time goes by even further we get

more and more longitudinal magnetization

now as we can see here we are gaining

longitudinal magnetization but by this

point we have lost all of our transverse

magnetization because although the

protons have regained some longitudinal

magnetization by this point they are

completely out of phase with one another

and all of those X Y vectors have

canceled one another out we are still

now regaining longitudinal relaxation or

T1 recovery along the z-axis which takes

a much longer period of time

now when the vectors are all aligned

with the magnetic field with the main

magnetic field we have regained 100 of

our longitudinal relaxation now

important to note that these two

processes happen independently of one

another if we know the free induction

decay of a certain tissue we can't

calculate the T1 recovery or the

longitudinal recovery of that tissue

they are completely independent of one

another

both longitudinal relaxation like we can

see here and free induction Decay

happened at different rates for

different tissues and it's those

differing rates that we use to generate

contrast within our image and lastly and

what's most important to remember is we

can only measure signal that is

perpendicular to the main magnetic field

so it's very difficult to measure

longitudinal magnetization unless we

flip that Vector again perpendicular to

the main magnetic field

now we can go about generating images by

using two separate parameters that will

exploit these differences in the free

induction Decay or T2 star Decay and T1

recovery or longitudinal relaxation now

the first parameter that we can use is

what's known as the time of echo now I'm

going to use these two knitting needles

to show two separate types of tissue now

we have the protons have been flipped

into the longitudinal Direction in both

of these tissues say CSF and fat

now what happens is we apply the radio

frequency pulse to 90 degrees our

protons are now processing perpendicular

to the main magnetic field at 90 degrees

now what happens is we start to lose

transverse magnetization as these start

to process out of phase with one another

we lose that T2 or the free induction

Decay because these are becoming out of

phase within that one another they were

initially in Phase providing maximum

signal that signal gets lost as we get

more and more out of phase now the time

of echo is the time from that RF pulse

at 90 degree RF pulse to the time that

we actually measure the signal being

generated by these tissues now given

more and more time the phase incoherence

will become more and more the difference

between these two tissues will become

more and more so as we wait a longer

period of time the difference between

these two tissues will become more and

more but the signal will become less and

less so it's a trade-off between getting

good signal and getting contrast between

these two tissues now that contrast is

based on the loss of transverse

magnetization

at the same time both of these tissues

are gaining longitudinal magnetization

in the z-axis and if we wait a really

long period of time we can see that they

will gain their longitudinal

magnetization at different rates but if

we wait long enough they will gain that

full net longitudinal magnetization

Vector we can then flip them again to 90

degrees with a second RF pulse the time

from that first RF pulse to the second

RF pulse is what's known as the time of

repetition or our TR time

if we wait a long period of time flip it

90 degrees and then wait another period

of time before measuring that signal

that time to Echo time from the RF pulse

to when we measure the differences in

Signal are going to be based on the loss

of transverse magnetization

now what happens if we wait a short

period of time a short TR time

we'll see that longitudinal

magnetization or longitudinal relaxation

occurs at different rates in fact the

longitudinal or T1 recovery happens much

faster than it does in water

now if we wait a short period of time

and don't allow the full neck

longitudinal magnetization or T1

recovery to happen what we'll see is the

longitudinal magnetization Vector in fat

is much longer than that of water now

when we apply a 90 degree RF pulse the

amount of transverse magnetization will

only be equal to the amount of

longitudinal recovery that has occurred

so our water will have a much smaller

net magnetization in the transverse

plane than the fact will so when we flip

this 90 degrees this is what's going to

happen the signal that is being

generated from fat is much more than the

signal that's being generated from water

the difference that we are seeing here

is because of that short time of

repetition it's because of the

differences in longitudinal relaxation

or the differences in T1 recovery so

when we make our time to repetition

short we're getting differences in

longitudinal recovery three or T1

differences we are not measuring the t2

differences between these two tissues

now I know this is a really difficult

concept and we have dedicated videos

specifically looking at the types of

relaxation and looking at t e and TR

times what I want to give you is an idea

of how we generate contrast in an image

and again I'm just showing you the front

cover of the puzzle that we are trying

to create you don't need to understand

these Concepts now but it's useful to

know where we're going in future

lectures

now we can manipulate the te and the TR

times as I've shown you now to generate

different contrast within our image as I

showed you in that example with a short

TR time the water lost its signal

because it wasn't gaining its

longitudinal relaxation as fast as the

fat was and that's what's generating a

T1 image where water like our CSF has a

low signal and fact like the

subcutaneous fattier has a high signal

when we had a long time to repetition we

allowed all of those tissues to fully

regain their magnetization in the

longitudinal plane before flipping them

into the 90 degrees and then having an

echo time that measured that transverse

magnetization that's what generates a T2

image where the differences between

water and fat now come from the

differences in the rate at which they

defaze in the transverse plane water

takes a very long time to de-phase and

the signal remains high in the

transverse plane unlike fat which

because of the spin spin interactions

that we're going to look at in a future

talk reduces the transverse

magnetization signal because fat D

phases relatively quickly compared to

water and we can see we get dark signal

in the fat coated axons in our white

matter we get bright signal in the water

in our CSF because of the differences in

the t2 relaxation or the free induction

Decay between those two tissues now the

way in which we Act generate these

images is more complicated than what we

covered here but the underlying

principle will always come back to the

time of Echo and the time of repetition

we still need to look at how we exactly

go about localizing the different

signals within the patient how we select

certain slices along the patient and

then how we encode the different X and Y

axis components of our image and in

order to do this we use what is known as

different pulse sequences and in this

module we're going to look at the main

pulse sequences the spin Echo sequence

the inversion recovery sequence as well

as gradient Echo sequences we will then

expand on these different sequences

looking at more advanced imaging

techniques we'll also look at Mr

spectroscopy as well as different types

of angiography in MRI imaging we'll end

off the module by looking at different

types of MRI artifacts as well as image

quality and safety within MRI imaging

now when we are generating signals

Within These different pulse sequences

we need a way of storing that data and

then ultimately using that data to

create an image now we use what is known

as k space to encode for the different

slices on our MRI image and we're going

to spend some time looking at how we go

about filling the data within a specific

case space and how we can use that case

space then to go about creating our

image stacking those K spaces on top of

one another in order to create a

scrollable image so I know this talk is

very complicated and if you're new to

MRI it's going to sound like a different

language and that's okay each and every

talk from now on is going to be looking

at a specific component of what we've

covered so hopefully you can use this

talk as the picture on the front of the

puzzle that we're trying to create and

when we go about building those

different sections on our puzzle the

different units within this module you

know where those units fit in on the

broad overarching picture now by the

time I've completed this entire physics

module there will be a question bank

that's linked Below in the first line of

the description you can use that

question Bank to test yourself with

actual past paper questions in Radiology

physics exams I've collated all of those

questions together and it's a great way

for you to test your knowledge and

identify knowledge gaps before heading

into a radiology Physics Exam so I hope

this has at least made some sense to you

use this as a springboard now going into

the following modules to go and build

your knowledge around MRI imaging so

until the first talk where we look at

the magnets in MRI imaging I'll see you

there goodbye everybody