T2 Relaxation, Spin-spin Relaxation, Free Induction Decay, Transverse Decay | MRI Physics Course #4

Summary

This educational video script delves into the intricacies of nuclear magnetic resonance (NMR) and relaxation processes in MRI. It explains how protons align in an external magnetic field and resonate at a specific frequency. The script then focuses on T2 relaxation, detailing the loss of transverse magnetization due to spin-spin interactions and the impact of magnetic field inhomogeneities. It uses the analogy of spinning basketballs to illustrate dephasing and discusses T2 star decay, highlighting the differences between T2 and T2 star relaxation times. The script also explores how T2 relaxation curves vary across tissues and the role of echo time (TE) in MRI imaging, emphasizing its importance in generating tissue contrast.

Takeaways

• 🧲 The process of nuclear magnetic resonance (NMR) involves placing protons in an external magnetic field, causing them to align and resonate at a specific frequency.
• 🌀 A radiofrequency (RF) pulse is applied perpendicular to the magnetic field, causing protons to resonate in phase and fan out from the longitudinal magnetization vector.
• 🔄 At 90 degrees, the net magnetization vector transitions from longitudinal to transverse, resulting in maximum transverse magnetization.
• 📉 T2 relaxation refers to the loss of transverse magnetization, which occurs due to dephasing of spins as the RF pulse is stopped.
• 🏀 Spin-spin relaxation, characterized by the dephasing of spins due to interactions, is a key component of T2 relaxation, with the rate depending on the tissue type.
• 🌐 T2 star decay accounts for the loss of signal due to both spin-spin interactions and magnetic field inhomogeneities, which are not solely due to spin interactions.
• 🛑 The MRI scanner's inability to create a perfectly homogeneous magnetic field contributes to T2 star decay, along with the presence of substances in the patient and local magnetic field disruptions.
• 🔄 A 180-degree RF pulse can be used to rephase spins that have dephased, effectively compensating for magnetic field inhomogeneities and recovering the T2 relaxation signal.
• 📊 T2 relaxation curves vary by tissue type, with water or CSF showing less dephasing and thus slower signal loss compared to fat, which has more rapid signal decay.
• ⏱ Adjusting the time to echo (TE) in MRI can manipulate the contrast between tissues by highlighting differences in T2 relaxation times, allowing for clearer differentiation in imaging.

Q & A

What is nuclear magnetic resonance (NMR)?

Nuclear magnetic resonance is a process where protons within an external magnetic field align with that field and resonate at a set frequency. A radiofrequency magnetic pulse is applied, causing the protons to start resonating in phase with one another, leading to a loss of longitudinal magnetization and a gain of transverse magnetization.

What are the two types of relaxation processes discussed in the script?

The two types of relaxation processes discussed are T2 relaxation, which is the loss of transverse magnetization, and T1 relaxation, which is the regaining of longitudinal magnetization.

What is meant by 'spin-spin relaxation'?

Spin-spin relaxation refers to the loss of transverse magnetization that occurs as protons or spins go out of phase with one another after the radio frequency pulse is stopped. This dephasing is primarily caused by interactions between the spins themselves.

How does the composition of fat affect its T2 relaxation?

Fat, composed of long chains of triglycerides, allows the molecules to bump into each other easily, leading to more frequent interactions between spins. This results in a quicker loss of phase and faster T2 relaxation, causing a rapid decrease in signal compared to tissues like CSF.

What is the significance of the T2 star decay in MRI?

T2 star decay in MRI accounts for the loss of signal due to both spin-spin interactions and magnetic field inhomogeneities. It is a measurable decay that occurs more quickly in tissues than the ideal T2 decay, which is solely due to spin-spin interactions.

Why do magnetic field inhomogeneities affect the T2 star decay?

Magnetic field inhomogeneities affect the T2 star decay because they cause protons at different locations to experience varying magnetic field strengths, leading to different precession rates and increased dephasing, which in turn accelerates the loss of transverse magnetization.

How does the 180-degree radio frequency pulse help in compensating for magnetic field inhomogeneities?

The 180-degree radio frequency pulse refocuses the dephasing spins by flipping their phase, allowing the faster and slower precessing spins to realign. This effectively compensates for the local field inhomogeneities, leading to a re-phasing of the spins and an increase in the transverse magnetization vector.

What is the purpose of the time to echo (TE) in MRI?

The time to echo (TE) in MRI is the time interval between the 90-degree RF pulse and the point at which the signal is sampled. It is crucial for controlling the contrast in the image by highlighting differences in T2 relaxation times between various tissues.

How does the choice of TE affect the MRI image contrast?

A shorter TE results in a higher signal but less contrast between tissues, as the differences in T2 relaxation times are not fully expressed. A longer TE allows for greater distinction between tissues based on their T2 relaxation properties, increasing the contrast in the image.

What is the relationship between T2 relaxation and the appearance of different tissues in MRI?

T2 relaxation times vary between different tissues, with some tissues like CSF having longer T2 times and appearing brighter, while others like muscle have shorter T2 times and appear darker. The MRI image contrast is directly influenced by these T2 differences.

Outlines

00:00

🧲 Understanding Nuclear Magnetic Resonance and T2 Relaxation

The paragraph introduces the concept of nuclear magnetic resonance (NMR), where protons align with an external magnetic field and resonate at a specific frequency. A radiofrequency pulse is applied to cause the protons to resonate in phase, leading to a loss of longitudinal magnetization and a gain in transverse magnetization. The lecture focuses on T2 relaxation, which is the loss of transverse magnetization. Spin-spin relaxation is explained as the process where protons lose phase coherence due to interactions with each other, leading to a decrease in transverse magnetization. The analogy of spinning basketballs is used to illustrate how spins can go out of phase when they interact with one another. The paragraph also discusses the difference between T2 relaxation and T2* decay, with the latter including signal loss due to magnetic field inhomogeneities. Examples of fat and CSF tissues are used to demonstrate how the rate of signal loss varies between different types of tissues due to their molecular structures and interactions.

05:02

🌀 Exploring T2* Decay and Its Causes

This paragraph delves deeper into T2* decay, which is the measurable decay of the transverse magnetization vector in MRI. It is distinguished from T2 decay by the fact that T2* includes signal loss due to magnetic field inhomogeneities, not just spin-spin interactions. The paragraph explains that T2* decay occurs more quickly in tissues due to these inhomogeneities. Three mechanisms causing magnetic field inhomogeneities are discussed: imperfections in the MRI scanner's magnetic field, substances within the patient that disrupt local magnetic fields, and the de-phasing of spins themselves. The paragraph emphasizes the importance of understanding T2* decay for imaging, as it affects the contrast and visibility of different tissues in MRI scans.

10:03

🔄 Compensating for T2* Decay with 180-Degree RF Pulse

The paragraph explains how the T2* decay, caused by local magnetic field inhomogeneities, can be compensated for in MRI imaging. It describes the process of applying a 180-degree radiofrequency (RF) pulse to re-phase the de-phasing spins, allowing for a more accurate measurement of T2 relaxation. The use of a 180-degree RF pulse effectively 'refocuses' the spins, leading to a temporary increase in the transverse magnetization vector. This technique is crucial for generating an echo, which is a signal that can be sampled to provide information about T2 relaxation. The paragraph also touches on the concept of time to echo (TE), explaining how varying this time can affect the signal strength and the contrast between different tissues in an MRI image.

15:04

📊 Impact of Time to Echo on MRI Contrast

This final paragraph discusses the practical implications of the time to echo (TE) on MRI imaging. It explains how different TE values can be chosen to highlight the T2 relaxation differences between tissues, which in turn affects the contrast in the resulting MRI images. The paragraph illustrates how a short TE results in high signal but low contrast between tissues, while a longer TE increases contrast by showing more significant differences in signal intensity. The importance of selecting the appropriate TE for imaging specific tissues is emphasized, as it directly influences the visibility and diagnostic utility of the MRI scan. The paragraph concludes by setting the stage for the next lecture, which will cover T1 relaxation and its implications for MRI imaging.

Keywords

💡Nuclear Magnetic Resonance (NMR)

Nuclear Magnetic Resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation, and is the fundamental principle behind Magnetic Resonance Imaging (MRI). In the script, NMR is described as the process where protons align with an external magnetic field and resonate at a set frequency, which is then perturbed by a radiofrequency pulse, causing the protons to fan out and change their magnetization.

💡Magnetization Vector

The magnetization vector refers to the collective magnetic moment of protons within a magnetic field. In the context of the video, it is described as aligning with the external magnetic field and then being manipulated to different angles (like 90 degrees) through radiofrequency pulses, which results in a change from longitudinal to transverse magnetization.

💡Transverse Magnetization

Transverse magnetization is the component of the magnetization vector that is perpendicular to the main magnetic field. The script explains that this form of magnetization is crucial for MRI as it is what generates the signal detected by the scanner. The process of T2 relaxation affects the transverse magnetization, leading to a loss of phase coherence among protons.

💡T2 Relaxation

T2 relaxation is the process by which transverse magnetization decays over time due to the dephasing of protons. The script describes T2 relaxation as involving two mechanisms: spin-spin relaxation, where protons lose phase coherence because of interactions with each other, and effects of magnetic field inhomogeneities, which also contribute to the decay of transverse magnetization.

💡Spin-Spin Relaxation

Spin-spin relaxation is a term used to describe the loss of phase coherence between protons due to their interactions with each other. In the script, it is likened to spinning basketballs on fingers; as they interact, they lose synchrony. This analogy helps to illustrate how the protons in different tissues, like fat and CSF, behave differently during T2 relaxation.

💡T2 Star Decay

T2 Star Decay refers to the observed signal decay in MRI that is not solely due to spin-spin interactions but also includes effects from magnetic field inhomogeneities. The script explains that T2 Star Decay occurs more quickly than T2 relaxation because it includes additional factors that cause the loss of transverse magnetization.

💡Magnetic Field Inhomogeneities

Magnetic field inhomogeneities refer to variations in the strength of the magnetic field within the scanner. The script mentions that these inhomogeneities can be caused by the scanner's limitations, substances within the patient, or the dephasing of spins themselves. These inhomogeneities affect the T2 Star Decay rate and can obscure differences in T2 relaxation times between tissues.

💡Echo

An echo, in the context of MRI, is a signal re-acquired after a specific delay (TE - time to echo) following a radiofrequency pulse. The script describes how a 180-degree radiofrequency pulse can be used to rephase protons that have dephased, creating an echo that can be sampled to compensate for magnetic field inhomogeneities and to highlight T2 relaxation effects.

💡Time to Echo (TE)

Time to Echo (TE) is the delay between the application of a radiofrequency pulse and the sampling of the MRI signal. The script explains how varying TE can change the contrast in MRI images by affecting the visibility of tissues with different T2 relaxation times. Short TE times result in high signal but low contrast, while longer TE times increase contrast by emphasizing differences in T2 relaxation.

💡Contrast

Contrast in MRI refers to the differences in signal intensity between various tissues, which allows them to be distinguished in the images. The script discusses how T2 relaxation and T2 Star Decay affect contrast, and how manipulating TE can enhance or reduce this contrast to better visualize different tissues, such as muscle, fat, and CSF.

Highlights

Nuclear magnetic resonance (NMR) involves placing protons in an external magnetic field, causing them to align and resonate at a set frequency.

A radiofrequency magnetic pulse is applied perpendicularly to initiate proton resonance and fanning out from the longitudinal magnetization vector.

At 90 degrees, the net magnetization vector reaches maximum transverse magnetization, losing longitudinal magnetization.

Relaxation processes are discussed, including T2 relaxation (loss of transverse magnetization) and T1 relaxation (regaining longitudinal magnetization).

T2 relaxation is the focus of the lecture, detailing the loss of transverse magnetization due to spin-spin interactions.

Spin-spin relaxation is characterized by protons going out of phase with each other, influenced by the tissue they're in.

Transverse Decay is introduced as the loss of net transverse magnetization vector due to proton dephasing.

An analogy of spinning basketballs is used to explain the dephasing process in T2 relaxation.

Fat and CSF are compared to illustrate how their molecular structures affect the rate of transverse magnetization loss.

T2 relaxation curves are plotted to show how different tissues lose phase at varying rates.

T2 star Decay is distinguished from T2 Decay, accounting for magnetic field inhomogeneities.

The concept of T2 star time constants is introduced, explaining the time it takes for 63% signal loss.

Three mechanisms causing magnetic field inhomogeneities are discussed: MRI scanner limitations, patient substances, and local spin dephasing.

T2 star Decay is shown to be influenced by the magnetic field strength experienced by protons at different locations.

A 180-degree radio frequency pulse is introduced as a method to rephase protons and compensate for local magnetic field inhomogeneities.

The echo mechanism in MRI is explained, showing how it can be used to regain T2 relaxation and account for magnetic field inhomogeneities.

The impact of time to echo (TE) on MRI signal and tissue contrast is discussed, showing how it can be manipulated to highlight T2 relaxation differences.

The lecture concludes with a preview of the next topic, T1 relaxation, and its implications for imaging tissue differences.

Transcripts

hello everybody and welcome back so in

the previous talk we looked at the

process of nuclear magnetic resonance

where we placed protons within an

external magnetic field and they aligned

with that magnetic field processing at a

set frequency we then applied a

perpendicular radiofrequency magnetic

pulse that caused those protons to start

resonating in Phase with one another and

Fanning out away from that longitudinal

magnetization Vector that net

magnetization Vector then gained more

and more transverse magnetization and if

we flip that net magnetization Vector to

90 degrees we completely lost the z-axis

magnetization the longitudinal

magnetization and at the same point

we've completely gained transverse

magnetization at 90 degrees we have our

maximum transverse magnetization Vector

so that process of nuclear magnetic

resonance and the application of a radio

frequency pulse caused loss of

longitudinal magnetization and gain of

transverse magnetization in the next two

talks we're going to look at the process

of relaxation which happens in two

separate independent mechanisms the

first is the loss of transverse

magnetization otherwise known as T2

relaxation the second process which is

independent of that first process is

what's known as T1 relaxation or the

gaining regaining of longitudinal

magnetization so in today's lecture

we're going to look at T2 relaxation

which is the loss of transverse

magnetization now there are multiple

different terms for this that I want to

introduce you to the first is what's

known as spin spin relaxation we've seen

in previous talks that the loss of

transverse magnetization comes from this

spins going out of phase with one

another as we stop that radio frequency

pulse the spins that were resonating in

Phase start to de-phase from one another

and the rate at which they defaze

depends on which tissue they're in now

that dephasing is primarily caused by

spins interacting with one another the

way I to remember this is that spin spin

has two s's here so spin spin is related

to T2 relaxation

now if you take that analogy of spinning

a basketball on your finger if I had

many people in the room and everyone was

spinning a basketball on their finger

and the basketballs bumped into one

another those basketballs will start to

lose some of their Spin and they will be

de-phasing or spinning at different

rates from the basketballs in the room

that is the process for the loss of

phase in T2 relaxation another term that

you may come across is what's known as

transverse Decay and this makes sense as

protons defaze they lose their

transverse or their net transverse

magnetization vector and we get a loss

of signal because it's a transverse

signal that we're measuring in our MRI

machine

so if you imagine these as basketballs

these protons here and they're bouncing

into one another and as the spins

interact with one another the spins with

different energy levels as well the

energy is transferred and those spins

become out of phase and that's the

predominant mechanism for the loss of

transverse magnetization so let's have a

look at an example here we have a unit

of fat represented by this orange color

here and a unit of CSF represented by

this blue color here we've applied a

radio frequency pulse that matches the

processional frequency of these net

magnetization vectors and we flip that

net magnetization Vector to 90 degrees

our maximum signal now what happens when

we turn off that radio frequency pulse

while those processing spins will now

start to de-phase and if you think about

what fat is made up of it's made up of

long chains of triglycerides where all

the molecules are joined together they

can bump into each other really easily

if we take our room full of people with

basketballs on their hands fat has got

chains of people holding hands with one

another and as they move around the room

they're much more likely to have their

spins or the basketballs bump into one

another

water or CSF has free people walking

around in the room free to move as they

please they're not joined to other

people in these long chains of fatty

acids so water they're less likely for

the spins to interact with one another

there's more free movement in the room

so you'll see that the phase of water or

CSF stays much more in Phase than fat

does let's have a look at these two

separate tissues and see how they behave

differently as they start to lose phase

in the transverse plane

as you see in CSF here the net

magnetization Vector is staying much

more in Phase the spins aren't

interacting as much as they are in

fattier and we see that the signal

generated from fat is lost much more

quickly than the signal generated from

CSF

so now we can draw these curves here

which are our T2 relaxation curves that

are dependent on the type of tissue

through which the spins are spinning

look in fact here how outer phase those

spins are and as we know as we get out

of phase our magnetization vectors in

the X Y plane start to cancel each other

out and after a period of time we're

getting complete loss of that signal in

fat the water has stayed relatively in

Phase with one another and although

we're still getting lots of signal

because they're not perfectly in Phase

that loss is much slower and for each

tissue we can plot that free induction

Decay curve for the different tissues

now you'll see that I've written T2 star

Decay here and not T2 Decay now whenever

you put an asterisk somewhere it means

that there's some terms and conditions

and the reason for this T2 star Decay is

because the actual measurable Decay that

we measure on the MRI machine the drop

off of signal that we were looking at at

the previous slide is not purely due to

the spins interacting with one another

T2 relaxation in an ideal world would

only be getting loss of that transverse

magnetization Vector from spins

interacting with one another spin spin

relaxation now in the real world we get

loss of signal because of spin spin

relaxation but we also get loss of

signal due to magnetic field in

homogeneities which we're going to look

at next before we move into that I want

to draw your attention to these T2 star

time constants here

at the beginning of our radio frequency

pulse once we flip those net

magnetization vectors to 90 degrees we

have maximum signal and all those

protons are in Phase with one another we

have a hundred percent of the transverse

signal at 90 degrees this is our

transverse magnetization vector

now as time goes by we get lots of that

signal because of the dephasing of those

atoms and it happens at different rates

depending on the tissue we're in

when 63 of that signal has been lost or

we have 37 of the transverse

magnetization Vector left that time

constant the amount of time it takes to

get to that point is what's known as T2

star or T2 star Decay and we can use

these values to get contrast in our

tissues later

now in an Ideal World we wouldn't want

T2 star we would want a T2 value which

would represent 63 loss in the

transverse magnetization Vector purely

due to spin spin interactions not due to

magnetic field in homogeneities and this

curve here would be known as our T2

relaxation curve we've seen that the t2

star relaxation curve happens much

quicker in tissues now if we have a look

at our MRI machine here in an Ideal

World the magnetic field would be

homogeneous it would be exactly the same

no matter where the protons are within

this magnetic field now there are three

separate mechanisms that make this

magnetic field inhomogeneous and cause

that T2 star Decay the first is that the

MRI scanner itself can't make a perfect

strength magnetic field that's equal all

the way through the transverse plane the

coils are going to have differing

magnetic field strains the further away

from the coils you get so that's the

first reason for magnetic field in a

motor geneties the second mechanism is

that there could be a substance within

the patient either metal or calcium or

dense cortical bone that causes

disruption in the local magnetic fields

here and that's why in a patient that

has a metal device you'll often see T2

signal is completely lost around that

device that's because of the localized

changes in the magnetic field strength

and the last thing is when spins start

to de-phase with one another the

magnetization vectors are becoming out

of phase with one another and they can

disrupt the local magnetic field as well

and so we don't get perfect magnetic

field lines in the longitudinal plane

here so all three of those mechanisms

cause the magnetic field to be in

homogeneous now because the field is

inhomogeneous a proton that is sitting

here will experience a different

magnetic field strength to a proton that

is sitting at a different location and

we've seen that when protons or spins

experience different magnetic field

strands they will spin at different

rates we look back to our alarm

frequency a different magnetic field

strength will cause the dephasing to be

increased because the rates of change of

those processional values will be

different between those two protons and

that's what's responsible for this T2

star effect occurring now our T2 star or

the free induction Decay Curve will

always be less than the t2 value the t2

relaxation value and in imaging we want

to try and compensate for this reduction

or increased rate of loss of transverse

magnetization and there actually is a

mechanism for which we can compensate

for these local field in homogeneous 80s

so let's have a look at how we go about

compensating for that T2 star decay when

we are trying to produce an image the

first thing we need to do is apply a 90

degree RF pulse that is perpendicular to

the main magnetic field once we've

applied that 90 degree RF pass and turn

it off we will get relaxation T2

relaxation where we get loss of

transverse magnetization now in an Ideal

World the transverse magnetization loss

will be June only due to spin spin

interactions where spins are

transferring energy and they start

becoming out of phase because of that

transfer of energy between the two spins

that is what's known as our T2 Decay or

T2 relaxation

now what actually happens in the real

world is we get spin spin interactions

which cause that loss of transverse

magnetization and we get local magnetic

field in homogeneities which causes this

T2 star Decay to occur so this is what

we want this T2 relaxation this is what

we actually measuring because of the

inhomogeneities within the magnetic

field

so what has actually happened here well

we've taken our longitudinal net

magnetization vector and flipped it to

90 degrees with that 90 degree RF pulse

we've completely lost longitudinal

magnetization and we've now got a

maximum transverse magnetization we've

got maximum T2 signal here now what's

going to happen is these spins in the

same voxel within our image are going to

defaze with one another if we look at it

end on they're going to D phase like

this some of them will move faster than

others and that's mainly due to the spin

spin interaction between the different

spins but we've also seen that there is

differing strengths of magnetic field

strength because of that local

inhomogeneity in the magnetic field now

the one that's experiencing a higher

magnetic field strength is going to

defaze quicker than the one that's

experiencing a lower magnetic field

strength so we've flipped it to 90

degrees and we're getting de-phasing of

these spins

now what happens is over time these

spins will defaze with one another and

they will also start gaining

longitudinal magnetization now the one

is dephasing faster than the other what

we want to do is be able to re-phase

these two spins with one another and the

way we do that is by applying what's

known as a 180 degree radio frequency

pulse it's the same radio frequency

pulses this 90 degree radio frequency

pulse same magnitude but for twice the

duration so what has happened now one

spin is de-phasing faster than another

Spin and we apply a 180 degree RF pole

so let me get this right here this is

the faster one the blue is the slower

one we apply a 180 degree radio

frequency pulse we are flipping those

spins now 180 degrees here is our main

magnetic field

now the leading spin is the slow Spin

and the trailing spin is the fast spin

we're spinning or processing in this

direction now what is going to happen

over time is the faster spin is going to

catch up with the slower or the lagging

Spin and if we wait the exact same

period of time between our 90 and 180

degree pulse what will happen is those

spins will now become in Phase with one

another because of that 180 degree spoon

and we have gained now that net

magnetization Vector in the transverse

plane our spins have re-phased with one

another and you can see that represented

by this graph here is that as the spins

start to re-phase with one another we

get an increase in that transverse

magnetization Vector because of that 180

degree flip and then allowing those

spins now to catch up with one another

and sync up giving us a maximum net

transverse magnetization vector and what

we can do then is sample the signal at

this point and if we sample the signal

at this point you'll notice that that

signal is the same as the t2 relaxation

the signal we're measuring now at the

time to Echo and you can see now why

it's called an echo is the same as what

we would have gotten if the loss of

transverse magnetization was only

because of spin to spin interactions or

T2 relaxation and it's this mechanisms

which we're going to look at later in a

pulse sequence called spin Echo

sequences that allows us to regain that

T2 relaxation and account for those

local inhomogeneities in the magnetic

field and we can do this for all the

different tissues all the different

voxels within our patient and plot these

values over time now importantly we can

place this 180 degree RF pulse wherever

we want to place it and then measure the

echo at the same distance between the 90

degree RF pulse and the 180 degree RF

boss this distance and this distance is

the same we can make this te time much

shorter or or we can make it much longer

a shorter te time will give us higher

signal and a longer te time is going to

give us lower signal and if we plot

those signals over time depending on the

different tissues that we're trying to

image we can see that T2 relaxation

curve it takes much longer in CSF

because those hydrogen protons are able

to move freely in fact you think of

people holding hands in the room

spinning basketballs in those long

triglyceride change the basketballs are

going to bump into each other and that

spin spin interaction is going to cause

a loss of transverse magnetization and

that happens even faster in muscle

now we've seen that we can choose the

time to Echo when we're going to sample

this tissue if we sample really early a

short time to Echo we flip that

longitudinal magnetization Vector into

90 degrees switch off the RF pulse and

immediately sample the tissue what we

get is a short time to Echo now you'll

see that the signal here is high for the

muscle for the fat and for the CSF we're

going to have a high signal and there's

going to be no contrast between these

tissues we've got very little difference

in the t2 relaxation times between these

tissues if we wait a longer period of

time and make our Echo slightly longer

you'll see that the signal has decreased

but the contrast between the various

different tissues has increased our

muscle signal is going to be much lower

it's going to be represented darker on

the MRI fat is lower than CSF but higher

than muscle and now CSF is still giving

us a bright signal value waiting or

prolonging the t2 times going to

increase the contrast between those two

tissues so you can see how changing te

time changes the contrast and that

contrast is based on the t2 relaxation

differences between these tissues now we

can wait even longer and have a third

time to Echo here where we've now got

very little signal and again we've lost

contrast here there's very slight

grayscale differences but now it's

difficult to tell the CSF from the fat

and from the muscle so if you wait too

long without time to Echo we're going to

lose that transverse magnetization

vector and not have any signal to detect

now hopefully this graph shows you that

change in the te will highlight the

differences in T2 relaxation differences

between the various different types of

tissues in the next talk we're going to

be looking at T1 relaxation and I'm

going to show you how we can use T1

relaxation differences in order to see

the T1 differences between tissues so

until that talk I'll see you all then

goodbye everybody