T1 Relaxation, Spin-lattice Relaxation, Longitudinal Recovery | MRI Physics Course #5

Summary

This educational video explores T1 relaxation in MRI, contrasting it with T2 relaxation. It explains T1 as spin-lattice interaction, where spins realign with the main magnetic field, regaining longitudinal magnetization. The video uses the analogy of a room with people and chairs to illustrate the concept and discusses how different tissues like fat and CSF have varying T1 relaxation times. It further explains how MRI machines use these differences to create T1-weighted images, highlighting the importance of understanding T1 and T2 relaxation for medical imaging.

Takeaways

• 🧲 T1 relaxation, also known as spin lattice relaxation, involves the interaction of spins with the lattice (non-spin components like proteins and macromolecules), leading to realigning with the main magnetic field and gaining longitudinal magnetization.
• 🌀 T2 relaxation, in contrast to T1, is about the loss of transverse magnetization due to spin-spin interactions causing de-phasing of the spins.
• 📈 T1 relaxation is associated with longitudinal recovery, where the net longitudinal magnetization is regained, as opposed to T2 relaxation, which is about transverse decay.
• 🚫 The rate of T1 relaxation varies among different tissues, with factors such as the presence of proteins or macromolecules influencing how quickly spins realign with the B0 field.
• 🧼 Fat tissues regain longitudinal magnetization more quickly due to their structure and the presence of more lattice components, unlike CSF which has fewer proteins and thus slower T1 relaxation.
• 📉 The T1 time constant measures the time it takes to regain 63% of the longitudinal magnetization vector, and it varies significantly between different types of tissues.
• 🌐 T1 contrast in MRI images is achieved by manipulating the time to repetition (TR) in the pulse sequence, which affects how much longitudinal magnetization different tissues have regained by the time of the next RF pulse.
• 🔄 The absence of T1 star relaxation compared to T2 star is due to the nature of how magnetic field inhomogeneities affect longitudinal versus transverse magnetization.
• 📊 By adjusting TR and TE (time to echo), MRI pulse sequences can be tailored to emphasize either T1 or T2 contrast, or a balance between them, known as proton density weighting.
• 📸 In T1-weighted MRI images, tissues with shorter T1 times, like fat, appear brighter, while those with longer T1 times, like CSF, appear darker.

Q & A

What is T1 relaxation in MRI?

T1 relaxation, also known as spin lattice relaxation, is the process by which the spins in a tissue realign with the main magnetic field after being perturbed, such as by a radio frequency pulse. This realignment results in the recovery of longitudinal magnetization.

How does T1 relaxation differ from T2 relaxation?

T1 relaxation involves the recovery of longitudinal magnetization, whereas T2 relaxation involves the loss of transverse magnetization due to de-phasing of spins. T2 relaxation is often referred to as transverse decay.

What is meant by the 'lattice' in the context of T1 relaxation?

The 'lattice' refers to the structural components, such as macromolecules and proteins, that do not have spin themselves but interact with the spins, causing them to realign with the main magnetic field.

Why is T1 relaxation also known as longitudinal recovery?

T1 relaxation is known as longitudinal recovery because it involves the recovery of longitudinal magnetization as the spins realign with the main magnetic field.

How does the presence of the lattice affect T1 relaxation?

The presence of the lattice, or non-spin components, influences the rate at which spins realign with the main magnetic field, thus affecting the speed of T1 relaxation.

What is the significance of the T1 time constant in MRI?

The T1 time constant represents the time it takes for a tissue to regain 63% of its longitudinal magnetization. It is a key parameter used to differentiate between tissues in MRI based on their T1 relaxation properties.

Why does fat appear differently than CSF in T1-weighted MRI images?

Fat regains longitudinal magnetization more quickly than CSF due to the presence of more lattice components and the nature of its triglyceride chains, which results in a brighter signal in T1-weighted images.

How does the time to repetition (TR) affect T1 contrast in MRI images?

A short TR time allows less time for longitudinal magnetization recovery, highlighting T1 differences between tissues. A longer TR time allows more time for recovery, reducing T1 contrast differences.

What is the role of time to echo (TE) in T2 relaxation, and how does it differ from its role in T1 relaxation?

In T2 relaxation, TE is the time at which the transverse magnetization signal is measured, and it affects the visibility of T2 contrast. In T1 relaxation, TE is less relevant because the focus is on longitudinal magnetization, which cannot be directly measured.

Why is there no T1 star relaxation effect as there is with T2 star relaxation?

T1 star relaxation does not occur because the magnetic field inhomogeneities that cause extra decay in T2 relaxation do not significantly affect the longitudinal magnetization recovery process in T1 relaxation.

Outlines

00:00

🧲 Introduction to T1 Relaxation

This paragraph introduces T1 relaxation, also known as spin lattice relaxation, which is the process where spins in a magnetic field realign with the main magnetic field, gaining longitudinal magnetization. It contrasts this with T2 relaxation, where spins lose transverse magnetization due to spin-spin interactions. The paragraph explains that T1 relaxation is also called longitudinal recovery because it involves the recovery of longitudinal magnetization. An example using MRI is provided, showing how different tissues like fat and CSF regain magnetization at different rates due to their interaction with the lattice, which consists of non-spin components like proteins and macromolecules.

05:03

🌀 Understanding T1 Relaxation Dynamics

This section delves deeper into the dynamics of T1 relaxation, explaining how the rate at which spins realign with the main magnetic field (B0) depends on the type of tissue. It uses the analogy of people in a room with chairs to illustrate how the presence of more 'lattice' components (like chairs) can cause spins to realign more quickly. The paragraph contrasts the T1 relaxation times of fat and CSF, with fat having a shorter T1 time due to its more complex structure and greater interaction with the lattice. It also discusses the simultaneous occurrence of T2 relaxation, which involves the loss of transverse magnetization as spins go out of phase with each other.

10:04

📈 T1 Relaxation and MRI Signal Contrast

The paragraph discusses how T1 relaxation contributes to the contrast seen in MRI images. It explains that while T2 relaxation is about the loss of signal due to transverse magnetization decay, T1 relaxation is about the gain of signal as longitudinal magnetization is regained. The T1 time constant is introduced as the time it takes for a tissue to regain 63% of its longitudinal magnetization. The paragraph also addresses why T1 star relaxation is not a concept like T2 star relaxation, due to the different effects of magnetic field inhomogeneities on longitudinal versus transverse magnetization.

15:06

🕒 Time to Repetition and T1 Relaxation in MRI

This final paragraph focuses on how the time to repetition (TR) in an MRI pulse sequence affects T1 relaxation and the resulting image contrast. It explains that a short TR time allows less time for tissues to regain longitudinal magnetization, leading to greater T1 contrast in the image. Conversely, a long TR time allows more time for tissues to regain magnetization, reducing T1 contrast and leading to proton density-weighted images. The paragraph concludes by setting the stage for the next discussion on how to manipulate MRI pulse sequences to emphasize either T2 or T1 contrast, or a balance between the two.

Keywords

💡T1 Relaxation

T1 relaxation, also known as spin-lattice relaxation, refers to the process where nuclear spins realign with the external magnetic field after an RF pulse in MRI. This regaining of longitudinal magnetization is key to creating contrast in MRI images. The concept is central to the video as it explains how different tissues regain their magnetization at different rates.

💡T2 Relaxation

T2 relaxation, also known as spin-spin relaxation, is the process where nuclear spins dephase and lose their transverse magnetization. This process is separate from T1 relaxation and contributes to the loss of signal over time. The video contrasts T2 relaxation (transverse decay) with T1 relaxation (longitudinal recovery) to explain how each process affects MRI imaging.

💡Spin-lattice Interaction

Spin-lattice interaction refers to the interaction between nuclear spins and their surrounding environment (the 'lattice') in T1 relaxation. This interaction helps spins realign with the external magnetic field, leading to the recovery of longitudinal magnetization. In the video, this interaction is compared to objects in a room that cause people to trip and fall, representing how spins are influenced by their surroundings.

💡Transverse Magnetization

Transverse magnetization is the magnetization of nuclear spins in a plane perpendicular to the external magnetic field, created by applying an RF pulse. It decays over time due to T2 relaxation, and its loss is a key factor in determining MRI signal strength. The video explains that transverse magnetization is lost as spins dephase, while longitudinal magnetization is regained through T1 relaxation.

💡Longitudinal Magnetization

Longitudinal magnetization refers to the alignment of nuclear spins with the external magnetic field (B0), and is regained during T1 relaxation. The video focuses on how this magnetization is gradually recovered after being disrupted by an RF pulse, and how the rate of recovery varies between tissues like fat and CSF, contributing to image contrast in MRI.

💡CSF (Cerebrospinal Fluid)

CSF, or cerebrospinal fluid, is a bodily fluid with low protein content, and in MRI, it takes longer to regain longitudinal magnetization during T1 relaxation. The video uses CSF as an example of a tissue with fewer structural components, explaining why it exhibits slower T1 relaxation compared to fat, which leads to distinct contrasts in MRI images.

💡Fat

Fat is a tissue rich in triglycerides, and it regains longitudinal magnetization faster than other tissues like CSF due to more lattice interaction. The video explains how the long chains of triglycerides contribute to quicker T1 relaxation, which is important for generating image contrast in MRI. Fat is often shown as bright in T1-weighted images due to this rapid recovery.

💡RF Pulse

An RF pulse, or radiofrequency pulse, is applied in MRI to flip the net magnetization vector, disrupting the longitudinal magnetization and creating transverse magnetization. The video discusses how this pulse initiates the processes of both T1 and T2 relaxation, which are responsible for regaining longitudinal magnetization and dephasing spins, respectively.

💡Time to Echo (TE)

Time to Echo (TE) is the time between the application of an RF pulse and the collection of the MRI signal. In T2 relaxation, a longer TE allows more dephasing to occur, highlighting differences in tissue properties. The video explains how adjusting TE affects signal strength and contrast in MRI images by emphasizing the decay of transverse magnetization.

💡Time to Repetition (TR)

Time to Repetition (TR) is the time between successive RF pulses in MRI, which influences the recovery of longitudinal magnetization. The video explains how different TR times affect the ability to measure T1 relaxation differences between tissues, with shorter TR times highlighting these differences more effectively in T1-weighted images.

Highlights

T1 relaxation, also known as spin lattice relaxation, involves the interaction of spins with the lattice, which includes structural components and macromolecules.

In T1 relaxation, spins realign with the main magnetic field, gaining longitudinal magnetization, in contrast to T2 relaxation where transverse magnetization is lost.

The rate of T1 relaxation varies by tissue type, influenced by the presence of proteins or macromolecules within the tissue.

Fat tissues regain longitudinal magnetization more quickly than CSF due to the presence of more structural components and triglyceride chains.

T1 relaxation is depicted as a gain of signal, in contrast to T2 relaxation, which is a loss of signal.

The T1 time constant represents the time taken to regain 63% of the longitudinal magnetization vector.

T1 relaxation differences are utilized to create T1 contrast within MRI images, highlighting variations in tissue types.

Magnetic field inhomogeneities cause T2 star relaxation due to the extra loss of decay from the T2 relaxation curve.

T1 relaxation is not affected by magnetic field inhomogeneities as it involves the gain of longitudinal magnetization, not phase differences.

Time to Echo (TE) in T2 relaxation highlights differences in signal decay, whereas Time to Repetition (TR) in T1 relaxation emphasizes differences in signal gain.

A short TR time in T1 relaxation results in a bright signal from fat and a dark signal from CSF, highlighting T1 differences.

A long TR time allows tissues to regain more longitudinal magnetization, reducing T1 contrast differences in the image.

Proton density weighted images negate both T1 and T2 differences by using long TR and short TE times.

Every MRI image has a combination of T2 and T1 contributions to contrast, which can be manipulated through pulse sequence parameters.

Understanding and manipulating TE and TR times are crucial for weighting MRI images towards T2, T1, or proton density contrasts.

Transcripts

hello everybody and welcome back today

we're going to be looking at the process

of T1 relaxation now this video follows

on from the previous video where we

looked at T2 relaxation so if you

haven't watched that one I'd highly

recommend watching that one first then

coming across to this video now T1

relaxation is also known as spin lattice

relaxation we saw in T2 relaxation it

was the interaction between spins spin

spin relaxation that caused the spins to

de-phase and ultimately lose transverse

magnetization and that's why T2

relaxation is often known as transverse

decay in T1 relaxation the spins

interact with what is known as the

lattice now the lattice is the

structural components the macromolecules

the proteins that don't have spin

themselves but when spins interact with

them it causes those spins to start to

gain longitudinal magnetization or start

to realign with the main magnetic field

now spin lattice relaxation is also

known as longitudinal recovery because

those spins are starting to realign with

the main magnetic field we are gaining

net longitudinal magnetization we're

recovering that longitudinal

magnetization in T2 relaxation we are

losing net transverse magnetization

that's why it's known as transverse

Decay so ultimately what is happening

here in T1 relaxation is the spins that

were in the transverse plane are now

realigning with the longitudinal plane

and we are regaining that longitudinal

magnetization Vector so let's have a

look at an actual example here we have

the MRI machine with two separate

tissues fat on the left and CSF on the

right

we've applied a radio frequency pulse

that has caused the net magnetization

Vector to flip to 90 degrees we have

lost now the longitudinal magnetization

vector and we're at maximum transverse

magnetization Vector now when we switch

off that radio frequency pulse at B1

pulse two things are going to happen and

these processes are separate we'll get

T2 relaxation and at the same time we're

going to get T1 relaxation independent

processes from one another

now we've looked at T2 relaxation where

those spins start to de-phase and we get

lots of transverse magnetization we get

transverse decay now we're going to look

at how longitudinal magnetization is

regained within the sample as that B1

radio frequency pulse is turned off

these spins will interact with the

lattice and we've talked about the

lattice it's the non-spin components

that cause those spins to realign with

the B naught field here

now the rate at which spins realign is

dependent on the type of tissue we've

looked at the example of people within a

room spinning a basketball on their

finger and we've said the basketball is

coming into contact with one another

spin spin interactions cause those

basketballs to spin out of phase that's

synonymous with T2 relaxation

now if you Picture People in the room

and there are chairs all over the room

or there are obstacles within the room

those chairs and obstacles aren't

spinning but the people walking around

can trip over those chairs interact with

the lattice within the room falling over

would cause the basketball to tip like

this into the longitudinal plane now in

CSF there are very few proteins or

macromolecules or structural components

very few chairs within the room so the

people walking around that room can walk

around freely the spin spin interactions

are less than say in fact and they're

less likely to trip over the lattice

within the room so in CSF T1 relaxation

also takes a long period of time now

we've looked at fat being long chains of

triglycerides and we've said it's like

people in the room holding hands with

one another and that's why T2 relaxation

happens much quicker in fact the

basketballs are much more likely to bump

into one another

now not only that but in fact there's

more lattice within the sample they're

more structural components non-spin

lattice components and that means that

fat gains longitudinal magnetization

quite quickly what also happens in fat

is the long triglyceride chains also

move in response to that radio frequency

pulse meaning that the spins are more

likely to come into contact with the

surrounding lattice again another reason

why T1 relaxation happens faster and fat

than it does in CSF so let's see what

happens over a period of time we wait a

period of time and we see that in fact

we regain some longitudinal

magnetization and the same things

happened in CSF now this Vector here if

we look in CSF we've got our net

magnetization Vector initially it was

along the B naught plane we flipped it

to 90 degrees and then over time that is

going to start to gain longitudinal

relaxation until ultimately lying

completely in the longitudinal plane

now as this process is happening as we

are regaining longitudinal magnetization

we are also getting T2 relaxation

happening at the same time where these

spins within the CSF are de-facing with

one another so when the CSF starts to

gain longitudinal magnetization at this

stage many of the CSF spins are out of

phase with one another and we've lost a

lot of net transverse magnetization

now this gaining of longitudinal

magnetization does account for some loss

in transverse magnetization but that

pales in comparison to the transverse

magnetization loss because of the deep

phasing of those spins

when we've regained some longitudinal

magnetization at this point we've likely

lost all of the transverse magnetization

because those spins are out of phase

with one another we can think of the net

magnetization Vector then as being just

this longitudinal component here that's

really important the transverse

component does not equal this part of

the vector because those spins are now

out of phase and because those spins are

out of phase the transverse component

has canceled each other out and we're

left with a net magnetization Vector in

the longitudinal plane now as the

tissues gain their longitudinal

magnetization we can use this x-axis

here as a proxy for the longitudinal

magnetization vector and that becomes

really important in T1 relaxation

as we wait more time we see that fat

again is gaining the longitudinal

magnetization faster than it is in CSF

and we can plot this on this graph here

the y-axis here being the longitudinal

magnetization the net longitudinal

magnetization and the y-axis ends in 100

here where we've got full recovery of

longitudinal magnetization and we can

see that fat is gaining that

longitudinal magnetization faster than

water is that's because in CSF there's

less lattice for interaction to occur

and not only in fact is there more

lattice but spin lattice interaction is

more likely to occur because of how

those triglycerides react to the

magnetic field as we wait more time we

can see now in fact we've regained 100

of that longitudinal magnetization and

the CSF sample is slowly regaining that

longitudinal magnetization and it's

these differences here that allow us to

get T1 contrast within an image we saw

in transverse relaxation that was

looking at T2 differences within the

image here now we're looking at how we

get T1 differences and that's what we're

going to focus on in this talk

now for the various different tissues

you can plot these on a graph the same

that we did with T2 relaxation

now we saw that T2 relaxation was a loss

of signal a decay in Signal T1

relaxation is a gain of signal its

longitudinal recovery we are gaining or

regaining that longitudinal

magnetization vector so here we can see

that fat gains faster than muscle and

muscle gains faster than CSF and again

we can use a time constant here known as

the T1 time constant now in T2 Decay we

looked at the time it took to lose 63 of

the transverse magnetization signal here

in T1 relaxation we're looking at the

time it takes to gain or regain 63

percent of the longitudinal

magnetization Vector that time is what's

known as the T1 time constant now this

isn't an arbitrary number 63 percent is

used in both of those equations because

there is an equation that looks at the

T1 and T2 relaxation constants and that

equation is out of the scope of this

lecture series but what you need to know

here is that the T1 time constant is

much longer in CSF than it is in fat now

why do I keep comparing CSF and fat the

predominant signal generated in the MRI

is either coming from water or it's

coming from fat that's where the most

free hydrogen atoms are available to

generate signal in MRI imaging now you

would have seen that in T2 relaxation we

had a concept known as T2 star why then

do we not get T1 star relaxation

what was causing T2 star relaxation T2

star was the extra loss of Decay away

from the t2 relaxation curve that was

due to the magnetic field in

homogeneities the differences in the

magnetic field strength throughout the

magnetic field caused the spins to

defaze faster than they would usually

just from spin spin interactions some

spins were experiencing a higher

magnetic field and therefore resonating

faster and some spins were experiencing

a lower magnetic field and therefore

resonating slower and because of the

differences in those speeds of resonance

or speeds of procession we got lots of

transverse magnetization

in T1 relaxation the magnetic field is

responsible for gaining longitudinal

magnetization and differences in

magnetic field strength will result in

slight differences in the longitudinal

relaxation however because the magnetic

field is inhomogeneous some of those

spins will experience a weaker magnetic

field and gain longitudinal

magnetization slower and some will

experience a stronger magnetic field and

gain longitudinal magnetization slightly

faster if we average out those

differences we're going to get gaining

of longitudinal magnetization at roughly

the average magnetic field strength and

that gaining of longitudinal

magnetization then will be equal to the

T1 time the regaining of longitudinal

magnetization has nothing to do with the

phase of the spins we saw that in T2

loss it has everything to do with the

phase of the spins and that magnetic

field in homogeneity whether it be

stronger or whether it be weaker

magnetic fields cause dephasing that

defacing doesn't affect this

longitudinal magnetization and we get a

time constant known as T1 that is the

average of that magnetic field so let's

then compare our T2 relaxation and T1

relaxation specifically looking at the

lens of time to Echo and time to

repetition we saw that in T2 relaxation

the te time highlighted the differences

in T2 relaxation between the different

tissues we can see here that changing

the te time in T2 relaxation highlighted

the t2 contrast differences between the

various tissues and if we used a really

short time to Echo we got high signal

but no contrast between those tissues we

negated the t2 differences between these

tissues but we still got signal from

that sample as we waited slightly longer

we have still got signal coming from the

sample but the signals differ because of

the differences in T2 relaxation and if

we waited even longer for a really long

te time we'd get very low signal and

very little contrast between the tissues

now when we look at T2 relaxation this

is something that we can directly

measure because we are looking at

transverse magnetization and its

transverse magnetization that we can

measure with the coils within the MRI

machine and that time to Echo is the

time that we actually measure that

signal now if we look at T1 relaxation

what are we gaining we're gaining

longitudinal magnetization and we can't

measure that longitudinal magnetization

because it's within the same plane as

our main magnetic field we can't place

coils there to measure that longitudinal

magnetization so how then do we go about

highlighting the differences in

longitudinal magnetization

the differences in longitudinal

magnetization rates is what is going to

give us the T1 contrast differences

within the tissues well in order to do

this we need to look at how we actually

go about creating these signals the

pulse sequence here the first thing we

do is apply a 90 degree RF pulse to lose

all of that longitudinal magnetization

and gain all of the transverse

magnetization we then sample the signal

at a time known as the time to Echo the

te time and as we've seen a very short

time to Echo results in high signal but

very little T2 differences in the tissue

the longer we wait for that time to Echo

the more the t2 differences are the more

those spins are allowed to de-phase at

their set rate for the tissue and we

highlight those T2 differences we then

wait a long period of time as all those

spins start to regain longitudinal

magnetization and lie in the

longitudinal plane then at a given

period of time we repeat that 90 degree

RF pulse that's our time to repetition

as we repeat that 90 degree RF files we

re-flip that net magnetization Vector

into the transverse plane so let's have

a look at what that means for the T1

relaxation times within our image

now importantly when we talk about T1

relaxation we're talking about the

gaining of the longitudinal

magnetization vector

if we look at CSF and fat for example

fat gains the longitudinal magnetization

Vector quicker than CSF does that's what

we've looked at already in this talk

now in this longitudinal plane we can

actually use this x-axis Vector value as

the net magnetization Vector for the

sample because the spins here in the t2

or the transverse plane have now defazed

the transverse plane has canceled

everything out we've got a net

magnetization Vector equal to this

x-axis value the same happens in CSF so

at any given period of time we've got

longitudinal magnetization vectors that

are equal to the x-axis value of that

longitudinal magnetization so that

period of time we've got a short

longitudinal magnetization Vector for

CSF and a long longitudinal

magnetization Vector for fat if we then

apply that 90 degree RF pulse at this

period of time which represents the

longitudinal magnetization vectors

differences between the CSF and the

fattier what will happen then is the net

longitudinal magnetization vectors for

CSF and for fat will be the y-axis value

for the transverse magnetization at the

time of repetition here you'll see that

now that we flip that Vector we flip the

longitudinal magnetization Vector the

differences in Signal between fat and

between CSF is quite large so let's now

look at two periods of time where we can

do the time to repetition within our T1

relaxation

as we do a short time to repetition we

get what we've just looked at here the

CSF has regained very little

longitudinal magnetization or MZ fat has

regained a lot of the longitudinal

magnetization here so what have we got

at this period tr1 we've got fat that

has regained a lot of longitudinal

magnetization and CSF which has only

regained a small amount of longitudinal

magnetization if we repeat the 90 degree

RF pulse at this stage the value of the

y-axis in the transverse plane is going

to be equal to the amount of

longitudinal magnetization that the

different tissues have gained at that

point so a short TR time means that we

haven't allowed full longitudinal

magnetization to occur and we've still

got differences between these tissues

now because we have flipped the

longitudinal magnetization Vector into

the transverse plane we can Now sample

that signal and if we have a very short

te time here we negate the t2

differences in the tissue we'll see that

the signal coming from fat is going to

be much higher than the signal coming

from CSF what we've done here is we've

highlighted the T1 relaxation

differences Within These tissues you can

see here the signal for fat is much

brighter signal for CSF is darker and

when we look at T1 weighted images we'll

see that CSF is dark and fat is bright

that's because of these shorter TR time

that's highlighting the T1 differences

in the tissues if we wait a longer

period of time and have a TR time that

is long we have allowed those tissues to

regain their longitudinal magnetization

and the longitudinal magnetization

Vector between the two different tissues

is going to be similar we then apply the

90 degree RF files and the signal from

those tissues is now very similar we can

see if we sample those signals at a very

short te time we'll have high signal

with very little T2 differences because

at te time is really short the t2

differences haven't had time to come

about and we've got very little T1

differences because we've allowed the

sample to regain the longitudinal

magnetization vector and this is a

sequence we're going to look at later

known as a proton density weighted image

where we negate the T1 differences from

a long time to repetition and we negate

the t2 differences by using a short te

time now changing the te results in

changes in T2 contrast and now you can

see that changing the TR the time to

repetition results in highlighting the

T1 differences in the next talk we are

going to look at how we use these t e

and TR times to weight our images to

weight them either towards the t2

contrast differences or towards the T1

contrast differences or somewhere in

between known as proton density

weighting now importantly every image

has some T2 contribution and some T1

contribution to contrast in the image so

if you want to learn how to do that join

me in the next talk where we will look

at weighting of MRI images until then

goodbye everybody