T1 Relaxation, Spin-lattice Relaxation, Longitudinal Recovery | MRI Physics Course #5
Summary
TLDRThis 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
- 𧲠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.
- π T2 relaxation, in contrast to T1, is about the loss of transverse magnetization due to spin-spin interactions causing de-phasing of the spins.
- π T1 relaxation is associated with longitudinal recovery, where the net longitudinal magnetization is regained, as opposed to T2 relaxation, which is about transverse decay.
- π« 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.
- π§Ό 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.
- π 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.
- π 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.
- π 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.
- π 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.
- πΈ 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
𧲠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.
π 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.
π 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.
π 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.
Mindmap
Keywords
π‘T1 Relaxation
π‘T2 Relaxation
π‘Spin-lattice Interaction
π‘Transverse Magnetization
π‘Longitudinal Magnetization
π‘CSF (Cerebrospinal Fluid)
π‘Fat
π‘RF Pulse
π‘Time to Echo (TE)
π‘Time to Repetition (TR)
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
Browse More Related Video
Wie funktioniert MRT? T1 und T2 Wichtung
T1, T2 and Proton Density Weighting | MRI Weighting and Contrast | MRI Physics Course #6
T2 Relaxation, Spin-spin Relaxation, Free Induction Decay, Transverse Decay | MRI Physics Course #4
Wie funktioniert MRT? Die Basics
MRI Physics | Magnetic Resonance and Spin Echo Sequences - Johns Hopkins Radiology
Brain MRI scan protocols, positioning and planning
5.0 / 5 (0 votes)