MRI physics overview | MRI Physics Course | Radiology Physics Course #1
Summary
TLDRThis introductory MRI physics module aims to build a comprehensive understanding of MRI imaging principles. It compares learning to assembling a puzzle, emphasizing the importance of gradually mastering individual concepts before integrating them for a complete picture. The script covers the basics, including the role of hydrogen atoms, the Cartesian plane for image localization, and the significance of magnetic moments and resonance frequencies. It also introduces key MRI parameters like TE and TR, which are crucial for generating image contrast, and hints at more complex topics like pulse sequences and k-space that will be explored in subsequent lectures.
Takeaways
- 🧩 The MRI physics module is designed to build a comprehensive understanding of MRI principles through multiple detailed talks.
- 🧭 The learning process is likened to solving a puzzle, where concepts are broken down and mastered before being combined for a complete picture.
- 🌐 MRI imaging uses the Cartesian plane to localize signals within the patient, with the Z-axis representing the longitudinal axis and the X-Y plane for transverse sections.
- 🌀 Nuclear magnetic resonance (NMR) is central to MRI, utilizing a large magnetic field to resonate hydrogen atoms, which are abundant and have a non-zero spin in the human body.
- 🧲 Hydrogen atoms, acting as tiny bar magnets, align and precess around their axis when subjected to an external magnetic field, creating a magnetic moment.
- 📊 The net magnetization vector, representing the combined magnetic moments of hydrogen atoms, is crucial for MRI imaging and is influenced by the external magnetic field.
- 🔄 The radio frequency (RF) pulse is applied perpendicular to the main magnetic field to move the net magnetization vector into the transverse plane, enabling signal measurement.
- 📉 The loss of transverse magnetization, or T2 decay, and the regaining of longitudinal magnetization, or T1 recovery, are key processes that occur independently and are exploited to generate image contrast.
- 🔄 The flip angle of the net magnetization vector, induced by the RF pulse, is a critical parameter in MRI imaging, affecting the signal generated and the resulting image contrast.
- 🔺 The time of echo (TE) and time of repetition (TR) are adjustable parameters that control the contrast in MRI images by manipulating the signal's sensitivity to T1 and T2 relaxation times.
- 🛠 Advanced imaging techniques, pulse sequences, and the use of k-space for data storage and image creation will be explored in further detail throughout the module.
Q & A
What is the main difference between MRI imaging and other imaging modalities like X-ray, CT, and ultrasound?
-The main difference is that in MRI imaging, the signal used to generate the image comes from within the patient, specifically from the hydrogen atoms in the body, whereas in X-ray, CT, and ultrasound, the signal comes from external sources or mechanical vibrations.
What is the Cartesian plane used for in MRI imaging?
-The Cartesian plane is used to localize the signal within the patient. It separates the image into three axes: the longitudinal (Z or z-axis), and the transverse (X-Y plane), which are essential for understanding the orientation of the imaging slices.
Why is the hydrogen atom used in MRI imaging?
-The hydrogen atom is used because it is abundant in the human body, and it has a non-zero spin, which means it acts as a tiny bar magnet with a north and south pole, contributing to the magnetic moment used in image generation.
What is nuclear magnetic resonance and how is it relevant to MRI imaging?
-Nuclear magnetic resonance is a phenomenon where certain atomic nuclei, like hydrogen, resonate in a large magnetic field. In MRI imaging, this principle is used to induce resonance in hydrogen atoms within the patient, which is essential for image formation.
How does the magnetic field influence the hydrogen atoms in the MRI scanner?
-The magnetic field causes the hydrogen atoms to align with it and precess around their own axis at a frequency determined by the type of atom and the strength of the magnetic field.
What is the significance of the net magnetization vector in MRI imaging?
-The net magnetization vector represents the combined magnetic moments of all the free hydrogen atoms in the body. It is crucial for MRI imaging as it is the vector that is manipulated and measured to generate the image.
What is the purpose of the radio frequency (RF) pulse in MRI imaging?
-The RF pulse is applied to move the net magnetization vector perpendicular to the main magnetic field, allowing for the measurement of the signal. It also causes the protons to precess in phase, which is necessary for generating a detectable signal.
What is the flip angle in the context of MRI imaging?
-The flip angle refers to the angle at which the net magnetization vector is tilted from its alignment with the main magnetic field to a position perpendicular to it by the RF pulse.
Why is it important to consider the time of echo (TE) and time of repetition (TR) in MRI imaging?
-The TE and TR are parameters that control the timing of the MRI pulse sequence, affecting the contrast in the images by exploiting differences in the relaxation times (T1 and T2) of various tissues.
What is the role of k-space in MRI imaging?
-K-space is a mathematical space used to encode the data for different slices in an MRI image. It is filled with data that is then used to reconstruct the image by stacking the k-spaces on top of one another.
What are the different types of pulse sequences mentioned in the script, and what is their significance?
-The script mentions spin echo, inversion recovery, and gradient echo sequences. These pulse sequences are used to generate images with different contrasts and are essential for understanding various tissue properties and diagnosing conditions.
Outlines
🧠 Introduction to MRI Physics Module
This paragraph introduces the MRI physics module, emphasizing the detailed nature of the course and the goal of building a conceptual understanding of MRI physics. The instructor likens learning MRI physics to assembling a puzzle, suggesting a step-by-step approach to grasp the subject. The module's content is structured around various talks that delve into specific topics, and the instructor presents a 3D model of an MRI machine to illustrate the complexity of its components. The fundamental difference between MRI and other imaging techniques is highlighted, with MRI relying on signals generated from within the patient, necessitating precise localization techniques like the Cartesian plane. The concept of nuclear magnetic resonance (NMR) is introduced, along with the importance of hydrogen atoms in MRI imaging due to their abundance and non-zero spin, which acts as tiny bar magnets within the body.
🌀 Understanding Net Magnetization and MRI Signal Generation
The second paragraph delves into the concept of net magnetization, explaining how the magnetic moments of hydrogen atoms combine to form a net magnetization vector within the sample. It discusses the alignment of hydrogen atoms in parallel or anti-parallel directions relative to the applied magnetic field and the resulting energy states. The paragraph further explains how the net magnetization vector is influenced by the MRI machine's magnetic fields and how the radio frequency (RF) pulse is used to move this vector into the transverse plane for signal measurement. The importance of matching the RF pulse frequency to the proton precession frequency is underscored, as is the process of signal induction in the receiver coil due to the movement of the net magnetization vector in the transverse plane.
📉 Exploring T2* Decay and T1 Recovery in MRI
This paragraph explores the processes of T2* decay and T1 recovery, which are crucial for generating contrast in MRI images. T2* decay refers to the loss of phase coherence among protons in the transverse plane, leading to a reduction in the net magnetization vector and signal intensity. The rate of this decay varies across different tissues, allowing for differentiation in the image. Simultaneously, the paragraph explains T1 recovery, which is the regaining of longitudinal magnetization over time. The rate of T1 recovery also varies among tissues and is independent of T2* decay. The paragraph highlights the importance of these processes in creating contrast within MRI images and mentions the use of different echo times (TE) and repetition times (TR) to manipulate image contrast.
🔄 The Role of TE and TR in Image Contrast
The fourth paragraph discusses the technical aspects of MRI imaging, focusing on the manipulation of time of echo (TE) and time of repetition (TR) to generate image contrast. It illustrates how varying TE and TR affects the signal intensity from different tissues, using the example of CSF and fat. The paragraph explains that a short TR can emphasize differences in T1 recovery, while a long TR, combined with an appropriate TE, can highlight differences in T2 decay. The importance of these parameters in creating T1-weighted and T2-weighted images is emphasized, with T1 images showing differences in longitudinal relaxation and T2 images reflecting differences in transverse relaxation.
🛠️ Advanced MRI Techniques and Image Generation
The final paragraph provides an overview of the advanced MRI techniques and pulse sequences that will be covered in the module, such as spin echo, inversion recovery, and gradient echo sequences. It also mentions the exploration of MR spectroscopy and various angiography techniques. The paragraph outlines the process of image generation, including the storage and utilization of data in k-space, which is essential for encoding different slices in the MRI image. The instructor assures that each component will be examined in detail in subsequent talks, helping to build a comprehensive understanding of MRI imaging. The paragraph concludes with an invitation to use the provided question bank for practice and knowledge assessment.
Mindmap
Keywords
💡MRI
💡Nuclear Magnetic Resonance (NMR)
💡Hydrogen Atom
💡Cartesian Plane
💡Magnetic Moment
💡Precession
💡Radio Frequency Pulse
💡Flip Angle
💡T1 and T2 Relaxation
💡Echo Time (TE) and Repetition Time (TR)
💡k-Space
Highlights
Introduction to the MRI physics module, emphasizing the detailed exploration of MRI physics concepts.
Learning MRI physics is compared to solving a puzzle, suggesting a structured approach to understanding complex topics.
Explanation of the MRI machine's structure, highlighting the layers representing different types of magnets used for imaging.
MRI's distinction from other imaging modalities due to its reliance on signals originating from within the patient.
Introduction of the Cartesian plane for localizing signals within the patient during MRI imaging.
The concept of nuclear magnetic resonance and its application in MRI using the hydrogen atom.
Explanation of the hydrogen atom's non-zero spin and its role as a tiny bar magnet within the body.
Description of the net magnetic moment generated by the combination of magnetic moments of free hydrogen atoms.
The influence of an external magnetic field on hydrogen atoms, causing them to align and precess.
The relationship between the strength of the magnetic field and the precessional frequency of hydrogen atoms.
Discussion on the alignment of hydrogen atoms in parallel or anti-parallel to the magnetic field and their energy states.
Introduction of the net magnetization vector and its significance in MRI imaging.
The process of moving the net magnetization vector perpendicular to the main magnetic field for signal measurement.
Explanation of the radio frequency pulse and its role in altering the net magnetization vector for image generation.
The concept of flip angle in MRI and its impact on the transverse magnetization of protons.
The use of Faraday's law of induction to measure the signal generated by the movement of the net magnetization vector.
Discussion on the loss of phase coherence and its effect on the signal generated from the transverse plane.
Introduction of T2 star curves and their significance in differentiating tissue types within the body.
The independent processes of transverse magnetization loss and longitudinal magnetization regain.
The importance of measuring signal perpendicular to the main magnetic field in MRI imaging.
Overview of generating image contrast by manipulating time of echo and time of repetition (TE and TR) parameters.
Explanation of T1 and T2 imaging, and how differences in tissue relaxation times contribute to image contrast.
Introduction to pulse sequences used in MRI, including spin echo, inversion recovery, and gradient echo sequences.
Discussion on advanced imaging techniques, MR spectroscopy, and angiography in MRI imaging.
Overview of k-space and its role in encoding data for different slices in MRI imaging.
Mention of a question bank for self-testing in radiology physics exams, linked in the description.
Transcripts
hello everybody and welcome to the MRI
physics module I can't wait to share the
upcoming talks with you now this course
consists of multiple different talks and
each one dives into a fair amount of
detail regarding that specific topic and
it's my hope that by the end of this
module you'll have a good conceptual
understanding as to how exactly MRI
physics works now I think about learning
MRI physics much like building a large
puzzle if I was to pour all the puzzle
pieces out on the table and pick up one
piece it'll be very difficult for me to
accurately place that piece where it
goes on the table what we want to do is
separate the puzzle into the edge pieces
find the corners separated into various
different color groups and then work on
each one of those groups individually
before combining them to give us the
overall picture now what I want to do
today is show you the front cover of the
puzzle we're trying to build show you
where we're going throughout this course
then we can take a step back and work on
each one of these individual sections
before putting them together and
hopefully having a good clear understand
scanning of how MRI physics works now as
you'll see here is a 3D model of the MRI
machine itself and you can see it's made
of multiple different layers and each
one of these layers represents a
different type of magnet that we're
going to use to generate our image now
if we look at the machine from side on
and then open up that machine we can see
where the patient lies within the MRI
machine
now MRI is different from X-ray and CT
Imaging as well as ultrasound imaging in
the fact that the signal that we use to
generate our image is actually coming
from within the patient and because the
signal is coming from within the patient
we need a way of localizing where
exactly that signal is coming from and
what we use is what's known as the
Cartesian plane we can separate this
image into three separate axes the first
by convention is the longitudinal axis
the axis that runs from head to toe
along the patient and that's always
labeled the Z or z-axis we can then cut
the patient in transverse section an
axial plane using the X Y axis here or
the X Y plane and that's what's known as
the transverse plane so we've got the
longitudinal plane and the transverse
plane and these are really important
Concepts to take forward into the
upcoming talks now in MRI imaging we use
a concept known as nuclear magnetic
resonance we use a large magnetic field
in order to induce resonance in certain
atoms within the patient and in MRI
imaging we use the hydrogen atom to do
this now the hydrogen atom is useful one
because it's abundant within the body
there are billions of hydrogen atoms
within the human body and two the
hydrogen atom has what's known as
non-zero Spin and atoms with non-zero
spin effectively act as Tiny bar magnets
within the body they have a north and a
South Pole and as a result have what's
known as a Magnetic Moment now the
Magnetic Moment In These diagrams is
represented by this Arrow here
now the arrow can actually be used as a
vector within the MRI machine it has
both Direction and magnitude and the
combination of the magnetic moments
amongst all the free hydrogen atoms
within the body is what's used to
generate the image now in conventional
MRI imaging we only use the hydrogen
atom to create our MRI imaging so we can
think of our patient as being a
combination of multiple different
hydrogen atoms that are moving randomly
within the body moving with Brownian
motion and the amount of movement is
determined by the temperature of that
patient
now because hydrogen protons have a
magnetic moment they will be influenced
by an external magnetic field much like
a compass aligns with the magnetic field
of the Earth's core we can also pass a
large magnetic field across the patient
that magnetic field will cause two
things to happen the first is that the
hydrogen atoms will align with the
magnetic field and the second is that
they will precess around their own axis
if you think of a spinning top on a
table experiencing Gravity the spinning
top processes like this around its own
axis the same thing is happening to
these hydrogen atoms within the patient
they're along the main magnetic field of
our MRI scanner and they process at a
certain frequency now that frequency is
determined by the type of atom so here
it's hydrogen and it's determined by the
strength of the magnetic field the
Precision frequency is directly
proportional to the strength of that
magnetic field higher the magnetic field
the higher the processional frequency
now don't worry this is starting to
confuse you we're going to look at each
one of these factors in isolation in the
coming talks now as you can see the
hydrogen atoms either align parallel to
the magnetic field or anti-parallel to
the magnetic field and in fact when we
look at quantum physics later the
hydrogen atom itself exists in both of
these states but for now what's
important is that the absolute number of
hydrogen atoms that exist in the
parallel Direction exceed those of that
in the anti-parallel direction and those
in the parallel direction are in a
slightly lower energy state to those in
the anti-parallel direction now we can
combine these magnetic moments to create
a net Magnetic Moment within the sample
that we are applying this magnetic field
to and as you can see there are more
magnetic moments in the parallel
Direction than they are in the
anti-parallel direction
secondly to note although the hydrogen
atoms are presetting at the same
frequency they are out of phase from one
another the X and Y vectors on each
individual Magnetic Moment here cancel
each other out you can see there's an
equal distribution within the X and Y
plane and what we get here is what's
known as net magnetization Vector we
combine all of these magnetic moments
here and we get the net magnetization
Vector now the net magnetization Vector
is along the longitudinal axis the
z-axis on the Cartesian plane there is
no X or Y value here because those
processional frequencies are out of
phase with one another
now we mustn't think of individual
hydrogen atoms when we are looking at
MRI imaging we need to think of the net
magnetization vector and how that is
influenced by changing magnetic fields
within the MRI machine so what we can do
is replace these hydrogen atoms with the
net magnetization Vector here
now within MRI imaging what we want to
measure is this net magnetization Vector
but we can't measure it along the
parallel Direction along the
longitudinal Direction here because our
main magnetic field strength is too
strong and it will interfere with our
measurement of this net magnetization
Vector what we want to do is move that
net magnetization Vector perpendicular
to our main magnetic field that will
allow us to measure that signal and
that's exactly what we do in MRI imaging
we have our main magnetic field that is
forcing those protons into the parallel
Direction what we then do is apply a
second magnetic field known as the radio
frequency pulse now the radio frequency
pulse acts in the perpendicular plane to
that main magnetic field and the radio
frequency pulse alternates at a
frequency that is equal to the
processional frequency of the protons if
the frequency of the radio frequency
pulse matches that of the process
additional frequency of the hydrogen
atoms within the patient two things will
happen the first is that the protons
will start to Fan out and become more
perpendicular with the main magnetic
field and the second is that the
processional frequencies of those
protons will start to process in Phase
our net magnetization Vector now will
get some transverse magnetization so
magnetization in the X Y plane so as we
apply this radio frequency pulse that
net magnetization Vector will start
gaining some transverse magnetization
and the angle at which we flip that net
magnetization Vector is what's known as
the flip angle in this example we
flipped it 90 degrees here now the
protons are all processing in Phase with
one another and they now align 90
degrees to the main magnetic field
now what we can do is place a small coil
here and the movement of a magnet as
we've seen with Faraday's law of
induction the movement of a magnet can
induce a current and it's the movement
of this net magnetization Vector that
induces a current within our receiver
coil that we then use that signal to
generate our image so we can see this
now Vector precessing in the transverse
plane in the X Y plane and we can
measure a signal based on the movement
of that Vector within the transverse
plane now this Vector is only moving in
this plane because of that radio
frequency pulse and importantly that
radio frequency pulse has to match the
processional frequency of the hydrogen
atoms if you're jumping on a trampoline
and someone is jumping at exactly the
same time as you you will get double
bounce you will get extra energy and you
will jump higher and higher if other
people are jumping on that trampoline
but not at the same time they're not
getting that extra energy they're
bouncing the same only when those
frequencies match will that energy be
transferred those protons start to
process in phase and the angle of
magnetization will start changing and
that angle changes dependent on the time
of the radio frequency pulse as well as
the amplitude of that radio frequency
pulse now that we've moved that Vector
into the transverse plane and we've
generated a signal we want to stop this
radio frequency pulse now we can see the
signal that has been generated here is
based on that net magnetization Vector
precessing at the frequency of the radio
frequency pulse now we don't actually
get a signal like this because what we
actually do is apply a radio frequency
pulse and then stop that radio frequency
pulse now what actually happens is the
net magnetization vectors are all
processing at that radio frequency pulse
and when we stop the radio frequency
pulse they will start to go out of phase
again and it's that loss of phase
coherence that will cause a net
magnetization Vector in the transverse
plane to get smaller and smaller so
let's have a look at an example here
here this Arrow here represents the net
magnetization Vector in the transverse
plane
as we stop that radio frequency pulse we
will see the various net magnetization
vectors start to become out of phase
with one another the more and more out
of phase they become the less our net
magnetization Vector in the transverse
plane will be and we see that the signal
that is generated becomes less and less
now this curve that we draw down like
that is what's known as the free
induction decayed curve or the t2 star
curve now importantly each and every
tissue within the body will have
different T2 star curves or different
free induction Decay curves if we look
at Water the free induction Decay is
very slow over time and if we look at
something like bone or fat the free
induction Decay is much faster and it's
those differences in loss of transverse
magnetization that we can use to start
generating contrast within our image and
we're going to look at that more within
this talk now this process is happening
simultaneously with a separate
independent process the loss of
transverse magnetization the loss of the
vector within the X Y plane is purely
because of that loss of phase between
the separate protons within the various
different tissues and the rate at which
we lose that transverse magnetization is
what's known as free induction Decay now
at the same time we are also gaining or
regaining the longitudinal magnetization
within our sample if we have the net
magnetization Vector perpendicular to
our main magnetic field we have lost all
of the longitudinal magnetization or the
net magnetization in the z-axis
as time goes by and that radio frequency
pulse has been turned off what will
happen is that net magnetization Vector
will slowly regain longitudinal
magnetization so we can see now as as
time goes by we will regain some
longitudinal magnetization the y-axis
here is representing the amount of MZ or
longitudinal magnetization along the
z-axis of our Cartesian plane along the
longitudinal axis of the patient
now importantly as we're gaining
longitudinal magnetization here we are
not losing transverse magnetization
because of the tilt of the protons we
lose transverse magnetization because of
those protons going out of phase with
one another that free induction Decay or
T2 star the loss of transverse
magnetization happens much quicker than
this regaining of the longitudinal
magnetization
now as time goes by even further we get
more and more longitudinal magnetization
now as we can see here we are gaining
longitudinal magnetization but by this
point we have lost all of our transverse
magnetization because although the
protons have regained some longitudinal
magnetization by this point they are
completely out of phase with one another
and all of those X Y vectors have
canceled one another out we are still
now regaining longitudinal relaxation or
T1 recovery along the z-axis which takes
a much longer period of time
now when the vectors are all aligned
with the magnetic field with the main
magnetic field we have regained 100 of
our longitudinal relaxation now
important to note that these two
processes happen independently of one
another if we know the free induction
decay of a certain tissue we can't
calculate the T1 recovery or the
longitudinal recovery of that tissue
they are completely independent of one
another
both longitudinal relaxation like we can
see here and free induction Decay
happened at different rates for
different tissues and it's those
differing rates that we use to generate
contrast within our image and lastly and
what's most important to remember is we
can only measure signal that is
perpendicular to the main magnetic field
so it's very difficult to measure
longitudinal magnetization unless we
flip that Vector again perpendicular to
the main magnetic field
now we can go about generating images by
using two separate parameters that will
exploit these differences in the free
induction Decay or T2 star Decay and T1
recovery or longitudinal relaxation now
the first parameter that we can use is
what's known as the time of echo now I'm
going to use these two knitting needles
to show two separate types of tissue now
we have the protons have been flipped
into the longitudinal Direction in both
of these tissues say CSF and fat
now what happens is we apply the radio
frequency pulse to 90 degrees our
protons are now processing perpendicular
to the main magnetic field at 90 degrees
now what happens is we start to lose
transverse magnetization as these start
to process out of phase with one another
we lose that T2 or the free induction
Decay because these are becoming out of
phase within that one another they were
initially in Phase providing maximum
signal that signal gets lost as we get
more and more out of phase now the time
of echo is the time from that RF pulse
at 90 degree RF pulse to the time that
we actually measure the signal being
generated by these tissues now given
more and more time the phase incoherence
will become more and more the difference
between these two tissues will become
more and more so as we wait a longer
period of time the difference between
these two tissues will become more and
more but the signal will become less and
less so it's a trade-off between getting
good signal and getting contrast between
these two tissues now that contrast is
based on the loss of transverse
magnetization
at the same time both of these tissues
are gaining longitudinal magnetization
in the z-axis and if we wait a really
long period of time we can see that they
will gain their longitudinal
magnetization at different rates but if
we wait long enough they will gain that
full net longitudinal magnetization
Vector we can then flip them again to 90
degrees with a second RF pulse the time
from that first RF pulse to the second
RF pulse is what's known as the time of
repetition or our TR time
if we wait a long period of time flip it
90 degrees and then wait another period
of time before measuring that signal
that time to Echo time from the RF pulse
to when we measure the differences in
Signal are going to be based on the loss
of transverse magnetization
now what happens if we wait a short
period of time a short TR time
we'll see that longitudinal
magnetization or longitudinal relaxation
occurs at different rates in fact the
longitudinal or T1 recovery happens much
faster than it does in water
now if we wait a short period of time
and don't allow the full neck
longitudinal magnetization or T1
recovery to happen what we'll see is the
longitudinal magnetization Vector in fat
is much longer than that of water now
when we apply a 90 degree RF pulse the
amount of transverse magnetization will
only be equal to the amount of
longitudinal recovery that has occurred
so our water will have a much smaller
net magnetization in the transverse
plane than the fact will so when we flip
this 90 degrees this is what's going to
happen the signal that is being
generated from fat is much more than the
signal that's being generated from water
the difference that we are seeing here
is because of that short time of
repetition it's because of the
differences in longitudinal relaxation
or the differences in T1 recovery so
when we make our time to repetition
short we're getting differences in
longitudinal recovery three or T1
differences we are not measuring the t2
differences between these two tissues
now I know this is a really difficult
concept and we have dedicated videos
specifically looking at the types of
relaxation and looking at t e and TR
times what I want to give you is an idea
of how we generate contrast in an image
and again I'm just showing you the front
cover of the puzzle that we are trying
to create you don't need to understand
these Concepts now but it's useful to
know where we're going in future
lectures
now we can manipulate the te and the TR
times as I've shown you now to generate
different contrast within our image as I
showed you in that example with a short
TR time the water lost its signal
because it wasn't gaining its
longitudinal relaxation as fast as the
fat was and that's what's generating a
T1 image where water like our CSF has a
low signal and fact like the
subcutaneous fattier has a high signal
when we had a long time to repetition we
allowed all of those tissues to fully
regain their magnetization in the
longitudinal plane before flipping them
into the 90 degrees and then having an
echo time that measured that transverse
magnetization that's what generates a T2
image where the differences between
water and fat now come from the
differences in the rate at which they
defaze in the transverse plane water
takes a very long time to de-phase and
the signal remains high in the
transverse plane unlike fat which
because of the spin spin interactions
that we're going to look at in a future
talk reduces the transverse
magnetization signal because fat D
phases relatively quickly compared to
water and we can see we get dark signal
in the fat coated axons in our white
matter we get bright signal in the water
in our CSF because of the differences in
the t2 relaxation or the free induction
Decay between those two tissues now the
way in which we Act generate these
images is more complicated than what we
covered here but the underlying
principle will always come back to the
time of Echo and the time of repetition
we still need to look at how we exactly
go about localizing the different
signals within the patient how we select
certain slices along the patient and
then how we encode the different X and Y
axis components of our image and in
order to do this we use what is known as
different pulse sequences and in this
module we're going to look at the main
pulse sequences the spin Echo sequence
the inversion recovery sequence as well
as gradient Echo sequences we will then
expand on these different sequences
looking at more advanced imaging
techniques we'll also look at Mr
spectroscopy as well as different types
of angiography in MRI imaging we'll end
off the module by looking at different
types of MRI artifacts as well as image
quality and safety within MRI imaging
now when we are generating signals
Within These different pulse sequences
we need a way of storing that data and
then ultimately using that data to
create an image now we use what is known
as k space to encode for the different
slices on our MRI image and we're going
to spend some time looking at how we go
about filling the data within a specific
case space and how we can use that case
space then to go about creating our
image stacking those K spaces on top of
one another in order to create a
scrollable image so I know this talk is
very complicated and if you're new to
MRI it's going to sound like a different
language and that's okay each and every
talk from now on is going to be looking
at a specific component of what we've
covered so hopefully you can use this
talk as the picture on the front of the
puzzle that we're trying to create and
when we go about building those
different sections on our puzzle the
different units within this module you
know where those units fit in on the
broad overarching picture now by the
time I've completed this entire physics
module there will be a question bank
that's linked Below in the first line of
the description you can use that
question Bank to test yourself with
actual past paper questions in Radiology
physics exams I've collated all of those
questions together and it's a great way
for you to test your knowledge and
identify knowledge gaps before heading
into a radiology Physics Exam so I hope
this has at least made some sense to you
use this as a springboard now going into
the following modules to go and build
your knowledge around MRI imaging so
until the first talk where we look at
the magnets in MRI imaging I'll see you
there goodbye everybody
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