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hello everybody and welcome to the MRI
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physics module I can't wait to share the
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upcoming talks with you now this course
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consists of multiple different talks and
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each one dives into a fair amount of
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detail regarding that specific topic and
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it's my hope that by the end of this
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module you'll have a good conceptual
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understanding as to how exactly MRI
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physics works now I think about learning
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MRI physics much like building a large
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puzzle if I was to pour all the puzzle
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pieces out on the table and pick up one
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piece it'll be very difficult for me to
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accurately place that piece where it
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goes on the table what we want to do is
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separate the puzzle into the edge pieces
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find the corners separated into various
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different color groups and then work on
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each one of those groups individually
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before combining them to give us the
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overall picture now what I want to do
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today is show you the front cover of the
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puzzle we're trying to build show you
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where we're going throughout this course
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then we can take a step back and work on
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each one of these individual sections
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before putting them together and
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hopefully having a good clear understand
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scanning of how MRI physics works now as
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you'll see here is a 3D model of the MRI
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machine itself and you can see it's made
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of multiple different layers and each
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one of these layers represents a
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different type of magnet that we're
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going to use to generate our image now
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if we look at the machine from side on
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and then open up that machine we can see
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where the patient lies within the MRI
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machine
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now MRI is different from X-ray and CT
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Imaging as well as ultrasound imaging in
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the fact that the signal that we use to
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generate our image is actually coming
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from within the patient and because the
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signal is coming from within the patient
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we need a way of localizing where
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exactly that signal is coming from and
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what we use is what's known as the
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Cartesian plane we can separate this
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image into three separate axes the first
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by convention is the longitudinal axis
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the axis that runs from head to toe
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along the patient and that's always
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labeled the Z or z-axis we can then cut
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the patient in transverse section an
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axial plane using the X Y axis here or
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the X Y plane and that's what's known as
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the transverse plane so we've got the
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longitudinal plane and the transverse
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plane and these are really important
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Concepts to take forward into the
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upcoming talks now in MRI imaging we use
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a concept known as nuclear magnetic
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resonance we use a large magnetic field
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in order to induce resonance in certain
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atoms within the patient and in MRI
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imaging we use the hydrogen atom to do
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this now the hydrogen atom is useful one
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because it's abundant within the body
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there are billions of hydrogen atoms
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within the human body and two the
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hydrogen atom has what's known as
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non-zero Spin and atoms with non-zero
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spin effectively act as Tiny bar magnets
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within the body they have a north and a
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South Pole and as a result have what's
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known as a Magnetic Moment now the
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Magnetic Moment In These diagrams is
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represented by this Arrow here
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now the arrow can actually be used as a
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vector within the MRI machine it has
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both Direction and magnitude and the
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combination of the magnetic moments
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amongst all the free hydrogen atoms
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within the body is what's used to
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generate the image now in conventional
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MRI imaging we only use the hydrogen
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atom to create our MRI imaging so we can
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think of our patient as being a
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combination of multiple different
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hydrogen atoms that are moving randomly
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within the body moving with Brownian
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motion and the amount of movement is
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determined by the temperature of that
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patient
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now because hydrogen protons have a
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magnetic moment they will be influenced
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by an external magnetic field much like
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a compass aligns with the magnetic field
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of the Earth's core we can also pass a
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large magnetic field across the patient
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that magnetic field will cause two
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things to happen the first is that the
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hydrogen atoms will align with the
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magnetic field and the second is that
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they will precess around their own axis
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if you think of a spinning top on a
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table experiencing Gravity the spinning
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top processes like this around its own
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axis the same thing is happening to
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these hydrogen atoms within the patient
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they're along the main magnetic field of
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our MRI scanner and they process at a
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certain frequency now that frequency is
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determined by the type of atom so here
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it's hydrogen and it's determined by the
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strength of the magnetic field the
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Precision frequency is directly
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proportional to the strength of that
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magnetic field higher the magnetic field
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the higher the processional frequency
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now don't worry this is starting to
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confuse you we're going to look at each
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one of these factors in isolation in the
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coming talks now as you can see the
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hydrogen atoms either align parallel to
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the magnetic field or anti-parallel to
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the magnetic field and in fact when we
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look at quantum physics later the
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hydrogen atom itself exists in both of
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these states but for now what's
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important is that the absolute number of
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hydrogen atoms that exist in the
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parallel Direction exceed those of that
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in the anti-parallel direction and those
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in the parallel direction are in a
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slightly lower energy state to those in
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the anti-parallel direction now we can
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combine these magnetic moments to create
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a net Magnetic Moment within the sample
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that we are applying this magnetic field
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to and as you can see there are more
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magnetic moments in the parallel
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Direction than they are in the
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anti-parallel direction
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secondly to note although the hydrogen
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atoms are presetting at the same
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frequency they are out of phase from one
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another the X and Y vectors on each
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individual Magnetic Moment here cancel
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each other out you can see there's an
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equal distribution within the X and Y
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plane and what we get here is what's
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known as net magnetization Vector we
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combine all of these magnetic moments
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here and we get the net magnetization
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Vector now the net magnetization Vector
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is along the longitudinal axis the
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z-axis on the Cartesian plane there is
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no X or Y value here because those
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processional frequencies are out of
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phase with one another
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now we mustn't think of individual
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hydrogen atoms when we are looking at
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MRI imaging we need to think of the net
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magnetization vector and how that is
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influenced by changing magnetic fields
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within the MRI machine so what we can do
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is replace these hydrogen atoms with the
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net magnetization Vector here
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now within MRI imaging what we want to
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measure is this net magnetization Vector
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but we can't measure it along the
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parallel Direction along the
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longitudinal Direction here because our
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main magnetic field strength is too
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strong and it will interfere with our
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measurement of this net magnetization
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Vector what we want to do is move that
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net magnetization Vector perpendicular
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to our main magnetic field that will
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allow us to measure that signal and
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that's exactly what we do in MRI imaging
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we have our main magnetic field that is
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forcing those protons into the parallel
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Direction what we then do is apply a
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second magnetic field known as the radio
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frequency pulse now the radio frequency
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pulse acts in the perpendicular plane to
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that main magnetic field and the radio
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frequency pulse alternates at a
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frequency that is equal to the
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processional frequency of the protons if
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the frequency of the radio frequency
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pulse matches that of the process
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additional frequency of the hydrogen
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atoms within the patient two things will
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happen the first is that the protons
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will start to Fan out and become more
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perpendicular with the main magnetic
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field and the second is that the
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processional frequencies of those
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protons will start to process in Phase
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our net magnetization Vector now will
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get some transverse magnetization so
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magnetization in the X Y plane so as we
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apply this radio frequency pulse that
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net magnetization Vector will start
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gaining some transverse magnetization
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and the angle at which we flip that net
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magnetization Vector is what's known as
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the flip angle in this example we
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flipped it 90 degrees here now the
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protons are all processing in Phase with
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one another and they now align 90
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degrees to the main magnetic field
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now what we can do is place a small coil
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here and the movement of a magnet as
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we've seen with Faraday's law of
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induction the movement of a magnet can
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induce a current and it's the movement
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of this net magnetization Vector that
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induces a current within our receiver
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coil that we then use that signal to
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generate our image so we can see this
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now Vector precessing in the transverse
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plane in the X Y plane and we can
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measure a signal based on the movement
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of that Vector within the transverse
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plane now this Vector is only moving in
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this plane because of that radio
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frequency pulse and importantly that
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radio frequency pulse has to match the
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processional frequency of the hydrogen
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atoms if you're jumping on a trampoline
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and someone is jumping at exactly the
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same time as you you will get double
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bounce you will get extra energy and you
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will jump higher and higher if other
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people are jumping on that trampoline
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but not at the same time they're not
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getting that extra energy they're
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bouncing the same only when those
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frequencies match will that energy be
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transferred those protons start to
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process in phase and the angle of
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magnetization will start changing and
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that angle changes dependent on the time
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of the radio frequency pulse as well as
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the amplitude of that radio frequency
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pulse now that we've moved that Vector
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into the transverse plane and we've
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generated a signal we want to stop this
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radio frequency pulse now we can see the
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signal that has been generated here is
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based on that net magnetization Vector
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precessing at the frequency of the radio
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frequency pulse now we don't actually
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get a signal like this because what we
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actually do is apply a radio frequency
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pulse and then stop that radio frequency
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pulse now what actually happens is the
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net magnetization vectors are all
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processing at that radio frequency pulse
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and when we stop the radio frequency
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pulse they will start to go out of phase
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again and it's that loss of phase
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coherence that will cause a net
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magnetization Vector in the transverse
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plane to get smaller and smaller so
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let's have a look at an example here
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here this Arrow here represents the net
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magnetization Vector in the transverse
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plane
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as we stop that radio frequency pulse we
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will see the various net magnetization
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vectors start to become out of phase
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with one another the more and more out
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of phase they become the less our net
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magnetization Vector in the transverse
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plane will be and we see that the signal
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that is generated becomes less and less
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now this curve that we draw down like
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that is what's known as the free
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induction decayed curve or the t2 star
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curve now importantly each and every
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tissue within the body will have
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different T2 star curves or different
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free induction Decay curves if we look
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at Water the free induction Decay is
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very slow over time and if we look at
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something like bone or fat the free
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induction Decay is much faster and it's
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those differences in loss of transverse
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magnetization that we can use to start
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generating contrast within our image and
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we're going to look at that more within
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this talk now this process is happening
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simultaneously with a separate
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independent process the loss of
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transverse magnetization the loss of the
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vector within the X Y plane is purely
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because of that loss of phase between
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the separate protons within the various
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different tissues and the rate at which
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we lose that transverse magnetization is
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what's known as free induction Decay now
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at the same time we are also gaining or
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regaining the longitudinal magnetization
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within our sample if we have the net
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magnetization Vector perpendicular to
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our main magnetic field we have lost all
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of the longitudinal magnetization or the
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net magnetization in the z-axis
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as time goes by and that radio frequency
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pulse has been turned off what will
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happen is that net magnetization Vector
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will slowly regain longitudinal
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magnetization so we can see now as as
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time goes by we will regain some
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longitudinal magnetization the y-axis
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here is representing the amount of MZ or
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longitudinal magnetization along the
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z-axis of our Cartesian plane along the
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longitudinal axis of the patient
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now importantly as we're gaining
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longitudinal magnetization here we are
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not losing transverse magnetization
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because of the tilt of the protons we
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lose transverse magnetization because of
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those protons going out of phase with
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one another that free induction Decay or
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T2 star the loss of transverse
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magnetization happens much quicker than
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this regaining of the longitudinal
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magnetization
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now as time goes by even further we get
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more and more longitudinal magnetization
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now as we can see here we are gaining
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longitudinal magnetization but by this
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point we have lost all of our transverse
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magnetization because although the
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protons have regained some longitudinal
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magnetization by this point they are
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completely out of phase with one another
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and all of those X Y vectors have
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canceled one another out we are still
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now regaining longitudinal relaxation or
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T1 recovery along the z-axis which takes
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a much longer period of time
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now when the vectors are all aligned
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with the magnetic field with the main
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magnetic field we have regained 100 of
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our longitudinal relaxation now
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important to note that these two
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processes happen independently of one
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another if we know the free induction
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decay of a certain tissue we can't
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calculate the T1 recovery or the
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longitudinal recovery of that tissue
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they are completely independent of one
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another
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both longitudinal relaxation like we can
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see here and free induction Decay
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happened at different rates for
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different tissues and it's those
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differing rates that we use to generate
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contrast within our image and lastly and
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what's most important to remember is we
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can only measure signal that is
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perpendicular to the main magnetic field
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so it's very difficult to measure
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longitudinal magnetization unless we
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flip that Vector again perpendicular to
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the main magnetic field
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now we can go about generating images by
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using two separate parameters that will
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exploit these differences in the free
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induction Decay or T2 star Decay and T1
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recovery or longitudinal relaxation now
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the first parameter that we can use is
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what's known as the time of echo now I'm
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going to use these two knitting needles
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to show two separate types of tissue now
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we have the protons have been flipped
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into the longitudinal Direction in both
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of these tissues say CSF and fat
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now what happens is we apply the radio
00:15:21
frequency pulse to 90 degrees our
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protons are now processing perpendicular
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to the main magnetic field at 90 degrees
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now what happens is we start to lose
00:15:33
transverse magnetization as these start
00:15:36
to process out of phase with one another
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we lose that T2 or the free induction
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Decay because these are becoming out of
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phase within that one another they were
00:15:46
initially in Phase providing maximum
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signal that signal gets lost as we get
00:15:51
more and more out of phase now the time
00:15:53
of echo is the time from that RF pulse
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at 90 degree RF pulse to the time that
00:15:59
we actually measure the signal being
00:16:01
generated by these tissues now given
00:16:04
more and more time the phase incoherence
00:16:06
will become more and more the difference
00:16:08
between these two tissues will become
00:16:10
more and more so as we wait a longer
00:16:13
period of time the difference between
00:16:14
these two tissues will become more and
00:16:16
more but the signal will become less and
00:16:19
less so it's a trade-off between getting
00:16:20
good signal and getting contrast between
00:16:23
these two tissues now that contrast is
00:16:25
based on the loss of transverse
00:16:26
magnetization
00:16:28
at the same time both of these tissues
00:16:31
are gaining longitudinal magnetization
00:16:34
in the z-axis and if we wait a really
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long period of time we can see that they
00:16:40
will gain their longitudinal
00:16:41
magnetization at different rates but if
00:16:43
we wait long enough they will gain that
00:16:45
full net longitudinal magnetization
00:16:48
Vector we can then flip them again to 90
00:16:51
degrees with a second RF pulse the time
00:16:54
from that first RF pulse to the second
00:16:57
RF pulse is what's known as the time of
00:17:00
repetition or our TR time
00:17:02
if we wait a long period of time flip it
00:17:05
90 degrees and then wait another period
00:17:08
of time before measuring that signal
00:17:10
that time to Echo time from the RF pulse
00:17:13
to when we measure the differences in
00:17:15
Signal are going to be based on the loss
00:17:17
of transverse magnetization
00:17:19
now what happens if we wait a short
00:17:22
period of time a short TR time
00:17:25
we'll see that longitudinal
00:17:27
magnetization or longitudinal relaxation
00:17:30
occurs at different rates in fact the
00:17:34
longitudinal or T1 recovery happens much
00:17:37
faster than it does in water
00:17:39
now if we wait a short period of time
00:17:42
and don't allow the full neck
00:17:44
longitudinal magnetization or T1
00:17:46
recovery to happen what we'll see is the
00:17:49
longitudinal magnetization Vector in fat
00:17:51
is much longer than that of water now
00:17:55
when we apply a 90 degree RF pulse the
00:17:58
amount of transverse magnetization will
00:18:00
only be equal to the amount of
00:18:03
longitudinal recovery that has occurred
00:18:05
so our water will have a much smaller
00:18:09
net magnetization in the transverse
00:18:10
plane than the fact will so when we flip
00:18:13
this 90 degrees this is what's going to
00:18:15
happen the signal that is being
00:18:17
generated from fat is much more than the
00:18:20
signal that's being generated from water
00:18:22
the difference that we are seeing here
00:18:25
is because of that short time of
00:18:27
repetition it's because of the
00:18:29
differences in longitudinal relaxation
00:18:32
or the differences in T1 recovery so
00:18:35
when we make our time to repetition
00:18:36
short we're getting differences in
00:18:38
longitudinal recovery three or T1
00:18:40
differences we are not measuring the t2
00:18:43
differences between these two tissues
00:18:45
now I know this is a really difficult
00:18:47
concept and we have dedicated videos
00:18:49
specifically looking at the types of
00:18:51
relaxation and looking at t e and TR
00:18:54
times what I want to give you is an idea
00:18:57
of how we generate contrast in an image
00:18:59
and again I'm just showing you the front
00:19:01
cover of the puzzle that we are trying
00:19:03
to create you don't need to understand
00:19:05
these Concepts now but it's useful to
00:19:07
know where we're going in future
00:19:09
lectures
00:19:10
now we can manipulate the te and the TR
00:19:13
times as I've shown you now to generate
00:19:16
different contrast within our image as I
00:19:19
showed you in that example with a short
00:19:20
TR time the water lost its signal
00:19:23
because it wasn't gaining its
00:19:25
longitudinal relaxation as fast as the
00:19:27
fat was and that's what's generating a
00:19:30
T1 image where water like our CSF has a
00:19:33
low signal and fact like the
00:19:35
subcutaneous fattier has a high signal
00:19:38
when we had a long time to repetition we
00:19:41
allowed all of those tissues to fully
00:19:44
regain their magnetization in the
00:19:46
longitudinal plane before flipping them
00:19:48
into the 90 degrees and then having an
00:19:50
echo time that measured that transverse
00:19:53
magnetization that's what generates a T2
00:19:56
image where the differences between
00:19:58
water and fat now come from the
00:20:00
differences in the rate at which they
00:20:02
defaze in the transverse plane water
00:20:05
takes a very long time to de-phase and
00:20:08
the signal remains high in the
00:20:10
transverse plane unlike fat which
00:20:13
because of the spin spin interactions
00:20:15
that we're going to look at in a future
00:20:16
talk reduces the transverse
00:20:18
magnetization signal because fat D
00:20:21
phases relatively quickly compared to
00:20:23
water and we can see we get dark signal
00:20:25
in the fat coated axons in our white
00:20:28
matter we get bright signal in the water
00:20:30
in our CSF because of the differences in
00:20:33
the t2 relaxation or the free induction
00:20:35
Decay between those two tissues now the
00:20:37
way in which we Act generate these
00:20:39
images is more complicated than what we
00:20:42
covered here but the underlying
00:20:43
principle will always come back to the
00:20:45
time of Echo and the time of repetition
00:20:47
we still need to look at how we exactly
00:20:50
go about localizing the different
00:20:52
signals within the patient how we select
00:20:54
certain slices along the patient and
00:20:57
then how we encode the different X and Y
00:20:59
axis components of our image and in
00:21:02
order to do this we use what is known as
00:21:04
different pulse sequences and in this
00:21:06
module we're going to look at the main
00:21:08
pulse sequences the spin Echo sequence
00:21:11
the inversion recovery sequence as well
00:21:13
as gradient Echo sequences we will then
00:21:16
expand on these different sequences
00:21:18
looking at more advanced imaging
00:21:20
techniques we'll also look at Mr
00:21:22
spectroscopy as well as different types
00:21:24
of angiography in MRI imaging we'll end
00:21:27
off the module by looking at different
00:21:29
types of MRI artifacts as well as image
00:21:31
quality and safety within MRI imaging
00:21:34
now when we are generating signals
00:21:36
Within These different pulse sequences
00:21:38
we need a way of storing that data and
00:21:41
then ultimately using that data to
00:21:43
create an image now we use what is known
00:21:45
as k space to encode for the different
00:21:48
slices on our MRI image and we're going
00:21:51
to spend some time looking at how we go
00:21:53
about filling the data within a specific
00:21:56
case space and how we can use that case
00:21:58
space then to go about creating our
00:22:00
image stacking those K spaces on top of
00:22:03
one another in order to create a
00:22:05
scrollable image so I know this talk is
00:22:08
very complicated and if you're new to
00:22:09
MRI it's going to sound like a different
00:22:11
language and that's okay each and every
00:22:14
talk from now on is going to be looking
00:22:16
at a specific component of what we've
00:22:18
covered so hopefully you can use this
00:22:21
talk as the picture on the front of the
00:22:23
puzzle that we're trying to create and
00:22:25
when we go about building those
00:22:26
different sections on our puzzle the
00:22:28
different units within this module you
00:22:30
know where those units fit in on the
00:22:32
broad overarching picture now by the
00:22:35
time I've completed this entire physics
00:22:37
module there will be a question bank
00:22:39
that's linked Below in the first line of
00:22:41
the description you can use that
00:22:43
question Bank to test yourself with
00:22:45
actual past paper questions in Radiology
00:22:47
physics exams I've collated all of those
00:22:50
questions together and it's a great way
00:22:52
for you to test your knowledge and
00:22:53
identify knowledge gaps before heading
00:22:55
into a radiology Physics Exam so I hope
00:22:58
this has at least made some sense to you
00:23:00
use this as a springboard now going into
00:23:03
the following modules to go and build
00:23:05
your knowledge around MRI imaging so
00:23:07
until the first talk where we look at
00:23:08
the magnets in MRI imaging I'll see you
00:23:11
there goodbye everybody
