MRI Physics | Magnetic Resonance and Spin Echo Sequences - Johns Hopkins Radiology
Summary
TLDRDr. Erin Gomez offers an insightful overview of magnetic resonance imaging (MRI), focusing on the basic spin echo sequence. She explains how protons in the body align with the MRI scanner's magnetic field, creating a net magnetization vector. The video delves into the effects of radio frequency pulses on protons, detailing how they induce changes in longitudinal and transverse magnetization, leading to the generation of signals through free induction decay. It also addresses T1 and T2 relaxation times, T2* effects, and how a 180-degree refocusing pulse combats signal decay, resulting in echo formation crucial for MRI imaging.
Takeaways
- đ§Č MRI Basics: An MRI scanner is essentially a giant magnet that influences protons within the body, aligning them with the external magnetic field (B0).
- đ Proton Behavior: Protons act like tiny bar magnets and usually align randomly but can be influenced by an external magnetic field, leading to a net magnetization vector along the body's z-axis.
- đ Precession and Spin: Protons spin along their axes (precession) at a frequency determined by the Larmor equation, which is dependent on the magnetic field strength and the gyromagnetic ratio of the nucleus.
- đĄ RF Pulses: Externally applied radiofrequency (RF) pulses can alter the alignment of protons, knocking them into a different plane and causing them to precess in sync (transverse magnetization).
- â±ïž T1 and T2 Times: T1 time is the point where 63% of longitudinal magnetization has recovered, while T2 time is when 63% of transverse magnetization has been lost. These times vary depending on tissue types.
- â T2 Star Effects: Inhomogeneities in the magnetic field cause protons to dephase rapidly, leading to signal decay known as T2 star effects, which appear as loss of signal in MRI images.
- đ Refocusing Pulses: 180-degree RF pulses can rephase protons that have dephased due to magnetic field inhomogeneity, creating an echo that the MRI scanner reads to generate images.
- đž Spin Echo Sequence: In a spin echo sequence, an initial 90-degree pulse is followed by a 180-degree refocusing pulse to produce echoes, which are used to form MRI images.
- â° Timing in MRI: Key timing parameters in MRI sequences include TE (Time to Echo) and TR (Time to Repetition), which define when signals are captured and when sequences are repeated.
- đŒïž Fast Spin Echo Imaging: This imaging technique uses multiple 180-degree pulses to capture successive echoes, reducing the effects of T2 star decay and enhancing image quality.
Q & A
What is the primary component of an MRI scanner?
-The primary component of an MRI scanner is a giant magnet that generates its own magnetic field, referred to as B0.
How do protons in the human body behave in the presence of an external magnetic field?
-Protons in the human body align with the external magnetic field, with a majority aligning parallel to the primary magnetic field, creating a net magnetization vector.
What is the significance of proton precession?
-Protons precess or spin along their axes in response to the magnetic field, and this precession frequency can be described by the Larmor equation, which relates it to the strength of the magnetic field and the gyromagnetic ratio.
What is the purpose of applying radio frequency (RF) pulses in an MRI scan?
-RF pulses are used to influence the protons by knocking them into an alternate plane and causing them to precess in phase, which changes their longitudinal magnetization.
What is the effect of a 90-degree RF pulse on the net magnetization vector?
-A 90-degree RF pulse causes the net magnetization vector to become perpendicular to its original orientation, eliminating longitudinal magnetization and generating a transverse magnetization vector.
What is the T1 time in the context of MRI?
-The T1 time refers to the point at which 63 percent of the longitudinal magnetization of a proton has been recovered after an RF pulse.
What is the T2 time, and how does it differ from T1 time?
-The T2 time is the point at which 63 percent of the transverse magnetization has been lost. It differs from T1 time in that it measures the loss of transverse magnetization rather than the recovery of longitudinal magnetization.
What are T2 star effects in MRI, and how do they affect image quality?
-T2 star effects are caused by inhomogeneities in the magnetic field, leading to dephasing of proton spins and signal decay. They can appear as diffuse loss of signal or 'black holes' in areas of the image where the magnetic field is particularly distorted.
How can T2 star effects be mitigated during an MRI scan?
-T2 star effects can be mitigated by applying a 180-degree refocusing RF pulse, which temporarily rephases proton precession and produces an echo that can be read by the MRI scanner.
What is the significance of the time to echo (TE) and time to repetition (TR) in MRI sequences?
-The time to echo (TE) is the moment that an echo is produced after a 180-degree refocusing pulse, and the time to repetition (TR) is the time between repetition of sequences. These timings are crucial for controlling the acquisition of MRI signals.
Outlines
đ§Č Introduction to MRI and Spin Echo Sequence
Dr. Erin Gomez introduces the concept of magnetic resonance imaging (MRI) by explaining the role of protons in the human body. She discusses how protons, found in water, fat, and sugars, act like tiny bar magnets. These protons align with the MRI scanner's magnetic field, creating a net magnetization vector. The MRI scanner's magnetic field, denoted as B0, influences the protons' alignment and their precession or spinning motion. The Larmor equation is mentioned to describe the relationship between the magnetic field strength and the protons' precession frequency. Dr. Gomez then explains how radio frequency (RF) pulses are used to manipulate the protons' alignment and create a transverse magnetization, which is the basis for MRI imaging.
đ Spin Echo Sequence and T1/T2 Times
The paragraph delves into the specifics of the spin echo sequence in MRI, starting with a 90-degree RF pulse that alters the net magnetization vector. This pulse causes the protons to move into a transverse plane, creating a signal through free induction decay. The recovery of longitudinal magnetization and the loss of transverse magnetization are described, with the T1 and T2 times being key parameters for different tissue types. T1 time is when 63% of the longitudinal magnetization has recovered, while T2 time is when 63% of the transverse magnetization has decayed. The concept of T2* effects, which are due to magnetic field inhomogeneities causing signal loss, is introduced. The paragraph concludes with a discussion of how a 180-degree RF pulse can be used to refocus the protons and create an echo, which is captured by the MRI scanner to form an image.
đ Multiple Echoes and Sequence Repetition
The final paragraph explains the process of applying multiple 180-degree refocusing pulses to capture multiple echoes, which are used to generate medical images. Each echo becomes weaker as the sequence progresses, until the signal dies out completely. At this point, the sequence must be restarted with another 90-degree pulse. The time between sequence repetitions is referred to as the TR or time to repetition. This paragraph summarizes the basic spin echo sequence and its importance in MRI imaging, highlighting the dynamic process of capturing echoes to create detailed images of the body's internal structures.
Mindmap
Keywords
đĄProton
đĄMagnetic Field (B0)
đĄPrecession
đĄLarmor Equation
đĄRadio Frequency (RF) Pulse
đĄLongitudinal Magnetization
đĄTransverse Magnetization
đĄSpin Echo Sequence
đĄT1 Time
đĄT2 Star Effects
Highlights
Protons in the body act like bar magnets due to their positive and negative poles.
MRI scanner operates as a giant magnet generating its own magnetic field, B0.
Protons align with the MRI scanner's magnetic field, creating a net magnetization vector.
Protons' precession or nuclear spin is influenced by the applied magnetic field's strength.
The Larmor equation relates the precession frequency to the magnetic field strength and gyromagnetic ratio.
Radio frequency (RF) pulses are used to manipulate protons' alignment and precession.
A 90-degree RF pulse flips the net magnetization vector, creating transverse magnetization.
Free induction decay is the process where the protons' transverse magnetization decays, inducing an electrical signal.
T1 and T2 times are recovery points for longitudinal and transverse magnetization, respectively.
T2* effects describe signal loss due to inhomogeneities in the magnetic field.
A 180-degree refocusing RF pulse is used to counteract T2* effects by rephasing proton precession.
Spin echo imaging uses multiple 180-degree pulses to capture multiple echoes, improving image quality.
The time between sequence repetitions is known as the TR (Time to Repetition).
The time at which an echo is produced is called TE (Time to Echo).
Spin echo sequences are fundamental in MRI for generating medical images.
Universal diagrams can be used to illustrate the events occurring in specific MR sequences.
Transcripts
hello my name is dr erin gomez and this
is a brief overview of magnetic
resonance and a basic mri spin echo
sequence
let's talk about protons
we have protons in the fat muscle and
sugars within our body and of course
within water
remember that a significant portion of
our bodies consists of water and that a
hydrogen atom is just a proton one
positron and one electron with a
positive and a negative pole
because of this each of these protons is
capable of acting like a bar magnet
usually the orientation of these protons
is random but they can be influenced by
an external magnetic field
at the most basic level an mri scanner
is a giant magnet and generates its own
magnetic field which we can call b0
when protons are placed within this
magnetic field they'll line up parallel
or anti-parallel to the primary magnetic
field with a small majority aligning
with the direction of the primary
magnetic field just going with the flow
this generates what is referred to as
the net magnetization vector
we can imagine this net magnetization
along the z axis the long axis or length
of the patient's body
in addition to aligning with the
magnetic field produced by the mri
scanner the protons in your body are
also spinning along their axes like
little tops or globes this is called
precession or nuclear spin
the speed or frequency of this axial
spin depends on the strength of the
applied magnetic field and can be
expressed by the larmor equation
simply put this equation states that the
precession frequency of a particle is
equal to the strength of the magnetic
field applied and the gyromagnetic ratio
which is a constant that is unique to
each specific nucleus or element
with the protons aligned with the main
magnetic field we can influence them
using externally applied radio frequency
or rf pulses when this happens the
protons are knocked down into an
alternate plane and also precessed
together in phase
the angle depends on the strength and
duration of the rf pulse
knocking the protons down into another
plane is a change in their longitudinal
magnetization
normally the majority of protons are
going with the flow and following the
direction of the external magnetic field
but with a little extra energy which we
can call excitation protons have the
ability to go against the current and
instead orient themselves in the
opposite direction against that of the
magnetic field this is called
anti-parallel
that's not all that happens with some
energy applied in the form of the rf
pulse the protons will also process
together in phase we can think of this
brief synchronization as the transverse
magnetization of the protons
to recap we've put some energy into the
system and temporarily convinced each of
these protons to sit down and get it
together
this doesn't last long much as if you
were knocked off of your feet or if i
yelled at my wild little children as
they ran haphazardly around their
playroom recovery is imminent
they'll behave for a short time but
they'll soon return my energy back to me
as the baseline state of disorder is
restored
much like my children the protons will
recover or return to their original
state of orientation with the magnetic
field and asynchronous procession
now that we've gone over what can happen
when we administer an rf pulse let's
talk specifically about what happens
during a typical spin echo sequence
remember the flip angle induced by an rf
pulse depends on the strength and
duration of the pulse
the thing being flipped is the net
magnetization vector at the beginning of
a standard spin echo sequence we apply a
90 degree pulse
this means that after the rf pulse has
been applied the net magnetization
vector is perpendicular to its original
orientation
this orientation is achieved by
eliminating longitudinal magnetization
and generating a transverse
magnetization vector by synchronizing
proton precession
during recovery longitudinal
magnetization increases and transverse
magnetization decreases the protons d
phase
this looks like a spiraling of the net
magnetic vector along the z axis
this spiraling of the net magnetization
vector induces an electrical signal by a
process called free induction decay
which is really just a throwback to the
high school physics principle of
inducing a current by rotating a
magnetic field search the depths of your
mind for the right hand rule
a few additional terms to note the
recovery of the longitudinal
magnetization of a proton occurs
exponentially
the point at which 63 percent of the
longitudinal magnetization has been
recovered is called the t1 time
the time at which 63 percent of the
transverse magnetization has been lost
is called the t2 time
the t1 and t2 time is unique to each
tissue type image
think about a class of children running
a foot race each will recover to their
baseline heart rate at a slightly
different time depending on their
physical fitness
we can take advantage of these unique
tissue properties and alter the mri
sequences to highlight them this is
called waiting and discussion of this is
for another time
that wasn't so bad was it seemed too
good to be true in a way it is there are
a few caveats and drawbacks to the
concept of free induction decay
number one it only applies to 90 degree
pulses
number two the signal decays very
rapidly and requires a very fast scanner
to detect
number three the dephasing of protons
occurs at a speed known as the t2 star
constant
this exponential decay in the
synchronization of proton spins is due
to the fact that each proton experiences
the magnetic field at a slightly
different strength meaning there is
never true uniformity in precession
these differences in precession end up
compiling leading to increasingly
asynchronous spins because each proton
already experiences the magnetic field
differently than its neighbors any in
homogeneity in the magnetic field makes
de-phasing and thus signal drop out even
worse
these are called t2 star effects
on mr imaging these t2 star effects can
appear as diffuse loss of signal or
black holes in areas where the magnetic
field is particularly distorted
because these effects are due to an
inhomogeneous magnetic field
we can liken them to distractions in a
child's environment
t2 star effects seem terrible isn't
there any way to fight them
fret not the answer is yes
the good news is that we can combat t2
star effects and their resulting signal
decay with the addition of another rf
pulse
to understand this we must remember that
although magnetic field in homogeneity
is inconvenient it is manageable in the
sense the differences in precession
speed that they cause are fixed and
predictable
as some protons lag behind their faster
counterparts we can apply a 180 degree
refocusing rf pulse that instructs all
of the protons to turn around and
process in the opposite direction
much like the classic tail of the
tortoise and the hair though the
tortoise is far behind the rabbit if we
ask both to turn around and head back to
the starting line of the race they'll
catch up to each other and arrive at the
same time due to the differences in
their speeds the crowd goes wild it's a
tie
when the proton procession sinks up
following the 180 degree pulse more
energy is released back into the system
this is called an echo and it is the
information collected by the mr scanner
which will eventually generate a medical
image
we can liken the 180 degree refocusing
pulse and the synchronous procession it
creates to an elementary school class
photo shoot
the teacher may need to raise her voice
in order to get the class to focus its
attention on the photographer and
achieve a yearbook worthy shot the echo
we can apply additional 180 degree
pulses to achieve multiple echoes photo
after photo after photo to continue
decreasing the t2 star effects
eventually however the students have
nothing left to give less and less
energy is yielded back with each echo
eventually dephasing occurs completely
and the echo dies out
once that happens the sequence must be
restarted again with another 90 degree
pulse
imaging in this manner is called spin
echo or fast spin echo imaging
we can use universal diagrams to depict
what happens with specific mr sequences
let's use one to recap the basic spin
echo sequence that we've just discussed
protons are aligned with the main
magnetic field b0 and are processing
randomly
a 90 degree rf pulse is applied
eliminating longitudinal magnetization
and producing a transverse magnetization
vector as protons process in phase
longitudinal recovery and transverse
decay occur producing a signal by a free
induction decay which is susceptible to
t2 star effects
a 180 degree refocusing pulse
temporarily rephases proton precession
producing an echo which can be read out
by the mr scanner
the moment that the echo is produced is
called the te or time to echo
we can apply multiple refocusing pulses
in an attempt to capture as many echoes
as possible the echoes become
successively weaker until the signal
dies out completely and the sequence
must be restarted
the time between repetition of sequences
is called the tr or time to repetition
that's all for now this concludes our
overview of magnetic resonance and the
basic mri spin echo sequence
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