MRI Physics | Magnetic Resonance and Spin Echo Sequences - Johns Hopkins Radiology

Summary

Dr. 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

• 🧲 MRI Basics: An MRI scanner is essentially a giant magnet that influences protons within the body, aligning them with the external magnetic field (B0).
• 🌀 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.
• 🔄 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.
• 📡 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).
• ⏱️ 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.
• ⛔ 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.
• 🔄 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.
• 📸 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.
• ⏰ 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.
• 🖼️ 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

00:00

🧲 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.

05:02

🕒 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.

10:03

🔁 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.

Keywords

💡Proton

A proton is a subatomic particle found in the nucleus of atoms, carrying a positive charge. In the context of MRI, protons in body tissues such as fat, muscle, and water act like tiny magnets that align with an external magnetic field. Their behavior under these conditions is critical to generating MRI images, as discussed in the video.

💡Magnetic Field (B0)

The magnetic field, denoted as B0, refers to the external magnetic field generated by the MRI scanner. This field causes protons in the body to align parallel or anti-parallel to it. The strength and uniformity of this magnetic field are essential for obtaining clear images, and it forms the foundation for proton alignment in MRI procedures.

💡Precession

Precession describes the wobbling or spinning motion of protons around their axis when exposed to a magnetic field. This movement depends on the strength of the magnetic field and plays a key role in MRI imaging, as the rate of precession affects how protons are manipulated during scanning sequences.

💡Larmor Equation

The Larmor Equation relates the frequency of proton precession to the strength of the applied magnetic field and a specific constant known as the gyromagnetic ratio. This equation is fundamental to understanding how MRI machines influence proton behavior to generate detailed images.

💡Radio Frequency (RF) Pulse

An RF pulse is an externally applied burst of radio waves that temporarily disrupts the alignment of protons with the magnetic field. By delivering energy to the protons, RF pulses cause them to change orientation and precess in sync, a critical step in creating the signals needed for MRI imaging.

💡Longitudinal Magnetization

Longitudinal magnetization refers to the alignment of protons along the direction of the magnetic field. It represents the baseline state of protons before an RF pulse is applied, and its recovery after the pulse is a key process in MRI scanning, as it helps form the basis of the final image.

💡Transverse Magnetization

Transverse magnetization occurs when protons are knocked into a plane perpendicular to the main magnetic field due to an RF pulse. This temporary state causes protons to precess together, generating a signal that is used to create the MRI image. Over time, transverse magnetization decreases as protons return to their original state.

💡Spin Echo Sequence

A spin echo sequence is a specific type of MRI imaging technique that involves applying a 90-degree RF pulse followed by a 180-degree refocusing pulse. This sequence helps counteract signal loss (T2 star effects) and improve image clarity by synchronizing proton precession, producing an 'echo' that is captured for image formation.

💡T1 Time

T1 time refers to the time it takes for 63% of the longitudinal magnetization to recover after an RF pulse. This value varies between different tissues, making it an essential factor in distinguishing between tissue types in MRI images. T1 time helps radiologists understand tissue properties based on their recovery rates.

💡T2 Star Effects

T2 star effects describe the rapid loss of synchronization between precessing protons due to small variations in the magnetic field. These effects result in signal decay and can appear as areas of signal loss or distortion in MRI images. The video explains how these effects can be mitigated by applying additional RF pulses to improve image quality.

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