Quiz 1: Lecture 2 Videos Flashcards
What does the z-axis indicate?
The direction of a magnetic field line
Instead of only two protons as shown in the model, how many may there be (ratio)?
There should only be two more protons pointing up (Ex: 82 pointing up, 80 pointing down)
Which protons have a net magnetic effect?
The two protons pointing up (in diagram) because their effects are not cancelled out
What happens if an “RF (90 degrees) pulse” is sent in with the correct strength and duration so that one of the two protons picks up energy to go into a higher state of energy (points down or “walks on its hands”)?
What happens to the longitudinal magnetization?
What happens to transversal magnetization?
The top left arrow moves to bottom right, under the right top arrow (both were top before)
It decreases to zero because the arrow pointing up is neutralized by the arrow pointing down
There is now a transversal magnetization that wasn’t there before as both protons are in phase (y axis line)
What happens with a RF 90 degree pulse?
No longer have a longitudinal magnetization, but have a transversal magnetization
What happens when a RF 90 degree pulse is switched off (2)?
How do their occur (2)?
Protons go back to their lower state of energy, Protons lose phase coherence
Both processes occur simultaneously and independently
How many protons are in which orientation when RF 90 degree pulse is switched off?
One proton goes back to the lower energy state, resulting in 4 protons pointing up, and 2 pointing down.
How is the net effect for longitudinal magnetization calculated?
(Transversal magnetization decreases at the same time)
Subtract the protons pointing up and the protons pointing down.
Ex: If 5 are pointing up and 1 is pointing down, there is a net longitudinal magnetization of “4”
If all of them are pointing up (5+1), net longitudinal magnetization is “6”
Why does the transversal magnetization decrease at the same time as longitudinal magnetization increases?
After the RF (90 degrees) pulse is switched off, the processing protons lose phase coherence
Sum vector:
When the RF (90 degrees) pulse is sent in, what happens to the longitudinal magnetic vector?
What happens after the RF pulse is switched off?
The longitudinal magnetic vector is tilted 90 degrees to the side
After the RF pulse is switched off, the transversal vector decreases while the longitudinal vector increases
What do vectors represent?
Forces of a certain size and a certain direction
What is the sum vector?
Adding up the longitudinal and transversal vectors pointing in different directions and coming up with a direction that is somewhere in between
Why is the sum vector important and what does it represent?
The sum vector represents the total magnetic moment of a tissue in general, and thus can be used instead of the two single vectors (that represent longitudinal and transversal magnetization separately)
Magnetic sum vector during relaxation…
Goes back to a longitudinal direction, in the end equaling the longitudinal magnetization (the whole system is processing, including the sum magnetic vector)
What can a changing magnetic force/moment induce?
An electrical current (the signal that we received and use in MR)
What happens to the signal in the RF (90 degrees) pulse experiment?
What is this signal called?
The signal disappears with time but has a constant frequency
Free Induction Decay (FID) Signal
The magnetic vectors…
Directly determine the MRI signal and signal intensity by inducing electric currents in the antenna
What terms can we use instead of “longitudinal” or “transversal magnetization”?
“Signal or signal intensity” at the axis of T1 and T2 curves
PRECESSION video:
Protons have a ____charge and possess a ____.
What do they have due to this?
Positive; Spin
A magnetic field (can be seen as little bar magnets with north-south pole)
What happens when you put protons into a strong external magnetic field?
They align with it, some parallel (pointing up), some anti-parallel (pointing down)
What do the protons do around the magnetic field lines?
They precess
What relationship is mathematically described in the Larmor equation?
The stronger the magnetic field, the higher the precession frequency
Parallel and anti-parallel protons can…
HOWEVER
Cancel each other’s forces out
As there are more parallel protons on the lower energy level (pointing up), we are left with some protons whose magnetic forces aren’t canceled
What do all the protons pointing up do?
Add up their forces in the direction of the external magnetic field
What happens when you put the patient in the MR magnet?
Can his magnetic field be measured directly?
He has his own magnetic field, which is longitudinal to the external field of the MR machine’s magnet
Because it is longitudinal, it can’t be measured directly
MRI Basics Alignment video:
What are the steps of an MRI examination?
Patient placed in a magnet, a radio wave sent in, the radio wave is turned off, the patient emits a signal (which is received and used for reconstruction of the picture)
What happens when we put a patient into the magnet of an MRI machine?
(Basic physics)
Atoms consist of…
In the nucleus…
What do protons possess?
A nucleus and a shell (made up of electrons)
In the nucleus, there are protons that have a positive electrical charge
A spin
What is a moving electrical charge?
What does it induce?
An electrical current
An electrical current induces a magnetic force/field (physics)
(Where there is an electrical current, there is also a magnetic field)
What are the two ways for protons to align with the magnetic field?
How are the alignments different?
Parallel or Anti-parallel
They are on different energy levels
What is the preferred state of alignment?
SO
The one that needs less energy
More protons are on the lower energy level, parallel to the external magnetic field and a smaller number is on the higher energy level (anti-parallel)
MRI Physics Magnetic Resonance and Spin Echo Sequences video:
Where do we have protons?
In the fat, muscle, and sugar within our body, and within water
A significant portion of our body consists of water and a hydrogen atom is a
Proton (one positron and one electron)
Knocking the protons down due to RF pulse into another plane is a change in their
Longitudinal magnetization
Because of the RF pulse, the protons will precess together in phase. This brief synchronization is
Transverse magnetization
What is the thing being flipped called?
Net Magnetization Vector
When 63% of longitudinal recovered, it’s called
T1 Time
The time when 63% of the transverse magnetization has been lost, it’s called
T2 Time
Free Induction Decay only applies to…
More caveats
How to combat?
90 degree pulses only
The signal decays rapidly
T2* constant (protons dephasing) (never true uniformity in precession)
^called T2* effects -diffuse loss of signal or “black holes”
How to combat T2* effects and signal decay: another RF pulse (180 DEGREES)
HOW DOES MRI SCAN WORK video:
MRIs use protons (abundant in human body). All protons spin creating a small magnetic charge. When a strong magnetic field is introduced (as in MRI machine), protons align with that field. MRI technician then introduced radio frequency pulse that disrupts proton and forces it either into 90 degree or 180 degree realignment with static magnetic field. Since the RF pulse pushes the proton against its nature, once it’s off the proton realigns which releases electromagnetic energy along the way. Differentiates various tissues based on how quickly they release energy after the pulse is turned off.
T1 and T2 Times video:
Which is normally longer, T1 or T2?
T1 is normally longer than T2
(T1 is 2-10 times as long as T2)
T1 defined as
The time when about 63% of the original longitudinal magnetization is reached
T2 is defined as
The time when transversal magnetization decreased to 37% of the original value
1/T1 is what?
1/T2 is what?
Longitudinal relaxation rate
Transversal relaxation rate
Water/liquids have a ____ T1 and a ____ T2.
T1 is long, T2 is long (tissues with high water content can have long relaxation times)
Fat has a ____ T1 and a ____ T2.
T1 is short, T2 is short
T1 Relaxation Time video:
T1 depends on (3)…
Tissue composition, structure and surroundings
T1 relaxation has something to do with…
The exchange of thermal energy (handed over from protons to the surroundings- lattice)
According to the Larmor frequency, the precessing protons..
Have a magnetic field that constantly changes directions and fluctuates
The lattice also has…
What do the protons want to do?
Its own magnetic fields
Want to hand energy over to the lattice to relax
When does T2 relaxation come about?
This has 2 causes:
When protons get out of phase
Inhomogeneities of the external magnetic field, Inhomogeneities of the local magnetic fields within the tissues
If there are no big differences in magnetic field strength within a tissue…
The protons stay in step for a long time before dephase so T2 is longer
