MRI fundamentals Flashcards
What is the most elementary chemical bases the technique draws on (in general)?
Protons within the atom’s nucleus spin around their axis -> movement of their charge creates electrical current -> forms a magnetic field (could be considered small magnets)
- they all have different directions of spin axis, prefer the direction that costs them less energy
What is precession? Can we measure it?
= movement of protons in which the top circle of the spin forms a cone shape
- can be measured as a procession frequency = how many times the protons precess per second
- speed depends on the strength of the magnetic field -> the stronger = the higher p.f.
What is the formula for precession frequency?
Larmor equation
w0 = precession frequency, Hz or MHz
B0 = strength of the external magnetic field, T
y = gyromagnetic ratio (differs for different material)
In the case of external magnetic field (direction Z) - what happens to the magnetic effects of the proton?
NOTE: pink arrow simbolizes the vector of magnetic field of a single proton at a specific snapshot of time
While aligned to external magnetic field some protons face one direction while some the oposite => cancelation of mag. effect
- there tends to be more protons facing one of these (very general) directions (e.g. facing up) BUT they some tilt to right, some to left => cancelation
- Only protons facing direction Z will sum up their magnetic influence => longitudinal magnetization (i.e. along/longitudinal to the external mag, field)
Look at longitudinal magnetization in a patient
We can use this new magnetic signal -> however how shall we measure it if the mag. force is in the same direction us our external mag. field
What change needs to be made in order to be able to measure the magnetic force?
We need to introduce magnetization transversal to the longitudinal external mag. field
- We use a radio frequency (RF) pulse = a short electromagnetic burst within the same frequency range as radio tends to receive
=> will disturb the aligned protons
What has to be taken into account when applying RF?
RF has to have the same speed/frequency as the protons it is trying to influence
- can be calculated via Larmor equation
-> if the same protons will be able to pick up some of the radio wave energy = resonance
So what happens to protons if we apply the correct RF-pulse?
Some protons will pick up the energy -> jump from lower to higher level -> turn to 90° or to 180°
=> the newly opposing protons will partially cancel out the remaining ones - magnetic force in the longitudinal axis decreases
=> precessing protons synch with the waveform of RF (they are “in phase”) -> now pointing in the transverse direction -> vectors of protons add up => Transversal magnetization
How does the transverse magnetization vector look like?
The vector doesn’t remain static, instead it moves with the precessing protons (comes towards antenna, way from it, towards, away) -> constantly changing magnetic vector induces electrical current in antenna which will have the precessing frequency (as the mag field moves close and away)
How can we know where in the body signal comes from?
We adjust the magnetic field to have different strenghts at different cross-sections of the patient -> precession frequency will be different for each -> also differentiating the MR signal
In general terms, what happens the moment we stop the RF signal?
Once RF gets switched off -> everything goes back to its original place
- Transversal relaxation
- Longitudinal relaxation
What is the longitudinal relaxation? How can we “describe” it?
= growth of the original longitudinal magnetization
- protons continuously go back to their original state -> to get to their lower energy state they give out what was received from RF to their surroundings = lettice (arrangement of atoms in 3D space)
- sometimes called spin-lettice-relaxation
-> no longer cancel out -> longitudinal magnetization grows until hits its original value
- We tend to use longitudinal relaxation time (T1) = to represent a CONSTANT of how long it takes for protons to get to their original place
What is the transversal relaxation?
After RF is switched off -> protons get out of phase
- since magnetic field applied on patient is NOT homogenous - protons will vary in their precessing frequency
-> since each proton can be influenced by the small mag. field of the neighbouring protons
- these internal variations are tissue specific
=> will soon be pushed out of phase again
SUMMARY: causes are inhomogeneities of external mag. field and inhomogeneities of the local tissue mag. field
How can we describe the transversal magnetization?
We may plot out the transversal relaxation time T2 which will show the time consant of how fast the transverse magnetization vanishes
- also called spin-spin relaxation
How long are the relaxation times?
It takes longer to achieve the original longitudinal magnetization than to loose transversal magnetization.
- So T1 takes longer than T2
- since end of magnetization is hard to determine - T1 = reaching 63% of the longitudinal magnetization, T2 = decreasing to 37% of the transeversal magnetization
NOTE:
- 1/T1 = longitudinal relaxation rate
- 1/T2 = transeversal relaxation rate
How could we use the previous concepts, to in theory, differentiate tissues?
In general, tissues with high water content tend to have longer T1 and T2 than more fatty tissues.
- NOTE: could be used in diagnostics considering diseased tissues tend to have higher water content
What in general influences T1?
In order to go back protons need to donate energy to the surrounding lattice
- magnetic field of protons fluctuates with Larmor frequency X lattice has its own magnetic field
=> if lattice mag. force fluctuates in the same frequency as Larmor frequency => energy transfer will be easy and fast
=> if different => will take longer
How is T1 influenced by water, medium-molecules/fat, magnetic field?
Water/liquid
- Due to fast movement of water molecules -> are on higher energy level than the protons at question -> will be harder to transfer energy to the water => longer T1
Fat
- medium-size molecules, and carbon bonds at the ends of the fatty acids have a magnetic field fluctuation near Larmor frequency -> easier to transfer energy => shorter T2
Magnetic field
- Stronger mag. field -> faster processing -> trouble handing out energy to lattice with slower fluctuations
How is T2 influenced by water as opposed to impure liquids containing larger molecules?
Water
- water molecules tend to move around fast -> mag. fields fluctuate a lot all over the place -> average themselves out -> no major net differences in the internal mag. field -> will take protons more time to get OUT of phase => longer T2
Larger molecules
- don’t move as much -> doesn’t cancel out the mag. fields -> bigger variations in mag. field -> larger differences in precession frequency => faster T2
What are we looking at in this picture?
Between longitudinal and transversal vector we find the sum vector => gives us the total magnetic moment of a tissue
- during relaxation goes back to longitudinal direction
- since th whole system is precessing the sum vector is actually completing a spiral motion
What do we mean by FID signal?
The MRI machine includes an “anthena”. As the sum vector approaches and goes away from the anthena it will generate an electrical current inside it
- further away = smaller signal, closer = bigger signal
=> generates the free induction decay (FID)
Why should we pay close attention to the specific pulse sequance we choose (i.e. how far away would the pulses be from one another)? What do we generate by applying TR shorter?
If we consider A and B where A represents a tissue with larger molecules (shorter Ts) and B being liquid-like tissue (longer Ts):
-> If we apply pulses with long duration between both tissues will have time to recover to its initial longitudinal magnetization and would generate the same transeversal magnetization after the next pulse
-> If we apply short “time to repeat” or TR tissue B won’t have enough time to recover completely -> once another pulse hits B will generate smaller transversal magnetization => this way we CAN differentiate the tissues
What is short and long TR? What do we attain from shorter TR?
TR short < 500 msec
TR long > 1500 msec
- By applying shorter TR we can vizualize differences in signal intensity between tissues, determined by the difference in T1 => produce T1-weighted image (tissue contrast is mainly due to differences in T1)
