MRI sequences and principles

Question 1

how is a signal in MRI received and used to form an image

Answer 1

• RF pulse causes processional protons to flip into the transverse plane
• this creates a net magnetic vector in the transverse plane
• the movement of this net magnetisation induces a current in the receiver coil
• this current forms the signal used to generate and image

Question 2

what 2 things does the angle of the net magnetisation of the flipped protons depend on

Answer 2

• time of RF pulse
• amplitude of RF pulse

Question 3

Explain what the free induction decay is / T2*

Answer 3

• once the flip angle has been applied, the RF pulse is stopped
• hence the flipped processional protons begin to get more and more out of phase and this gradually reduces the net magnetisation vector in the transverse plane
• this gradual decay of net magnetisation in the transverse plane is known as free induction decay

Question 4

know that different tissues have different FID/ T2* decays e.g bone is slow and fat is fast

Question 5

the loss of net magnetisation in the transverse plane is purely due to the loss of phase between the separate photons within the various tissues (FID)

Question 6

simultaneous to loss of net magnetisation in the transverse vector, there is longitudinal relaxation / T1 recovery.

The loss of net magnetisation in transverse plane (T2*) happens much fast than T1 recovery (as protons in the transverse plane get out of phase very quickly and eventually cancel each other out so there is 0 net magnetisation in the x,y axis)

Question 7

when all the vectors after having been flipped, return back to the main magnetisation, there will be maximum magnetisation in the z axis/ longitudinal axis

Question 8

know that FID/T2* and T1 recovery happen INDEPENDANT of each other ( e.g you cannot calculate T1 recovery from T2* decay)

different tissues have different T1/T2 rates.

Question 9

in which direction ONLY can signal be measure

Answer 9

only when its perpendicular to the main magnetisation field

Question 10

what is time of echo (TE)

Answer 10

the time taken from the moment RF pulse is given, and the signal is measured from the tissue

Question 11

As you wait a longer time, the more out of phase the precession of the protons of different tissues get hence this increases the contrast between the different tissues BUT it decreases the overall signal (so its a tradeoff)

(it is a tradeoff between getting good signal and getting good contrast between different tissues)

Question 12

what is the time of repetition (TR)

Answer 12

the time taken from the first RF pulse to the 2nd RF pulse.

Question 13

explain how short or long TR times affect the visuals in a t1 and t2 weighted image

Answer 13

(use fat and water to explain)

T1 WEIGHTED (short TR)=
-fat has a shorter TR time than water, hence after the protons have been flipped, fat is much faster to reach full longitudinal magnetisation recovery compared to water

• by the time fat has fully recovered to the longitudinal plane, water is halfway there BUT if you apply an RF pulse whilst this happens a larger signal from fat is made compared to water (as fat has been flipped a larger angle than water)
• because there is a larger signal from fat, it is brighter and water is darker as there is a smaller signal (forming t1 weighted image)

T2 WEIGHTED (long TR)=
- there is a long time of repetition which allows both fat and water to fully recover to the magnetisation in the longitudinal plane

• this means the only factor affecting signal size is the rate of dephasing of the different tissues (water has slower dephasing so signal lasts longer compared to fat ) (difference in FID/T2*)
• hence fat is seen darker and water is seen brighter on a T2 weighted image

Question 14

what 2 things does the strength of the percussion of the protons depend on

Answer 14

1. type of atom
2. strength of main magnetic field

Question 15

how do you calculate the strength of the percussion also known as the lamour frequency

what is the units of each factor

Answer 15

f = gydromatic ratio of atom x magnetic field strength.

e.g (42.5 (hydrogen) x 3T

frequency = MHZ
gyromatic ratio = MHz/T
field strength = T

Question 16

why is there no magnetisation in the transverse plane due to the protons

Answer 16

• despite all the protons having the same frequency ( 42.5 x field strength)
• they are all out of phase which means all net magnetisation is cancelled out in the x,y plane
• and u have a net magnetisation along the z axis

Question 17

you only get transverse magnetisation when the protons are in phase, when they are in phase, they are undergoing RESONANCE

Question 18

if a recivever coil is placed perpendicular to the main magnetic field, you can measure a signal.

what is the amplitude of the signal proportional to?

Answer 18

the net magnetisation of the in-phase precessing protons in the transverse plane

(know that the depending on the length of time that the RF pulse is given, it can reach up to an angle of 90 degrees (which is max amplitude))

Question 19

how does the lamour frequency vary due to the gradient applied to the main magnetic field

Answer 19

• all protons have the same gyromatic radio
• but application of gradient varies the strength of the main magnetic field going from left to right
• so using the equation, the lamour frequency would be increasing going to the right (where there is stronger magnetic field) and decreasing going left (where there is decreasing magnetic field strength)

Question 20

hence you can you the lamour frequency equation to calculate the require frequency of the RF pulse to flip the protons in the area/ slice you wish to visualise

Question 21

when you apply an RF pulse, it takes time for the now in-phase precessing photons to gain transcverse magnetisation.

if you apply an RF pulse that matches the processional frequency for a certiain period of time, it raises the net magnetisation vector by a certain value e.g 45 degrees and it provides a certain signal.

If you wait the same about of time but continually apply an RF pulse, it will add up to a 90 degree signal and you will have gained max transverse magnetisation .

Note the time taken for 45 degree flip is half the time take for 90 degree but the 45 degree flip produces the signal of 70% of the signal from 90 degree.

why would this be useful

Answer 21

there will be cases where you need a short flip angle because you cannot afford to wait long for the full net magnetisation of a 90 degree flip angle

Question 22

explain the mechanism for T2 relaxation / spin-spin relaxation / transverse decay

Answer 22

• as the spins (with different energy levels) in the transverse plane interact with one another, energy is transferred and they become out of phase with each other
• as they become out of phase, this causes the gradual loss of the net transverse magnetisation

Question 23

why does water have longer dephasing compared fat

Answer 23

• fat is made of triglycerides that are chained, this makes it much more likely for the interaction of particles to transfer energy and go out of phase
• water are single free moving molecules, they are less likely to interact to transfer energy and take longer to go out of phase

Question 24

hence compare the signal formed from fat vs water

Answer 24

water e.g CSF, has a greater average amplitude with a longer lasting FID

Fat decays much faster and decreases in amplitude quickly

Question 25

you can plot a FID curve for the different types of tissue e.g fat, CSF, muscle

Question 26

what is the difference between t2* and t2 decay

Answer 26

T2 = decay PURELY due to spin-spin interaction T2* = decay due to spin-spin interaction and taking into account magnetic field inhomogeneity

Question 27

specifically what is the T2* decay

Answer 27

the amount of time taken for only 37% of the transverse magnetisation of different tissues left is its T2* decay value time taken for different tissues to go from 100% to 37% net magnetisation in the transverse plane we can use the t2* value to get contrast in tissues

Question 28

in an ideal world, we would have T2 decay purely due to spin-spin interaction and we would have a homogenous magnetic field (same no matter where the protons are in the magnetic field)

Question 29

what are the 3 reasons for magnetic field inhomogeneity

Answer 29

1. MRI scanner cannot make equal magnetic field (coils will have different magnetic field strengths the further away u get) 2. can be substance in the patient e.g metal, cortical bone etc that causes disruption in the local magnetic field 3. when spin starts to rephrase with one another, the magnetisation vectors begin to get out of phase with one another and this can also affect local magnetic field

Question 30

why/how does inhomogeneity affect T2 decay (forming T2*decay)

Answer 30

- when protons experience different magnetic field streghts, they spin at different rates (larmour frequency) - different MFS will cause the dephasing to be increased because rates of change of processional values will be different between the 2 protons - causing T2* decay

Question 31

note that the T2* decay will always be less than the T2 decay

Question 32

how is T2* decay caused and how do you go about compensating for T2* decay / the increase in rate of transverse decay due to inhomogeneity

Answer 32

- once you apply 90 degree flip and stop the RF pulse, you begin to get T2 decay - ideally T2 decay is only due to spin-spin interactions BUT we actually get spin-spin interaction and local inhomonogeity causing T2* decay - spins (within the same voxel) become out of phase, (some faster than other) mainly due to spin-spin interaction but we know there are difference in magnetic field strength due to local inhomogeneity - whatever spin is experiencing higher MFS (due to inhomogeneity) will dephase faster and simultaneously they gain longitudinal magnetisation - to put these spins back in phase, you apply a 180 DEGREE RF pulse (same as 90 degree but twice the duration) - now the slow spin is the leading spin and the faster spin is behind - as you wait the same amount of time between the 90 and 180 degree pulse, the faster spin eventually catches up to the slower one (that got placed ahead) and they will become in phase - you now get an increase in the net magnetisation in the transverse plane as they become in phase and as you sample the signal at the point in which both spins become in phase again, you will get a signal at the same amplitude as the 'ideal' T2 decay

Question 33

a shorter TE time will provide a higher signal (as it is earlier along the T2 decay) a longer TE time will provide a lower signal (as it is towards the end of T2 decay)

Question 34

describe the difference in terms of signal and contrast depending on how long you TE is

Answer 34

if you have a very short time of echo, all the tissues will have very high signals BUT you will see very little contrast as there is not much dephasing If you have a moderate time of echo, the tissue signals will be decreased (CSF, Fat, Muscle) but you will see good contrast due to dephasing At a very long time of echo tissue signals will be very low and you will see close to no contrast between them.

Question 35

what causes the contrast seen in decay

Answer 35

contrast is due to the differences in T2 decay between different tissues

Question 36

why is T1 relaxation / longitudinal recover also known as spin-lattice relaxation

Answer 36

- spins interact with the lattice (structural components that dont have spin themselves but when spins interact with them, it causes them to gain longitudinal magnetisation/ align with the main magnetic field)

Question 37

why does fat have a faster longitudinal recovery than water

Answer 37

- water has less 'lattice' meaning spin-lattice interact is decreased and so the dephasing of the spins is much slower - fat has a lot of lattice = high spin-lattice interact increasing rate of dephasing and longitudinal recovery (lattice 'trips' the fat molecules into the longitudinal plane) in general fat also has increased spin-spin interaction due to its structure of long chains of triglycerides

Question 38

be aware that t1 recovery does not really contribute to the rate of t2 decay as t2 decay is mainly due to dephasing of spins. Though t1 recovery has slight effect, it is negligible in comparison

Question 39

as you begin to gain longitudinal magnetisation, it is likely that all transverse magnetisation is gone as all the spins have gone out of phase with each other, and so in that moment of time, the only net magnetisation is in the longitudinal plane.

Question 40

what allows the contrast in T1 weighted images to be seen

Answer 40

the differences in t1 recovery between different tissues

Question 41

what is the t1 time constant

Answer 41

time taken for 63% of longitudinal recovery for different tissues

Question 42

why is there no T1* but there is T2*

Answer 42

- we know that t2* is due to the increase in decay due to the local inhomonegeity as well as the spin-spin interaction - in t1, the inhomogeneity does cause changes as spins experiencing low MFS recover slower and spins experience high MFS recover faster - but if you average out the differences in the time taken for longitudinal recovery, it comes to the average T1 time (inhomonegeity affects spin phases (decay) more than recovery)

Question 43

why can u not measure longitudinal magnetisation

Answer 43

because its in the same plane to the main magnetic field

Question 44

you cannot measure longitudinal magnetisation, so how would you go about highlighting the differences in longitudinal magnetisation?

Answer 44

- differences in longitudinal magnetisation rates is what gives the T1 contrast differences within tissues - first apply 90 degree pulse to lose all longitudinal magnetisation - depending on TE, you can get high or low contrast of T2 decay (if high TE you give longer time for dephasing which increases T2 contrast) - then, wait a long time to allow spins to regain longitudinal magnetisation - at a specific time, you repeat the 90 degree RF pulse (time of repetition TR) - this brings about a transverse signal equal to the x axis value of the longitudinal magnetisation (e.g fat reaches full longitudinal magnetisation so experiences full 90 degree flip but CSF is 45 in x- axis so experiences 45 degree flip) - overall fat now has high signal and CSF has low signal

Question 45

so how does TR time affect ability to view contrast on T1 image

Answer 45

- short TR = high contrast as fat recovers fast and CSF recovers slow so you can see clearly the difference between the 2 - long TR = low contrast as time has been given for all tissues to recover in longitudinal plane, so differences are harder to see

Question 46

remember that T2 and t1 do not occur at the same rate. T2 decay due to dephasing is much faster (transverse net magnetisation is lost as soon all spin go out of phase) than T1 recovery due to gain in longitudinal magnetisation

Question 47

TE determines the contrast seen in T2 decay. TR determines the contrast seen in T1 recovery.

Question 48

how is a proton weighted image formed/ what forms it

Answer 48

- if there is long TR and short TE, there is going to be very little contrast - hence the only indicator of differences in contrast in based on the number of protons that are available for nuclear magnetic resonance - after you wait a long time for repetition, the net magnetisation in the transverse plane will be purely based on amount of hydrogen available to exhibit nuclear magnetic resonance - so the differences seen in transverse magnetisation, is due to the differences in proton number in - the different tissues - hence it forms an image with contrast purely based on proton numbers

Question 49

what does fat and water and bone and ,muscle look like on a proton weighted image and why

Answer 49

fat and water = bright muscle and bone = dark fat and water have a much higher hydrogen content

Question 50

in general, why does muscle have a low transverse magnetisation

Answer 50

doesnt have as much hydrogen (which is what gets flipped) to begin with