MRI fundamental function Flashcards
longitudinal axis = z axis (patient head to toe)
transverse plane = y x (cross section of patient)
why is hydrogen targeted in MRI and what characteristic of it makes it suitable for mri
hydrogen is abundant in body
- has non zero spin which allows it to act as mini bar magnets due to their north and south pole creating magnetic moment
what is the precession frequency of the hydrogen atoms directly proportional to (besides the type of atom it is)
the strength of the magnetic field in the machine
what is B0?
main magnetic field
- main magnetisation applied to all hydrogen atoms
what is M0?
NET magnetisation (cancelling of the parallel and anti parallel direction of hydrogen spin creating a NET direction of magnetisation following the original main magnetic field b0)
why cant net magnetisation be measure along the longitudinal direction/axis? which axis do u do it in instead
because the main magnetic field runs along the longitudinal axis and will interfere with measurements of net magnetisation in the same direction, you have to move the magnetisation perpendicular to the longitudinal axis in order to measure a net magnetisation
YOU CAN ONLY MEASURE NET MAGNETISATION PERPENDICULAR TO THE MAIN MAGNETIC FIELD (IN THE TRANSVERSE PLANE)
what is required for a radio frequency pulse to flip protons into the perpendicular plane and what 2 things happen when rf pulse is applied
protons must have the same frequency as the rf being fired.
- protons with correct frequency flip (amount is known as flip angle) to yx axis and those flipped being spinning IN PHASE
give a rough description of how the receiver coil gets a signal to help form the image
according to faradays law, movement of a magnet can induce a current thus,
- movement of the net magnetisation section (in the perpendicular plane/flipped angle), induces current within receiver coil that can be used to form an image
describe how the stopping of RF pulse causes free induction decay (T2*)
when RF pulse is paused, protons begin going OUT OF PHASE, the more out of phase they become, the less the net magnetisation in the transverse plane thus signal becomes less and less (each tissue in body has different T2* curve)
compare speed of FID/T2* in fat and water
T2* for water = slow
for fat = fast
difference in loss of transverse magnetisations what allows contrast difference of tissue to be seen
as free induction decay/t2* is happening there is a simultaneous gain in longitudinal magnetisation (t1 recovery).
loss of transverse magnetisation is due to protons going out of phase NOT because the flip angle is decreasing.
free induction decay/t2* happens much QUICKER than regain of longitudinal magnetisation
T1 recovery and t2* decay are completely INDEPENDANT of one another, they both cannot be measured simultaneously
What is TE and how how does the aftermath of this allow identification of different tissues
(time echo) time between RF pulse applied and the signal from transverse net magnetisation recieved
- after the RF pulse stops, the tissues spin increasingly out of phase, the difference in the speed of loss in transverse magnetisation/gain in longitudinal magnetisation of the different tissues allows identification but signal in the transverse plane is simultaneously being lost
what is TR and how can this help enhance signal for a certain tissue
time of repetition, time between 1st RF pulse and 2nd RF pulse
- fat loses transverse magnetisation/gains longitudinal magnetisation faster than water (t2*)
- when another RF pulse is applied, the size of the net magnetisation of the tissue is equal to the amount of longitudinal recovery that has occurred
- therefore if fat decays faster/gains longitudinal magnetisation faster than water, if there is a short TR and the 2nd RF pulse is applied, then there will be a larger signal coming from fat
therefore, time of TR is important if you need to identify certain tissues as if you have short TR you can receive larger net magnetisation in transverse plane from fat
(this measures the tissues T1 differences not T2 as you are utilising the t1 recovery and applying a short TR to get a larger signal from fat)
explain why fat or water is bright/dark in t1/t2 images
T1 = bright fat/dark water
- faster longitudinal recovery of fat compared to water (t1 recovery)(utilises time of echo)
T2 = dark fat/bright water
- 2nd RF pulse (TR) applied after both fat and water reached maximum longitudinal recovery but after 2nd rf pulse, the water decays slower in transverse plane than fat so water signal is larger.
what 2 factors of the main coil affects B0 (main magnetic field strength)
- number of coils
- strenght of current running through coikl
what property must the main coil have to withstand high currents and thus high resistance
superconductivity
what is used to maintain low temps for superconductivity (-4 degrees kelvin)
liquid helium
what is quenching in mri
temperature for superconudctivity not reached therefore resistance causes increase in temperature and converts liquid helium into gas that should be released (safety mechanism)
what do radiographers do if the mri machine is not homogeneous?
shimming
what are the 2 types of shims found in the machine
passive shim - ferromagnetic metal wedge placed in bore to manipulate magnetic field
active shim - has its own electric supply in which current gets altered to adjust homogeneity of bore
explain how gradient coils form a gradient that changes the STRENGTH of the magnetic field (not direction)
- 2 gradient coils on opposite ends of z axis
- one forms magnetic field opposite to b0, one forms magnetic field along b0
- field opposite b0 decreases b0 and field along increases b0
- together it forms a gradient of varing magnetic field strengths along b0/z axis
stronger the magnetic field the faster the precessional frequency of proton
what is the isocentre
part of magnetic field strenght that has the same value as the original b0 (unaffected by either end of gradient coils)
RF coil produces magnetic field perpendicular to main magnetic field.
how does the rf pulse affect the protons with matching frequencies (with gradient applied)
- range of precessions due to gradient coils
- protons with precession that matched the applied RF pulse fan out more and more (flip angle) and spin IN PHASE
- the longer the RF pulse applied, the larger the flip angle
classic vs quantum mechanical model of spin
spin angular momentum
gyromagnetic ratio corrolates the magnitude of magnetic moment and spin of proton
what is the gyromagnetic ration of a proton/hydrogen proton
42.5 megahtz PER TESLA
how do you calculate the larmor frequency
gyromagnetic ratio (42.5) x B0 (Whatever Tesla strength the machine is)
time taken to form 40 degree flip angle from RF pulse is half the time taken to form 90 degree flip angle yet the 40 degree flip angle manages to produces 70% signal of the 90 degree one
(this comes in use for sequences that require quicker speed and utilise the smaller flip angles)
explain spin-spin relaxation (mechanism) that causes t2 relaxtion/transverse decay thus use this to explain why fat decays faster than water
- precessing spins of the many protons bump into each other and become out of phase
- fat molecular makeup has many atoms surrounding therefore chances of protons colliding are increased and it gets out of phase faster (increased spin interaction)
- water molecules are spaced out, less chance of bumping, decays slower (decreased spin interaction)
whats the difference between t2 and t2* decay, which one is faster at decaying?
- t2* = time taken to reach 37%/ lose 63% magnetisation in transverse plane
- t2 = time taken to lose 63% transverse magnetisation/reach 37% purely due to spin spin interaction (not due to inhomogeneity)
- t2* transverse magnetisation is faster than T2 because it accounts for inhomogeneity as well which affect the dephasing time taken
what are 2 reasons for mri field inhomogeneity
- gradient coils form different strength magnetic field the further away they are from the coil
- substance in patient e.g metal causing disruption in magnetic field
- dephasing of the spins can also affect the local magnetic field
note that due to inhomogeneity, the dephase of spins can be affected as they will process differently according to the magnetic field. This is accounted for/effect t2* decay
how do you compensate for t2* decay
- apply 90 degree rf pulse
- different tissue begin decay faster than each other due to inhomogeneity and atomical nature
- 180 rf pulse applied (x2 of the initial 90) flipping the precessions so the slower spin is now leading and the faster one is now trailing, eventually the faster spin catches up with the slower one getting in phase and producing a transverse magnetisation/signal that would be equal to that of a regular T2 decay
- note the TE is received after the same time it took to apply the 180 rf after the initial 90 rf
how does the time echo affect signal received
longer the TE, the smaller the signal
knowing that csf transverse magnetisation decays slower than fat, how can you use this to identify the different structures/ how can you adjust the TE to produce contrast b/w the 2
- the longer the TE, the more the signal decreases due to dephasing
- fat dephases faster than water
- if you retrieve the signal after a longer TE, signal of fat should be low compared to water (as csf takes much longer to dephase)
- there fore with longer TE there will still be a higher signal (bright) from csf compared to fat
- but if you wait too long for TE, it will be hard to identify contrast between any tissue as they will all have low contrast
t1 spin-lattice recovery, what is it and compare the recovery b/w fat and water
- dephasing of spins and recovery into longitudinal magnetisation
- lattice structure of molecule can increase dephasing of spins from transverse magnetisation
- fat has increase surrounding lattice/molecules compared to water so it dephases faster and returns to longitudinal magnetisation quicker than water does
difference between time constant in t1 and t2
t2= time taken to lose 63% transverse magnetisation
t1= time taken to gain 63% longitudinal magnetisation
t1* doesnt occur as the inhomogeneity affecting the dephasing of spins into longitudinal magnetisation are balanced out as some due to it will dephrase faster and some slower
explain how utilisation of time of repetition allows contrast b/w tissue found in T1 recovery
- fat decays faster than csf/ fat gains longitudinal magnetisation faster than csf
- the amount of longitudinal magnetisation is equal to the amount of transverse magnetisation when TR is applied
- if TR is short, fat has increased signal / greater contrast found b/w tissue as fat gained more longitudinal magnetisation than csf so when TR is applied, larger signal is found from fat
- if TR is done after long time, all tissue will have high signal as they all will have regained longitudinal magnetisation
what 5 things determines size of signal?
- proton density (amount of tissue
- relaxation time (properties of tissue)
- blood flow (motion)
- type of pulse sequence e.g GE, SPE
- pulse sequence parameter e.g TR,TE
- NOTE THAT T2 decay occurs very quickly compared to t1 recovery
- to highlight t2 decay contrast you need to adjust TE (short but not too short) (water is bright)
- to highlight t1 recovery contrast you need to adjust TR (short but not too short) (fat is bright)
what type of image would a long TR and fairly long TE create? what tissues would be bright
- csf is bright (due to long TE (csf decays slow) and long TR (regain of T1 done))
- fat is bright due to long TR (gains longitudinal recovery fastest and less signal than csf with long TE but still significant)
- not very visible muscle (mainly due to lack of protons in comparison to csf and fat not due to how tr and te effects it)
why might csf appear brighter than fat despite long TE and TR?
csf has more protons than fat
how is a proton density image formed
- you wait for all tissues to reach max regain in longitudinal magnetisation
- when you now apply TR/another pulse, the difference in signal recieved in transverse plane will pure be based on the number of protons available in the tissue to exhibit nuclear magnetic resoannace
- TE is collected as soon as it flips to avoid t2 decay from happening
what does a proton density image highlight
fat and water (as they both have highest protons available)
- muscle is comparatively darker as it starts with a much lower longitudinal vector so when its flipped into transverse, not much signal is recieved
LONG TR AND LONG TE = T2 IMAGE
SHORT TR AND SHORT TE = T1 IMAGE
LONG TR AND SHORT TE = PROTON DENSITY IMAGE
what is the range for long/short TR and TE times
2000 ms+ is a long TR
anything in hundreds is a short TR
10-30ms is short/normal TE
80-160ms+ is long TE
