MRI fundamental function Flashcards
(53 cards)
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
