MRI overview Flashcards
Imaging in MRI relies on the quantum properties of the hydrogen atom
what is SPIN in MRI
’ the intrinsic angular momentum of subatomic particles’
- spin of the singular proton in hydrogen atoms act as a bar magnet, creating a magnetic field around it
’ proton is the magnet and the spin is the property of the magnet (spinning) ‘
what is the orientation of the magnetic north in MRI described as?
probabilistically
combined magnetic field of hydrogen atoms in body cancels out
what happens when hydrogen atoms within body are inserted into a large external magnetic field
- distribution of ‘tiny magnets’ changed, augmenting the number of atoms aligned with the external magnetic field
how is an MRI signal created/what is the source of the MRI signal
- distribution of ‘ tiny magnets’ change (from 0/neutral) as hydrogen atoms from body enter large external magnetic field (MRI)
- augmentation of number of atoms aligned with external magnetic field
- imbalance is what generates the MRI signal
( tiny magnets get manipulated into make signals that can become images)
what is used to ‘nudge’ the protons that are aligned with the external magnetic field?
magnetic radio frequency pulse
what and how generates the magnetic radio frequency pulse
set of coils within machine
- alternation of current though coils at specific frequencies
how does the magnetic radio frequency affect the alignment of the protons
- nudge misaligns hydrogen bar magnets to shift their magnetic field properties perpendicular to large magnetic field made by machin
how do the hydrogen bar magnets / protons induce current
- after being misaligned by magnetic radio frequency, they naturally want to align their orientation back to their og position ( with external magnetic field)
- hence they decay in a spiralling motion which causes a changing magnetic field
- with nearby coils, this changing magnetic field induced a current within the coils and induce clear signals
( MRI machines can use the same coils to send the ‘ nudge pulses’ and read signals)
what is done for higher contrast and resolution in terms of coils in MRI?
some machines use separate coils to transmit pulses and receive signals
- receiver coils can be placed much closer to the body maximising the strength of the signal
how can you increase the number of hydrogen atoms in the body to align with external magnetic field?
increase strength of external magnetic field
how does current, strength of magnetic field and number of aligned hydrogen atoms relate?
- increase external magnetic field —> increases number of aligned hydrogen atoms —–> induced a larger current as it spirals back/decays
how does increase current, signal and image quality relate
increase current from decay —> increase signal strength —> increased/ improved image quality recieved
what is a superconductor
resistance drops to 0 at temperatures close to absolute zero / -273k
theoretically, how can superconductor material affect an electric current?
- an electric current could travel in a loop of superconducting material indefinitely without needing a power source
with use of a superconducting material, where would the main consumption of energy fall to
to keep the coiled cooled down so the current can travel endlessly
what is the most commonly superconducting material in MRI
Nb-Ti (niobium titanium)
what is used to cool down the coils in MRI
liquid helium
due to heat, liquid helium can evaporate, what is used nowadays to prevent the helium evaporating and escaping
vacuum - sealed chamber
’ zero-boil off ‘ machine
what type of cycle is used in a vacuum sealed chamber
- electric refrigerant cycle
- keeps helium in liquid phase and magnets cool enough to maintain them in superconducting state
what frequency do the protons in the hydrogen atoms rotate at in a 1.5/3.0 Tesla
1.5 T = 64 MHz
3.0 T = 128 MHz
how can a bar magnet/proton be ‘nudged’ by a magnetic radiofrequency
by a magnetic radio frequency at the same frequency of the proton in a 1.5/3 T so at 64/128 MHZ
how does field strength and frequency corrolate
stronger field = higher frequency
what is gradient coil and how is it used in MRI
- separate set of electromagnets that form gradient of magnetic field strength (y= field strength x= length along tube)
- allows advantage in imaging individual slices as using the gradient can allow you to selectively nudge atoms along gradient by applying corresponding frequency
what is T1 relaxation
- how quickly atoms re-align themselves with large magnetic field after a nudging pulse
what is T2 relaxation
- hydrogen atoms dont realign with magnetic field uniformly
- in tissue, hydrogen interacts with each other and surroundings
- after nudging pulse is sent, interaction causes spin to fall out of uniformity
- so T2 is the sum of the decaying signals
t1 and t2 ARE NOT EQUAL,and dependant on the tissue
you can use T1 and T2 properties to your advantage, how?
- to emphasise T1, send pulses rapidly and listen to signal immediatly (T2 doesnt have enough time to take place) (e.g fatty tissue)
- to emphasise T2, send pulses slowly and listen longer allowing dephasing to occur (e.g spinal fluid)
what do MRIs sample to form imaged instead of pixels
striped patterns
what is the physical interpretation of the striped patterns used to for MRI images
- slice seen as a grid of rotating hydrogens, rotated phases coloured by greyscale
- when in unison, greyscale is all white
- when out of phase 180, greyscale is all black
- different phases = different shades of grey
- alternate set of gradient coils precisely change phase of rotating hydrogen to create striped patterns in all directions etc
- by adding increasing patterns, image begins to form
