introduction to MRI Flashcards
basic hardware of MRI machine
- primary magnet
- gradient coils
- radiofrequency RF coils
what are the 3 magnet types
- permanent
- resistive
- superconductive
list features of a permanent magnet
- limited field strength
- 20-30 tonnes
- iron, cobalt, nickel used
- low power consumption
- they generate a magnetic field without an external power source
list features of a resistive magnet
- require stable supply to create B0
- water cooling required
- field strength = 0.3T
- uses electric current flowing through coil of wire to generate magnetic field
what does current produce in resistive magnet
- produces heat through resistive turns
can strength of resistive magnet be managed
yes, can be adjusted by changing the current
list features of superconductive magnets
- use a coil that when cooled to low temperatures, loses all electrical resistance
- allows persistent strong magnetic field to be maintained with minimal energy loss
- can generate strong and stable magnetic fields = ideal for high field applications
- always ON
materials of superconducting magnet
niobium/ titanium alloy
what are gradient coils used for
used to spatially encode MR signals produced by photons in the body, allowing creation of 2D and 3D images
gradient coils role
- used to introduce small, controlled magnetic field gradients in addition to main magnetic field
- provide slops within field, 3 orthogonal X Y Z
- noisy
what are radiofrequency RF coils
responsible for sending RF pulses into patient’s body during MRI scans
- these pulses are used to excite protons in body causing them to emit MR signals
what is passive shielding
involves the use of materials and physical barriers to block/ absorb electromagnetic interference without requiring external power source
what is active shielding
employs active electronic components such as amplifiers and feedback control systems to actively counteract and cancel out unwanted electromagnetic interference
what are faraday cages designed to do
designed to block electromagnetic fields
- conducted from electrically conductive materials such as copper, aluminium/ steel
basic principles of MRI
- hydrogen nuclei protons are MR active
- behave like random mini bar magnets (N + S pole)
- protons aligned in same direction as magnetic field
- creates net magnetisation in direction of magnetic field
what is spin?
refers to the intrinsic property of certain atomic nuclei, particularly hydrogen nuclei, that makes them suitable for detection in MRI
what is the principle element that detects signals
protons
- tissues with more water will give higher signal
two possible orientations of spin
- parallel
- anti-parallel
what is the energy difference between parallel and anti-parallel
directly proportional to the strength of B0 (magnetic susceptibility)
how can an MRI scanner be made - superconducting?
- selection of superconducting material
- formation of superconducting wire coils
- cooling system
- integration into magnet structure
3 advantages of the superconducting MRI scanner
- higher magnetic field strength
- stability and persistent current mode
- energy efficiency
3 disadvantages of superconducting MRI scanners
- high initial cost
- high maintenance
- long ramp-up time to reach optional magnetic field strength
what are RF pulses used to do
manipulate the magnetic properties of (hydrogen) nuclei within body
what is B0
the main magnetic field
what happens before RF pulse applied
protons within the body are randomly orientated with respect to main magnetic field (B0)
- this is the resting state
RF pulse application
- RF pulse is short burst of radiofrequency energy applied perpendicular to B0
- frequency of RF pulse matches resonant frequency of protons
what is resonance and tipping
when RF pulse matches frequency of protons it causes them to absorb energy and enter higher energy state (known as resonance)
- RF pulse tips magnetic moments of protons away from direction of main magnetic field
flip angle
the angle at which the magnetic field is tipped is called the flip angle
- the flip angle is a critical parameter that determines amount of magnetization tipped into transverse plane
what happens when the RF pulse is turned off
the tipped magnetic moments begin to rotate back towards B0
- while they are precessing they emit radiofrequency signals known as free induction decay (FID)
what does free induction decay contain
these signals contain information about local tissue characteristics
signal detection
these emitted signals are detected by the coils in MRI system and are used to create detailed images of internal structure
what are relaxation times
it describes how long the magnetisation takes to get back to equilibrium after an RF pulse
what are the two types of relaxations
- T1 recovery
- T2 relation
what is T1 recovery
- refers to recovery process in MRI that is associated in T1W images
- process where energy absorbed by the excited protons/ spins are released back into surrounding lattice re-establishing thermal equilibrium
what type of magnetisation does T1 recovery refer to
- longitudinal magnetisation is returned to its equilibrium
what is T2 relaxation also known as
spin-spin relaxation
what is T2 relaxation
- whenever spin comes close to each other, an interaction takes place which affects their individual magnetic moment and relative phase angle (phase scrambling)
- with time phase scrambling increases until it reaches a point where spins run out of phase thus reaching ZERO magnetisation
what does ‘phase’ refer to
represents position/ location of signal in space
what does ‘magnitude’ refer to
represents the amplitude/ strength of signal
what is T2 star
when T2 relaxation time is influenced by both magnetic field inhomogeneities and other effects (like susceptibility effects) the resulting decay is referred to as T2 star
what is T2 star sensitive to
- certain substances such as deoxy blood, iron, air
- substances that can cause magnetic field variations
