MRI lecture notes - Summaries on how MRI works and the physics behind it PDF

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Summaries on how MRI works and the physics behind it...

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Lecture 1 MR Active Nuclei =Hydrogen

Nucleus Mass number is odd (number of protons and neutrons that exist in the atom) = a charged particle e.g. 1 proton, 2 proton & 1 neutron, 3 proton & 2 neutrons - Protons are targeted – (tiny magnets) are attracted to the main magnetic field when entered into the MRI environment – causing them to align with it

Magnetic moment

- Hydrogen Protons= most commonly used/most abundant proton in the human body (83% water = H2O) - Mass Number = 1 (1 proton) Spinning MR active nuclei – aligning their axis of rotation to the applied magnetic field (B0) Described as linear.

B0

Main Magnetic field - Always on (superconductor) - Influences low energy protons to align with magnetic field - Unit = Tesla / Gauss (1 T = 10,000 G)

No Magnetic field = randomly orientated magnetic moments

Strong magnetic field (B0) = magnetic moments align parallel (spin up) or antiparallel (spin down) to B0 = 2 energy states

Tesla (T)

** if the proton has a higher energy than B0 It will align against the main magnetic field – Antiparallel magnetic moment T = the strength of B0 1 tesla = 2 million proton (roughly) 6 more protons will be aligned with the field (spin up) than against (spin down) when exposed to B0 Increase Tesla = more protons align with the field – a greater range of proton energies can be influenced

Stronger the magnetic field the more protons are pointing in the direction of the field.

Net Magnetisation Vector (NMV)

 Clinical scanners = 3T  Research = 7T  Earth magnetic field = 0.5G / 0.00005T  Refrigerator magnet = 10G/ 0.0001T The Direction that the protons move to when aligned to the main magnetic field Net sum of protons pointing in the direction of the magnetic field Spin up – Spin down = NMV

Increase B0 = Increase NMV – most commonly used

B1

Advantages/ disadvantages

Energy States (2)

Decrease Temp (thermal energy of the nuclei)= Increase NMV --- Not typically used as you patient temp limits this Radiofrequency (RF) magnetic field - Turned on and off = acquires the image - Frequency is matched (resonated) with a particular proton - Creates an activation of the protons (more protons activated = a brighter signal) - Moves the NMV out of alignment with the main magnetic field  high equipment cost & set up  high contrast sensitivity (increasing B0= Increasing fringe to soft tissue field = Greater shielding required) differences (can tell  Scan acquisition complexity (lots apart tissues with very of variables that can be changed) similar densities)  Long scan times – 20 – 40 minuets  Safer than other imaging modalities  Significant image artifacts (non-ionizing radiation)  Claustrophobia –60cm bore = a lot smaller than CT  Biomedical implant magnetisation – rotation or heating Spin Down Nuclei Spin Up nuclei - High energy Nuclei - Low energy Nuclei - Align their magnetic moments - Always more spin up than spin down antiparallel to the external field ----ONLY when in the - Create the image main magnetic field. **Increase B0 = Increase Spin Down = (the RF pulse will Increase SNR (Greater protons there to

produce a more detailed image) change that) - Align magnetic Moments parallel to the external fields - large B0 filed - Will give a higher signal to noise ration – more signal = a better image - Increase B0 = Decrease Spin up Component of the NMV parallel to B0 = Longitudinal Magnetization Longitudinal NMV will be parallel to the longitudinal axis when a patient is in a Axis MZ magnetic field but there are no RF frequencies present Transverse axis Component of NMV 90o (perpendicular) to B0 = Transverse Magnetization MXY - When the RF fields are activated - NMV moves from the longitudinal to the transverse field - giving energy to the protons – moving from the spin up to the spin down Precession The spin/ rotation of a hydrogen nucleus on its axis. A secondary spin to B0

Precession Frequency

The frequency = how many times (how fast) the nucleus rotates per second around B0 Unit = Megahertz (MHz) 1 MHz = 1 million cycles/ 360 rotations per second

= Lamour Equation – calculates how fast the nucleus Increase B0 = Increase Lamour Frequency precesses Increase the filed strength = increase rotation – increase Lamour frequency

Gyro-magnetic ratio (y)

** The Lamour Frequency must be matched to the FR pulse Frequency == this causes Resonance = the absorption of energy Constant – for each nuclei Unit = MHz/T Hydrogen y = 42.58MHZ/T 1T = 42.58 MHz 2T = 85.16 MHz (42.48 x 2) 3T = 127.74 MHz (42.58 x 3) Ect……..

Resonance

Excitation

RF influence

** at 1 tesla, the hydrogen proton will rotate at 42.58 million times per second Increase magnetic field strength = increase the gyromagnetic ration – rotate faster Need to match the RF magnetic field to the same frequency at which a particular body part will absorb energy (resonance) – to get a maximum amount of signal -- if you put a 127.74 frequency into the body – only hydrogen protons will absorb the energy and move out of alignment as no other atom has the same gyro-magnetic ratio = Active selection of the hydrogen protons the absorption or emission of energy only occurs at matching frequencies RF frequency much match with Lamour frequency / The application of RF pulse causes resonance absorption of energy = the nucleus has gained energy, so it becomes excited = Increases Spin down Nuclei add RF pulse = NMV move out of alignment with B0 Simultaneous actions when RF is turned ON: Absorption Flipping The excited protons RF energy is move absorbed by the Net magnetisation protons when the away from the main frequencies are magnetic field – matched moving away from the longitudinal axis to the transverse plane

Simutneous actions when RF is truned OFF: Re-transmittion Realign Spin back to their Release of energy original orientation – from the protons at realign with the the resonance main/external frequency magnetic field (B0)

Flip Angle

Spin in phase At any specific time, they are pointing int eh same direction - individual magnetic moments are in the same plane on the processional path

dephase Move away from the transverse plane back into the longitudinal plane = a loss of signal - need to acquire the signal before this hapens The angle at which the NMV moves out of alignment with B0

- Dependant on amplitude & duration of the RF pulse - Controlled by the pulse sequence – MRI radiographer - The amount of magnetisation in the transverse plane is responsible for the MRI signal 180 degrees 90 degrees - all of the NMV in the longitudinal - same number of spin plane up and spin down protons - No signal - Pointing against the main magnetic - no NMV in longitudinal plane – all field (180o) in transverse plane - when the RF frequency is left on for too long – many many protons - NMV rotates at the have been given a large amount of Lamour frequency energy – much much more spin - Maximum possible down protons signal can be acquired when NMV is in the transverse plane and there is phase coherence

MRI signal

Horizontal component

Relaxation Process

Phase coherence + NMV rotating at Larmor frequency = the receiver coil receives the signal If the NMV is not processing entirely within the transverse plane = the signal magnitude is reduced If the NMV is processing in the longitudinal plane (0 or 180 degrees) there will be no signal in the coil 0 horizontal component = 100% in y axis = no signal You want to push the NMV into the transverse plane

- Occurs when the RF switches off - return to original state NMV returns to the longitudinal plane = an increase in the amount of the spin up protons.

T1 water = T2 water = 2500ms - Used as a reference point *** T1 (longitudinal) and T2 (transverse) occur at the same time *** T1 is always bigger/faster than T2 (except water) *** Water has the longest decay/recovery rate - emission of energy T1 Recovery = movement of the net magnetisation vector back to the longitudinal plane - Cut off time = when 63% of the longitudinal magnetisation has been recovered – (otherwise it would take too long to fully recover)

*** 63% of the signal is recovered – the recovery times are recorded  Small molecules = Long T1 E.g. water  Large molecules = Short T1 E.g. proteins, fats Different recovery rates form the T1 weighted image T1 weighted images ==== SHORT TR – so that neither fat nor water has time to return to B0  if the TR is too long the contrast is produced by the difference in spin density  TR controls the amount of T1 weighting  contrast dependant on differences in the T1 times of fat and water Fat recovers quicker than water (the time it takes for the NVM to move back to the longitudinal axis) - T1 fat = 220ms - T1 water = 2500ms There is more longitudinal magnetisation (recovery to B0) in fat than water before the 2nd RF pulse After the 2nd RF pulse (-- after saturation) The fat and water are saturated to different angles  Fat = high signal = bright  Water = Low signal = Dark

T2 decay (transverse relaxation) = Dephasing of the protons (decrease in the amount of magnetisation) - Time it takes for 63% of the transverse magnetization to be lost == TIME CONSTANT - *** 37% of the signal is recovered - FID – free induction decay (signal) is produced through the release of energy as the protons revert to their original states T2 weighted images (contrast) ===== LONG TE – so that fat decays more and allows the water to start to decay – putting off a brighter signal - differences in dephasing decay between tissues - controls the amount of de-phasing occurs before the signal is acquired - T2 fat = 90ms - T2 water = 2500ms

Magnetic Field Inhomogeneity

 Fat = low signal = dark  Water = high signal = bright Protons in different locations process at different frequencies due to their spins being exposed to slightly different magnetic field strength = different spin dephasing Slower magnetic field (B0 -ab) = slower procession /rotation Faster magnetic field (B0 +ab) = faster procession/rotation

T2*

MRI Pulse

Inhomogeneities = T2* decay Occurs in the presence of = T2 decay + Dephasing due to the external B0 magnetic inhomogeneities - Always faster than T2 – faster dephasing occurring - change in the magnetic field strength / processional frequency within a scan protons in a higher magnetic field will spin/precess faster == shim coils are used to try and create uniformity – within 10 parts per million Spin Echo: used to maximise the signal

Sequences Repetition Time (TR)

Gradient Echo: shortens the scan time Time from one repetition to the next = Unit (ms) - Determines amount or T1 relaxation allowed to occur at the end of one RF pulse sequence Increase TR = the more time for the net magnetization vector to recover (T1 recovery)

Time to Echo (TE)

The time from the application of the RF pulse to the peak of the signal induced in the coil Unit (ms) - Determines the amount of T2 Decay that has occurred - determined how much decay is allowed before the image Is read  Long phase = both water and fat will dephase = a small signal  Short phase = water will still be dephasing (bright signal) and fat will have fully de-phased (weak signal)

Lecture 2 Image contrast

Proton/ spin density (PD)

High signal Low signal - Large Transverse - small transverse component – component – most of the Most of the NVM is in the NMV is still in the longitudinal component (closer transverse plane (sending to 0o) - Dark out a signal) - Bright Number of protons per unit of volume – how many protons are there in that section? Contrast = dependant on the difference in proton densities between tissues & the arear being examined

Saturation

 Tissues with high proton density = bright E.g. Brain  Tissues with low proton density = dark E.g. Cortical Bone Long TR = counteracts T1 weighting Short TE = counteracts T2 weighting When the NMV is pushed past 90 degrees - partial saturation – T1 - complete saturation = 180 degrees

T1 = dark fluid T2 = bright fluid Fat will recover quicker that water Short TR = T1 = Fat projects most of the signal – fat has recovered more than water TR – 400 -500ms Long TE = T2 = Water projects most of the signal- water is allowed to start to decay while fat has almost fully decayed TE - 80 – 120ms

Instrumentation Process to produce an image 1. nuclear alignment (main magnet – aligning the magnetic moment) 2. Radiofrequency excitation (RF coils) 3. Spatial encoding – gradient coils (localisation of the signal to a positioning the body) = in the X,Y & Z planes 4. Image formation – Using FID signals

Hardware

Patient Table

** the patient is not moved during the scan 1. A magnet 2. RF coil 3. Gradient coil 4. Image processor 5. computer system

Criteria: - support the patient - Allow the patient to be moved into the bore

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Main Magnet

comfortable attachment locations for coils immobilisation devices accept up to 130kg capable of moving to the imaging position within 1mm (create a high image accuracy)

Safety: - mechanism to evacuate the patient rapidly – couch can be undocked from the magnet - Couches constructed of non-magnetic material Generates the static magnetic field Properties:  Magnetic field strength (B0) - High field strength = increase SNR (signal to noise ratio) - however this increases costs, bioeffects and the fringe field - Between 1.5T – 3T  Homogeneity (uniformity of field) – to reduce artefacts - = less that 10ppm (parts per million) – Achieved using shimming process - Greater that 10 ppm = inhomogeneity = artefacts  Temporal stability (uniformity over time) - constant field strength over time

Magnets

 Bore size - 1m bore size - add gradient and RF coils – reduces the size to 60cm Permanent: Field strength = 0.25 – 0.5T = smaller RF signal & longer scan times with low SNR Ferromagnetic substances used – alloy, nickel, cobalt (alnico) Advantages: - no power supply required - required little maintenance - large bore Resistive: Field strength = 0.1T – 3T Current is passed through a solenoid to produce a magnetic field – produces a significant amount of heat  increase loops = increase field strength  Increase current flow = increase field strength  Decrease radius of loops = increase field

strength – require a small bore advantages: - Considerably uniform - lighter weight than permanent - low capital cost - can turn off the magnetic field Superconducting: Field strength = 1.5T – 7T Allows current flow without high temperatures  NO electronic power is required if kept at liquid helium temperatures  Magnetic field is Never turned off (except in emergencies) – as this will heat up the liquid helium = quenching Advantages: - High field strength = high resolution studies - high field uniformity – 1ppm in a 40cm3 volume Disadvantages: - High initial capital and siting costs - cryogen costs (liquid helium needing to be replaced) - difficult to turn off in an emergency - extensive fringe field Quenching Loss of superconductivity and resistance due to an increase in temperature - if uncontrolled – will cause explosive boiling of the liquid helium Radiofrequency RF pulse transmitted at: coils - the resonant frequency for resonance to occur - perpendicular to the main field Receiver coils: - receive the RF emitted (FID signal) - receiver loops in transverse plane to get the signal volume coil: - transmits and received the MR signal (transceiver) - pretty much the entire MR machine - coil size chosen to match anatomy = maximises SNR - can isolate body parts e.g. head imaging, extremity imaging – or do a whole body image Surface coils: - placed close to the anatomy being imaged - Improves SNR = increase spatial resolution Phased Array coils: - Multiple coils and receives combined to create improved SNR + coverage - not commonly used clinically

Shim coils: - Used to correct for field inhomogeneities = shimming - additional solenoids outside the main magnet Passive shimming:  when the magnet is shimmed with the metal Active shimming:  shimming performed with loops of current carrying wire  extra loop of wire = shim coil  used more than passive Gradient coils:

Produce a linear slope of magnetic field strength to define the spatial coordinates of a sample Adjusted using varying distances between the loops Loops closer together at one end = stronger and gradually move apart – weakening the Magnetic field strength

Electronic indexing MRI computer

- Gradient is altered by changing the current = producing different Lamour frequencies at different sections of the body = localisation of individual slices - switched on (expansion) and off (contraction) rapidly - causes the ‘thumping’ noise Y-coil Z-coil X-coil Varying magnetic field: Varying Varying magnetic magnetic field: Front to back Left to right field: Head to toe Indexing of slice locations means that the patient is not needed to be moved during the scan Controls all aspects of imaging process: 1. enter patient demographics 2. timing of RF excitation (TR & TE) 3. Gradient application

4. Data acquisition 5. Image reconstruction --- Fourier transformation on FID 6. image display Requirements: - High capacity - fast & handle high rate of data acquisition - Transmission, storage and display of images

Lecture 3 MRI essentials

1. Signal as high as possible 2. Contrast as high as possible 3. Examination time as short as possible Manipulative parameters = - TE - TR - Flip angle Spin echo pulse *** MOST COMMON ----- Maximum signal sequence Protons must be processing in phase when the signal is collected - Minimises the T2* effects 1. 90-degree pulse is applied - NVM moves into transverse plane - Protons spin in phase 2. 90-degree pulse is removed - FID signal – dephasing of protons - T2 decay - Tissues with different T2 relaxation times dephase at different rates - Fast = the leading edge - Slow = trailing edge 3. 180-degree pulse is applied - when the spins are in phase (the leading and the trailing edge become superimposed over one another) the signal is acquired

Gradient echo pulse sequence

90 degree = moving away from each other 180 = flip over so now they are rephasing towards each other = they will come together to go become in phase *** LESS COMMON ------ Shorter exam time Variable excitation pulse / flip angle

Reduce flip angle = shorter exam time Reduce flip angle = less spin echo = weaker signal 1. RF excitation pulse applied - transverse component created 2. RF pulse is removed - FID signal and T2 dephasing

Gradient field

TR / TE

TAU

3. Gradient field is applied - rephases the magnetic moments - a signal is collected from the coils An increasing or decreasing range of frequencies along a plane to create a gradient through the patient - used for slice selection - used in X, Y and Z axis E.g. the head has a high frequency and the feet have a low frequency causing a slope

TR = Time between 2 difference sequences (time between 2 different 90-degree excitation pulses)

TE = Time between the initialisation of the sequence and the acquisition of the signal (i.e. spin echo)

 Controls T2 weighting (dephasing)  Controls T1 weighting (mvt. of  Long TE = Maximises T2 NVM) weighting – allows water to decay  Short TR = maximising T1 and for a signal to be produced off weighting – Fat recovers faster it than water – if left too long the  Short TE = Maximising PD fat will return to the B0.  Long TR = Maximising PD - The time It takes for the NMV to dephase after the 90-degree excitation is removed Or ...