MRI

FSE/TSE

(fast spin echo/turbo spin echo) • Multiple 180 pulses and echoes also follow each 90 pulse • Multiple phase encoding lines acquired during each TR interval, significantly reduces imaging time • Echoes no longer diminishing in size as different phase encoding gradients are applied with each pulse • Echo train length (ETL)/turbo factor= # of echoes acquired in TR interval • ETL typically4-32 for routine, >200 for rapid imaging or echo planar techniques

encoding directions

1. Slice selection: locating a slice within the scan plane selected 2. Frequency encoding: spatially locating/encoding along the axis of the anatomy 3. Phase encoding: spatially locating/encoding signal along the short axis of anatomy

multiecho sequence

• Spin-system is excited by a 90 pulse • After time delay (T), one or several 180 pulses follow • Leads to the formation of an echo • Time between 90 pulse and the peak of the echo is called the echo time TE=2T • TR= time between two complete pulse sequences • Several pulses = decreasing amplitudes • T2* obtained from FID o T2* removes MF inhomogeneity • T2 calculated from peaks of echo amplitudes (spin spin decay) • Several 180 pulses create echoes of decreasing amplitude

phase encoding

• Step 0 (baseline: no phase-encoding gradient applied), total signal from A and B o So(t) = A sin ωt + B sin ωt = (A+B) sin ωt • Step 1( phase encoding gradient applied, diphase spins along vertical axis) gradients turned off • Pixel A signal unchanged = A sin ωt • Pixel B inverted (180°) = −B sin ωt • Total signal from A&B: S1(t) = A sin ωt − B sin ωt = (A−B) sin ωt

signal display

• two voxels in x-y plane of a slice of the brain • voxel A: region of grey matter • Voxel B: region of white matter • Aim: distinguish these two structures (voxels) o the basis of signals recived following T1 and T2 relaxation • T1 is significantly longer than T2 and within a magnetic field strength of 1.5T • A: grey matter T1= 900ms, T2=100ms • B: white matter T1 780ms, T2=90ms

MR facility safety zones

• zone I: general public (0.5mT) • Zone II: public area, MR safety screen required • Zone III: near magnet room where fringe, gradient, RF MF are sufficiently strong to present a physical hazard to unscreened pts/personnel • Zone IV: MR magnet room, al ferromagnetic objects excluded

phase encoding

• Sum of two sine waves o Two pixels, A and B, same frequency encoding column in image, different gradient-induced phase shifts o A generates MR signal A(t) = sin(ωt + φA) B generates B(t) = sin(ωt + φB) o Image measures sum A(t) + B(t) o Looking at sum only ♣ Sine wave of certain frequencies and phase impossible from single observation to sort out individual contributions made by waves A and B o Making two observations with A and B shifted by different phases (two SUMS) determine individual contributions

acoustic noise

• Superconductive scanners refrigeration system pump for liquid helium: quiet, low f, rhythmic background noise present • Loud noises during scan: vibration of gradient coils • Each pulse sequence produces own noises based on gradient waveforms • Ear plugs or headphones during scan in mandatory • Noise reduction o Pulse sequence modification o Mechanical methods

time frame

• TR: repetition time = time between 90 RF pulses • TE (echo time) = time between 90 RF pulse and central point of detection

FSE/TSE advantages

o As # of echoes required , scan time , SNR and T2 blurring o Time saving may allow lengthening of TR, more time for recovery of longitudinal magnetisation improves the SNR o # of phase encoding steps = improvement in spatial resolution o susceptibility-induced signal losses , so FSE superior to CSE for imaging of skull base and around metal objects

why gradient echo

o Dephasing gradient applied before frequency-encoding or readout gradient o Aim: obtain an echo when readout gradient is applied and data acquired o Dephasing stage of readout gradient is inverse sign of readout gradient during acquisition o Sequence show better tissue contrast between white and grey matter o Maintain better SNR as short TR o Only one RF-pulse is applied, reverse gradient calculated, echo recorded relatively quickly, allowing use of short TE times o Combination of short TR and TE = rapid signal acquisition, gradient echo sequences form the basis of most rapid imaging

T1 relaxation

o Effect of 90 RF pulse at resonance frequency on T1 relaxation o As grey matter has a longer T1 than white matter, it will take longer for it's signal to re-grow to it's maximum value

T1 signal display

o If the signal is measured to maximise the T1 contrast TE is recorded when the T1 difference is the greatest o Magnitude of signal intensities can be equated to separate levels on the display scale o Grey matter will appear darker than white matter

T2 dephase

o Loss of phase coherence from the xy plane o Spin system: slightly unequal distribution of angular momentum in z-direction = net magnetisation (M) o 90 pulse rotates entire system converting initial asymmetry of the z-components into asymmetry of transverse xy components resulting in a temporary statistical phase coherence o now tipped into the xy plane and RF pulse over, transverse spin components and their vector sum (Mxy) will precess within the same plane of the Larmour frequency o any process that disrupts either other number or relative positions of these transverse components will result in T2 relaxation • 2 dipoles are interacting • distance and angles they are on when they interact • 100% coherence at the start and then this relaxes and moves back to z plane • if the material is more dense the spin-spin relaxation will be much faster as the energy can be dispersed quicker

measuring T2

o MR signal: a small electric current induced in receiver coil by the precession of the net magnetisation (M) during resonance current induced by each sweep of M past the receiver coil o A voltage is generated in the coil proportional to the rate of change of the magnetic field (dB/dt) o The induced current opposes the applied field • How to measure T2 o T1 relaxation o Static local field disturbances o Angular momentum exchanges between spins

order of k-space

o SSD: slope of slice select gradient determines which slice is excited o Phase: determines which line or drawer to fill with data o Frequency: f in echoes are digitised to acquire data points which fill the line of k-space that was selected by the phase encoding gradient

function of RF coils

o Transmitter1 perpendicular to static magnetic ♣ Generates oscillating/rotating magnetic field B1 perpendicular to static magnetic field B0 ♣ B1 oscillation matches natural precession of nuclear spins near Larmour frequency, energy is deposited into spin system change net alignment ♣ B1 on for a few seconds (RF pulses) o Receiver ♣ Rare parallel imaging applications ♣ Detect MR signal, magnetic flux from excited spin induces electric current, frequency and phase information extracted

T2 relaxation

o after RF pulse is switched off o T2 for white matter is shorter than grey matter, takes less time for loss of coherence to occur o Pulse is chosen to maximise T2 contrast, so magnitudes of 2 signals are separate levels on grey scale o Grey matter will appear lighter than white matter

importance of field homogeneity

o directly relates to image quality and various artefacts o poor MF homogeneity: shading, spatial distortion, blurring, intensity loss, curved slice profiles, zebra banding o shimming: make main MF B0 more homogenous o passive shimming: small pieces of sheet metal or ferromagnetic pellets at various locations within the scanner bore, creates static homogeneity but gets interrupted by patient in bore o active shimming: currents are directed through specialised coils to generate corrective MF ♣ superconducting: located within the liquid helium-containing crystal ♣ resistive: mounted on support structure with g-coils, on room temp inner walls of scanner

FSE/TSE disadvantages

o susceptibility induced signal means FSE images are less likely to detect small areas of calcification or haemorrhage o other limitations: overly bright signals from fat on T2 weighted images, overly bright CSF on spin density weighted images o increased tissue heating from multiple 180 pulses limits FSE use on infants and small children

Flip angle

• The amount of rotation the net magnetisation (M) experiences during application of a RF pulse • Effect of RF pulse with amplitude B1 and duration to create a flip angle ()

Fourier transform

separate frequencies in the image slice, crucially giving the amplitude =signal intensity levels in an image

Magnetic moment

• A vector quantity that measures the tendency of object with a small magnetic dipole/current loop to interact with external MF induced by the solenoid • Hydrogen atoms intrinsic magnetic properties visualised as emanating from a tiny bar magnet with north and south poles (dipoles) magnetic dipole moment • Magnetic moment () will seek to align with an externally applied magnetic field (B0) • Gyromagnetic ratio ()= /S (MHz/Tesla) • The amount the particle is gyrating according to its inherent magnetic moment

spin echo

• A: B0, a few spins cluster together enough to give a NMV (red arrow) • B: B1 is applied, NMV is flipped into the transverse plane • C: dephasing T2, some particles are losing their energy and losing E a bit faster • D: another pulse is applied (pancake flips 180 deg) now 270deg from start. • E: rephasing: give it more E and it will start to rephase F: echo formation, protons are precessing in phase again, NMV will create a current

magnetic shielding

• Active and passive magnetic shielding methods used to reduce fringe fields and protect main MF from outside influences

phase and frequency encoding

• After 90 RF pulse, all spins in selected slice precess at the same frequency 0 and they all in phase (aligned to NMV • Following application of GY gradient pixels in top row will experience a higher MF spins start precessing faster, middle row will be unchanged and lower pixels will precess slower • Rows will be out of phase (in B) top row leading and bottom row lagging the phase of the unaffected central row

k-space

• An array of numbers representing spatial frequencies in MR image • Cells of K-space are commonly displayed on rectangular grid with principle axis kx and ky corresponding x and y axis • K-axes however represent spatial frequencies in the x and y directions rather than positions • These do not correspond to one to one individual pixels in the image • Each row corresponds to the echo data obtained from a single application of phase encoding gradient

effective field

• B1 field is never perfect, even if exactly Larmour frequency is applied, not all parts of image experience this frequency

pixel location

• CANNOT frequency and phase encode simultaneously, must be consecutive • Having selected slice through patient (z-axis), next step is to locate x and y spatial coordinates within that slice • Achieved by o First phase encoding for the y-axis and then frequency encoding for x-axis o First phase encoding for x-axis, and then frequency encoding for y-axis

Spin state

• Cannot know the exact direction of a particles spin at any point in time • Can only measure angular momentum along a single direction • L + half spin up or parallel • L - half spin down or anti-parallel • Difference between these energy states just get amplified when B0 is applied • Absence of external magnetic field: two separate spin states for hydrogen are not observable (degenerate) • External MF (B0): quantum-field interaction occurs allowing two separate states to be measured • Nearly all spins exist in a weighted superimposition of both states simultaneously • A non uniform magnetic field is used to check is particle is + or -. The difference is due to the rotation of the protons rotating charged particles have

frequency encoding

• Define location either 1) within slice or 2) between slices • Gf, Gx, Bx begins on the left of the image at position x=0 and increases linearly along the horizontal axis • Uses centre frequency to determine slice position and frequency span (F) to determine slice thickness • Apply a second gradient orthogonal to SSG during RF echo acquisition GX • Resulting MF o B(x) = B0 + x · Gx • F of received RF signal depends on x-position of spins • Fourier transform determines signal strength as a function of F and as a function of x-position • We have localised the signal in the z-direction by applying Gz during RF injection and we have localised the signal in the x-direction by applying the gradient Gx during echo acquisition

spatial encoding

• Differences in frequency: MF gradients alter main MF • Differences in phase: imaging gradient results in gain/loss in phase, persists even after gradient turned off • Differences in signal timing: collecting signals at different time offsets • Distance from receiver coils: proximity to surface receiver coils, closer to voxel= stronger signal

slice selection

• Different frequency RF pulses are used to select different slices • Field variation in y means than spins in various axial planes precess at different frequencies and are excited by different frequencies • Field gradient superimposed over B0 field so field strength and Larmour frequency increases linearly in z-direction • RF signal in resonance for only a small slice of tissue, experiences 90 flip and subsequent phase coherence • Any spin echo received must originate from this thin slice, as any section above or below DN have phase coherence and cant emit an echo • If no gradient applied (Bz=0), entire volume excited by RF signal • Gradient applied, original Larmour frequency excites the slice at z=0 • Slightly increasing the RF, a slice in + z-direction is excited • ω0(z) = γ · (B0 + Bz(z)) = γ · (B0 + z · Gz) • B0 =1.5 T and we apply a gradient Gz =10 mT/m. Recall that the gyromagnetic ratio for hydrogen = 42.58 MHz/T. 1. What is the RF frequency for the slice at z = 0? 2. If we tune the RF generator to produce 63.891 MHz, which slice parallel to the xy-plane is in resonance? • So the 90° and 180° pulses only affect this slice. By choosing an RF frequency that deviates from the original no- gradient Larmor frequency, we can choose the location of the slice to be excited. • Volumetric MR image is obtained by acquiring one cross-sectional slice and then moving to a subsequent slice by repeating the acquisition with a different RF frequency.

gradient echo

• FID: sine wave oscillating at Larmour frequency damped by exponential decay (relaxation) • Gradient echo: manipulation of FID signal by applying external dephasing gradient field across tissue • 2: process reversed, rephrasing gradient applied, reversing/undoing phase scramble • small gradient echo is generated • gradient is applied to change the signal and get another echo (spin-spin interactions haven't been altered) • T2: transverse decay, spin-spin, energy is moved between its spins (is effected by T1 decay as this changes the angular momentum) (both transfers and releases energy) (T1 only releases energy) • T2*: MF B1 inhomogeneity that accounts for more decay energy (spin loses some energy when there is inhomogeneity) • we want T2 not T2* because we want info about the tissue in the body (T2 and T2* are 2 different signals) • spins precess rapidly and accumulates phase due to location in a stronger portion of the gradient • more slowly precessing spins in a weaker part of the gradient • faster spins have a much greater phase accumulation • reversal of direction halfway through corresponds to gradients being applied with opposable polarities • faster spins spin faster but in opposite direction • Both return to starting line a same time with net phase shift of 0

MF gradient

• Gradient field differs in magnitude or direction between two points in space • Magnitude of gradient (G)= change in field divided by change in distance • Gradient is a vector both magnitude and direction • Convention: direction of main magnetic field = z axis • Gradient= how the magnetic field changes in space o MR magnets have gradient fields due to ♣ Field imperfections (inhomogeneities) ♣ Field distortion from object in the scanner • Gradient coils: set of electromagnets embedded in the body of the MR magnet assembly o Electrical current passes through coils main magnetic field focally distorted, creating magnetic gradients o Gradient coils alter main MF predictability in x, y and z directions so used to spatially encode the MR signal

phase encoding

• Gradients turned off, frequency returns to normal, phase shift persists • All imaging gradients temporarily change resonant frequencies of spin while gradient is applied • Gradient turned off: spins back to original precession frequencies • However, spins have gained/lost phase relative to reference state permanent phase shift

phase and frequency encoding

• Gy turned off, all spins in original MF precess with same frequency 0 but phase difference maintained • When signal is to be read out, Gx gradient is turned on and this allows frequency encoding in x direction (D) Compared to pixels in the central column (remain unchanged), pixels in RH column precess at a higher frequency (ahead of phase), pixels in LHS precess at lower f (behind phase)

Frame of reference

• In real time, the precession at any MHz makes M appear as a blur • Application of rotating/oscillating B1 field is applied to spin system • B1 arrow on plane is shown in one position only for clarity but it is rotating as fast as the spins so should be ablur • Rotating frame of reference o B1 is rotating because it needs to turn the particle in x and y directions ♣ This allows the particle to gyrate in x and y o If B1 is stationary M won't precess 1. Effect of the main field B0 has disappeared 2. Diagram placed B1 along the x'-axis, by adjusting the phase of transmission, could be along y'-axis 3. M has begun a new precession around B1 since it no longer exists in this frame of reference, the only MF acting on M is B1

NMV

• Individual spins are precessing around external MF axis at equilibrium, net magnetisation (vector sum of all spins) does not precess. • Individual spins are out of phase with one another, so large component along z-axis but no component along x or y axis • NMV remains stationary and pointed in z direction

2D fourier transform

• MR image is hundreds of pixels in each dimension, requires hundreds of phase encoding steps to decode • # of echoes acquired = # of phase encode steps (196 or 256) • phase shift between rows are not multiples of 180, but vary from echo to echo depending on phase encoding step size • each echo has been acquired using a different set of inter-row phase shifts, spectral amplitudes (heights of bars) for each row cannot be added or subtracted to compute individual pixel values as before • to sort out where each signal in a frequency column has originated, a second Ft must be performed

decoding signal

• MR scan contains >4million voxels, each generating its own tiny signal • MR signals from multiple voxels, often entire slices recorded all at once • Voxels: different materials with different spin densities, relaxation times • Main MF B0 often distorted by gradients to change frequency and phase of spins as a function of spatial position • Resulting total MR signal = sum of thousands of FIDs and echoes arising from individual voxels with different amplitudes, frequencies and phases • To separate huge array of overlapping and interfering signals: frequency encoding, phase encoding, variations of signal timing and exploiting the knowledge of coil location and sensitivity • Fourier transform methods: data sorted according to differences in frequency and accumulated phase

Fc

• MR signal derived from both water and fat protons- frequencies differ • Scanner must be turned to a single resonance for proper slice positioning and fat saturation • Even after coil tuning, pt to pt variations in susceptibility may change this value by a few dozen Hz potentially resulting in a few mm of error in spatial location

precession vs resonance

• Precession o Constant that is always there • Resonance o Found by adding energy into a nuclear spin system near the Larmour/resonance frequency o Slight energy losses occur incoherent EM interactions, resonance dies out but not precession

RF shielding

• Prevent interfering RF waves from entering or leaving the scanner room • Mandatory RF shielding o Prevent extraneous electromagnetic radiation from contaminating/distorting the MR signal o Prevent electromagnetic radiation generated by MR scanner from causing interference in nearby devices • Faraday cage: encircles entire room (walls, floor, ceiling)

gradient coils

• Produce calibrated distortions of main MF in x, y, or z directions • Current is passed through coils, creates secondary MF • Gradient field distorts main MF in a predictable pattern, causing resonance f of protons to vary as a function of position spatial encoding • 3 sets of g-coils used in x, y and z directions o each coil set driven by independent power amplifier, creates GF whose z- component varies linearly along x, y and z directions o z: circular coils o x/y: saddle coil configuration • if the gradient is played out during slice selection and again during signal readout, a slice can be selected perpendicular to the gradient direction • x/y gradients provide augmentation in the z direction to B0 field as a function of left-right or anterior-posterior in the gantry • Produce calibrated distortions of main MF in x, y, or z directions • Current is passed through coils, creates secondary MF • Gradient field distorts main MF in a predictable pattern, causing resonance f of protons to vary as a function of position spatial encoding • 3 sets of g-coils used in x, y and z directions o each coil set driven by independent power amplifier, creates GF whose z- component varies linearly along x, y and z directions o z: circular coils o x/y: saddle coil configuration • if the gradient is played out during slice selection and again during signal readout, a slice can be selected perpendicular to the gradient direction • x/y gradients provide augmentation in the z direction to B0 field as a function of left-right or anterior-posterior in the gantry

T2

• RF pulse (B1) is removed, happens simultaneously with T1 relaxation • Transverse components of magnetisation (Mxy) decay/diphase • T2 relaxation = transverse relaxation = spin-spin relaxation • Exponential recovery • T2 is always faster than T1 because it had T1 influence and other interactions • Time required for transverse magnetisation to fall to 37% of initial value

bandwidth

• Range of frequencies involved in transmission or reception of electronic signal • Value is an operator-selectable parameter, typically 50kHz • This total bandwidth is apportioned to pixels (width=w) along the frequency encoding direction equally

spin echo with other pulses

• SE can be generated by any 2 succesive RF pulses • 90-180 pair produces max echo signal • other options produce other effects STE: stimulated echo • practical applications o 180 rephrasing pulse gives a true T2 signal without T2* o choosing correct parameters (TE and TR) images weighted in T1, T2 or proton density o major disadvantage of T2 weighted SE: long TR resulting in prohibitive acquisition times o SE can be used in clinical practice to obtain good quality anatomical T1 weighted images, faster types of sequence is preferred to obtain T2 weighted images

spatial localisation

• Sample in homogenous field all regions have same Larmour frequency, no spatial information • When gradient is imposed on sample, MR signal contains information about spatial location of resonating spins • Rotate the gradient in all three dimensions, distinguish different tissue contents in the body as well as locate them in all 3 dimensions • Z-gradient: along the long axis, applied with RF pulse to choose a slice axial images (slice select gradient) • Y-gradient: vertical axis, encodes location along one axis of slice determined by RF pulse coronal images (phase encoding gradient • X-gradient: horizontal axis, encodes location along other axis in slice sagittal images (frequency encoding gradient)

spin echo

• Single RF pulse generates FID • 2 successive pulses produces spin echo (SE) • TE: time between middle of 1st RF pulse and peak of spin echo • SE: regeneration of spin phase information apparently lost during FID • FID signal not destroyed, disorganised as individual spins lose phase coherence • Applying second RF pulse, certain dephased FID components can be refocused into SE • B1 switched off, spins diphase in x-y plane no longer a cluster. Some spins move faster than others (out of phase faster from net magnetisation alignment, M) • After time delay (T), system exposed to 180 pulse, refocus initialised • Faster spins lie behind slower ones (opposite) but they catch up, thus TE=2T o Flips into receiver coil and we measure the signal • System resets, another 90 RF pulse is applied, ready for detection

slice thickness

• Slice-select gradient: along axis perpendicular to plane of desired slice linear variation of resonance frequencies in that direction • Transmitting single frequency RF pulse signals from infinitely thin line in patient • In practice, a pulse is used with a range of frequencies (bandwidth of frequencies) • Stronger gradients produce thinner slices and visa versa • Deviations of a few PPM from Larmour frequency no longer meets resonance condition • However, boundary of excited slice is not abrupt • Some phase coherence is achieved a small distance from the center of the plane • RF signal has a bandwidth • RF pulse is used to excite particular band of frequencies • Slice thickness can be decreased by either o Using a narrow bandwidth o increasing slope of magnetic field gradient

Precession

• The angular momentum resist change, so instead of twisting it traces out a circle, this is the Larmour precession • Proton is always precessing in the earths MF but is greater in strong MRI machine • Angular momentum results from an intrinsic quantum property (spin) • Objects possessing momentum maintain their motion unless acted upon by an external force • Angular momentum: strong resistance to changing orientation or direction of motion • MF create torque (twisting force) perpendicular to the field direction of angular momentum • Nucleus does not tip over, instead deflected into circular path perpendicular to field: precession, angular frequency (0) • Precession frequency is proportional to MF strength and gyromagnetic ratio (particle specific constant incorporating size, mass and spin • External MF B produces a torque on M resulting in precession at an angular freqeuncy • Larmour equation (w=yB0)

MF gradient

• This is a weak MF • Applying a single gradient: frequency variation of protons as a function of position along direction of gradient • MF difference 10mT across 1m MF varies from point to point, each position has its own resonance frequency

T1

• Time for 63% recovery of longitudinal (z-direction) • B1 pulse is removed • Net magnetisation (M) grows/returns to it's original maximum M0 value parallel to B0 • T1 relaxation = longitudinal relaxation - thermal relaxation = spin-lattice relaxation • Energy transfer from spin to environment (proteins, lipids, membranes) • • T1 is longer than T2, this is determined on o Size and motion of the molecule on which the hydrogen nucleus resides • Small, rapidly rotating molecules (free water) have long T1 and T2 times • As molecular motion slows (proteins and dense solids) T2 shortens and T1 increases

coils

• Transmits gradient to nuclei o Produces oscillating field in resonance with nuclei o Causes torque that tips the magnetisation out of equilibrium into the transverse plane • Same coil receives the signal o Detects electromotive force/oscillating voltage as nuclear magnetisation precesses around in the coil, signal gradually decaying away with time (FID) as spins come back to thermal equilibrium o This induces a current

RF coils

• Transmits to the nuclei o Produces an oscillating field in resonance with nuclei o Causes a torque that tips the magnetisation out of equilibrium into the transverse plane • Same coil receives the signal o Detects electromotive force/oscillating voltage as nuclear magnetisation precesses around in the coil, with signal gradually decaying away with time (free induction decay) as spins come back to thermal equilibrium • Coil positions o Main (B00 coils ♣ Principle magnet windings plus superconducting shim and shield coils o Shim: improve homogeneity ♣ Gradient: imaging, active shields ♣ RF: transmits B1 field ♣ Patient: detect MR signal or transmit/receive

relaxation

• Turning on B1 rotates the magnetisation down to 90n net magnetisation (M) starts precessing around B0 and then turn B1 off

RF field: B1

• When M begins to precess nuclear magnetic resonance is occurring • At equilibrium no scanning is taking place o M is aligned with B0 • During MR imaging, M is purposely tipped out of alignment allowing it to precess around the direction of B0 • M has both longitudinal Mz and transverse Mxy components • Since M is precessing, Mz and Mxy are a function of time • To tip out of alignment second MF needs to be applied o B1 rotating/oscillating at Larmour frequency • Applied perpendicular to MF B0 • With continued application of B1 field, M precesses at continually larger angles away from B0 (tip angle) • M can be rotated any amount of degrees • After every 360 M returns to its original alignment with B0 Precession vs resonance

Solenoid

• coil of current that creates a magnetic field, particles align along the direction of the MF

x/y gradient coil

• coils in finger print pattern, double saddle coil configuration with more arcs and curves added o simplest from of golay coil consists of 4 inner and 4 outer arcs on cylinder surface connected by 8 straight wires running parallel to z-axis o current along inner arcs are responsible for creating required gradient o straight wires, parallel to z axis, serve as a return pathway for current o advanced configurations use fingerprint coil design

Coupled Pendulums

• coupled due to electromagnetic field • 2 influence one another, they are always moving • superimposition when they are moving in opposing directions

Shielding

• eddy currents generated in nearby conductor when changing MF is present • source of changing MF: either imaging gradients or RF coils • active shielding: actively shielded gradients precent eddy currents from being generated • additional set of shield coils placed around gradient coils used for imaging reduces eddy currents by order of magnitude

z-gradient coil

• maxwell coil pair 2 loops with currents travelling in opposite directions o current through coil related to strength of gradient field o direction of gradient field: RHS rule, thumb = direction of current o z-gradient produced using 2 coils carrying current in opposite directions o gradient is 0 at magnet isocentre but increased linearly in -z and +z directions o when added to B0 results in a gradually increasing gradient in z axis

Free induction decay

• peak when current is pointing in the direction of the coil

fringe fields

• peripheral MF outside the magnetic bore • larger fringe fields extend for several meters around the MR scanner (3D) • pacemaker affected by MF > 0.5mT (5G) • smaller fields (1-3G) affect nearby CR/MRI scanners • fringe fields of 10G affect computers, 30G watch affected and credit card

Spin

• pure quantum mechanical property that comes from solving relativistic equations • the particle (proton) has an inherent magnetic property (dipole) • intrinsic angular momentum (magnetism) of a particle (PARTICLE IS NOT SPINNING) • a lone proton not rotating possesses spin (S), this is the magnetic moment and can point in any direction • protons, neutrons, whole nuclei and electrons al posses spin (angular momentum), often represented by spinning balls • They are NOT spinning/rotating • Spin is a fundamental property of nature • Spin (intrinsic angular momentum, S) interacts with EMF but classic angular momentum (L) interacts with gravitational fields • Magnitude of spin in quantised limited set of discrete values • Protons, neutrons and electrons all have spin= ½ • Hydrogen nucleus has a spin of half, this is the same spin as a single proton • Only nuclei on non-zero spins can absorb and emit electromagnetic radiation and undergo resonance when placed in a MF