Magnetic Resonance Imaging
FRCR Physics Lectures
Anna Beaumont
Basic MR Physics
MRI (very brief) summary MRI imaging consists of placing the patient inside a large
magnetic field.
This field causes protons in water molecules to align with/against the field.
Radiofrequency pulses are used to “excite” the protons
Energy subsequently released by these protons is measured and turned into an image
How large a field?Tesla - unit of magnetic field strengthGauss - unit of magnetic field strength1G = 0.0001 T
Earth’s magnetic field ~ 0.5G or 50μT
MRI scanners ~ 1 – 3T or 10-30kG
What types of tissue? Fluids – cerebrospinal fluid (CSF), synovial fluid,
oedema; Water based tissue – muscle, brain, cartilage, kidney; Fat based tissues – fat, bone marrow.
– Fat based tissues have some special MR properties, which can cause artefacts.
– Fluids are separated from other water based tissues because they contain very few cells and have a different appearance on images.
– Pathological tissues frequently have either oedema or a proliferating blood supply, so their appearance can be a mixture of water based tissues and fluids.
Nuclear Spin The hydrogen nucleus consist of a proton.
Each proton has a positive charge and spins like a top.
This circulating charge is like a small loop of current.
A moving charge has an associated magnetic field.
Proton behaves like a tiny rotating magnet, represented by vectors. Tiny field that is generated is known as its magnetic moment.
No external magnetic field
Random orientation
no net magnetisation
Apply magnetic field
Majority of magnetic moments align with field (think of a compass needle aligning to the Earth’s magnetic field)
net magnetisation M0
B0
Net Magnetisation
Energy States: the Quantum Mechanically bit
Energy levels related to magnetic field
For a proton there are two states– Spins opposing field are high energy (‘spin down’)– Spins aligned with field are low energy (‘spin up’)– Population difference exists– Slightly more dipoles point spin up than spin down (lazy
protons!)– Difference is ~ 3 out of 1 million protons at 1T and
S.T.P. (3ppm)
0BE
E
NN exp
increase the difference in population (sensitivity)by increasing B0 or decreasing temperature
Classical Physics
Spin causes precession around B0
(Resonance) Larmor frequency:
At 1.5 Tesla and 1H frequency is 63.8 MHz
(Radio-frequency, RF) At 1.0 Tesla and 1H frequency is
42.6 MHz ( γ= gyromagnetic ratio)
B0
00 B
Tilting of spin axis splits magnetic vector, m, into two components– Longitudinal, mz
– Transverse, mxy
Spins align parallel /anti-parallel with B0– Produce net longitudinal
magnetisation Mz
Protons precess independently, out of phase– Mxy point in all different
directions– Net transverse magnetisation
Mxy=0
B1 Field Net magnetisation is very small, e.g. 1μT.
– Cannot measure whilst lying parallel to B0
– Can measure if ‘flipped’ into transverse plan perpendicular to B0
Exchange of energy between two systems at a specific frequency is called resonance.
Protons spin at the Larmor frequency. This frequency is in the Radio frequency (RF) range.
A pulse of RF at the right frequency can be absorbed by the protons and put them in a different energy state, e.g. a spin moves from the lower energy state to the higher one.
The system then relaxes back to an equilibrium state and electromagnetic energy
is emitted, which can then be detected and provides a signal.
Application of B1field (RF pulse) B1 applied at the
resonance frequency
Complicated spiral motion in stationary or laboratory frame of reference
B0
B1
Transverse plane
www.olympusmicro.com
Rotating Frame of Reference Spins are ‘tipped’ into the
transverse plane
‘flip angle’,α, is determined by B1 (field strength), tp
(duration of pulse)– α = γB1tp
– 90˚ pulse: flips M0 to transverse plane
– 180˚ pulse: twice duration/ double strength: flips M0 through 180˚
B1
B0
Relaxation Mechanisms I
B1 pulse is then removed
Spins begin to dephase
This is called transverse relaxation or decay
B0
Recording MR Signal Receiver coil sees oscillating
magnetic field which induces a varying voltage
Sinusoidal waveform without relaxation
Coil measures signal in transverse plane– Only Mxy produces an MR signal,
Mz does not.
– Because Mxy is produced by tipping Mz the signal produced by the 90˚ pulse depends on Mz immediately before that pulse is applied.
(1) without relaxation, signal is sinusoidal
(2) real signal is attenuated (sinc function) due to relaxation (FID)
z
y
x
Free Induction Decay (FID) The signal at this stage is called the FID. Relaxation occurs due to interactions between spin-
lattice and spin-spin. FID is attenuated by characteristic relaxation time T2*
Magnitude signal measured in coil
Decay envelope due to T2*
T2* DecaySignal loss called T2* decay
– T2 (effective T2) due to inhomogeneities* in B0
– T2 (natural T2) due to spin-spin interactions (Neighbouring protons exert a tiny magnetic field which
alters the rate of precession, causes dephasing)Summation of both effects:
*Even if the magnet were perfect, the presence of the patient will always cause
local inhomogeneities
'
11
*
1
222 TTT T2* T2
T2 Decay (Spin-Spin)
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500TE (ms)
Mxy
(a
.u.)
20
Tt
xy eMM
Mxy is magnetisation in transverse plane– After 90° pulse it
is at maximum value M0
– Decays to zero as
t – At t = T2 signal is
37% (e-1) of initial value
T2 values are unrelated to field strength
Causes of Spin-Spin Relaxation Local variation of magnetic field is greatest in solids
& rigid macromolecules
Dipoles in compact bone, tendons, teeth dephase quickly → very short T2
Effect is least in free water, urine, CSF. Lighter molecules in rapid thermal motion – smoothes out local field → long T2
Water bound to surface of proteins & in fat have a shorter T2 than free water
Relaxation Mechanisms II Spins return to equilibrium
– Spin-Lattice relaxation
This is called T1 relaxation or recovery
This requires a loss of energy
B0
T1 Recovery (Spin-Lattice)
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500
TR (ms)
Mz
(a.u
.)
)1( 10
Tt
z eMM
Mz is magnetisation in longitudinal plane– After 90° pulse it
is zero– Recovers to
maximum value M0 as t
– At t = T1 signal is 63% (1-e-1) of M0
– T1 increases as B0 increases
Causes of Spin-Lattice RelaxationLarge, slow moving molecules most effective at
removing energy from excited dipoles– Fat, also water bound to surface of proteins→ short T1
Small, lightweight molecules ineffective at removing energy from excited dipoles– Water, urine, CSF → long T1
Atoms in solids are relatively fixed and least effective at removing energy– Bone, teeth →very long T1
Typical Relaxation TimesMaterial T1 (ms) T2 (ms)
Fat 250 80
Liver 400 40
White Matter 650 90
Grey Matter 800 100
CSF 2000 150
Water 3000 3000
Bone, Teeth Very long Very short
*Abnormal tissue has higher PD, T1 & T2 than normal tissue, due to increased water content or vascularity
Summary 90° excitation pulse B1
Spins tipped into xy plane→ in phase
B1 removed Spins dephase (T2*) Spins return to alignment
with B0 (T1) T2 is tissue-specific & always
shorter than T1
Process repeated hundreds of times to make an image
Signal Characteristics
Peak signal is proportional to (and pixel brightness depends on):–Proton density (no. of protons per
mm3) in the voxel.–Gyromagnetic ratio of the nucleus–Static field strength, B0
Ref: From Picture to Proton, McRobbie et al
Signal Characteristics Only mobile protons give signals – those in large molecules or
effectively immobilised in bone do not
Greater part of signal due to body water (free or bound to molecules)
Air produces no signal and is always black.
Fat has a higher PD than other soft tissues
Grey matter has a higher PD than white matter
However; tissues do not vary greatly in PD
Spin-Echo SequenceT2 decay can be reversed in a spin-echo
experiment– Initial 90° pulse– Dipoles in phase– Dipoles begin to dephase, at different
speeds, some lag– 180° refocusing pulse reverses the sense
of the spins – Refocus to produce the echo
Signal has decayed by T2 only
Spin-Echo
1. Spins dephase: fast and slow
2. Apply 180° at t = TE/2
3. Echo at t = TE
T2* FID
T2 decay
FID refocused to giveSpin-Echo
Time 0 TE/2 TE
RF 90° 180°
Contrast in MRI MRI offers excellent soft-tissue contrast which
can be manipulated.
T2 contrast can be altered by varying the echo-time (TE).
T1 contrast can be altered by varying repetition time (TR)– Time between two 90º pulses
Flip angle, α, can also be varied (Gradient echo imaging – see lecture 6)
Image Contrast (‘weighting’) For T2-weighted imaging
– Use a long TE and long TR– Often known as ‘pathology’ scans because collections of
abnormal fluid are bright against the darker normal tissue.
For T1-weighted imaging– Use a short TR and short TE– Often known as ‘anatomy’ scans as they show most
clearly the boundaries between tissues. To minimise either the above effects
– Long TR and short TE– Image signal now determined by the density of spins
present i.e. Proton density weighted. In gradient-echo sequences the ‘flip angle’ is also
varied (more on that in lecture 6)
Fat-Water: T2 Contrast T2-weighting is
controlled by TE Water appears
brighter than Fat
TE
time
Mxy
Fat-Water: T1 Contrast
TR
time
Mz
T1-weighting is controlled by TR
Water appears darker than Fat
The Opposing Effects of T1 & T2T1 & T2 are mutually antagonistic
Tissues with long T1 often have long T2 & vice versa.
Images cannot be weighted for T1&T2
If TE & TR not chosen correctly, tissues with different relaxation times can produce equal signal
TE is always shorter than TR A short TR usually < 500 ms A long TR usually >1500 ms A short TE usually < 30ms A long TE usually > 90ms
Choice of TR & TE for conventional SE sequence
TR
TE
Short (< 40ms) Long (>75 ms)
Short (< 750ms) T1 weighted Not useful
Long (> 1500 ms) PD-weighted T2 weighted
From Picture to Proton
Example in Brain
T2-weighted
FSE: TE/TR = 100 ms/4 s
T1-weighted
SE: TE/TR = 9/380 ms
PD-weighted
FSE: TE/TR = 19 ms/3 s
Example in Prostate
PD-weightedT2-weighted
Summary: important pointsMRI measures the hydrogen content of individual
voxels in each transverse slice of the patient & represents it as a shade of grey or colour in the corresponding image pixel on the screen
The patient is placed in a strong electromagnetic field for an MRI scan
Hydrogen nuclei (protons) in the body align themselves parallel or antiparallel with the magnetic field
Summary: important points For each transverse image slice, a short, powerful
radiosignal is sent through the patient’s body, perpendicular to the main magnetic field.
The hydrogen nuclei, which have the same frequency as the radiowave, resonate with the RF wave.
The hydrogen atoms return to their original energy state, releasing their excitation energy as an RF signal, (the MR signal), when the input radiowave is turned off. The time this takes, relaxation time, depends on the type of tissue.
Summary: important points The time and signals are computer analysed and an image is
reconstructed.
Soft tissue contrast is high. The range of T1 and T2 values in soft tissue is even wider than the range of CT numbers.
Bone and air do not produce artefacts.
MRI is non-invasive, contrast media being required only for specialised techniques
Ionising radiation is not involved.
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