Magnetic Resonance Imaging
Dr Sarah Wayte
University Hospital of Coventry &
Warwickshire
Siemens MR Scanner
GE MR Scanner
Receiver Coils
‘Typical’ MR Examination
• Surface coil selected and positioned
• Inside scanner for 20-30min
• Series of images in different orientations &
with different contrast obtained
• It is very noisy
MRI in Cov & Warwickshire
Year
No of scanners
Field Strength
1987
1
0.5/1.0T
1997
1
1.0T
2007
7
2012
8
5x1.5T, 3.0T
0.35T
6x1.5T, 3.0T,
1.5T extremity
1.5T Extremity
Wide Bore 1.5T
What is so great about MRI?
• By changing imaging parameters (TR and
TE times) can alter the contrast of the
images
• Can image easily in ANY plane
(axial/sag/coronal) or anywhere in between
Spatial Resolution
• In slice resolution = Field of view / Matrix
– Field of view typically 250mm head
– Typical matrix 256
– In slice resolution ~ 0.98mm
• Slice thickness typically 3 to 5 mm
• High resolution image
– FOV=250mm, 512 matrix, in slice res~0.5mm
– Slice thickness 0.5 to 1mm
Any Plane
Any Plane
• Magnetic field varied
linearly from head to toe
• Hydrogen nuclei at
various frequency from
head to toe (ωo=γBo)
• RF pulse at ωo gives slice
through nose (resonance)
• RF pulse at ωo+  ω
gives slice through eye
RF wave
ωo+ω
ωo
ωo - ω
Slice selection gradient
Sagittal/Coronal Plane
• Sagittal slice: vary
gradient left to right
• Coronal slice: apply
vary gradient anterior
to posterior
• Combination of sag &
coronal gradient can
give any angle
between
Image Contrast
TR=525ms TE=15ms
TR=2500ms TE=85ms
Image Contrast
• Depends on the pulse sequence timings
used (TR/TE)
• 3 main types of contrast
– T1 weighted
– T2 weighted
– Proton density weighted
• Explain for 90 degree RF pulses
TR and TE
• To form an image have to apply a series of 90o pulses (eg
256) and detect 256 signals
• TR = Repetition Time = time between 90o RF pulses
• TE = Echo Time = time between 90o pulse and signal
detection
TR
TR
90-----Signal-------------90-----Signal-----------90-----Signal
TE
TE
TE
Bloch Equation
• Bloch Equations BETWEEN 90o RF pulses
Signal=Mo[1-exp(-TR/T1)] exp(-TE/T2)
• TR<T1, TE<<T2, T1 weighted
• TR~3T1, TE<T2, T2 weighted
• TR~3T1, TE<<T2, Mo or proton density
weighted
TR
TR
90-----Signal-------------90-----Signal-----------90-----Signal
TE
TE
TE
PD/T1/T2 Weighted Image
T1 weighted
– Water dark
– Short TR=500ms
– Short TE<30ms
T2 weighted
– Water bright
– Long TR=1500ms
(3xT1max)
– Long TE>80ms
PD weighted
– Long TR=1500ms
(3xT1max)
– Short TE<30ms
T1/T2 Weighted Image
TR = 562ms
TR = 4000ms
TE = 20ms
TE = 132ms
T1/T2 Weighted
TR=525ms TE=15ms
TR=2500ms TE=85ms
Proton Density/T2
TR = 3070ms
TR = 3070ms
TE = 15ms
TE = 92ms
Proton Density/T2
TR = 3070ms
TR = 3070ms
TE = 15ms
TE = 92ms
Lumbar Spine Images
Disc protrusion L5/S1. Degenerative changes bone.
Axial Images of L Spine
Imaging Time (Spin Warp)
• 1 line of image (in k-space) per TR
Imaging time = TR x matrix x Repetitions
• Reps typically 2 or 4 (improves SNR)
• E.g. TR=0.5s, Matrix=256, Reps=2 Image
time = 256s = 4min 16s
• During TR image other slices
• Max no slices = TR/TE
– e.g. 500/20=25 or 2500/120=21
Speeding Things Up 1
• Spin warp T2 weighted image, 256 matrix,
3.5s TR, 2reps
• Imaging time = 3.5 x 256 x 2 ~ 30min!!!
• Solution: acquire 21 lines k-space per 90o
pulse
Speeding Things Up 2
• With 21 signals per 90o pulse for 256
matrix, 3.5s TR, 2reps
Imaging time = 3.5 x 256 x 2/21 ~ 1min 25s
• All images I’ve shown so far use this
technique
(Fast spin echo or turbo spin echo)
Even Faster Imaging
• How fast? 14-20 images in
a breath-hold (30 images @
3T)
• Use < 90 degree flip (α)
• Some Mz magnetisation
remains to form the next
image, so TR<20ms
• Drawback- less
magnetisation/signal in
transverse plane
Mz
Signal = MoCosα
T1 Breath-hold Images
14 slices in 23s breath-hold (t1_fl2d_tra_bh)
TR=16.6ms, TE=6ms α=70o
T2 breath-hold images
19 slice in 25s breath-hold (t2-trufi_tra_bh)
TR=4.3ms TE=2.1ms α=80o
30 Images in 20s Breath-hold
Echo Planar Imaging
Takes TSE/FSE to the extreme by acquiring
64 or 128 image lines (signals) following a
single 90 degree RF pulse
Image matrix size (64)2 or (128)2 (poor
resolution)
EPI Imaging
• Each slice acquired in
10s of milliseconds
• Lower resolution
• More artefacts
www.ph.surrey.ac.uk
EPI Imaging
• Each slice acquired in ~10ms
• Used as basis for functional MRI (fMRI)
• Images acquired during ‘activation’ (e.g. finger
tapping) and rest. Sum active and rest and subtract
Right motor cortex
excited with left
finger tapping, in
close proximity
with tumour
www.icr.chmcc.org
Functional MRI (fMRI)
• Concentration of oxyhaemoglobin brighter (longer
T2* than de-oxyhaemoglobin) Subtracted image of
bright ‘dots’ of activated brain
• Super-impose dot image over ‘anatomical’ MR
image
fMRI of patient with tumour
near right motor cortex
Active area with left finger
tapping
Shows right motor cortex close
too, but not overlapping tumour
www.ich.ucl.ac.uk
Imaging Blood Flow
• Apply series of high flip angle pulses very quickly (short
TR)
• Stationary tissue does NOT have time to recover, becomes
saturated
• Flowing blood, seen no previous RF pulses, high signal
from spins each time
Flip
TR
Flip
MIPs of Base Image
Abnormal MIP with AVM
MRA Base Images
• 72 slices through head
• Brain tissue ‘saturated’
high signal from moving
blood
• Processed by computer to
produce Maximum
Intensity Projections
(MIPs)
• Maximum signal along
line of site displayed
Diffusion Imaging
• Uses EPI imaging
technique with additional
bi-polar gradients in x, y &
z directions
• Bi-polar gradients also
varied in amplitude
• No diffusion – high signal
• More diffusion- lower
signal
T2 & EPI Images: Stroke?
Different Amp Diffusion Gradient:
Ischemic Stroke?
Amp = 0
Stroke reduces diffusion
Bright on diffusion weighted image
Amp = 500
Amp = 1000
Diffusion Co-efficient Map & Images
Diffusion co-efficient map
Intensity α Diffusion Co-efficient
Diffusion image
Intensity α 1/Diffusion (& T2)
Anisotropic Diffusion
Diffusion gradient
Diffusion gradient
Anisotropic Diffusion: Diffusion
tensor imaging
• Anisotropic diffusion in white
matter tracks
• Apply diffusion gradients in 1215 direction
• ‘Track’ white matter track
direction by diffusion
anisotropy
Brainimaging.waisman.wisc.edu
www.cimst.ethz.ch