magnetic resonance imaging dr sarah wayte university hospital of coventry & warwickshire
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Magnetic Resonance Imaging
Dr Sarah Wayte
University Hospital of Coventry & Warwickshire
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Siemens MR Scanner
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GE MR Scanner
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Receiver Coils
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‘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
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MRI in Cov & WarwickshireYear No of scanners Field Strength
1987 1 0.5/1.0T
1997 1 1.0T
2007 7 5x1.5T, 3.0T
0.35T
2012 8 6x1.5T, 3.0T,
1.5T extremity
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1.5T Extremity
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Wide Bore 1.5T
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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
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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
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Any Plane
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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
Slice selection gradient
ωo+ω ωo ωo - ω
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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
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Image ContrastTR=525ms TE=15ms TR=2500ms TE=85ms
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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
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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
90-----Signal-------------90-----Signal-----------90-----Signal
TRTR
TE TE TE
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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
90-----Signal-------------90-----Signal-----------90-----Signal
TRTR
TE TE TE
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PD/T1/T2 Weighted ImagePD weighted
– Long TR=1500ms (3xT1max)
– Short TE<30ms
T1 weighted
– Water dark
– Short TR=500ms
– Short TE<30ms
T2 weighted
– Water bright
– Long TR=1500ms (3xT1max)
– Long TE>80ms
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T1/T2 Weighted Image
TR = 562ms
TE = 20ms
TR = 4000ms
TE = 132ms
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T1/T2 WeightedTR=525ms TE=15ms TR=2500ms TE=85ms
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Proton Density/T2
TR = 3070ms
TE = 15ms
TR = 3070ms
TE = 92ms
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Proton Density/T2
TR = 3070ms
TE = 15ms
TR = 3070ms
TE = 92ms
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Lumbar Spine ImagesDisc protrusion L5/S1. Degenerative changes bone.
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Axial Images of L Spine
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Imaging Time (Spin Warp)
• 1 line of image (in k-space) per TRImaging 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
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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
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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)
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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
Signal = MoCosα
Mz
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T1 Breath-hold Images 14 slices in 23s breath-hold (t1_fl2d_tra_bh)
TR=16.6ms, TE=6ms α=70o
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T2 breath-hold images19 slice in 25s breath-hold (t2-trufi_tra_bh)
TR=4.3ms TE=2.1ms α=80o
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30 Images in 20s Breath-hold
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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)
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EPI Imaging• Each slice acquired in
10s of milliseconds• Lower resolution• More artefacts
www.ph.surrey.ac.uk
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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
www.icr.chmcc.org
Right motor cortex excited with left finger tapping, in close proximity with tumour
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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
www.ich.ucl.ac.uk
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
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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
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MIPs of Base Image
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Abnormal MIP with AVM
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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
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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
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T2 & EPI Images: Stroke?
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Different Amp Diffusion Gradient: Ischemic Stroke?
Amp = 0
Amp = 500
Amp = 1000
Stroke reduces diffusion
Bright on diffusion weighted image
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Diffusion Co-efficient Map & Images
Diffusion image
Intensity α 1/Diffusion (& T2)
Diffusion co-efficient map
Intensity α Diffusion Co-efficient
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Anisotropic Diffusion
Diffusion gradient Diffusion gradient
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Anisotropic Diffusion: Diffusion tensor imaging
• Anisotropic diffusion in white matter tracks
• Apply diffusion gradients in 12-15 direction
• ‘Track’ white matter track direction by diffusion anisotropy
Brainimaging.waisman.wisc.edu
www.cimst.ethz.ch