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 & 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

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

Slice selection gradient

ωo+ω ωo ωo - ω

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 ContrastTR=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

90-----Signal-------------90-----Signal-----------90-----Signal

TRTR

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

90-----Signal-------------90-----Signal-----------90-----Signal

TRTR

TE TE TE

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

T1/T2 Weighted Image

TR = 562ms

TE = 20ms

TR = 4000ms

TE = 132ms

T1/T2 WeightedTR=525ms TE=15ms TR=2500ms TE=85ms

Proton Density/T2

TR = 3070ms

TE = 15ms

TR = 3070ms

TE = 92ms

Proton Density/T2

TR = 3070ms

TE = 15ms

TR = 3070ms

TE = 92ms

Lumbar Spine ImagesDisc protrusion L5/S1. Degenerative changes bone.

Axial Images of L Spine

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

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

Signal = MoCosα

Mz

T1 Breath-hold Images 14 slices in 23s breath-hold (t1_fl2d_tra_bh)

TR=16.6ms, TE=6ms α=70o

T2 breath-hold images19 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

www.icr.chmcc.org

Right motor cortex excited with left finger tapping, in close proximity with tumour

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

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

Amp = 500

Amp = 1000

Stroke reduces diffusion

Bright on diffusion weighted image

Diffusion Co-efficient Map & Images

Diffusion image

Intensity α 1/Diffusion (& T2)

Diffusion co-efficient map

Intensity α Diffusion Co-efficient

Anisotropic Diffusion

Diffusion gradient Diffusion gradient

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