Magnetic Resonance Imaging

Dr Sarah Wayte

University Hospital of Coventry & Warwickshire

MRI Machine

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

2001 2 1.0T, 1.5T

2002 3 2x1.5T, 1.0T

2006 6 5x1.5T, 1.0T

Coventry ‘Super’ Hospital

Opened July 2006

• 1.5T scanner (installed 2004 moved)

• 3.0T scanner (scanning June 07?)

• 0.35T open magnet (permanent magnet weighing 17.5tonnes, scanning Sept 07?)

• (1.5T scanner in private hospital & 3 others in surrounding area)

Open 0.35T Ovation

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

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

TR=498ms TE=12ms

TR=2743ms TE=96ms

Axial Slices

• Slice selection gradient applied from head to toe

• Spins at various frequency from head to toe (fo=γBo)

• RF pulse at fo gives slice through nose (resonance)

• RF pulse at fo + f gives slice through eye

RF wave

Slice selection gradient

fo+f fo fo - f

Sagittal/Coronal Slices

• Sagittal slice apply slice selection gradient left to right

• Coronal slice apply slice selection gradient anterior to posterior

• Combination of sag & coronal can give any angle between etc

Image ContrastTR=525ms TE=15ms TR=2500ms TE=85ms

Image Contrast

• Depends on the pulse sequence timings used

• 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

Imaging Sequence: (Spin Warp)RF

Slice Selection Gradient

Frequency Encoding Gradient

Signal

time

time

time

time

timePhase Encoding Gradient

K-Space

Phase Encoding

kx

ky

K-Space to Real Space

kx

ky

2D

FT

K-Space to Real Space

Imaging Time (Spin Warp)

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, use 90_signal_signal_signal…. sequence of long TE time. Typical 21 signals per 90o pulse

• 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)

Echo Planar Imaging

Takes TSE/FSE to the extreme by acquiring 64 or 128 signals following a single 90 degree RF pulse

Image matrix size (64)2 or (128)2 (poor resolution)

Echo Planar Imaging

Phase Encoding

kx

ky

Frequency encoding

EPI Imaging• Each slice acquired in

10s of mili-seconds• Lower resolution• More artefacts

www.ph.surrey.ac.uk

EPI Imaging• Each slice acquired in 10s of ms• 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

EPI Imaging• Concentration of de-oxyhaemoglobin (longer T2* than

oxyhaemoglobin) brighter

• Subtracted image of bright ‘dots’ of activated brain

• Super-impose dot image over ‘anatomical’ MR image

www.ich.ucl.ac.uk

fMRI of candidate for epilepsy surgery.

Active area in verb generation task

Shows left-hemisphere localisation of language tasks

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: Stroke?

Amp = 0

Amp = 500

Amp = 1000

Diffusion Direction

Diffusion gradient Diffusion gradient

Diffusion Co-efficient Map : T2

T2 weighted image

Intensity α T2

Diffusion weighted image

Intensity α Diffusion

Propeller: Another method of sampling K-space

• Sampled k-space in rows so far

• Propeller samples k-space in a ‘propeller’ pattern

• Over-sampling centre k-space means in-sensitive to motion

ky

kx

Propeller Imaging

www.gemedical.com

Spin warp FSE Propeller

Propeller Gradients?

kx

ky

Inversion Recovery Sequence• This sequence has a 180o RF pulse which inverts

all the magnetization before the standard 90o pulse and signal detection

• TI = Inversion time = Time between 180o and 90o pulses

180----90--Signal----------180----90--Signal

TI

TR TE

TI

Inversion Recovery

• Due to inverting 180o pulse, magnetization is recovering from a negative value Mz(t)=Mo[1-2exp(-TI/T1)]

• With correct TI (1=2exp(-TI/T1) or TI=ln2T1) can eliminate signal from a tissue type completely

Mz

t

Fat

Brain

CSF

Inversion Recovery

y

x

B0 Brain

CSF

Fat

y

x

y

x

180o inversion pulse

STIR = Short TI Inversion Recovery

• TI= 130ms at 1.0T, so Mz of fat=O at this point> NO SIGNAL

Mz

t

Fat

Brain

CSF

TI

STIR

y

x

B0

Invert Magnetisation

Brain

CSF

Fat

y

x

y

x

At Time TI

Fat at zero

Flip 90 degrees

Image with short TE

STIR Image

TI=130ms

TR=4 450ms

TE=29ms

FLAIR

• FLAIR=Fluid Attenuated Inversion Recovery

• TI= 2500ms at 1.0T, so Mz of water=O at this point NO SIGNAL

Mz

t

TI

Fat

Brain

CSF

FLAIR

y

x

B0

Invert Magnetisation

Brain

CSF

Fat

y

x

y

x

At Time TI

No Water Signal

Flip 90 degrees

Image with long TE

FLAIRTI=2500ms TR=9000ms TE=105ms TR=2743ms TE=96ms

Even Faster Imaging

• How fast? 14-19images in a breath-hold

• 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

Dynamic Breast Tumour Imaging

• Another fast imaging technique using <90 degree pulses

1. Acquire anatomical images to locate tumour

2. Acquire at 1min or 30s intervals, 64-88 (2.0-1.5mm) images through the breast whilst injecting contrast agent

3. Draw region of interest over the tumour & look at how the contrast arrives and leaves the tumour

Breast Tumour Imaging

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

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

MIPs of Base Image

Abnormal MIP with AVM