# Magnetic Resonance Imaging

## Presentation on theme: "Magnetic Resonance Imaging"— Presentation transcript:

Magnetic Resonance Imaging
Dr Sarah Wayte University Hospital of Coventry & Warwickshire

GE MR Scanner

‘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 1.0T 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 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 Signal Signal 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 Signal Signal 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 TE = 20ms TR = 4000ms TE = 132ms

T1/T2 Weighted TR=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 Images Disc protrusion L5/S1. Degenerative changes bone.
L5/S1 slight bulge, no harm to theca or nerves.

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

Imaging time = 3.5 x 256 x 2/21 ~ 1min 25s
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? images in a breath-hold (30 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

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

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

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 Flip TR

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?
Stroke reduces diffusion Bright on diffusion weighted image Amp = 0 Amp = 500 Amp = 1000

Diffusion Co-efficient Map & Images
Diffusion image Intensity α 1/Diffusion (& T2) Intensity α Diffusion Co-efficient

Anisotropic Diffusion