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

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Presentation on theme: "Magnetic Resonance Imaging Dr Sarah Wayte University Hospital of Coventry & Warwickshire."— Presentation transcript:

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

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4 GE MR Scanner

5 Receiver Coils

6 ‘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

7 MRI in Cov & Warwickshire YearNo of scannersField Strength /1.0T T x1.5T, 3.0T 0.35T x1.5T, 3.0T, 1.5T extremity

8 1.5T Extremity

9 Wide Bore 1.5T

10 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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13 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

14 Any Plane

15 Magnetic field varied linearly from head to toe Hydrogen nuclei at various frequency from head to toe (ω o =γB o ) RF pulse at ω o gives slice through nose (resonance) RF pulse at ω o +  ω gives slice through eye RF wave Slice selection gradient ω o +  ω ω o ω o -  ω

16 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

17 Image Contrast TR=525ms TE=15msTR=2500ms TE=85ms

18 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

19 TR and TE To form an image have to apply a series of 90 o pulses (eg 256) and detect 256 signals TR = Repetition Time = time between 90 o RF pulses TE = Echo Time = time between 90 o pulse and signal detection Signal Signal Signal TR TE

20 Bloch Equation Bloch Equations BETWEEN 90 o RF pulses Signal=Mo[1-exp(-TR/T1)] exp(-TE/T2) TR

21 PD/T1/T2 Weighted Image PD 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

22 T1/T2 Weighted Image TR = 562ms TE = 20ms TR = 4000ms TE = 132ms

23 T1/T2 Weighted TR=525ms TE=15msTR=2500ms TE=85ms

24 Proton Density/T2 TR = 3070ms TE = 15ms TR = 3070ms TE = 92ms

25 Proton Density/T2 TR = 3070ms TE = 15ms TR = 3070ms TE = 92ms

26 Lumbar Spine Images Disc protrusion L5/S1. Degenerative changes bone.

27 Axial Images of L Spine

28 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

29 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 90 o pulse

30 Speeding Things Up 2 With 21 signals per 90 o 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)

31 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 Signal = M o Cosα Mz

32 T1 Breath-hold Images 14 slices in 23s breath-hold (t1_fl2d_tra_bh) TR=16.6ms, TE=6ms α=70 o

33 T2 breath-hold images 19 slice in 25s breath-hold (t2-trufi_tra_bh) TR=4.3ms TE=2.1ms α=80 o

34 30 Images in 20s Breath-hold

35 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)

36 EPI Imaging Each slice acquired in 10s of milliseconds Lower resolution More artefacts

37 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

38 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

39 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 FlipTR Flip

40 MIPs of Base Image

41 Abnormal MIP with AVM

42 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

43 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

44 T2 & EPI Images: Stroke?

45 Different Amp Diffusion Gradient: Ischemic Stroke? Amp = 0 Amp = 500 Amp = 1000 Stroke reduces diffusion Bright on diffusion weighted image

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

47 Anisotropic Diffusion Diffusion gradient

48 Anisotropic Diffusion: Diffusion tensor imaging Anisotropic diffusion in white matter tracks Apply diffusion gradients in direction ‘Track’ white matter track direction by diffusion anisotropy Brainimaging.waisman.wisc.edu


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