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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 Receiver Coils

5 ‘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

6 MRI in Cov & Warwickshire YearNo of scannersField Strength 198710.5/1.0T 199711.0T 200121.0T, 1.5T 200232x1.5T, 1.0T 200665x1.5T, 1.0T

7 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)

8 Open 0.35T Ovation

9 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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11 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

12 Any Plane TR=498ms TE=12ms TR=2743ms TE=96ms

13 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

14 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

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

16 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

17 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 90-----Signal-------------90-----Signal-----------90-----Signal TR TE

18 Bloch Equation Bloch Equations BETWEEN 90 o RF pulses Signal=Mo[1-exp(-TR/T1)] exp(-TE/T2) TR { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/14/4261348/slides/slide_18.jpg", "name": "Bloch Equation Bloch Equations BETWEEN 90 o RF pulses Signal=Mo[1-exp(-TR/T1)] exp(-TE/T2) TR

19 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

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

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

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

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

24 Imaging Sequence: (Spin Warp) RF Slice Selection Gradient Frequency Encoding Gradient Signal time Phase Encoding Gradient

25 K-Space Phase Encoding kxkx kyky

26 K-Space to Real Space kxkx kyky 2D FT

27 K-Space to Real Space

28 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

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

32 Echo Planar Imaging Phase Encoding kxkx kyky Frequency encoding

33 EPI Imaging Each slice acquired in 10s of mili-seconds Lower resolution More artefacts www.ph.surrey.ac.uk

34 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

35 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

36 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

37 T2 & EPI Images: Stroke?

38 Different Amp Diffusion Gradient: Stroke? Amp = 0 Amp = 500 Amp = 1000

39 Diffusion Direction Diffusion gradient

40 Diffusion Co-efficient Map : T2 T2 weighted image Intensity α T2 Diffusion weighted image Intensity α Diffusion

41 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 kyky kxkx

42 Propeller Imaging www.gemedical.com Spin warp FSE Propeller

43 Propeller Gradients? kxkx kyky

44 Inversion Recovery Sequence This sequence has a 180 o RF pulse which inverts all the magnetization before the standard 90 o pulse and signal detection TI = Inversion time = Time between 180 o and 90 o pulses 180----90--Signal----------180----90--Signal TI TR TE TI

45 Inversion Recovery Due to inverting 180 o 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

46 Inversion Recovery y x B0B0 Brain CSF Fat y x y x 180 o inversion pulse

47 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

48 STIR y x B0B0 Invert Magnetisation Brain CSF Fat y x y x At Time TI Fat at zero Flip 90 degrees Image with short TE

49 STIR Image TI=130ms TR=4 450ms TE=29ms

50 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

51 FLAIR y x B0B0 Invert Magnetisation Brain CSF Fat y x y x At Time TI No Water Signal Flip 90 degrees Image with long TE

52 FLAIR TI=2500ms TR=9000ms TE=105ms TR=2743ms TE=96ms

53 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 = M o Cosα Mz

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

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

56 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

57 Breast Tumour Imaging

58 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

59 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

60 MIPs of Base Image

61 Abnormal MIP with AVM


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