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Magnetic Resonance Imaging

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

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


3 Receiver Coils

4 ‘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

5 MRI in Cov & Warwickshire
Year No of scanners Field Strength 1987 1 0.5/1.0T 1997 1.0T 2001 2 1.0T, 1.5T 2002 3 2x1.5T, 1.0T 2006 6 5x1.5T, 1.0T

6 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)

7 Open 0.35T Ovation

8 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


10 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

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

12 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 fo+f fo fo - f Slice selection gradient

13 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

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

15 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

16 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

17 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

18 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

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

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

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

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

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

24 K-Space ky Phase Encoding kx

25 K-Space to Real Space ky 2D kx FT

26 K-Space to Real Space

27 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

28 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

29 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)

30 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)

31 Echo Planar Imaging ky Phase Encoding kx Frequency encoding

32 EPI Imaging Each slice acquired in 10s of mili-seconds
Lower resolution More artefacts

33 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

34 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 fMRI of candidate for epilepsy surgery. Active area in verb generation task Shows left-hemisphere localisation of language tasks

35 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

36 T2 & EPI Images: Stroke?

37 Different Amp Diffusion Gradient: Stroke?

38 Diffusion Direction Diffusion gradient Diffusion gradient

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

40 Propeller: Another method of sampling K-space
ky 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 kx

41 Propeller Imaging Propeller Spin warp FSE

42 Propeller Gradients? ky kx

43 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 TI TI Signal Signal TR TE

44 Inversion Recovery Mz t
Fat Brain CSF t 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

45 Inversion Recovery B0 Brain CSF Fat y y y x x x 180o inversion pulse

46 STIR = Short TI Inversion Recovery
Mz Fat Brain CSF t TI TI= 130ms at 1.0T, so Mz of fat=O at this point> NO SIGNAL

47 STIR Flip 90 degrees Image with short TE Invert Magnetisation
B0 Brain CSF Fat y y y x x x Flip 90 degrees Image with short TE Invert Magnetisation At Time TI Fat at zero

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

49 FLAIR Mz t FLAIR=Fluid Attenuated Inversion Recovery
Fat Brain CSF t TI FLAIR=Fluid Attenuated Inversion Recovery TI= 2500ms at 1.0T, so Mz of water=O at this point NO SIGNAL

50 FLAIR Flip 90 degrees Image with long TE Invert Magnetisation
B0 Brain CSF Fat y y y x x x Flip 90 degrees Image with long TE Invert Magnetisation At Time TI No Water Signal

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

52 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 Mz Signal = MoCosα

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

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

55 Dynamic Breast Tumour Imaging
Another fast imaging technique using <90 degree pulses Acquire anatomical images to locate tumour Acquire at 1min or 30s intervals, ( mm) images through the breast whilst injecting contrast agent Draw region of interest over the tumour & look at how the contrast arrives and leaves the tumour

56 Breast Tumour Imaging

57 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

58 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

59 MIPs of Base Image

60 Abnormal MIP with AVM

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