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Magnetic Resonance Imaging Magnetic field gradients.

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Presentation on theme: "Magnetic Resonance Imaging Magnetic field gradients."— Presentation transcript:

1

2 Magnetic Resonance Imaging

3 Magnetic field gradients

4 Gradient echo

5 Gradient Amplitude Time M xy T 2 * envelope Equal areas

6 Maxwell Pair Net B field from coils Local fields Current paths B o a3 a Maxwell Pair for dz dB z

7 Straight wires B o y z Increases B o Reduces B o Straight wires for dy dB z

8 Conventional Waveform Minimum slew rate waveforms Gradient field: Acoustic noise One of the biggest challenges at high field Acoustic noise –Ear defenders and foam to reduce bone conduction –Gradient system decoupled from bed (and room) –Modification of gradient waveforms –Novel gradient coil designs

9 Gradients can be combined to produce gradients in arbitrary directions

10 Slice selection using gradients Sample x x1x1 x2x2  (x 1 )  (x 2 ) BzBz

11 Slice selection

12 Imaging in gradients Sample x Frequency Signal (spectrum) x1x1 x2x2 B(x 1 ) B(x 2 ) BzBz 11 22 PROJECTION

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14 Projection Reconstruction

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16 Basic reconstruction X-ray beam is attenuated forming a projection

17 Projections are collected from various directions

18 The projection tells you that the beam was attenuated with equal probability anywhere along its path

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21 Projection Reconstruction Diffusion imaging Radial Spin Echo and TSE Scan PR Low sensitivity to motion Low sensitivity to susceptibility artifacts

22 Projection Reconstruction Sodium Imaging Human Skeletal Muscle: Sodium MR Imaging and Quantification- Potential Applications in Exercise and Disease Constantinides, Gillen, Boada, Pomper, Bottomley, Radiology

23 K-space Image

24 K space Units: rad/ m Increasing spatial frequency

25 Projection Reconstruction

26 With MRI we can control how we sample k-space Time RF G slice G phase G read kyky kxkx Spin-warp pulse sequence Spin-warp k-space trajectory A B C ABC n n Signal

27 Phase encoding Phase encoding Readout gradient Initial phase during readout depends on phase encoding amplitude

28 Phase encoding Initial phase during readout depends on phase encoding amplitude Phase encoding Readout gradient

29 With MRI we can control how we sample k-space Time RF G slice G phase G read kyky kxkx Spin-warp pulse sequence Spin-warp k-space trajectory A B C ABC n n Signal

30 Spin warp imaging Wicklow, Washington University, St. Louis

31 Roger Ordidge, UCL

32 Ultrashort TE- Tendons

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34 Motion artefacts Steve Keevil UMDS Anesthetic

35 EPI

36 Diffusion EPI

37 Diffusion

38 EPI- fetus Fetal Brain Placenta Fetal Lung Fetal Liver

39 EPI- gut

40 EPI - fMRI Ludman, Mason, O’Donoghue, Morris, Nottingham Cochlear Stimulation


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