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FMRI acquisition

FMRI acquisition. Richard Wise FMRI Director wiserg@cardiff.ac.uk +44(0)20 2087 0358. Why do we need the magnet?. d. Inside an MRI Scanner. z gradient coil. r.f. transmit/receive. x gradient coil. super conducting magnet. subject. gradient coils. Common NMR Active Nuclei.

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FMRI acquisition

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  1. FMRI acquisition Richard Wise FMRI Director wiserg@cardiff.ac.uk +44(0)20 2087 0358

  2. Why do we need the magnet? d

  3. Inside an MRI Scanner z gradient coil r.f. transmit/receive x gradient coil super conducting magnet subject gradient coils

  4. Common NMR Active Nuclei Isotope Spin % g I abundance MHz/T 1H 1/2 99.985 42.575 2H 1 0.015 6.53 13C 1/2 1.108 10.71 14N 1 99.63 3.078 15N 1/2 0.37 4.32 17O 5/2 0.037 5.77 19F 1/2 100 40.08 23Na 3/2 100 11.27 31P 1/2 100 17.25

  5. Nuclear Spin M magnetic moment M=0 spin If a nucleus has an unpaired proton it will have spin and it will have a net magnetic moment or field

  6. Resonance • If a system that has an intrinsic frequency (such as a bell or a swing) can draw energy from another system which is oscillating at the same frequency, the 2 systems are said to resonate

  7. Spin Transitions High energy Low energy

  8. The Larmor Frequency ω = γ B Frequency  Field strength 128 MHz at 3 Tesla

  9. Tissue magnetization B0 M 90º RF excitation pulse

  10. Tissue magnetization B0 M 90º RF excitation pulse MR signal ω = γ B

  11. Tissue magnetization B0 M 90º RF excitation pulse . MR signal ω = γ B

  12. Tissue magnetization B0 90º RF excitation pulse MR signal ω = γ B signal Signal decay: time constant T2 time

  13. Tissue contrast: TE &T2 decay EchoAmplitude Long T2 (CSF) Medium T2 (grey matter) Contrast Short T2(white matter) TE

  14. T2 Weighted Image

  15. T2 Weighted Image T2/ms CSF 500 8090 grey matter 7080 white matter 1.5T SE, TR=4000ms, TE=100ms SE, TR=4000ms, TE=100ms

  16. Tissue magnetization B0 M M Magnetization recovery: time constant T1 time

  17. Tissue magnetization B0 M M Magnetization recovery: time constant T1 time

  18. Tissue contrast: TR & T1 recovery Short T1 (white matter) Mz Medium T1 (grey matter) Long T1 (CSF) Contrast TR

  19. T1 Weighted Image SPGR, TR=14ms, TE=5ms, flip=20º

  20. T1 Weighted Image T1/s R1/s-1 0.7 1.43 white matter 1 1 grey matter CSF 4 0.25 1.5T SPGR, TR=14ms, TE=5ms, flip=20º SPGR, TR=14ms, TE=5ms, flip=20º

  21. Long TR Short TR T1 PD Short TE Long TE T2

  22. From Frequencies to Images • Vary the field by position • Decode the frequencies to give spatial information

  23. Gradient coils z gradient coil r.f. transmit/receive x gradient coil super conducting magnet subject gradient coils

  24. Image formation Fourier Transform time frequency Signal Spectrum

  25. n n 2 x 2 The Fourier Transform FFT

  26. Slice selection RF excitation ω = γ B time frequency 0 G

  27. (Gradient echo) Pulse sequence

  28. The Pulse Sequence Controls • Slice location • Slice orientation • Slice thickness • Number of slices • Image resolution • Field of view (FOV) • Image matrix • Echo-planar imaging • Image contrast • TE, TR, flip angle, diffusion etc • Image artifact correction • Saturation, flow compensation, fat suppresion etc

  29. T2* : pleasure …..

  30. T2* : ….. and pain

  31. T2* contrast

  32. T2* contrast • Field variation across the sample • Decay of summed NMR signal

  33. GE-EPI is T2* weighted

  34. Wilson et al Neuroimage 2003

  35. Neural activity Signalling Vascular response BOLD signal Vascular tone (reactivity) Autoregulation Blood flow, oxygenation and volume Synaptic signalling arteriole B0 field glia Metabolic signalling venule Neural activity to FMRI signal

  36. FMRI and electrophysiology Logothetis et al, Nature 2001

  37. Haemodynamic response balloon model % -1 undershoot initial dip Buxton R et al. Neuroimage 2004

  38. Blood oxygenation Bandettini and Wong. Int. J. Imaging Systems and Technology. 6:133 (1995) Bandettini and Wong. Int. J. Imaging Systems and Technology. 6:133 (1995)

  39. Rest O2 Sat 100% 80% 60% O2 O2 O2 • Active: 40% increase in CBF, 20% increase in CMRO2 • O2 Sat 100% 86% 72% CMRO2 = OEF  CBF

  40. CMRO2: CBF ratio Hoge R et al

  41. Signal evolution • Deoxy-Hb contribution to relaxation • Gradient echo R2* (1-Y) CBV Y=O2 saturation b~1.5 S = Smax . e-TE/R2* • Longer TE, more BOLD contrast but less signal and more dropout/distortion. TE=T2*

  42. Vessel density 500 m 100 m Harrison RV et al. Cerebral cortex. 2002

  43. Resolution Issues • Spatial Resolution • How close is the blood flow response to the activation site (CBF better?) • Most BOLD signal is on the venous side • EPI is “low res” • Dropout and distortion • Slice orientation • Slice thickness • Temporal Resolution

  44. Factors affecting BOLD signal? • Physiology • Cerebral blood flow (baseline and change) • Metabolic oxygen consumption • Cerebral blood volume • Equipment • Static field strength • Field homogeneity (e.g. shim dependent T2*) • Pulse sequence • Gradient vs spin echo • Echo time, repeat time • Resolution

  45. Physiological baseline • Baseline CBF, • But CBF CMRO2 unchanged (Brown et al JCBFM 2003) • BOLD response  Cohen et al JCBFM 2002

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