Chapter 3

1. Chapter 3 Principles of nuclear magnetic resonance and MRI Review

2. Basic Physics of MRI • Nuclei line up with magnetic moments either in a parallel or anti-parallel configuration. • In body tissues more line up in parallel creating a small additional magnetization M in the direction of B0. Nuclei spin axis not parallel to B0 field direction. Nuclear magnetic moments precess about B0.

3. Basic Physics of MRI • Frequency of precession of magnetic moments given by Larmor relationship f = g x B0 f = Larmor frequency (mHz) g = Gyromagnetic ratio (mHz/Tesla) B0 = Magnetic field strength (Tesla) g~ 43 mHz/Tesla Larmor frequencies of RICs MRIs 3T ~ 130 mHZ 7T ~ 300 mHz 11.7T ~ 500 mHz

4. Basic Physics of MRI NMRable Nuclei • Body 1H content is high due to water (>67%) • Hydrogen protons in mobile water are primary source of signals in fMRI and aMRI

5. Basic Physics of MRI • M is parallel to B0 since transverse components of magnetic moments are randomly oriented. • The difference between the numbers of protons in the parallel (up here) and anti-parallel states leads to the net magnetization (M). • Proton density relates to the number of parallel states per unit volume. • Signal producing capability depends on proton density. B0

6. Basic Physics of MRI Frequency of rotation of M about B1 determined by the magnitude (strength) of B1. RF pulse duration and strength determine flip angle Basic RF Pulse Concepts RF Pulse strength duration

7. Basic Physics of MRI FID = Free Induction Decay • 90° RF pulse rotates M into transverse (x-y) plane • Rotation of M within transverse plane induces signal in receiver coil at Larmor frequency. • Magnitude signal dependent on proton density and Mxy. FID magnitude decays in an exponential manner with a time constant T2. Decay due to spin-spin relaxation.

8. Need for 180° Pulse - Spin Echo • FID also diminishes due to local static magnetic field inhomogeneity • Some spins precess faster and some slower than those due to B0 90° 0 180° time TE/2- TE/2+ • 180 ° RF pulse reverses dephasing at TE (echo time) • Residual decay due to T2 Spin Echo Signal TE

9. Nuclear Magnetic Resonance (NMR) Signal: Spin Echo (SE) TR (repetition time) = time between RF excitation pulses 90o 90o 180o FID Spin Echo TE/2 TE/2 TE = time from 90o pulse to center of spin echo

10. Developing Contrast Using Weighting • Contrast = difference in image values between different tissues • T1 weighted example: gray-white contrast is possible because T1 differs between these two types of tissue

11. T1 and T2 • T1-Relaxation: Recovery • Recovery of longitudinal orientation of M along z-axis. • ‘T1 time’ refers to time interval for 63% recovery of longitudinal magnetization. • Spin-Lattice interactions. • T2-Relaxation: Dephasing • Loss of transverse magnetization Mxy. • ‘T2 time’ refers to time interval for 37% loss of original transverse magnetization. • Spin-spin interactions,and more.

12. Tissue T1 (ms) T2 (ms) Grey Matter (GM) 950 100 White Matter (WM) 600 80 Muscle 900 50 Cerebrospinal Fluid (CSF) 4500 2200 Fat 250 60 Blood 1200 100-200 Properties of Body Tissues T1 values for B0 ~ 1Tesla. T2 ~ 1/10th T1 for soft tissues

13. Basic Physics of MRI: T1 and T2 T1 is shorter in fat (large molecules) and longer in CSF (small molecules). T1 contrast is higher for lower TRs. T2 is shorter in fat and longer in CSF. Signal contrast increased with TE. • TR determines T1 contrast • TE determines T2 contrast.

14. Contrast, Imaging Parameters T1W T2W r - proton density SE – spin echo imaging GRE – gradient echo imaging Short TEs reduce T2W Long TRs reduce T1W

15. Making an Image k-space (frequency domain)A k-space domain image is formed using frequency and phase encoding

16. k-space Image space ky y kx x Acquired Data Final Image Two Spaces FT-1 FT MRI task is to acquire k-space image then transform to a spatial-domain image. kx is sampled (read out) in real time to give N samples. ky is adjusted before each readout. MR image is the magnitude of the Fourier transform of the k-space image

17. The k-space Trajectory Equations that govern 2D k-space trajectory kx = g 0tGx(t) dt if Gx is constant kx = gGxt ky = g 0t’Gy(t) dt if Gy is constant ky = gGyt’ The kx, ky frequency coordinates are established by durations (t) and strength of gradients (G).

18. Simple MRI Frequency Encoding: RF Excitation Slice Selection (Gz) Frequency Encoding (Gx) digitizer on Readout Exercise drawing k-space manipulation

19. The k-space Trajectory Frequency Encoding Gradient (Gx) Move to left side of k-space. (0,0) ky Digitizer records N samples along kx where ky = 0 kx

20. Simple MRI Frequency Encoding: Spin Echo Excitation Slice Selection Frequency Encoding (Gx) digitizer on Readout Exercise drawing k-space representation

21. The K-space Trajectory 180 pulse Digitizer records N samples of kx where ky = 0

22. 90 180 Excite Slice Select Frequency Encode Phase Encode digitizer on Readout Frequency and Phase Encoding for 2D Spin Echo Imaging kx ky

23. The 2D K-space Trajectory 180 pulse Digitizer records N samples of kx and N samples of ky

24. Gradient Echo Imaging • Signal is generated by magnetic field refocusing mechanism only (the use of negative and positive gradient) • Signal intensity is governed by S = So e-TE/T2* • Can be used to measure T2* value of the tissue • R2* = R2 + R2ih +R2ph (R2=1/T2) • Used in 3D and BOLD fMRI

25. MRI Pulse Sequence for Gradient Echo Imaging E. Excitation Slice Selection Frequency Encoding Phase Encoding digitizer on Readout Ernst angle (E) for optimum SNR .

26. FLASH Pulse Sequence TR2 TR1 B1 refocus Gz TRN Gy crusher crusher Gx acquire TRN/2 TRN TRN/2 B1 Gz Gy TR2 crusher crusher Gx TR1 2D Gradient Echo RF (10-15 degrees) Short TR (10-50 msec) N= 256 (2.5-13 sec per slice) Fig. 3.19. Courtesy of Peter Jezzard.

27. 3D Sequence (Gradient Echo) acq kz Gx read Gy phase Select & phase Gz ky kx B1 RF Scan time = NyNzTR Good for high resolution T1W images of brain

28. 3D T1W brain image 0.8mm spacing Time = 25 min

29. 2D Echo Planar Imaging (EPI) a) b) B1 refocus Gz Gy Gx acquire 2d Gradient Echo Entire 2D slice within one TR 64x64 or 128x128 Time per slice (30-50 msec) Whole volume (2-4 sec) Good for fMRI studies Fig. 3.20. Courtesy of Peter Jezzard.

30. FLASH Image T2* Weighted TE = 30 msec CSF is bright Signal loss and distortions due to local differences in magnetic field Sources of Contrast in Brain - Endogenous - BOLD - Exogenous - could be contrast agent (Gd based) - Other - Susceptibility R2* = net T2 relaxation rate = 1/T2* R2* = R2tis + R2ih + R2BOLD + R2suc Fig. 3.23 courtesy of Peter Jezzard.

31. Chapter 8 Quantitative Measurements Using fMRI Review

32. T2* fMRI Signal

33. 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 From Neural Activity to fMRI Signal End bouton dendrite Complex relationship between change in neural activity and change in blood flow (CBF), oxygen consumption (CMRO2) and volume (CBV).

34. positive BOLD response 3 initial dip post stimulus undershoot 2 overshoot 1 Deoxyhemoglobin BOLD response, % 0 time BOLD signal stimulus fMRI Bold Response Model • Initial dip 0.5-1sec • Overshoot peak 5-8 sec • + BOLD response 2-3% • Final undershoot variable Figure 8.1. from textbook.

35. Graded BOLD Response • Graded change in signal for a) BOLD and b) perfusion (CBF). • 3 minute visual pattern stimulation with different luminance levels. • Note max BOLD change of 2-3 % and max CBF change of 40-50 %. Figure 8.2. from textbook. N=12 subjects.

36. Perfusion vs. Volume Change • 30 second stimulation • 3-second intervals • DCBF rapid • DCBV slow BOLD volume assessed using exogenous tracer that remains in blood. In rat experiments TC for DCBV similar to that for BOLD overshoot. Mandeville et al., 1999 Figure 8.4. from textbook.

37. 511 keV + 511 keV Measurement of Cerebral Blood Flowwith PET or MRI (Arterial Spin Labeling - ASL) PET Method • Uses magnetically labeled arterial blood water as an endogenous flow tracer • Potentially provide quantifiable CBF in classical units (mL/min per 100 gm of tissue) O-15 H20 Detre et al., 1992

38. inversion slab imaging plane Arterial Spin Labelling z (=B0) • ASL is an example of a motion contrast • IMAGEperfusion = IMAGEuninverted – IMAGEinverted • Perfusion is useful for clinical studies: how much blood is getting to a region, how long does it take to get there? excitation blood y x inversion white matter = low perfusion Gray matter = high perfusion www.fmrib.ox.ac.uk/~karla/

39. Chapter 5 Hardware for MRI Review

40. 3T Siemens Trio • 60 cm patient bore • 40 mT/m max gradient amplitude per axis • 200 T/m/sec slew rate • 2nd order active shimming • ~0.30 ppm B0 homogeneity over 40 cm sphere • self shielded • Shielding • Shims • Field Strength

41. MRI Scanner Anatomy • A helium-cooled superconducting magnet generates the static field. • Always on: only quench field in emergency. • niobium titanium wire. • Coils allow us to • Make static field homogenous (shims: solenoid coils) • Briefly adjust magnetic field (gradients: solenoid coils) • Transmit, record RF signal (RF coils: antennas)

42. Superconductor Magnet

43. 3T magnet RF Coil gradient coils (inside) Necessary Equipment Magnet Gradient Coils RF Coil

44. Magnet Shim coil Gradient coil RF coil Magnet Shielding and Shimming Iron Shielding Subject • Shims • superconducting • static • room temperature Figure 5.2 from textbook.

45. Gradient Coils Sounds generated during imaging due to mechanical stress within gradient coils.

46. Current and Gradient Pulse Shape c a • a. gradient current supplied (short rise time induces eddy currents) • b. eddy currents oppose changing field w/o compensation • c. gradient current supplied with eddy current compensation • d. potential field vs time with eddy current compensation d b Jerry Allison.

47. dB dt dB/dt Effect (more eddy currents) Peripheral Nerve Stimulation • dB/dt -- dE/dt • dt is gradient ramp time • dB/dt largest near ends of gradient coils • spatial gradient of dE/dt also important

48. dB/dt / E-Field Characteristics of Stimulation • Not dependent on B0 • Gradients - 40mT/m (larger Bmax for longer coil) • Gradient Coil Differences - strength (increases dB) and length (head vs. body determines site) • Rise Time - shorter rise time means larger shorter dt and therefore larger dB/dt • Other • Disruption of nearby medical electronic devices • Subject Instructions • Don’t clasp hands - closed circuit, lower threshold • Report tingling, muscle twitching, painful sensations

49. MRI Scanner Components

50. Exciter XMTR Synthesizer T/R RF switch RCVR Preamp Coil Network Host A/P RAM Amps Gx, Gy, Gz Shim coils Shim Pulse driver Synthesizer, A/P programmer XMTR, RCVR, T/R Gradient coils Schematic of MRI System Same or different transmit and receive coil. A/P - Array Processor RF, Shim, Gradient Coils inside magnet All but Host, RAM, and A/P in equipment room Figure 5.1b from textbook.