MR physics and safety for fMRI

1. MR physics and safety for fMRI Massachusetts General Hospital Athinoula A. Martinos Center Lawrence L. Wald, Ph.D.

2. Outline: Monday Oct 18 (LLW): MR signal, Gradient and spin echo Basic image contrast • Wed. Oct 20 (JJ): • Encoding the image • Monday Oct 25 (LLW): • Fast imaging for fMRI, artifacts • Wed. Oct 25 (LLW): • fMRI BOLD contrast • Mon. Nov. 1 (JJ): • Review, plus other contrast mechanisms for fMRI (CBV, CBF)

3. Fast MR ImagingTechniques • Why, introduction • How: Review of k-space trajectories • Different techniques (EPI, Spiral) • Problems from B0 Susceptibility artifacts

4. Why fast imaging Capture time course, (e.g. hemodynamic) eliminate artifact from motion (during encode.)

5. Magnetization vector durning MR encode RF time Voltage (Signal) Mz time

6. Review of Image encoding,journey through kspace Two questions: 1) What does blipping on a gradient do to the water magnetization. 2) Why does measuring the signal amplitude after a blip tell you info about the spatial frequency composition of the image (k-space).

7. Aside: Magnetic field gradient Bo Gx x Bo + Gx x Field from gradient coils Uniform magnet Total field z x

8. Step two: encode spatial info. in-plane Bo along z y “Frequency encoding” x BTOT = Bo + Gz x v = gBTOT = g(Bo + Gx x) Bo B Field (w/ x gradient) x Signal u uo Freq. with gradient without gradient

9. y y2 z y1 How does blipping on a grad. encodespatial info? Bo  Gy all y locs process at same freq. all y locs process at same freq. Bo B Field (w/ z gradient) y spins in forehead precess faster... y1 y2 y = BTOT = Bo yGy y = y=Bo y (Gy 

10. y y2 z y1 z 90° x y x y x y y x o How does blipping on a grad. encodespatial info? Bo y = y=Bo y (Gy  after RF After the blipped y gradient... z z z position y1 position 0 position y2

11. How does blipping on a grad. encodespatial info? y The magnetization vector in the xy plane is wound into a helix directed along y axis. Phases are ‘locked in’ once the blip is over.

12. y The bigger the gradient blip area, the tighter the helix y = y=Bo y (Gy  Gy large blip medium blip small blip

13. uniform water What have you measured? Consider 2 samples: 1 cm signal is as big as if no gradient no signal observed

14. Measurement intensity at a spatial frequency... ky 1/1.2mm = 1/Resolution 1/2.5mm 10 mm 1/5mm 1/10 mm kx

15. ky kx Fourier transform 1 / Resx FOVx = matrix * Resx 1 / FOVx

16. 1/1.2mm = 1/Resolution 1/2.5mm 1/5mm 1/10 mm kx Sample 3 points in kspace Gy t Gy t More efficient! Frequency and phase encoding are the same principle!

17. Conventional “Spin-warp” encoding ky RF t Gz “slice select” t Gy “phase enc” t kx “freq. enc” (read-out) Gx a2 t a1 S(t) t one excitation, one line of kspace...

18. Image encoding,“Journey throughkspace”The Movie…

19. ky kx Fourier transform 1 / Resx FOVx = matrix * Resx 1 / FOVx

20. Conventional “Spin-warp” encoding ky RF t Gz “slice select” t Gy “phase enc” t kx “freq. enc” (read-out) Gx a2 t a1 S(t) t one excitation, one line of kspace...

21. “Echo-planar” encoding ky RF t Gz t Gy t kx Gx etc... S(t) (no grads) T2* t T2* one excitation, many lines of kspace...

22. Bandwidth is asymmetric in EPI ky • Adjacent points in kx have short • Dt = 5 us (high bandwidth) • Adjacent points along ky are taken • with long Dt (= 500us). (low bandwidth) • The phase error (and thus distortions) are in the phase encode direction. kx

23. RF t Gz t Gy t Gx Characterization of EPI performance length of readout train for given resolution or echo spacing (esp) or freq of readout… ‘echo spacing’ (esp) esp = 500 us for whole body grads, readout length = 32 ms esp = 270us for head gradients, readout length = 17 ms

24. What is important in EPI performance? Short image encoding time. Parameters related to total encoding time: 1) echo spacing. 2) frequency of readout waveform. Key specs for achieving short encode times: 1) gradient slew rate. 2) gradient strength. 3) ability to ramp sample. Good shimming (second order shims)

25. Susceptibility in MR The good. The bad. The ugly.

26. Enemy #1 of EPI: local susceptibility gradients Bo field maps in the head

27. Enemy #1 of EPI: local susceptibility gradients Bo field maps in the head

28. What do we mean by “susceptibility”? In physics, it refers to a material’s tendency to magnetize when placed in an external field. In MR, it refers to the effects of magnetized material on the image through its local distortion of the static magnetic field Bo.

29. Ping-pong ball in water… Susceptibility effects occur near magnetically dis-similar materials Field disturbance around air surrounded by water (e.g. sinuses) Bo Field map (coronal image) 1.5T

30. Bo map in head: it’s the air tissue interface… Sagittal Bo field maps at 3T

31. Susceptibility field (in Gauss) increases w/ Bo Ping-pong ball in H20: Field maps (DTE = 5ms), black lines spaced by 0.024G (0.8ppm at 3T) 1.5T 3T 7T

32. Other Sources of Susceptibility You Should Be Aware of… Those fillings might be a problem…

33. Local susceptibility gradients: 2 effects • Local dephasing of the signal (signal loss) within a voxel, mainly from thru-plane gradients • Local geometric distortions, (voxel location improperly reconstructed) mainly from local in-plane gradients.

34. z 90° y x 1) Non-uniform Local Field Causes Local Dephasing Sagittal Bo field map at 3T 5 water protons in different parts of the voxel… z slowest fastest T = 0 T = TE

35. Local susceptibility gradients: thru-plane dephasing in grad echo EPI Bad for thick slice above frontal sinus… 3T

36. Thru-plane dephasing gets worse at longer TE 3T, TE = 21, 30, 40, 50, 60ms

37. Problem #2 Susceptibility Causes Image Distortion in EPI To encode the image, we control phase evolution as a function of position with applied gradients. Local suscept. Gradient causes unwanted phase evolution. The phase encode error builds up with time. Dq = g BlocalDt y Field near sinus y

38. Susceptibility Causes Image Distortion y Field near sinus y Conventional grad. echo, Dqa encode time a 1/BW

39. Susceptibility in EPI can give either a compression or expansion Altering the direction kspace is transversed causes either local compression or expansion. choose your poison… 3T whole body gradients

40. Susceptibility Causes Image Distortion Echoplanar Image, Dqa encode time a 1/BW z 3T head gradients Field near sinus Encode time = 34, 26, 22, 17ms

41. ky kx EPI and Spirals ky kx Gx Gx Gy Gy

42. EPISpirals Susceptibility: distortion, blurring, dephasing dephasing Eddy currents: ghosts blurring k = 0 is sampled: 1/2 through 1st Corners of kspace: yes no Gradient demands: very high pretty high

43. EPI and Spirals EPI at 3T Spirals at 3T (courtesy Stanford group)

44. Nasal Sinus B0

45. Nasal Sinus + mouth shim B0

46. Effect of Ear & Mouth Shim on EPI B0 From P. Jezzard, Oxford

47. { Reduced k-space sampling Folded images in each receiver channel Reconstruction: SENSE SMASH Folded datasets + Coil sensitivity maps With fast gradients, add parallel imaging Acquisition:

48. (iPAT) GRAPPA for EPI susceptibility 3T Trio, MRI Devices Inc. 8 channel array b=1000 DWI images iPAT (GRAPPA) = 0, 2x, 3x Fast gradients are the foundation, but EPI still suffers distortion

49. Encoding with RF… 4 fold acceleration of single shot sub-millimeter SE-EPI: 23 channel array 23 Channel array at 1.5T With and without 4x Accel. Single shot EPI, 256x256, 230mm FOV TE = 78ms

50. Extending the phased array to more channels:23 channel “Bucky” array for 1.5T