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Principles of MRI Physics and Engineering

Principles of MRI Physics and Engineering. Allen W. Song Brain Imaging and Analysis Center Duke University. Part III.1 Some fundamental acquisition methods And their k-space view. k-Space Recap. Equations that govern k-space trajectory:. Kx = g /2 p  0 t Gx(t) dt.

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Principles of MRI Physics and Engineering

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  1. Principles of MRI Physics and Engineering Allen W. Song Brain Imaging and Analysis Center Duke University

  2. Part III.1 Some fundamental acquisition methods And their k-space view

  3. k-Space Recap Equations that govern k-space trajectory: Kx = g/2p 0tGx(t) dt Ky = g/2p 0tGx(t) dt These equations mean that the k-space coordinates are determined by the area under the gradient waveform

  4. A 2x2 Matrix k-Space (data space) Image Space S(1,1) S(1,2) S(2,1) S(2,2) I(1,1) I(1,2) I(2,1) I(2,2) S(1,1) S(1,2) S(2,1) S(2,2)

  5. Gradient Echo Imaging • Signal is generated by magnetic field refocusing mechanism only (the use of negative and positive gradient) • It reflects the uniformity of the magnetic field • Signal intensity is governed by S = So e-TE/T2* where TE is the echo time (time from excitation to the center of k-space) • Can be used to measure T2* value of the tissue

  6. MRI Pulse Sequence for Gradient Echo Imaging Excitation Slice Selection Frequency Encoding Phase Encoding digitizer on Readout

  7. K-space view of the gradient echo imaging Ky 1 2 3 . . . . . . . n Kx

  8. Spin Echo Imaging • Signal is generated by radiofrequency pulse refocusing mechanism (the use of 180o pulse ) • It doesn’t reflect the uniformity of the magnetic field • Signal intensity is governed by S = So e-TE/T2 where TE is the echo time (time from excitation to the center of k-space) • Can be used to measure T2 value of the tissue

  9. MRI Pulse Sequence for Spin Echo Imaging 180 90 Excitation Slice Selection Frequency Encoding Phase Encoding digitizer on Readout

  10. K-space view of the spin echo imaging Ky 1 2 3 . . . . . . . n Kx

  11. Inversion Recovery Imaging

  12. Time History of MR Signal S = So * (1 – 2 e –t/T1) So S = So * (1 – 2 e –t/T1’) -So

  13. Pulse Sequence for Inversion Recovery 180o RF GRE or SE Readout Gz

  14. Part III.2 Image Contrast Mechanisms

  15. The Concept of Contrast (or Weighting) • Contrast = difference in RF signals — emitted by water protons — between different tissues • T1 weighted example: gray-white contrast is possible because T1 is different between these two types of tissue

  16. MR Signal MR Signal T2 Decay T1 Recovery 1 s 50 ms

  17. Proton Density Contrast • Technique: use very long time between RF shots (large TR) and very short delay between excitation and readout window (short TE) • Useful for anatomical reference scans • Several minutes to acquire 256256128 volume • ~1 mm resolution

  18. MR Signal MR Signal T2 Decay T1 Recovery Proton Density Contrast 1 s 50 ms

  19. Proton Density Weighted Image

  20. T2* and T2 Contrast • Technique: use large TR and intermediate TE • Useful for anatomical and functional studies • Several minutes for 256x256X128 volumes, or ~several seconds to acquire 646420 volume • 1mm resolution for anatomical scans or 4 mm resolution [better is possible with better gradient system, and a little longer time per volume]

  21. MR Signal MR Signal T2 Decay T1 Recovery T2* and T2 Contrast 1 s 50 ms

  22. T2 Weighted Image

  23. T1 Contrast • Technique: use intermediate timing between RF shots (intermediate TR) and very short TE, also use large flip angles • Useful for anatomical reference scans • Several minutes to acquire 256256128 volume • ~1 mm resolution

  24. MR Signal MR Signal T2 Decay T1 Recovery T1 Contrast 1 s 50 ms

  25. T1 Weighted Image

  26. Inversion Recovery for Extra T1 Contrast S = So * (1 – 2 e –t/T1) So S = So * (1 – 2 e –t/T1’) -So

  27. Inversion Recovery (CSF Attenuated) T2

  28. In summary, TR controls T1 weighting and TE controls T2 weighting. Short T2 tissues are dark on T2 images, but short T1 tissues are bright on T1 images.

  29. Other Imaging Methods • Can “prepare” magnetization to make readout signal sensitive to different physical properties of tissue • Flow weighting (bulk movement of blood) • Diffusion weighting (scalar or tensor) • Perfusion weighting (blood flow into capillaries) • Magnetization transfer (sensitive to proteins in voxel) • Temperature

  30. MR Angiogram • Time-of-Flight Contrast • Phase Contrast

  31. Acquisition Excitation Saturation No Flow Medium Flow High Flow No Signal Medium Signal High Signal Vessel Vessel Vessel Time-of-Flight Contrast

  32. Time to allow fresh flow enter the slice 90o 90o RF Excitation Gx Saturation Image Acquisition Gy Gz Pulse Sequence: Time-of-Flight Contrast

  33. Blood Flow v Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G T 2T 0 Time Phase Contrast (Velocity Encoding)

  34. 90o RF Excitation G Gx Phase Image Acquisition -G Gy Gz Pulse Sequence: Phase Contrast

  35. MR Angiogram

  36. Diffusion Weighted Imaging Sequences Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G T 2T 0 Time

  37. Excitation 90o RF G -G Gx Image Acquisition Gy Gz Pulse Sequence: Gradient-Echo Diffusion Weighting

  38. Pulse Sequence: Spin-Echo Diffusion Weighting 180o 90o RF G G Excitation Gx Image Acquisition Gy Gz

  39. Advantages of DWI • The absolute magnitude of the diffusion • coefficient can help determine proton pools • with different mobility • 2. The diffusion direction can indicate fiber tracks

  40. Diffusion Anisotropy

  41. Determination of fMRI Using the Directionality of Diffusion Tensor

  42. Display of Diffusion Tensor Using Ellipsoids

  43. Diffusion Contrast

  44. Perfusion/Flow Weighted Arterial Spin Labeling Coil Tagging Imaging Plane Transmission

  45. Perfusion/Flow Weighted Arterial Spin Labeling with Pulse Sequences Pulse Tagging Imaging Plane Alternating Inversion Alternating Inversion EPISTAR EPI Signal Targeting with Alternating Radiofrequency FAIR Flow-sensitive Alternating IR

  46. Pulse Sequence: Perfusion Imaging 180o 180o 90o RF Gx Image Gy Alternating Proximal Inversion Odd Scan Even Scan Gz 90o 180o 180o RF Gx Image Gy Odd Scan Alternating opposite Distal Inversion Gz Even Scan

  47. Advantages of ASL Perfusion Imaging • It can non-invasively image and quantify • blood delivery • Combined with proper diffusion weighting, • it can assess capillary perfusion

  48. Perfusion Contrast

  49. Diffusion and Perfusion Contrast Perfusion Diffusion

  50. Other Interesting Types of Contrast • Perfusion weighting: sensitive to capillary flow • Diffusion weighting: sensitive to diffusivity of H2O • Very useful in detecting stroke damage • Directional sensitivity can be used to map white matter tracts • Also useful in functional MRI to determine the signal origin • Flow weighting: used to image blood vessels (MR angiography) • Magnetization transfer: provides indirect information about H nuclei that aren’t in H2O (mostly proteins)

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