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Ultrashort Echo Time Imaging

Ultrashort Echo Time Imaging. 02/02/07. Short T2 Imaging. Solids, Proteins, Macromolecules …. Signal Increase in Short T2 Components. Signal Decrease in Short T2 Components. Shortening T1 of short T2 components Shortening T2s of long T2 components Addition of new short T2 tissue

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Ultrashort Echo Time Imaging

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  1. Ultrashort Echo Time Imaging 02/02/07

  2. Short T2 Imaging Solids, Proteins,Macromolecules…

  3. Signal Increase in Short T2 Components Signal Decrease in Short T2 Components • Shortening T1 of short T2 components • Shortening T2s of long T2 components • Addition of new short T2 tissue • Examples: calcification, hemorrhage, iron deposition, malignant melanoma • Increasing T1 or T2 of tissue • Loss of order in highly structured tissue (e.g. collagen) • Examples: edema, inflammation, infection, infarction, tumors Short T2 Imaging

  4. Problems with imagingshort T2 tissues • Echo Time • RF pulses • Readout duration • Off-resonance saturation effects

  5. . . . • - - • . . . • - - Short T2 Long T2 Short T2 Long T2 Time Time Echo Time Long T2 majority Short T2 majority

  6. RF Excitation

  7. Readout Duration

  8. FATSAT pulses • MT pulses • Multi-slice and multi-echo imaging Line-width of short T2 components

  9. Short T2 majority Long T2 majority • . . . • - - • . . . • - - Short T2 Long T2 Short T2 Long T2 Time Time Short TE Imaging • Magic Angle Imaging • Basic UTE Imaging with no T2-contrast • Long-T2 suppression

  10. Magic Angle Imaging • MR signal from collagen shows directional dependence • Dipolar interactions are minimized at a certain orientation w.r.t. B0 • Not always possible to achieve the desired orientation.

  11. Basic UTE Imaging Sequence 2 1 • Half Fourier excitation • PR acquisitions

  12. Basic UTE Imaging Sequence • No T2-contrast • Problems when majority of components are long T2 • Need long T2 suppression

  13. Long T2 Suppression • Long 90 pulse followed by a dephaser • Only T2 >> T (pulse duration) are excited • Problems: • Sensitive to B0 and B1 homogeneity • No reference image

  14. Long T2 Suppression • Reduction in Mz is due to T2 and bandwidth of RF • Can also suppress fat with a dual-band pulse • Problems: • B0 and B1 inhomogeneity

  15. Long T2 Suppression

  16. Long T2 Suppression

  17. Long T2 Suppression • Inversion Recovery to suppress long T2 components • Image when the null is reached • Problems: • Hard to choose a single T1 for all long T2 comp • Signal recovery during acquisition  Long TR

  18. . . . • - - Short T2 Long T2 Long T2 Suppression 1st acq. 2nd acq. Time • Acquire images at two different TEs • Problems: • B0 Inhomogeneity, susceptibility • Subtraction adds noise to short T2 data

  19. Long T2 Suppression Patellar tendon TE = 100 us TE = 2.3ms Subtraction

  20. SPI SPRITE SPI and SPRITE • Does not encode in time • Resolution limited by grad amp. • T1 contrast with high flip angle and step duration on the order of T1

  21. Possible Artifacts • Air-tissue interfaces with short T2* • Susceptibility effects, flow-dephasing effects lead to problems with the later echo subtraction • Short T2 material in the coil and clothes may show up • Fat-water boundaries are problematic. (Later echo can be out-of-phase) • Motion artifacts • RF switching and eddy currents

  22. TE = 80 us TE = 6 ms Examples

  23. Examples

  24. Examples

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