Ultrashort Echo Time Imaging

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

9. Readout Duration

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

11. 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

12. 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.

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

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

15. 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

16. 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

17. Long T2 Suppression

18. Long T2 Suppression

19. 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

20. . . . • - - 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

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

22. 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

23. 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

24. TE = 80 us TE = 6 ms Examples

25. Examples

26. Examples