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					Ultrashort Echo Time Imaging



            02/02/07
                        Short T2 Imaging




Solids, Proteins, Macromolecules…
                          Short T2 Imaging

Signal Increase in Short T2 Components
1. Shortening T1 of short T2 components
2. Shortening T2s of long T2 components
3. Addition of new short T2 tissue

Examples: calcification, hemorrhage, iron deposition, malignant melanoma


Signal Decrease in Short T2 Components
1. Increasing T1 or T2 of tissue
2. Loss of order in highly structured tissue (e.g. collagen)

Examples: edema, inflammation, infection, infarction, tumors
Problems with imaging short T2 tissues



1. Echo Time

2. RF pulses

3. Readout duration

4. Off-resonance saturation effects
                      Echo Time


Long T2 majority                  Short T2 majority


             . . . Short T2               . . . Short T2
             - - - Long T2                - - - Long T2




      Time                            Time
RF Excitation
Readout Duration
Line-width of short T2 components




• FATSAT pulses
• MT pulses
• Multi-slice and multi-echo imaging
                   Short TE Imaging
     Short T2 majority              Long T2 majority


                . . . Short T2               . . . Short T2
                - - - Long T2                - - - Long T2




         Time                              Time



1. Magic Angle Imaging
2. Basic UTE Imaging with no T2-contrast
3. Long-T2 suppression
                 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.
         Basic UTE Imaging Sequence




                  1         2




• Half Fourier excitation
• PR acquisitions
       Basic UTE Imaging Sequence




• No T2-contrast

• Problems when majority of components are long T2

• Need long T2 suppression
              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
             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
Long T2 Suppression
Long T2 Suppression
              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
              Long T2 Suppression

                                     . . . Short T2
             1st acq.
                                     - - - Long T2
                   2nd acq.




                              Time

• Acquire images at two different TEs

Problems:
• B0 Inhomogeneity, susceptibility
• Subtraction adds noise to short T2 data
              Long T2 Suppression


                    Patellar tendon




TE = 100 us          TE = 2.3ms       Subtraction
                   SPI and SPRITE
             SPI                          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
                 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
             Examples




TE = 80 us        TE = 6 ms
Examples
Examples

				
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