In vivo MRI of Fast Relaxing Spins Using a Swept Radiofrequency

1. In vivo MRI of Fast Relaxing Spins Using a Swept Radiofrequency Djaudat Idiyatullin, Curt Corum, Jang-Yeon Park, Michael Garwood Center for Magnetic Resonance Research, Cancer Center, and Department of Radiology, University of Minnesota

2. What means the “fast relaxing spins”? Range of relaxation times (T2):10-3 – 10-5 s Dark zone for regular imaging sequences (gradient echo) Imaging of fast relaxing spins T2 ~ t 90 Echo , slice selection → impractical Excitation → acquisition (FID)

3. Imaging of fast relaxing spins BLAST (Back-projection Low Angle Shot Technique) Sensitivity, contrast: low Problems: first points in k-space, distortion the excitation profile in image space 1 G acq

4. Imaging of fast relaxing spins BLAST (Back-projection Low Angle Shot Technique) Sensitivity, contrast: low Problems: first points in k-space, distortion the excitation profile in image space 1 G acq Amplitude and frequency modulated pulses Hyperbolic secant (HSn) pulses low peak power to excite a large bandwidth, flat excitation profile 1 RF-c G

5. 1 RF-c G acq Imaging of fast relaxing spins BLAST (Back-projection Low Angle Shot Technique) Sensitivity, contrast: low Problems: first points in k-space, distortion the excitation profile in image space 1 G acq Amplitude and frequency modulated pulses Hyperbolic secant (HSn) pulses low peak power to excite a large bandwidth, flat excitation profile Interleaved excitation and sampling Sensitive to spins with a very short T2 energy of the signal distributed in time Problem: it is not a regular FID spins + sweep excitation (< 90o) → linear system

6. Excitation, x(t) Spin system, h(t) Response FT FT Conjugate multiplication System spectrum Correlation method for linear system

7. Excitation, x(t) Spin system, h(t) Response FT FT Conjugate multiplication System spectrum Correlation method for linear system Simulated data for HS4 pulse

8. SWeep Imaging with Fourier Transform (SWIFT) HSn pulses Flip angle < 90 degree TR ~ Tp Bw=sw=2πN/Tp Projection reconstruction

9. First in vivo SWIFT 3D image Slices of 3D image of the feet sw = 20kHz128 x 64 x64 4T

10. Sensitivity to short T2 MIP of 3D images of empty 16-element TEM head coilsw = 32kHz128x128 x 644T

11. Sensitivity to short T2 MIP of 3D images of plastic toy in the breast coil sw = 39kHz128x128 x 128D=25cm4TThe breast coil’s building material has T2 ~ 0.3 ms.

12. Selected slices of 3D images of a normal mandible and surrounding areas in a 48-year-old man (4T). SWIFT sw = 62 kHz, 256 x 128 x 64 Gradient-echo sw = 80 kHz, TE = 3ms 256 x 256 x 64

13. demineralization plaque pulp dentin root cementum Direct MRI of the teeth 3D MRI of decayed molar tooth obtained with SWIFT (10 min) sw = 62 kHz, 4.7T

14. Conclusions • fast;The method avoids the delays and gradient switching, and also time for an excitation pulse (it’s combined with the acquisition period). (b) sensitive to short T2 ;any T2 > 1/sw. (c) reduced motion artifacts;Because the SWIFT method has no “echo time” it is expected to be less sensitive to motion and flow artifacts than conventional MRI methods. (d) reduced signal dynamic range;The different frequencies are excited sequentially the resulting signal is distributed in time, leading to a decreased amplitude of the acquired signal. This allows more effective utilization of the dynamic range of the digitizer. (e) quiet.The SWIFT method uses a small step when changing gradients between projections, and the fast gradient switching that creates loud noise can be avoided.

15. Fast and quiet MRI using a swept radiofrequency Djaudat Idiyatullin, Curt Corum, Jang-Yeon Park, Michael Garwood Journal of Magnetic Resonance 181 (2006), available online Acknowledgments.This research was supported by NIH grants RR008079 and CA92004. The authors would like to thank Dr. Ivan Tkac for helping with reconstruction software implementation and Dr. Jutta Ellermann for assistance in conducting the experiments