1 / 17

Active Detuning of Inductively Coupled Surface Coils

Active Detuning of Inductively Coupled Surface Coils. Jolinda Smith Lewis Center for Neuroimaging University of Oregon. Inductive coupling. Tuned coil is inductively coupled to a matching loop No physical connection to tuned coil. Inductive coupling. Easy to construct Balanced

armand-dyer
Download Presentation

Active Detuning of Inductively Coupled Surface Coils

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Active Detuning of Inductively Coupled Surface Coils Jolinda Smith Lewis Center for Neuroimaging University of Oregon

  2. Inductive coupling • Tuned coil is inductively coupled to a matching loop • No physical connection to tuned coil

  3. Inductive coupling • Easy to construct • Balanced • Especially useful for implanted and cryocooled coils • Kuhns, P. L., M. J. Lizak, et al. (1988). "Inductive Coupling and Tuning in Nmr Probes - Applications." Journal of Magnetic Resonance78(1): 69-76. • Hoult, D. I. and B. Tomanek (2002). "Use of mutually inductive coupling in. probe design." Concepts in Magnetic Resonance15(4): 262-285.

  4. Need for decoupling • When using separate transmit and receive coils, they must be decoupled from each other. • Failure to do so results in nonuniform flip angles and image artifacts. Decoupled Not decoupled

  5. Approaches to decoupling • Geometric decoupling • Passive detuning with crossed diodes • Optical detuning with photodiodes • Active detuning with pin diodes

  6. Pin diode detuning • Active detuning uses pin diodes to detune the receive coil during the transmit phase. • Biasing pin diode creates resonant circuit; coupling to this circuit shifts resonance of coil L C

  7. Use a third coil • Third coil is switched into resonance by biasing on a pin diode • Coupling of detuning coil with receive coil shifts resonance peak • Analogous to pin diode detuning of capacitively coupled coils Wong W H, Rath A R, “Detunable coil assembly and method of detuning RF coil for MRI”, US Patent no. 6,552,544 (2003)

  8. Inductively coupled coil with active detuning Pin diode off Receive coil diameter = 1 inch Detuning coil diameter = 1.25 inches Pickup coil diameter = 0.75 inches Height of stack = 0.5 inches Pin diode on

  9. Inductively coupled coil with Helmholtz pair transmit Without detuning With detuning Spin echo images of water-filled phantom -51 dB S21, transmit coil on port 1, receive coil on port 2 -26 dB

  10. Inductively coupled coil with birdcage transmit coil Without detuning With detuning Spin echo images of water-filled phantom -49 dB S21, transmit coil on port 1, receive coil on port 2 -15 dB

  11. Small surface coils with birdcage transmit coil Capacitively coupled coil with active detuning Inductively coupled coil with passive detuning Inductively coupled coil with active detuning Spin echo images of water-filled phantom SNR maps

  12. Human finger joint images 1 2 1) 3D VIBE, res = 0.26 mm, sl th = 0.5 mm. 2) 3D FLASH, res = 0.20 mm, sl th = 0.3 mm. 3) TSE, res = 0.20 mm, sl th = 0.5 mm. 4) TOF3D, res = 0.20 mm, sl th = 1 mm. 3 4

  13. Ex vivo mouse brain at 3T 3D flash, in plane resolution 75 m, slice thickness 100 m, TR = 68 ms, TE = 13 ms, flip angle = 30°, 64 slices, 32 averages, total scan time = 9 hours

  14. Conclusions • Inductively coupled coils may be actively detuned by adding a third detuning coil controlled by pin diodes • These coils are easy to construct and show no loss in SNR compared to coils using other methods of decoupling.

  15. Acknowlegments • Ray Nunnally, LCNI • Scott Watrous, LCNI • Cliff Dax, TSA, University of Oregon • Felicia Katz, California Institute of Technology

  16. Geometric decoupling Well aligned 1° offset 2° offset -45 dB -37 dB -31 dB

  17. Geometric decoupling • Place receive coil orthogonal to transmit RF field • Advantages: conceptually simple, no additional components needed • Disadvantages: Alignment must be precise

More Related