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 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 Resonance 78(1): Hoult, D. I. and B. Tomanek (2002). "Use of mutually inductive coupling in. probe design." Concepts in Magnetic Resonance 15(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. DecoupledNot decoupled

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

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

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)

Inductively coupled coil with active detuning 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 off Pin diode on

Inductively coupled coil with Helmholtz pair transmit With detuningWithout detuning -26 dB -51 dB S 21, transmit coil on port 1, receive coil on port 2 Spin echo images of water- filled phantom

Inductively coupled coil with birdcage transmit coil Without detuningWith detuning -15 dB -49 dB S 21, transmit coil on port 1, receive coil on port 2 Spin echo images of water- filled phantom

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

Human finger joint images 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

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

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.

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

Geometric decoupling Well aligned1° offset2° offset -45 dB-37 dB-31 dB

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