Active Detuning of Inductively Coupled Surface Coils

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Active Detuning of Inductively Coupled Surface
Coils

• Jolinda Smith
• Lewis Center for Neuroimaging
• University of Oregon

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Inductive coupling

• Tuned coil is inductively coupled to a matching
  loop
• No physical connection to tuned coil

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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) 69-76. Hoult, D. I. and B. Tomanek
(2002). "Use of mutually inductive coupling in.
probe design." Concepts in Magnetic Resonance
15(4) 262-285.
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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
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Approaches to decoupling

• Geometric decoupling
• Passive detuning with crossed diodes
• Optical detuning with photodiodes
• Active detuning with pin diodes

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

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Acknowlegments

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

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

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