Download (Figure 1) display a varactor diode tunable

Autotuning Electronics for Varactor Tuned, Flexible
Interventional RF Coils
ROSS VENOOK, GARRY G
INTRODUCTION
†
OLD ,
BOB HU, GREIG SCOTT
Department of Electrical Engineering, Stanford University, Stanford, CA USA
† Department of Radiology, Stanford University, Stanford, CA USA
μ-CONTROLLER:
We chose an Atmel 9028515 μ-controller,
with its serial peripheral interface (SPI) and on-board analog
comparator. Standard C code was compiled by CodeVisionAVR
for use in the μ-controller.
Recent work with flexible, interventional coils has resulted in very
high local SNR [1]. The same increased coil-sample coupling that
yields SNR benefit also results in greater loading effects on coil
performance. Specifically, coil detuning that overshadows the gains
in SNR can result from modest variations in loading conditions.
This is especially true for deployable/flexible coils whose geometry
is not well controlled.
PLL SYNTHESIZER: This block is composed of a MC145170-2
PLL, a Mini-circuits POS-100 VCO, a four-pole active Butterworth
low-pass filter with a LT1677 single-supply opamp, and a LT1227
tri-stateable current feedback buffer (CFB).
Typically, interventional and intravascular receivers require tuning
and matching circuitry in order to reap the SNR benefits of tissue
proximity [2, 3]. Though the cited examples have external tuning
and matching boxes, work has been done to develop DC-bias tuning
methods using varactor diodes closer to the receiver [4, 5].
PHASE COMPARATOR:
The AD835 high speed multiplier with
an RC lowpass filter at the output performs the phase comparison.
AD9665 ultra-fast voltage comparators eliminate amplitude
sensitivity.
Clinical applicability would be increased by automation of
interventional coil tuning. This work presents a method for push-ofa-button tuning of varactor tuned RF receiver coils.
PREVIOUS WORK
The schematic and picture below (Figure 1) display a
varactor diode tunable receiver. This receiver was used
as the coil in the experiments presented here, as well as
in previous work [1,2]. The DC bias is provided by a
potentiometer in the diagram, but the new scheme uses
a 12 bit serial DAC.
a)
150pF
20K
C
9V
10K
8
DC bias,
RF isolate
20K
75nH
Flex coil
C
8
<360nH
22 or 68pF
Varactor
Q spoil
Rcv
Port
Pull wire
2.5 cm
Figure 5 shows the impedance curves for the tunable receiver under a
progression of loading and tuning conditions. Loading (holding the
coil in a closed fist) shifts the center frequency by as much as
2.8MHz from the tuned, unloaded case (Figure 5a). The automatic
system then, at the press of a button, re-tunes the peak to 63.8MHz
(Figure 5b). Total tuning time is under 300ms.
~15 cm
2 turns
Figure 1: Varactor tuned flexible interventional coil.
a) Circuit schematic. b) Picture of coil.
PRINCIPLES OF OPERATION
Reference signal
TUNING BY PHASE COMPARISON
The impedance of a parallel circuit is purely resistive
on resonance, and is capacitive and inductive if
resonant at frequencies above and below the Larmor
frequency, respectively. If a current is applied at the
Larmor frequency to a resonant circuit in series with a
reference capacitor (Figure 3), the voltages across
them will have a difference in phase of 90 degrees
under tuned conditions. On resonance there is a DC
null at the output of a multiplier of these two signals
(Figure 4). We tune the circuit to find this null.
The PLL Synthesizer is SPI-controlled to switch the
phase comparator reference signal (Figure 3) from 63.9
MHz when tuning to 90 MHz while receiving. Also,
the CFB is tri-stated during receive mode to reduce
PLL synthesizer interference.
The TR Switch uses PIN diodes and a λ/4 impedance
transformer to provide proper impedance isolation
during tuning and receiving modes. The μ-controller
biases the diodes through RF chokes.
Va
Cref
_
+
_
Vb
+
F->90
Vo=0
Vo
Filter
F->180
Vo<0
Vo ~ |Va||Vb|cos(Φ)
Figure 3: Phase Comparator schematic. A Larmor
frequency signal produces multiplier DC output
according coil impedance. Φ is the voltage phase
difference between Cref and the tuned receiver.
SYSTEM INTEGRATION
We used a μ-controller with a serial peripheral
interface (SPI) to orchestrate the PLL synthesizer, TR
switches, and DAC, and to perform the tuning
algorithm. This implementation provides a simple
push-button tuning scheme.
F->0
Vo>0
coil
Figure 5: Impedance curves. a) Coil loaded by
human fist, detuned to 66.6MHz. b) Same loading,
automatically tuned to 63.8MHz. c) Different human
fist loading, detuned to 62.55MHz. d) Automatically
tuned to 63.8MHz.
600
DC output (mV)
Tuning and matching can be accomplished via a
network of passive components anywhere between the
receiver and the pre-amplifier. However, to best match
the receiver, the tuning elements should be as close as
possible to the coil.
manual
tune
RESULTS
Figure 2: Complete system schematic. See below for modular explanations.
Most of the work with invasive receivers indicates the
need for tuning and matching. These networks serve
the dual purpose of centering the tuned receiver at the
Larmor frequency and matching the impedance of the
receiver for optimal SNR performance with the preamplifier. It is well known that these qualities of the
receiver are of consequence [6].
MATERIALS
400
200
0
REFERENCES
-200
-400
-600
55
57
59
61
63
Frequency (MHz)
65
67
69
Figure 4: Phase Comparator DC output as a
function of applied reference frequency. Coil tuned
to 63.9MHz.
Tuning is accomplished via a 12-bit SPI DAC. The μcontroller uses the DAC to alter the reverse bias on the
receiver’s varactor, stepping up until the phase
comparator reports a zero DC output voltage.
(corresponding to the tuned state.)
[1] Gold, G. et al., Proc. ISMRM, 84, 2001.
[2] Scott, G. et al., Proc. ISMRM, 20, 2001.
[3] Atalar, E. et al., MRM, 36: 596, 1996.
[4] Quick, H. et al., MRM, 41: 751, 1999.
[5] Boskamp, E., Radiology, 157: 449, 1985.
[5] Hoult, D. et al., JMR 24:71-85, 1976.
ACKNOWLEDGMENTS
NIH: CA79728-01; CA95882-01; 1R01 CA92409-01
1R24 CA92862-01
GE Medical Systems