Download RF current element design for independent control of current

RF Current Element Design for Independent
Control of Current Amplitude and Phase in
Transmit Phased Arrays
KRISHNA N. KURPAD,1 STEVEN M. WRIGHT,1 EDDY B. BOSKAMP2
1
Department of Electrical Engineering, Magnetic Resonance Systems Laboratory, Texas A&M University, College
Station, TX 77843
2
Applied Science Laboratory, GE Healthcare Technologies, 3200 N. Grandview Boulevard, Waukesha, WI 53188
ABSTRACT: Both the optimization of B1 field homogeneity in high-field MRI and the
implementation of the exciting new theory of transmit SENSE can be accomplished by individual and independent control of the amplitude and phase of current on the elements or rungs
of a radio frequency (RF) transmit coil array. One way of achieving this is by using the concept
of the active rung, which consists of one rung of a volume coil connected across the output
terminals of an RF power MOSFET. The RF power MOSFET is used as a voltage-controlled
current source and drives RF current through the rung. The amplitude and phase of the RF
current are controlled by the amplitude and phase of the RF control voltage. In this article, we
demonstrate that the active rung may be used as a current element by tuning the rung to series
resonance. We then demonstrate, using field measurements, that the current induced by
adjacent rungs is suppressed by greater than 15 dB in a current element relative to a resonant
loop representing a single element of the well known TEM coil. Thus, the active rung configuration enables significantly greater isolation and independence between rungs than in conventional designs where each rung is fed by a voltage source. Measurements and theory demonstrate a dynamic range of independent control of the rung current amplitude of 17 dB. We
conclude that the degree of suppression is dependent on the size of the MOSFET output
parasitic capacitance. The active rung should find application as a building block in the
construction of parallel transmit coils.
© 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part
B (Magn Reson Engineering) 29B: 75– 83, 2006
KEY WORDS: high-field MRI; RF current source; active rung; MOSFET; phased array
transmit coil
INTRODUCTION
by the RF currents on the coil elements or rungs. The
amplitude and phase of the B1 field contribution of
each rung are governed, respectively, by the amplitude and phase of the rung current. Volume coils, such
as birdcage coils (1) and TEM coils (2), are designed
to support a sinusoidal amplitude distribution of rung
currents, which results in the generation of a resultant
B1 field that is homogeneous in the unloaded case.
However, the introduction of a dielectric load such as
a human body or head is known to perturb B1 field
homogeneity at high-static magnetic (B0) fields.
At high B0 fields (3T and higher), the wavelength
of the B1 field in a dielectric medium, such as human
In volume transmit coils used in MRI, the radio frequency (RF) transverse magnetic (B1) field is created
Received 20 April 2005; revised 1 November 2005;
accepted 6 November 2005
Correspondence to: Krishna N. Kurpad; E-mail: [email protected]
Concepts in Magnetic Resonance Part B (Magnetic Resonance
Engineering), Vol. 29B(2) 75– 83 (2006)
Published online in Wiley InterScience (www.interscience.wiley.
com). DOI 10.1002/cmr.b.20059
© 2006 Wiley Periodicals, Inc.
75
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KURPAD, WRIGHT, AND BOSKAMP
tissue, is comparable to the cross-sectional dimension
of the human body. The superposition of the B1 fields
from multiple sources results in the formation of
interference patterns, with the characteristic bright
spot in the center. There is therefore considerable
interest in the development of radio frequency volume
coils that are capable of generating uniform B1 field at
high frequency. This requires a parallel transmit system with independent control of the amplitude and
phase of the rung currents to enable optimization of
B1 field homogeneity (3–5). The recent development
of the theory of transmit SENSE (5, 6) and the consequent renewal of interest in multidimensional RF
pulses (7), which allows selective excitation of specific regions of interest and optimization of B1 field
homogeneity (4), has further contributed to the increased interest in parallel transmit coils (5).
A parallel transmit coil with independent control of
the rung current amplitude and phase may be achieved
by ensuring that the rungs are well decoupled from
each other. Decoupling between the elements of a
receive coil is accomplished using a combination of
preamplifier and capacitive or inductive decoupling
mechanisms. Preamplifier decoupling works on the
principle of suppression of element currents induced
by the spins, thus preventing cross-talk, and is well
suited for parallel receive coil design because the
measured parameter is the spin-induced emf and not
the induced current. However, in parallel transmit
arrays, large current amplitudes are required to generate the desired B1 field. A decoupling mechanism
for transmit arrays would therefore be required to
suppress current induced by neighboring rungs, while
simultaneously allowing large currents to be driven
through the rungs. Recently, Zhu et al. (5) have suggested a transmit phase array design. Inductive or
capacitive decoupling mechanisms have been used to
decouple the nearest neighbor elements. Though inductive decoupling is effective for nearest neighbor
elements, it is not effective for distant neighbor elements. The above design relies on the loading to
improve isolation between the distant neighbor elements and may not be reliable.
An alternative decoupling mechanism that involves the use of an active device such as a transistor
integrated with the rung is the topic of discussion of
this article. We refer to such an arrangement as an
“active rung” (8, 9). The active rung is based on the
principle of the antennafier element (10). The use of
active devices such as BJTs and MOSFETs to drive
integrated antenna elements has been investigated before (11–13). Studies have shown that such an arrangement makes the feed network of antenna arrays
less sensitive to the effects of mutual impedance than
Figure 1 Large signal equivalent circuit of an n-channel
enhancement mode RF power MOSFET (a). The three
terminals of the MOSFET are the gate (G), the drain (D),
and the source (S). The MOSFET is represented as a voltage
controlled current source. The rung is represented by a
series RL circuit (b). The rung impedance is determined by
the variable capacitor, CT, in series with the rung (c).
passive arrays because of the unidirectional nature of
the active devices (11). In this work, we develop
further the concept of the active rung as an element of
a volume coil. We demonstrate the key concept of
independent control of the rung current amplitude and
phase enabled by suppression of induced currents in
an active rung.
THEORY
The RF power MOSFET is a transistor that behaves as
a voltage-controlled current source, when placed in
the saturation region of its DC characteristics (14).
The large signal equivalent circuit of the N-channel
MOSFET is shown in Fig. 1 (15, 16). In the common
source configuration, the gate (G) is the input terminal, the drain (D) is the output terminal, and the
source (S) is the ground terminal. In this configuration, the drain-source current, ID, is proportional to the
applied gate-source voltage, VGS. The constant of
proportionality is called the transconductance and is
denoted by gfs. Cis, Cos, and Crs are, respectively, the
input, output, and feedback parasitic capacitances of
the MOSFET.
The transistorized array element or active rung
may be implemented by connecting one end of the
rung to the drain of the MOSFET and the other end to
the common source. This arrangement is illustrated by
the schematic in Fig. 1. The parasitic output capacitance, Cos, of the MOSFET forms a parallel path to
ground for RF current. For maximum current to be
driven through the rung, the rung impedance to
ground should be small compared with the reactance
offered by Cos. This may be achieved by tuning out
the rung inductance with series capacitors. Such an
arrangement where RF current is driven directly
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
DESIGN FOR CURRENT AMPLITUDE AND PHASE
77
quency as the current on the adjacent rung to simulate
the situation in resonant volume coils such as TEM
coils (2), the loop impedance, Zp, is then a real quantity and is equal to Rloop. Rloop will, in general, be
larger than Rrung by a few ohms as it includes the
MOSFET resistances, not shown in the equivalent
circuit of Fig. 1. The emf induced in the current
element due to the adjacent rung is governed by
Faraday’s law of electromagnetic induction and remains unchanged for a given current on the adjacent
rung. The amplitude of the induced current in the
resonant loop is therefore given by
Figure 2 Equivalent output circuit of a MOSFET with the
rung tuned to series resonance (a). In this case, the rung
forms a low impedance path to ground for the driven current. The loop formed by the rung and the output capacitance (Cos) is substantially off resonance and contributes to
suppression of induced current. The equivalent output circuit of a MOSFET with the rung tuned as a TEM element
(b) shows that the loop formed by the rung and Cos is low
impedance for both the driven and induced currents.
兩I p兩 ⫽
兩E ind兩
R loop
[3]
The ratio of the induced currents in the two cases is
then
I ratio ⫽
兩I s兩 R loop
⫽
兩I p兩
兩Z s兩
[4]
Expressed in decibels,
through the active rung by the RF power MOSFET
may be referred to as a current element.
The schematic in Fig. 1 shows that the current
element consists of two loops. One loop is formed by
the RF current source and the rung, and the other is
formed by the rung and Cos. When a current element
is placed adjacent to a rung carrying current at the
Larmour frequency, the rung induces an emf in the
current element, as shown in Fig. 2(a). The emf induced in the current element by the adjacent rung may
then be represented as a voltage source, Eind, which
drives a current through the loop formed by Cos and
the rung. Because the current source has high internal
resistance, it is an open circuit to induced current.
The complex impedance of the loop formed by the
series tuned loop and Cos is given by
Z s ⫽ R rung ⫺ jX Cos
[1]
where Rrung is the series resistance of the rung. The
rung impedance is a pure resistance because of the
series tuning of the rung. The amplitude of the induced current in the loop is given by
兩I s兩 ⫽
兩E ind兩
兩Z s兩
[2]
2
where 兩Zs兩 ⫽ 冑Rrung
⫹ X2Cos.
If the current element is replaced by a loop (see
Fig. 2b) that is tuned to resonance at the same fre-
I ratio兩 dB ⫽ 20关log 10 共Rloop 兲 ⫺ log10 共兩Zs 兩兲兴
[5]
The amplitude of current induced in a current
element depends on the magnitude of impedance presented to the induced emf. The impedance magnitude
is, in turn, governed by the magnitude of Cos as Rrung
is, ideally, a constant and small compared to XCos.
Therefore, from Eqs. [4] and [5], the current induced
in a current element is small compared with that
induced in a resonant loop, and the extent of suppression is determined by the value of the parasitic output
capacitance of the MOSFET.
METHODS
Design and Construction
The active rung design is similar to the concept of
active integrated antennas, where the radiating element is a part of the output circuit of the amplifier.
The circuit schematic of the active rung design is
shown in Fig. 3. The BLF245 (Philips Semiconductors, Eindhoven, Netherlands), a RF power MOSFET
with rated maximum output power of 30 watts and 11
dB gain, was used to drive RF current through the
rung. The rung was connected between the drain and
source terminals of the MOSFET. Because the MOSFET was used in a common source configuration, the
source terminal was tied to ground. The amplitude
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
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KURPAD, WRIGHT, AND BOSKAMP
Figure 3 Circuit schematic of the RF current source. The array element or rung is connected
across the output terminals of the MOSFET. The MOSFET drives current directly through the rung.
AA⬘ is the plane at which the rung impedance is measured with and without the MOSFET in the
circuit. CB is the DC block capacitance.
and phase of the RF rung current was determined by
the amplitude and phase of the RF voltage that appeared across the gate-source terminals of the MOSFET. This was achieved by providing the MOSFET
drain with a DC supply voltage of 28V, which placed
the MOSFET in the saturation region of its DC characteristic, where it behaves as a voltage-controlled
current source. The gate was also provided with a DC
bias voltage of 3.6V, thus setting up the MOSFET for
class AB operation. An explanation of the DC characteristics and classes of operation of the RF power
transistor may be found in literature (14, 17).
The rung was broken up into five segments by four
chip capacitors, each of 47 pF capacitance. This was
done to increase the uniformity of current amplitude
along the z direction and also to raise the self-resonance frequency of the rung higher. The other end of
the rung was connected to ground by a trimmer capacitor (Johanson, Boonton, NJ). The range of the
trimmer was chosen such that the rung impedance, as
measured from the drain terminal of the MOSFET,
varied from capacitive to inductive through a series
resonance point. An input matching network was implemented to match the input impedance of the MOSFET to the source impedance of 50⍀ to ensure maximum power transfer into the MOSFET and also to
prevent reflections back into the cable, which would
result in formation of standing waves, leading to cable
coupling problems.
The active rung design was implemented on a
printed circuit board (PCB). The PCB layout was
fabricated using a PCB prototyping machine (Protomat C60, LPKF Laser and Electronics, Wilsonville,
OR). A picture of a completed PCB showing the
MOSFET, the DC bias circuits, the input circuit, and
the output circuit, which is a part of the rung, is
displayed in Fig. 4. The PCB was mounted on a
cylindrical acrylic former (292 mm inner diameter,
Figure 4 Picture of the PCB implementation of the active rung showing the output circuit (1), the
drain bias circuit (2), the MOSFET (3), the input matching network (4 ), and the gate bias circuit (5 ).
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
DESIGN FOR CURRENT AMPLITUDE AND PHASE
79
and mounted on the spring loaded fixture. The current
induced in the probe was transferred to the measurement apparatus by a RG316 coaxial cable through a
lattice balun that was mounted on the plastic rod at the
mouth of the cylindrical cavity. The balun was used to
prevent currents induced on the cable shields from
entering the measurement apparatus.
Experiments
Figure 5 A drawing of the side view of the active rung
construction, showing the placement of the magnetic field
probe (a) and a frontal view showing the manner in which
the PCB containing the RF current source is mounted on the
cylindrical acrylic former.
305 mm length) whose outer surface was wrapped in
a thin copper foil. The copper foil was used not only
as a RF shield but also as the ground plane. The rung
consisted of a length of copper tape that spanned the
length of the former and was placed directly under the
PCB. The rung was supported by a rectangular acrylic
strip that was 19 mm wide and 305 mm long. One end
of the rung was connected to the output trace on the
PCB through openings created in the former, the RF
shield, and the PCB. The other end of the rung was
connected to the RF shield by the trimmer capacitor
described above. The drawing in Fig. 5(a) illustrates
the active rung arrangement. Figure 5(b) shows the
front view of the active rung arrangement mounted on
the cylindrical former.
The RF rung current measurement apparatus consisted of a magnetic field probe mounted on a mechanism that would allow the probe to be moved in both
the axial and azimuthal directions. This enabled the
probe to be placed directly under the current element
of interest at any position along its length. The probe
placement is illustrated in Fig. 5(a). The mechanism
consisted of a plastic rod that was placed coincidental
to the axis of the cylindrical former. A spring-loaded
fixture, which pressed against the inner cylinder, was
mounted on the rod and fastened firmly to it. The
magnetic field probe was fabricated on a small copper-clad PC board as a 10 ⫻ 10-mm trace of copper
In conventional amplifier design, the parasitic output
capacitance (Cos) of the MOSFET is compensated by
the output matching network. However, in the active
rung design, there is no output matching network. To
determine the effect of rung tuning using the series
trimmer capacitor on the rung impedance as seen by
the MOSFET, the rung impedance was measured for
various settings of the trimmer capacitor, CT, both
with and without the MOSFET in the circuit at the
plane AA⬘ shown in the schematic of Fig. 3. The
impedance measurements were made at the 3 T Larmour frequency of 128 MHz.
A unique property of the active rung design is the
ability to suppress current induced by an adjacent
current-carrying conductor, thereby allowing a large
dynamic range of independent control of the amplitude and phase of the current on the active rung. To
demonstrate this key property, two sets of three experiments were performed to obtain the desired coupling data. The first set of experiments was performed
with the test rung tuned to series resonance at 128
MHz, when the active rung behaves as a voltagecontrolled current element.
The second set of corresponding experiments was
performed with the test rung tuned so as to simulate a
resonant loop of a TEM coil. The TEM elements are
essentially transmission line elements that form resonant loops with the RF shield. The TEM elements are
strongly coupled to each other. When the active rung
is tuned such that the rung forms a resonant loop with
Cos, the arrangement is similar to a TEM element
excited by a voltage source. The experiments are
described below.
Experiment A. The test rung was tuned to series
resonance at 128 MHz, the drive port to the active
rung was terminated in 50⍀, and the DC supply to the
MOSFET was turned on. The frequency of the RF
current on the adjacent conductor was set to 128 MHz
and its amplitude was set at several incremental values by ramping up the input voltage amplitude. The
magnetic field probe was placed under the test element, and the spectrum analyzer readings of the field
sensed by the probe were recorded. The purpose of
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
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KURPAD, WRIGHT, AND BOSKAMP
Figure 6 Plots of measured rung impedance with the MOSFET in the circuit and biased (i) and
without the MOSFET in the circuit (ii). The coincident dip in the two plots is the area in which the
MOSFET behaves as a true current source with respect to the rung.
this experiment was to measure the current induced in
the test element. It was expected that the current
induced in a current element would be significantly
less than that induced in a TEM element.
Experiment B. With the adjacent conductor carrying
no current, the current on the test rung was ramped up.
The magnetic field probe was again placed under the
test rung. The purpose of this experiment was to
determine the magnitude of current driven in element
A under no influence from the adjacent conductor.
The result expected from this experiment was a curve
that represented the ideal response of the B1 field to
the amplitude of the input RF control voltage.
Experiment C. The amplitude of RF current on the
test rung was ramped up with the current on the
adjacent conductor set to its maximum amplitude of
1.6 A. The magnetic field probe was placed under the
test element. The purpose of this experiment was to
determine the dynamic range of independent control
of current amplitude on the test rung for the worst
case scenario of the adjacent conductor carrying current at maximum amplitude. The result expected from
this experiment was a significantly larger dynamic
range of independent control of the B1 contribution of
a current element compared with the TEM element.
RESULTS AND DISCUSSION
The plots of the rung impedance magnitude for various settings of the trimmer capacitor, both with (curve
[i]) and without (curve [ii]) the MOSFET in the circuit, are shown in Fig. 6. The region where the two
curves coincide and exhibit a dip in the impedance
magnitude, as indicated in Fig. 6, represents the region of series resonance tuning of the rung. In this
region, Cos has no effect on the magnitude of rung
current. Curve (ii) displays an impedance peak at a
rung impedance measurement of (2.7 ⫹ j15.8)⍀. This
peak may be attributed to the resonant loop formed by
the rung inductance and Cos. These results validate the
output circuit model discussed in section 2.
Two important inferences may be made from these
results. First, maximum current is driven through the
rung when the rung impedance is small compared
with the reactance presented by Cos. This occurs when
the rung is tuned to series resonance and the Cos path
appears as an open circuit to the driven current. Second, Cos forms a closed loop with the rung, which is
resonant at 128 MHz for a different setting of the rung
impedance. This can be viewed as a single loop of a
TEM coil that is excited by a voltage source. Equivalently, the loop formed by Cos and the series resonant
rung (at 128 MHz) is resonant at a frequency that is
significantly shifted from the Larmour frequency.
As shown in Fig. 7, the current induced in the test
rung that is tuned to series resonance by an adjacent
current-carrying conductor is, on average, less than
that induced in the test rung that is tuned as a TEM
element by the same adjacent conductor carrying the
same current amplitude by 15.52 dB. From the impedance measurements, the resistance of the series
resonant test rung is measured to be 0.4⍀. The impedance of the test rung tuned as a TEM element is
measured to be (2.7 ⫹ j15.8)⍀. Therefore, the impedance of the loop formed by the series resonant test
rung and Cos at a frequency of 128 MHz is (0.4 ⫺
j15.8)⍀, and the impedance of the TEM element at
the same frequency is 2.7⍀. Substituting these values
in Eqs. [4] and [5], the current in the TEM element is
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
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81
Figure 7 Plots of B1 field measurements made using the magnetic field probe placed directly
under test rung. The series tuned and parallel tuned measurements were made with the test rung
tuned as a current element and TEM element, respectively. The average difference in dB between
the measurements is shown.
found to be greater than that in the series resonant test
rung by 15.34 dB. The calculated and measured values are in good agreement.
Two important inferences can be made from the
above results. First, the active rung design with the
rung tuned to series resonance suppresses induced
current by a significant amount when compared to the
TEM element. This may be attributed to the significant shift in the resonance frequency of the induced
current loop from that of the driven current path.
Second, the magnitude of suppression depends entirely on the value of Cos. The smaller the value of
Cos, the greater the suppression of induced current.
The plots shown in Fig. 8 are measured values of
the magnetic field sensed by the magnetic field probe
generated by the current on the test rung that is tuned
to series resonance at 128 MHz in the presence of an
adjacent conductor carrying varying current amplitudes at the same frequency. Plot (a) is the magnetic
field sensed by the probe due to the current induced on
the test rung. Plot (b) represents the magnetic field
sensed by the probe due to current driven in the test
rung in the absence of the adjacent conductor, and
plot (c) represents the resultant magnetic field sensed
by the probe due to the current driven into the test
rung with the adjacent conductor carrying maximum
current. This represents the worst-case scenario for
coupling between neighboring current elements.
Plot (c) can be divided into three regions as shown
in Fig. 8. In region 1, the field sensed by the probe is
Figure 8 Curves showing dynamic range of independent control of RF current on the current
element. (a) The sensed field due to induced current on the test rung tuned to series resonance; (b)
the sensed field due to driven current on the test rung with the adjacent conductor removed from the
circuit; and (c) the sensed field due to driven current on the test rung with the adjacent conductor
carrying maximum current. Region 1 of curve (c) is induced-current dominated, and region 3 is
driven-current dominated. The current probe senses the resultant current due to driven and induced
currents on the test rung in region 2.
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
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KURPAD, WRIGHT, AND BOSKAMP
Figure 9 Curves show the effect of induced currents on the dynamic range of independent control
of current amplitude and phase on the test rung tuned as a TEM element. (a) The sensed field due
to induced current on the test rung; (b) the sensed field due to driven current on the test rung; and
(c) the sensed field due to driven current on the test rung under worst-case conditions of maximum
current on the adjacent conductor. This set of curves shows that there is no region dominated by the
driven current on the test rung.
predominantly due to the current induced on the test
rung by current on the adjacent conductor. Region 2 is
where the magnitude of current driven in the test rung
is comparable to that induced by the adjacent conductor. Hence, the field sensed by the probe is the resultant field due to the resultant of the driven and induced
currents on the test rung. Region 3 is where the driven
current in the test rung dominates the induced current
due to the adjacent conductor, and the magnetic field
probe senses the field predominantly due to current
driven on the test rung. The dynamic range of independent control of the current amplitude, as seen in
Fig. 8, is represented by region 3 and is 17dB.
The curves in Fig. 9 correspond to those in Fig. 8
and are due to the test rung, tuned as a TEM element.
Comparison of plots (a) and (b) in this case shows that
the induced current is, on average, only 5.2 dB down
from the driven current. This is only a factor of 1.8
less than the driven current. For the worst-case scenario of maximum current on the adjacent conductor,
the driven current on the test rung never completely
dominates the induced current. This is borne out by
the fact that plot (c) can be divided into only two
regions, whereas region 1 is induced current dominated and region 2 represents the resultant field due to
the driven and induced currents.
It is evident from the above results that the degree
of suppression of induced currents determines the
dynamic range of independent control of the amplitude and phase of the rung current. A TEM element is
strongly coupled to similar adjacent elements and
therefore does not exhibit independent control. The
active rung with the rung tuned to series resonance, on
the other hand, exhibits a large range of independent
control of the rung current amplitude and phase as a
result of the key property of suppression of induced
current from similar adjacent rungs. Further, the degree of suppression of induced current is limited by
the parasitic output capacitance of the MOSFET.
The results presented above show that with the RF
current source implementation, the induced currents
on neighboring elements may be suppressed, though
the mutual impedances and the induced emfs remain
the same. This serves to reduce the influence of neighboring array elements on the amplitude and phase of
the current on any given element or rung. This result
has significant implications for the design of transmit
coils for high-field MRI.
CONCLUSION
In this article, we described the development of an RF
current source integrated with an array element or
rung, which we refer to as the active rung. Series
resonance tuning of the rung makes the effects of
parasitic MOSFET capacitance negligible. The amplitude and phase of the current on the rung may be
controlled accurately by the amplitude and phase of
the input RF control voltage. The existence of separate paths for the driven and induced currents within
the MOSFET results in suppression of induced current. The degree of suppression is largely dependent
on the size of the MOSFET parasitic output capaci-
Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b
DESIGN FOR CURRENT AMPLITUDE AND PHASE
tance. Induced current suppression is an important
feature that is necessary for the design of parallel
transmit arrays.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support from the
National Science Foundation (BES0101059). The authors
would also like to thank Mr. John Lorbiecki for building the
acrylic former and Mr. LeRoy Blawat for stimulating conversations on RF circuit design.
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Concepts in Magnetic Resonance Part B (Magnetic Resonance Engineering) DOI 10.1002/cmr.b