U.S. patent application number 11/609275 was filed with the patent office on 2008-06-12 for integrated low noise amplifier and balun for mri receivers.
Invention is credited to William E. Burdick, Michael A. de Rooij, Eladio Clemente Delgado, Randy Otto John Giaquinto.
Application Number | 20080136414 11/609275 |
Document ID | / |
Family ID | 39426867 |
Filed Date | 2008-06-12 |
United States Patent
Application |
20080136414 |
Kind Code |
A1 |
de Rooij; Michael A. ; et
al. |
June 12, 2008 |
INTEGRATED LOW NOISE AMPLIFIER AND BALUN FOR MRI RECEIVERS
Abstract
An integrated balun-low noise amplifier (LNA) system is included
in a magnetic resonance imaging system. The integrated balun-LNA
system conditions electromagnetic signals received from at least
one RF receiver coil. The integrated balun-LNA system is configured
to be enclosed in a balun housing. The size of the integrated
balun-LNA system minimizes component area and volume thereof on or
near the RF receiver coil and thus, minimizes interference with the
magnetic flux field.
Inventors: |
de Rooij; Michael A.;
(Schenectady, NY) ; Giaquinto; Randy Otto John;
(Burnt Hills, NY) ; Burdick; William E.;
(Niskayuna, NY) ; Delgado; Eladio Clemente; (Burnt
Hills, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Family ID: |
39426867 |
Appl. No.: |
11/609275 |
Filed: |
December 11, 2006 |
Current U.S.
Class: |
324/322 |
Current CPC
Class: |
G01R 33/3621 20130101;
G01R 33/422 20130101; H03F 2200/294 20130101; G01R 33/3685
20130101; H03F 3/195 20130101 |
Class at
Publication: |
324/322 |
International
Class: |
G01R 33/36 20060101
G01R033/36 |
Claims
1. A magnetic resonance imaging (MRI) system comprising: at least
one magnet for generating a magnetic field; at least one gradient
coil for manipulating the magnetic field generated by the at least
one magnet by way of a gradient field; and at least one RF receiver
coil to receive electromagnetic signals from the manipulated
magnetic field; at least one integrated balun-preamplifier system
configured to condition the received electromagnetic signals, said
at least one integrated balun-preamplifier system comprising a
low-noise amplifier (LNA) to amplify the received electromagnetic
signals and a balun to block unwanted common mode signals; and
wherein the at least one integrated balun-preamplifier system is
enclosed in a balun housing.
2. (canceled)
3. The MRI system of claim 1 wherein the balun further comprises a
common-mode inductor coil and common mode capacitor and the LNA
includes an input matching inductor.
4. The MRI system of claim 3 wherein the common-mode inductor coil
and the input matching inductor are oriented at 90 degrees to one
another to minimize mutual magnetic coupling there between.
5. The MRI system of claim 3 wherein the input matching inductor is
configured to include one of a single input and a differential
input.
6. The MRI system of claim 3 wherein the balun housing further
comprises: a first chamber to substantially house the LNA; a second
chamber to substantially house the common-mode inductor coil and a
remaining part of the LNA; and an internal shield separating the
first and second chambers.
7. The MRI system of claim 6 wherein the internal shield blocks
feedback induced signals between the first chamber and the second
chamber.
8. The MRI system of claim 1 wherein the at least one integrated
balun-preamplifier system further comprises an energy storage
device to provide temporary power to the LNA.
9. The MRI system of claim 1 wherein the at least one integrated
balun-preamplifier system further comprises a transmit protection
bias circuit configured to power a transmit protection circuit.
10. The MRI system of claim 9 wherein the transmit protection
circuit further comprises one of a PIN diode, a MOSFET, a MEMS
protection device, and a MEMS switch, to reduce power consumption
in the transmit protection circuit.
11. The MRI system of claim 9 wherein the transmit protection
circuit is powered by one of a negative current and a negative
voltage.
12. The MRI system of claim 9 wherein the at least one integrated
balun-preamplifier system further comprises a signal output
configured to transmit the electromagnetic signals, power the LNA,
and power the transmit protection circuit.
13. The MRI system of claim 12 wherein the at least one integrated
balun-preamplifier system further comprises at least one diode to
selectively transmit power through the signal output to the LNA and
the transmit protection circuit.
14. The MRI system of claim 12 wherein the signal output further
comprises a co-axial cable.
15. The MRI system of claim 1 wherein the LNA is powered by a
positive voltage.
16. An integrated balun-low noise amplifier (LNA) module
comprising: a balun enclosure; a preamplifier partially located in
a first chamber of the balun enclosure; a common mode inductor
located in a second chamber of the balun enclosure, the second
chamber also including a remaining portion of the preamplifier; and
an internal shield separating the first chamber and the second
chamber to reduce transmission of magnetic and electric fields
between the first and second chambers.
17. The integrated balun-LNA module of claim 16 wherein the
integrated balun-LNA module conditions electromagnetic signals
received from a receiver coil in an MRI system.
18. The integrated balun-LNA module of claim 17 wherein the common
mode inductor further comprises a signal line configured to
transmit the electromagnetic signals, power the preamplifier, and
power a transmit blocking circuit.
19. The integrated balun-LNA module of claim 18 further comprising
at least one diode to separate power transmission through the
signal line to the preamplifier and the transmit blocking
circuit.
20. The integrated balun-LNA module of claim 18 wherein the
transmit blocking circuit further comprises one of a PIN diode, a
MOSFET, a MEMS protection device, and a MEMS switch to reduce power
consumption therein.
21. The integrated balun-LNA module of claim 18 wherein the signal
line further comprises a co-axial cable to provide power and
transmission of an output data.
22. A method of constructing an integrated balun-low noise
amplifier (LNA) module for conditioning electromagnetic signals in
an MRI system comprising the steps of: forming a balun housing
positioning a low-noise amplifier partially into a first chamber of
the balun housing; positioning a remaining portion of the low-noise
amplifier and a common mode inductor into a second chamber of the
balun housing; positioning an internal shielding mechanism between
the first chamber and the second chamber; positioning an external
shielding mechanism at opposing ends of the balun housing and
around the balun housing; and electrically connecting the low-noise
amplifier, the common mode inductor, the internal shielding, and
the external shielding.
23. The method of claim 22 further including the step of
electrically connecting a transmission protection circuit.
24. The method of claim 22 further including the step of
electrically connecting a common-mode capacitor to the external
shielding mechanism.
25. The MRI system of claim 3 wherein the balun housing further
comprises: a first chamber to substantially house the common-mode
inductor coil and a portion of the LNA; a second chamber to
substantially house the remaining portion of the LNA; and an
internal shield separating the first and second chambers.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to receiver
electronics in a magnetic resonance imaging (MRI) system, and, more
specifically, to an integrated balun-low noise amplifier (LNA)
system.
[0002] MRI uses radio frequency pulses and magnetic field gradients
applied to a subject in a strong homogenous magnetic field to
produce viewable images. When a substance such as human tissue is
subjected to a uniform magnetic field (polarizing field B.sub.0),
the individual magnetic moments of the spins in the tissue attempt
to align with this polarizing field, but precess about it, in
random order, at their characteristic Larmor frequency. If the
substance, or tissue, is subjected to a magnetic field (excitation
field B.sub.1) which is in the x-y plane and which is near the
Larmor frequency, the net aligned moment, or "longitudinal
magnetization", M.sub.Z, may be rotated, or "tipped", into the x-y
plane to produce a net transverse magnetic moment M.sub.t. A signal
is emitted by the excited spins after the excitation signal B.sub.1
is terminated and this signal may be received and processed to form
an image.
[0003] When utilizing these signals to produce images, magnetic
field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed.
Typically, the region to be imaged is scanned by a sequence of
measurement cycles in which these gradients vary according to the
particular localization method being used. The resulting set of
received MR signals are digitized and processed to reconstruct the
image using one of many well known reconstruction techniques.
[0004] MR receiver coils receive the electromagnetic signals
emitted from the patient and use the acquired signals for image
reconstruction. Before image reconstruction occurs, the
electromagnetic signals received by the receiver coil elements are
amplified and filtered to produce an analog signal that can be
further processed into an image. One circuit component in conveying
the electrical signal from the receiver coil element to the analog
conversion portion of the receiver is the balun. Typically, these
baluns are constructed as stand-alone components. Such a design can
be problematic in that the baluns use large circuit components,
that when located in, on, or near the receiver, can interfere with
the magnetic flux field and thus reduce the quality and quantity of
signals captured by the receiver. Thus, a balun design that
minimizes the adverse impact of the balun geometry on the image
produced by the MRI system would be beneficial.
[0005] Another component in the analog conversion chain of the
receiver coil elements is the preamplifier. Similar to the balun,
the size of the preamplifier can also interfere with the magnetic
flux field and reduce the quality and quantity of signals captured
by the receiver. This can be especially problematic as the quantity
of receiver coil elements in a phased array increases and the
diameter of the coil elements decreases. The decreased diameter of
the coil elements results in a higher density of coil elements for
a fixed receiver geometry, as there is an increase in the ratio of
the electronic component volumetric surface area to receiver coil
element area. This increased density of electronic components
located near the receiver coil elements effects a higher likelihood
of there being an adverse impact on image quality due to the higher
relative disturbance of the magnetic flux field.
[0006] Therefore, a receiver electronics package that minimizes
component area and volume thereof so as to reduce impact on the
magnetic field on or near the receiver coil elements is strongly
desired.
BRIEF DESCRIPTION OF THE INVENTION
[0007] The present invention is directed to an apparatus and method
for integrating receiver electronics in a magnetic resonance
imaging (MRI) system into a single device to overcome the
aforementioned drawbacks. An integrated balun-LNA system is
constructed to be enclosed within a standard balun to minimize
volume and impact on the signals acquired by the RF receiver coils.
Such a system would integrate electronic and passive components and
use advanced electronic packaging technologies to achieve this
minimization, thereby reducing the impact of the receiver
electronics on image quality.
[0008] Therefore, in accordance with one aspect of the present
invention, a magnetic resonance imaging (MRI) system includes at
least one magnet for generating a magnetic field, at least one
gradient coil for manipulating the magnetic field generated by the
at least one magnet by way of a gradient field, and at least one RF
receiver coil to receive electromagnetic signals from the
manipulated magnetic field. Also included in the MRI system is at
least one integrated balun-preamplifier system configured to
condition the received electromagnetic signals, wherein the at
least one integrated balun-preamplifier system is enclosed in a
balun housing.
[0009] In accordance with another aspect of the present invention,
an integrated balun-low noise amplifier (LNA) module includes a
balun enclosure, a preamplifier partially located in a first
chamber of the balun housing, and a common-mode inductor located in
a second chamber of the balun housing, the second chamber also
including a remaining portion of the preamplifier. Integrated
balun-low noise amplifier (LNA) module also includes an internal
shield separating the first chamber and the second chamber to
reduce transmission of magnetic fields between the first and second
chambers.
[0010] In accordance with yet another aspect of the present
invention, a method of constructing an integrated balun-LNA module
is set forth. The method includes the steps of forming a balun
housing, positioning a low-noise amplifier LNA partially into a
first chamber of the balun housing, and positioning a remaining
portion of the low-noise amplifier and a common-mode inductor into
a second chamber of the balun housing. The method also includes the
steps of positioning an internal shielding mechanism between the
first chamber and the second chamber, positioning an external
shielding mechanism at opposing ends of the balun housing and
around the balun housing, and electrically connecting the low-noise
amplifier, the balun, the internal shielding, and the external
shielding.
[0011] Various other features and advantages of the present
invention will be made apparent from the following detailed
description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The drawings illustrate one embodiment presently
contemplated for carrying out the invention.
[0013] In the drawings:
[0014] FIG. 1 is a schematic of a magnetic resonance imaging (MRI)
system according to one embodiment of the present invention.
[0015] FIG. 2 is a cross-sectional side view of a balun as known in
the prior art
[0016] FIG. 3 is a cross-sectional side view of an integrated
balun-LNA system according to the present invention.
[0017] FIG. 4 is an exploded perspective view of the integrated
balun-LNA system of FIG. 3.
[0018] FIG. 5 is a schematic circuit diagram of the integrated
balun-LNA system of FIG. 3.
[0019] FIG. 6 is a detailed cross-sectional side view of a balun
shield according to one embodiment of the present invention.
[0020] FIG. 7 is an end plan view of the balun shield of FIG.
6.
[0021] FIG. 8 is a side cross-sectional view of an additional
embodiment of the balun shield of FIG. 6.
[0022] FIG. 9 is a side cross-sectional view of an additional
embodiment of the balun shield of FIG. 6.
[0023] FIG. 10 is a side cross-sectional view of an additional
embodiment of the balun shield of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Referring to FIG. 1, the major components of a magnetic
resonance imaging (MRI) system 10 incorporating the present
invention are shown. This system is merely exemplary and one
skilled in the art will readily understand that variations are not
only possible, but frequently occur between various embodiments.
For example, while some components are shown as a separate
component, it may very well be incorporated into another
component.
[0025] As shown in FIG. 1, the operation of MRI system 10 is
controlled from an operator console 12 which includes a keyboard or
other input device 13, a control panel 14, and a display screen 16.
The console 12 communicates through a link 18 with a separate
computer system 20 that enables an operator to control the
production and display of images on the display screen 16. The
computer system 20 includes a number of modules which communicate
with each other through a backplane 20a. These include an image
processor module 22, a CPU module 24 and a memory module 26, known
in the art as a frame buffer for storing image data arrays. The
computer system 20 is linked to disk storage 28 and removable
storage 30 for storage of image data and programs, and communicates
with a separate system control 32 through a high speed serial link
34. The input device 13 can include a mouse, joystick, keyboard,
track ball, touch activated screen, light wand, voice control, or
any similar or equivalent input device, and may be used for
interactive geometry prescription.
[0026] The system control 32 can include a set of modules connected
together by a backplane 32a. These include a CPU module 36 and a
pulse generator module 38 which connects to the operator console 12
through a serial link 40. It is through link 40 that the system
control 32 receives commands from the operator to indicate the scan
sequence that is to be performed. The pulse generator module 38
operates the system components to carry out the desired scan
sequence and produces data which indicates the timing, strength and
shape of the RF pulses produced, and the timing and length of the
data acquisition window. The pulse generator module 38 connects to
a set of gradient amplifiers 42, to indicate the timing and shape
of the gradient pulses that are produced during the scan. The pulse
generator module 38 can also receive patient data from a
physiological acquisition controller 44 that receives signals from
a number of different sensors connected to the patient, such as ECG
signals from electrodes attached to the patient. And finally, the
pulse generator module 38 connects to a scan room interface circuit
46 which receives signals from various sensors associated with the
condition of the patient and the magnet system. It is also through
the scan room interface circuit 46 that a patient positioning
system 48 receives commands to move the patient to the desired
position for the scan. The pulse generator module 38 may also be
located directly in the scan room.
[0027] The gradient waveforms produced by the pulse generator
module 38 are applied to the gradient amplifier system 42 having
Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a
corresponding physical gradient coil in a gradient coil assembly
generally designated 50 to produce the magnetic field gradients
used for spatially encoding acquired signals. The gradient coil
assembly 50 forms part of a magnet assembly 52 which includes a
polarizing magnet 54 and RF coil 56. A transceiver module 58 in the
system control 32 produces pulses which are amplified by an RF
amplifier 60 and coupled to the RF coil 56 by a transmit/receive
switch 62. The resulting signals emitted by the excited nuclei in
the patient may be sensed by the same RF coil 56 or may by sensed
by a separate receiver coil system comprising an array of multiple
receive coils that are embedded into a structure that is worn by
the patient (i.e., a surface coil). The amplified MR signals are
demodulated, filtered, and digitized in the receiver section of the
transceiver 58. The transmit/receive switch 62 is controlled by a
signal from the pulse generator module 38 to electrically connect
the RF amplifier 60 to the RF coils 56 during the transmit mode and
to connect the preamplifier 64 to the coil 56 during the receive
mode. The transmit/receive switch 62 can also enable a separate RF
surface coils (not shown) to be used in either the transmit or
receive mode.
[0028] One circuit component in conveying the electrical signal
from the RF coils 56 to the analog conversion portion of the
receiver is the balun. Shown in FIG. 2 is a balun 65 as is commonly
known in the prior art. As shown, balun 65 is constructed as a
stand-alone component. Such a design can be problematic in that
baluns use large circuit components, that when located in, on, or
near the RF coils, can interfere with the magnetic flux field and
thus reduce the quality and quantity of signals captured by the RF
coils. Additionally, when the balun 65 is a stand-alone component,
additional space is required to house a separate preamplifier.
[0029] To maintain the quality and quantity of signals captured by
the RF coils 56 of FIG. 1 or the surface coils in a receive mode
(generally referred to as receiver coils 56 hereinafter), receiver
electronics that are connected to the receiver coils 56 are
preferably minimized in volume to reduce interference with the
magnetic flux field. That is, as shown in FIG. 3, preamplifier 64
is integrated within balun 66 to form an integrated
balun-preamplifier system 68 or module. Preamplifier 64, as
identified in FIG. 3, includes the input matching inductor 80 as
well as amplifier circuitry that is shown in greater detail in FIG.
5. Similarly, balun 66 includes the components of common-mode
inductor 69, balun shield 70, and capacitor 94. In a preferred
embodiment, the preamplifier 64 is configured as a low noise
amplifier (LNA) 64 that operates to amplify the weak
electromagnetic signals received by the receiver coils 56 (shown in
FIG. 1), by connecting the LNA 64 directly into the received
signal. The integrated balun-LNA system/module 68 reduces the
volumetric impact of the LNA 64 and balun 66 on the magnetic flux
of signals received by the receiver coils, by reducing the overall
volume of the two components as compared to when each of the balun
66 and the LNA 64 are separately housed and electrically connected
to the receiver coils. To achieve this volume reduction, the
integrated balun-LNA system 68 is enclosed within a typical balun
66 (i.e., enclosed within balun housing 70), with the LNA 64 partly
located in a first chamber 72 thereof and the common-mode inductor
69 and remainder of the LNA 64 located in a second chamber 74
thereof. Separating the first chamber 72 from the second chamber 74
is an internal shield 76 configured to block feedback induced
signals between the first chamber and the second chamber and the
circuit therein.
[0030] The balun shield 70 is constructed to enclose integrated
balun-preamplifier system 68. Balun shield 70 is constructed of an
electrically conductive rigid tube that includes an end cover/plate
78 on each end thereof. The electrically conductive rigid tube
preferably is comprised of a plexiglass tubing surrounded by a
copper sleeve, although it is envisioned that other similarly
suited materials can also be used. Each of the end covers 78 joins
with the electrically conductive rigid tube of balun shield 70 to
form an external shield that further blocks feedback induced
signals and externally generated signals. The end covers 78 also
include a miniature electronics package (not shown) thereon that
includes printed circuit boards, electronic components, and passive
devices, as will be explained in greater detail below.
[0031] LNA, generally referred to as 64 in FIG. 4, includes input
matching inductor 80 and LNA circuitry, which is shown in greater
detail in FIG. 5. Referring back to FIG. 4, one component of LNA 64
is field effect transistor (FET) 100, which is preferably mounted
to internal shield 76 and located in second chamber 74. Input
matching inductor 80 is made up of two separate inductor coils 82,
84. Inductor coils 82, 84 are arranged such that the magnetic flux
of inductors 82, 84 is well contained inside the volume of the
coils. This configuration ensures minimal variation of the
inductance when enclosed by balun shield 70. Input matching
inductor 80 provides input matching between the receiver coil 56 of
FIG. 1 and an amplifier chip (e.g., PHEMT), and works in
conjunction with a capacitor.
[0032] Connected to the LNA 64 is common-mode inductor 69 of balun
66, which functions as a passive device in the integrated balun-LNA
system 68. As shown in FIG. 4, common-mode inductor 69 is
preferably in the form of a common mode choke coil made by winding
a co-axial cable in the form of a cylindrical spiral, the co-axial
cable having a non-magnetic core. The common-mode inductor 69 is
configured to have a high reactance to the imaging frequency when
common-mode currents flow through it and a low reactance to the
imaging frequency when differential-mode currents flow through it,
and thus, together with the common-mode capacitor 94, forms a
parallel resonant circuit that exhibits a high impedance to
common-mode currents at the imaging frequency, thereby effectively
decoupling the receiver coils 56 of FIG. 1 from the MRI system 10
by preventing electromagnetic interference in the analog conversion
portion of the transceiver 58, also shown in FIG. 1. Also in a
preferred embodiment, the common-mode inductor 69 (i.e., common
mode choke coil) is oriented 90 degrees to the input matching
inductor coils 82, 84 of the LNA 64. Such a construction minimizes
mutual magnetic coupling between the coils.
[0033] Referring still to FIG. 4, the co-axial cable that forms
common-mode conductor 69 also forms, in part, signal line 88, along
with additional co-axial cable added thereto. In one embodiment,
the signal line 88 is configured as a triple-function signal line
88 that transmits electromagnetic signals into the balun-LNA system
68, powers the LNA 64 in one mode, and also provides a bias for a
transmit protection circuit 90 (shown in FIG. 5) in another mode.
Signal line 88 can be composed of any material suitable for
transmitting both power and communication signals, although in a
preferred embodiment, it is a co-axial cable comprised of a
non-magnetic conductive material.
[0034] The functions of the signal line 88 are separated by way of
one or more diodes contained in the circuit 96 of the integrated
balun-LNA system as shown in FIG. 5. Referring to FIG. 5, an
exemplary circuit 96 is diagrammed for the integrated balun-LNA
system of the present invention. As shown therein, diodes 92 are
used to separate the power functions of the signal line 88 and
control power transmission to the LNA. That is, diodes 92 connect
the LNA to signal line 88 to power LNA by way of a positive DC
voltage and disconnect LNA by way of a negative DC current.
Additionally, diodes 92 provide a bias to transmit power to
transmit protection circuit 90 by way of transmit protection bias
circuit 89.
[0035] The circuit 96 in the integrated balun-LNA system shown in
FIG. 5 is preferably a single 3D component that combines the
individual circuits of the separate balun, preamplifier, and
transmit protection bias circuit 89. The integrated circuit 96 is
mounted to or integrated with end covers 78 and internal shield 76
of the balun shield 70, shown in FIG. 3, and is minimized by way of
flexible printed circuits (patterned metal on polymide), organic
substrates (thin laminate PCBs), electronic components or devices
(thin, bare, or packaged ICs), and/or passive devices (resistors,
capacitors, inductors formed within or attached to the PCBs, etc.).
The integrated circuit 96 is designed to allow for powering of the
LNA and transmit protection circuit 90 through the signal line 88,
thus eliminating the need for additional power sources or
lines.
[0036] As shown in FIG. 5, transmit protection circuit 90 is
located apart from the circuit 96 of the integrated balun-LNA
system and is connected by signal line 88. In this embodiment, the
transmit protection circuit 90 is located adjacent to the receiver
coil 56 shown in FIG. 1. Alternatively, it is also envisioned that
transmit protection circuit by incorporated into circuit 96 and
into the overall design of the integrated balun-LNA system 68 shown
in FIG. 3. As shown in FIG. 5, transmit protection circuit 90 also
contains one or more PIN diodes 98. PIN diode 98 is configured to
transmit a negative DC current from signal line 88 to the transmit
protection circuit 90 and reduce power consumption therein. Rather
then implementing a PIN diode 98, it is also envisioned that the
transmit protection circuit 90 could implement a MEMS switch,
MOSFET, or other MEMS protection device or semiconductor switch
that reduces power consumption and transmits a voltage to the
transmit protection circuit 90.
[0037] In another embodiment, it is also envisioned that an energy
storage device be included in the circuit 96 of the integrated
balun-LNA system. In one embodiment, a capacitor can be used to
temporarily store energy. The capacitor functions to maintain
performance of the integrated balun-LNA system by reducing
interference that can be caused during switching in the signal
line, as can occur during powering of the LNA and the transmit
protection circuit. For example, the capacitor provides a temporary
source of power to the LNA to maintain operation thereof during the
transmit protection phase of the signal line operation. As an
additional element, filters can be employed to further help mask
this switching between the LNA and the transmit protection circuit.
While a capacitor has been described for reducing interference
during switching of the signal line powering, it is also envisioned
that other suitable devices can be employed for the same
purpose.
[0038] Referring back to FIG. 3, other additional components can
also be added to the integrated balun-LNA system 68. In one
embodiment of the present invention, a Schottky diode 101 is
included to disconnect the LNA 64 from signal line 88 during a
transmit phase.
[0039] Beyond any modifications implemented in the circuitry of the
integrated balun-LNA system 68, modifications can also be made to
the balun shield 70. To further enhance the performance of the
balun shield 70, it is envisioned that a capacitive element can be
integrated into the balun shield 70 design. As shown in FIG. 6, end
cover 78 of the balun shield 70 is constructed as a two conductor
parallel plate capacitor 102 that, together with balun shield 70,
forms a parallel resonant circuit. Parallel plate capacitor 102
includes an inner conductor plate 104 and an outer conductor plate
106 comprised of a non-magnetic conductive material. As one
example, conductors 104, 106 comprise aluminum conductors. Between
the inner and the outer conductor plates 104, 106 is a dielectric
substrate 108 that maintains an electric field between the
conductor plates 104, 106. In one embodiment, the dielectric
substrate 108 is comprised of Kapton, FR4, or a BaTi (barium
titanium) based ceramic. It is envisioned, however, that other
materials may form the dielectric substrate 108. Inner and outer
conductor plates 104, 106 adhere to dielectric substrate 108 via,
for example, an adhesive or through bonding by means such as
sputtering of the conductors onto the substrate or by thermal
bonding.
[0040] An additional capacitor unit 130 can be attached to the
parallel plate capacitor 102 to increase or tune capacitance in the
balun shield 70. In one embodiment, the capacitor unit 130 is
mounted to the inward facing surface 132 of inner conductor plate
104. A first terminal 134 of the capacitor unit 130 is attached to
the inner conductor plate 104. A second terminal 136 of the
capacitor unit 130 is connected to the outer conductor plate by way
of vias 138 in the parallel plate capacitor 102. Alternatively, the
capacitor unit 130 can be mounted to the outward facing surface 140
of the outer conductor plate 106. In this configuration, first
terminal 134 of the capacitor unit 130 is connected to the outer
plate capacitor 106 and second terminal 136 is connected to the
inner conductor plate 104 by vias 138 through the outer conductor
plate 106.
[0041] FIG. 7 shows the inner conductor plate 104 and the outer
conductor plate 106 configured to substantially overlap each other
to ensure proper shielding to the electrical devices and circuitry
contained in the balun shield 70. The overlapping inner and outer
conductor plates 104, 106 of the parallel plate capacitor 102
provide a reduction in the gap between the end cover 78 and the
balun shield 70 that is typically present when a physical resonant
capacitor is attached to the end cover 78 of a balun shield 70. In
one embodiment, the inner conductor plate 104 is configured to be
fitted inside the cylindrical balun shield 70. As such, the
circumference 110 of the inner conductor plate 104 is sized to be
smaller than the circumference 112 of the balun shield 70. The
circumference 114 of outer conductor plate 106 is constructed to
overlap the circumference 112 of the balun shield 70. In another
embodiment, as shown in FIG. 8, outer conductor plate 106 can
further include an outer lip 116 that extends axially inward over a
small portion of the balun shield 70 to form a cup-like structure
that encircles the balun shield 70. Such a construction allows the
circumference 114 of outer conductor plate 106 to be seam soldered
to the circumference 112 of the balun shield 70 to create a solid
connection between the end cover 78 and the balun shield 70 and
ensure quality shielding.
[0042] As also shown in FIG. 8, the inner conductor plate 104 is
further configured to extend axially inward from the end cover 78.
The inner conductor plate 104 includes an inward extending portion
118 that runs along the circumference thereof to form a cup-shaped
inner conductor plate 104. The inner conductor plate 104 fits
within the cylindrical balun shield 70 and extends along an inner
surface 120 thereof. Such a cup-shaped inner conductor plate 104
allows for additional capacitance to be added into the overall
design of the balun shield 70.
[0043] Additional methods and mechanisms are also envisioned to
program and adjust desired capacitance into the balun shield 70. In
one embodiment, the radius and overall surface area of the inner
conductor plate 104 can be reduced to effect capacitance in the
parallel plate capacitor 102. In an additional embodiment,
capacitance of the parallel plate capacitor 102 can be programmed
by selective patterning and pattern interconnection on the end
cover 78 to obtain a desired capacitance amount. Additional surface
mount components can also be mounted to the inner conductor plate
104 to affect capacitance. In yet another embodiment, capacitance
can be programmed by changing the thickness of the dielectric
substrate 108 between the inner conductor plate 104 and the outer
conductor plate 106.
[0044] Also in one embodiment, parallel plate capacitor 102 is
constructed to allow for passage of signal line 88 therethrough.
Preferably, the signal line 88 can be guided through the parallel
plate capacitor 102 through a passage 128 positioned near the outer
circumference of the parallel plate capacitor 102, as shown in FIG.
8. Alternatively, it is envisioned that signal line 88 can be
guided through a passage positioned in the center of the parallel
plate capacitor 102.
[0045] FIG. 9 shows an additional embodiment of the balun shield 70
design where a non-magnetic conductive screw 122 is included in the
balun shield 70. Non-magnetic conductive screw 122 is inserted
axially through the parallel plate capacitor 102 and is configured
to allow frequency tuning in the balun 66. That is, an operator can
adjust the depth of penetration of the non-magnetic conductive
screw 122 through the parallel plate capacitor 102 and into the
interior cavity 124 of the balun 66 to adjust the inductance of the
common-mode inductor 69 (shown in FIG. 4) and enhance the frequency
of the balun 66. By tuning the frequency of the parallel resonant
circuit created by the parallel plate capacitor 102 and common-mode
inductance to match the imaging frequency, common mode currents of
the imaging frequency can be significantly reduced from propagating
down the signal line 88 and degrading the desired electromagnetic
signal. As shown in FIG. 9, in one embodiment, the non-magnetic
conductive screw 122 is electrically connected to the outer
conductor plate 106 and electrically isolated from the inner
conductor plate 104, so as to prevent shorting there between. In an
alternate embodiment shown in FIG. 10, the non-magnetic conductive
screw 122 is electrically connected to the inner conductor plate
104 and electrically isolated from the outer conductor plate 106.
The electrical connection of the non-magnetic conductive screw 122
to either the inner or outer conductor plate 104, 106 allows the
balun to resist electromagnetic field generated disturbances.
[0046] To provide mechanical support to the non-magnetic conductive
screw 122, a threaded non-magnetic flange 126 can be included in
the parallel plate capacitor 102. As shown in FIG. 9, the flange
126 is attached to the outer conductive plate 106 to support the
non-magnetic conductive screw 122 as it passes through the parallel
plate capacitor 102. While the construction of the flange 126 and
its exact dimensions can vary, it is envisioned that the flange 126
is able to sufficiently support the non-magnetic conductive screw
122.
[0047] Referring back to FIG. 6, parallel plate capacitor 102
integrated into end cover 78 is envisioned as being included in
balun shield 70 of a standard, stand-alone balun. However, parallel
plate capacitor is also envisioned as being included in the end
cover of the integrated balun-LNA system 68 described in detail
above and shown in FIGS. 3 and 4. Regardless of the exact system it
is incorporated with, the integration of the parallel plate
capacitor 102 into the balun shield 70 helps to reduce the number
of components in the balun and reduce manufacturing steps.
[0048] Therefore, in accordance with embodiment of the present
invention, a magnetic resonance imaging (MRI) system includes at
least one magnet for generating a magnetic field, at least one
gradient coil for manipulating the magnetic field generated by the
at least one magnet by way of a gradient field, and at least one RF
receiver coil to receive electromagnetic signals from the
manipulated magnetic field. Also included in the MRI system is at
least one integrated balun-preamplifier system configured to
condition the received electromagnetic signals, wherein the at
least one integrated balun-preamplifier system is enclosed in a
balun housing.
[0049] In accordance with another embodiment of the present
invention, an integrated balun-low noise amplifier (LNA) module
includes a balun enclosure, a preamplifier partially located in a
first chamber of the balun housing, and a common-mode inductor
located in a second chamber of the balun housing, the second
chamber also including a remaining portion of the preamplifier.
Integrated balun-low noise amplifier (LNA) module also includes an
internal shield separating the first chamber and the second chamber
to reduce transmission of magnetic fields between the first and
second chambers.
[0050] In accordance with yet another embodiment of the present
invention, a method of constructing an integrated balun-LNA module
is set forth. The method includes the steps of forming a balun
housing, positioning a low-noise amplifier LNA partially into a
first chamber of the balun housing, and positioning a remaining
portion of the low-noise amplifier and a common-mode inductor into
a second chamber of the balun housing. The method also includes the
steps of positioning an internal shielding mechanism between the
first chamber and the second chamber, positioning an external
shielding mechanism at opposing ends of the balun housing and
around the balun housing, and electrically connecting the low-noise
amplifier, the balun, the internal shielding, and the external
shielding.
[0051] The present invention has been described in terms of the
preferred embodiment, and it is recognized that equivalents,
alternatives, and modifications, aside from those expressly stated,
are possible and within the scope of the appending claims.
* * * * *