Note: Descriptions are shown in the official language in which they were submitted.
CA 02482801 2011-03-09
RADIO FREQUENCY GRADIENT AND SHIM COIL
Related Applications
This application claims the benefit of United States Provisional
Application Serial No. 60/373,808, filed, April 19, 2002, and entitled "RF
GRADIENT AND SHIM COIL."
This application claims the benefit of United States Provisional
Application Serial No. 60/378,111, filed, May 14, 2002, and entitled "SHIM
GRADIENT AND PARALLEL IMAGING COIL."
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Government Rights
The present subject matter was partially supported by the National
Institute of Health (NIH) under Agency Grant Numbers NIH RO1-CA76535 and
P41 RR08079. The United States government may have certain rights in the
invention.
Technical Field
The present subject matter pertains generally to medical imaging and
more specifically to surface and volume coils for magnetic resonance imaging
and spectroscopy procedures.
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Backuo
In a magnetic resonance imaging system, for example, a radio frequency
magnetic field unit, or coil, is positioned in the bore of a magnet. The
object to
be imaged is placed within the magnetic field unit. The magnetic field unit is
driven by an excitation signal that stimulates a nuclear induction (free
induction
decay) signal in the object, which, in turn, is received by a radio frequency
coil.
The nuclear induction signal includes information characteristic of the object
being imaged. The information in the induction signal can be used to identify
chemicals and to diagnose diseases.
Different radio frequency magnetic field units are used to image different
portions of a patient depending on such variables as, for example, the patient
size
and shape and the biomedical region of interest. Thus, for any particular
imaging application, the magnetic field unit selected is typically a
compromise
between performance, size, cost and availability. Consequently, the images
resulting from the use of a particular radio frequency magnetic field unit may
be
inadequate for their intended purpose.
Brief Description of the Drawings
In the drawings, like numerals describe substantially similar components
throughout the several views. Like numerals having different letter suffixes
represent different instances of substantially similar components.
Fig. 1 includes a schematic of a magnetic resonance system according to
one embodiment of the present subject matter.
Fig. 2A includes a perspective view of a volume coil according to one
embodiment of the present subject matter.
Fig. 2B includes a section view along cut line 2B-2B of Fig. 2A.
Fig. 2C includes a view of connections for electrically connecting a
current element of the volume coil to circuitry according o one embodiment.
Fig. 3A includes a view of a volume coil according to one embodiment
of the present subject matter.
Fig. 3B includes a section view along cut line 3B-3B of Fig. 3A.
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Fig. 3C includes a view of a current element according to one
embodiment of the present subject matter.
Fig. 4 includes a view of a current element according to one embodiment
of the present subject matter.
Fig. 5 includes a flow chart of a method according to one embodiment of
the present subject matter.
Fig. 6 includes a flow chart of a method according to one embodiment of
the present subject matter.
Detailed Description
In the following detailed description, reference is made to the
accompanying drawings which form a part hereof, and in which is shown by
way of illustration specific embodiments in which the invention may be
practiced. These embodiments are described in sufficient detail to enable
those
skilled in the art to practice the invention, and it is to be understood that
the
embodiments may be combined, or that other embodiments may be utilized and
that structural, logical and electrical changes may be made without departing
from the spirit and scope of the present invention. The following detailed
description is, therefore, not to be taken in a limiting sense, and the scope
of the
present invention is defined by the appended claims and their equivalents. In
the
drawings, like numerals describe substantially similar components throughout
the several views. Like numerals having different letter suffixes represent
different instances of substantially similar components.
Fig. 1 includes a schematic of system 100 according to one embodiment.
In system 100, magnet 10 provides a static magnetic field for magnetic
resonance imaging. Disposed within the magnetic field is coil 20A. Coil 20A
provides a radio frequency field for both exciting the region of interest and
for
detecting signals from the region of interest. Coil 20A is connected to switch
25
and control 55. In one embodiment, control 55 includes an impedance control.
Switch 25, sometimes referred to as a transmit-receive switch, is
connected to receiver 40 and transmitter 50, each of which are connected to
system console 60. Console 60 is also connected to switch 25 by line 45.
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Console 60 applies a signal to line 45 to select between a receive mode and a
transmit mode for coil 20A. When in transmit mode, as determined by a control
signal on line 45, console 60 supplies a radio frequency signal to transmitter
50
and the output of transmitter 50 is delivered to coil 20A. Transmitter 50
provides a radio frequency excitation signal to coil 20A. Console 60, in one
embodiment, modulates a signal, or radio frequency current, delivered to coil
20A. In various embodiments, transmitter 50 provides an excitation signal
having a modulated amplitude, frequency or phase. The specimen is subjected
to a static magnetic field concurrent with both the transmit and receive mode.
Console 60, in one embodiment, includes a processor or controller.
Receiver 40, in one embodiment, includes a signal detector. Transmitter 50, in
one embodiment, includes a power amplifier. In one embodiment, coil 20A
includes a dedicated receiver coil connected to one or more receiver channels.
In one embodiment, coil 20A includes a dedicated transmitter coil connected to
one or more transmitter channels. In one embodiment, two or more coils 20A
are used with at least one coil dedicated for transmitting and at least one
coil
dedicated for receiving.
In one embodiment, transmitter 50 includes a single transmitter having
multiple output channels, each of which is selectively operable. For example,
in
one embodiment, transmitter 50 provides a quadrature signal to coil 20A via
multiple independent channels. In one embodiment, transmitter 50 of the figure
represents multiple transmitters, each connected to a separate current element
of
coil 20A.
Following excitation, console 60 transitions to a receive mode. When in
receive mode, console 60 provides a signal on line 45 to instruct switch 25 to
connect coil 20A with an input of receiver 40. Receiver 40 supplies an
electrical
signal to console 60 based on the received signal generated by coil 20A. Coil
20A generates an electrical signal based on a received signal which is
generated
by the specimen.
In one embodiment, receiver 40 includes multiple receive channels, each
of which is selectively operable. For example, in one embodiment, receiver 40
receives multiple signals from coil 20A via multiple independent channels. In
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one embodiment, receiver 40 of the figure represents multiple receivers, each
connected to a separate current element of coil 20A.
The current flowing in coil 20A, in various embodiments, is modulated
by console 60, transmitter 50 or switch 25. The radio frequency current
flowing
in coil 20A, in one embodiment, is circularly polarized. Circular polarization
entails sequentially driving individual segments of the coil in a manner that
creates a circularly polarized field within the coil. Circular polarization is
also
referred to as quadrature drive. In various embodiments, the phase, frequency
or
amplitude of the radio frequency current is modulated.
Control 55 is connected to coil 20A and console 60. Console 60 supplies
a signal to control 55 to select a value of a parameter of a particular
current
element or group of current elements. In one embodiment, the parameter
includes impedance. For example, in one embodiment, a program executing on
console 60 determines the impedance of one or more current elements of coil
20A. In various embodiments, the impedance of a current element is adjusted by
changing a dielectric constant between conductors of the current element, by
changing an inductance, by changing capacitance or by changing a resistance.
In
one embodiment, control 55 provides a direct current control signal to an
adjustable component of coil 20A.
In one embodiment, at least one phase shifter is connected to at least one
current element of the coil. A phase shifter allows control of the phase of a
signal propagating on the coil. Phase shifters, in various embodiments,
include
delay lines, PIN diodes or reactive components that allow selective control of
a
phase.
In one embodiment, the coil of the present subject matter is tuned to a
particular resonant frequency by adjusting a PIN diode or other devices. In
one
embodiment, the coil of the present subject matter is detuned from a
particular
resonant frequency by adjusting a PIN diode or other devices.
In one embodiment, the coil of the present subject matter is tuned to
multiple resonant frequencies.
Console 60 is connected to memory 75, user input 80, printer 65 and
display 70. Memory 75 provides storage for data and programming accessible to
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console 60. Memory 75, in various embodiments, includes random access
memory, read only memory, removable storage media, optical media, magnetic
media or other digital or analog data storage. User input 80 includes a user
accessible input device, including, for example, a keyboard, a mouse or other
pointing device, an optical device, touch sensitive screen or microphone.
Printer
65 includes hardware for producing a printed output, or paper copy, and in
various embodiments, includes a laser printer, dot matrix printer or an ink
jet
printer. Display 70 includes hardware for generating a visible image based on
data from console 60, and in one embodiment, includes a liquid crystal
display,
cathode ray tube display or other computer monitor. Other functions and
features may be present in a magnetic resonance imaging console.
Fig. 2A includes a perspective view of coil 20B according to one
embodiment of the present subject matter. Coil 20B, as shown in the figure,
includes eight current elements arranged in the shape of a cylindrical wall.
Each
current element includes a pair of linear conductors separated by a
dielectric.
The dielectric, in one embodiment, is disposed along at least a portion of the
length of the linear conductors. The conductors arranged substantially in
parallel
with each other and substantially in parallel with a central axis of the
volume
enclosed by the cylindrical wall. Dielectric 110 is disposed between an inner
conductor 120A and an outer conductor 115A. In the embodiment shown, each
current element includes an inner conductor 120A and an outer conductor 11 5A,
each in the form of a conductive strip, and inlaid on dielectric 110. A view
of
current element 130A is illustrated in section view Fig. 2B. In one
embodiment,
inner conductor 120A and outer conductor 11 5A of a current element are
mounted on a surface of dielectric 110. '
For each current element 130A, as shown in Fig. 2B, inner conductor
120A is aligned with outer conductor 115A. In one embodiment, inner
conductor 120A includes a conductor having a circular cross section as, for
example, a wire, rod or tube. In the embodiment illustrated, both inner
conductor 120A and outer conductor 115A are foil strips. In one embodiment,
outer conductor 11 5A includes a screen, mesh or perforated conductive
material.
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Inner conductor 120A and outer conductor 11 5A, in one embodiment, are
fabricated of copper. Other, conductive materials are also contemplated,
including, for example, aluminum or semiconductor materials. In one
embodiment, inner conductor 120A and outer conductor 11 5A include thin
conductive plating applied by semiconductor fabrication methods, including for
example, electroplating, vapor deposition, or etching or by adhesive bonding.
Each current element 130A of coil 20B is sufficiently spaced apart from
an adjacent current element to be electrically reactively decoupled. In one
embodiment, each current element 130A is sufficiently close that adjacent
current elements are reactively coupled.
Dielectric 110, in one embodiment, includes polytetrafluoroethylene
(PTFE) or Teflon or other non-conductive material. In the embodiment
illustrated, dielectric 110 is a continuous section of tubular material,
however,
discrete segments may also be held in alignment to create a volume coil.
In one embodiment, the dielectric includes air, liquid or other fluid.
Coil 20B of Fig. 2A is suited for use as a volume coil, such as, for
example, a head coil or body coil. Coil 20B of Fig. 2A can be used as a
transmit
coil, a receive coil or both and in one embodiment, the coil is used for
parallel
imaging.
Fig. 2C illustrates one embodiment of electrical contacts or connections
for connecting current element 130A of coil 20A with the structure and
circuitry
of Fig. 1. In the figure, outer conductor 11 5A is connected to terminal 117
by
link 116 and inner conductor 120A is connected to terminal 122 by link 121.
Terminals 117 and 122, in one embodiment, each include a binding post or other
threaded fastener. Links 116 and 121, in one embodiment, each include a
segment of electrical wire. The wire is soldered to a lug affixed to a
terminal
and either an inner conductor or outer conductor. In one embodiment, links 116
and 121 include conductive traces on an insulator. Other contacts or
electrical
connections are also contemplated, such as, for example, a cable electrically
connected to the coil and fitted with an electrical connector. In one
embodiment,
an electrical connection includes a soldered connection. In one embodiment, a
component (such as a capacitor, PIN diode or both) is soldered between inner
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conductor 120A and outer conductor 11 5A. Radio frequency signal and direct
current (DC) control leads are also soldered across the gap between inner
conductor 120A and outer conductor 11 5A. The radio frequency leads are
attached to a TR switch, preamplifier, power amplifier or some combination
thereof. The DC leads, in one embodiment, are attached to a PIN diode driver
or
voltage bias source. The connection point, in one embodiment, is positioned
across a gap disposed anywhere on the current element.
Fig. 3A illustrates coil 20C according to the present subject matter. In
the figure, coil 20C includes twenty current elements 130B arranged about a
volume. More or less than twenty current elements are contemplated for other
embodiments. Each current element 130B is sufficiently close to an adjacent
element 130B to be reactively coupled. Each current element 130B includes an
outer conductor 115B and in inner conductor 120B arranged in parallel
alignment, with each aligned on axis 160 through the center of the volume.
Dielectric 11 OB is disposed between each current element 130B.
In the embodiment shown, each current element 130B includes outer
conductor 115B. The segments are electrically connected together at connector
140. Connector 140, in one embodiment, also provides an electrical connection
to coil 20C. Other electrical connections to coil 20C are provided at
connector
130B at one end and at connector 150 at a second end. In one embodiment, a
discontinuity is provided at a point along the length of the current element,
such
as, for example, gap 165 disposed near the midpoint. Gap 165, in various
embodiment, provides improved current distribution for the current element or
improved shielding of contact points for the inside sample volume. An
electrical
connection, in various embodiments, includes a binding post, a soldered joint,
or
other electrical connector. In one embodiment, the outer conductor is split
into
more than two segments. In one embodiment, some current elements have
multiple segment outer conductors and other current elements have a single
segment outer conductor.
In one embodiment, at least one inner conductor 120B is segmented or
split. At the gaps between segments or splits, a contact point may be provided
for connecting components. Examples of components include capacitors,
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inductors, PIN diodes, voractors, radio frequency cable attachment points, DC
control lines or other components.
Fig. 3B includes a section view along cut line 3B-3B of the embodiment
in Fig. 3A. In the figure, outer conductors 11 5B are set atop a surface of
dielectric 110B and inner conductors 120B are set below a surface of
dielectric
1 l OB. The figure illustrates a segmented dielectric with two segments shown
as
well as two portions of segments. Adjacent dielectric segments, in one
embodiment, are joined together by an adhesive, mechanical fastener or by
other
fastening means. Adjacent dielectric segments, in one embodiment, are
continuous. Adjacent outer conductor segments 11 5B are separated by a space,
or slot.
Fig. 3C illustrates current element 130B according to one embodiment,
such as that shown in Figs. 3A and 3B. The figure illustrates two segments, or
sections, of outer conductor 115B separated by gap 165. Each segment of outer
conductors 115B is connected by electrical component 155. In one embodiment,
component 155 includes a reactive component, such as, for example, an
electrical capacitor. In one embodiment, component 155 includes an electrical
wire, an inductor, a resistor, or other combination of passive or active
electrical
components. In one embodiment, electrical component 155 is adjustable such
that an electrical parameter or quality can be selectively varied. Electrical
component 155 is connected to outer conductor 115B at connectors 140.
Connector 140, in one embodiment, includes a solder point. In one embodiment,
current element 130B includes connection points disposed on either side of a
gap
in the current element between outer conductor 115B and inner conductor 120B.
In one embodiment, current element 130B includes connection points disposed
at a strategically selected point on the current element. In one embodiment,
current element 130B includes connectors 145 disposed at ends of outer
conductor 115B. A portion of inner conductor 120B is illustrated and includes
a
connector 150.
In one embodiment, one or more radio frequency signals are provided to
coil 20B or received from coil 20B. In one embodiment, one or more control
signals are provided to coil 20B or received from coil 20B. The radio
frequency
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signals or control signals are electrically connected to coil 20B across gaps
positioned at 140, 145 or 150 or at other positions selected on a current
element.
Electrical component 155, in various embodiments, includes a PIN diode,
a transistor, a voractor, a phase shifter or other active electrical
component.
Electrical component 155, in various embodiments, includes a capacitor, an
inductor, a filter, a TR switch, a preamplifier circuit or a power amplifier
feed
point. In one embodiment, electrical component 155 includes circuitry to
adjust
the electrical coupling between the segments of outer conductor 115. For
example, in one embodiment, electrical component 155 includes a voltage
biasing circuit to adjust a PIN diode connected between the segments. As
another example, in one embodiment, electrical component 155 includes a
circuit to adjust modulation of a transistor connected between the segments.
In
one embodiment, component 155 includes a voractor or silicon controlled
rectifier.
Fig. 4 illustrates current element 130C according to one embodiment of
the present subject matter. In the figure, current element 130C includes outer
conductor 115C and inner conductor 120C separated by an air dielectric. In the
figure, outer conductor 115C includes a thin copper foil connected to
insulator
copper foil end plates 170. Inner conductor 120C includes a metallic rod or
shaft
or copper foil strip.
The present subject matter can be used for active shimming of radio
frequency fields or for selecting a slice plane or volume in a specimen under
observation. The present subject matter can be operated under control of
console
60 or operated manually or operated by other controlling circuitry. The
following describes methods of using the present subject matter.
Fig. 5 illustrates a flow chart of method 500 according to one
embodiment of the present subject matter. The method starts at 505 and
proceeds to 510 where the coil is positioned about a patient or other nuclear
magnetic resonance active sample. In one embodiment, the coil is placed to
generate a field at the region of interest. In one embodiment, this entails
positioning coil 20A within magnet 10, as illustrated in Fig. 1, and placing a
body or specimen within the volume. In one embodiment, a body coil according
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to the present subject matter is built into a bore of a magnet. At 515, a
radio
frequency field is generated within the volume. The driving signal for the
radio
frequency field is provided by transmitter 50 of Fig. 1 and connected to coil
20A
by switch 25 in response to a signal on line 45 from console 60. The
transmitter
signal, in one embodiment, is divided in a power splitter to feed multiple
current
elements. At 520, the image uniformity or other desired parameter within the
volume, or at the region of interest, is measured. In one embodiment, the
parameter includes a particular measure of field homogeneity. In one
embodiment, the parameter is selected to optimize imaging at the region of
interest. At 525, an inquiry is presented to determine if the measured
parameter
is satisfactory. If not satisfactory, then the procedure continues at 530
wherein
the coil is adjusted. Coil adjustment can be performed, for example, by
changing an inductance value, a capacitance value or a resistance value or
other
the value of other components or circuit functions for one or more current
elements of the coil. An impedance adjustment will result in a change of
amplitude, phase or frequency of the current flowing in the coil. Following
adjustment of the coil (for example, adjusting the impedance) at 530,
processing
continues at 515 where the radio frequency field is again generated. If the
field
or other measurement criteria are met following the inquiry at 525, then the
method ends at 540. In one embodiment, the measurement criteria may entail
determining if the field is sufficiently homogenous. The foregoing procedure
is
a description of negative feedback.
By way of example, in a 4 tesla (T) magnet, when imaging the heart, a
radio frequency field dependant signal artifact may obscure the image.
According to the present subject matter, the radio frequency field generated
by
the coil is manipulated to remove the artifact and improve the heart image
uniformity. The independently controllable current elements are adjusted to
compensate for radio frequency field inhomogeneities created by radio
frequency wave propagation and loss phenomena in the anatomy. In one
embodiment, the radio frequency field (sometimes referred to as the B1 field)
is
produced by the coil at the Larmour frequency. Radio frequency shimming, in
the manner described herein, is used to adjust, manipulate, or steer the radio
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frequency field to approximately optimize the field for a nuclear magnetic
resonance measurement at a region of interest.
In one embodiment, the present subject matter is used to produce a
desired radio frequency field gradient. The radio frequency field gradient, in
one
embodiment, allows for selective excitation of the imaging volume, as
illustrated, for example, by method 600 of Fig. 6. In the figure, the
procedure
begins at 605 and proceeds to 610 where the coil is positioned at a region of
interest. The region of interest may include a portion of a human body or
other
specimen to be imaged. At 615, a slice or volume is selected for signal
acquisition. The slice, in one embodiment, includes a three dimensional volume
in the region of interest. At 620, an impedance or other property of one or
more
current elements is adjusted to select a particular slice or volume in the
object.
At 625, the data is acquired for the selected slice or volume. The method ends
at
630.
The amplitude and phase of the field gradient can be changed during an
imaging scan by a variety of signal acquisition protocols. Gradient selection,
in
the manner described herein, can be used to improve the B1 field over each
slice
or volume element in a multiple slice scan.
In one embodiment, the impedance of a current element is adjusted by
changing an impedance. The impedance can be changed, for example, by
adjusting a dielectric in the core of an inductor or by changing the spacing
of
windings or by other means of changing the inductance. In one embodiment, the
capacitance of a current element is adjusted by changing a capacitor. The
impedance can be changed by adjusting a dielectric between plates of the
capacitor or by changing the spacing on the plates or by other means of
changing
the capacitance. In one embodiment, the impedance is changed by physically
adjusting a core or dielectric element.
In one embodiment, control of the procedures shown in Fig. 5 and Fig. 6
are executed by a console, a processor or other circuitry adapted to execute a
procedure. For example, in one embodiment, console 60 in Fig. 1 includes a
processor executing programming to adjust an impedance or select a region of
interest for imaging.
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The impedance of each current element of the present subject matter is
independently adjustable. For example, in one embodiment, the impedance of a
first current element can be increased while that of a second current element
can
be reduced without regard for the impedance of the first current element. In
one
embodiment, the impedance of multiple current elements in a group are adjusted
as a unit. For example, in one embodiment, a threaded shaft is rotated to move
a
core within an inductor for a number of current elements, thereby changing the
dielectric constant and thus, the impedance. Individual current elements can
be
adjusted independently to achieve a particular value or parameter and yet, as
a
whole a group of current elements can be adjusted to achieve a particular
strategy.
The radio frequency field within the volume of a coil according to the
present subject matter is dependent, in part, on the electrical properties of
the
anatomy or other sample to be imaged. For example, in a magnetic field of 7 T,
the wavelength in air is approximately one meter (m) whereas the wavelength in
human brain tissue is approximately 12 centimeters (cm). Thus, upon
introduction of a human head into the volume of the coil, the radio frequency
magnetic field within the head load is distorted by the electrical properties
of the
head. These anatomy, or load dependent distortions will often result in a non-
uniform image. To create a more homogenous, or uniform image in such a coil,
the impedance of one or more current elements can be independently adjusted to
compensate for load dependent B1 field distortions. The current elements can
be
adjusted individually or as part of a group of current elements.
Alternative Embodiments
Variations of the above embodiments are also contemplated. For
example, in one embodiment, the present subject matter is adapted for use with
imaging systems, such as, for example, spectroscopy systems, magnetic
resonance imaging systems, nuclear magnetic resonance imaging systems,
functional magnetic resonance imaging systems, and electron spin resonance
systems. In one embodiment, the present subject matter is adapted for used
with
a technology utilizing a radio frequency coil.
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In various embodiments, the present subject matter includes a solenoidal
coil, a planar (surface) coil, a half-volume coil, a volume coil, a quadrature
coil
or a phased array coil, each of which include one or more current elements as
described herein. For example, a surface coil, in one embodiment, includes a
plurality of parallel current elements in adjacent alignment.
In one embodiment, a first radio frequency coil is used to transmit an
excitation signal and a second radio frequency coil is used to receive a
signal
from the object or specimen under investigation.
In one embodiment, the present subject matter is adapted for parallel
imaging. In parallel imaging, a plurality of one or more independent current
elements are used to receive a signal. The signals received by each current
element are combined through post processing to form a composite image. In
one embodiment, a processor or console receives the plurality of signals and
compiles the image. In one embodiment, an excitation signal is provided by one
or more current elements and each current element is reactively decoupled from
an adjacent current element.
In one embodiment, the present subject matter includes programming to
cause an imaging system to perform shimming or gradient selection. The
programming is adapted to run on a processor or console connected to a radio
frequency coil. The programming may include instructions for operation by the
processor or console.
In one embodiment, the impedance or other coil control component are
manually adjustable. In one embodiment, the present subject matter includes a
computer-accessible or machine-accessible storage medium with instructions
and data to execute a method described herein.
In one embodiment, the present subject matter includes a plurality of
current elements as described herein. In various embodiments, the current
elements include wave guides, cavities, transmission line segments, microstrip
segments or coaxial line segments.
In one embodiment, the present subject matter is used for interactive
image optimization or negative feedback optimization.
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In one embodiment of the present subject matter, one current element is
electromagnetically decoupled from an adjacent current element. While, in
some embodiments, a measurable amount of coupling may exist between
adjacent current elements, nevertheless, it is understood that adjacent
current
elements are adequately decoupled for certain purposes, such as, for example,
performing parallel imaging.
Decoupling, in one embodiment, includes physically separating adjacent
current elements by a distance sufficient to reduce electromagnetic coupling.
By
introducing adequate physical separation, the field from one current element
will
have a de minimis effect on the field of an adjacent current element.
Electronic
circuitry can also be used to decouple current elements. For example, in one
embodiment, a suitably sized capacitor or inductor provides substantial
decoupling of adjacent current elements.
In one embodiment, each current element may be described as a discrete
resonant current element in that elements do not rely on current flowing in an
end-ring for proper operation. The current path in a current element is
substantially confined to the inner and outer conductors and current
significant to
the operation of the coil does not flow in an end-ring structure. However, in
one
embodiment, an end ring is provided.
In one embodiment, a parameter associated with a radio frequency field
is measured to gauge performance of the coil. Parameters may be measured in-
situ or in conjunction with the development of a magnetic resonance image or
nuclear magnetic resonance spectra. In one embodiment, the parameter is
determined interactively in an iterative process of measuring and adjusting an
adjustable component of a current element of the coil. For example, in one
embodiment, the parameter includes field homogeneity. In one embodiment, the
parameter includes signal intensity. For nuclear magnetic resonance, the
signal
amplitude of images or spectra is used as parameter. In one embodiment, the
parameter includes determining how much power is needed to achieve a
predetermined intensity in a region. In one embodiment, the parameter includes
signal to noise ratio. Other parameters include the field of view, relaxation
constants (such as T1 and T2), echo time (TE) and repetition time (TR).
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In one embodiment, a phase shifter is used to adjust the current phase in
individual current elements. A phase shifter, in various embodiments, includes
a
delay element, capacitor or a PIN diode circuit.
In various embodiments, radio frequency transmit signal amplitude is
controlled by the power amplifier gain. Receiver signal amplitude, in one
embodiment, is controlled by the gain of a preamplifier.
In various embodiments, the frequency of a radio frequency signal is
controlled by an inductor or capacitor. In one embodiment, capacitance can be
provided by a discrete capacitor or distributed capacitance. In one
embodiment,
inductance can be provided by a discrete inductor or distributed inductance.
For purposes of shimming, according to one embodiment, a component
of a current element is adjusted to establish a desired radio frequency field
within the coil. The field can be manipulated to provide a suitable bias to
compensate for body-caused artifacts. During the study, the bias is
maintained.
For purposes of gradient selection, according to one embodiment, the
bias is switched to progress across a region of interest over a period of
time. By
sweeping the bias across the region of interest, individual slices or volumes
can
be selected at different times.
For purposes of parallel imaging, according to one embodiment, the
current elements are electromagnetically decoupled. Transmitting using such a
coil, in one embodiment, includes driving each current element directly from a
single transmitter signal divided and distributed to the elements by means of
a
power splitter rather than relying on inductive coupling for signal
propagation.
In one embodiment, multiple power amplifiers are dedicated to respective
current elements in the coil.
In one embodiment, the present subject matter includes a plurality of
discrete resonant current elements each disposed about a region of interest.
Each
current element includes pair of parallel conductors that are separated by a
dielectric. Each current element includes an adjustable component.
In various embodiments, the adjustable component includes a
capacitance, an inductance, a voractor, a PIN diode, or a phase shifter. In
one
embodiment, a preamplifier (or receiver) is connected to a current element. In
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one embodiment, a transmitter is connected to a current element. In one
embodiment, a radio frequency filter circuit is connected to a current
element. In
one embodiment, a transmit-receive switch is connected to the first current
element. In one embodiment, a combiner is connected to two or more current
elements. In one embodiment, a power splitter is connected to two or more
current elements. In one embodiment, a component control line connected to the
first current element and is adapted to control the adjustable component. The
component control line, in various embodiments, includes a direct current or
alternating current control signal. In one embodiment, a pair of current
elements
are electromagnetically decoupled.
In one embodiment, a system includes a radio frequency coil and a
console connected to an adjustable component of a current element of the coil.
The console is adapted to control the adjustable component.
In one embodiment, the current element is connected to the console by a
transmitter. In one embodiment, the current element is connected to the
console
by a receiver. In one embodiment, the current element is connected to the
console by a control line that is connected to the adjustable component. In
one
embodiment, the console includes programming to provide radio frequency field
shimming. In one embodiment, the console includes programming to select a
radio frequency field gradient. In one embodiment, the console includes
programming to provide parallel signal excitation. In one embodiment, the
console includes programming to provide parallel signal reception. Parallel
signal excitation and reception are used with parallel imaging.
In one embodiment, a method according to the present subject matter
includes positioning a sample relative to a radio frequency coil. In one
embodiment, this entails placing the sample adjacent the coil. In one
embodiment, this entails placing the sample within a volume of the coil. The
method includes comparing a parameter with a predetermined value. The
measured parameter includes a radio frequency field dependant parameter
associated with nuclear magnetic resonance. If the measured nuclear magnetic
resonance parameter is unsatisfactory, then the method entails adjusting the
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adjustable component to achieve a satisfactory nuclear magnetic resonance
value.
In various embodiments, the method includes adjusting an impedance, a
capacitance, an inductance, a PIN diode or adjusting a phase. In one
embodiment, the method includes adjusting a preamplifier connected to a
current
element. In one embodiment, the method includes adjusting a transmitter
connected to a current element. In various embodiments, the method includes
adjusting a radio frequency filter circuit, a 'transmit-receive switch, or a
component control line connected to a current element
In one embodiment, a method according to the present subject matter
includes positioning a sample relative to a radio frequency coil having a
first
current element and a second current element: The method includes adjusting
adjustable components of the first current element the second current element
to
achieve a satisfactory nuclear magnetic resonance value and processing a
signals
received from the two current elements using a parallel imaging routine.
In one embodiment, the method includes an article having a machine-accessible,
storage medium including stored data, wherein the data, when accessed, results
in a machine performing a method. The method includes determining a
parameter of a field in a region of interest proximate a radio frequency coil
and
adjusting an impedance of a first current element at a time when an impedance
of a second current element remains fixed. The impedances are adjusted such
that the impedances cause the parameter to be satisfied. In one embodiment,
the
parameter includes determining image uniformity. In one embodiment, the
second impedance is adjusted without affecting the first impedance. In one
embodiment, a reactive component is adjusted.
Conclusion
The above description is intended to be illustrative, and not restrictive.
Many other embodiments will be apparent to those of skill in the art upon
reviewing the above description.
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