U.S. patent application number 11/209450 was filed with the patent office on 2006-01-05 for assymetric radio frequency magnetic line array.
This patent application is currently assigned to Regents of the University of Minnesota. Invention is credited to Gregor Adriany, Kamil Ugurbil, J. Thomas Vaughan.
Application Number | 20060001426 11/209450 |
Document ID | / |
Family ID | 22831036 |
Filed Date | 2006-01-05 |
United States Patent
Application |
20060001426 |
Kind Code |
A1 |
Vaughan; J. Thomas ; et
al. |
January 5, 2006 |
Assymetric radio frequency magnetic line array
Abstract
An apparatus comprises a radio frequency magnetic field unit to
generate a desired magnetic field. In one embodiment, the radio
frequency magnetic field unit includes a first aperture that is
substantially unobstructed and a second aperture contiguous to the
first aperture. In an alternative embodiment, the radio frequency
magnetic field unit includes a first side aperture, a second side
aperture and one or more end apertures. In one embodiment of a
method, a current element is removed from a radio frequency
magnetic field unit to form a magnetic field unit having an
aperture. In an alternative embodiment, two current elements
located opposite from one another in a radio frequency magnetic
field unit are removed to form a magnetic filed unit having a first
side aperture and a second side aperture.
Inventors: |
Vaughan; J. Thomas;
(Stillwater, MN) ; Adriany; Gregor; (Minneapolis,
MN) ; Ugurbil; Kamil; (Minneapolis, MN) |
Correspondence
Address: |
Schwegman, Lundberg, Woessner & Kluth, P.A.
P.O. Box 2938
Minneapolis
MN
55402
US
|
Assignee: |
Regents of the University of
Minnesota
|
Family ID: |
22831036 |
Appl. No.: |
11/209450 |
Filed: |
August 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
10637261 |
Aug 8, 2003 |
6958607 |
|
|
11209450 |
Aug 23, 2005 |
|
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|
09919479 |
Jul 31, 2001 |
6788056 |
|
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10637261 |
Aug 8, 2003 |
|
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|
60222144 |
Jul 31, 2000 |
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Current U.S.
Class: |
324/318 |
Current CPC
Class: |
G01R 33/34046 20130101;
G01R 33/3453 20130101 |
Class at
Publication: |
324/318 |
International
Class: |
G01V 3/00 20060101
G01V003/00 |
Claims
1. An apparatus comprising: a volume coil including a plurality of
current elements, the volume coil having an aperture formed by
removal or displacement of one or more current elements from a
regular or symmetric pattern or arrangement of current
elements.
2-116. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/637,261, filed Aug. 8, 2003, which is a
continuation of U.S. patent application Ser. No. 09/919,479, filed
Jul. 31, 2001, which claims priority under 35 U.S.C. 119(e) from
U.S. Provisional Application Ser. No. 60/222,144, filed Jul. 31,
2000, which applications are incorporated by reference herein.
FIELD
[0002] This invention relates to radio frequency magnetic field
units suitable for use in connection with an imaging and/or
spectroscopy system.
BACKGROUND
[0003] Radio frequency magnetic field units, such as volume coils,
are used in connection with imaging and/or spectroscopy systems,
such as but not limited to magnetic resonance imaging systems,
nuclear magnetic resonance imaging systems, functional magnetic
resonance imaging systems, and electron spin resonance systems.
[0004] A problem with many cylindrical form volume coils is that
they provide limited access to the coil volume. These cylindrical
form volume coils can be accessed only through the ends of the
cylinders or between the radio frequency (RF) current carrying
rungs or loops. The "between the rung" or lateral access is further
limited when the coil is shielded. A Faraday shield on a birdcage
for example, completely screens the lateral walls of the coil
cylinder with typically a copper clad, etched circuit board
material. The result is a "copper can." Similarly, the transverse
electromagnetic (TEM) coil circuits are composed of a cylindrical
symmetrical array of conductor rungs in parallel resonance with and
enclosed by a copper resonant cavity. The limited access provided
by end access or "between the rung" access to the coil volume
affects a subject confined to the coil volume and physicians or
technicians treating or interacting with the subject. Some subjects
are claustrophobic and cannot tolerate confinement in a volume
coil, while some medical procedures, such as brain surgery, require
access to the subject during imaging. For these and other reasons
there is a need for the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1A is an illustration of some embodiments of an
apparatus including a radio frequency magnetic field unit according
to the teachings of the present invention;
[0006] FIG. 1B is an illustration of some embodiments of an imaging
unit including a radio frequency magnetic field unit according to
the teachings of the present invention;
[0007] FIG. 2A is an illustration of some embodiments of an
alternative embodiment of an apparatus including a radio frequency
magnetic field unit according to the teachings of the present
invention;
[0008] FIG. 2B is an illustration of some alternative embodiments
of an imaging unit including a radio frequency magnetic field unit
according to the teachings of the present invention;
[0009] FIG. 2C is an illustration of some embodiments of the
apparatus including the radio frequency magnetic field unit shown
in FIG. 2A configured for use in a clinical setting;
[0010] FIG. 3A-3D are illustrations of some embodiments of the
structure of a volume coil according to the teachings of the
present invention; and
[0011] FIG. 4 is an illustration of one embodiment of a current
element suitable for use in connection with the radio frequency
magnetic field units of the present invention.
[0012] FIGS. 5A, 5B, 5C, 5D compare lumped element resonant
circuits to transmission line analogues.
[0013] FIGS. 6A and 6B show alternative circuit models for a tuned
TEM resonator according to the present subject matter.
DESCRIPTION
[0014] In the following detailed description of the invention,
reference is made to the accompanying drawings which form a part
hereof, and in which are shown, by way of illustration, specific
embodiments of the invention which may be practiced. In the
drawings, like numerals describe substantially similar components
throughout the several views. These embodiments are described in
sufficient detail to enable those skilled in the art to practice
the invention. Other embodiments may be utilized and structural,
logical, and electrical changes may be made without departing from
the scope of the present invention. The following detailed
description is not to be taken in a limiting sense, and the scope
of the present invention is defined only by the appended claims,
along with the full scope of equivalents to which such claims are
entitled.
[0015] Radio frequency magnetic field units that include an
aperture that is substantially unobstructed and located in a radio
frequency magnetic field unit and radio frequency magnetic field
units that include side apertures are described. When a radio
frequency magnetic field unit that includes a first aperture that
is substantially unobstructed is used in connection with an imaging
system, the medical benefits associated with the use of an imaging
system can be extended to claustrophobic subjects. When a radio
frequency magnetic field unit that includes side apertures is used
in connection with an imaging system, the medical benefits
associated with the use of an imaging system can be extended to
subjects that have difficulty fitting into a standard radio
frequency magnetic field unit. Methods for transforming a radio
frequency magnetic field unit that lacks an aperture that is
substantially unobstructed into a radio frequency magnetic field
unit that has an aperture that is substantially unobstructed, and
methods for transforming a radio frequency magnetic field unit that
lacks side apertures into a radio frequency magnetic field unit
that has side apertures are also described.
[0016] In addition, including an aperture in a radio frequency
magnetic filed unit, such as a coil, allows parts of the anatomy to
project from the coil (e.g. nose, or arms). This allows the rest of
the coil to be much smaller and fit much closer to a subject.
Small, close fitting coils improve image signal efficiency which
results in images of higher resolution being acquired in less time
using less power.
[0017] FIG. 1A is an illustration of some embodiments of an
apparatus 100 comprising a radio frequency magnetic field unit 102
according to the teachings of the present invention. The radio
frequency magnetic field unit 102 includes a first aperture 104 and
a second aperture 106.
[0018] The radio frequency magnetic field unit 102 generates a
desired magnetic field 108. The desired magnetic field 108 is not
limited to a magnetic field having a particular magnitude and
direction. Preferably, the desired magnetic field 108 has a
magnitude and direction suitable for use in imaging an object, such
as a human head, in an imaging system, such as but not limited to a
magnetic resonance imaging system, a magnetic resonance
spectroscopy system, a functional magnetic resonance imaging
system, or an electron spin resonance system.
[0019] The radio frequency magnetic field unit 102 is not limited
to a particular type of radio frequency magnetic field unit. In one
embodiment, the radio frequency magnetic field unit 102 is a TEM
cavity resonator. A TEM cavity resonator includes one or more
current elements having controllable elements, such as inductors
and capacitors, that are varied to tune the transmission line
resonator. In some embodiments, TEM cavity resonators include two
open ends. In alternative embodiments, TEM cavity resonators
include one open end and one closed end.
[0020] The radio frequency magnetic field unit 102 is not limited
to a particular number of current elements (shown in FIG. 4).
Current elements 110-115 shown in FIG. 1A are illustrations only
and no attempt is being made to depict the detailed components of
the current elements. In some embodiments, the radio frequency
magnetic field unit 102 includes current elements 110-115. The
current elements 110-115 are preferably arranged such that none of
the elements 110-115 obstruct the second aperture 106, but the
current elements 110-115 are not limited to a particular
arrangement. In some embodiments, the current elements 110-115 are
asymmetrically arranged, physically disconnected from one another
and reactively coupled. In alternative embodiments, the current
elements 110-115 are arranged to "enclose" a substantially
cylindrical volume. In some embodiments, each of the current
elements 110-115 is a resonant current element inductively coupled
to at least one other current element. In alternative embodiments,
each of the current elements 110-115 is a resonant current element
capacitively coupled to at least one of the current elements
110-115. When used in connection with an imaging system (shown in
FIG. 1B), the magnetic filed unit 102 is tuned to a frequency
suitable to image a particular object or subject.
[0021] The radio frequency magnetic field unit 102 is not limited
to a particular shape or volume. Preferably, the shape and volume
of the radio frequency magnetic field unit 102 approximate the
shape and volume of the object or subject to be imaged. In one
embodiment, the radio frequency magnetic field unit 102 has a
substantially cylindrical shape, including a diameter and a length
sufficient to receive a human head. In an alternative embodiment,
the radio frequency magnetic field unit 102 has a substantially
cylindrical shape that includes a longitudinal axis 116 and a
surface 118 that is substantially parallel to the longitudinal axis
116. The surface 118 need not be continuous. The current elements
110-115 are arranged substantially parallel to the longitudinal
axis.
[0022] The first aperture 104 provides a port for introducing an
object or subject into the radio frequency magnetic field unit 102.
For example, a human head (not shown) can be introduced into the
radio frequency magnetic field unit 102 at the first aperture 104.
The head is preferably oriented within the radio frequency magnetic
field unit 102 such that the eyes are directed toward the second
aperture 106. With this orientation, the subject avoids the
claustrophobic effects often experienced by subjects introduced
into a radio frequency magnetic field unit that lacks a second
aperture that is substantially unobstructed. The first aperture 104
is not limited to a particular alignment with respect to the radio
frequency magnetic field unit 102. In one embodiment, the first
aperture 104 has a center of mass point 120 that is substantially
aligned with the longitudinal axis 116. Such an alignment permits
easy introduction of the subject into the radio frequency magnetic
field unit 102. The first aperture 104 is formed at an end of the
radio frequency magnetic field unit 102. An end of the radio
frequency magnetic field unit 102 is located at an end of the
current elements 110-115.
[0023] The first aperture 104 is preferably contiguous to the
second aperture 106. A contiguous second aperture 106 permits
relatively easy introduction of a subject into the radio frequency
magnetic field unit 102 and reduces the likelihood that the subject
will experience claustrophobic effects during imaging by providing
a contiguous open space that includes the first aperture 104 and
the second aperture 106. The second aperture 106 also allows a
subject to see outside the radio frequency magnetic field unit 102
and allows a physician or technician access to the eyes, nose and
mouth of the subject.
[0024] The second aperture 106 comprises an area 122 including an
unobstructed area 124 and a potentially obstructed area. An area is
unobstructed, if the area is substantially transparent. An area is
obstructed, if the area is not substantially transparent.
Preferably, the area 122 does not include an obstructed area. The
area 122 is not limited to a particular size.
[0025] The second aperture 106 has a center of mass point 130 (not
drawn to scale) and a first aperture axis 132. In one embodiment,
the first aperture axis 132 passes through the center of mass point
130, intersects the longitudinal access 116 and is substantially
perpendicular to the longitudinal access 116. In one embodiment,
the second aperture 106 subtends an arc 134 having an arc length
136 of between about 0.degree. and about 90.degree. as traced out
by the first aperture axis 132 rotating about the longitudinal axis
118. The second aperture 106 subtending an arc 134 having an arc
length 136 of greater than 0.degree. and about 90.degree. reduces
claustrophobic effects in a human subject. However, an arc length
136 of greater than about 90.degree. increases the difficulty in
generating the desired magnetic field 108.
[0026] The second aperture 106 permits the manufacture of a radio
frequency magnetic field unit 102 that closely fits the head of a
human subject having a large nose. A radio frequency magnetic field
unit that lacks the second aperture 106 must be sized to
accommodate the large nose of a subject and therefore cannot be
designed to closely fit the head of a human subject having a large
nose. Since a close fitting radio frequency magnetic field unit
produces higher quality images than a larger loosely fitting radio
frequency magnetic field unit, the radio frequency magnetic field
unit 102 including the second aperture 106 produces higher quality
images than a radio frequency magnetic field unit that lacks the
second aperture 106.
[0027] An imaging unit 139 can be mounted on the radio frequency
magnetic field unit 102 to provide a communication link to the
second aperture 106. The imaging unit 139 is located with respect
to the second aperture 106 such that the imaging unit 139 provides
a communication link to a subject whose head is positioned in the
radio frequency magnetic field unit 102. The imaging unit 139 is
not limited to a particular type of imaging unit. In one
embodiment, the imaging unit 139 comprises a mirror. In an
alternative embodiment, the imaging unit 139 comprises a prism. In
still another alternative embodiment, the imaging unit 139
comprises a projection system.
[0028] In some embodiments, one or more apertures 144 and 145 are
formed on a side of the radio frequency magnetic field unit 102 to
permit access to a subject's ears. These apertures can be formed by
removing a current element from a radio frequency magnetic field
unit. In other embodiments, an auditory communication device 146 is
attached to one or more of the one or more apertures 144 and 145 to
communicate with a subject or provide auditory protection for the
subject. The communication device 146 is preferably capable of
providing active or passive auditory protection.
[0029] A radio frequency magnetic field unit lacking a second
aperture can be transformed into the radio frequency magnetic field
unit 102 that includes the second aperture 106. In one embodiment
of a method to transform a radio frequency magnetic field unit
lacking a second aperture into the radio frequency magnetic field
unit 102 that includes the second aperture 106, one current element
is removed from the radio frequency magnetic field unit lacking a
second aperture to form the radio frequency magnetic field unit 102
that includes the second aperture 106. Removing one current element
from a radio frequency magnetic field unit lacking a second
aperture creates a void in the radio frequency magnetic field unit
lacking a second aperture. This void provides an area in which to
form the second aperture 106. After removing a current element from
the radio frequency magnetic field unit lacking a second aperture,
currents to produce the desired magnetic field 108 are calculated
for the remaining current elements. In an alternative embodiment,
two or more adjacent current elements are removed from a radio
frequency magnetic field unit lacking a second aperture to form the
radio frequency magnetic field unit 102 that includes the second
aperture 106. Removing two or more adjacent current elements from
an the radio frequency magnetic field unit lacking a second
aperture creates a void in the radio frequency magnetic field unit
lacking a second aperture. This void provides an area in which to
form the second aperture 106 of the radio frequency magnetic field
unit 102. After removing two or more current elements from the
radio frequency magnetic field unit lacking a second aperture,
currents to produce the desired magnetic field 108 are calculated
for the remaining electronic circuits.
[0030] FIG. 1B is an illustration of some embodiments of an imaging
unit 140 including a radio frequency magnetic field unit 102
according to the teachings of the present invention. The imaging
unit 140 includes a static field magnetic field unit 142 and the
radio frequency magnetic field unit 102 located within the static
field magnetic field unit 142. Preferably, the static field
magnetic field unit 142 produces a magnetic field having a high
magnetic field strength. A high magnetic field strength enables the
production of high resolution images by the imaging unit 140.
However, the radio frequency magnetic field unit 102 is not limited
to use in connection with a particular static magnetic field or a
static field magnetic field unit that produces a particular
magnetic field strength. The radio frequency magnetic field unit
102 is suitable for use in connection with any static field magnet
used in connection with an imaging unit.
[0031] FIG. 2A is an illustration of some embodiments of an
apparatus 200 comprising a radio frequency magnetic field unit 202
according to the teachings of the present invention. The radio
frequency magnetic field unit 202 includes a pair of end apertures
204 and 205, a first side aperture 206 and a second side aperture
208.
[0032] The radio frequency magnetic field unit 202 generates a
desired magnetic field 210. The desired magnetic field 210 is not
limited to a magnetic field having a particular magnitude and
direction. Preferably, the desired magnetic field 210 has a
magnitude and direction suitable for use in imaging an object, such
as a human body, in an imaging system, such as but not limited to a
magnetic resonance imaging system, a functional magnetic resonance
imaging system or an electron spin resonance system.
[0033] The radio frequency magnetic field unit 202 is not limited
to a particular type of radio frequency magnetic field unit. In one
embodiment, the radio frequency magnetic field unit 202 is a TEM
cavity resonator. A TEM cavity resonator includes one or more
current elements having controllable elements that are varied to
tune the transmission line resonator. In one embodiment, the radio
frequency magnetic field unit 202 comprises a first group of
current elements 212 and a second group of current elements 214. In
one embodiment, the first group of current elements 212 include at
least one current element, such as current elements 216-218, and
the second group of current elements 214 include at least one
current element, such as current elements 220-222. The first group
of current elements 212 and the second group of current elements
214 are preferably arranged such that none of the current elements
216-218 or the current elements 220-222 obstruct the first side
aperture 206 or the second side aperture 208. In one embodiment,
the first group of current elements 212 are separated from the
second group of current elements 214 by a separation distance 228
of between about 15 centimeters and about 30 centimeters which is
the area available to form the first side aperture 206 and the
second side aperture 208. A separation distance of less than about
15 centimeters is insufficient to permit extremities, such as arms
or legs, or excess body mass, of a subject to fit into the first
side aperture 206 and the second side aperture 208. A separation
distance 228 of greater than about 30 centimeters results in the
radio frequency magnetic field unit 202 having a volume
significantly greater than necessary to receive a human body. When
used in connection with an imaging system (shown in FIG. 2B), the
magnetic filed unit 202 is tuned to a frequency suitable to image a
particular object or subject.
[0034] A tunable TEM resonator according to the invention has a
cavity and a set of transmission line segments which provide a high
frequency magnetic field in the cavity. Circuitry including the
distributed impedance of all the segments together determines the
field frequency.
[0035] A preferred form of segment is a length of coaxial
transmission line, wherein the center conductor's length is
interrupted intermediately, so that the circuitry, of which it
forms part, incorporates it as a half-wave resonator balanced with
respect to a virtual ground plane of the cavity.
[0036] The first side aperture 206 and the second side aperture 208
permit the extremities or excess body mass of a subject (not shown)
to be positioned outside the radio frequency magnetic field unit
202 when the subject is located inside the radio frequency magnetic
field unit 202. The first side aperture 206 and the second side
aperture 208 are substantially parallel to the first group of
current elements 212 and the second group of current elements 214.
The first side aperture 206 and the second side aperture 208 are
preferably free of physical obstructions. A physical obstruction is
a structure that prevents the extremities or excess body mass of a
subject from extending into and through the first side aperture 206
or the second side aperture 208. The first side aperture 206 and
the second side aperture 208 also permit the radio frequency
magnetic field unit 202 to receive subjects larger than an inside
diameter 230 of the radio frequency magnetic field unit 202 without
increasing the inside diameter 230 of the radio frequency magnetic
field unit 202. For many subjects, the first side aperture 206 and
the second side aperture 208, by allowing extremities or excess
body mass to extend outside the radio frequency magnetic field unit
202, increase the subject's comfort when positioned inside the
radio frequency magnetic field unit 202. A comfortable subject
tends to move less during imaging, and therefore fewer imaging
retakes are required and higher quality images are obtained when
the subject is imaged. In addition, the smaller, closer coil
improves image quality significantly for body coils. The smaller
sized coil can be made to resonate efficiently at the high
frequencies required for high field strength imaging.
[0037] The radio frequency magnetic field unit 202 is not limited
to a particular shape or volume. Preferably, the shape and volume
of the radio frequency magnetic field unit 202 approximate the
shape and volume of the object or subject to be imaged. For
example, a substantially cylindrical radio frequency magnetic field
unit having a length 232 of about 100 centimeters and the diameter
230 of about 60 centimeters has a shape that approximates the shape
of a human body. In one embodiment, the radio frequency magnetic
field unit 202 has a substantially cylindrical shape, including a
diameter and a length sufficient to receive an adult human
body.
[0038] In an alternative embodiment, the radio frequency magnetic
field unit 202 has a substantially cylindrical shape that includes
a longitudinal axis 234 and surfaces 236 and 238 that are
preferably curved and substantially parallel to the longitudinal
axis 234. The surfaces 234 and 236 need not be continuous. The
first group of current elements 212 including the at least three
current elements 216-218 and the second group of current elements
214 including the at least three current elements 220-222 are
arranged to "enclose" a substantially cylindrical volume.
[0039] The end aperture 204 provides a port for introducing an
object or subject into the radio frequency magnetic field unit 202.
For example, a human body (not shown) can be introduced into the
radio frequency magnetic field unit 202 at the end aperture 204.
The end aperture 204 is not limited to a particular alignment with
respect to the radio frequency magnetic field unit 202. In one
embodiment, the end aperture 204 includes a center of mass point
240 that is substantially aligned with the longitudinal axis
234.
[0040] In one embodiment, the first side aperture 206 and the
second side aperture 208 are contiguous to end aperture 204. A
contiguous relationship between the first side aperture 206, the
second side aperture 208 and the end aperture 204 permits easy
introduction of a subject into the radio frequency magnetic field
unit 202. The first side aperture 206 has a width or separation
distance 228 and the second side aperture 208 has a width or
separation distance 244. The width or separation distance 228 is
preferably about equal to the width or separation distance 244.
[0041] In some embodiments, the radio frequency magnetic field unit
202 includes a top-half 247 and a bottom-half 248, the top-half 247
capable of being mechanically attached and detached to the
bottom-half 248 at the first side aperture 206 or the second side
aperture 208. In one embodiment, an attachment device 249, such as
a hinge or flexible bracket, attaches the top-half 247 to the
bottom half 248.
[0042] A radio frequency magnetic field unit lacking a first side
aperture and a second side aperture can be transformed into the
radio frequency magnetic field unit 202 including the first side
aperture 206 and the second side aperture 208. In one embodiment of
a method to transform a radio frequency magnetic field unit lacking
a first side aperture and a second side aperture into a radio
frequency magnetic field unit 202 that includes the first side
aperture 206 and the second side aperture 208, two non-adjacent
current elements are removed from the radio frequency magnetic
field unit lacking a first side aperture and a second side
aperture. Preferably, the two non-adjacent current elements are
located opposite from one another. Removing two non-adjacent
current elements from the radio frequency magnetic field unit that
lacks a first side aperture and a second side aperture creates two
voids in the radio frequency magnetic field unit. These voids
provide areas in which to form the first side aperture 206 and the
second side aperture 208. After removing two non-adjacent current
elements from the radio frequency magnetic field unit lacking a
first side aperture and a second side aperture, currents to produce
the desired magnetic field 210 are calculated for the remaining
current elements.
[0043] FIG. 2B is an illustration of some embodiments of an imaging
unit 250 including the radio frequency magnetic field unit 202
according to the teachings of the present invention. The imaging
unit 250 includes a static-field magnetic field unit 252 and the
radio frequency magnetic field unit 202 located within the
static-field magnetic field unit 252. Preferably, the static-field
magnetic field unit 252 produces a magnetic field having a high
magnetic field strength. A high magnetic field strength enables the
production of high resolution images by the imaging unit 250.
However, the radio frequency magnetic field unit 202 is not limited
to use in connection with a static-field magnetic field unit or a
static-field magnetic field unit that produces a particular
magnetic field strength. The radio frequency magnetic field unit
202 is suitable for use in connection with any static-field
magnetic field unit used in connection with an imaging unit.
[0044] FIG. 2C is an illustration of some embodiments of the
apparatus including the radio frequency magnetic field unit 202
shown in FIG. 2A configured for use in a clinical setting. As can
be seen in FIG. 2C, a subject easily and comfortable fits into a
close fitting radio frequency magnetic field unit 202 which makes
radio frequency magnetic field unit 202 particularly well suited
for use in heart, lung and breast imaging applications.
[0045] Although the embodiments described above were directed to
radio frequency magnetic field units for use in connection with
imaging a human head and body, the radio frequency magnetic field
units 102 and 202 are not limited to use in connection with imaging
a human head and body. The radio frequency magnetic field units 102
and 202 are suitable for use in connection with imaging a wide
range of subjects including but not limited to human extremities,
such as arms, legs, joints, hands and feet, non-human subjects,
such as dogs, cats, mice, rats, horses, and primates and the
extremities of those non-human subjects.
[0046] FIG. 3A-3D are illustrations of some embodiments of the
structure of a volume coil 300 according to the teachings of the
present invention. The volume coil 300 includes a cavity wall 301,
which is not shaded so that the underlying structure of the volume
coil 300 can be seen. The volume coil 300 shown in FIG. 3A-3D
includes current elements 302-308. The volume coil 300 shown in
FIG. 3A includes a radio frequency conductive front end ring 310
and a radio frequency conductive backplane 312. The radio frequency
conductive front end ring 310 and the radio frequency conductive
backplane 312 are coupled to the current elements 302-308.
[0047] The volume coil 300 shown in FIG. 3B includes a radio
frequency conductive front end ring 314 having a gap 316 and a
radio frequency conductive backplane 318 truncated to the current
elements 302 and 308. The radio frequency conductive front end ring
314 and the radio frequency conductive backplane 318 are coupled to
the current elements 302-308.
[0048] The volume coil 300 shown in FIG. 3C includes a radio
frequency conductive front end ring 310 and a radio frequency
conductive back end ring 319. The radio frequency conductive front
end ring 310 and the radio frequency conductive back end ring 319
are coupled to the current elements 302-308.
[0049] The volume coil 300 shown in FIG. 3D includes a radio
frequency conductive front end ring 314 having a gap 316 and a
radio frequency conductive back end ring 320 having a gap 322. The
radio frequency conductive front end ring 314 and the radio
frequency conductive back ring 320 are coupled to the current
elements 302-308.
[0050] As can be seen in FIGS. 3A-3D, the volume coil 300 includes
an aperture 324 formed between the current elements 302 and 308.
Also, as can be seen in FIG. 3A-3D, the aperture 324 is formed by
removing a current element 326 (shown by a dashed line) from a
regular or symmetrical arrangement of current elements that
includes current elements 302-308 and current element 326 (shown by
a dashed line). In an alternative embodiment, the current element
326 is removed from the top 328 of the volume coil 300. In another
alternative embodiment, the current element 326 is displaced
(rather than removed) to form an the aperture 324.
[0051] Each of the end rings 310, 314, 319 and 320 comprise an open
end 330 of the volume coil 300 and each of the backplanes 312 and
318 comprise a closed end 332 of the volume coil 300.
[0052] The volume coil 300 includes an impedance. In one
embodiment, an adjustable impedance is included in each of the
current elements 302-308. The adjustable impedance, in one
embodiment, is a capacitance. The adjustable impedance, in an
alternative embodiment, is an inductance.
[0053] In one embodiment, the cavity wall 301 comprises return
elements of the current elements 302-308. In an alternative
embodiment, the cavity wall 301 comprises a slotted shield. As can
be seen in FIG. 3A-3D, the cavity wall 301 includes an aperture in
line with the missing or displaced current element 326.
[0054] The volume coil 300 is suitable for use in imaging a wide
range of objects and subjects including but not limited to heads,
ankles, feet, and other extremities.
[0055] Each of the radio frequency magnetic field units 102 and 202
and the volume coils 300 described above is suitable for use as a
double tuned coil, a multiply tuned coil, a circularly polarized
coil, a coil doubly tuned by the Vaughan method and an actively
detuned coil. A double tuned coil is driven at two frequencies. A
multiply tuned coil is driven at multiple frequencies. A circularly
polarized coil is driven to impart a circularly polarized radio
frequency magnetic field. The Vaughan method of doubly detuning a
coil is described in U.S. Pat. No. 5,557,247 titled "High Frequency
Volume coils for Nuclear magnetic Resonance Applications" which is
hereby incorporated herein by reference. Each of the radio
frequency magnetic field units 102 and 202 and the volume coil 300
are capable of being actively detuned/retuned for use with a local
receiving coil by adjusting the current elements included in the
coil. The current elements are adjusted by changing the impedance
of the current elements.
[0056] FIG. 4 is an illustration of one embodiment of a current
element 400 suitable for use in connection with the radio frequency
magnetic field units of the present invention. The current element
400 includes a shield or cavity wall section 402 resonant with a
conductor 404. The cavity wall section 402 is formed from a
conductive material and the conductor 404 is formed from a
conductive material. In one embodiment, the cavity wall section 402
is formed from a conductive mesh. A plurality of current elements
400 can be arranged to form an "enclosure." In one embodiment, a
plurality of current elements 400 are arranged to form a
cylindrical enclosure (not shown). In a cylindrical enclosure, the
shield or cavity wall section 402 is oriented to the outside of the
enclosure and the conductor 404 is oriented to the inside of the
enclosure. Current elements, such as current element 400, are
further described in U.S. Pat. No. 5,557,247 which is hereby
incorporated by reference herein.
[0057] Transmission line theory was used to describe the tuned TEM
resonator as a transmission line tuned coaxial cavity resonator.
Alternatively, the TEM resonator can be approximated as a balanced
comb-line, band-pass filter using a lumped element circuit of FIG.
6A. The lumped elements in this circuit approximate the distributed
element coefficients of the transmission line circuit. Analysis of
this lumped element filter circuit model adhering to methods in the
literature for "bird-cage" resonators gives inaccurate results. A
more accurate approach considers the lumped element filter's
distributed stripline analogue in FIG. 6B. This network is a
quarter wave (as in FIGS. 5A and 5C) comb-line filter interfaced
with its mirrored image at the virtual ground plane of symmetry
indicated by the dotted line. Each coaxial element, due to its
split central conductor, therefore is a resonant half wave line
(mirrored quarter wave pair, as in FIGS. 5B and 5D wave pair) whose
bisected center conductor 11 is grounded at both ends to a cavity.
The elements 9 are coupled via the TEM slow wave propagation h the
cavity. The performance characteristics of this distributed
structure are calculated from TEM assumptions.
[0058] Because the TEM coil has no endring currents (as does the
birdcage), sections of the TEM coil can be removed entirely to
provide maximum access with minimal impact to the compensated RF
field of the invention volume coil. Because the TEM coil return
current is parallel to the coil rungs, the return paths can be
discretized to narrow, unobtrusive conductors such as 1 cm strips
of transparent screen. The integrity of the TEM cavity is thus
approximately maintained while providing through the rung access,
in addition to the entirely unobstructed access provided by removal
of both an element and its corresponding return path on the
cavity.
[0059] Although specific embodiments have been described and
illustrated herein, it will be appreciated by those skilled in the
art, having the benefit of the present disclosure, that any
arrangement which is intended to achieve the same purpose may be
substituted for a specific embodiment shown. This application is
intended to cover any adaptations or variations of the present
invention. Therefore, it is intended that this invention be limited
only by the claims and the equivalents thereof.
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