U.S. patent application number 15/402326 was filed with the patent office on 2017-05-04 for combined mri and radiation therapy equipment.
This patent application is currently assigned to Siemens PLC. The applicant listed for this patent is Siemens PLC. Invention is credited to Simon James Calvert.
Application Number | 20170120074 15/402326 |
Document ID | / |
Family ID | 44147403 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170120074 |
Kind Code |
A1 |
Calvert; Simon James |
May 4, 2017 |
COMBINED MRI AND RADIATION THERAPY EQUIPMENT
Abstract
In a combined MRI and radiation therapy system, a magnet
structure and radiation therapy equipment are provided. The magnet
structure comprises a single substantially cylindrical field coil
structure comprising a number of superconducting coils joined by a
support structure and extending axially of a central region. An
outer vacuum chamber encloses the field coil structure in an
evacuated volume. A cooling arrangement comprising cooling tubes is
in thermal contact with the superconducting coils and receives a
cryogen flowing through the cooling tubes. The radiation therapy
equipment comprises a gamma radiation source rotatable about an
axis of the field coil structure to direct a radiation beam
substantially radially through the field coil structure. Parts of
the field coil structure and the outer vacuum chamber are
transparent to radiation emitted by the gamma radiation source
whereby the radiation beam is directed through the field coil
structure and the outer vacuum chamber without substantial
interference.
Inventors: |
Calvert; Simon James;
(Oxfordshire, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Siemens PLC |
Camberley |
|
GB |
|
|
Assignee: |
Siemens PLC
Camberley
GB
|
Family ID: |
44147403 |
Appl. No.: |
15/402326 |
Filed: |
January 10, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14112741 |
Oct 25, 2013 |
|
|
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15402326 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B 2562/17 20170801;
A61N 5/1082 20130101; A61N 2005/1055 20130101; A61B 5/055 20130101;
G01R 33/3815 20130101; A61N 5/1081 20130101; G01R 33/3804 20130101;
A61N 5/1077 20130101; A61N 5/1039 20130101; A61N 2005/1094
20130101; G01R 33/4808 20130101 |
International
Class: |
A61N 5/10 20060101
A61N005/10; G01R 33/48 20060101 G01R033/48; G01R 33/38 20060101
G01R033/38; A61B 5/055 20060101 A61B005/055; G01R 33/3815 20060101
G01R033/3815 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 2012 |
EP |
PCT/EP2012/053981 |
Claims
1. A cylindrical superconducting magnet comprising: an inner coil
and an outer coil, aligned along an axis and housed within a
generally cylindrical outer vacuum container, and a thermosyphon
cooling arrangement; thermosyphon cooling arrangement comprising a
cryogen tank that is smaller than the inner coil and the outer
coil, and a thermosyphon tube extending around the outer coil in
thermal contact with its surface, the thermosyphon tube comprising
a bore connected to the cryogen tank at its two ends, one end of
the bore entering the cryogen tank at an upper location, above a
maximum liquid cryogen level, the other end of the bore entering
the cryogen tank at a lower location below a minimum liquid cryogen
level, and the cryogen tank is positioned within the generally
cylindrical shape of the OVC, and a portion of the outer coil is
not in thermal contact with the thermosyphon tube below the minimum
liquid cryogen level over a length of the portion.
2. A cylindrical superconducting magnet according to claim 1
wherein the portion subtends an angle at the axis wherein the angle
lies in the range of 40.degree.-120.degree..
3. A cylindrical superconducting magnet according to claim 1
wherein a higher pressure of cryogen is provided at the end of the
bore entering the cryogen tank at the lower location than the
cryogen pressure at the end of the bore entering the cryogen tank
at the upper location.
4. A cylindrical superconducting magnet according to claim 1,
further comprising a further thermosyphon tube extending around the
inner coil in thermal contact with its surface, the thermosyphon
tube comprising a bore connected to the cryogen tank at its two
ends, one end of the bore entering the cryogen tank at an upper
location, above a maximum liquid cryogen level, the other end of
the bore entering the cryogen tank at a lower location below a
minimum liquid cryogen level.
5. A cylindrical superconducting magnet according to claim 1,
comprising a plurality of inner coils and a plurality of outer
coils, each provided with a thermosyphon tube each extending around
the respective coil in thermal contact with its surface, wherein
the thermosyphon tubes are connected to the cryogen tank though
manifolds, a single tube extending from each manifold to the
cryogen tank at respective upper and lower locations.
6. A cylindrical superconducting magnet according to claim 1,
wherein a thermally conductive cooling strap or thermally
conductive member is provided, extending around the outer coil,
extending at least over the portion of the outer coil which is not
in thermal contact with the thermosyphon tube below the minimum
liquid cryogen level over a length of the portion.
Description
RELATED APPLICATION
[0001] The present application is a divisional application of
application of Ser. No. 14/112,741, filed on Oct. 25, 2013.
BACKGROUND
[0002] The present disclosure relates to a combined MRI and
radiation therapy system. In particular, it relates to such a
system which is compact, inexpensive and employs a background
magnetic field for MRI imaging with high magnetic flux.
[0003] Recently, attempts have been made to combine imaging systems
with therapy systems, particularly in the field of radiation
therapy, as such combined systems allow localization of tumors as,
or immediately before, the treatment beam is applied. This ensures
that the treatment beam is correctly targeted, in turn meaning that
treatment may be more effective and that unintentional irradiation
of healthy tissue is minimized.
[0004] Certain radiation therapy systems utilize highly penetrating
gamma-like radiation to kill cancerous tissue. Gamma radiation is
generally regarded as electromagnetic radiation having a wavelength
of between 10.sup.-10 m and 2.times.10.sup.-13 m, or quantum energy
in the range 10.sup.4 eV to 5.times.10.sup.6 eV. High energy x-rays
also fall within this range, and the present description should be
understood in the sense that "gamma radiation" includes all
electromagnetic radiation of sufficient energy to be useful in
radiation therapy applications.
[0005] Gamma radiation is not perturbed by magnetic fields and can
only be screened by the use of significant amounts of dense
material such as lead or concrete. Radiation of this type is
normally generated by either small linear accelerators or by
gamma-emitting radioactive sources such as colbalt-60. Since linear
accelerators are affected by background magnetic fields, the second
of these options is preferred in the present preferred embodiment,
as the magnetic field required by the MRI system will not interfere
with the generation of gamma radiation using a radioactive
source.
[0006] Previously, separate MRI and radiation therapy systems were
used but this was found to be far from ideal. Problems with organ
motion and image registration led to poor utilization of the
available radiation dose and accidental necrosis of viable
tissue.
[0007] More recently, some superconducting magnet configurations
have been developed for combined MRI and radiation therapy
applications, using split magnets with a rotating gamma source in
the gap between the two parts of the magnet. The resulting complex,
cumbersome designs have relatively low field and poor homogeneity
due to the necessarily large axial distance between the center-most
coils of the magnet. The need to accommodate a rotating Gamma
source in the gap means that the problem of supporting the two
cryostat halves with respect to each other presents many
difficulties. Mechanical difficulties associated with restraining
the forces generated between the two halves of the magnet lead to
further cumbersome arrangements.
SUMMARY
[0008] The present preferred embodiments accordingly provide
combined MRI and radiation therapy systems.
[0009] In a combined MRI and radiation therapy system, a magnet
structure and radiation therapy equipment are provided. The magnet
structure comprises a single substantially cylindrical field coil
structure comprising a number of superconducting coils joined by a
support structure and extending axially of a central region. An
outer vacuum chamber encloses the field coil structure in an
evacuated volume. A cooling arrangement comprising cooling tubes is
in thermal contact with the superconducting coils and receives a
cryogen flowing through the cooling tubes. The radiation therapy
equipment comprises a gamma radiation source rotatable about an
axis of the field coil structure to direct a radiation beam
substantially radially through the field coil structure. Parts of
the field coil structure and the outer vacuum chamber are
transparent to radiation emitted by the gamma radiation source
whereby the radiation beam is directed through the field coil
structure and the outer vacuum chamber without substantial
interference.
[0010] The above, and further, objects, characteristics and
advantages of the present preferred embodiments will be more
apparent from the following description of those certain
embodiments thereof, in conjunction with the appended drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows an axial cross-section of a combined MRI and
radiation therapy system of the present preferred embodiments
invention;
[0012] FIG. 2 shows a cut-away view of a coil arrangement with
cooling and support structures, suitable for inclusion in a
combined MRI and radiation therapy system of the present preferred
embodiments; and
[0013] FIG. 3 shows an axial view of a coil arrangement with
cooling, similar to that shown in FIG. 2, and suitable for
inclusion in a combined MRI and radiation therapy system of the
preferred embodiments invention.
DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0014] For the purpose of promoting an understanding of the
principles of the invention, reference will now be made to
preferred exemplary embodiments/best mode illustrated in the
drawings and specific language will be used to describe the same.
It will nevertheless be understood that no limitation of the scope
of the invention is thereby intended, and such alterations and
further modifications in the illustrated embodiments and such
further applications of the principles of the invention as
illustrated as would normally occur to one skilled in the art to
which the invention relates are included herein.
[0015] The present exemplary embodiments particularly relate to
essentially cylindrical magnets. The term "axial" is used herein to
denote direction parallel to the axis of a cylindrical magnet,
while the term "radial" is used to indicate direction perpendicular
to axial directions.
[0016] The present exemplary embodiments provide a closely
integrated MRI and radiation therapy system without compromising
the functionality or either and, in particular, without
compromising the performance, appearance and cost of the
superconducting magnet.
[0017] The present exemplary embodiments provide a magnet
configuration which enables a combined MR and radiation therapy
system to be realized without resort to a split magnet. This will
allow a superconducting magnet of magnetic flux density 1.5T or 3T
or more to be used thus enabling better spatial imaging resolution
and/or faster imaging than is possible on existing systems, in
which the mechanical forces between parts of a split magnet limit
the magnetic flux density which can be used in such combined MRI
and radiation therapy systems.
[0018] The cost of the proposed system is expected to be inherently
lower than for the known split magnet designs, for example due to
the simpler manufacturing method for more conventional coil
geometry; and simpler, un-split, cryostat design.
[0019] FIG. 1 illustrates an axial cross-section through a combined
MRI and radiation therapy system 10 of an exemplary embodiment of
the present invention.
[0020] The MRI system includes a magnet structure 100 which
includes shield coils 102 and field coils 104, the field coils
including end coils 106 and inner coils 108. The field coils 104
are held in their relative positions by a field coil support
structure 110. Shield coils 102 are of greater diameter than the
field coils, and are held in their relative positions by field coil
supports 112. Cooling tubes 114 are provided in thermal contact
with the coils 102-108. The cooling tubes carry a cryogen and serve
to cool the coils to below the superconducting transition
temperature of the material of the coils. For example, the coils
may be cooled to approximately 4K. Surrounding the magnet structure
100 is a thermal radiation shield 120 which is cooled by suitable
means (not shown) to a temperature intermediate the temperature of
the coils and the ambient temperature. Surrounding the thermal
radiation shield 120 is an outer vacuum container (OVC) 122, which
is at ambient temperature. The volume within the OVC, including the
volume between the thermal radiation shield and the magnet
structure 100, is evacuated to a hard vacuum, as is conventional,
to provide thermal insulation between the magnet structure 100, the
thermal radiation shield 120 and the OVC 122. Solid insulation 124
may be provided in the volume between the OVC 122 and the thermal
radiation shield 120. The presence of thermal radiation shield 120
and solid insulation 124 is not a requirement of the present
exemplary embodiment, and they may be omitted provided that
sufficient cooling is provided to cool the coils 104, 102 to below
their superconducting transition temperature.
[0021] Connection 126 is a passageway allowing the cooling pipes
114 to pass out of the OVC to connect to a remote reservoir of
cryogen fluid. Such arrangement will be discussed in more detail
below. The inner volume of connection 126 is evacuated, typically
being exposed to the evacuated inner volume of the OVC. The MRI
system also comprises a gradient coil assembly 130 which, as is
well known, contains electromagnet coils which generate oscillating
magnetic field gradients in mutually perpendicular directions. The
MRI system will also comprise conventional control systems, power
supplies, RF coils for generating and receiving high frequency
magnetic fields to generate MRI images, and other equipment, as is
conventional in itself, but such components do not directly relate
to the present exemplary embodiment, and so are not illustrated or
described in detail herein.
[0022] The radiation therapy equipment 200 includes a gamma source
202, for example containing cobalt-60 mounted on a gimbal
arrangement 203 which allows the source 202 to rotate about axis
A-A and to pivot about a gimbal pivot indicated at 204. This
equipment is essentially at ambient temperature.
[0023] A suitable radiation shielding 206 may be provided,
sufficient to receive gamma radiation 206 emitted from the gamma
source 202 in any of its possible locations. In the illustrated
arrangement, the gamma source 202 may be constrained to rotate only
in a lower semicircular arc, with radiation shielding 206 provided
in a corresponding upper semicircular arc. Radiation shielding may
be a layer of lead or concrete of sufficient thickness to absorb a
beam of gamma radiation 208 emitted by the gamma source 202.
[0024] Patient 300 is schematically illustrated in FIG. 1, and will
be positioned on a patient bed (not shown), enabling the patient to
be moved 302 in an axial direction relative to the magnet structure
and the radiation therapy equipment on the patient bed.
Conventional patient beds are known, constructed of materials which
are transparent to gamma radiation. Such gamma-transparent
materials include materials of low atomic number such as beryllium,
carbon and aluminum. Resin-impregnated carbon fiber may be used.
Such a gamma-transparent patient bed should be used in systems of
the present exemplary embodiment.
[0025] In the illustrated embodiment, the arrangement of coils 102,
104 is such that the OVC may be "waisted"--that is, an axially
central portion of the OVC has a lesser outer diameter d1 than the
outer diameter d2 of the axially outer portions of the OVC. This
provides a toroidal cavity 210, and the gimbal arrangement 203 and
the gamma source 202 are substantially accommodated within the
toroidal cavity 210. The exemplary embodiment does not, however,
require such a waisted OVC, and may be embodied by a cylindrical
OVC with the gimbal arrangement and gamma source being arranged
radially further from axis A-A than the outer surface of the OVC
122.
[0026] Gimbal arrangement 203 comprises a pivoting gimbal ring 212
which retains the radiation source 202, and is able to rotate 204
by a limited amount about gimbal pivot 204, which joins the
pivoting gimbal ring 212 to a rotating gimbal ring 214 arranged to
rotate about axis A-A. An outer fixed gimbal ring 216 is in a fixed
relative position compared to the OVC, and bearings 208, such as
ball bearings or rollers, are provided between the outer fixed
gimbal ring 218 and the rotating gimbal ring 214 to allow the
latter to rotate within the former. Using the gimbal arrangement,
it is possible to arrange the gamma beam 208 to reach the patient
300 from any angle within the range of rotation of the gamma
source, and at any angle within the range of rotation about the
gimbal pivot 204. Such gimbal arrangements are known in themselves
in combination with known split magnet MRI systems of the prior
art.
[0027] The present exemplary embodiment proposes an optimal magnet
configuration suitable for the realization of a combined MRI and
radiation therapy system. In such a system, a patient 300 is
typically imaged to locate a tumor to be treated. The tumor would
then be targeted with radiation from radiation source 202 by
suitably positioning the source 202 with respect to the patient.
Two available dimensions: rotation about axis A-A in an X-Y plane,
and inclination of the source 202 with respect to the XY plane by
rotation of the pivoting gimbal ring 212 about the gimbal pivot 204
are provided by the gimbal arrangement 203. A third dimension of
relative motion between the patient and the source 202 is provided
by motion 302 of the patient in the Z direction by movement of the
patient table. As is conventional, multiple irradiation steps may
be performed on a tumor, from different angles, to ensure a high
radiation dose in the tumor, with a tolerably low radiation dose in
healthy tissue. Following irradiation of the tumor, the patient
would be re-imaged by the MRI system of the apparatus to ensure
that all of the tumor has been killed. Further irradiation steps
may be performed if the MRI imaging reveals that not all of the
tumor has been killed. MRI imaging is capable of clearly
distinguishing between live and dead tissue.
[0028] As the gamma radiation source and the gamma radiation itself
are not affected by the presence of a magnetic field, the radiation
therapy may be performed while the background magnetic field of the
MRI system, generated by coils 102-108, is present. It may be found
possible to perform imaging as the radiation therapy is performed,
with the gradient coil assembly 130 generating oscillating magnetic
fields at the same time.
[0029] The combined MRI and radiation therapy system of the
exemplary embodiment allows accurate targeting of tumors in very
mobile organs such as the lungs. The beam targeting and collimation
(conventional in themselves, and not described in detail here) can
be adjusted, almost in real time, to compensate for organ movement
during therapy. This allows effective use of the allowable patient
dose by avoiding re-dosing parts of the tumor which are already
dead. The ability of the present exemplary embodiment to employ a
magnet structure 100 generating high magnetic flux density enables
improved imaging over conventional combined MRI and radiation
therapy systems using a split magnet, which has a background field
of much lower magnetic flux density.
[0030] In order to allow a single essentially cylindrical magnet
structure 100 which permits application of radiation therapy of a
patient while in position within the magnet, it must be possible to
apply the radiation beam 208 through the structure of the magnet
100 including the OVC 122, and the thermal radiation shield
120.
[0031] This is achieved, according to a feature of the present
exemplary embodiment, by elimination of virtually all dense
structures from those parts of the magnet, OVC and thermal
radiation shield which lie within a path of the radiation beam 208
which may be selected for use in a radiation therapy irradiation
step. Those parts may be conveniently referred to as a "central
region" 180.
[0032] Conventionally, MRI magnets have been constructed by winding
coils of superconducting wire onto an aluminum former. The magnet
of the present exemplary embodiment does not employ an aluminum
former, but uses an alternative assembly process to ensure that the
magnet structure does not impede the passage of the radiation beam
208. In the illustrated embodiment, and as will be discussed in
further detail with reference to FIG. 2, this is achieved by
assembling the magnet structure 100 from alternating cylindrical
annular components, being impregnated coils 106, 104 and support
tubes 110. The support tubes can be complete cylinders, or may have
cut-outs extending partially or intermittently around the
circumference. Other coil support structures may be employed, for
example a mounting cylinder having coils mounted by their radially
outer surfaces onto the radially inner surface of the cylinder.
[0033] The central support tube 110, through which the radiation
beam 208 must be able to pass, may include a region which is
transparent to the gamma radiation of radiation beam 208. This may
be achieved by forming the central support tube from a suitable
gamma-transparent material, or including a "window" of suitable
material within the appropriate part of the central support tube.
Suitable gamma-transparent materials for forming such windows or
transparent support tubes include materials with very low atomic
number, such as beryllium and graphite/carbon composites.
[0034] A similar structure may be applied to a corresponding part
of a mounting cylinder, where used.
[0035] Cooling of the superconducting magnet coils 102-108 is
achieved by passing cryogen coolant through cooling pipes 114 in
thermal contact with superconducting coils 102, 106, 108. The
cooling pipes are joined by a manifold (not visible in FIG. 1)
comprised of suitable gamma-transparent materials such as those
described above, at least where such manifold lies within a path of
the radiation beam 208 which may be selected for use in a radiation
therapy irradiation step. The cooling pipes 114 are provided with
cryogen from a remote reservoir such as will be discussed with
reference to FIGS. 2 and 3. The reservoir is preferably kept away
from any path of the radiation beam 208 which may be selected for
use in a radiation therapy irradiation step, and is preferably
vacuum insulated by being located within a volume open to the
evacuated interior of the OVC.
[0036] The OVC 112 includes gamma-transparent inner and outer tube
sections 130 manufactured from materials such as graphite
composites or CFRP. Such materials are already used to manufacture
X-ray transparent patient beds for radiation therapy systems and CT
scanners. In the illustrated embodiment, the outer
gamma-transparent tube section 130 occupies only the central
"waisted" portion of the outer cylindrical surface of the OVC,
while the inner gamma-transparent tube section 130 occupies the
entire inner cylindrical surface of the OVC. In alternative
embodiments, the inner gamma-transparent tube section 130 may
occupy only a central part of the inner cylindrical surface of the
OVC, or the outer gamma-transparent tube section 130 may occupy the
entire inner cylindrical surface of the OVC, as appropriate to the
construction of the OVC. The gamma-transparent tube sections 130
should occupy at least all of those parts of the OVC which lie
within a path of the radiation beam 208 which may be selected for
use in a radiation therapy irradiation step.
[0037] Similarly, gamma-transparent sections 132 should be provided
in the thermal radiation shield 120 at least for all of those parts
of the thermal radiation shield 120 which lie within a path of the
radiation beam 208 which may be selected for use in a radiation
therapy irradiation step. Choice of material for the
gamma-transparent sections 132 of the thermal radiation shield
should favor materials which are also thermally conductive, and
relatively opaque to infra-red radiation, to enable the thermal
radiation shield to block infra-red radiation from reaching the
cryogenically cooled superconducting magnet coils 102, 106, 108.
Alternatively, while being less preferred, cut-outs may be provided
in the thermal radiation shield, such that the thermal radiation
shield is absent in any path of the radiation beam 208 which may be
selected for use in a radiation therapy irradiation step.
[0038] Preferably, in a finished combined MRI and radiation therapy
system according to the present exemplary embodiment, looks covers
will be provided to enclose the entire system in an aesthetically
attractive outer cover. The provision of a waisted section of the
OVC and the location of the radiation source 202 within the annular
cavity 210 provided by the waisted section means that the final
appearance of the system will be similar to that of a conventional
cylindrical-magnet MRI system. This may reduce anxiety in the
patient, and will increase the chances of effective imaging and
treatment as the patient may be more relaxed than if confronted
with the appearance of conventional combined MRI and radiation
therapy systems.
[0039] FIG. 2 shows a cutaway view of a magnet structure 100
comprising a coil arrangement with cooling and support structures,
suitable for inclusion in a combined MRI and radiation therapy
system of the present exemplary embodiment such as illustrated in
FIG. 1. Features corresponding to features illustrated in FIG. 1
carry corresponding reference numerals. An example beam of gamma
radiation 208 is illustrated for reference, along with a
representation of the OVC 122 in phantom.
[0040] In this example, shield coils 102 are mounted by their
radially outer surfaces to a radially outer support 142 which may
be composed of resin-impregnated glass fiber, for example. The
radially outer supports are mounted within the OVC by appropriate
means, not illustrated. The field coils 104 are bonded by their
axial extremities to annular or cylindrical support structures 110.
As discussed above, at least the central support structure is of
gamma-transparent material, at least in those portions which lie
within a path of the radiation beam 208 which may be selected for
use in a radiation therapy irradiation step. This structure is
supported by the support structures within the OVC 122 by
appropriate means, not illustrated.
[0041] FIG. 2 particularly illustrates a cooling arrangement
suitable for use in the present exemplary embodiment. The cooling
arrangement is one commonly referred to as a "cooling loop". A
remote cryogen reservoir 144 is provided, vacuum insulated from
ambient temperature by inclusion within a vacuum region 146 exposed
to the interior of the OVC 122. Cryogen reservoir 144 contains a
cryogen 148 which is arranged to circulate through cooling pipes
114. In the illustrated example, liquid cryogen is gravity-fed
through a feed pipe 150 to a lower manifold 152. The lower manifold
152 joins feed pipe 150 to a first cooling circuit 114a, which is
in thermal contact with field coils 106, 108, and to a second
cooling circuit 114b, which is in thermal contact with shield coils
102. Cryogen passes through first and second cooling circuits 114a,
114b, extracting heat from the coils. This will cause the cryogen
to expand, and possibly boil. This reduction in density will cause
the cryogen to rise within the cooling circuit to an upper manifold
154 which joins upper parts of first and second cooling circuits
114a, 114b to the cryogen reservoir 144. The gravity feed of
cryogen into the feed pipe 150 will displace the cryogen through
upper manifold 154 back into the cryogen reservoir 144. Preferably,
an active cooling arrangement such as a cryogenic refrigerator is
provided to re-cool the cryogen returned to the cryogen reservoir,
in preparation for re-circulation through the first and second
cooling circuits. Unlike the example of FIG. 1, cooling pipes 114
are not provided in thermal contact with each coil. Rather,
thermally conductive straps 156 are provided, each in thermal
contact with at least one cooling pipe 114 and at least one coil
106, 108. Coils are cooled by thermal conduction through cooling
straps 156 to cooling pipe 114, which is cooled by circulation of
cryogen as described above. The thermally conductive straps 156 may
be solid straps of a conductive material such as aluminum or
copper, or may be a braid or laminate of such materials, or any
other suitable thermally conductive material. Similarly, the
cooling pipes 114 may be of aluminum or copper or any other
suitable thermally conductive material. The material of the cooling
straps 156 and the cooling pipes is preferably non-magnetic. The
cooling straps do not extend into the axially central region of the
magnet structure which lies within a path of the radiation beam 208
which may be selected for use in a radiation therapy irradiation
step.
[0042] Feed pipe 150 and upper and lower manifolds 152, 154 need
not be of thermally conductive materials. As shown in FIG. 2, the
upper and lower manifolds 152, 154 of the cooling arrangement
extend across an axially central region of the magnet assembly, in
the region where radiation beams 208 will be directed during
radiation therapy treatment. To prevent these manifolds from
interfering with the radiation therapy beam 208, those portions 158
of the manifolds which lie within a path of the radiation beam 208
which may be selected for use in a radiation therapy irradiation
step are composed of a gamma-transparent material, such as those
gamma-transparent materials described above. Ceramic materials may
be preferred for the gamma-transparent parts of the manifolds.
[0043] Accordingly, all parts of the magnet structure, the OVC and
the thermal radiation shield, if any, in the central region 180
within any path of the radiation beam 208 which may be selected for
use in a radiation therapy irradiation step, are transparent to the
gamma radiation emitted by the radiation source 202, such that a
radiation beam 208 may be directed through the structure of the MRI
magnet and its cryostat to act upon patient 300 without substantial
interference from the structure of the MRI magnet and its
cryostat.
[0044] FIG. 3 shows an axial end-view of the structure of FIG. 2.
Features corresponding to features shown in FIG. 1 or FIG. 2 carry
corresponding reference numerals. The arrangement of feed and
return pipes in the remote reservoir 144 differs from that in FIG.
2, and shows an alternative arrangement. Either arrangement may be
used, or any other which ensures a higher pressure of cryogen at
the opening of the feed tube 150 as compared to the pressure at the
opening of the return tube. A certain lower section 160 of the
shield coils 102, and a corresponding certain upper section of the
shield coils are cooled by conduction cooling through a thermally
conductive cooling strap or thermally conductive member extending
around each shield coil. This simplifies the tubing required and
enables a gravity fed return even from the uppermost point of the
shield coil cooling circuit.
[0045] In alternative embodiments, the radiation source 202 and
gimbal arrangement 203 may be positioned within the OVC, in the
central region 180, in which case it is not necessary to provide
gamma-transparent portions 131 of the OVC.
[0046] In some embodiments, the field coils 102 may not be
required. In some embodiments, the gimbal arrangement may not
include the facility to tilt the radiation source 202, such that
the radiation source is constrained to rotate about the axis of the
essentially cylindrical the field coil structure 104 in the central
region 180, so as to direct a beam of radiation essentially
radially through the essentially cylindrical field coil structure
104.
[0047] The present exemplary embodiments therefore provide a
combined MRI and radiation therapy system comprising a non-split
superconducting magnet with a `gamma-transparent` central region.
This is achieved by using gamma-transparent materials, typically
using materials of low atomic number, preferably carbon or graphite
composites, in the OVC, thermal shield and magnet support
structure.
[0048] Cooling of the coils of the superconducting magnet is
provided by the use of a cooling loop arrangement, where the upper
and lower collector manifolds are linked by radiation transparent
elements at the mid-plane.
[0049] The optional realization of a `waisted` cryostat design
enables the accommodation of a gantry carrying a radiation source
and collimator mounted such as to occupy much the same space
envelope as a conventional cylindrical MRI magnet.
[0050] Although preferred exemplary embodiments are shown and
described in detail in the drawings and in the preceding
specification, they should be viewed as purely exemplary and not as
limiting the invention. It is noted that only preferred exemplary
embodiments are shown and described, and all variations and
modifications that presently or in the future lie within the
protective scope of the invention should be protected.
* * * * *