Method and apparatus for magnetic resonance imaging

Magnet assembly for use in medical magnetic resonance imaging includes means for increasing flux generation in the gap region to provide the capability of scanning smaller volume regions of a patient at increased levels of scanning resolution. The means for increasing flux generation is mechanical or electromagnetic, is coupled to each of the polar regions and maintains the gap region sufficiently large and unobstructed to allow for access to the patient by several persons during scanning. Tapered outer walls of the polar region proximate the gap region further enhance accessibility to the patient during scanning.

FIELD OF THE INVENTION

The present invention relates to magnets for medical magnetic resonance imaging, and more particularly, to such magnets having magnetic zoom capabilities and an open configuration that enables magnetic resonance imaging during surgery.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging techniques are currently used to obtain images of various portions of an anatomical region of interest. A magnetic resonance imaging magnet assembly generates magnetic field gradients to spatially encode the nuclear magnetic resonance (NMR) signals from an anatomical region which is positioned in the path of the field gradients. The NMR signals are detected and then processed to obtain images that provide an accurate representation of anatomical features and soft tissue contrast of the region of interest.

Early magnet assemblies for performing magnetic resonance imaging on a patient required that the patient be positioned in a narrow, substantially enclosed gap region. These magnet assemblies induced claustrophobic reactions in the patient and also prevented another person, such as a medical attendant or physician, from having easy access to the patient while a region of the patient was scanned to obtain a magnetic resonance image.

Recently, open type magnetic resonance imaging magnet assemblies have been developed. These open assemblies have a large gap region for receiving a patient, are configured to be less confining and also permit greater access to the patient during scanning. For example, magnet assemblies with open areas on four sides of the patient, such as those described in U.S. patent application Ser. No. 07/993,072, filed Dec. 18, 1992, and U.S. patent application Ser. No. 08/975,913, MRI APPARATUS, Gordon Danby, John Linardos, Jevan Damadian and Raymond V. Damadian, filed Nov. 21, 1997, both assigned to the assignee of the present invention and incorporated by reference herein, have been proposed which provide for imaging volumes large enough to conduct surgery therein.

U.S. Ser. No. 07/993,072 also discloses magnet assemblies configured in the form of a room with only the polar regions of the magnet visible in the room, such as projecting from either the horizontal or vertical walls of the room. These magnet assemblies further reduce claustrophobic stress for the patient and allow others even greater access to the patient during scanning. In particular, these magnet assemblies provide that one or more persons can have access to the patient while the patient is positioned between the poles of the magnet assembly during scanning. This accessibility enables a physician to perform surgical procedures on the patient that are guided by the images obtained from scanning desired anatomical region of the patient. The images obtained using open magnet assemblies, however, may not necessarily have sufficient resolution to be useful for guiding surgery in an anatomical region, which generally is smaller than the anatomical region that the magnet assembly is scanning.

Therefore, there exists a need for an open magnet assembly for magnetic resonance imaging which allows several persons to have access to a patient while the patient is undergoing scanning and furthermore provides a capability of increasing the resolution of scanning over a more limited region of interest of the patient, as desired, simply and conveniently while maintaining access to the patient substantially unimpeded and without requiring that the patient be moved.

SUMMARY OF THE INVENTION

In accordance with the present invention, a magnet assembly for use in medical magnetic resonance imaging provides a sizable gap region in which a patient can be received and allows for substantially unimpeded access to the patient while the patient is undergoing scanning of any region of interest. The magnet assembly has a capability to scan a first relatively large volume region of the patient at a first scanning resolution and the capability to scan a second, smaller volume region of the patient at higher scanning resolutions than the first scanning resolution.

In a preferred embodiment, the magnet assembly comprises a ferromagnetic yoke configured as a frame and conformed to the structure of an ordinary room. The frame includes a pair of opposing vertical ferromagnetic elements and a pair of opposing pole supports, each of which forms one side of the frame, which is the flux return path. The pole supports support respective ferromagnetic poles which face each other and are axially aligned with each other. Each of the poles includes a first body portion which is adjacent to the pole support and has a rectangular box structure. Each of the poles further includes a second body portion which extends away from the first body portion and terminates at a gap facing surface. The second body portion is a trapezoidal box structure which includes opposing walls which extend from and are in the same plane as the longer sides of the rectangular first body portion and tapered walls which extend towards the center of the pole at the same angle with respect to the shorter walls of the first body portion. The facing surfaces of the respective poles are spaced apart to define a gap region therebetween for receiving a portion of a patient and each have a magnet field gradient coil support mounted thereto. The gap region and the tapered walls of the poles which are in proximity to the gap region provide for open access to the patient during scanning.

In one aspect of the invention, means for increasing magnetic flux generation in the gap region is coupled to each of the poles. Such increasing magnetic flux generation means, or magnetic zoom means, allows for higher resolution scanning of a smaller volume region of a patient in comparison to the scanning resolution and the volume region of the patient which would be scanned, respectively, when the magnetic zoom means is not utilized. The magnetic zoom means in the magnet assembly decreases the distance between the facing surfaces of the structures of the magnet assembly which extend furthest from the respective poles into the gap region, or the gap distance of the magnet assembly, during higher resolution scanning and, alternatively, also during scanning without magnetic zoom, without substantially impeding access to the patient.

The magnetic zoom means comprises a mechanical magnetic zoom means or an electromagnetic magnetic zoom means, or both, and either of these magnetic zoom magnetic zoom means can be provided in the magnet assembly axially or non-axially axially symmetrical about the center of the poles. The mechanical magnetic zoom means is a ferromagnetic structure which extends or is extendible from the facing surface of each pole into the gap region. The electromagnetic magnetic zoom means comprises a support containing a distribution of conducting coils which is coupled to the facing surface of each pole and extends or is extendible into the gap region.

In a preferred embodiment of either magnet assembly, each pole includes a hollowed cylindrical region in which a piston formed from ferromagnetic material is received in tight fitting relation to the surface of the pole which defines the hollowed region. The piston is coupled to a magnetic zoom operating assembly which is coupled to the adjoining pole support. The operating assembly can position each of the pistons simultaneously and identically at a plurality of positions extending into the gap region to provide for higher resolution scanning of a more limited volume region of the patient in comparison to the region defined by the facing surfaces of the poles. The facing end surfaces of the pistons define the more limited volume region. The surfaces of the pole and the piston which face each other remain in substantial contact with each other at all times to provide a sufficiently large flux contact area.

In a further preferred embodiment, the hollowed cylindrical region of each of the poles receives a first ferromagnetic piston having a hollowed cylindrical region and a second ferromagnetic piston which is disposed in the hollowed region of the first piston. The first piston is in tight fitting relation to the surface of the pole defining the hollowed region and to the outer surface of the second piston facing the first piston. The first and second pistons are each coupled to the magnetic zoom operating assembly. The operating assembly can independently position each of the first and second pistons simultaneously and identically, at various distances extending into the gap region to provide for higher resolution scanning of a more limited region of a patient and adjustability of the magnet fields within the gap region when the higher resolution scanning is performed. The surfaces of the pole and the first piston which face each other, and the surface of the first piston and the second piston which face each other, remain in substantial contact with each other at all times to provide a sufficiently large flux contact area.

In a further embodiment, a multiple axis patient bed is located in the gap region so that the patient can be positioned at almost any desired angle in relation to the facing surfaces of the poles.

In another aspect of the invention, independent electromagnetic zoom means are positioned within the gap region by a mechanical support means and are separate and independent from the poles of a magnet assembly. The independent electromagnetic zoom means are arranged in the gap region to define a volume region of the patient through which an increased magnetic flux density is directed to provide for higher resolution scanning in that region.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A magnet assembly in accordance with the present invention is configured to define a sufficiently large gap region which allows several medical personnel to have easy access to a patient positioned in the gap region while an anatomical region of the patient is scanned to obtain magnetic resonance images. The scanning can include scanning of a first volume region of the patient at a first scanning resolution and utilizing a magnetic zoom capability of the magnet assembly, which increases magnetic field strength in the gap region, to scan a volume region of the patient which is smaller than the first volume region, at a higher scanning resolution than the first scanning resolution.

FIG. 1illustrates a perspective view of a magnetic resonance magnet assembly in a surgery room10, which provides the capability of scanning various anatomical regions of interest of a patient in the room10at a plurality of scanning resolutions for generating magnetic resonance images, in accordance with the present invention. The magnet assembly, preferably, is a room size magnet and the room10is a magnetic resonance imaging operating room in which surgical procedures on a patient can be guided by magnetic resonance images. It is to be understood that a magnet assembly having the magnetic zoom capability of the present invention can have other suitable configurations which do not conform to the shape of a room.

Referring toFIG. 1, the magnet assembly includes a ferromagnetic upper pole support12and a ferromagnetic lower pole support14. Ferromagnetic elements16and18are disposed between and at the ends of the pole supports12and14. The ferromagnetic elements16and18support the upper pole support12above the lower pole support14. The pole supports12and14and the ferromagnetic elements16and18, thus, form four sides of a rectangular ferromagnetic yoke or frame, which is the flux return path.

Preferably, each of the ferromagnetic elements16and18is a steel slab comprised of multiple sections about nine feet tall, about ten feet wide and about one foot thick, and each of the pole supports12and14is a steel slab comprised of multiple sections about sixteen feet long, about ten feet wide and about one foot thick. Consequently, the upper pole support12lies approximately nine feet above the lower pole support14and the inwardly facing surfaces of the ferromagnetic elements16and18are spaced apart from one another by a distance of approximately fourteen feet.

Ferromagnetic gusset plates20are provided at the corners of the frame formed by the ferromagnetic elements16and18and the pole supports12and14. The gusset plates20reinforce the frame.

Referring to FIG.1and also toFIGS. 2A and 2B, which show vertical cross-sections ofFIG. 1at the lines2A—2A and2B—2B, respectively, the magnet assembly further comprises an upper ferromagnetic pole22which projects downwardly from the upper pole support12and a lower ferromagnetic pole24which projects upwardly from the lower pole support14. Both of the poles22and24and the pole supports12and14are aligned axially and are also symmetrical about an imaginary medial plane S which extends between the ferromagnetic elements16and18. The poles22and24further define a polar axis26which extends between the pole supports12and14and about which the poles22and24and the pole supports12and14are symmetrical.

The poles22and24as illustrated inFIGS. 1,2A and2B are covered with a shroud (not shown) which substantially conforms to the geometrical structure of the outer facing surfaces of the poles22and24. A more detailed description of the elements contained within or which can be associated with the top and bottom polar regions of the magnet assembly, in accordance with present invention, is provided below in connection with the description ofFIGS. 3,4A,4B,5A,5B,7and8. For clarity, the poles22and24are described at this point only in terms of their outer surfaces, which would be visible to a person in the room and to which the shroud would substantially conform when the magnet assembly is not utilized to obtain images using its magnetic zoom capability.

Referring again toFIGS. 1,2A and2B, the pole22includes a rectangular box shaped body portion28which is adjacent to the pole support12. The body portion28includes shorter outer side walls31which are parallel to the ferromagnetic elements16and18and longer outer side walls32which are orthogonal to the ferromagnetic elements16and18. The pole22further includes a trapezoidal box shaped body portion30which is integral with and extends downwards towards the opposing pole24from the body portion28. The body portion30includes opposing longer walls33which are in the same plane as and extend from the side walls32of the body portion28. The body portion30further includes opposing tapered outer side walls34, each of which extends towards the polar axis26at the same angle in relation to the walls31of the body portion28. The taper of the walls34accordingly decreases the lengthwise dimension of the outer walls33as the outer walls33extend away from the body portion28. The outer walls33and34of the body portion30terminate in the same plane, which is parallel to the plane S, to form a rectangular pole tip facing surface38. The facing surface38, for example, can have a length of about 72 inches and a width of about 48 inches.

It is to be understood that the pole22can be constructed so that the facing surface38has another shape, such as circular or elliptical, and that the body portions28and30would be constructed accordingly to obtain such shape and also to maintain a pair of opposing walls which face the ferromagnetic elements16and18and each taper towards the polar axis26. The tapered walls34of the pole22are suitably angled in relation to the polar axis26to maximize access to a patient56which is received in the gap region42between the poles22and24of the magnet assembly.

The pole24is identical in construction to the pole22, with like structures having like reference numerals, and is not described in detail below. For clarity of reference, the facing surface on the pole24is designated below by reference numeral40. The facing surfaces38and40of the poles22and24, respectively, define a magnet gap region42therebetween which is sufficiently large to receive the body of a patient. For ease of reference, a gap distance is referred to below as the distance between the surfaces of the polar regions of the magnet assembly which extend furthest into the gap region42towards the plane S. Also, a pole separation distance is referred to below as the distance between facing surfaces of ferromagnetic structures, such as the surfaces38and40, of the respective polar regions.

Apertures44and46are defined in the pole supports12and14, respectively. A magnetic zoom operating assembly48is coupled to the surfaces of the pole supports12and14which oppose the gap region42. The assembly48covers the apertures44and46. The structure and operation of the operating assembly48in relation to the apertures44and46and a mechanical flux generation increasing means, or so-called mechanical magnetic zoom means, which can be coupled to each pole of the magnet assembly to provide magnetic zoom capability, is discussed in greater detail below in connection withFIGS. 3,4A and4B.

An upper electromagnetic coil50encircles the pole22at the juncture of the body portion28with the upper pole support12. A corresponding lower electromagnetic coil52encircles the pole24at the juncture of its body portion28with the lower pole support14. The coils50and52, alternatively, can be resistive or superconductive.

The gap region42further includes a patient support or bed54of which at least a portion is positioned between the surfaces38and40and on which the patient56is positioned lying down. One or more radio frequency (RF) transmitting and receiving antennae59are also included in the gap region42, preferably in proximity to a region of interest of the patient56which will be scanned for obtaining magnetic resonance images.

The poles22and24, the coils50and52, the antennae59, the operating assembly48and electronic components which are coupled to the poles22and24, such as magnetic field gradient coils, are linked to a conventional magnetic resonance imaging system58. The system58includes elements such as a DC power supply for energizing the coils50and52, a gradient coil power supply for energizing the magnetic field gradient coils and RF transmitters and receivers which are linked to the antennae59. The system58further includes magnetic resonance imaging hardware and software, such as a microprocessor linked to a memory, that transforms the magnetic resonance signals detected from a region of interest which is scanned into magnetic resonance images. Further, an image display or image data download device, such as a video monitor60, is connected to the microcontroller in the system58and suitably mounted inside the interior of the room10so that a physician62or another attendant (not shown) who may be involved in performing medical procedures, such as surgery, on the patient56and is standing at least partially in the gap region42near the patient56, can observe the magnetic resonance images of the patient56in real time, while performing the medical procedures.

Control apparatus64, such as a keyboard, joystick, mouse or speech recognition control module, is also linked to the system58, such as by hardwire or infrared radiation link, and disposed as near to the patient56as suitable. The control apparatus64allows the physician62, from within the room10, to control the type of scanning performed on the patient56and, in particular, to utilize the magnetic zoom capability of the magnet assembly to obtain higher resolution scanning of a smaller, more defined volume region of the patient56than the region scanned when the magnetic zoom capability is not utilized. In addition, the monitor60can include touch-sensitive elements that similarly allow one to control the type of scanning that the magnet assembly performs. Such computer control elements are well known in the magnetic resonance imaging art and are not described further herein.

The room10further includes a raised floor66which is supported above the lower pole support14by a set of braces68. The floor66extends over the top of the coils52and around the body portion28of the pole24. Ceiling suspension support members72suspend a ceiling70beneath the upper pole support12. Wall coverings74cover the inwardly facing surfaces of the ferromagnetic elements16and18and other walls76which define the room10. The floor66, the ceiling70and the wall coverings74preferably are formed from non-magnetic materials such as polymeric materials, wood fibers, paper and cementitious materials such as concrete, plaster, plasterboard and the like. The exposed, inwardly facing surfaces of the floor66, the walls74and the ceiling70desirably are formed from standard architectural materials and have the appearance of ordinary room walls. The floor66may be continuous with a floor78of a building in which the room10is located. The wall coverings74may be continuous with the walls76of the building. Likewise, the ceiling70may be continuous with a ceiling (not shown) which is part of the building.

Thus, the space within the magnet assembly and enclosed by the floor66, the ceiling70and the wall coverings74constitutes part of an ordinary room, i.e., the room10. The frame of the magnet assembly, which is defined by the pole supports12and14and the ferromagnetic elements16and18, is disposed outside of the room10. Also, the coils50and52are disposed outside of the room10. The patient56or another person inside of the room10sees the poles22and24protruding into the room10from the ceiling70and the floor66, but otherwise considers the room10to be an ordinary room. The shrouds which cover and conceal the poles22and24desirably are formed from non-magnetic materials, such as polymeric materials. Thus, a patient perceives the magnetic resonance imaging magnet assembly as entirely open and non-claustrophobic.

Each of the ferromagnetic elements16and18is disposed about seven feet from the polar axis26as measured from the polar axis26to any ferromagnetic element in a direction perpendicular to the polar axis26. The disposition of the ferromagnetic elements16and18at a substantial distance from the polar axis26allows an adult human patient to be positioned on the support54, such as a five-axis bed, in a generally horizontal position with her body extending along the medial plane S. The bed54, preferably, can be translated, as seen from the perspective ofFIG. 1, in any direction in a plane orthogonal to the flux elements16and18and also orthogonal to the facing surfaces38and40. The bed54also can be rotated up to 360° in either direction in a plane parallel to the plane S and clockwise or counterclockwise about an axis of rotation defined by a line extending between and orthogonal to the ferromagnetic elements16and18. Thus, a patient can be disposed in any radial direction with any part of her body in relation to the surfaces38and40, and essentially any part of a normal human patient can be imaged.

Moreover, the space around the poles22and24, as enabled by the tapering of the walls34, provides an unobstructed working space sufficient to accommodate the physician62or one or more persons, such as other physicians, nurses or attendants. This space is unobstructed by any portion of the frame of the magnet assembly and extends entirely around the poles22and24and the polar axis26. Thus, apart from any obstructions that the patient support54or the patient56herself can create, the attendants can have access to the patient56from all directions. This working space extends to the region of the magnet assembly between the coils50and52, which includes the portion of the working space disposed above the lower coil52and below the upper coil50. The tapered walls34of each of the poles22and24also advantageously provide additional working space in the vicinity of the patient56. As such, the magnet assembly affords a degree of access to the patient56that is essentially the same as the degree of access provided in an ordinary operating room, with only a slight obstruction caused by the poles22and24themselves.

The room10also, preferably, is surrounded with a continuous or substantially continuous electrically conductive shield, commonly referred to as a Faraday shield, which shields the working space and the gap region42from radio frequency interference to prevent interference with the magnet resonance imaging procedure. The pole supports12and14and the ferromagnetic elements16and18of the magnet frame are electrically conductive and thus, individually, form portions of the Faraday shield. The floor66, the walls76and the ceiling70of the room10are provided with conductive elements, such as conductive mesh80, as shown in FIG.1. The conductive mesh80may be electrically connected to the frame of the magnet assembly by a wire or bonding strap (not shown), which connects the mesh80to the frame.

A door82and a window84of the room10, each of which penetrates one of the walls76, are also provided with conductive coverings, such as a mesh in the door82and a conductive film on the window84. These conductive coverings desirably are also connected to the remainder of the Faraday shield.

The equipment disposed inside of the room10, and hence inside of the Faraday shield, are suitably designed for low radio frequency (RF) emission. For example, the video monitor60may be provided with an enclosure having a conductive shield which is grounded to the frame. Also, fixtures such as overhead lights (not shown) that are secured to the ceiling70may be provided with similar shielding. Equipment for performing medical procedures on a patient or any other type of conventional medical equipment also may be disposed inside the room, within the interior of the magnet frame.

In ordinary or normal mode operation of the magnet assembly, in other words, when the magnetic zoom capability of the magnet assembly is not utilized in accordance with the present invention, the pole supports12and14, the ferromagnetic elements16and18and the poles22and24are arranged to provide a path of low magnetic reluctance for the flux that the coils50and52generate. The flux is relatively concentrated in the poles22and24and in regions of the upper and lower pole supports12and14adjacent to the polar axis26. Thus, the magnetic field achievable in the gap region42at a volume region of the patient56defined by the area of the surfaces38and40facing the plane S, in the normal mode of the magnet assembly, is limited by magnetic saturation of the ferromagnetic material in the magnet assembly and the pole separation distance. In the normal mode, the pole separation distance is the distance between the surfaces38and40and is, preferably, equal to about 36 inches.

In accordance with present invention, means for increasing flux generation in the gap region42is coupled to each of the poles22and24to provide a high resolution scanning mode of operation of the magnet assembly, or a so-called magnetic zoom mode, that allows for higher resolution scanning of a smaller region of the patient, in comparison to the region scanned and the scanning resolution attainable under the normal mode of operation of the magnet assembly.

In one aspect of the invention, a mechanical means for increasing flux generation in the gap region42is coupled to each of the poles22and24of the magnet assembly.FIG. 3illustrates one embodiment of the magnet assembly including a mechanical magnetic zoom means comprising a ferromagnetic piston88, which can be extended into the gap region42from the poles22and24in the magnetic zoom mode.FIG. 3shows the piston88in the magnet assembly from the perspective of a vertical cross-section through the top polar region of the magnet assembly, which includes the top plate support12, the top pole22and the electromagnetic coil50, as shown in FIG.2B. It is noted that the bottom polar region, which includes the plate support14, the bottom pole24and the electromagnetic coil52, would have a structure that is identical to the top polar region and that the top and bottom polar regions are symmetrically aligned about the polar axis26and symmetrical about the plane S. Therefore, for conciseness, only the top polar region is described in detail below.

Referring toFIG. 3, the body portions28and30of the pole22are hollowed axially symmetrically about the polar axis26to a constant diameter W to define a hollow cylindrical volume region86within the pole22. The hollowed region86extends lengthwise through the entire pole22, from the surface38to the surface of the pole support12adjacent to the body portion28, the distance between the former and latter being equal to t. The aperture44in the pole support12also has been hollowed about the polar axis26to the same constant diameter W to define a hollow cylindrical volume region extending through the entire thickness of the support12. The inner surfaces of the pole support12which define the aperture44and the inner surfaces of the pole22which define the region86are, therefore, aligned with each other.

The hollowed region86contains the piston88. The piston88is in the shape of a cylinder bounded lengthwise by an end surface90which faces the assembly48and an end surface92which faces the gap region42. The outer surface of the piston88has a constant diameter equal or substantially equal to W and the distance between the end surfaces90and92is equal to L. Thus, the outer surface of the piston88has a constant diameter which is substantially equal to the diameter W of the region86and the aperture44.

An annular ferromagnetic structure called a shim bar94is disposed on the surface38. The shim bar94is mounted at the outer perimeter of the surface38and has a beveled inner surface which faces the pole center. The shim bar94is a conventional component positioned around the periphery of the pole22to compensate for normal magnetic field fall off at the periphery, thereby increasing the volume of uniform and homogenous magnetic field in the gap region42.

An insulative support96is mounted on the portion of the surface38which the shim bar94circumscribes. The support96contains magnetic field gradient coils98which can conduct electrical current and develop magnetic field gradients to spatially encode the region of interest being scanned according to well known techniques that are not a part of this invention.

An insulative support100is mounted on the surface92of the piston88. The support100also contains magnetic field gradient coils102which can conduct electrical current and develop magnetic field gradients. The support100with the coils102has the same thickness as the support96, and operates in the same manner as the support96with the coils98. The support100and96are each electrically coupled (not shown) to the system58and are independently controllable by the system58.

Ends106of two connecting rods104are each rigidly secured to the end surface90of the piston88. The connecting rods104extend from the end surface90, through the aperture44and are connected at opposite ends108to a means for piston positioning110which is contained in the magnetic zoom operating assembly48.

Encircling the rods104adjacent to the ends108are stop means or cylinders109which are rigidly connected to the rods104. The stop cylinders109have a diameter which is wider than the apertures in the piston positioning means110through which the rods104pass. Reinforced supports119rigidly mount the piston positioning means110to the surface of the assembly48which opposes the plane S.

The piston positioning means110is compartmentalized into two chambers by the piston head117. The assembly48further includes a controllable piston actuating means or pump112which is coupled to the two chambers of the piston positioning means110via the lines114and115, respectively. The piston head117and all penetrations of the piston positioning means110and the pump112, such as the lines114and115, have air tight seals.

In a preferred embodiment, the combination of the piston positioning means110, the pump112and the lines114and115constitutes a conventional hydraulic positioning device that is controllable by control signals that a microcontroller, such as a microcontroller in the system58, transmits to the pump112. The pump112can control fluid flow over the lines114and115to maintain the rods104at, or to move the rods104to, a predetermined position in relation to the plane S. The positioning means110is a conventional hydraulic support which can maintain the rods104stationary or move them towards or away from to the plane S, based on the fluid that the pump112supplies to or receives from either of the chambers of the positioning means110.

Based on the control signals transmitted to the pump112, the pump112can operate to receive a predetermined amount of fluid from the positioning means110over the line114and supply a predetermined amount of fluid to the positioning means110over the line115so as to retract the connecting rods104into the positioning means110a predetermined length, thereby causing the piston88to be moved the predetermined length away from the plane S. On the other hand, the actuating means112can operate to supply a predetermined amount of fluid under pressure to the positioning means110over the line114and receive a predetermined amount of fluid from the positioning means110over the line115to force the connecting rods104away from the positioning means110a predetermined length, thereby causing the piston88to be moved the predetermined length towards the plane S. When the pump112does not supply fluid to or receive fluid from the positioning means110, the rods104and thus the pistons88, are maintained in place at the same distance away from the plane S.

The piston positioning means110is of a sufficient size and is suitably positioned within the assembly48and the connecting rods104are of sufficient length to permit the piston positioning means110to controllably retain the connecting rods104when the connecting rods104are positioned such that: (i) the end surface38is in the same plane as the end surface92of the piston88; and (ii) the piston88is extended into the gap region42to a maximum extent, which would constitute a maximum level of magnetic zoom for the magnet assembly. When at least a portion of the end surfaces92of the pistons88are extended into the gap region42, the pole separation distance is the distance between the end surfaces92of the pistons88and the gap distance is the distance between the facing surfaces of the supports100which are mounted on the respective surfaces92. The gap distance at the maximum level of magnetic zoom is about 12 inches.

It is to be understood that the assembly48can contain other suitable mechanical devices for controllably positioning the connecting rods104at different positions in the gap region42in relation to the plane S in accordance with present invention, such as, for example, a pneumatic piston positioning system.

The dimensions of the piston88and the hollowed region86provide that the outer surface of the piston88is, preferably, in substantial contact with the surface of the pole22which defines the region86. Also, when at least a portion of the piston88is within the aperture44, the outer surface of the piston88which is within the aperture44is preferably in substantial contact with the surface of the pole support12which defines the region44. The diameter W of the piston88, the cylindrical hollow region86and the aperture44is suitably set to define a smaller size volume region of the patient54which is to undergo higher resolution scanning in the magnetic zoom mode. The diameter W, preferably, is about 24 inches and can be larger or smaller, as desired.

It is to be understood that the piston88may assume other shapes, such as an elliptical or rectangular body shape, and that the hollowed regions in the pole and the aperture in the pole support would have a corresponding structure which would ensure close contact between the surfaces of the piston which face the pole and the pole support and the surfaces of the pole and the pole support which define the hollowed region and the aperture, respectively.

In a preferred embodiment, the length L of the piston88is sufficient to ensure that when at least a portion of the piston88is positioned within the gap region42, the outer surface of the piston88contacts a large area of the surface of the pole22which defines the region86. The length of the piston88, preferably, provides that when the piston88vertically protrudes into the gap region42to the maximum extent, thereby providing the maximum magnetic zoom, a large flux contact area between the facing surfaces of the piston88and the pole22equal to πW×t is maintained. This large flux contact area maximizes the amount of transfer of the flux that the coil50generates and is directed into the portion of the gap region42which is defined between the end surfaces92of the respective pistons88. The quality of the ferromagnetic material used in the pole22and the amount of field strength required for achieving a predetermined level of scanning resolution in the magnetic zoom mode determines the amount of flux contact that would be required.

The operation of the magnet assembly ofFIGS. 1,2A and2B including the embodiment of the polar region illustrated inFIG. 3at both the top and bottom polar regions is, for conciseness, described below for the most part with respect to the movement of the piston88in the pole22towards and away from the medial plane S. It is to be understood that the piston88in the pole24is identical in structure and operation to the piston88in the pole22, and that each of the pistons88would move simultaneously and identically towards and away from the medial plane S during magnetic zoom mode operation of the magnet assembly.

Referring toFIGS. 1 and 3, the patient56is positioned in the gap region42on the support54with the center of an anatomical region of interest intersected by the polar axis26. In the normal mode of operation of the magnet assembly, which is ordinarily initially performed, the piston88is positioned completely within the pole22and the end surface92is in the same plane as the surface38. The coils98and102in the supports96and100are both energized for scanning. The gap distance in the normal mode is the distance between the facing surfaces of the supports100and96, which are in the same plane, and provides substantially unimpeded access to the patient56.

Magnetic resonance images in the normal operation mode are obtained by scanning a relatively large volume region of the patient56. The large volume region is defined based on the combined surface area of the end surfaces38and92which face the plane S. The scanning resolution is defined in relation to the entire surface area of the end surfaces38and92and the pole separation distance, which is the distance between the end surfaces38and92of the opposing poles22and24. The magnetic field strength of the magnet assembly generated by the coils50and52also determines the resolution of the scanning and, for simplicity, it is assumed to be constant in both the normal and the magnetic zoom modes of operation.

The operation of the magnet assembly in the normal mode may be performed as the patient56undergoes surgery in a region near or within the anatomical region being scanned. As the need arises, the physician62can, via the controller64, command the magnet assembly to operate in the magnetic zoom mode.

In the magnetic zoom mode, a higher level of scanning resolution within a smaller volume region of the patient56, which is defined by the surface area of the surface92which faces the patient56, is obtained. Upon initially receiving a command to operate in the magnetic zoom mode rather than in the normal mode, the controller in the system58would transmit control signals to the operating assembly48, particularly to the pump112, to cause the piston positioning means110to move the piston88a predetermined distance towards the plane S into the gap region42. The positioning means110forces the connecting rods104and, in turn, the piston88into the gap region42at smooth and non-abrupt increments based on the amount of fluid that the pump112supplies to one of the chambers of the positioning means110over the line114and the amount of fluid that the pump112receives from the other chamber of the positioning means110over the line115. Similarly, the positioning means110provides that the piston88can be retracted from the gap region42in smooth and non-abrupt increments based on the fluid received therefrom and supplied thereto by the pump112over the lines114and115, respectively. Also, in the magnetic zoom mode, the system58energizes only the coils102in the piston88.

The surgeon62can command the system58to locate the piston88to various preset positions within the gap region42to achieve respective higher levels of scanning resolution, as desired. For example, if the surgeon62desires to view images of the same smaller region of the patient56at various preset levels of increased scanning resolution, the surgeon62can command the system58, via the controller64, to locate the piston88further into the gap region42. At a higher scanning resolution level, the pole separation distance is the distance between the surfaces92of the opposing pistons88in the poles22and24with the pistons88within the gap region42. The movement of the pistons88into the gap region42also decreases the gap distance. At the maximum magnetic zoom, the pole separation distance is about 12 inches.

If the positioning means110malfunctions, such that the positioning means110cannot controllably retain the rods104, the stop cylinders109on the rods104would prevent the rods, and hence the piston88, from moving closer than a predetermined distance away from the plane S. The stop cylinders109prevent the rods104from emerging from the piston positioning means110beyond a predetermined extent at the apertures where the rods104are received. The reinforced supports119in combination with the assembly48can support the weight of the piston88and the piston positioning means110. Thus, the patient56is protected from injury which would be caused if the piston88of the pole22accidentally fell onto the patient56.

The movement of the pistons88of the poles22and24into the gap region42causes magnetic flux to be applied through a volume region defined between the surfaces92of the opposing pistons88. The smaller pole separation distance in the magnetic zoom made, in comparison to the normal mode, provides for an increase in the magnetic field strength at the region of interest positioned in the gap region42between the surfaces92. Although at least a portion of the piston88protrudes from the pole22into the gap region42in the magnetic zoom mode, the length of the piston88is sufficient to maintain a sufficiently large area of contact with the pole22. This large flux contact ensures the flux from the coil50is efficiently transferred into the piston88and through the smaller pole separation distance of the gap region42in the magnetic zoom mode. Further, the smaller gap distance in the maximum magnetic zoom level, in comparison to that of the normal mode, does not substantially impede access to the patient56by others, such as to interfere with surgery that is being performed on the patient56.

The combination of a high level of flux transference, provided by the large flux contact area between the piston88and the pole22, and the movement of the piston88further into the gap region42to decrease the pole separation distance and the gap distance of the magnet assembly, advantageously operates to produce higher magnetic fields through the smaller region of interest in the form of an increased flux density. The increased flux density in the smaller region of the patient56provides for higher resolution scanning within that smaller region, because the detected radiation signals at the antennae59for the smaller scanned region would have a higher radio frequency and a higher signal-to-noise ratio.

In one alternative embodiment, a series of different transmitting and receiving coils on antennae, each of which is tuned for the frequency of the corresponding preset piston location, provides the frequency appropriate to the preset position of the pistons88. In another alternative embodiment, a single receiving and transmitting coil or antenna can be tuned to multiple frequencies.

The radiation signals that are detected when the magnet assembly is operated in the magnetic zoom mode are processed to obtain magnetic resonance images in a manner similar to that performed to obtain magnetic resonance images when the magnet assembly is not operated in the magnetic zoom mode.

In one embodiment, when the microcontroller in the system58receives a command for moving the pistons88, the microcontroller automatically de-energizes all of the coils, including the coils50,98and102, and then moves the pistons88to the next desired position with respect to the plane S, and then re-energizes all of the coils. Alternatively, the pistons88can be moved in a full field condition, while all of the coils are energized.

In one preferred embodiment of the magnetic zoom mode, the radio frequency coils59can be disposed in greater proximity to the region of interest being scanned to obtain further improvements in the scanning resolution.

In another preferred embodiment, a plurality of hollowed regions and apertures20can be defined in the poles and the pole supports to receive a plurality of pistons, respectively, in a magnet assembly, in accordance with the present invention, to provide that a plurality of smaller volume regions of a patient can be scanned individually, or in combination, at higher scanning resolution levels in the magnetic zoom mode.

FIG. 4Aillustrates an alternative embodiment of the top polar region of the magnet assembly shown inFIG. 3including another ferromagnetic piston118which provides for adjusting the magnetic fields generated when the magnet assembly is operated in the magnetic zoom mode. Like reference numerals are used to refer to elements having similar and, preferably, identical structural and functional characteristics as those described above in connection with FIG.3.

Referring toFIG. 4A, the body portions28and30of the pole22are hollowed axially symmetrically about the polar axis26to a constant diameter Y to define a hollow-cylindrical volume region116within the pole22. The hollowed region116extends from the surface28to the surface of the pole support12adjacent to the body portion28and has a length equal to t. The aperture44defined in the pole support12also has been hollowed about the polar axis26to the same constant diameter Y. The inner surface of the pole support12which defines the aperture44and the inner surface of the pole22which defines the region116are, thus, aligned with each other.

The hollowed region116contains a piston118which is comprised of ferromagnetic material. The piston118is in the shape of a hollowed cylinder bounded lengthwise by an end surface120which faces the assembly48and an end surface122which faces the gap region42. The outer surface of the piston118, which extends between the end surfaces90and92, has a constant diameter equal or substantially equal to Y. The inner surface of the piston118, which extends between the end surfaces120and122and defines a hollowed region86A within the piston118, has a constant diameter equal or substantially equal to W. The distance between the end surfaces120and122is equal to M. Thus, the outer surface of the piston118has a constant diameter which is substantially equal to the diameter Y of the region116and the aperture44.

Ends128of two connecting rods130are each rigidly secured to the end surface120of the piston118. The connecting rods130extend from the end surface120, through the aperture44and are connected at opposite ends132to a second piston positioning means134which is contained in the operating assembly48. The rods130further include stop cylinders135at the ends132which are similar in structure and operation as the stop cylinders109. Also, the piston positioning means134is rigidly connected to the assembly48by reinforced supports121which are similar in structure and operation to the supports119. The pump112is coupled to the piston positioning means134over the lines136and137. The piston positioning means134is similar in structure and operation to the piston positioning means110.

The combination of the piston positioning means134, the actuating means112and the lines136and137, like the combination of the piston positioning means110, the actuating means112and the line114, constitutes a conventional hydraulic positioning device that is controllable by signals that a microcontroller, such as the microcontroller in the system58, supplies to the positioning means134. Based on the control signals supplied to the pump112, the pump112supplies a predetermined amount of fluid under pressure to and receives a predetermined amount of fluid from the piston positioning means134over the lines136and137to hold the rods130stationary or to move the rods130towards or away from the medial plane S. Thus, the assembly48provides for independent control of the positioning of the piston118in relation to the plane S.

The hollowed region86A of the piston118contains the piston88therein. Therefore, the pole22includes a pair of axially symmetric concentric pistons. The outer surface of the piston118is, preferably, substantially in contact with the surface of the pole22which defines the region116. Also, the outer surface of the piston88is, preferably, substantially in contact with the surface of the piston88which defines the region86. When at least a portion of the piston118is within the aperture44, the outer surface of the piston118which is within the aperture44, is substantially in contact with the adjacent facing surface of the pole22which defines the region44. Consequently, the facing surfaces of the pistons88and118, the pole22and the pole support12provide a low reluctance path for flux.

The diameter W of the piston88and the width of the end surfaces120and122of the piston118, which is defined as the difference between Y and W, are suitably set to define the size of the smaller volume region of the patient56which is to undergo higher resolution scanning. The values for Y and W are selected to provide for suitable adjustment of the uniformity of the magnetic field that passes through the gap region42between the facing surfaces of the support100mounted on the surfaces92at the higher resolution scanning levels attainable in the magnetic zoom mode. The diameters W and Y, preferably, are about 24 and 30 inches, respectively.

It is to be understood that the piston118and the hollowed region86A which it defines may assume other shapes, such as an elliptical or rectangular box. The hollowed regions in the poles and the apertures in the pole supports would have a corresponding structure to receive the pistons88and118which also would have corresponding structures. This correspondence in structure would maintain as close contact between the facing wall surfaces of the pistons and the poles as possible.

The piston positioning means134is of a sufficient size and is suitably positioned within the assembly48and the connecting rods130are of sufficient length for the piston positioning means134to controllably retain the connecting rods134when the connecting rods134are positioned such that: (i) the face surface122is aligned in the same plane as the surface38of the pole22; and (ii) the piston118is extended into the gap region42to a necessary extent in relation to the extent that the piston88is extended the gap region42to provide suitable adjustment of the magnetic field in the magnetic zoom mode of operation for the magnet assembly.

In operation of a magnet assembly of the present invention including top and bottom polar regions as shown inFIG. 4Ain the magnetic zoom mode, the piston positioning means134independently controls the position of the piston118in relation to the plane S to adjust the magnetic field that is applied through the smaller volume region of the patient in accordance with the level of magnetic zoom applied. The piston118acts as a tunable shim bar for the piston88. The amount that the piston118is moved towards or away from the plane S in relation to movement of the piston88towards or away from the plane S to adjust the magnetic field strength is determined automatically based on values stored in the memory of the system58, such as in a ROM lookup table. These values are calculated to account for the different field strengths that the coils of a magnet assembly generate and the increased field strength that is obtained when the piston88is moved a predetermined distance into the gap region42.

FIG. 4Bshows an alternative embodiment of the polar region shown inFIG. 4Awhich has the same components, except that the hollowed region116in the pole22and the aperture44, although aligned with each other, are not axially symmetric about the polar axis26. This arrangement of the pistons88and118translates the region of interest in which scanning in the magnetic zoom mode is performed toward the edge of the pole22and away from the polar axis26or the pole center. An off-pole center magnetic zoom feature may be desirable in particular surgical applications where scanning of a first region in the normal operation mode of the magnet assembly is desired and scanning of a second smaller region at a higher scanning resolution and at a region of the patient which is shifted from the polar axis26is also desired without having to the move the patient on the support80or the support80itself. This feature is particularly suitable for delicate surgical procedures which require that the patient be maintained absolutely stable throughout and for which it is desired to scan a smaller region of the patient in the magnetic zoom mode and also to scan a larger region, which is not concentric with the smaller region, at a lower scanning resolution in the normal mode of operation.

In another aspect of the invention, magnetic zoom capability in a magnet assembly is provided by coupling an electromagnetic magnetic zoom means to each of the poles. It is also to be understood that the electromagnetic magnetic zoom means can be coupled to each of the poles alone or in combination with a suitable mechanical magnetic zoom means which is also coupled to each of the poles. In one preferred embodiment, the electromagnetic magnetic zoom means may be superconducting.

FIG. 5Aillustrates an alternative embodiment of the top polar region of a magnet assembly as shown inFIG. 2Bincluding an electromagnetic magnetic zoom means146. Referring toFIG. 5A, the pole22has the same structure as described above in relation to the embodiment ofFIG. 3, except that the hollowed region86is completely filled with ferromagnetic material and the surface28also includes the surface portion of the filled hollowed region which faces the medial plane S. An insulative support140is mounted to the surface38. The support140contains magnetic field gradient coils142which have the same structure and operate in the same manner as the coils98in the support100, described above. The support140includes several sets143of threaded recesses144in the surface which faces the gap region42, as more clearly shown inFIG. 6, which is a plan view of the surface of the support140which faces the gap region42. The sets143of recesses144are dispersed on the surface of the support140which faces the gap region42.

The electromagnetic magnetic zoom means is a cylindrical disc support146. The support146includes threaded apertures148arranged in the same spatial configuration as the recesses144of one of the sets143of the recesses144. Threaded ferromagnetic or steel bolts150, which are threaded through the apertures148and into one of the sets143of the recesses144, securably mount the support140to the support146and cause the respective facing surfaces to be in close contact with each other. The plurality of the sets143of the recesses144enables the support146to be mounted at different locations on the support140in relation to the polar axis26.

The support146further comprises high density superconducting coils152contained in cryostats154which are arranged in the support146in a manner well known in the art. The coils152may be circular, elliptical or rectangular in shape. The coils152determine the thickness of the support146. The support146further includes a suitable electrical signal coupling means (not shown) that allows for connection to the system59.

In operation, when magnetic zoom operation is desired, the physician62or another attendant initially secures the support146to the support140at a selected position in relation to the polar axis26by screwing the steel bolts150through the apertures148and into one of the sets143of the recesses144. The set143that is selected would oppose a region of the patient56for which scanning at a higher resolution is desired. When the system56receives a command to operate in the magnetic zoom mode, the microcontroller provides that a current is initially supplied to the coils152to bias the cryostats154. When suitably powered by the bias current, the coils152significantly increase the magnetic field strength through the gap region42and a volume region of the patient defined by the surfaces of the supports146which would be coupled to each of the poles22and24and face the plane S. The gap distance for this embodiment of the magnet assembly is the distance between the facing surfaces of the supports146. This gap distance, like the gap distances for the embodiments of the magnet assemblies operated with magnetic zoom and discussed above, does not substantially impede access to the patient by others.

In one preferred embodiment, a plurality of electromagnetic zoom supports146can be mounted on the support140simultaneously in accordance with the present invention, and one or more of the supports146can be utilized to provide higher scanning resolutions at regions of the patient56which face the faces of the pairs of the supports146, respectively.

In an alternative embodiment, shown inFIG. 5B, the support146A can have an increased thickness such that the coils152are positioned closer to the patient54, a region of which would be positioned in the plane S. The positioning of the coils152closer to the patient54increases the magnetic field strength through the smaller region of interest defined by the support146A and narrows the gap distance within the gap region42. The support146A also may contain ferromagnetic material to further increase the magnetic field strength through the smaller region being scanned.

In still another alternative embodiment, the support146may be formed only from ferromagnetic material or permanent magnet material and not include the cryostats154containing the coils152. The support146would be attached to the support140in the same or similar manner as described above inFIG. 5AorFIG. 5Busing the bolts150. The thickness of the support146would determine the increase in the scanning resolution obtained for a region of the patient defined by the surface area of the surface of the support140which faces the plane S.

Thus, operation in the magnetic zoom mode operation can be achieved by attaching an identical ferromagnetic structure to the facing surfaces38and40of the poles22and24, respectively, as desired, so that the structure extends a predetermined distance into the gap region42. Alternatively, an electromagnetic zoom means can be coupled to the surface of a ferromagnetic structure which faces the gap region42, where the ferromagnetic structure is removably attachable to the surface of the polar region facing the plane S, to provide for even higher resolution scanning.

FIG. 7shows a further preferred embodiment of the magnet assembly as shown in FIG.3and including an electromagnetic magnetic zoom means coupled to the piston88. Referring toFIG. 7, the support100is suitably modified to include recesses100A in the surface of the support100which faces the gap region42. The recesses100A are disposed in the support100so that they can be aligned with the apertures148in the support146. The magnet assembly further includes an RF coil assembly support160containing RF receiving and transmitting coils or antennae which are linked (not shown) to the system58. The RF support160includes aperture162which are disposed so that they can be aligned with the apertures148in the support. The configuration of the recesses100A, the apertures148and the apertures162enables the support146to be mounted to the support100and the support160to be mounted to the support146using the bolts150. In a preferred embodiment, the support146can be used interchangeably in a magnet assembly which includes the piston88as shown inFIG. 7, and in a magnet assembly which does not include a piston coupled to each of the poles, as shown in FIG.5A.

FIG. 8shows still a further embodiment of the magnet assembly, as shown inFIG. 2B, including electromagnetic magnetic zoom means147A and147B for increasing magnetic field strength in the gap region42. The poles22and24have the same structure as described above in relation to the embodiment ofFIG. 3, except that the hollowed regions86are completely filled with ferromagnetic material and the surfaces38and40also include the surface portion of the filled hollowed region which faces the medial plane S. The insulative supports140are mounted to the surfaces38and40, respectively, as above.

The electromagnetic magnetic zoom means147A and147B comprise identical cylindrical discs, which are independent and separate structures from those structures which comprise the polar regions. Flexible support arms149attach the discs147A and147B to, for example, the bed support54. The support arms149, alternatively, can be secured to the floor66of the room10. The flexible support arms149can be positioned such that the discs147A and147B can be positioned at a plurality of positions in relation to the patient56and the polar axis26. The discs147A and147B, preferably, can easily be positioned symmetrical about the plane S.

The discs147A and147B comprise high density superconducting coils152contained in cryostats154which are arranged in a manner well known in the art. Suitable electrical signal coupling means (not shown) link the discs147A and147B to the system58to provide for energization of the coils152therein. When the coils152in the discs147A and147B are energized, a higher level of scanning resolution of a volume region of the patient56, which is defined between the facing surfaces of the discs147A and147B, is obtained.

Consequently, a magnet assembly in accordance with the present invention can provide for higher resolution scanning of a smaller region of the patient in the gap region, in comparison to the region that is scanned and the resolution of scanning that is obtained in the normal mode operation, by coupling a mechanical or electromagnetic magnetic zoom means, or both, to each of the poles to face the other pole and at a desired position in relation to the polar axis26, or by positioning independent electromagnetic magnetic zoom means in the gap region proximate a desired region of the patient.