Abstract:
A CT scanner ( 10 ) for obtaining a medical diagnostic image of a subject includes a stationary gantry ( 12 ), and a rotating gantry ( 16 ) rotatably supported on the stationary gantry ( 12 ) for rotation about the subject. A fluid bearing ( 18 ) is interposed between the stationary and rotating gantries ( 12 ) and ( 16 ), respectively. The fluid bearing ( 18 ) provides a fluid barrier ( 110 ) which separates the rotating gantry ( 16 ) from the stationary gantry ( 12 ). In a preferred embodiment, the fluid bearing ( 18 ) provides for quieter CT scanner operation at high rotational speeds. Moreover, eliminating the physical contact between the gantries minimizes wear and optimizes longevity.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the art of medical diagnostic imaging. It finds particular application in conjunction with computed tomography (CT) scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications. 
     Generally, CT scanners have a defined examination region or scan circle in which a patient, phantom or like subject being imaged is disposed. A thin beam of radiation is transmitted across the examination region from an radiation source, such as an x-ray tube, to oppositely disposed radiation detectors. The source, or beam of radiation, is rotated around the examination region while data is collected from the radiation detectors receiving x-ray radiation passing through the examination region and the subject disposed therein. Rotation of the radiation source is often achieved by mounting the radiation source to a rotating gantry which is rotated on a stationary gantry. 
     The sampled data is typically manipulated via appropriate reconstruction processors to generate an image representation of the subject which is displayed in a human-viewable form. Commonly, the x-ray data is transformed into the image representation utilizing filtered back projection. A family of rays extending from source to detector is assembled into a view. Each view is filtered or convolved with a filter function and backprojected into an image memory. Various view geometries have been utilized in this process. In a rotating, fan-beam-type scanner in which both the source and detectors rotate (i.e. a third generation scanner), each view is made up of concurrent samplings of an arc of detectors which span the x-ray beam when the x-ray source is in a given position to produce a source fan view. Alternately, with stationary detectors and a rotating source (i.e. a fourth generation scanner), a detector fan view is formed from the rays received by a single detector array as the x-ray source passes behind the examination region opposite the detector. 
     In any event, accurate reconstruction is dependant upon acquiring data views from a range of accurately resolved angular orientations or positions of the source as it rotates about the examination region. Reconstruction algorithms have been developed which use data collected over numerous helical rotations, 360 degrees of source rotation, 180 degrees plus the angle or spread of the fan of radiation, and the like. Therefore, scan times are constrained by the speed of rotation of the source. 
     In previously developed CT scanners, commonly the rotating gantry is supported on the stationary gantry via a mechanical bearing including rolling elements or balls interposed between two raceways. However, with increased rotational speed of the rotating gantry, noise levels associated which such mechanical bearings reach unacceptable levels. In continuously rotating systems, friction related heating can restrict the length of scans. Moreover, the accompanying friction causes wearing of parts in physical contact with one another thereby incurring disadvantageous maintenance requirements and a limited lifetime. 
     In another type of CT scanner, the rotating gantry is suspended via electromagnetic levitation. However, such a technique tends to be unstable and employs complex feedback controls to maintain stability. Moreover, the size and cost associated with such a system can be prohibitive when rotating loads of the size desired for many CT scanners, e.g., in the neighborhood of 1000 lbs. 
     The present invention contemplates a new and improved gantry suspension technique which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a CT scanner for obtaining a medical diagnostic image of a subject is provided. The CT scanner includes a stationary gantry, and a rotating gantry rotatably supported on the stationary gantry for rotation about the subject. A fluid bearing is interposed between the stationary and rotating gantries. The fluid bearing provides a fluid barrier which separates the rotating gantry from the stationary gantry. 
     In accordance with a more limited aspect of the present invention, the fluid barrier is a gas. 
     In accordance with a more limited aspect of the present invention, the gas is air. 
     In accordance with a more limited aspect of the present invention, the fluid barrier is a liquid. 
     In accordance with a more limited aspect of the present invention, the liquid is oil. 
     In accordance with a more limited aspect of the present invention, the CT scanner further includes a reservoir containing the fluid which is supplied from the reservoir to the fluid bearing to create the fluid barrier. 
     In accordance with a more limited aspect of the present invention, the fluid is supplied to the fluid bearing through distribution pads which distribute the fluid between the stationary and rotating gantries. 
     In accordance with a more limited aspect of the present invention, the distribution pads have a beveled edge along a leading side thereof relative to a direction of rotation of the rotating gantry. 
     In accordance with a more limited aspect of the present invention, the CT scanner further includes opposing surfaces on the stationary and rotating gantries which face one another across the fluid barrier. The opposing surfaces define the shape of the fluid bearing. 
     In accordance with a more limited aspect of the present invention, the shape of the fluid bearing is symmetrical with respect to an axial plane which is normal to an axis of rotation of the rotating gantry. 
     In accordance with a more limited aspect of the present invention, the shape of the fluid bearing is defined by two conic sections which meet at the axial plane to form an annular V-shaped trough. 
     In accordance with a more limited aspect of the present invention, the CT scanner further includes a recovery system which collects fluid escaping from the fluid bearing. 
     In accordance with a more limited aspect of the present invention, the CT scanner further includes a radiation source attached to the rotating gantry. The radiation source produces a beam of penetrating radiation which irradiates the subject as the rotating gantry rotates. A cooling system circulates a cooling fluid past the radiation source. The cooling fluid draws heat from the radiation source as the cooling fluid is circulated past the radiation source. A heat exchanger, interfacing the cooling system with the recovery system, transfers heat from the cooling fluid in the cooling system to the fluid collected by the recovery system. 
     In accordance with a more limited aspect of the present invention, the recovery system returns fluid collected thereby to the fluid bearing. 
     In accordance with another aspect of the present invention, a method of rotating a source of radiation about an axis is provided. The method includes suspending a second gantry from a first gantry while the second gantry has the source of radiation mounted thereto. Thereafter, a fluid is fed in between the first and second gantries such that they are spaced apart from one another by a layer of the fluid, and the second gantry is then rotated. 
     In accordance with a more limited aspect of the present invention, the method further includes controlling the rate at which the fluid is fed in between the first and second gantries. 
     In accordance with a more limited aspect of the present invention, the method further includes storing a reserve of the fluid such that the fluid is fed in between the first and second gantries from the stored reserve. 
     In accordance with a more limited aspect of the present invention, the method further includes generating dynamic forces which radially stabilize the second gantry as it rotates. 
     In accordance with a more limited aspect of the present invention, the method further includes collecting the fluid as it escapes from in between the first and second gantries, and returning the fluid for further use. 
     In accordance with a more limited aspect of the present invention, the method further includes transferring heat from a cooling fluid which cools the radiation source to the collected fluid. 
     One advantage of the present invention is faster CT scanner speeds and correspondingly reduced scan times. 
     Another advantage of the present invention is quieter CT scanner operation. 
     Yet another advantage of the present invention is extended bearing life with reduced maintenance due to the elimination of frictional wearing of contacting parts. 
     Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a CT scanner in accordance with aspects of the present invention; 
     FIG. 2A is a partially cut away diagrammatic illustration showing the interface of rotating and stationary gantries of a CT scanner in accordance with aspects of the present invention; 
     FIG. 2B is enlarged view of the partially cut away portion shown in FIG. 2A; 
     FIG. 3 is a cross-sectional view showing a fluid bearing in accordance with aspects of the present invention; 
     FIG. 4A is a partial side view of a fluid bearing in accordance with aspects of the present invention; 
     FIG. 4B is a cross-sectional view taken along section line A—A in FIG. 4A; 
     FIG. 5 is a diagrammatic illustration showing asymmetric feeding of a fluid bearing in accordance with aspects of the present invention; and, 
     FIG. 6 is a diagrammatic illustration of a CT scanner with fluid recover system in accordance with aspects of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a CT scanner  10  includes a first stationary gantry  12  which defines an examination region  14 . A second rotating gantry  16  is suspended from or otherwise supported on the stationary gantry  12  by a fluid bearing  18  for rotation about the examination region  14 . A radiation source  20 , such as an x-ray tube, is arranged on the rotating gantry  16  for rotation therewith. The radiation source  20  produces a beam of penetrating radiation  22  that passes through the examination region  14  as the rotating gantry  16  rotates. A collimator and shutter assembly  24  forms the beam of penetrating radiation  22  into a thin fan shape and selectively gates the beam  22  on and off. Alternately, the radiation beam  22  is gated on and off electronically at the source  20 . In any event, a subject support  30 , such as a couch or the like, suspends or otherwise holds a subject being examined or imaged at least partially within the examination region  14  such that the fan-shaped beam of radiation  22  cuts a cross-sectional slice through the region of interest of the subject. 
     Optionally, the subject is successively re-positioned such that neighboring cross-sectional slices are taken in consecutive indexed fashion to produce a three-dimensional volume of slices. Alternately, as is the case with continuous helical CT, concurrently with the rotation of the second gantry  16 , the support  30 , and consequently the subject thereon, are translated along a central horizontal axis of the examination region  14 . In this manner, the source  20  follows a helical path relative to the subject. In another preferred embodiment, the support  30  remains stationary while the first gantry  12  is translated or otherwise moved relative to the subject such that the source  20  follows a helical path relative thereto. 
     In the illustrated fourth generation CT scanner, a ring of radiation detectors  40  is mounted peripherally around the examination region  14  on the stationary gantry  12 . Alternately, a third generation CT scanner is employed with an arc of radiation detectors  40  mounted on the rotating gantry  16  on a side of the examination region  14  opposite the source  20  such that they span the arc defined by the fan-shaped beam of penetrating radiation  22 . Regardless of the configuration, the radiation detectors  40  are arranged to receive the radiation emitted from the source  20  after it has traversed the examination region  14 . 
     In a source fan geometry, an arc of detectors which span the radiation emanating from the source  20  are sampled concurrently at short time intervals as the source  20  rotates behind the examination region  14  to generate a source fan view. In a detector fan geometry, each detector is sampled a multiplicity of times as the source  20  rotates behind the examination region  14  to generate a detector fan view. The paths between the source  20  and each of the radiation detectors  40  are denoted as rays. 
     The radiation detectors  40  convert the detected radiation into electronic projection data. That is to say, each of the radiation detectors  40  produces an output signal which is proportional to an intensity of received radiation. Optionally, a reference detector may detect radiation which has not traversed the examination region  14 . A difference between the magnitude of radiation received by the reference detector and each radiation detector  40  provides an indication of the amount of radiation attenuation along a corresponding ray of a sampled fan of radiation. In either case, each radiation detector  40  generates data elements which correspond to projections along each ray within the view. Each element of data in the data line is related to a line integral taken along its corresponding ray passing through the subject being reconstructed. 
     With detector view geometry, each view or data line represents a fan of rays having its apex at one of the radiation detectors  40  collected over a short period of time as the source  20  rotates behind the examination region  14  from the detector. With source view geometry, each view or data line represents a fan of rays having an apex at the source  20  collected by concurrent sampling of all the radiation detectors  40  spanning the fan of radiation. 
     A gantry acquisition memory board  50 -receives the sampled data from the radiation detectors  40 . The gantry acquisition memory board  50  optionally shuffles the data to transform it from a detector fan geometry to a source fan geometry, or vice versa, and performs a ripple filtering operation before passing the data to an image processor  60  which reconstructs image representations of the subject. 
     The image processor  60  processes the data from the gantry acquisition memory board  50  and backprojects it into an image memory  70 . More specifically, the image processor  60  performs mathematical manipulations which convolve each data set with an appropriate filter or convolution function for the view format. The image processor  60  of the preferred embodiment includes a convolver  64  which convolves the data sets and a backprojector  66  which backprojects the convolved data sets into the image memory  70 . Ultimately, a video processor  80  selectively retrieves slices, projections, three-dimensional (3D) renderings, and other image information from the image memory  70  and appropriately formats an image representation for depiction on a human viewable display  90 , such as a video monitor or the like. 
     Optionally, for those applications wherein other than parallel projection data is collected, the image processor  60  includes a rebinning processor  62 . Initially, the electronic data generated by the radiation detectors  40  and sampled by the gantry acquisition memory board  50  is fed to the rebinning processor  62 . The rebinning processor  62  converts each data line from its fan-beam or otherwise divergent format to a parallel-beam format. Thereafter, the image processor  60  implements a conventional reconstruction algorithm, such as a convolution and filtered back projection algorithm. Examples of suitable image processing techniques which are optionally employed (including back projection, rebinning, and other reconstruction techniques) are found in commonly owned U.S. Pat. Nos. 4,965,726; 5,262,946; 5,384,861; 5,396,418; 5,481,583; 5,485,493; and 5,544,212; all incorporated herein by reference. 
     With reference to FIGS. 2A,  2 B, and  3  and continuing reference to FIG. 1, in a preferred embodiment, the stationary gantry  12  and rotating gantry  16  suspended thereon interface with one another through a fluid bearing  18  interposed therebetween. The fluid bearing  18  provides a thin layer of fluid or a fluid barrier  110  that separates opposing surfaces of the stationary gantry  12  and rotating gantry  16 . Surfaces  12   a  and  12   b  of the stationary gantry  12  and surfaces  16   a  and  16   b  of the rotating gantry  16  face one another respectively across the fluid barrier  110  in spaced apart relation such that the shape of the fluid bearing  18  is defined thereby. In preferred embodiments, for example, the gap between gantries or the thickness of the fluid barrier  110  is approximately 0.0175-0.03 mm with the larger thicknesses being employed in conjunction larger diameter rotating gantries. Optionally, the fluid is gas or alternately liquid. More specifically, the fluid is air or oil. Regardless, in this manner, the rotating gantry  16  is freely rotated while being suspended from the stationary gantry  12  without direct contact of the gantries and without any ball bearings, roller bearings, or other mechanical bearings being interposed therebetween. 
     In a preferred embodiment, a fluid stored under pressure in a reservoir  120  is fed or supplied to the fluid bearing  18  to create the fluid barrier  110 . In one preferred embodiment, a pressure of approximately 690 kPa is used in conjunction with a gas fluid (e.g., air). Alternately, with a liquid fluid (e.g., oil) less pressure is employed. The fluid reserves and pressure are maintained in the reservoir  120  via a fluid pump which supplies the fluid thereto. As a safety feature, in the case of system failure, malfunction or other sudden system shutdown, the reserve fluid supply stored in the reservoir  120  is maintained at a level sufficient to continue suspension of the rotating gantry  16  until it comes to rest. 
     The fluid from the reservoir  120  is fed or supplied to the fluid bearing  18  through an array of orifices  130  circumferentially arranged about the fluid bearing  18 . The orifices  130  provide for fluid communication between the reservoir  120  and the fluid bearing  18 . Preferably, orifice inserts  132  having selected inner diameters are secured within the orifices  130  to limit or control the fluid flow therethrough. In a preferred embodiment, the orifice inserts  132  have an inner diameter of approximately 0.2 mm. Alternately, variable aperture valves, porous material inserts, or other like controls are used to adjust fluid flow to the desired level. Additionally, slot feeds are optionally substituted for the orifices  130 . 
     With further reference to FIGS. 4A and 4B, in a preferred embodiment, the stationary gantry  16  includes distribution pads  140  which are arranged around the gantry&#39;s inner diameter. The pads  140  are optionally secured by a layer of adhesive  142 . As the fluid is being supplied through the orifices  130 , it encounters and traverses the distribution pads  140  which diffuse and distribute the fluid into the fluid bearing  18  to create the fluid barrier  110 . Preferably, the pads  140  are made of a porous medium or a rigid web of, e.g., carbon or other suitable material, which evenly distributes the fluid flow over its surface. 
     Optionally, as best seen in FIG. 4A, the distribution pads  140  have a beveled, sloped, or otherwise angled edge  144  along a leading side thereof relative to the direction of rotation of the rotating gantry  16 . The angled edges  144  generate a desired pressure distribution or aerodynamic/hydrodynamic forces which stiffen and stabilize the rotating gantry  16  against radial forces thereby restricting lateral movement of the axis of rotation of the rotating gantry  16 . 
     In a preferred embodiment, the shape of the fluid bearing  18  is symmetrical with respect to an axial plane which is normal to the axis of rotation of the rotating gantry  16 . See FIG.  3 . More specifically, the shape of the fluid bearing  18  is defined by two conic sections which meet at the axial plane to form an annular V-shaped trough. In this manner, the sloping conic sections of the fluid bearing  18  serve to stiffen and stabilize the rotating gantry  16  against axial forces thereby restricting the position and orientation of the rotating gantry  16  to the axial plane in which it rotates. That is to say, the axial forces experienced by surfaces  16   a  and  16   b  of the rotating gantry  16 , due to the radial feeding of fluid under pressure to the bearing  18 , tends to center the rotating gantry  16  in the track formed by surfaces  12   a  and  12   b  of the stationary gantry  12 . 
     As the fluid flows into the channel to create fluid barrier  110 , the flow splits. Fluid flowing toward the apex where there is no fluid outlet forms a high pressure or maximum lift zone. 
     The fluid pressures are self-centering. That is, if the rotating gantry  16  should start to shift parallel to its axis of rotation, the channel and hence fluid barrier  110  will become narrower in the direction of travel. Narrowing of the channel in the direction of travel increases the pressure while widening the other side of the channel reduces pressure. This pressure differential creates a force which urges the rotating gantry  16  back toward its original center. 
     In a preferred embodiment, to counter the weight of the load (i.e., the weight of the rotating gantry  16  and attached components), fluid is asymmetrically fed to the fluid bearing  18  by asymmetrically locating the orifices  130  circumferentially around the fluid bearing  18  and/or by asymmetrically controlling the fluid flows through the orifices  130 . To provide lift, more fluid is fed to the fluid bearing  18  from beneath the rotating gantry  16  than from above. In a preferred embodiment, for example, as shown in FIG. 5, a net lift is provided by having fluid fed to the fluid bearing  18  from feed points  130   a  while no fluid is fed from above the rotating gantry  16  at feed points  130   b . Likewise, the beveled edges  144  on the distribution pads  140  are asymmetrically arranged and/or the angle of the bevels are asymmetrically selected to generate or provide aerodynamic/hydrodynamic forces with a net positive lift acting on the rotating ring  16 . Optionally, the asymmetric arrangement of fluid feed and asymmetric arrangement of bevels are employed in lieu of or in conjunction with one another to achieve the desire lift which counters the weight of the load. Baffles are also optionally added adjacent to annular discharge areas of the channel defining the fluid barrier  110  to increase pressure, at least at selected locations. Analogously, an air outlet passage is optionally tapped into the high pressure zone at the apex of the fluid barrier  110  in regions where relative pressure is to be reduced. 
     With further reference to FIG. 6, in an alternate embodiment preferably used in conjunction with a liquid fluid bearing  18 , a fluid recovery system is employed to capture or collect fluid escaping or otherwise leaving the fluid bearing  18 . The fluid recovery system includes a pair (front and back) of annular recovery chambers  200  which empty into a collection reservoir  210 . Preferably, the annular recovery chambers  200  are housed inside and defined by non-contact bearing seals  202  at the front and back of the fluid bearing  18 . The non-contact seals  202  serve to contain in the annular recovery chambers  200  fluid escaping the bearing  18  and to seal the bearing  18  from outside contaminants without providing physical contact between the stationary gantry  12  and the rotating gantry  16 . 
     The bottoms of the annular recovery chambers  200  are open to the collection reservoir  210  such that gravity drains fluid which has trickled down to the bottom of the recovery chambers  200  from the same into the collection reservoir  210 . Preferably, a fluid pump  220  pumps or recirculates the collected fluid from the collection reservoir  210  to the reservoir  120  where it is stored under pressure for feeding the fluid bearing  18 . In a closed system or circulation loop, pumping fluid from the collection reservoir  210  creates a negative pressure which tends to draw or pull fluid from the recovery chambers  200 . That, in turn, creates a negative pressure in the recovery chambers  200  which tends to draw or pull fluid from the bearing  18 . In this manner, circulation of the fluid is encouraged. 
     Optionally, through ports, conduits, or the like, such as the depicted vacuum hoses  230 , a negative pressure, preferably small, is applied to the annular recovery chambers with a vacuum pump or similar negative pressure producing apparatus (not shown). The negative pressure in the annular recovery chambers  200  further aids in maintaining the circulation of the bearing fluid buy promoting the draw or pull of fluid from the fluid bearing  18  into the annular recovery chambers  200 . 
     In an alternate embodiment, the radiation source  20  (e.g., an x-ray tube) is fluid cooled, for example, by a liquid such as oil circulating in a cooling system. More specifically, the cooling system circulates a cooling fluid in a loop  240  past or over the radiation source  20  such that the cooling fluid absorbs heat generated by the operation of the radiation source  20  to thereby cool the same. To remove the absorbed heat from the cooling fluid, a heat exchanger  250  (e.g., a fluid to fluid heat exchanger) interfaces the cooling system with the recovery system. That is to say, the loop  240  of the cooling system circulates the cooling fluid through the heat exchanger  250  where heat from the cooling fluid is removed. 
     In the illustrated embodiment, the fluid recovery system also includes a circulation loop  260  which passes through the heat exchanger  250  wherein heat is transferred from the cooling fluid in loop  240  to the fluid in circulation loop  260 . In the illustrated example, the circulation loop  260  saps, drains, or otherwise taps off fluid from the fluid bearing  18  and returns it to one or both of the annular recovery chambers  200 . The fluid thereafter collected in the collection reservoir  210  is cooled via a separate heat exchanger (not shown) which is optionally remotely located. 
     In an alternate embodiment, the fluid supply system is supported on the rotating gantry  16  rather than the stationary gantry  12  as shown. That is to say, the reservoir  120 , orifices  130 , fluid distribution pads  140 , etc., are supported on the rotating gantry  16 . Preferably, the fluid bearing  18  having a rotating gantry side fluid supply system is gas or air based. While increasing the weight load on the rotating gantry  16 , certain other advantages are achieved. Namely, space is conserved by mounting the fluid pump for supplying the reservoir  120  on the rotating gantry  16 . Additionally, the pump is preferably positioned or mounted to counter balance the radiation source  20  and thereby provide even distribution of the weight on the rotating gantry  16 . 
     In another alternative, at the interface of the gantries, the roles of the track (formed by surface  12   a  and  12   b ) and the follower/guide (formed by surfaces  16   a  and  16   b ) are optionally reversed. That is to say, a recessed track is optionally defined by or formed on the rotating gantry  16  while a protruding follower/guide is defined by or formed on the stationary gantry  12 . 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.