Abstract:
A CT scanner ( 10 ) for obtaining a medical diagnostic image of a subject includes a stationary gantry ( 12 ), and a rotating gantry ( 14 ) rotatably supported on the stationary gantry ( 12 ) for rotation about the subject. A fluid bearing is interposed between the stationary gantry ( 12 ) and the rotating gantry ( 14 ) by means of radial and axial fluid bearing pads, ( 100 ) and ( 102 ) respectively. The fluid bearing provides a fluid barrier which separates the rotating gantry ( 14 ) from the stationary gantry ( 12 ). In a preferred embodiment, the fluid bearing 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 rotating gantry applications. 
     Generally, CT scanners have a defined examination region or scan circle in which a patient, or subject being imaged is disposed. A thin fan beam of radiation is transmitted across the examination region from an radiation source, such as an x-ray tube, to an oppositely disposed array of radiation detectors. The x-ray tube and associated power supply and cooling components are rotated around the examination region while data is collected from the radiation detectors. 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. Various hardware geometries have been utilized in this process. In third generation scanners, both the source and detectors rotate around the subject. In a fourth generation scanner, the x-ray source rotates and the detectors remain stationary. The detector array typically extends 360° around the subject in a ring outside of the trajectory of the x-ray tube. 
     In previously developed CT scanners, commonly the rotating gantry is supported on the stationary gantry via a large diameter mechanical bearing including rolling elements or balls interposed between two raceways. The bearing was typically on the order of three quarters of a meter to two meters in diameter. Mechanical bearings typically have a small amount of play or clearance between the races and the rotating elements. The mechanical play permits the x-ray tube and detectors in a third generation scanner to move axially and radially and permits the plane of rotation to cant. Accurate reconstruction, typically to a resolution on the order of millimeters is dependant upon acquiring data from accurately resolved positions of the source and the detectors. 
     In helical volume scanning, CT fluoroscopy or other real time imaging techniques, and high speed imaging, the x-ray tube gantry rotates continuously at high speed. However, with increased rotational speed of the rotating gantry, noise levels associated with mechanical bearings reach unacceptable levels. In continuously rotating systems, friction related heating can restrict the length of scans. Moreover, the accompanying fiction causes wearing of parts in physical contact with one another thereby incurring increased play and noise, disadvantageous maintenance requirements, and a limited lifetime. 
     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 diagnostic imaging apparatus is provided. An x-ray source is mounted on a rotating gantry. The rotating gantry or a stationary gantry includes at least one smooth, annular bearing race. Fluid bearing pads are mounted to the other of the gantries, having a porous face that contacts the smooth annular bearing race. A pump supplies a bearing fluid to the pads to be ejected therefrom. 
     In accordance with another aspect of the present invention, A diagnostic imaging apparatus is provided. A plurality of individual bearing pads are mounted to a stationary gantry, at least some of them being mounted for individual radial adjustment. A rotating gantry including at least one bearing race is separated from the bearing pads by a thin layer of air as it rotates. An x-ray tube is mounted on the rotating gantry. 
     In accordance with another aspect of the present invention, a method of diagnostic imaging is provided. A rotating gantry is rotated about an imaging region. Fluid bearings are created between the rotating gantry and a plurality of fluid bearing pads. A bias of at least some of the bearing pads is adjusted. An image representation of a subject in an imaging region is reconstructed by irradiating the subject and reconstructing detected radiation. 
     One advantage of the present invention is faster CT tri scanner speeds and correspondingly reduced scan times. 
     Another advantage of the present invention is quieter CT scanner operation. 
     Another advantage of the present invention is extended bearing life with reduced maintenance. 
     Another advantage resides in a CT scanner with a larger gantry and bore. 
     Yet another advantage resides in the simplicity of the support structure. 
     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. 2 is a frontal view of a main rotor and gantry support structure, in accordance with aspects of the present invention; 
     FIG. 3 is a side view of the main rotor and gantry support; 
     FIG. 4 is an alternate embodiment of the CT scanner gantry and rotor; 
     FIG. 5A is detail view of a radial bearing pad support, in accordance with aspects of the present invention; 
     FIG. 5B is a detail view of an axial bearing pad support, 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 stationary gantry  12  and a rotating gantry  14  which define an examination region  16 . The rotating gantry  14  is suspended from the stationary gantry  12  for rotation about the examination region  16 . A radiation source  20 , such as an x-ray tube, is arranged on the main rotor  16  for rotation therewith. The radiation source  20  produces a beam of penetrating radiation  22  that passes through the examination region  16  as the rotating gantry  16  is rotated by an external motor (not illustrated) about a longitudinal axis of the examination region  16 . 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  16  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 repositioned 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 rotating gantry  14 , the support  30 , and consequently the subject thereon, are translated along a central horizontal axis of the examination region  16 . In this manner, the source  20  follows a helical path relative to the subject. 
     In the illustrated third generation CT scanner, an array of radiation detectors  40  is mounted peripherally across from the source on the rotating gantry. Alternately, a fourth generation CT scanner is employed with a stationary ring of radiation detectors (not shown) mounted on the stationary gantry  12 . Regardless of the configuration, the radiation detectors are arranged to receive the radiation emitted from the source  20  after it has traversed the examination region  14 . 
     The radiation detectors  40  convert the detected radiation into electronic projection data. That is, each of the radiation detectors  40  produces an output signal which is proportional to an intensity of received radiation. Each radiation detector  40  generates data elements which correspond to projections along a corresponding ray within the view. Each element of data in a projection or data line is related to a line integral taken along its corresponding ray passing through the subject being reconstructed. 
     With source view geometry, as is typical with third generation scanners, 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. 
     An acquisition memory  50  receives the sampled data from the radiation detectors  40 . The acquisition memory  50  optionally performs filtering or other operations before passing the data to a reconstruction processor  60  which reconstructs image representations of the subject. 
     The reconstruction processor  60  processes the data from the acquisition memory board  50  and backprojects it into an image memory  70 . The reconstruction processor  60  of the preferred embodiment includes a convolver  62  which convolves the data lines and a backprojector  64  which backprojects each convolved data line into the image memory  70 . An image 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, CCD display, active matrix display, or the like. 
     With reference to FIG. 2, the rotating gantry  14  is suspended from the stationary gantry  12  by a plurality of fluid bearing pads. In the preferred embodiment, two distinct types of bearing pads are utilized. Radial pads  100  contact the outer polished circumferential bearing surface or race  102  of the rotating gantry. Axial pads  104  contact proximal and distal flat circumferential faces or races  106  of the rotating gantry  14 . A supply manifold supplies fluid to the bearing pads  100 ,  104  from a pressure reservoir  110 . Fluid from the reservoir  110  is used to create fluid bearings between the pads  100 ,  104  and the races  102 ,  106 , thereby effectively levitating the rotating gantry on micro-thin cushions of fluid. In the preferred embodiment, the fluid is air. Fluids, such as water and oil, are also contemplated. Preferably the reservoir  110  is kept at about 400 kPa, by a fluid compressor  112 . The air bearing layers of the preferred embodiment are approximately 0.5 mm thick. The air bearings of the preferred embodiment provides a near-frictionless surface for the main rotor on which to rotate. 
     In the preferred embodiment, the air bearings each have a metal housing  120  that defines an air distribution passage  122  connected with the pressure manifold and the pressure reservoir  110 . A porous pad  124  permits air to escape at a rate which maintains a fluid layer of about 0.5 mm between the porous pad and the race  102  ( 106 ). In order to minimize the possibility of introducing particles into the system (such as dust, dirt, etc.) that may degrade the bearing pads  124 , air taken in to the compressor is first filtered by an air filter  130 . A common problem with air compressors is that moisture in the air tends to condense when the air is compressed. In order to minimize the moisture introduced into the bearing system, an air line water trap  132  dehumidifies the compressed air. 
     In the event of power loss to the system while in operation, the reservoir  110  provides a sufficient buffer to sustain the bearings for a sufficient duration to decelerate the rotating gantry  14 . This feature helps to make the system less susceptible to power outages, extending the life of the bearing pads. 
     In the preferred embodiment, four radial pads  100  contact the main rotor on its outer circumferential surface  102 . These four bearing pads  100  keep the rotating gantry  14  stationary in x and y-directions, as illustrated in FIG.  2 . Given an arbitrary load force, the bearings actively respond to counteract the load. A force that pushes the rotating gantry  14  against one or more pads causes the air gap between the race  102  and that pad  100  to narrow. As the thickness of the bearing decreases, the pressure increases, stiffening the bearing and counteracting the load force. Similarly, the bearings located on the rotor opposite the direction of the load force increase in thickness, decreasing their pressure. Thus, load forces tend to be canceled by the bearings. 
     Typical load forces also include the weight of the rotating gantry  14  including the x-ray tube, its power supply, its cooling system, detectors, and the like. Since this load force is always present, it has been contemplated to asymmetrically distribute the air bearing between radial pads  100 . Optionally, more bearing fluid or higher bearing pressure can be supplied to the lower two radial bearing pads  100  to counteract the force of gravity. 
     Similar to the radial bearing pads  100 , the axial bearing pads  104  cancel forces in the z-direction, as illustrated in FIGS. 2 and 3. Longitudinal and canting displacements of the rotating gantry apply pressure in the z-direction to one or more of the axial bearing pads  104 . As a result, the corresponding bearing is compressed, increasing its pressure, counteracting the displacement force. By virtue of the preferred arrangement of bearing pads  100 ,  104 , load forces applied to the rotating gantry  14  are counteracted by one or more of the fluid bearings. Opposing pressures of the bearings induced by such displacement return the rotating gantry  14  to positional equilibrium in its original position. 
     The bearing pads  100 ,  104  are not necessarily stationary. In an alternate embodiment as illustrated in FIG. 4, the bearing pads  100 ′,  104 ′ are attached to a rotating gantry  14 ′ and rotate therewith. A stationary gantry  12 ′ of this alternate embodiment supports an inner race  102 ′ and oppositely disposed longitudinal races  106 ′. The radial pads  100 ′ form fluid bearings with the inner circumferential race  102 ′ and the axial pads  104 ′ form bearings along the side races  106 ′. 
     With reference to FIG. 5A, and continuing reference to FIGS. 2 and 3, the radial bearing pads  100  are fixed with radial fixture assemblies, illustrated in detail in FIG. 5A. A radial pad  100  is secured to the stationary gantry  12  with a radial ball stud  140 . Between an interface of the radial pad  100  and the radial ball stud  140  is a spring element, such as a Belville washer  142 . The Belville washer  142  is flexible, has a selected, fixed spring constant in the preferred embodiment. During machine setup, the radial pads  100  are positioned adjacent the rotating gantry race  102  in an original configuration. The radial pads  100  are held in position by the radial ball studs  110 . The studs are tightened to a desired tension and locked with a preload lock nut  144 . Setup of the radial pads  100  determines the stiffness of the bearings in relation to the rotating gantry  14 , defining the operating characteristics of the fluid bearings. The bearing stiffness is at least 3.5×10 9  Pa with 7.0×10 9  Pa being preferred 
     Alternately, the Belville washer can be eliminated and the torque applied to the ball stud controlled precisely. As another alternative, other spring biasing mechanisms are contemplated, such as torsion springs, coil springs, fluid springs, and the like. 
     In the preferred embodiment, two of the four radial pads  100  are fitted with Belville washers or other still springs as discussed above. The remaining two pads  100  are supported in a fixed position without springs. 
     With continuing reference to FIGS. 2 and 3, the axial bearing pads  104  are fixed with axial fixture assemblies, as illustrated in detail in FIG.  5 B. Each axial pad  104  is secured to a support arm  150  with an axial ball stud  152 . A Belville washer  154  is disposed between an interface of the axial pad  104  and the axial ball stud  152 . The Belville washer  154  is similar to the washers used in the radial support assembly. Again, other springs and pressure controlling devices are contemplated. During machine setup, the axial pads  104  are positioned adjacent the races  106 . The axial pads  104  on one side are moved by the axial ball studs  152  without Belville washers to define the plane of rotation. The studs on the other side are tightened to the tension set by the Belville washers and secured with an axial preload lock nut  156 . Setup of the axial pads  104  determines the original stiffness of the bearing in relation to the rotating gantry  14 . 
     Optionally, load measuring sensors are included in each of the axial and radial support assemblies. Data gathered by such sensors is used to adjust bearing distribution to counteract load forces. Alternately, such data could be used as a quality control check by an operator, ensuring that the CT scanner is operating within tolerable load limits. 
     Numerous other devices in addition to the x-ray tube  20  and the detector array  40  are mounted on the rotating gantry  14 . These include a coolant circulating system for the x-ray tube and high voltage generators for the x-ray tube. In order to minimize distortion of the rotating gantry  14 , devices are mounted thereon such that their centers of gravity are in the plane of the scan beam  22  and their collective center of gravity is at the geometric center of the rotating gantry. As the rotating gantry  14  approaches high speeds (500-600 RPM) stresses of centripetal acceleration can distort the bearing races if components are not balanced adequately. 
     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.