Abstract:
A method of manufacturing a collimator mandrel having variable attenuation characteristics is presented. The manufacturing process includes the placement of a layer of attenuating material on a core of base material. The layer of attenuating material is relatively thin and varies in thickness circumferentially around the core. The collimator mandrel may be manufactured by placing a cast about a core of non-attenuating material, filling a void between the cast and the core with an attenuating material, allowing the material to cure, and removing the cast from the assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is continuation of and claims priority of U.S. Ser. No. 10/604,634 filed Aug. 6, 2003, now U.S. Pat. No. 7,031,434 the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to computed tomography (CT) diagnostic imaging systems and, more particularly, to a method of manufacturing a collimator mandrel having variable attenuation characteristics. 
   Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image. 
   Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. 
   Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction. 
   Pre-patient collimators are commonly used to shape, or otherwise limit the coverage, of an x-ray or radiation beam projected from an x-ray source toward a subject to be scanned. Typically, the CT system will include a pair of collimator mandrels, each of which is mounted on an eccentric drive, such that the collimators may be positioned relative to one another to define a non-attenuated x-ray or radiation path. For example, by increasing the relative distance between the collimators, the width of the x-ray or radiation beam that impinges on the subject increases. In contrast, by moving the collimators closer to one another, the x-ray or radiation beam narrows. The eccentrics are designed to position the collimator mandrels with respect to one another and relative to an x-ray focal point to modulate the width of an x-ray or radiation path that bisects the collimators. 
   Collimators are frequently implemented to provide variable patient long axis (z-axis) coverage when a curvilinear detector assembly is used to detect radiation passing from the x-ray source through and around the subject during data acquisition. Conventional collimator mandrel configurations utilize a solid rod of attenuating material such as tungsten that is machined with a slight increase in diameter in the center of the mandrel relative to its ends. However, as the detector size increases in the z-axis, the constraints on the collimator tighten. Moreover, the collimator must be constructed to accommodate the increase in detector size while limiting x-ray coverage. Increased x-ray coverage increases patient radiation dose and degrades image quality due to the increased scatter in the reconstructed image. Accordingly, the collimator mandrel must be constructed to have a complex shape to accommodate the increase in detector size. 
   One known manufacturing process requires that the solid tungsten rod be machined to provide the complex shape necessary to achieve the desired beam shaping. Tungsten is a rigid material that is highly absorptive of x-rays. As such, tungsten is considered well-suited for collimator assemblies in CT systems. The rigidity of the tungsten, however, makes machining of a solid tungsten rod to have a complex shape difficult and time consuming. Moreover, machining with a precision required for a CT collimator can be difficult thereby compromising system performance. 
   Therefore, it would be desirable to have an accurate and repeatable manufacturing process capable of providing a precise and complex-shaped collimator mandrel for a CT system. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention is a directed to a manufacturing process overcoming the aforementioned drawbacks. The present invention provides a repeatable and precise process of constructing a collimator mandrel for a CT system. A rod of rigid material is positioned within a cast. The cast defines a void circumferentially around the rod which serves as a layout or pattern for an attenuating layer of epoxy, resin, or other material. Epoxy or other material is then deposited within the void and is allowed to cure. After curing, the cast is removed, and a complexly shaped collimator mandrel results. Alternatively, a thin layer of variable thickness may be deposited or sputtered directly on the outer surface of the rod to provide the complex shape desired. 
   Therefore, in accordance with one aspect of the present invention, a method of manufacturing a collimator mandrel for a CT imaging system includes the steps of forming a core of base material and applying a tapered layer of attenuating material to the core. 
   In accordance with another aspect of the invention, a CT collimator mandrel comprises a solid cylindrical rod positioned within a layer of attenuating material. The mandrel is formed by shaping a bulk of supporting material into a core and positioning the core in a cast such that a non-uniform void is created between an outer surface of the core and an inner surface of the cast. The mandrel is further formed by injecting attenuating material into the void and removing the cast upon curing of the attenuating material. 
   According to yet another aspect, a process of constructing a mandrel for a CT imaging system is provided and includes the steps of forming a solid cylindrical rod of first material and depositing a layer of second material designed to substantially block x-rays on the cylindrical rod. 
   Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a pictorial view of a CT imaging system. 
       FIG. 2  is a block schematic diagram of the system illustrated in  FIG. 1 . 
       FIG. 3  is a perspective view of a pair of collimator mandrels in a first position and forming a collimator assembly for use with the CT imaging system shown in  FIG. 1 . 
       FIG. 4  is a side elevational view of the collimator assembly shown in  FIG. 3  in the first position such that a minimum aperture is formed between the pair of mandrels. 
       FIG. 5  is a perspective view of the pair of collimator mandrels in a second position. 
       FIG. 6  is a side elevational view of the collimator assembly shown in  FIG. 5  in the second position such that a maximum aperture is formed between the pair of mandrels. 
       FIG. 7  is cross-sectional view of one assembly used to construct a collimator mandrel in accordance with the present invention. 
       FIG. 8  is a pictorial view of a CT system for use with a non-invasive package inspection system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention will be described with respect to the blockage, detection, and conversion of x-rays. However, one skilled in the art will appreciate that the present invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The present invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems. 
   Referring to  FIGS. 1 and 2 , a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of the gantry  12 . Detector array  18  is formed by a plurality of detectors  20  which together sense the projected x-rays that pass through a medical patient  22 . Each detector  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
   Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to an x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detectors  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 . 
   Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  and gantry  12 . Particularly, table  46  moves portions of patient  22  through a gantry opening  48 . 
   Referring to  FIG. 3 , a collimator assembly  50  having a pair of collimator mandrels  52  and  54  that are constructed to collimate x-rays projected toward a patient and detector assembly or array. Each collimator mandrel  52 ,  54  is designed to be rotated along a lengthwise axis by pivot assemblies  56 . As will be described in greater detail below, collimator mandrel  52  is rotated clockwise and collimator mandrel  54  is rotated counterclockwise to define the width of the aperture  58  that is formed between the pair of mandrels. However, one skilled in the art would readily recognize that other rotational orientations are possible and contemplated to achieve a desired aperture shape and/or width. 
   X-rays are projected from an x-ray tube toward the collimator assembly  50 . The mandrels  52 ,  54  are positioned relative to one another to define an aperture size tailored to the specific CT study to be carried out. In this regard, each mandrel is designed and constructed of material to block or prevent passage of those x-rays that are not passed through aperture  58 . As such, each mandrel  52 ,  54  has a complexly-shaped outer layer  60 ,  62  of attenuating material. That is, each outer layer extends circumferentially around a rod  64 ,  66  of base material and a non-constant diameter. The rods  64 ,  66  form a solid and rigid base for the layers of attenuating material. Preferably, the rods are constructed of steel, but other materials are possible. The attenuating layers may be fabricated from tungsten or other attenuating epoxy or alloy. 
   As shown, each rod  64 ,  66  has a circular or constant diameter. In contrast, each mandrel, as a result of the non-circular attenuating layer, has a complex shape. This complexity in shape allows the collimator assembly to provide a more variable aperture size without a change in the collimator assembly itself. Simply, in one preferred embodiment, the mandrels  52  and  54  have oblong or egg-like cross-sectional shapes that extends the entire length of rods  64  and  66 , respectively. However, the manufacturing process described herein allows for other mandrel shapes as well as varying attenuating layer thickness along the length of the rods. 
   Referring now to  FIG. 4 , a side view of the collimator assembly  50  illustrates a first or minimum aperture size that can be achieved by dynamically controlling the rotation of the mandrels  52  and  54 . In the relative position illustrated, each mandrel has been rotated to maximize the amount of attenuating material  60 ,  62  axially positioned between each rod  64 ,  66 . As a result, the size of aperture  58  is affected to control the expanse and coverage of x-ray beams  16  projected toward the patient (not shown) and detector assembly  18 . 
   In  FIG. 5 , the collimator assembly  50  is shown with a maximum aperture size. To achieve a maximum in the size of aperture  58 , eccentrics  56  rotate each mandrel  52  and  54  such that the thinnest amount of attenuating material is positioned adjacent the x-ray path through the aperture  58 . As a result, more of the x-ray beam is allowed pass through the collimator assembly unaltered by mandrels  52  and  54 . Eccentric assemblies  56  may be rotated mechanically by a user or, preferably, by a controller mechanism that is electronically controlled to rotate the mandrels based on a desired aperture size. Further, while  FIG. 5  illustrates rotation of both mandrels compared to that shown in  FIG. 3 , one mandrel may be rotated while the other mandrel remains stationary. Additionally, since each mandrel may be rotated independently by eccentrics  56 , one mandrel may be rotated more than the other mandrel. As a result, the number of aperture sizes that is possible is a function of the degree change in attenuating material thickness around each rod. Moreover, one mandrel may have a layer of attenuating material that is dimensionally different from the layer of attenuating material around the other mandrel. In this regard, the number of aperture sizes available is increased. 
     FIG. 6  is a side view similar to that of  FIG. 4  but illustrates a second or maximum aperture size that is achieved as a result of the relative rotation of both mandrels  52  and  54 . The position of each rod  64  and  66  remains fixed, but each mandrel is caused to rotate along a lengthwise axis through the center of the rod. As a result, the thickness of the attenuating layer placed in the x-ray path is variably controlled to fit the particulars of the CT study. As is shown, aperture  58  has a much larger size in  FIG. 6  than in  FIG. 4 ; therefore, the x-ray path therebetween is much larger which allows for greater coverage in the z-direction on detector  18 . 
   The collimator mandrel profile illustrated in  FIGS. 3-6  represents one embodiment of the shape each collimator mandrel may have. However, as will be described, the manufacturing process disclosed herein is capable of constructing other-shaped mandrels than that illustrated in  FIGS. 3-6 . For example, the mandrels could be constructed to have lobes or other geometrical shapes to achieve the desired aperture shape. 
   Shown in  FIG. 7  is a cross-sectional view illustrating the construction of a collimator mandrel in accordance with the present invention. The construction process begins with the formation of a cylindrically or other shaped rod  68  of base material having a constant cross-section. The rod  68  is constructed to have an eccentric pivot  70  on each end to support rotation of the mandrel once assembled and fit in the CT system. As noted above, the rod is preferably constructed of a solid, rigid material, i.e. steel, that is designed to receive and support a layer of attenuating material, such as tungsten, lead, a high atomic weight alloy, or epoxy laden with high atomic weight material. Rod  68  is placed is a cast  72  that envelops the rod. The cast  72  envelopes the rod such that a void  74  is created circumferentially around the outer surface of the rod  68  between the inner surface of cast. The void defines the dimensions, thickness, and shape of a layer of attenuating material to be deposited or otherwise formed to the outer surface of the rod. 
   In the example illustrated in  FIG. 7 , a highly attenuative epoxy or resin is deposited in void  74  and is allowed to cure. Once cured, the cast is removed and a tapered layer of attenuating material affixed to the outer surface of the rod results. However, use of a cast and the filling of a void between the cast and rod illustrates only one technique for forming a complexly shaped mandrel. For example, a thin layer of tungsten or other attenuative layer could be vapor or chemically deposited about the rod in a controlled manner such that a non-circular cross-sectioned or other complex shaped mandrel is constructed. In another embodiment, a thin layer of attenuating material could be sealed against the rod or core material using adhesive, glues and other intermediaries. Further, given the cast layer provides the x-ray attenuation, other attenuating materials other than tungsten may be used. As a result, the non-tungsten layer with improved machinability could be sealed against the rod and machined to provide the desired complex shape. 
   Referring now to  FIG. 8 , package/baggage inspection system  100  includes a rotatable gantry  102  having an opening  104  therein through which packages or pieces of baggage may pass. The rotatable gantry  102  houses a high frequency electromagnetic energy source  106  as well as a detector assembly  108  having scintillator arrays comprised of scintillator cells. A conveyor system  110  is also provided and includes a conveyor belt  112  supported by structure  114  to automatically and continuously pass packages or baggage pieces  116  through opening  104  to be scanned. Objects  116  are fed through opening  104  by conveyor belt  112 , imaging data is then acquired, and the conveyor belt  112  removes the packages  116  from opening  104  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  116  for explosives, knives, guns, contraband, and the like. 
   Therefore, in accordance with one embodiment of the present invention, a method of manufacturing a collimator mandrel for a CT imaging system includes the steps of forming a core of base material and applying a tapered layer of attenuating material to the core. 
   In accordance with another embodiment of the invention, a CT collimator mandrel comprises a solid core positioned within a layer of attenuating material. The mandrel is formed by shaping a bulk of supporting material into a core and positioning the core in a cast such that a non-uniform void is created between an outer surface of the core and an inner surface of the cast. The mandrel is further formed by injecting attenuating material into the void and removing the cast upon curing of the attenuating material. 
   According to yet another embodiment, a process of constructing a mandrel for a CT imaging system is provided and includes the steps of forming a solid cylindrical rod of first material and depositing a layer of second material designed to substantially block x-rays on the cylindrical rod. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.