Abstract:
The present invention is directed to a CT imaging system utilizing a pre-subject cone-angle dependent filter to optimize dosage applied to the scan subject for data acquisition. The cone angle dependent pre-subject filter is designed to have a shape that is thicker for outer detector rows and thinner for inner detector rows. As a result, x-rays corresponding to the outer detector rows undergo greater filtering than the x-rays corresponding to the inner detector rows.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to computed tomography (CT) technology, and more particularly, to a method and apparatus for optimizing the dosage applied to a scan subject to acquire imaging data. Specifically, the present invention is directed to a cone angle dependent pre-subject filter. 
     Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a scan subject, such as a patient. 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 then produces a separate electrical signal indicative of the attenuated beam received by that detector element. The electrical signals are then transmitted to a data processing unit for analysis and ultimately image reconstruction. 
     Generally, the x-ray source and the detector array are rotated with a gantry within an imaging plane and around the scan 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 detecting the light energy from an adjacent scintillator. 
     There has been a general desire toward reducing radiation exposure in such systems. Reduction of radiation dosage to scan subjects is therefore desirable on CT systems. A number of imaging techniques have been developed to reduce the radiation dose directed toward a scan subject for data acquisition. However, these imaging techniques often result in higher signal-to-noise ratios and poor image quality. 
     It would therefore be desirable to design an imaging system that optimizes the dose of radiation projected to the scan subject for data acquisition without jeopardizing image quality. 
     BRIEF DESCRIPTION OF INVENTION 
     The present invention is directed to a CT imaging system utilizing a cone angle dependent pre-subject filter to optimize dosage applied to the scan subject for data acquisition. The cone angle dependent pre-subject filter is designed to have a variable shape. In one embodiment the shape is thicker for outer detector rows and thinner for inner detector rows. As a result, x-rays corresponding to the outer detector rows undergo greater filtering than the x-rays corresponding to the inner detector rows which also evens noise distribution. All of which overcome the aforementioned drawbacks. 
     Therefore, in accordance with one aspect of the present invention, a cone angle dependent pre-subject filter for use with a radiation emitting imaging device is provided. The filter includes a flat surface as well as a concave surface. A number of sidewalls connecting the flat surface and the concave surface in a single solid structure are also provided. 
     In accordance with another aspect of the present invention, a radiation emitting imaging device includes a rotatable gantry having an opening defined therein for receiving a subject to be scanned. The device further includes a subject positioner configured to position the subject within the opening as well as a high frequency electromagnetic energy projection source configured to project high frequency electromagnetic energy to the subject. The imaging device further includes at least one filtering device configured to filter high frequency electromagnetic energy projected to the subject. The filtering device is formed of a bulk of filtering material having a non-uniform attenuation. The imaging device also includes a detector array having a plurality of detectors to detect high frequency electromagnetic energy passing through the subject and to output a plurality of electrical signals indicative of an intensity of the high electromagnetic energy detected: A data acquisition system is provided and connected to the detector array and configured to receive a plurality of electrical signals. An image reconstructor connected to the data acquisition system is provided and configured to reconstruct an image of the subject from the plurality of signals received by the data acquisition system. 
     In accordance with a further aspect of the present invention, a cone angle dependent pre-subject filter includes means for receiving high frequency electromagnetic energy. The filter further includes means for increasing attenuation of high frequency electromagnetic energy flux in a first region as well as means for decreasing attenuation of high frequency electromagnetic energy flux in a second region. 
     In accordance with yet another aspect of the present invention, a method of manufacturing a pre-subject filter for use with a radiation emitting imaging device includes the step of defining a block of filtering material. The method further includes shaping the block to have a linear surface and fashioning the block to have a curvilinear surface. 
    
    
     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 CT system detector array. 
     FIG. 4 is a perspective view of a detector from FIG.  3 . 
     FIG. 5 is illustrative of various configurations of the detector of FIG. 4 in a four-slice mode. 
     FIG. 6 is a cross-sectional view of a pre-subject filter in accordance with one embodiment of the present invention. 
     FIG. 7 is a plot of noise distribution corresponding to filters of varying designs. 
     FIG. 8 is a plot of a predicted dosage based on the varying designs referenced in FIG.  7 . 
     FIG. 9 is a pictorial view of one embodiment of a non-invasive baggage/package imaging system incorporating the present invention. 
    
    
     DETAILED DESCRIPTION 
     The operating environment of the present invention is described with respect of a four-slice computed tomography (CT) system. However, it will be appreciated by those of ordinary skill in the art that the present invention is equally applicable for use with other multi-slice configurations. Moreover, the present invention will be described with respect to the detection and conversion of x-rays. However, one of ordinary skill in the art will further appreciate, that the present invention is equally applicable for the detection, conversion, and convergence of other high frequency electromagnetic energy. Additionally, the present invention will be described with respect to a “third generation” CT scanner, but is applicable with other generation CT scanners as well. 
     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 . A pre-subject filter  15  is disposed between source  14  and patient  22  to filter the x-rays received by patient  22 . Detector array  18  is formed by a plurality of detectors  20  which together sense the projected x-rays that pass through the 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 or other data entry device. 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  3 , 6  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 . 
     As shown in FIGS. 3 and 4, detector array  18  includes a plurality of detectors  20 . Each detector  20  includes a two-dimensional photodiode array  52  and a two-dimensional scintillator array  56  positioned above the photodiode array  52 . A collimator (not shown) is positioned above the scintillator array  56  to collimate x-ray beams  16  before such beams impinge upon scintillator array  56 . Photodiode array  52  includes a plurality of photodiodes  60 , deposited or formed on a silicon chip. Scintillator array  56 , as known in the art, is positioned over the photodiode array  52 . Photodiodes  60  are optically coupled to scintillator array  56  and are capable of transmitting signals representative of the light output of the scintillator array  56 . Each photodiode  60  produces a separate low level analog output signal that is a measurement of the attenuated beam entering a corresponding scintillator  57  of scintillator array  56 . Photodiode output lines  76  may, for example, be physically located on one side of detector  20  or on a plurality of sides of detector  20 . As shown in FIG. 45, photodiode output lines  76  are located on opposing sides of the photodiode array  52 . 
     In one embodiment, as shown in FIG. 3, detector array  18  includes detectors  20 . Each detector  20  includes a photodiode array  52  and scintillator array  56 , each having an array size of 16×16. As a result, arrays  52  and  56  have 16 rows and 912 columns (16×57) detectors each, which allows 16 simultaneous slices of data to be collected with each rotation of gantry  12 . The scintillator array  56  is coupled to the photodiode array  52  by a thin film of transparent adhesive (not shown). 
     Switch arrays  80  and  82 , FIG. 4 are multi-dimensional semiconductor arrays having similar width as photodiode array  52 . In one preferred embodiment, the switch arrays  80  and  82  each include a plurality of field effect transistors (FET). Each FET is electrically connected to a corresponding photodiode  60 . The FET array has a number of output leads electrically connected to DAS  32  for transmitting signals via a flexible electrical interface  84 . Particularly, about one-half of the photodiode outputs are electrically transmitted to switch array  80  and the other one-half of the photodiode outputs are electrically transmitted to switch array  82 . Each detector  20  is secured to a detector frame  77 , FIG. 3, by mounting brackets  79 . 
     Switch arrays  80  and  82  further include a decoder (not shown) that controls, enables, disables, or combines photodiode output in accordance with a desired number of slices and slice resolutions. In one embodiment defined as a 16-slice mode, decoder instructs switch arrays  80  and  82  so that all rows of the photodiode array  52  are activated, resulting in 16 simultaneous slices of data available for processing by DAS  32 . Of course, many other slice combinations are possible. For example, decoder may also enable other slice modes, including one, two, and four-slice modes. 
     Shown in FIG. 5, by transmitting the appropriate decoder instructions, switch arrays  80  and  82  can be configured in the four-slice mode so that the data is collected from four slices of one or more rows of photodiode array  52 . Depending upon the specific configuration of switch arrays  80  and  82  as defined by the decoder, various combinations of photodiodes  60  of the photodiode array  52  can be enabled, disabled, or combined so that the slice thickness may consist of one, two, three, or four rows of photodiode array elements  60 . Additional examples include a single slice mode including one slice with slices ranging from 1.25 mm thick to 20 mm thick, and a two slice mode including two slices with slices ranging from 1.25 mm thick to 10 mm thick. Additional modes beyond those described are contemplated. 
     Now referring to FIG. 6, a cross-sectional view of the cone angle dependent pre-subject filter  15  is shown. Filter  15  includes a bottom surface  86  and a concave top surface  88 . Sidewalls  90  connect the bottom surface and the convex top surface in a single solid structure. Filter  15  is formed from a filtering material  92  that, in one embodiment, has a constant density. Convex Concave top surface  88  is fabricated to have a continuous and smooth face. 
     Preferably, filter  15  is fabricated to have a thickness at a generally end region  94  that exceeds a thickness at a generally center region  96 . That is, a maximum thickness is enjoyed at each end of the filter whereas a minimum thickness exists in the center region. As a result, the noise index at each generally end region  94  exceeds the noise index of the general center region  96 . In one embodiment, filter  15  may comprise a number of thin slabs of filtering material that are stacked together such that the thickness of the filter at the end regions  94  exceeds the thickness of the center region  96  and vice-versa. Alternately, filter  15  could be equivalently formed from a bulk material having non-uniform density such that the filter has a uniform shape yet non-uniform attenuation. For example, the density of the material forming the end regions may be less than the density of the material forming the center region resulting in a varying attenuation profile of the filter. Moreover, the filter may be fabricated from more than one material with varying degrees of density. 
     In the reconstruction process of multi-slice CT, the measured projection data is first weighted by a set of weighting functions prior to the filtered back-projection. These weighting functions serve the purpose of interpolation to estimate a set of projections at the plane of reconstruction (POR). For multi-slice CT, one of the major sources of image artifacts is the cone beam effect. It should be noted that the projection data collected by the detector row closer to the center of the detector are nearly parallel to the POR and are essentially fan-beam sampling. For the projection data collected by the detector rows further away from the detector center, the samples are significantly non-coplanar with the POR. With two-dimensional back-projection hardware, the discrepancy between the actual x-ray path and the x-ray path assumed by the back-projection process often causes imaging artifacts. This type of artifact is commonly referred to as “cone beam artifact” referring to the cone beam nature of the data collection. 
     Helical weighting functions have been implemented such that projection samples with larger cone angles contribute less to the final reconstructed image. This is accomplished by assigning less weight to the data projection samples collected by the outer detector rows. For example, one of the weighting schemes for an eight slice 5:1 pitch helical reconstructions assigns the following relative weights to the eight detector rows: 0.125, 0.25, 0.375, 0.5, 0.5, 0.375, 0.25, 0.125. Different weights could be assigned however depending upon the reconstruction algorithm. It should be noted that the contribution from the outermost rows is only one-fourth of the contribution from the center rows. Because the final reconstructed image is obtained by the summation (back-projection) of signals from all detector rows, variance in the final image is the weighted sum of the variances of the projection samples of all detector rows. Since human anatomies do not change quickly over a short distance along the patient long axis, noise in the samples of all detector rows can be assumed approximately equal. Because the contribution from the outer detector rows is much less than the contribution from the inner detector rows, the efficiency of the sample utilization is not optimized. However, if the noise in the outer detector rows is increased, the impact of the noise on the final reconstructed image is much smaller than if the noise in the inner detector rows is increased. As a result, the x-ray flux to the inner detector rows may be increased and the x-ray flux to the outer detector rows may be reduced to obtain an overall improvement in terms of noise and dosage to the patient. Utilization of a cone angle dependent pre-subject filter similar to that shown in FIG. 6 increases the x-ray flux to the inner detector rows and reduces the x-ray flux to the outer detector rows yielding a reconstructed image with fewer artifacts as well as reduced x-ray to the patient. 
     Referring now to FIG. 7, noise distributions from several filter-shaped designs are shown with respect to detector row number for an eight slice helical scan. The noise level at the innermost detector rows (rows  3  and  4 ) is assumed to be uniform and the noise levels for the other detector rows are normalized accordingly. To ensure artifact-free image when the x-ray focal spot moves (due to mechanical or thermal expansion), the filter shape should be continuous and smooth along the z axis. The several filter-shaped designs differ from one another in the thickness of the generally end regions. As shown, the noise index increases as the thickness of each end region increases. 
     Referring now to FIG. 8, the relative x-ray dosage to patient for the several filter designs characteristically depicted in FIG. 7 are shown. Specifically, the fraction of total dosage projected to the patient decreases as the thickness of the filter is increased. For example, filter shape  1  provides a relative dose of 0.87 whereas filter shape  6  provides a relative dose of approximately 0.85. That is, the radiation detected by the outer rows of detector array  18 , FIG. 3, decreases as thickness of the filter end regions increase. 
     The present invention may be incorporated into a CT medical imaging device similar to that shown in FIG.  1 . Alternatively, however, the present invention may also be incorporated into a non-invasive package or baggage inspection system, such as those used by postal inspection and airport security systems. 
     Referring now to FIG. 9, 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 . A filter  107  similar to that cross-sectionally shown in FIG. 6 is also housed within gantry  102 . 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, etc. 
     Therefore, in accordance with one embodiment of the present invention, a cone angle dependent pre-subject filter for use with a radiation emitting imaging device is provided. The filter includes a flat surface as well as a convex concave surface. A number of sidewalls connecting the flat surface and the concave surface in a single solid structure are also provided. 
     In accordance with another embodiment of the present invention, a radiation emitting imaging device includes a rotatable gantry having an opening defined therein for receiving a subject to be scanned. The device further includes a subject positioner configured to position the subject within the opening as well as a high frequency electromagnetic energy projection source configured to project high frequency electromagnetic energy to the subject. The imaging device further includes at least one filtering device configured to filter high frequency electromagnetic energy projected to the subject. The filtering device is formed of a bulk of filtering material having a non-uniform attenuation. The imaging device also includes a detector array having a plurality of detectors to detect high frequency electromagnetic energy passing through the subject and to output a plurality of electrical signals indicative of an intensity of the high electromagnetic energy detected. A data acquisition system is provided and connected to the detector array and configured to receive a plurality of electrical signals. An image reconstructor connected to the data acquisition system is provided and configured to reconstruct an image of the subject from the plurality of signals received by the data acquisition system. 
     In accordance with a further embodiment of the present invention, a cone angle dependent pre-subject filter includes means for receiving high frequency electromagnetic energy. The filter further includes means for increasing attenuation of high frequency electromagnetic energy flux in a first region as well as means for decreasing attenuation of high frequency electromagnetic energy flux in a second region. 
     In accordance with yet another embodiment of the present invention, a method of manufacturing a pre-subject filter for use with a radiation emitting imaging device includes the step of defining a block of filtering material. The method further includes shaping the block to have a linear surface and fashioning a block to have a curvilinear surface. 
     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.