Abstract:
A CT detector capable of energy discrimination and direct conversion is disclosed. The detector includes multiple layers of semiconductor material with the layers having varying thicknesses. The detector is constructed to be segmented in the x-ray penetration direction so as to optimize count rate performance as well as avoid saturation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is continuation of and claims priority of U.S. Ser. No. 10/848,877 filed May 19, 2004, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates generally to diagnostic imaging and, more particularly, to a multi-layer direct conversion CT detector capable of providing photon count and/or energy data with improved saturation characteristics.  
         [0003]     Typically, in radiographic imaging systems, an x-ray source emits x-rays toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be interchangeably used to describe 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-rays. 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.  
         [0004]     In some computed tomography (CT) imaging systems, the x-ray source and the detector array are rotated about a gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-rays as a 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 a photodiode for receiving the light energy from an adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. 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.  
         [0005]     Conventional CT imaging systems utilize detectors that convert radiographic energy into current signals that are integrated over a time period, then measured and ultimately digitized. A drawback of such detectors is their inability to provide data or feedback as to the number and/or energy of photons detected. That is, conventional CT detectors have a scintillator component and photodiode component wherein the scintillator component illuminates upon reception of radiographic energy and the photodiode detects illumination of the scintillator component and provides an electrical signal as a function of the intensity of illumination. While it is generally recognized that CT imaging would not be a viable diagnostic imaging tool without the advancements achieved with conventional CT detector design, a drawback of these detectors is their inability to provide energy discriminatory data or otherwise count the number and/or measure the energy of photons actually received by a given detector element or pixel. Accordingly, recent detector developments have included the design of an energy discriminating, direct conversion detector that can provide photon counting and/or energy discriminating feedback. In this regard, the detector can be caused to operate in an x-ray counting mode, an energy measurement mode of each x-ray event, or both.  
         [0006]     These energy discriminating, direct conversion detectors are capable of not only x-ray counting, but also providing a measurement of the energy level of each x-ray detected. While a number of materials may be used in the construction of a direct conversion energy discriminating detector, semiconductors have been shown to be one preferred material. A drawback of direct conversion semiconductor detectors, however, is that these types of detectors cannot count at the very high x-ray photon flux rates typically encountered with conventional CT systems. The very high x-ray photon flux rate ultimately leads to detector saturation. That is, these detectors typically saturate at relatively low x-ray flux levels. This saturation can occur at detector locations wherein small subject thickness is interposed between the detector and the radiographic energy source or x-ray tube. It has been shown that these saturated regions correspond to paths of low subject thickness near or outside the width of the subject projected onto the detector fan-arc. In many instances, the subject is more or less circular or elliptical in the effect on attenuation of the x-ray flux and subsequent incident intensity to the detector. In this case, the saturated regions represent two disjointed regions at extremes of the fan-arc. In other less typical, but not rare instances, saturation occurs at other locations and in more than two disjointed regions of the detector. In the case of an elliptical subject, the saturation at the edges of the fan-arc is reduced by the imposition of a bowtie filter between the subject and the x-ray source. Typically, the filter is constructed to match the shape of the subject in such a way as to equalize total attenuation, filter and subject, across the fan-arc. The flux incident to the detector is then relatively uniform across the fan-arc and does not result in saturation. What can be problematic, however, is that the bowtie filter may not be optimum given that a subject population is significantly less than uniform and not exactly elliptical in shape. In such cases, it is possible for one or more disjointed regions of saturation to occur or conversely to over-filter the x-ray flux and create regions of very low flux. Low x-ray flux in the projection will ultimately contribute to noise in the reconstructed image of the subject.  
         [0007]     A number of imaging techniques have been developed to address saturation of any part of the detector. These techniques include maintenance of low x-ray flux across the width of a detector array, for example, by using low tube current or current that is modulated per view. However, this solution leads to increased scanned time. That is, there is a penalty that the acquisition time for the image is increased in proportion to the nominal flux needed to acquire a certain number of x-rays that meet image quality requirements. Other solutions include the implementation of an over-range algorithm that is used to generate replacement data for the saturated data. However, these algorithms may imperfectly replace the saturated data as well as contribute to the complexity of the CT system.  
         [0008]     It would therefore be desirable to design a direct conversion, energy discriminating CT detector that does not saturate at the x-ray photon flux rates typically found in conventional CT systems.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0009]     The present invention is directed to a multilayer CT detector that can be made to perform at very high count rates that overcomes the aforementioned drawbacks.  
         [0010]     A CT detector capable of energy discrimination and direct conversion is disclosed. The detector includes multiple layers of semiconductor material of varying thicknesses throughout the detector. In this regard, the detector is constructed so as to be segmented in the x-ray penetration direction to optimize count rate performance as well as avoid saturation.  
         [0011]     The CT detector supports not only x-ray photon counting, but energy measurement or tagging as well. As a result, the present invention supports the acquisition of both anatomical detail as well as tissue characterization information. In this regard, the energy discriminatory information or data may be used to reduce the effects of beam hardening and the like. Further, these detectors support the acquisition of tissue discriminatory data and therefore provide diagnostic information that is indicative of disease or other pathologies. For example, detection of calcium in a plaque in a view is possible. This detector can also be used to detect, measure, and characterize materials that may be injected into a subject such as contrast agents and other specialized materials such as targeting agents. Contrast materials can, for example, include iodine that is injected into the blood stream for better visualization. A method of fabricating such a detector is also disclosed.  
         [0012]     Therefore, in accordance with one aspect of the present invention, a direct conversion CT detector includes multiple direct conversion layers designed to directly convert radiographic energy to electrical signals representative of energy sensitive CT data. The detector also includes an electrical signal collection layer sandwiched between adjacent direct conversion layers.  
         [0013]     In accordance with another aspect, the present invention includes a CT system having a rotatable gantry having a bore centrally disposed therein, a table movable fore and aft through the bore and configured to position a subject for CT data acquisition, and a radiographic energy projection source positioned within the rotatable gantry and configured to project radiographic energy toward the subject. The CT system also includes a detector array disposed within the rotatable gantry and configured to detect radiographic energy projected by the projection source and impinged by the subject. The detector array includes a plurality of detector cells, wherein each cell has a stacked arrangement of semiconductor layers in a direction generally that of energy projection and designed to provide energy sensitive data acquired from the subject in response to receiving radiographic energy.  
         [0014]     According to another aspect, the present invention includes a CT detector having a first means and a second means for directly converting radiographic energy to electrical signals. The detector also has means for receiving electrical signals interstitially positioned between the first means for directly converting and the second means for directly converting.  
         [0015]     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  
       [0016]     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.  
         [0017]     In the drawings:  
         [0018]      FIG. 1  is a pictorial view of a CT imaging system.  
         [0019]      FIG. 2  is a block schematic diagram of the system illustrated in  FIG. 1 .  
         [0020]      FIG. 3  is a perspective view of one embodiment of a CT system detector assembly.  
         [0021]      FIG. 4  is a perspective view of a CT detector.  
         [0022]      FIG. 5  is illustrative of various configurations of the detector in  FIG. 4  in a four-slice mode.  
         [0023]      FIG. 6  is a partial perspective view of a two-layer director in accordance with the present invention.  
         [0024]      FIG. 7  is a cross-sectional view of  FIG. 6  taken along lines  7 - 7  thereof.  
         [0025]      FIGS. 8-10  illustrate cross-sectional views of direct conversion detectors in accordance with several additional embodiments of the present invention.  
         [0026]      FIG. 11  is a cross-sectional schematic view of that shown in  FIG. 10  illustrating signal feedthroughs that are created in another embodiment of the invention.  
         [0027]      FIG. 12  is a pictorial view of a CT system for use with a non-invasive package inspection system.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     The operating environment of the present invention is described with respect to a four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the present invention is equally applicable for use with single-slice or other multi-slice configurations. Moreover, the present invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the present invention is equally applicable for the detection and conversion of other radiographic energy.  
         [0029]     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 assembly  18  on the opposite side of the gantry  12 . Detector assembly  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 not only the intensity of an impinging x-ray beam but is also capable of providing photon or x-ray count data, 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 .  
         [0030]     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  review 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 .  
         [0031]     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 .  
         [0032]     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 .  
         [0033]     As shown in  FIGS. 3 and 4 , detector assembly  18  includes a plurality of detectors  20 , with each detector including a number of detector elements  50  arranged in a cellular array. A collimator (not shown) is positioned to collimate x-rays  16  before such beams impinge upon the detector assembly  18 . In one embodiment, shown in  FIG. 3 , detector assembly  18  includes 57 detectors  20 , each detector  20  having an array size of 16×16. As a result, assembly  18  has 16 rows and 912 columns (16×57 detectors) which allows 16 simultaneous slices of data to be collected with each rotation of gantry  12 .  
         [0034]     Switch arrays  54  and  56 ,  FIG. 4 , are multi-dimensional semiconductor arrays coupled between cellular array  52  and DAS  32 . Switch arrays  54  and  56  include a plurality of field effect transistors (FET) (not shown) arranged as multi-dimensional array and are designed to combine the outputs of multiple cells to minimize the number of data acquisition channels and associated cost. The FET array includes a number of electrical leads connected to each of the respective detector elements  50  and a number of output leads electrically connected to DAS  32  via a flexible electrical interface  58 . Particularly, about one-half of detector element outputs are electrically connected to switch  54  with the other one-half of detector element outputs electrically connected to switch  56 . Each detector  20  is secured to a detector frame  60 ,  FIG. 3 , by mounting brackets  62 .  
         [0035]     It is contemplated and recognized that for some applications, the count rate limitation of the FET arrays may make them less desirable. In this regard, as will be described, each detection pixel or cell is connected to a channel of electronics.  
         [0036]     Switch arrays  80  and  82  further include a decoder (not shown) that enables, disables, or combines detector element outputs in accordance with a desired number of slices and slice resolutions for each slice. Decoder, in one embodiment, is a decoder chip or a FET controller as known in the art. Decoder includes a plurality of output and control lines coupled to switch arrays  54  and  56  and DAS  32 . In one embodiment defined as a 16 slice mode, decoder enables switch arrays  54  and  56  so that all rows of the detector assembly  18  are activated, resulting in 16 simultaneous slices of data for processing by DAS  32 . Of course, many other slice combinations are possible. For example, decoder may also select from other slice modes, including one, two, and four-slice modes.  
         [0037]     As shown in  FIG. 5 , by transmitting the appropriate decoder instructions, switch arrays  54  and  56  can be configured in the four-slice mode so that the data is collected from four slices of one or more rows of detector assembly  18 . Depending upon the specific configuration of switch arrays  54  and  56 , various combinations of detectors  20  can be enabled, disabled, or combined so that the slice thickness may consist of one, two, three, or four rows of detector elements  50 . 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.  
         [0038]     As described above, each detector  20  is designed to directly convert radiographic energy to electrical signals containing energy discriminatory data. The present invention contemplates a number of configurations for these detectors. Notwithstanding the distinctions between each of these embodiments, each detector does share two common features. One of these features is the multilayer arrangement of semiconductor films or layers. In a preferred embodiment, each semiconductor film is fabricated from Cadmium Zinc Telluride (CZT). However, one skilled in the art will readily recognize that other materials capable of the direct conversion of radiographic energy may be used. The other common feature between the various embodiments is the use of interstitial or intervening metallized films or layers separating the semi-conducting layers. As will be described, these metallized layers are used to apply a voltage across a semiconductor layer as well as collect electrical signals from a semiconductor layer.  
         [0039]     It is generally well known that photon count rate performance of a semiconductor is a function of the square of the thickness of the detector and the radiographic energy deposition process is exponential. The count rate performance for a CZT detector may be defined by:  
         T   TR     =         L   2       V   ⁢           ⁢     μ   e         .         
 
         [0040]     From this definition, assuming a thickness of L=0.3 cm and an electric field V of 1000 V/cm, and with a μ e  of about 1000, a maximum count rate of 1.0 megacounts may be achieved. In other words, the count rate of a CZT semiconductor layer that is 3 mm thick may have a count rate performance in the range of 1.0 megacounts/sec. However, as will be described, constructing a direct conversion semiconductor detector with multiple layers as opposed to a single thicker layer can improve count rate performance.  
         [0041]     Referring now to  FIG. 6 , a portion of a two-layered CZT or direct conversion detector  20   a  in accordance with one embodiment of the present invention is shown in perspective. Detector  20   a  is defined by a first semiconductor layer  62  and a second semiconductor layer  64 . During the fabrication process, each semiconductor layer  62 ,  64  is constructed to have a number of electronically pixilated structures or pixels to define a number of detection elements  65 . This electronic pixilation is accomplished by applying a 2D array  67 ,  69  of electrical contacts  65  onto a layer  62 ,  64  of direct conversion material. Moreover, in a preferred embodiment, this pixilation is defined two-dimensionally across the width and length of each semiconductor layer  62 ,  64 .  
         [0042]     Detector  20   a  includes a contiguous high voltage electrode  66 ,  68  for semiconductor layers  62 ,  64 , respectively. Each high voltage electrode  66 ,  68  is connected to a power supply (not shown) and is designed to power a respective semiconductor layer during the x-ray or gamma ray detection process. One skilled in the art will appreciate that each high voltage connection layer should be relatively thin so as to reduce the x-ray absorption characteristics of each layer and, in a preferred embodiment, is a few hundred angstroms thick. As will be described in greater detail below, these high voltage electrodes may be affixed to a semiconductor layer through a metallization process.  
         [0043]     Referring now to  FIG. 7 , a cross-sectional view of  FIG. 6  taken along line  7 - 7  thereof illustrates the relative thickness of each semiconductor layer  62 ,  64 . Similar to the high voltage electrode layers  66 ,  68 , the 2D arrays  67 ,  69  should also be minimally absorbent of radiographic energy. Each array or signal collection layer is designed to provide a mechanism for outputting the electrical signals created by the semiconductor layers to a data acquisition system or other system electronics. One skilled in the art will appreciate that several hundred interconnects (not shown) are used to connect each contact  65  with the CT system electronics.  
         [0044]     In addition, as shown in  FIG. 7 , the thickness of the semiconductor layers  62 ,  64  is different from one another. In this regard, more x-rays are deposited in semiconductor layer  62  than in semiconductor layer  64 . For example, assuming that semiconductor layer  62  has a thickness of one millimeter (mm) and semiconductor  64  has a thickness of 2 mm, semiconductor layer  62  is expected to absorb about 78% of the x-rays whereas the second semiconductor layer  64  is expected to absorb about 22% of the x-rays. Further, it is expected that the first semiconductor layer  62  is to experience a maximum count rate that is approximately nine times faster than that of a single layer semiconductor 3 mm thick. However, the first semiconductor layer  62  measures only approximately 78% of the total flux thereby providing an 11.5 times increase in effective max count rate performance compared to a single semiconductor layer 3 mm thick. The second semiconductor layer  64  is expected to have a count rate that is 2.25 times faster than that of a single 3 mm thick semiconductor but measures only approximately 22% of the total flux, thereby, providing an equivalent or effective max count rate that is approximately 10.2 times that expected to be experienced with a single layer of semiconductor material 3 mm thick. As a result of the improved count rates of the segmented detector described above relative to a single layer of semiconductor material, detector  20   a  may be constructed to provide a tenfold increase in count rate performance.  
         [0045]     The above dimensions are illustrative of the improvement in maximum count rate that may be experienced with a two layer detector. However, it is contemplated that more than two layers may be used to construct a CT detector with improved count rate characteristics. For example, a single 0.43 mm layer is expected to absorb about 54% of x-rays received and, as such, has a maximum count rate of approximately 40 times that of a single layer, 3.0 mm thick semiconductor. However, a 0.43 mm layer absorbs only approximately 54% of the total flux to provide an equivalent or effective max count rate of approximately 92 times that of a single semiconductor layer that is 3 mm thick. Additional layers may be added to provide an overall count rate increase of 9200%.  
         [0046]     Referring now to  FIG. 8 , another contemplated design for a CZT or direct conversion detector is shown. In this embodiment, detector  20   b  also includes a pair of semiconductor layers  74 ,  76 . In contrast to the previously described embodiment, detector  20   b  includes a single, common signal collection layer or 2D contact array  78 . This single, yet common array  78  is designed to collect electrical signals from both semiconductor layers  74 ,  76  and output those electrical signals to a DAS or other system electronics. In addition, detector  20   b  includes a pair of high voltage electrodes  80 ,  82 . Each high voltage electrode effectively operates as a cathode whereas the contacts of the 2D array  78  operate as an anode. In this regard, the voltage applied via high voltage connections  80 ,  82  creates a circuit through each semiconductor layer to the signal collection contacts array  78 .  
         [0047]     Yet another contemplated embodiment is illustrated in  FIG. 9 . As shown in this embodiment, detector  20   c  includes four semiconductor layers  84 ,  86 ,  88 , and  90 . Detector  20   c  further includes two electrically conductive lines or paths  92 ,  94  that are electrically connected to high voltage electrodes  87 ,  89 ,  91  as well as collection contact arrays  93 ,  95 . Electrically conductive path  92  receives and translates electrical signals from contact arrays  93 ,  95 . In this regard, a single data output is provided to the CT system&#39;s electronics. Similar to a single signal collection lead, a single high voltage connection  94  is used to power the four semiconductor layers  84 - 90  via electrodes  87 ,  89 ,  91 . Detector  20   c  only requires a single high voltage connection.  
         [0048]     Referring to  FIG. 10 , a monolithic embodiment of the present invention is shown. Similar to the embodiment of  FIG. 7 , detector  20   d  includes four semiconductor layers  96 - 102 . Each semiconductor layer  96 - 102  is connected to a pair of electrically conductive layers. In this regard, one electrically conductive layer is used for application of a voltage whereas the other electrically conductive layer is used for collection of the electrical signals generated by the respective semiconductor layers. To minimize the number of electrically conductive layers, detector  20   d  utilizes an alternating electrically conductive layer architecture. That is, every other electrically conductive layer is used for high voltage connection with the other electrically conductive layers used for signal collection. In this regard, electrically conductive layers  104 ,  106 , and  108  are used for application of a relatively high voltage whereas layers  110  and  112  include contacts for signal collection. As such, high voltage collection layers  104  and  108  are used to apply a voltage to semiconductor layers  96  and  102 , respectively. High voltage connection layer  106  is used to apply a voltage to semiconductor layers  98  and  100 .  
         [0049]     As described above, in a preferred embodiment, each semiconductor layer is constructed from CZT material. One skilled in the art will appreciate that there are a number of techniques that may be used to construct such a semiconductor. For example, molecular beam epitaxy (MBE) is one method that may be used to grow each thin layer of CZT material. One skilled in the art will appreciate that a number of techniques may be used to metallize the semiconductor layers to provide the electrically conductive connections heretofore described.  
         [0050]     Further, metallization may also be used to provide signal feedthroughs for the collection contacts as illustrated in  FIG. 11 . As shown, a single layer of semiconductor material  114  is sandwiched between an array  116  of collection contacts and a high voltage electrode layer  118 . Prior to metallization of the semiconductor layer  114  to form collection contact array  116  and high voltage electrode layer  118 , holes  120  may be etched or otherwise formed in semiconductor  114 . The holes  120  may then be metallized to provide a signal feed path  122  from a respective collection contact  124 . The signal feedthroughs or conductive paths  122  are constructed within holes  120  so as to not be in contact with the near-contiguous high voltage electrode layer  118 . In this regard, signal runs may extend vertically or in the x-ray reception direction throughout the detector to a bus (not shown) designed to translate the electrical signals emitted by the individual collection contacts  124  to the CT system&#39;s electronics. As a result, a stacked arrangement of a series of thin stacked layers in the x-ray direction is formed.  
         [0051]     Referring now to  FIG. 12 , package/baggage inspection system  126  includes a rotatable gantry  128  having an opening  130  therein through which packages or pieces of baggage may pass. The rotatable gantry  128  houses a high frequency electromagnetic energy source  132  as well as a detector assembly  134 . A conveyor system  136  is also provided and includes a conveyor belt  138  supported by structure  140  to automatically and continuously pass packages or baggage pieces  142  through opening  130  to be scanned. Objects  142  are fed through opening  130  by conveyor belt  138 , imaging data is then acquired, and the conveyor belt  138  removes the packages  142  from opening  130  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  142  for explosives, knives, guns, contraband, etc.  
         [0052]     Therefore, a direct conversion CT detector includes multiple direct conversion layers designed to directly convert radiographic energy to electrical signals representative of energy sensitive CT data. The detector also includes an electrical signal collection layer sandwiched between adjacent direct conversion layers.  
         [0053]     The present invention also includes a CT system having a rotatable gantry having a bore centrally disposed therein, a table movable fore and aft through the bore and configured to position a subject for CT data acquisition, and a radiographic energy projection source positioned within the rotatable gantry and configured to project radiographic energy toward the subject. The CT system also includes a detector array disposed within the rotatable gantry and configured to detect radiographic energy projected by the projection source and impinged by the subject. The detector array includes a plurality of detector cells, wherein each cell has a stacked arrangement of semiconductor layers in a direction generally that of energy projection and designed to provide energy sensitive data acquired from the subject in response to receiving radiographic energy.  
         [0054]     The present invention further includes a CT detector having a first means and a second means for directly converting radiographic energy to electrical signals. The detector also has means for receiving electrical signals interstitially positioned between the first means for directly converting and the second means for directly converting.  
         [0055]     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.