Patent Number: 
Section: description

Referring initially to FIG. 2, an imaging system in accordance with the present invention is shown and generally designated 10. As shown in FIG. 2, the system 10 includes an X-ray source 12 configured to produce a spectrum of X-ray radiation 14. An optional collimator 16 may be provided to collimate the radiation 14 emitted from the X-ray source 12 into one or more beams 18a-c. As such, each beam 18 emanates from the X-ray source 12 in a slightly different direction, and consequently, along a separate path 20a-c. It is to be appreciated that the use of three beams 18 is merely exemplary and that as many beams 18 as desired may be used in accordance with the present invention. In detail, as shown in FIG. 2, beam 18a initially travels substantially along path 20a, beam 18b initially travels substantially along path 20b and beam 18c initially travels substantially along path 20c.  Referring still to FIG. 2, a detector array 22 is shown positioned to receive the beams 18 from the source 12. Specifically, the detector array 22 is shown having three detectors 24a-c, with detector 24a positioned to receive beam 18a, detector 24b positioned to receive beam 18b and detector 24c positioned to receive beam 18c. For the present invention, an object 26 can be interposed between the X-ray source 12 and the detector array 22 to thereby allow the beams 18 to be modified by passing through the object 26 before reaching the detectors 24. In accordance with the present invention, the detectors 24 can be any type of detector known in the pertinent art capable of receiving radiation and producing an electrical signal that is proportional to the intensity of the radiation received. For example, the detectors 24 can be solid state detectors (separate or having a charge couple detector structure), gas-filled detectors or scintillators with photo-multipliers. Preferably, each detector 24 is a small-area X-ray detector. For the present invention, the output of each detector 24 is electrically wired to a computer (not shown) to allow the signals generated by the detectors 24 to be processed. Also shown in FIG. 2, the X-ray source 12 can be slideably mounted on a circular track 28 that extends around the object 26. Additionally, as shown, each detector 24 or the entire detector array 22 can be slideably mounted on the track 28. As such, the X-ray source 12 and detectors 24 can be moved either continuously or incrementally around the track 28 and relative to the object 26. The dashed lines in FIG. 2 show an exemplary second position for the X-ray source 12 and detectors 24. By moving the X-ray source 12, each radiation beam 18 emanating from the X-ray source 12 can be caused to successively travel on different paths 20 through the object 26. For example, as shown in FIG. 2, when X-ray source 12 is in the initial position represented by the solid lines, beam 18a travels substantially along path 20a, and when X-ray source 12 is moved to a second position represented by dashed lines, beam 18a travels substantially along path 20d. Similarly, beam 18b travels substantially along path 20e and beam 18c travels substantially along path 20f when the X-ray source 12 is in the position indicated by dashed lines. Accordingly, the detector array 22 can be moved in conjunction with the X-ray source 12 to allow each detector 24 to track a single X-ray beam 18, as that X-ray beam 18 travels on successive paths 20 through the object 26. An important aspect of the present invention is that the X-ray radiation 14 is filtered between the X-ray source 12 and the detectors 24. By cross-referencing FIGS. 2 and 3, it can be seen that a wheel 30 having attached filters 32, 34, 36 and 38 can be used to successively filter each X-ray beam 18a-c on each path 20. For the present invention, the relative position of each filter 32, 34, 36; 38 with respect to the other filters 32, 34, 36, 38 is inconsequential. As further shown, a motor 40 having a shaft 42 can be used to rotate the wheel 30 and filters 32, 34, 36, 38 to successively filter each beam 18 four times while the beam 18 travels substantially along a single path 20, with each of the four filtrations occurring with a different filter 32, 34, 36, 38. Accordingly, each time a beam 18 is moved to a new path 20, the wheel 30 is rotated through one complete revolution to once again filter the beam 18 four times. Alternatively, the wheel 30 can be located between the X-ray source 12 and the collimator 16 (this configuration not shown). As shown, a bracket 44 can be used to attach the motor 40 to the X-ray source 12 to allow the wheel 30, the filters 32, 34, 36, 38, the motor 40 and the shaft 42 to travel with the X-ray source 12 as the source 12 moves along the track 28 relative to the object 26. Each time a beam 18 is successively filtered four times, four different electrical signals are produced by a detector 24. For the present invention, a computer processor (not shown) can be configured to manipulate the four electrical signals created for each path 20 to produce an image signal for the path 20. For example, each path 20 can be used to produce an image signal that represents a single pixel in the final image. Or stated another way, a computer process can be configured to subtract, pixel by pixel, the digital images of each pair of the four images obtained with the filtered beam 18. The two digital difference images are further subtracted to finally produce the contrast enhancement image. Once an image signal is established for each desired path 20, conventional tomography techniques known in the pertinent art can be used to combine all the image signals (one image signal for each path 20) into a composite image that shows the internal features of the object 26. Referring now to FIG. 3, a filter set having four different filters 32, 34, 36, 38 is mounted on the wheel 30 to allow each beam 18 on each path 20 to be successively filtered four times. As further detailed below, a unique filter set is designed for use with a specific contrast agent that is prescribed for introduction into the object 26. Specifically, the chemical constituents and thickness of each filter 32, 34, 36, 38 is determined with reference to the specific contrast agent that is being used. FIG. 4 shows an exemplary filter 32 having layers 46, 48, 50 and 52. Specifically, the filter 32 can include an optional transparent layer 46, a filtering layer 48, an optional additional balance layer 50 and an optional protective layer 52. It is to be appreciated that each filter 32, 34, 36, 38 will have different layers 46, 48, 50, 52, the layers 46, 48, 50, 52 differing in both chemical makeup and thickness. For the present invention, the optional transparent layer 46 can be included to support as well as protect the other layers 48, 50, 52. The optional protective layer 52 can be included to protect the other layers 48, 50 from corrosion or other environmental factors. The function of the filtering layer 48 and the additional balance layer 50 are discussed below. As seen by cross-referencing FIGS. 3 and 4, a metal ring 54 can be used to hold the layers 46, 48, 50, 52 together and attach them to the wheel 30. When used in conjunction with a contrast agent containing a chemical element having a KEDGE, CONTRAST AGENT, a filter set is constructed in accordance with the present invention having a filter 32 with a filtering layer 48 that contains a chemical element having a KEDGE that is greater than KEDGE, CONTRAST AGENT, and a filter 34 with a filtering layer 48 that contains a chemical element having a KEDGE that is greater than or equal to KEDGE, CONTRAST AGENT. Further, the filter set is constructed in accordance with the present invention having a filter 36 with a filtering layer 48 that contains a chemical element having a KEDGE that is less than or equal to KEDGE, CONTRAST AGENT, and a filter 38 with a filtering layer 48 that contains a chemical element having a KEDGE that is less than KEDGE, CONTRAST AGENT. For most contrast agents, both the filter 36 and the filter 38 include a filtering layer 48 that contains the same chemical element that is used in the contrast agent (i.e. a chemical element having a KEDGE, CONTRAST AGENT), but the filtering layer 48 of filter 36 may differ in thickness from the filtering layer 48 of filter 38. The invention includes specific chemical elements and thickness"" sufficient to create filter sets for various contrast agents as shown in Table 1. Referring back to FIG. 2, in the operation of the present invention, a contrast agent is first introduced into the object 26. Once introduced, the contrast agent will be selectively absorbed or localized in specific regions to thereby establish portions of the object 26 having differing concentrations of contrast agent. Table 1, below, lists a number of suitable contrast agents that are either in current use for imaging portions of the human body or are contemplated for future use. It is to be appreciated that conventional methods of administering the contrast agent that are known in the pertinent art can be employed. Further, it is anticipated that the present invention is applicable to the imaging of a non-human object 26, such as a structural component for a machine or device (not shown). In this case, a material in the structural component can be used as a contrast agent and a suitable filter set constructed accordingly. Once a contrast agent has been introduced, the object 26 can be placed between the X-ray source 12 and the detector array 22 as shown in FIG. 2. Next, the X-ray source 12 is located at a first position and activated to produce one or more beams 18a-c travelling through the object 26 on a first set of paths 20a-c. Next, the wheel 30 containing the filters 32, 34, 36, 38 is rotated to successively interpose each of the four filters 32, 34, 36, 38 between the X-ray source 12 and the object 26 to filter each of the beams 18 with each of the four filters 32, 34, 36, 38. This results in the production of four intensity-proportional signals by a detector 24 for each beam 18. It is to be appreciated that the four signals will be temporally spaced from each other, the spacing corresponding to the time the beam 18 strikes the wheel 30 between adjacent filters 32, 34, 36, 38. Referring now to FIG. 5A, a typical emission spectrum for a conventional X-ray source 12 that has passed through a portion of the body having no contrast agent is shown by curve 56. When the spectrum represented by curve 56 reaches a detector 24, an electronic signal that is approximately proportional to the area under curve 56 (the intensity of the emission) is produced. Curve 60 in FIG. 5A represents the spectrum that results after radiation produced by a typical X-ray source 12 is passed through a portion of the body having exemplary contrast agent, Gd, in the absence of filters. Referring now to FIG. 5B, curve 58 represents the spectrum that results after radiation producing curve 56 in FIG. 5A is now passed through filter 38 and a portion of the body having no contrast agent. In this case, filter 38 has a filtering layer 48 having a chemical element with a KEDGE of approximately 49 keV. Accordingly, the electronic signal produced by a detector 24 when filter 38 is interposed between the X-ray source 12 and the detector 24 will be approximately proportional to the area under curve 58. Filter 36, in general, has a filtering layer 48 having the same chemical element that is used in the contrast agent. In this example, filter 36 has a chemical element with a KEDGE, CONTRAST AGENT of approximately 50 keV. The spectrum received by a detector 24 when filter 36 is interposed between the X-ray source 12 and the detector 24 is approximately represented as curve 60. Accordingly, the signal produced by a detector 24 when filter 36 is interposed between the X-ray source 12 and the detector 24 will be approximately proportional to the area under curve 60. For the present invention, the electrical signal produced by a detector 24 while filter 38 is interposed along a path 20 containing no contrast agent can be subtracted from the electrical signal, after digitization, produced by the detector 24 while the filter 36 is interposed along the same path 20 to produce the second intermediary difference signal. This second intermediary difference signal simulates the image signal that would be obtained if a quasi-monochromatic beam having an average energy slightly below KEDGE, CONTRAST AGENT were to be passed through the object 26. More specifically, the second intermediary difference signal produced for paths 20 through a portion of the body having no contrast agent simulates the exemplary quasi-monochromatic spectrum shown in FIG. 6A and designated 62. It is to be appreciated that the curve of the quasi-monochromatic spectrum shown in FIG. 6A and designated 62 represents a resultant area obtained by subtracting the area under curve 58 (FIG. 5B) from the area under curve 60 (FIG. 5A). The resultant spectrum is essentially the same as the spectrum when a beam of narrow energy band, or a quasi-monochromatic beam, were used as the source. Similarly, the second intermediary difference signal produced for paths 20 through a portion of the body having a contrast agent simulates the quasi-monochromatic spectrum shown in FIG. 6B and designated 64. In a similar fashion, the processor subtracts the digital signal produced by the detector 24 with the filter 34 interposed along the path 20 from the digital signal produced by the detector 24 with the filter 32 interposed along the path 20 to produce a first intermediary difference signal. A curve representing the spectrum that results after radiation producing curve 56 in FIG. 5A is now passed through filter 32 and a portion of the body having no contrast agent is shown in FIG. 5C and designated curve 65. It is to be appreciated that the first intermediary difference signal simulates the image signal that would be obtained if a quasi-monochromatic beam having an average energy slightly above KEDGE, CONTRAST AGENT were to be passed through the object 26. More specifically, the first intermediary difference signal produced for paths 20 having no contrast agent simulates the exemplary quasi-monochromatic spectrum shown in FIG. 6A and designated 66. Similarly, the first intermediary difference signal produced for paths 20 having contrast agent simulates the exemplary quasi-monochromatic spectrum shown in FIG. 6B and designated 68. Next, the processor subtracts the second intermediary difference signal from the first intermediary difference signal to produce an image signal for the path 20. More specifically, the image signal produced for paths 20 having no contrast agent simulates the difference in intensity between spectrum curve 66 and spectrum curve 62 in FIG. 6A. Similarly, the image signal produced for paths 20 having a contrast agent simulates the difference in intensity between spectrum curve 68 and spectrum curve 64 in FIG. 6B. This final difference is the data to be processed for tomography or angiography. The final difference signal strongly varies with concentration and thickness of the contrast element due to the variation of absorption. This results in an enhanced contrast image between the region with the contrast agent and the region without. Referring now to FIG. 7, the effect of additional balance layers 50 in a filter set is shown. Specifically, FIG. 7 compares the quasi-monochromatic signal that is simulated without additional balance layers 50 (curve 70) and the quasi-monochromatic signal that is simulated with additional balance layers 50 (curve 72). The curve 72 was generated for a filter set having a filter 32 with a filtering layer 48 that includes 140.0 xcexcm of 65Tb and an additional balance layer 50 of 260.0 xcexcm of 65Tb and a filter 34 that includes 152.0 xcexcm of 64Gd and an additional balance layer 50 of 260.0 xcexcm of 65Tb. With cross reference to Table 1 and FIG. 7, these two filters 32, 34 can be used in a filter set in conjunction with the contrast agent Gd to generate the first intermediary difference signal. As shown in FIG. 7, the use of additional balance layers 50 reduces the non-zero difference of the filter transmission outside the energy pass band. Of course, this effect is obtained by paying the price of reducing the radiation intensity within the pass band (by a factor of about two, in this case). In practice, the additional balance layer 50 is designed to provide a compromise between the enhancement of the quality of monochromatization (i.e. a thicker additional balance layer 50 providing better balance) and the intensity level within the energy pass band (i.e. a larger number of photons to provide a better Signal-To-Noise ratio). Referring back to FIG. 2, once image signals are obtained for the first set of paths 20a-c, the X-ray source 12 and collimator 16 can be moved to a second position (shown by dashed lines) to cause the beams 18a-c emanating from the collimator 16 to travel along a new set of paths 20d-f. While the X-ray source 12 and collimator 16 are at the second position, the wheel 30 is again rotated to successively interpose each of the four filters 32, 34, 36, 38 between the X-ray source 12 and the object 26 to again filter each of the beams 18a-c with each of the four filters 32, 34, 36, 38. Again, four intensity-proportional signals are produced by a detector 24 for each beam 18. For the present invention, these four signals can be manipulated by a processor (not shown) to produce an image signal for each new path 20d-f. This process of moving the X-ray source 12 and producing an image signal for each new path 20 can be repeated as desired. Further, it is to be appreciated that the X-ray source 12 can be moved continuously around the object 26. When this technique is used, the wheel 30 containing the filters 32, 34, 36, 38 can be rotated continuously as the X-ray source 12 moves. By rotating the wheel 30 very rapidly, each beam 18 can be filtered four times before significant movement of the beam 18 occurs. Thus, in effect, each beam 18 remains on a single path 20 while the successive filtration takes place. Once an image signal is produced for all paths 20 of interest, conventional tomography techniques can be used to combine all the image signals (one image signal for each path 20) into a composite image that shows the internal features of the object 26. While the particular imaging systems and methods as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.