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 plan view of a conventional rectangular CT detector matrix comprised of square-shaped detector cells.
FIG. 2 is a plan view of a CT detector matrix with staggered detector channels.
FIG. 3 is a pictorial view of a CT imaging system.
FIG. 4 is a block schematic diagram of the system illustrated in FIG. 1.
FIG. 5 is a plan view of a CT detector matrix with detector cells having diagonal edges according to one aspect of the invention.
FIG. 6 is a plan view of a single exemplary detector cell in accordance with one aspect of the present invention.
FIG. 7 is a graph illustrating a z-axis comparison of a conventional CT detector matrix and the CT detector matrix of FIG. 5.
FIG. 8 is a plan view of a CT detector matrix with diamond-shaped detector cells according to another aspect of the invention.
FIG. 9 is a graph illustrating a comparison in z-axis profile between a conventional rectangular-shaped detector cell and a diamond-shaped detector cell.
FIG. 10 is a plan view of a CT detector matrix with detector cells having diagonal edges and showing interpolated data points according to one aspect of the invention.
FIG. 11 is a plan view of a CT detector matrix with detector cells having diagonal edges and showing two position focal spot deflection according to one aspect of the invention.
FIG. 12 is a plan view of a CT detector matrix with detector cells having diagonal edges and showing three position focal spot deflection according to one aspect of the invention.
FIG. 13 is a plan view of a CT detector matrix with detector cells having diagonal edges and showing interpolated data points with two position focal spot deflection according to one aspect of the invention.
FIG. 14 is a plan view of a CT detector matrix with detector cells having diagonal edges and showing four position focal spot deflection according to one aspect of the invention.
FIG. 15 is a plan view of a CT detector matrix with detector cells having diagonal edges and showing diagonal focal spot deflection according to one aspect of the invention.
FIG. 16 is a pictorial view of a CT system for use with a non-invasive package inspection system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 3 and 4, an exemplary computed tomography (CT) imaging system 10 is shown as including a gantry 12 representative of a “third generation” CT scanner. One skilled in the art will appreciate that the present invention is applicable with other configured CT scanners, such as those generally referred to as first generation, second generation, fourth generation, fifth generation, sixth generation, etc. scanners. Further, the present invention will be described to a CT detector cell geometry that is applicable with energy integrating cells as well as photon counting and/or energy discriminating cells.
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 or plane 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.
As alluded to above, the present invention is directed to a CT detector comprised of individual detector cells or pixels. These cells are defined by an active surface or area and convert x-rays into a form that may be processed for image reconstruction. In this regard, the cells may, through a scintillator-photodiode combination, convert x-rays to light, detect the light, and provide an electrical signal to a data acquisition system for image reconstruction. The present invention, however, is not limited to scintillator-photodiode constructions. That is, as will be illustrated below, the present invention is also applicable with direct conversion detector cells that directly convert x-rays to electrical signal.
Additionally, the invention is applicable with conventional energy integrating cells as well as photon counting/energy discriminating cells. In a conventional integrating cell, the output of the scintillator or other x-ray conversion component is the product of the energy of the x-rays received and the number of x-rays received. Thus, there is no separation of the number of x-rays received from the energy level of the individual x-rays. Thus, it is possible, with energy integrating detector cells, for one cell to provide an output equal to that of another cell despite the one cell receiving more x-rays than the another cell. This equality in outputs is a result of the energy level of the x-rays received by the “another” cell being greater than the x-rays received by the “one” cell.
To provide photon count and/or energy discriminating information, CT detectors are increasingly being formed of energy discriminating and/or photon counting cells. These ED/PC detectors are capable of providing photon count and energy level information. Despite the differences between conventional energy integrating detectors and ED/PC detectors, there remains a need to improve spatial coverage/sampling density in both cases. Therefore, the present invention is applicable with both general types of detectors and, in fact, is not limited to a particular type of detector. Additionally, this invention is not limited to detectors for CT systems.
To achieve a CT detector with improved spatial coverage, detector cells with diagonal edges or perimeter walls is proposed. An exemplary construction is illustrated in FIG. 5. As shown, a CT detector 20 is defined by an array or matrix 50 of detector cells 52. As shown, each detector cell 52 has a non-rectangular shape. This non-rectangularity increases the spatial coverage of the detector in the z direction. However, effective spatial coverage in each “row” of the x direction is slightly decreased as compared to the conventional detector of FIG. 1. That is, detector array 50 is shown acquiring a resolution of eight data points in the column direction and four data points per row, whereas the detector 2 of FIG. 1 is shown acquiring three data points in the column direction and twelve data points per row. Alternatively, if the orientation of the detector were rotated, spatial coverage could be increased in the x direction.
Despite the non-rectangularity in the geometry of each detector cell, as illustrated, the detector cells in each column (channel) are uniformly aligned with one another. This eases the fabrication process relative to the staggered-channel approach illustrated in FIG. 2. Also, as shown, most of the detector cells are similarly shaped. However, because of the non-rectangularity of the detector cells, irregular shaped sections of the matrix must be accounted for. This is achieved by specially-shaped cells 53 that are constructed to “fill” the matrix. A skilled artisan will appreciate that each “specially-shaped” cell 53 may include multiple cells to fill the matrix.
Referring now to FIG. 6, a single exemplary detector cell 52 according to one aspect of the invention is shown. The detector cell 52 has an active area 54 that is generally parallel to the plane of x-ray projection (not shown) during data acquisition. In the exemplary illustration, the active area 54 is defined by four perimeter walls or edges 56. As shown, the exemplary cell has the shape of a rhombus. In this regard, the angle, α1, formed by the intersection of edges 56(a) and 56(b) is acute. Likewise, the angle, α2, between edges 56(c) and 56(d), is acute. Conversely, the angle, β1, at the intersection of edges 56(a) and 56(c) and the angle, β2, formed at the intersection of 56(b) and 56(d) are each obtuse. In short, edges 56(b) and 56(c) are not perpendicular to the plane of gantry rotation as in conventional rectangular shaped cells; however, channel edges 56(a) and 56(d) are perpendicular to the plane of gantry rotation. In this regard, the diagonal edges 56(b) and 56(c) extend in the x-z plane whereas edges 56(a) and 56(d) extend only in the z direction.
The geometry of the detector cell can be more generally described as follows. As shown, the z boundaries of the detector cell are formed by straight diagonal edges. Thus, with the cell pitch in the z direction referenced “a” and the cell pitch in the x direction referenced “b”, the diagonal boundary makes an angle α with the x axis such that:
tan (α)=a/(2b) (Eqn. 1).
For a=b, alpha is approximately 26.5 degrees. However, one skilled in the art will appreciate that the present invention is not limited to the case where a=b. For example, in one preferred embodiment, b=a√{square root over (3)}/2. In this case, which was found to be particularly optimal for sampling density, alpha is 30 degrees. With an alpha of 30 degrees, a hexagonal lattice detector matrix or array would result. Other values for alpha are of course contemplated.
As a result of edges 56(b) and 56(c) being in the x-z plane, the sampling density of the overall detector is improved, as illustrated in FIG. 7. Specifically, as illustrated, the z axis profile of a conventional rectangular detector cell is enveloped by the collective profiles of the diagonally edged cells illustrated in FIGS. 5-6.
Not only does the present invention provide a detector cell geometry with improved spatial coverage, it does so without requiring significant variants to conventional detector fabrication techniques. Specifically, the detector cell illustrated in FIG. 6 can be fabricated using two cuts in a cutting process. That is, after making a straight cut, i.e., edges 56(a) and 56(d), the wafer or bulk of x-ray converting material need only be rotated acutely a fixed degree of rotation followed by a second cut. Thus, instead of making four ninety degree cuts, a detector according to one embodiment of the present invention can be formed with two ninety degree cuts and two acute (less than ninety degree) diagonal cuts. This can be done without requiring a significant change to a typical cutting setup.
Referring now to FIG. 8, a CT detector 20 having an array 50 of detector cells 52 shaped according to another embodiment of the present invention is shown. In this embodiment, each of the detector cells 52 is diamond-shaped. Thus, four diagonal edges rather than two, as in the cell shown in FIG. 6, define each cell. One advantage of the cell geometry illustrated in FIG. 8 is that there is substantial sample overlap in the x and z directions. Moreover, the z axis profile is narrower than that of conventional rectangular detector cells. One skilled in the art will appreciate that fabrication of the diamond-shaped detector cell can be carried out with a conventional wire-saw process.
Referring to FIG. 9, the axial profile of a diamond-shaped cell relative to a rectangular-shaped cell is illustrated. As shown, notwithstanding the more narrow profile, the sampling coverage of the diamond-shaped cell is equal to that of a conventional rectangular-shaped cell.
As discussed above, an advantage of the present invention is its unique applicability with x-ray focal spot deflection techniques (sometimes referred to as x-ray “wobble”). X-ray focal spot deflection is essentially a linear displacement of the effective focal spot of a projected x-ray beam in either an X or Z direction with respect to scan subject position. This deflection takes place for each projection position of the x-ray source about the gantry of a CT system. One manner of producing such a displacement is to move or tilt the cathode of an x-ray source back and forth a certain distance within the plane of rotation of the anode of the x-ray source. The result is an x-ray focal spot being moved back and forth across a linear distance of the x-ray detector. The x-ray source may be projecting the entire length of the deflection distance, or may simply project at various discrete locations, such as end points or a center of the deflection distance. Likewise, while many deflection techniques involve moving the x-ray focal spot ½ the total desired deflection distance in either direction off center, it is recognized that deflection can also include moving the focal spot the entire deflection distance in one direction off center. Regardless of the particular embodiment used, x-ray focal spot deflection produces the ability to acquire increased views in the direction of deflection.
Another technique commonly employed to provide the appearance of increased data acquisition is interpolation. FIG. 10 shows a diagonal-cut detector 118 in accordance with the present invention. Each solid data point 120 represents data acquired from an actual non-deflected x-ray focal spot projection. Therefore, as discussed above, detector 118 provides increased spatial resolution in a z or column direction. To increase the effective resolution in the x direction, additional data points may be interpolated 122 during data processing. In other words, the output of two neighboring detectors 124 may be averaged to provide an additional data point usable as though another detector existed therebetween. Likewise, the output of vertically aligned detectors 126 may be used for interpolation; or the output of four surrounding detectors 124, 126 may be used to interpolate a data point. The result of this post-processing is a matrix of data points which has increased z direction resolution due to the detector shape 118 and increased effective x resolution due to the interpolation.
FIG. 11 shows a detector 128 experiencing x-ray focal spot deflection. As initially apparent, the total number of data acquisitions 130, 132 is equal to the total number of data points from FIG. 10. The distinction however, is that the data acquisitions 130, 132 all represent unique data actually acquired by the detector 128, rather than data points interpolated from other acquisitions. Thus, reconstructed images may be more precise. The deflected x-ray focal points of FIG. 11 are produced by an x-ray source configured to deflect the projected focal point half a detector cell width to the left 130 and right 132. The detector 128 is then programmed or configured such that its sample rate (and the coordination thereof with the deflection period of the x-ray source) results in data acquisitions occurring when the x-ray beam is focused at the end points 130, 132 of the focal deflection. In other words, the detector acquires multiple data samplings during a projection period 130, 132 of focal deflection (whether or not the x-ray source projects during the entire focal distance or only at the end points 130, 132). Therefore the combined effective detector resolution of the two acquisitions in the x direction is equal to that in the z direction.
Similarly, FIG. 12 shows a detector 134 with three position x-ray focal spot deflection incidence thereon. The deflected x-ray focal points of FIG. 12 are produced by an x-ray source configured to deflect the projected focal point two-thirds of a detector cell width to the left 136 and two-thirds of a detector cell width to the right 140 in the x or “in-plane” direction. The timing of the detector 134 sample rate is such that acquisitions occur when the focal point is at the left end position 136, the center position 138, and the right position 140. This embodiment of the present invention results in approximately one and a half times the x plane spatial resolution and twice the z plane spatial resolution of conventional detectors with non-deflected x-rays.
FIG. 13 shows an alternative in which both focal spot deflection and interpolation are used. The resulting x plane and z plane resolutions are approximately equal to those of the embodiment of FIG. 12. However, in the embodiment of FIG. 13, a two position focal spot deflection of ±⅓ of a detector cell width results in actual data acquisitions at points 144 and 146. A third data point 148 is interpolated from an average of some combination of neighboring data acquisitions 144, 146, as described above.
The embodiment of FIG. 14 shows the incidence of a four position focal spot deflection technique on a detector 150. In such a case, the detector 150 outputs data acquisitions when the x-ray focal spot is at the end points 152, 158 and two inner points 154, 156 of a deflection pattern of ± three fourths of an x direction detector cell width. This embodiment exhibits twice the x plane and z plane resolution of a conventional detector system, such as shown in FIG. 1.
FIG. 15 illustrates an alternative embodiment in which focal deflection occurs in a diagonal direction. It is thus appreciated that the present invention is applicable in MR systems which provide deflection in a variety of directions, including x plane, z plane, diagonal, and cross-diagonal (90 degrees from the deflection direction shown in FIG. 15). Furthermore, it is recognized that the distances of deflection should be ± fractions of the width of the detector cells along the direction of the deflection in order to evenly “space” the acquired focal positions to increase effective spatial resolution. In short, many combinations of various deflection directions and distances, different numbers of focal spot positions, post-acquisition interpolations, and detector acquisition sampling rates and timing patterns are encompassed by the present invention.
Also, the present invention may be incorporated in medical scanners, such as that shown in FIGS. 3-4, or non-medical scanners. Referring now to FIG. 10, package/baggage inspection system 100 incorporating the present invention 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 detector cells similar to those described herein. 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.
As noted above, the present invention is not limited to a particular type of detector cell. In this regard, it is contemplated that the invention can be applied to energy integrating, photon counting, or energy discriminating constructions. Thus, the invention is applicable with scintillators or direct conversion x-ray conversion material, charge collectors, such as photodiodes, charge-storage devices, charge collection anodes or cathodes, as well as, anti-scatter, collimator, and reflector grids.
As described herein and appreciable by one skilled in the art, the present invention provides a detector cell geometry that enables overlapping samples in the z and/or x directions without requiring additional data acquisition system channels. Moreover, the active area of each cell is equivalent to those of conventional detector cells. The detector cells can be fabricated with slight modification of a conventional wire-saw process; thus, fabrication costs are comparable to conventional detector cells. Moreover, since the diagonal and diamond-shaped cells described herein can be fabricated using wire-saw cuts of the same pitch, only a single wire-saw setup is required. Additionally, the detector cell is applicable with flying-focal-spot deflection techniques, e.g., x-direction wobble, for improved sampling resolution, as discussed above. Further, for the embodiment illustrated in FIG. 6, the channel edges of each cell are aligned with the channel edges of each other detector cell in the channel. Thus, a conventional ID scatter grid may be used. Also, one skilled in the art will appreciate that the present invention is applicable with CZT photon counting detectors. In such a case, the scintillator is not diced in a manner described above. The charge collection electrodes are formed with overlapping rows.
Therefore, the invention includes a CT scanner having a rotatable gantry, an x-ray source to project x-rays from the gantry, an x-ray detector disposed opposite the x-ray source, and a data processing unit to acquire data from the x-ray detector. The x-ray detector has an array of detector cells that each have one perimeter side that isn't parallel to two other perimeter sides. A program on the data processing unit causes one or both of an interpolation of x-ray detector data or an x-ray oversampling to occur.
An x-ray detector is also disclosed. The detector includes an array of x-ray detector cells configured to convert radiation projected from an x-ray source into data signals. Each detector cell has a number of perimeter sides wherein an angle of intersection formed between a pair of the perimeter sides is acute. The sampling rate of the array is set so that multiple data acquisitions are output during a projection period of the x-ray source.
The invention is also embodied in a method for implementing an x-ray detection system. The method includes the steps of providing a scintillator array having a number of divisions at a first angle and a number of divisions at a second angle more than 90 degrees from the first angle, connecting outputs of the scintillator array to a data acquisition system, and programming the data acquisition system. When executing the program, the data acquisition system should acquire a matrix of data samples from the scintillator array having a number of values in a column direction and a number of values in a row direction and then augment the matrix of data samples with additional values in the row direction.
In addition, the present invention encompasses a method for acquiring x-ray incidence data. The method includes projecting deflecting radiation from an x-ray source towards a detector during a projection period and sampling a set of acquisition data from the detector. The set of acquisition data is indicative of the incidence of radiation on portions of the detector having an edge that is not parallel to either the slice direction or the subject direction. The method also includes integrating other data values with set of acquisition data to increase the apparent sampling resolution in the slice direction or the subject direction.
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.
Patent Application
20070248208
