Abstract:
Apparatus for producing multiple image slice data responsive to incident radiation passing through an object. The apparatus includes a detector array having a plurality (p) of parallel rows of detector elements which receive such incident radiation and generate signals therefrom, each of which rows is characterized by a width measured prior to collimation in the direction perpendicular to a long dimension thereof; and signal processing circuitry which receives signals from the detector elements and which combines the signals in a first combination mode and in at least m additional combination modes. In the first combination mode, the circuitry forms a set of n groups of rows, each such group of rows having an effective group width substantially equal to the effective group width of each of the other groups in the set. In each of the m additional combination modes, the circuitry forms different sets of n groups of rows, each such set having a different effective group width common to all groups in the set.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to transmission computerized tomographic (CT) systems, and specifically to segmented array detectors for use in such systems to simultaneously acquire data from multiple axial slices. 
     BACKGROUND OF THE INVENTION 
     CT scanning systems and methods are well known in the art, particularly for medical imaging and diagnosis, but also in other field of imaging, for example, industrial quality control. 
     CT scanners generally create images of one or more sectional slices through a subject&#39;s body. A radiation source, such as an X-ray tube, irradiates the body from one side thereof. A collimator, generally adjacent to the X-ray source, limits the angular extent of the X-ray beam, so that radiation impinging on the body is substantially confined to a planar region defining a cross-sectional slice of the body. At least one detector (and generally many more than one detector) on the opposite side of the body receives radiation transmitted through the body substantially in the plane of the slice. The attenuation of the radiation that has passed through the body is measured by processing electrical signals received from the detector. 
     Typically, in commonly-used third- and fourth-generation CT scanners, the X-ray source (or multiple sources) is mounted on a gantry, which revolves about a long axis of the body. In third-generation scanners, the detectors are likewise mounted on the gantry, opposite the X-ray source, while in fourth-generation scanners, the detectors are arranged in a fixed ring around the body. Either the gantry translates in a direction parallel to the long axis, or the body is translated relative to the gantry. By appropriately rotating the gantry and translating the gantry or the subject, a plurality of views may be acquired, each such view comprising attenuation measurements made at a different angular and/or axial position of the source. Commonly, the combination of translation and rotation of the gantry relative to the body is such that the X-ray source traverses a spiral or helical trajectory with respect to the body. The multiple views are then used to reconstruct a CT image showing the internal structure of the slice or of multiple such slices, using methods known in the art. 
     The lateral resolution of the CT image, or specifically, the thickness of the slices making up the image, is generally determined by the angular extent of the radiation beam or of the individual detectors, whichever is smaller. The use of thick slices is advantageous in increasing the signal/noise ratio, and thereby reducing the time needed to acquire the data needed to reconstruct an image. But images reconstructed using thick slices have poor resolution in the axial direction and are more susceptible to partial volume artifacts, i.e., imaging errors that are introduced when a single volume element (voxel) within a slice contains two types of tissue having different attenuation coefficients. 
     Smaller detectors are generally used, therefore, to improve axial resolution and reduce partial volume artifacts. Excessive reduction of the extent of the detector, however, leads to degradation of the signal/noise ratio and decreases the throughput of the imaging system. Using very small detectors can also reduce the system&#39;s dose efficiency, i.e., increase the relative amount of radiation to which the portion of the body being imaged is exposed, because the angular extent of the X-ray beam irradiating the body will typically extend somewhat beyond the bounds of the detectors. Radiation outside these bounds is “wasted,” since it is not used in forming the CT image. 
     In order to improve throughput, as well as increase axial resolution and utilize the X-ray source more efficiently, various inventors have described the use of differently configured detector arrays. Such arrays typically include a plurality of radiation detectors, such as scintillator-photodiodes, which receive radiation simultaneously from a radiation source and are thereby used to acquire multiple views and/or multiple slices simultaneously. Spiral modes of translation and rotation, as mentioned above, are frequently combined with multi-slice image acquisition to cover a greater volume of the body in less time with improved axial resolution. 
     For example, U.S. Pat. No. 4,965,726, to Heuscher, et al., whose disclosure is incorporated herein by reference, described a CT scanner with a plurality of segmented detector arrays. Each array includes a plurality of rows of radiation-sensitive cells. The rows may have different dimensions in a lateral direction, perpendicular to the long dimension of the rows, and the effective lateral dimensions of the rows may be varied by moving collimators adjacent thereto, so as to provide slices of the same or different lateral thicknesses. Multiple detectors may be grouped together in the lateral direction to provide thicker slices, so as to improve the signal/noise ratio and throughput of the scanner, while reducing partial volume artifacts relative to slices of comparable thickness that are acquired using a single detector having an equivalent lateral dimension. 
     U.S. Pat. No. 5,241,576, to Lonn, whose disclosure is likewise incorporated herein by reference, similarly describes a CT scanner including an array of detector elements for the purpose of acquiring thick-slice images with reduced partial volume artifacts. The array includes a plurality of detector elements, wherein each such element includes a set of sub-elements disposed along the slice thickness (lateral) dimension. The signal output of each sub-element is processed individually, generally by taking the log of the signal and applying a weighting factor thereto. The processed outputs of the plurality of sub-elements belonging to a single element are then summed together to form a combined thick-slice signal. 
     U.S. Pat. No. 5,430,784 to Ribner, et al., whose disclosure is also incorporated herein by reference, describes a CT scanner and detector array having a plurality of rows of identical detectors, which are connected together by a controllable switching matrix. This switching matrix is controlled to interconnect a predetermined number of successive detector sub-elements, in order to produce combined signals corresponding to one or more slices of a desired thickness. 
     U.S. Pat. No. 4,417,354, to Pfeiler, whose disclosure is incorporated herein by reference, describes a CT scanner including a detector array that is mounted to pivot about a lateral axis, perpendicular to an image slice acquired by the array. The array is pivoted in order to increase the effective resolution within the image slice, but only a single image slice is provided, and no suggestion is made of changing the slice thickness by pivoting the array about a transverse axis. 
     Similarly, U.S. Pat. No. 5,493,593, to Müller et al., whose disclosure is incorporated herein by reference, describes a scanner for CT microscopy including a tiltable detector array, which is also shifted horizontally in order to maximize the utilization of the array. Only a single image slice is provided, however, without suggestion of changing the slice thickness. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide detector arrays for simultaneous acquisition of multiple image slices of variable thickness. Preferably, at a given time all the slices are of substantially equal thickness, and the thickness may be accurately and conveniently controlled. 
     In one aspect of the present invention, the thickness is controlled in successive steps of increasing thickness. 
     In another aspect of the present invention, the number of switching and summation elements needed for combining adjacent elements of such detector arrays to form slices of a desired thickness is minimized. 
     In another aspect of the invention, the outputs of the elements are combined to provide slices of variable thickness. Preferably, one or two thick slices are provided together with one or two thin slices wherein the ratio between the thickness of the two types of slices is greater than 2:1, more preferably, greater than 3:1 and most preferably greater than 5:1. In a preferred embodiment of the invention, the ratio of the thicknesses is 9:1 or 10:1. 
     Preferably, the detector arrays comprise radiation detectors and are used in CT scanner systems. 
     In preferred embodiments of the present invention, a detector array, for use in an imaging system, comprises a plurality of rows of detectors, each row characterized by a width in a lateral dimension perpendicular to the long axis of the row, which long axis is typically aligned in the imaging system in a generally circumferential direction relative to a body being imaged. The detectors in each row receive radiation from a corresponding slice of an object being imaged and generate signals in response to the radiation. Preferably the array includes rows having different widths, more preferably at least four different widths. A minimum slice thickness is defined by a lateral dimension of a slice, i.e., a dimension of a slice measured in a direction parallel to the lateral dimension of the rows, corresponding to a row of the array having the smallest width. Switching circuitry associated with the array selectively combines signals generated by adjacent detectors in different rows, so as to produce sum signals corresponding to multiple slices having a common thickness, which is a selectable value equal to or greater than the minimum slice thickness. 
     Preferably the switching circuitry produces sum signals corresponding to at least four laterally-adjacent, substantially contiguous slices, all of which have a common thickness. Acquiring multiple, contiguous image slices simultaneously, as described herein, generally increases the throughput and/or resolution of the imaging system. In general, multi-slice imaging also increases the dose efficiency of the system, i.e. reduces the relative amount of radiation to which the body is exposed in producing a CT image, particularly when thin image slices are acquired, since using multiple rows of detectors reduces the relative amount of “wasted” radiation falling outside the bounds of the detectors. 
     In preferred embodiments of the present invention, the switching circuitry may combine the signals using any appropriate circuit elements and computational algorithms known in the art. Preferably, the signals undergo a log operation, as is known in the art, and are digitized and summed. However, other suitable operations may similarly be used, and the order of performing the operations may be varied, as desired to reduce the cost and/or number of components in the imaging system, subject to the requirement that the system produce images of a desired quality. It will be understood that image quality includes measures of resolution, signal/noise ratio, artifacts, particularly partial volume artifacts, and other measures known in the art. 
     In some preferred embodiments of the present invention, the detector array comprises four adjacent central rows, having a common lateral width, which is the smallest width of rows in the array, and a plurality of peripheral rows, symmetrically arranged on both sides of the four central rows. The widths of the peripheral rows are preferably integer multiples of the width of the central rows, wherein the widths of the rows preferably generally increase with the lateral distance of the rows from the central rows. 
     In some preferred embodiments of this type, the fifth and sixth rows, which are peripherally adjacent to the four central rows on either side thereof, have a common width generally twice that of the central rows. The seventh and eighth rows, which are peripherally adjacent to the fifth and sixth rows, respectively, have a common width generally twice that of the fifth and sixth rows. Ninth, tenth and additional pairs of rows, if desired, are similarly added peripherally as desired, each additional row having width generally twice that of its inner neighbor. 
     In accordance with such preferred embodiments, when four slices of the minimum thickness are desired, switching circuitry selects signals only from the four central rows. To produce four slices of a greater thickness, approximately twice the minimum thickness, the switching circuitry selects signals from the fifth and sixth rows to produce two such slices, and combines signals generated by mutually adjacent first and second rows of the four central rows and by mutually-adjacent third and fourth rows thereof, to produce two more such slices. To produce four slices of approximately four times the minimum thickness, the switching circuitry selects signals from the seventh and eighth rows to produce two such slices, and combines signals from the first, second and fifth rows (the fifth row being adjacent to the first row) to produce a third such slice, and from the third, fourth and sixth rows to produce a fourth slice. Four slices of thickness corresponding to the width of any of the rows of the array, up to the largest such width, may similarly be produced. 
     In this and other preferred embodiments of the present invention, the slice thicknesses are generally measured at the location of the object being imaged. These thicknesses depend primarily upon the width of the corresponding row or combined rows of detectors, but are also affected by other elements of the imaging system in which the detector array is used. Such elements typically include a radiation source and beam-control optics, such as collimator slits. Therefore, while the slice thicknesses are approximately proportional to the row widths, other aspects of the system must be taken into account to determine the slice thicknesses accurately. 
     Thus, in the context of the present invention, it will be understood that the use of the term “substantially” or “approximately” in stating a detector width dimension, for example to say that the width of rows of detectors are substantially equal, or that the thickness of one row is substantially an integer multiple of another row, means that the detector thickness is such that the slices traversed by the radiation beam has approximately the stated dimension or ratio. This includes any correction to the detector width which might be required, for example to correct for the difference between the effective thickness a eff  and the thickness that would be obtained if focal point  78  were infinitesimal or for other geometrically caused variation between the effective slice width and the detector width. Such correction is generally very small compared to the detector and slice dimensions of interest. 
     It will be appreciated that the preferred embodiments described above allow a wide range of choices of slice thickness to be produced by the detector array and switching circuitry, relative to the number of rows in the array. For example, an array having ten rows of detectors, in accordance with a preferred embodiment of the present invention of the type described above, will be capable of producing four slices having a common thickness, which is variable from a minimum slice thickness up to a maximum thickness approximately eight times the minimum slice thickness. By comparison, an array composed of rows of detectors having equal row widths would require 32 rows of detectors in order to produce such an 8:1 range of slice thicknesses, and would also require considerably more complex and costly switching circuitry to accomplish this purpose. In other, analogous preferred embodiments of the present invention, arrays having greater or lesser numbers of rows may be used to similar advantage. 
     Furthermore, in the preferred embodiments of the present invention described above, the selection of slice thicknesses is accomplished without the use of any additional mechanical aperture, collimator or beam optics. 
     In other preferred embodiments of the present invention, however, a linear aperture whose width is variable in a lateral direction, i.e., the direction parallel to the lateral dimension of the rows of the array, is used in conjunction with the detector array and switching circuitry to produce slices having desired thicknesses. In one preferred embodiment of this sort, two central rows of the array have the smallest width, and peripheral rows having greater widths are symmetrically arranged on both sides of these two central rows. The aperture is constructed and aligned so that when it is fully open, all rows of the array are exposed to radiation from the object. When the aperture is narrowed, however, it masks all or portions of successive peripheral rows in the array, preventing radiation from impinging thereon. The aperture thus controls the effective widths of the peripheral rows by covering or exposing desired portions thereof. 
     Signals from mutually-adjacent detectors in different, selected rows of the array are combined by the switching circuitry, as described above, to produce multiple image slices having a common, desired thickness. By appropriately varying the aperture from a minimum to a maximum opening dimension, while controlling the switching circuitry, the thickness of the multiple slices is increased approximately by multiples of the minimum slice thickness, as defined by the smallest row width and/or minimum aperture. The addition of the aperture allows a wider variety of thickness multiples to be produced, for a given number of rows, than in the preferred embodiments described above in which only the detector array and switching circuitry are used to determine the slice thicknesses. 
     In some preferred embodiments of the present invention, the linear aperture may be narrowed sufficiently to mask all peripheral rows of the array and portions of the two central rows. In this configuration, signals from the two central rows are selected by switching circuitry to produce two thin image slices. 
     It will be appreciated that by producing two adjoining, thin image slices rather than a single slice of comparable thickness, the dose efficiency of the imaging system is generally enhanced. To maximize this efficiency, the aperture is adjusted so as to generally match a minimum lateral extent of a beam of radiation that is irradiating the object and impinging on the array. This minimum lateral extent is generally determined by the geometry of a radiation source irradiating the object. In this case, the minimum slice thickness is approximately equal to half the lateral extent of the beam of radiation. If only a single slice of this minimum thickness were produced, then only about half of the radiation irradiating the subject would be used in creating the image, so that the dose efficiency would similarly be about 50% less. 
     Preferably, when the linear aperture is opened to its maximum opening dimension, the switching circuitry combines signals from selected rows of the array to produce four slices having a common thickness, corresponding to the width of outermost rows of the array. Additionally or alternatively, signals from each of the two central rows may be combined with signals from all other, peripheral, rows on its respective side of the array, out to and including the outermost row, so as to produce two slices have a maximum possible thickness. 
     In one such preferred embodiment of the present invention, the array comprises eight rows of detectors, wherein the two central rows have the smallest width, and the peripheral rows, having greater widths, are symmetrically arranged on both sides of the central rows. The array is coupled to a switching network comprising fourteen switches, which may be of any suitable type known in the art. The array and the switches are coupled to four adders, whose outputs are used to produce two or four image slices of desired thickness, as described above. 
     In still other preferred embodiments of the present invention, the detector array is mounted on a movable base, which shifts the array laterally, relative to the object being imaged, along an axis perpendicular to the long axes of the rows of the array. In such preferred embodiments, rows of different widths may be arranged asymmetrically relative to a central axis of the array defined by one or more rows of the smallest width, unlike the preceding embodiments, in which the array is generally symmetrical about such an axis. Preferably the movable base is used in conjunction with switching circuitry and a variable aperture, as described above, to produce multiple slices of varying thicknesses, while keeping the slices commonly centered on a central plane, in a fixed relation to the object, regardless of the thickness of slices that is chosen. 
     The use of the movable base in conjunction with other aspects of the present invention allows an array having a reduced number of rows to be used in producing a desired combination of slice widths. In other words, the use of the movable base allows a greater variety of slice thicknesses to be produced, relative to the number of rows in the detector. 
     In some preferred embodiments of the present invention, a CT imaging system includes a detector array, as described above, wherein an X-ray tube irradiates the body of a subject, and the detector array is positioned on the opposite side of the body to the tube, so that the detectors receive radiation that has been transmitted through the body. The detector array preferably comprises scintillators and photodiodes or other suitable X-ray detectors known in the art and produces a plurality of sectional image slices, preferably four such slices, although different numbers of slices may similarly be produced. Preferably a collimator is associated with the X-ray tube so as to limit the extent of the X-ray beam irradiating the body to a region of the body containing the slices. 
     In some of these preferred embodiments, the detector array is planar, i.e., all the detectors are substantially in a single plane. In other preferred embodiments, however, the detector array is arcuate, having a radius of curvature approximately equal to the distance of the array from the X-ray tube, which preferably emits a fan-shaped beam whose angular extent generally corresponds to the angle subtended by the arcuate array. In still other preferred embodiments, in which the CT imaging system preferably comprises a fourth-generation CT scanner, the detector array generally describes a ring, substantially surrounding the body. It will be appreciated that the various arrangements of rows having different widths, as described above, as well as the accompanying switching circuitry, aperture, movable base and other aspects of the present invention, may equally be applied to planar and arcuate detector arrays. 
     Preferably, the X-ray tube is mounted on a gantry or other suitable apparatus, which revolves about an axis passing through the body. In preferred embodiments of the present invention in which the detector array is planar or arcuate, the array is preferably mounted on the gantry, opposite the X-ray tube, so as to revolve around the body, as is known in the art with regard to third-generation CT scanners. Alternatively, in preferred embodiments of the present invention in which the detector array describes a ring, the array preferably remains rotationally stationary, and only the X-ray tube revolves around the body. In either of these cases, the position of the gantry and detector array translates laterally relative to the body in a direction parallel to the axis, preferably by translational motion of the body relative to the gantry and array. The revolution of the gantry and the translation of the gantry and the detector array relative to the body allow multiple angular views and multiple sectional slices to be acquired. 
     In alternative preferred embodiments of the present invention, a tiltable, planar detector array comprises a plurality of rows of detectors, all such rows having generally equal widths. The array is coupled to a mechanical tilting device, which controllably tilts the array about a tilt axis substantially parallel to the long axes of the rows. A normal orientation of the tiltable array is defined by a plane that is perpendicular to a line passing through a focal point of the radiation source and perpendicularly intersecting the tilt axis of the array. An effective row width, common to all the rows, is defined by geometrical projection of the lateral dimension of the row onto the plane of the normal orientation. It will thus be understood that as the array tilts away from the normal orientation, the effective widths of the rows decrease, substantially in proportion to the cosine of a tilt angle thereof. 
     The detectors in each row of the array receive radiation from a corresponding slice of an object being imaged. When the array is substantially in the normal orientation, the detectors have effective widths equal to the full widths of the rows, and thus receive radiation from equal, relatively thick slices of the object. When the array is tilted relative to this direction, however, the effective widths of the detectors are smaller, and therefore, the detectors receive radiation from equal, relatively thinner slices. By tilting the array to various tilt angles, multiple slices having a plurality of different thicknesses are defined. 
     In still other preferred embodiments of the present invention, a detector array comprises a plurality of rows of detectors having substantially equal widths (and providing substantially equal slice widths), wherein each such row may be controllably tilted about its respective long axis. Preferably all the rows in this preferred embodiment are co-planar and contiguous with their immediate neighbors, when the rows are in a normal orientation, as described in reference to the preceding embodiments. Preferably all rows are tilted to substantially equal angles, so that they define multiple slices having equal, variable thicknesses. 
     In one such preferred embodiment of the present invention, the rows are moved laterally relative to one another, in a direction perpendicular to their long axes. When the rows are tilted, they are then also moved closer together, so as to maintain substantial contiguity of the thinner slices defined by this tilted orientation. 
     In still further preferred embodiments of the present invention, a detector array comprises four parallel rows of detectors, including two outer rows and two inner rows, preferably all of equal width, each row corresponding to a respective image slice. The outer rows are adapted to act as a linear aperture with respect to the inner rows, i.e., the outer rows are mounted so that they may be translated laterally to overlap and thus mask portions of the widths of the inner rows. In this way, the thicknesses of the two slices corresponding to the two inner rows are controlled. Preferably, an adjustable aperture or collimator slit masks portions of the widths of the outer rows, so that all four of the outer and inner rows may have any desired effective widths, preferably equal effective widths. 
     It will be understood that while the above preferred embodiment is described in terms of four rows of detectors, generating four image slices of preferably equal thicknesses, the principle of using one or more rows of the detector array to variably overlap and mask one or more other rows may similarly be applied in other, different preferred embodiments of the present invention, as well. For example, the array may include more than four rows of detectors, so as to produce more than four image slices. The effective widths of these more than four rows of detectors may be controlled by the above principle of overlapping and masking rows, or by other means as described above with reference to other preferred embodiments of the invention. 
     In yet a further alternative embodiment of the invention a plurality of rows of detectors having equal widths is provided. The outputs of the detectors are summed such that a single wide slice and a single thick slice are produced. Alternatively, one wide slice, flanked by two narrow slices are produced. Further alternatively, a single thin slice flanked by two thin slices are produced. Preferably, these slice widths are produced without masking the detectors. Furthermore, the thicker slices are preferably produced by adding the outputs of a plurality of equal sized detectors while the thinner slices are produced by utilizing the output of a single row of detectors or a sum of a lesser number of detectors than that utilized for producing the thicker slice(s). 
     In yet another preferred embodiment of the invention, a plurality of rows of different widths is provided, with a thin row or rows at the center and wider rows on one side of or preferably flanking the thin row or rows. In this embodiment of the invention one or more thin slices are provided at the center of the group of rows and thick slices are provided by combining the outputs of detectors of adjoining wider rows. The thin slices may be provided by the detectors of a single row or by combing signals from detectors of adjacent thin rows. In a preferred embodiment of the invention, ratios of 3:1, 5:1, 8:1 or more, such as 10:1 are provided between the thin slices and the thicker slices. 
     There is therefore provided, din accordance with a preferred embodiment of the present invention, apparatus for producing multiple image slice data responsive to incident radiation passing through an object, including: 
     a detector array, including a plurality, p, of parallel rows of detector elements, which receive the incident radiation and generate signals in response thereto, each of which rows is characterized by a width, measured in a direction perpendicular to a long dimension thereof; and 
     signal processing circuitry, which receives signals from the detector elements and which combines the signals in a first combination mode to form a set of n groups of rows, each such group of rows having an effective group width substantially equal to the effective group width of each of the other groups, and which further combines the signals in at least m additional combination modes to form different sets of n groups of rows, each such set having a different effective group width common to all groups in the set, wherein p=n+2(m−1). 
     Preferably, in at least one of the combination modes, at least one of the n groups of rows includes at least one row having a width different from all the rows in at least one other of the n groups of rows. 
     Preferably, each group of rows includes mutually adjoining rows, and the n groups of rows in each of the combination modes are mutually exclusive. 
     There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for producing multiple image slice data responsive to incident radiation, including: 
     a detector array, including a plurality of parallel rows of detector elements, which generate signals responsive to radiation incident thereon, each of which rows is characterized by a width, measured in a direction perpendicular to a long dimension thereof; and 
     signal processing circuitry, coupled to the array, which receives signals from at least four of the rows of the array and produces four or more channels of output data, each such channel including data derived from signals generated by detector elements in one or more rows of the array selected by the circuitry for inclusion of data therefrom in said channel, 
     wherein each row is characterized by an effective row width, defined by a geometrical projection of the portion of the width of the row that is exposed to the radiation, onto a plane that is substantially perpendicular to a direction of propagation of the radiation incident on the array, and 
     wherein each channel of output data is characterized by an effective channel width, defined by the sum of the effective widths of the one or more rows selected by the circuitry for inclusion of data therefrom in the channel, and 
     wherein the effective channel widths of all of the four or more channels are substantially equal, and 
     wherein the number of different effective channel widths that may be selected by the signal processing circuitry is equal to at least half the number of rows in the array, less one. 
     Preferably, the signal processing circuitry includes switching circuitry, which alternately selects different rows for inclusion of data therefrom in each of the four or more channels, thereby varying the effective channel widths thereof; and two or more adders, each respectively associated with one of the four or more channels, and each of which sums the signals generated by adjacent detectors in two or more respective, adjoining rows of the array that are selected by the circuitry for inclusion of data therefrom in the channel. 
     Preferably, the array includes two central rows having a common width smaller than or equal to the widths of all the other rows, and the widths of all the rows are substantially equal to integer multiples of the width of the central rows. 
     Additionally or alternatively, the apparatus includes an adjustable slit or linear aperture, having an aperture that is variable in a direction perpendicular to the long dimension of the rows, which may be variably closed to mask portions of the widths of the rows, thereby varying the effective row widths. 
     Additionally or alternatively, the apparatus includes a movable base on which the detector array is mounted, which base moves the array in a direction perpendicular to the long dimension of the rows. 
     Additionally or alternatively, the apparatus includes at least one mechanical tilting device, which controllably tilts a row of the array about a tilt axis substantially parallel to the long dimension of the rows, wherein the effective row width is varied by controlling the at least one tilting device. 
     There is moreover provided, in accordance with another preferred embodiment of the present invention, apparatus for producing multiple image slice data, responsive to incident radiation, including: 
     a detector array including at least three detector elements disposed in a lateral direction, which detector elements generate signals responsive to radiation incident thereon, wherein each detector element is characterized by a width, measured in the lateral direction, and wherein at least two of the at least three detector elements have substantially different widths; and 
     circuitry, coupled to the array, which selects a first exclusive group including one or more detector elements and sums the signals generated by the detector elements in the first group to produce a first channel of output data, and which selects a second exclusive group including at least two detector elements and sums the signals generated by the detector elements in the second group to produce a second channel of output data, 
     wherein each detector element is characterized by an effective detector width, defined by the portion of the width of the detector element that is exposed to the radiation, and 
     wherein each channel is characterized by an effective channel width, defined by the sum of the effective detector widths of the one or more detector elements in the group that is selected to produce the channel, and 
     wherein the effective channel widths of the first and second channels are substantially equal. 
     Preferably, the widths of all of the at least two detector elements having substantially different widths are integer multiples of the width of the one of the at least two detector elements having the smallest width. 
     There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for producing multiple image slice data responsive to incident radiation, including: 
     a detector array, including at least a first detector element and a second detector element disposed in a lateral direction, which detector elements generate signals responsive to radiation incident thereon, wherein each detector element is characterized a width in the lateral direction, 
     wherein each detector element is characterized by an effective detector width, defined by the portion of the width of the detector element that is exposed to the radiation, and 
     wherein the second detector element is shifted in the lateral direction relative to the first detector element, so as to overlap and mask a portion of the first detector, thereby altering the first detector&#39;s effective width. 
     Preferably, the apparatus includes an adjustable slit or aperture, which masks a portion of the second detector, thereby altering the second detector&#39;s effective width. 
     There is additionally provided, in accordance with a preferred embodiment of the present invention, apparatus for producing image slice data responsive to incident radiation, including: 
     a detector array, including one or more rows of detector elements, which generate signals responsive to radiation incident thereon; and 
     at least one mechanical tilting device, which controllably tilts at least one of the one or more rows about a tilt axis thereof, which tilt axis is substantially parallel to the long dimension of the at least one row, 
     wherein the one or more rows are characterized by a width, measured in a direction perpendicular to the tilt axis thereof, and 
     wherein an effective width of the at least one row, defined by a geometrical projection of the width thereof onto a plane that is substantially perpendicular to a direction of propagation of the radiation incident on the row, is varied by controlling the at least one mechanical tilting device. 
     Preferably, the one or more rows include a plurality of rows, and the mechanical tilting device tilts all of the plurality of rows in the array, more preferably tilting all the rows of the array by a common angle, and most preferably tilting the entire array about a common tilt axis. 
     Alternatively or additionally, the at least one mechanical tilting device includes a plurality of such devices, which tilt about different, respective axes, wherein each row of the array is preferably tilted about its own respective axis. 
     Preferably, the apparatus includes a motion control mechanism, which controls a distance between adjoining rows of the array when they are tilted, so that geometrical projections of the rows onto the plane that is substantially perpendicular to the direction of propagation of the radiation incident on the array are substantially contiguous. 
     There is further provided, in accordance with still another preferred embodiment of the present invention, a CT scanner, for producing images of multiple sectional slices through an object, including: 
     a radiation source, which irradiates the object from a first side thereof; and 
     apparatus for producing image slice data, as described above, 
     wherein the detector array is positioned on a second side of the object, opposite to the first side. 
     There is also provided, in accordance with yet another preferred embodiment of the present invention, a detector array switching network, including: 
     a plurality of detector elements, which generate signals responsive to incident radiation; 
     a plurality of switches; and 
     two output channels, 
     wherein a first detector element is connected to a first output channel, and 
     wherein a second detector element adjacent to the first detector element, is connected by a first switch to the first output channel and alternatively by a second switch to a second output channel, and 
     wherein a third detector element adjacent to the second detector element, is connected by a third switch to the first output channel and alternatively by a fourth switch to the second output channel, and 
     wherein a fourth detector element adjacent to the third detector element, is connected by a fifth switch to the second output channel, and 
     wherein the switches are controlled so that one of the second, third and fourth detector elements is connected to the second output channel, and other detector elements, if any, between the first detector element and the detector element connected to the second output channel are connected to the first output channel, together with the first detector element. 
     There is further provided, in accordance with an additional preferred embodiment of the present invention, a detector array switching network, including first and second sub-networks, each such sub-network substantially in accordance with the switching network described above, 
     wherein the respective first detector elements of the first and second sub-networks are mutually adjacent, and 
     wherein the network is substantially symmetrical about a central axis, defined by a border between the respective first detector elements. 
     There is also provided, in accordance with a preferred embodiment of the present invention, a method for producing multiple-slice images of an object, including: 
     irradiating the object; 
     receiving and processing signals generated in response to radiation transmitted through a volume of the object; 
     dividing the volume into a plurality of substantially contiguous, parallel object slices, each of which slices has a thickness approximately determined by the width of a respective detector that generates signals responsive to radiation transmitted therethrough; 
     producing four or more substantially contiguous sectional image slices having a common thickness, each such image slice corresponding to one or more adjoining object slices; and 
     reconstructing an image of the volume, said image including at least the four or more sectional image slices, 
     wherein the object slices comprise at least two central slices of substantially equal thickness, which thickness is less than the thickness of each the other object slices, and at least two peripheral slices whose thickness is greater than the thickness of the central slices, and 
     wherein producing four or more image slices having a common thickness comprises producing at least four image slices of thickness substantially equal to the thickness of the two central slices. 
     There is further provided, in accordance with a preferred embodiment of the invention, a method for producing multiple-slice images of an object, comprising: 
     irradiating the object; 
     receiving and processing signals generated in response to radiation transmitted through a volume of the object; 
     dividing the volume into a plurality of substantially contiguous, parallel object slices, each of which slices has a thickness approximately determined by the width of a respective detector that generates signals responsive to radiation transmitted therethrough; 
     producing two or more substantially contiguous sectional image slices having at least two different thicknesses, each such image slice corresponding to one or more adjoining object slices; and 
     reconstructing an image of the volume, said image including at least the two or more sectional image slices, 
     wherein the thickness of the widest reconstructed slice and the thickness of the thinnest image slice have a ratio of at least 3:1, 
     In preferred embodiments of the invention the ratio is at least 5:1 or 8:1. In a preferred embodiment of the invention, the ratio is about 10:1. 
     There is further provided, in accordance with a preferred embodiment of the invention, a method for producing multiple-slice images of an object, comprising: 
     irradiating the object; 
     receiving and processing signals generated in response to radiation transmitted through a volume of the object; 
     dividing the volume into a plurality of substantially contiguous, parallel object slices of equal width, each of which slices has a thickness approximately determined by the width of a respective detector that generates signals responsive to radiation transmitted therethrough; 
     producing two or more substantially contiguous sectional image slices having at least two different thicknesses, each such image slice corresponding to one or more adjoining object slices; and 
     reconstructing an image of the volume, said image including at least the two or more sectional image slices. 
     There is further provide, in accordance with a preferred embodiment of the invention a method for producing multiple-slice images of an object, comprising: 
     irradiating the object; 
     receiving and processing signals generated in response to radiation transmitted through a volume of the object; 
     dividing the volume into a plurality of substantially contiguous, parallel object slices, each of which slices has a thickness approximately determined by the width of a respective detector that generates signals responsive to radiation transmitted therethrough; 
     producing two or more substantially contiguous sectional image slices having at least two different thicknesses, at least one such image slice corresponding to a plurality of adjoining object slices; and 
     reconstructing an image of the volume, said image including at least the two or more sectional image slices. 
     In a preferred embodiment of the invention, the object slices are of substantially the same thickness. 
     In various preferred embodiments of the invention, the image slices comprise a single thin slice and a single thick slice or a single thin slice and two thick slices or a single thick slice and two thin slices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: 
     FIG. 1 is a schematic, partly sectional representation of a CT scanner, in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a schematic representation of a detector array, in accordance with a preferred embodiment of the present invention; 
     FIG. 3A is a cross-sectional view of the CT scanner shown in FIG. 1; 
     FIG. 3B is a cross-sectional view of a CT scanner similar to that shown in FIG. 3A, but including a detector array in accordance with an alternative preferred embodiment of the present invention; 
     FIG. 3C is a cross-sectional view of a CT scanner in accordance with still another preferred embodiment of the present invention; 
     FIGS. 4A-4D are sectional representations of the detector array of FIG. 2, together with schematic representation of switching circuitry associated therewith, in accordance with a preferred embodiment of the present invention; 
     FIGS. 5A-5D are sectional representation of a detector array and a mechanical aperture associated therewith, in accordance with a preferred embodiment of the present invention; 
     FIGS. 6A-6D are sectional representations of a detector array, together with an associated mechanical aperture and movable base, in accordance with a preferred embodiment of the present invention; 
     FIGS. 7A-7B are sectional representations of a tiltable detector array, in accordance with a preferred embodiment of the present invention; 
     FIGS. 8A-8B are sectional representations of an array of tiltable rows of detectors, in accordance with another preferred embodiment of the present invention; 
     FIGS. 9A-9E are sectional representations of a detector array and a mechanical aperture associated therewith, in accordance with a preferred embodiment of the present invention; 
     FIG. 10 is a schematic representation of switching circuitry associated with the detector array shown in FIGS. 9A-9E, in accordance with a preferred embodiment of the present invention; 
     FIG. 11 is a schematic representation of a detector array and a mechanical aperture associated therewith, in accordance with still another preferred embodiment of the present invention; and 
     FIG. 12 shows two ways in which the outputs of detectors having the same width may be combined in accordance with a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic side view of a CT scanner  20  in accordance with a preferred embodiment of the present invention. An X-ray tube  22  irradiates a region of the body  24  of a subject being imaged. The angular extent of a beam of radiation  26  is preferably restricted by an adjustable collimator  28 . X-rays transmitted through body  24  are received by a multi-slice detector array  30 , as will be described below. The lateral dimension of array  30 , i.e., the dimension parallel to long axis  25  of body  24 , is exaggerated in FIG. 1 for the purpose of clarity in the explanation that follows below. Array  30  may be mounted as shown to a movable base  32 , which moves and aligns the array relative to an axis defined by X-ray tube  22 . In some preferred embodiments of the present invention, collimator  28  and/or a mechanical aperture  34  limits the angular extent of the radiation beam striking array  30 . Preferably, collimator  28  is adjusted so as to limit the angular extent of beam  26  to the region of body  24  being imaged by array  30 , and minimize irradiation of other regions of the body. Base  32  and/or aperture  34  are particularly useful in conjunction with certain preferred embodiments of the present invention, such as those shown in FIGS. 5A-5D,  6 A- 6 D and  9 A- 9 E and described hereinbelow. The base and the aperture are not essential to the operation of array  30 , but are shown in FIG. 1 by way of illustration. 
     Reference is now made additionally to FIG. 2, which is a schematic view of array  30 , viewed from above in the perspective of FIG.  1 . As shown in FIG. 2, array  30  comprises a plurality of rows  50 ,  52 ,  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66  and  68 , each such row comprising a plurality of detector elements  70 . Detector elements  70  may comprise any suitable type of radiation-sensitive detectors, for example photodiodes or other detectors known in the art. Preferably, multiple elements  70  are fabricated and/or mounted together on a common substrate, although alternatively elements  70  may be discrete elements, without a common substrate. 
     Along a direction parallel to the long axis  72  of array  30 , detector elements  70  preferably all have a substantially equal dimension, or pitch, as shown in FIG.  2 . In the direction perpendicular to axis  72 , however, some of the rows have different widths. Central rows  58  and  60  have the smallest width, while peripheral rows have widths equal to or greater than this smallest width, and exterior rows  50  and  68  have the greatest widths. In the preferred embodiment of the present invention shown in FIG. 2, all rows have widths that are integral multiples of the width of the central rows, wherein if the width of rows  58  and  60  is taken to be equal to 1, the remaining rows have the following widths: 
     Rows  56 ,  62 —width=1 
     Rows  65 ,  64 —width=2 
     Rows  52 ,  66 —width=4 
     Rows  50 ,  68 —width=8 
     The reasons for this choice of proportions will be explained below. 
     FIG. 3A is a cross-sectional view of the scanner shown in FIG.  1 . Array  30  is mounted in CT scanner  20  with its long dimension, indicated by axis  72 , transverse to long axis  25  (shown in FIG. 1, and perpendicular to the plane of FIGS. 3A and 3B) of body  24 . Each element  70  of array  30  receives radiation that has traversed body  24  along a linear path from a focal point  78  of X-ray tube  22  to the element, and generates an electrical signal indicative of the attenuation of tissue in the body intercepted by this path. Array  30  and X-ray tube  22 , along with ancillary apparatus, such as collimator  28 , are mounted on gantry  74 . The gantry revolves around an axis substantially parallel to axis  25 , so that array  30  can capture views from different angles with respect to this axis. Body  24  is further translated laterally relative to gantry  74 , in a direction substantially parallel to axis  25 , so that different cross-sectional portions of body  24  may be imaged. 
     FIG. 3B shows an alternative preferred embodiment of the present invention, wherein array  76  is arcuate, rather than planar. The arrangement of rows and detector elements in array  76 , however, is identical to that of array  30 , and in all other respects, the preferred embodiment shown in FIG. 3B is substantially identical to that shown in FIGS. 1,  2  and  3 A. The radius of curvature of array  76  is generally equal to the distance from the array to focal point  78 . Thus, all elements  70  in array  76  subtend substantially equal angles of beam  26  in the transverse direction. As is known in the art, this equality of angles is useful in reducing angular distortion in the image of body  24  that is produced by CT scanner  20 . Although the following preferred embodiments of the present invention will be described with reference to planar detectors arrays, it will be appreciated that arcuate arrays may similarly be used in such embodiments. 
     FIG. 3C shows still another, alternative, preferred embodiment of the present invention, wherein array  77  describes a ring shape, substantially surrounding body  24 . In this preferred embodiment, scanner  20  is preferably a fourth-generation CT scanner. As in the embodiment of FIG. 3B, the arrangement of the rows and detector elements  70  in array  77  is substantially identical to that of array  30 . In other respects, array  77  is used in system  20  in a manner substantially similar to that shown in FIGS. 1 and 2 and described herein, except that whereas arrays  30  and  76  preferably revolve around body  24  on gantry  74 , array  77  is preferably substantially stationary. 
     Referring again to FIG. 1, the signals generated by elements  70  are processed by pre-processing circuitry  80  and then transferred via a switching network  82  to a data acquisition system (DAS)  84 . A reconstructor  86  receives data from DAS  84  and applies algorithms, as are known in the art, to reconstruct images showing internal structures within body  24 . These images are preferably displayed by a display unit  87 . A processor  88  receives these images and, optionally, records them in mass memory, prints them on hard-copy media and performs other data and display processing functions known in the art. Processor  88  preferably includes a computer, which controls other components of CT scanner  20 , including collimator  28 , aperture  34 , movable base  32  and gantry  74 . 
     Pre-processing circuitry  80  may be of any type known in the art, and may be integrated on a common substrate with array  30 , or contained on a separate substrate or circuit board. Preferably the pre-processing circuitry includes analog pre-amplifiers. 
     Switching network  82  preferably selects the rows of array  30  from which data are to be acquired, and adds together signals generated by elements in selected, adjacent rows. The switching network may be integrated with array  30  on a common substrate or contained on a separate substrate or circuit board. 
     Although in the preferred embodiment shown in FIG. 1, switching network  82  receives signals from array  30  after processing by pre-processing circuitry  80 , in other preferred embodiments of the present invention, the switching network may select and add together signals from adjacent rows before the signals are pre-processed. Such embodiments may generally be advantageous in reducing the number of components in the system, and thus reducing the system&#39;s cost, particularly if switching network  82  is integrated on a common substrate with array  30 . Switching of signals before pre-processing however, may also tend to introduce a greater degree of noise into the signals. 
     In still other preferred embodiments of the present invention, switching network  82  may be eliminated, and instead signals from all the rows of array  30  may be acquired separately by DAS  84 , and then signals from adjacent rows may be selected and added together by software processing. 
     DAS  84  preferably digitizes signals received from switching network  82 , using analog-to-digital (A/D) conversion circuitry known in the art. Preferably a logarithm operation is then performed on the digitized signals, for example using look-up tables. 
     The foregoing order of operations, wherein signals generated by elements  70  are first summed, then digitized and then undergo a logarithm operation, is advantageous in that it reduces the number of electronic components required in the system, and thus reduces the cost of the system, as well. In other preferred embodiments of the present invention, however, the order of these operations may be different. 
     For example, in one such preferred embodiment, pre-processing circuitry  80  also includes a logarithmic amplifier for each active detector, which results in reduced partial volume artifacts, as is known in the art. Switching network  82  then serializes, selects and adds together signals, and network  82  digitizes the signals, as described above. 
     In other preferred embodiments of the present invention, pre-processing circuitry  80  may include analog-to-digital (A/D) conversion circuitry. Switching network  82  includes digital circuitry, as is known in the art, which serializes, selects and sums the signals. A logarithm operation may be performed by logarithmic amplifiers included in pre-processing circuitry  80 , as described above, or alternatively may be performed digitally, for example by look-up table. Such preferred embodiments will tend to be costly to produce, since they must generally include multiple A/D conversion circuits, but they will generally have the advantage of improved signal/noise ratio. 
     As illustrated by FIG. 1, by way of example, the rows and/or combinations of rows selected by switching network  82  define substantially parallel image slices  102 ,  104 ,  106  and  108  within the angular extent of X-ray beam  26 . Slice  102  is reconstructed, by reconstructor  86 , using data derived from row  52 ; slice  104  is reconstructed using data from rows  54 ,  56  and  58 ; slice  106  from rows  60 ,  62  and  64 ; and slice  108  from row  66 . Data from rows  50  and  68  are not used in this case. Preferably collimator  28  and aperture  34  are adjusted so as to limit the angular extent of beam  26  to angle subtended by slices  102 ,  104 ,  106  and  108 , so as to reduce unwanted radiation dosage, but this adjustment is not necessary to the operation of the preferred embodiment shown here, as long as radiation traversing body  24  can reach all the rows selected by the switching network. 
     It will be understood that the thickness of each of the image slices, i.e., its lateral dimension measured along axis  25 , is generally determined approximately by the width of the row or the sum of the widths of the multiple rows defining the slice. Thus, thinner or thicker slices may be produced by appropriate selection of the rows of the array. Preferably all four slices  102 ,  104 ,  106  and  108  are of substantially equal thicknesses. 
     The actual slice thicknesses are determined only approximately by the row widths, because the thicknesses also depend on optical qualities of other elements of scanner  20 , such as X-ray tube  22  and collimator  28 . Generally, however, these other elements have only minor effect on the slice thicknesses, as will be illustrated by the following example: 
     Assuming focal point  78  of X-ray tube  22  to have a dimension f, and rows  56  and  62 , the narrowest rows of array  30 , to have width w, the effective thickness a eff  of an image slice defined by row  56  or  62  will typically be given (neglecting the generally insignificant effect of collimator  28 ) by: 
     
       
           a   eff =1/ M*SQRT[w   2 +( M− 1) 2   f   2 ] 
       
     
     where a eff  is measured at the center of rotation of gantry  74 , and M, the magnification, is the ratio of the distance from focal point  78  to array  30 , over the distance from the focal point to the center of rotation. Taking typical values of w=2 mm, f=1 mm and M=2, we find that a eff =1.1 mm, rather than 1 mm, which would be the slice thickness if focal point  78  were infinitesimal. It will be appreciated that when wider rows of the array are used, the effect of the other elements of scanner  20  on the slice thickness will be even less significant. 
     As noted earlier, it will be understood that the use of the term “substantially” or “approximately” in stating a detector width dimension, for example to say that the width of rows of detectors are substantially equal, or that the thickness of one row is substantially an integer multiple of another row, means that the detector thickness is such that the slices traversed by the radiation beam has approximately the stated dimension or ratio. This includes any correction to the detector width which might be required, for example to correct for the difference between the effective thickness a eff  and the thickness that would be obtained if focal point  78  were infinitesimal or for other geometrically caused variation between the effective slice width and the detector width. Such correction is generally very small compared to the detector and slice dimensions of interest. 
     The size of focal point  78 , together with other geometrical factors, such as the position of collimator  28  and the size of its aperture, also determines a minimum extent of beam  26  at the center of rotation, as is known in the art. In typical CT system geometries, such as that shown in FIG. 1, this minimum extent is significantly larger than the dimension f of the focal point. Thus, if only a single slice of a minimum thickness, for example 1 mm, is acquired by scanner  20 , substantial radiation will pass through body  24  outside the bounds of this single slice. The system in this case will have a relatively low dose efficiency. On the other hand, when multiple slices are acquired simultaneously, as described herein, more or substantially all of the radiation incident on body  24  is captured by detector array  30  and used in creating the CT image, so that dose efficiency is increased. 
     FIGS. 4A-4D schematically illustrate the row-adding function of switching network  82  with respect to array  30 , which is shown in cross-section in the figure. In these figures, switching network  82  includes two adders  90  and  92  and four output channels,  94 ,  96 ,  98  and  100 , corresponding to four image slices, labeled slice A though slice D, respectively. Adders  90  and  92  may be of any suitable type known in the art, for example multi-input analog operational amplifiers or digital adder circuits (in the case that the signals received by network  82  have first been digitized). Pre-processing circuitry  80  is omitted in these figures, for simplicity of illustration (and, as described above, because pre-processing may be performed after switching). As described above, switching network  82  may receive the signals from elements  70  before they are pre-processed. 
     In FIG. 4A, channels  94 ,  96 ,  98  and  100  receive signal data from rows  56 ,  58 ,  60  and  62 , respectively. In this case, image slices A through D have the smallest possible thickness, corresponding to detector row width=1, where the width of rows  58  and  60  ( as well as  56  and  62 ) has been taken to be equal to 1, as described earlier. In this case, CT scanner  20  will produce images having the highest available resolution in the lateral direction, although possibly at the expense of lower volume coverage for a given scanning duration and/or reduced signal/noise ratio. 
     In FIG. 4B, channels  94  and  100  receive signal data from rows  54  and  64 , which have width=2. Channel  96  receives data derived by summing signals using adder  90 , wherein the signal from each element in row  56  is summed with that from an adjacent element in row  58 , so that channel  96  corresponds to an effective row width=2, i.e., the sum of the widths of rows  56  and  58 ; and channel  98  similarly receives data summer by adder  92  from rows  60  and  62 . In this case, slices A through D have a common thickness that is approximately twice that of the slices produced in the configuration shown in FIG.  4 A. 
     FIG. 4C shows still another configuration, in which each of channels  94 ,  96 ,  98  and  100  receives data corresponding to an actual or effective row width=4, producing image slices as illustrated by slices  102 ,  104 ,  106  and  108  in FIG.  1 . 
     FIG. 4D shows another configuration, in which all the rows of array  30  are used to produce slices having a maximal thickness, approximately eight times as thick as the slices defined by the configuration of FIG.  4 A. The configuration of FIG. 4D will generally enable CT scanner  20  to operate at its highest throughput rate and highest signal/noise ratio. 
     It will be appreciated that detector array  30  as illustrated by FIG. 2, operating in accordance with FIGS. 1,  3 A and  4 A- 4 D, enables CT scanner  20  to acquire four image slices, of equal thickness, simultaneously. The slice thicknesses may be varied electronically, without the use of moving parts, over a range of approximately 1:8. Array  30 , however, includes only ten rows of detector elements  70 , so that the complexity and cost of pre-processing circuitry  80  and switching network  82  may be reduced relative to comparable circuitry that must be used for producing multiple slices of similarly variable thicknesses in conjunction with other detector arrays known in the art. 
     It will further be appreciated that although the preceding preferred embodiment, as well as other preferred embodiments of the present invention described below, is shown to produce four image slices with four alternative choices of slice thickness, the principles of the present invention may similarly be applied to produce a greater number of slices and a greater or smaller range of thicknesses. The number of slices and the thicknesses thereof, in such embodiments of the present invention, are generally dependent on the number of rows in the detector array and the construction and function of a switching network associated therewith. 
     FIGS. 5A-5D illustrate an alternative preferred embodiment of the present invention, wherein a detector array  100  has structure and function generally similar to those of array  30 , but the rows of array  100  have different relative widths, specifically: 
     Central rows  112 ,  114 —width=1 
     Rows  110 ,  116 —width=2 
     Rows  108 ,  118 —width=4 
     Rows  106 ,  120 —width=3 
     Rows  104 ,  122 —width=10. 
     In the preferred embodiment shown in FIGS. 5A-5D, mechanical aperture  34  is controlled to selectively mask some of the rows in array  100 . Preferably, collimator  28  is also adjusted so as to limit the angular extent of beam  26  to the extent of aperture  34  shown in the figures. Collimator  28  may, alternatively, be used instead of aperture  34  for this purpose. As shown in FIGS. 5A-5C, this selective masking may include limiting the effective widths of some of the rows. Switching circuitry similar to network  82 , as illustrated in FIGS. 4A-4D, selects and adds together signals from adjacent rows of array  100 , so as to produce four slices labeled slice A, B, C and D, as above. 
     Thus, in FIG. 5A, aperture  34  is narrowed laterally so as to mask substantially one half of the widths of rows  110  and  116 , and the effective widths of these rows are then substantially equal to 1, like rows  112  and  114 . In this case, four relatively thin slices A-D are produced, corresponding to row width=1. 
     In FIG. 5B, aperture  34  is opened so that rows  10  and  116  are fully exposed, and substantially one fourth of the widths of rows  108  and  118  are masked, so that these rows have effective width=3. Signals from rows  110  and  112  are combined in slice B, thus producing a similar effective width=3, and likewise rows  114  and  116  in slice C. 
     In FIG. 5C, aperture  34  is opened still further, so as to mask substantially 60% of the widths of rows  104  and  122 , and expose all other rows fully. In this case, the four slices have thickness corresponding to effective row width=7. 
     Finally, in FIG. 5D, aperture  34  is fully open, and the four slices have thickness corresponding to effective row width=10. 
     It will be appreciated that preferred embodiments of the present invention that make use of a variable aperture, such as collimator  28  or mechanical aperture  34 , in the manner described here can typically generate a wider range of choices of slice thicknesses than can embodiments that use electronic switching alone, such as that shown in FIGS. 4A-4D. 
     FIGS. 6A-6D illustrate an alternative preferred embodiment of the present invention, using a detector array  130 , which has structure and function generally similar to those of arrays  30  and  122 , but having rows of different relative widths, which vary asymmetrically about a central axis of the array parallel to the rows. Specifically: 
     Rows  138 ,  140 —width=1 
     Row  136 —width=2 
     Row  134 —width=4 
     Rows  132 ,  142 ,  144 —width=8. 
     In the preferred embodiment shown in FIGS. 6A-6D, as in the preceding embodiment, mechanical aperture  34  is controlled to mask some of the rows in array  130 , so as to limit their effective widths. Movable base  32  further controls the lateral position of array  130 , relative to an axis  146  perpendicular to the surface of the array and passing through focal point  78 . Switching circuitry similar to that illustrated in FIGS. 4A-4D selects and adds together signals from adjacent rows of array  130 , so as to produce four slices labeled slice A, B, C and D, as above. 
     In FIG. 6A, mechanical aperture  34  masks portions of rows  136  and  142 , so that these two rows have effective width=1, and four image slices are produced having a minimum thickness corresponding to this width. 
     In FIG. 6B, movable base  32  shifts the position of array  130 , so that a common edge of adjoining rows  136  and  138  is substantially aligned with axis  146 . Mechanical aperture  34  opens asymmetrically, so as to mask portions of rows  134  and  142 . Four image slices corresponding to effective row width=2 are thus produced. 
     In FIG. 6C, movable base  32  shifts array  130  still further, so that a common edge of adjoining rows  134  and  136  is substantially aligned with axis  146 , and aperture  34  opens so as to mask portions of rows  132  and  142 . Four image slices are produced corresponding to effective row width=4. 
     In FIG. 6D, aperture  34  is fully opened, and movable base  32  shifts array  130  back to align a common edge of adjoining rows  140  and  142  with axis  146 . Four image slices are produced corresponding to effective row width=8. 
     It will be appreciated that array  130 , as shown in FIGS. 6A-6D, achieves the same range and values of slice thicknesses as does array  30 , as illustrated by FIGS. 4A-4D; but array  130  includes only seven rows of detector elements, while array  30  has ten rows. Thus, array  130  can achieve resolution that is comparable to that of array  30 , but with substantially fewer detector elements in the array, and a correspondingly simpler switching network. 
     FIGS. 7A and 7B show an alternative preferred embodiment of the present invention, in which a planar detector array  150  comprises a plurality of detectors  70  arranged in four rows  152 ,  154 ,  156  and  158  of equal widths. Array  150  may be used in CT scanner  20  in place of detector array  30  shown in FIG.  1 . Array  150  is mounted on a pivot  160 , which rotates about an axis parallel to the long axes of the rows, preferably under the control of processor circuitry, such as processor  88 . The array is coupled to pre-processing, DAS and reconstructor circuitry similar to that illustrated in FIG. 1, but switching network  82  may be eliminated. 
     As shown in FIG. 7A, when array  150  is oriented so that the plane of the array is substantially perpendicular to axis  146  (as described in reference to FIGS.  6 A- 6 D), CT scanner  20  will produce four image slices having a common thickness T, determined by the width of the rows. As FIG. 7B shows, however, when array  150  is tilted, due to rotation of pivot  160 , the thickness of the slices is reduced to a value approximately equal to Tcosθ, where θ is the angle of rotation of the array relative to its starting position. By rotating array  150  through an angle θ=82.8°, the slice thickness may be reduced to approximately T/8. Provision must be made, for example in reconstructor  86 , for small differences that will arise in the relative strengths of the signals among the four rows and in the corresponding slice thicknesses, due to rows  156  and  158  being closer to focal point  78  than rows  154  and  152 . 
     FIGS. 8A and 8B show still another preferred embodiment of the present invention, wherein a detector array  170  comprises a plurality of tiltable rows  172 ,  174 ,  176  and  178 , each of which comprises a plurality of detector elements  70 . Array  170  may be used in CT scanner  20  in place of detector array  30  shown in FIG.  1 . Rows  172 ,  174 ,  176  and  178  have substantially equal widths. Each row is independently fixed to a pivot  180 , which allows the row to tilt about a row axis substantially parallel to the row&#39;s long dimension. Preferably, pivots  180  are mounted on movable pivot mounts  184 , and are rotated about the respective row axes by transmission belts  182 , or other suitable rotation transmission devices. Mounts  184  and belts  182  are coupled to a motion control mechanism  186 , which is preferably controlled by a computer, such as processor  88 . 
     As shown in FIG. 8A, when rows  172 ,  174 ,  176  and  178  are oriented so as to define a plane that is substantially perpendicular to axis  146  (as described above), CT scanner  20  will produce four image slices having a common thickness, determined by the width of the rows. As FIG. 8B shows, however, when rows  172 ,  174 ,  176  and  178  are tilted, due to rotation of pivots  180 , the thicknesses of the slices are reduced, as was described above with reference to FIG.  7 B. Preferably all the rows are tilted by a common angle, so that the thicknesses of the slices are substantially equal. 
     Preferably, as shown in FIG. 8B, motion control mechanism  186  reduces the distance between mounts  184  when the rows are tilted. In this way, the slices may be maintained in substantial contiguity, i.e., without intervening spaces that are not imaged in between the image slices, regardless of changes in the thickness of the slices. 
     It will be appreciated that in the preferred embodiments of the present invention shown in FIGS. 7A,  7 B,  8 A and  8 B and described above, image slices may be produced having substantially any desired thickness, by appropriately tilting the array or rows in the array, as long as the desired thickness is less than or equal to a maximum thickness, determined by the width of the rows of the array. Furthermore, although all the rows of array  150  in FIGS. 7A and 7B and of array  170  in FIGS. 8A and 8B are shown as having substantially equal widths, in other preferred embodiments of the invention, rows of different widths may be provided so as to produce slices of different thicknesses. 
     It will further be appreciated that in the preferred embodiment of the present invention shown in FIGS. 8A and 8B, the rows of array  170  need not all be tilted by an equal angle, as illustrated in FIG. 8B, but may rather be tilted by different angles, so as to produce slices of different thicknesses. Such varying slice thicknesses are useful in certain CT imaging modalities, for example, in CT imagining of the lungs, in which high- and low-resolution slices may be interspersed so as to reduce the radiation dose to which the body is exposed. 
     Furthermore, while tilting the detectors allows for a wide range of variation in the width of the slices, this range can be further increased by utilizing, in addition to such tilting, combination of rows as shown in FIGS. 4-6 and  9 - 11 . One way these two methods could be combined is for the combination of rows to provide a first, coarser slice width and for the tilting to provide a finer variation on the combination width. 
     FIGS. 9A-9E show still another preferred embodiment of the present invention, wherein a detector array  190  has structure and function generally similar to those of arrays  30  and  102 , and operates in conjunction with mechanical aperture  34 , in a manner similar to that described above with reference to the preferred embodiment shown in FIGS. 5A-5D. The rows of array  190 , however, have the following relative widths: 
     Central rows  198 ,  200 —width=1 
     Rows  196 ,  202 —width=1.5 
     Rows  194 ,  204 —width=2.5 
     Rows  192 ,  206 —width=5 
     FIG. 10 schematically shows a switching network  210  that receives signals from array  190  and selectively combines these signals to produce the slices shown in FIGS. 9A-9E. It will be appreciated that the network comprises two substantially identical and independent portions: a first portion coupled to rows  192 ,  194 ,  196  and  198 , and a second portion coupled to rows  200 ,  202 ,  204  and  206 . Network  210  is configured so that either two or four image slices may be simultaneously produced. 
     Thus, as shown in FIG. 9A, aperture  34  is narrowed laterally so as to mask substantially one half of the widths of rows  198  and  200 , and the effective widths of these rows are then substantially equal to 0.5. Switches S 1 , S 2 , S 8  and S 9 , shown in FIG. 10, are held in an open position, and two thin slices, A and B, are produced and acquired respectively by receiving an output from row  198  via adder A 1  and an output from row  200  via adder A 3 . The remaining switches are closed, and the outputs of adders A 2  and A 4  are not used. 
     In FIG. 9B, aperture  34  is opened so that rows  198  and  200  are fully exposed, and substantially one third of the widths of rows  196  and  202  are masked, so that these rows have effective width=1. Switches S 3 , S 6 , S 7 , S 10 , S 13  and S 14  are closed, while the remaining switches are held open. Four slices having thickness corresponding to width=1 are thus produced and acquired via adders A 1 -A 4 . 
     In FIG. 9C, aperture  34  is opened still further, so as to expose substantially all of rows  194  and  204  (as well as rows  196 ,  198 ,  200  and  202  in between them). Switches S 1 , S 4 , S 7 , S 8 , S 11  and S 14  are closed, while the remaining switches are held open. The outputs of rows  196  and  198  are combined by adder A 1 , and those of rows  200  and  202 , by adder A 3 . Four slices having thickness corresponding to effective row width=2.5 are thus produced and acquired via the adders. 
     In FIG. 9D, aperture  34  is fully open. Switches S 1 , S 2 , S 5 , S 8 , S 9  and S 12  are closed, while the remaining switches are held open. Four slices having thickness corresponding to effective row width=5 are thus produced. 
     Finally, FIG. 9E illustrates a configuration in which two slices, having thickness corresponding to effective row width=10, are produced. In this case, the switches are maintained in the same positions as were described above with reference to FIG.  9 D. The outputs of adders A 1  and A 2  are combined, preferably by means of a software operation carried out by DAS  84 , for example, to produce slice A, and the outputs of adders A 3  and A 4  are similarly combined to produce slice B. 
     It will thus be appreciated that array  190 , having eight rows, together with switching network  210 , is capable of producing two or four slices simultaneously, having an available range of five different slice thicknesses. Other preferred embodiments of the present invention, similar to that illustrated in FIGS. 9A-D and  10  but generally including detector arrays having a greater number of rows than array  190 , can similarly produce more than four slices simultaneously. 
     FIG. 11 illustrates schematically yet another preferred embodiment of the present invention, in which a detector array  220  comprises four parallel rows of detectors: inner rows  222  and  224 , and outer rows  226  and  228 , each row corresponding to a respective image slice. Preferably all four rows have equal widths. As shown in the figure, outer rows  226  and  228  are mounted and positioned relative to inner rows  222  and  224  so that the outer rows may be translated laterally to overlap and mask portions of the widths of the inner rows. Preferably, aperture  34  and/or collimator  28  (as shown in FIG. 1) similarly masks portions of the widths of outer rows  226  and  228 . 
     It will thus be appreciated that by translating rows  226  and  228  and correspondingly opening or closing aperture  34  (and/or collimator  28 ), the four image slices may be adjusted to substantially any desired thickness, up to a maximum corresponding to the full width of the rows. Preferably, outer rows  226  and  228  and aperture  34  and/or collimator  28  are positioned so that all four of the outer and inner rows have substantially equal effective widths. However, the principle described here of using one or more rows of the array to overlap and mask, and thus control the effective width of, one or more other rows, may similarly be used in other preferred embodiments of the present invention in which the array includes a greater or lesser number of rows, and produces image slices having equal or different thicknesses. 
     Although the above preferred embodiments have been described with reference to detector elements having substantially equal pitch sizes, wherein pitch is measured in a direction substantially parallel to long array axis  72 , it will be appreciated that the principles of the present invention may similarly be applied to arrays of detectors having two or more different pitch sizes. Signals from adjacent detectors within a row of the array may also be combined, using switching circuitry and/or methods similar to those described above, or other circuitry and methods known in the art. 
     FIG. 12 shows a first combination of detectors in different rows to produce composite slice widths especially suitable for lung imaging. In a preferred embodiment of the invention, 10 small detectors rows  300  each having a row width of 1-2 mm are utilized. In a preferred embodiment of the invention, the outputs of corresponding detectors in 9 of the rows are added together to form data for a thick slice while the data for the 10th row is used to form a thin slice. Alternatively, the outputs of all the rows are added together to form a thick slice and the outputs of one row is used to provide a thin slice. Alternatively, two thin slices may be provided in this manner, which thin slices can be either adjacent slices or formed of the outputs of rows at the ends of the group of rows. Further, alternatively, a single thin slice may be either at the center of the group of rows or at the edge of the group. 
     In a further alternative embodiment of the inventions non-uniform slices are produced using combinations of the outputs of detectors in non-uniform rows. In these embodiments for example, the row configurations of FIGS. 1,  2 ,  4 ,  5 ,  6  or  9  may be utilized. For example, in these configurations, signals from detectors in the two central thin rows may be combined to form a single relatively thin slice and signals from detectors in a plurality of adjacent outer detector rows may be combined to form two (or more) thick slices. These tin and thick slices may have any ratio, consistent with the available widths, but preferably a large ratio, as described above, is provided as required, for example, for lung images. Alternatively, two (or four) thin slices are provided utilizing the separate outputs of the detectors of the central two (or four) rows and thick slices are provided by summing the outputs of the detectors in the outside rows. Of course, if the ratio between the width of detectors in the various rows is large enough for the application, no summing is necessary. 
     Alternatively, only detectors on one side of the center of the row grouping are irradiated and only a single thin slice and a single thick slice is formed. Alternatively, the detectors on one side of the center line of the row configurations of FIGS. 1,  2 ,  4 ,  5 ,  6  or  9  may be omitted. 
     In a further preferred embodiment of the invention, a greater or lesser number of rows may be provided, such that the ratio between the slices is less then 9:1 or 10:1 described above. For example, if 6 equal rows are provided, then ratios of 6:1, 5:1 or less can be achieved. If a larger number of rows is provided, then more than one wide grouping of rows and more than one narrow grouping of rows may be achieved. 
     Preferred embodiments of the present invention have been described with reference to CT scanners and CT imaging of the human body, and are preferably used in the context of third- and fourth-generation CT scanners. The inventive principles of the present invention may be similarly applied, however, to CT scanners applied to industrial quality control and other applications, as well as to other imaging systems and methods. 
     It will be appreciated that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.