Abstract:
Variable-Resolution X-ray (VRX) techniques boost spatial resolution of a Computed Tomographic (CT) scanner in the scan plane by two or more orders of magnitude by reducing the angle of incidence of the x-ray beam with respect to the detector surface. A multi-arm multi-angle VRX detector for targeted CT scanning allows for “target imaging” in which an area of interest is scanned at higher resolution than the remainder of the subject, yielding even higher resolution for the target area than that obtained from prior VRX techniques. In one embodiment, the VRX-CT detector comprises four quasi-identical arms, each containing six 24-cell modules made of individual custom CdWO4 scintillators optically-coupled to custom photodiode arrays. The maximum scan field is 40 cm for a magnification of 1.4. A significant advantage of the four-arm geometry is that it can transform quickly to a two-arm or single-arm geometry for comparison studies and other applications.

Description:
This application claims priority to provisional patent application Ser. No. 60/771,177 filed Feb. 7, 2006, titled VARIABLE RESOLUTION X-RAY CT DETECTOR WITH TARGET IMAGING CAPABILITY. 

   The present invention was developed at least in part with funding received from the National Institutes of Health grant number EB-00418. The U.S. government may have certain rights in this invention. 

   FIELD 
   This invention relates to the field of Computed Tomographic (CT) x-ray scanners. More particularly, this invention relates to a Targeting Variable Resolution (TVRX) CT x-ray scanner. According to the invention, the spatial resolution over a portion of a target zone can be greatly increased by reducing the projected size of CT detector elements in a detector array, such as by angulating a portion of the detector array or by using a stair-case detector array design. 
   BACKGROUND 
   Computed Tomographic x-ray scanners (referred to herein as “CT scanners”) have been in clinical use since the early 1970s. Generally, a CT scanner uses a rotating x-ray beam and detector to make cross-sectional (or three-dimensional) images of human anatomy and other subjects. A major disadvantage of current CT scanners is the inability to substantially increase the spatial resolution when the size of the subject under examination decreases. Moreover, the spatial resolution of prior CT scanners cannot be substantially increased when one wishes to target a specific area of anatomy for detailed examination. 
   Prior variable resolution (VRX) techniques have boosted spatial resolution of CT detectors in the scan plane by two or more orders of magnitude. Generally, this has been accomplished by decreasing the angle of incidence of the x-ray beam relative to the surface of the detector array, thereby reducing the projected detector spacing. In prior single-arm angled detector geometries, there have been problems with non-symmetry from one side to the other side of the detector array. Also, in prior systems, the total variation in the sampling aperture has been relatively large and the maximum local system magnification has been large, which reduced overall system resolution unless extremely small focal spots were employed. Dual-arm detectors have offered an improvement over single-arm detectors by providing left-right symmetry, reduction in the total variation in the line-spread function (LSF), and reduction of the maximum system magnification. However, prior dual-arm detectors have had a gap or discontinuity at the central detector cells where the two arms meet, and there have been problems with inter-arm x-ray scattering. 
   Thus, prior VRX-CT scanners have provided increased spatial resolution by reducing the detector angle to reduce its projected size and the corresponding size of the scan circle. However, if the scan circle was reduced too much, the object being imaged no longer fit inside the scan circle. To reduce the scan circle even smaller would produce reconstruction artifacts created by structures outside the scan circle which are not sampled in all views (projections). 
   What is needed, therefore, is a VRX-CT x-ray scanner having an improved geometry that provides increased spatial resolution without the problems associated with prior single-arm and dual-arm scanner designs. 
   SUMMARY 
   One embodiment of the present invention provides a four-arm VRX-CT scanner which overcomes the problems of prior multi-arm scanner designs. The two inner arms of the four-arm detector continue to reduce in angle over a progressively-smaller target scan circle exhibiting progressively-higher spatial resolution, while the two outer arms increase in angle so as to span the minimum acceptable scan circle. In this manner, the scanner produces a complete set of scan projections that all span the entire object but have increased resolution inside a target circle and lower resolution outside the circle. This yields high-quality reconstructions inside the target circle. Accordingly, the multi-arm geometry of the present invention allows “targeted imaging” with the accompanying extremely-high local spatial resolution. 
   In one preferred embodiment, the invention provides an apparatus for generating x-ray images of a subject. The apparatus includes an x-ray radiation source for directing x-ray radiation along a radiation axis toward the subject, a detector array for receiving the x-ray radiation as altered by the subject, and means for processing signals generated by the detector array to generate a human-perceivable image of the subject. The detector array of this embodiment comprises a plurality of array arms, where each array arm includes a plurality of detector cells. The detector cells receive and detect the x-ray radiation at a spatial resolution which is dependent at least in part on the number of detector cells in each array arm and the orientation of the array arms with respect to the radiation axis of the x-ray radiation source. 
   The plurality of array arms include a first array arm and a second array arm. The first array arm is operable to be positioned at a first angle with respect to the radiation axis, such that the first array arm has a first spatial resolution determined at least in part by a value of the first angle. The first array arm includes a first portion of the detector cells for receiving and detecting x-ray radiation that passes through a first zone of the subject. The second array arm is operable to be positioned at a second angle with respect to the radiation axis, such that the second array arm has a second spatial resolution determined at least in part by a value of the second angle which may be different from the value of the first angle. The second array arm includes a second portion of the detector cells for receiving and detecting x-ray radiation that passes through a second zone of the subject. 
   In a preferred embodiment, the detector array also includes a third array arm and a fourth array arm. The third array arm of this embodiment is operable to be positioned at a third angle with respect to the radiation axis, such that the third array arm has a third spatial resolution determined at least in part by a value of the third angle. The third array arm includes a third portion of the detector cells for receiving and detecting x-ray radiation that passes through the first zone of the subject. The fourth array arm is operable to be positioned at a fourth angle with respect to the radiation axis, such that the fourth array arm has a fourth spatial resolution determined at least in part by a value of the fourth angle which may be different from the value of the third angle. The fourth array arm includes a fourth portion of the detector cells for receiving and detecting x-ray radiation that passes through the second zone of the subject. 
   In the four-arm embodiment of the detector array, the first and second array arms are preferably disposed to one side of the source radiation axis, and the third and fourth array arms are disposed to an opposite side of the source radiation axis. Also in this embodiment, the first array arm and the third array arm form a vertex of the detector array, which vertex is disposed on or adjacent the radiation axis. In some configurations of the four-arm embodiment, the value of the first angle is substantially equivalent to the value of the third angle, and the value of the second angle is substantially equivalent to the value of the fourth angle, thereby providing array symmetry about the radiation axis. 
   The targeted VRX-CT imaging provided by the various embodiments of the invention has many areas of application including increasing the resolution of structural details in bodily organs, tumors and other neoplasms, vascular structures, bone structure in the spine, long bones and skull, microcalcifications in breast imaging, intervertebral disks, ligaments, tendons and other connective tissues. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention are apparent by reference to the detailed description in conjunction with the figures, wherein elements are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
       FIG. 1  depicts a four-arm targeting variable-resolution CT x-ray scanner according to a preferred embodiment of the invention; 
       FIG. 2  depicts a front view of the detector array of the four-arm variable-resolution CT x-ray scanner shown in  FIG. 1 ; 
       FIG. 3  depicts the mapping of an object point to VRX space according to a preferred embodiment of the invention; 
       FIG. 4  depicts minimum target field diameter vs. outer field of view (FOV) according to a preferred embodiment of the invention; 
       FIG. 5  depicts maximum cut-off frequency vs. outer FOV according to a preferred embodiment of the invention; 
       FIGS. 6A and 6B  depict high-resolution VRX CT reconstructions of a section of a plasticized human forearm; 
       FIG. 7  depicts a high-resolution VRX CT reconstruction of a human thigh; 
       FIG. 8  depicts a low-resolution VRX CT reconstruction of a human thigh; 
       FIG. 9  depicts reconstructions of a VRX CT scan in the target zone made with an original high-resolution data set (left), a target data set (center) and a low-resolution data set (right); 
       FIG. 10  depicts a dual-arm TVRX-CT scanner according to an alternative embodiment of the invention; and 
       FIG. 11  depicts a dual-arm TVRX-CT scanner according to another alternative embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   A preferred embodiment of a four-arm targeting VRX CT scanner system  10  is shown in  FIG. 1 . In this embodiment, the system  10  includes an x-ray radiation source  12 , slice collimator  14 , detector array  18 , analog-digital converter  26  and computer system  28 . X-ray radiation from the x-ray source  12  passes through a subject  16  positioned within a scan field  32  between the source  12  and the detector array  18 . As described in more detail below, after the x-ray radiation passes through and is modified by the subject  16 , the radiation is received and detected by the detector array  18 . The analog-digital converter  26  converts the detected analog signals from the detector array  18  into digital signals that are processed by the computer system  28  to generate images of the subject  16 . In one embodiment, a rotating table supports the subject  16  during a scan. In another embodiment, the subject remains stationary and the x-ray source  12 , slice collimator  14  and detector array  18  are rotated around the subject  16 . In configurations wherein the A/D converter  26  is physically located with the detector array  18 , the A/D converter  26  may also rotate around the subject  16  with the array  18 . 
   In a preferred embodiment, the x-ray source  12  comprises a radiographic x-ray tube, such as model number G-1582BI manufactured by Varian Medical Systems, which operates at a nominal anode input power of about 60 kW with a bias voltage of about 60 kV(peak) and generates a focal spot size of about 0.6 mm. In an alternative embodiment, the x-ray source  12  comprises a micro-focus x-ray tube, such as model SB-80-250 manufactured by Source-Ray, Inc., which operates at about 80 kV(peak) and generates a focal spot size of about 36 μm by 65 μm. 
   The slice collimator  14  confines and directs the x-ray radiation to a specific region or “slice” of the subject  16 . In a preferred embodiment, the slice collimator  14  has an adjustment range for slice thickness ranging from about 0-10 mm and is typically set to about 0.5 mm. In some embodiments of the invention, the collimator  14  comprises a multi-slice collimator for directing the x-ray radiation simultaneously to more than one “slice” of the subject  16 . 
   In the embodiment of  FIG. 1 , the detector array  18  comprises four detector arms, including two outer arms  20   a  and  20   b  and two inner arms  22   a  and  22   b . The inner array arms are also referred to herein as a first array arm  22   a  and a third array arm  22   b . The outer array arms are also referred to herein as a second array arm  20   a  and a fourth array arm  20   b . In a preferred embodiment, the inner arms  22   a - 22   b  and outer arms  20   a - 20   b  are each about 14.4 cm in length, providing a total array length of 55.6 cm (when the arms are aligned end-to-end). As shown in  FIG. 1 , the first array arm  22   a  includes a first end  22   a   1  and a second end  22   a   2 , the second array arm  20   a  includes a first end  20   a   1  and a second end  20   a   2 , the third array arm  22   b  includes a first end  22   b   1  and a second end  22   b   2 , and the fourth array arm  20   b  includes a first end  20   b   1  and a second end  20   b   2 . In preferred embodiments, each array arm  20   a - b  and  22   a - b  is operable to pivot about its first end. As shown in  FIG. 1 , the first end  22   a   1  of the first array arm  22   a  is disposed adjacent the first end  22   b   1  of the third array arm  22   b  to form a central vertex  34  of the array  18 . 
   In one exemplary embodiment, each detector arm comprises six detector modules. Each of the six detector modules of this embodiment includes twenty-four CdWO 4  crystal-photodiode scintillator cells  24 . Thus, in this exemplary embodiment, the array  18  includes a total of 576 cells. It will be appreciated that the array  18  could comprise any number of modules and cells. Thus, the invention is not limited to any particular number or arrangement of cells or modules in the detector array  18 . So as not to overcomplicate  FIG. 1 , only two cells  24  are depicted therein. The preferred center-to-center spacing of the scintillator cells  24  is 1 mm. This provides a maximum scan field of about 40 cm with a magnification of 1.4. A typical 360° scan time is about four seconds using a radiographic x-ray tube for the x-ray source  12  and about 20 seconds using a micro-focus tube. 
   In an alternative embodiment of the invention, the arms  20   a - 20   b  and  22   a - 22   b  each include a multi-row discrete detector array, or a flat-panel detector array comprising detector cells arranged in a two-dimensional grid. Examples of such multi-row detector arrays include 64-row detectors. Examples of such flat-panel detector arrays include 1000×1000 cell arrays and 2000×2000 cell arrays. 
   As shown in  FIG. 1 , the first (inner) arm  22   a  of the array  18  is positioned at a first angle ψ 1a  relative to the central radiation axis of the source  12 , and the third (inner) arm  22   b  is positioned at a third angle ψ 1b  relative to the central radiation axis. In accordance with the invention, the angles ψ 1a  and ψ 1b  may be varied from near zero degrees up to 90 degrees, with the selection of angles ψ 1a  and ψ 1b  depending on the size of the target zone  30  of the subject  16  and the desired detector resolution within the target zone  30 . The second (outer) arm  20   a  of the array  18  is positioned at a second angle ψ 2a  relative to the radiation axis of the source  12 , and the fourth (outer) arm  20   b  is positioned at a fourth angle ψ 2b  relative to the radiation axis. In preferred embodiments of the invention, the angles ψ 2  and ψ 2b  may be varied from the value of the angles ψ 1a  and ψ 1b  up to 90 degrees, where the selection of angles ψ 2a  and ψ 2b  depend on the overall size of the subject  16  and the desired detector resolution outside the target zone  30 . As the angles ψ 1a  and ψ 1b  are adjusted to move the second ends  22   a   2  and  22   b   2  of the inner arms  22   a - 22   b , the positions of the first ends  20   a   1  and  20   b   1  of the outer arms  20   a - 20   b  are moved accordingly so that no part of the outer arms  20   a - 20   b  blocks the projected “field of view” of the inner arms  22   a - 22   b  and also so that there is no gap or discontinuity in the detected signal at the junctions where the outer arms  20   a - 20   b  meet the inner arms  22   a - 22   b.    
   As discussed in more detail hereinafter, the value of the first angle ψ 1a  between the first arm  22   a  and the radiation axis may be different from the value of the third angle ψ 1b  between the third arm  22   b  and the radiation axis. Also, the value of the second angle ψ 2a  between the second arm  20   a  and the source axis may be different from the value of the fourth angle ψ 2b  between the fourth arm  20   b  and the source axis. Thus, the number of combinations of positions of the array arms  20   a - 20   b  and  22   a - 22   b  is literally infinite. 
     FIG. 2  depicts a view in the X-direction of the detector array  18 . (See the XYZ coordinate axis indicator in  FIG. 1 .) This is an example of a view looking toward the array  18  from the position of the source  12 . The array  18  depicted in  FIG. 2  has fewer cells shown than would be present in the preferred embodiment of the invention so as not to overcomplicate the representation of the array features. This view most clearly indicates that the projected spacing of the cells in the two inner arms  22   a - 22   b  is smaller than the projected spacing of the cells in the two outer arms  20   a - 20   b  when the arms are angled as shown in  FIG. 1 . In the configuration depicted in  FIG. 2 , ψ 1a =ψ 1b ≈30° and ψ 2a =ψ 2b =90°. Generally, for any of the array arms, as the angle ψ between an array arm and the source axis decreases, the spatial image resolution of the array arm increases according to a resolution improvement factor of 1/sin(ψ). 
   As shown in  FIG. 1 , the analog/digital (A/D) converter  26  receives analog sample signals from the detector cells  24  and converters the analog sample signals into digital sample signals. The digital sample signals are provided to the computer system  28  for image processing. In a preferred embodiment, the A/D converter  26  is a 16-bit device that samples the detector signals every 2.5 mS. As described in more detail below, the computer system  28  executes software applications to calibrate the system  10  and to process the digital sample signals to generate images of slices of the subject  16 . 
   Prior to scanning a subject  16  and constructing images, a calibration procedure is performed to specify the geometry of the four detector arms  20   a - 20   b  and  22   a - 22   b . In a preferred embodiment of the invention, the calibration procedure involves moving an x-ray “shadow” of a metal pin across the entire detector array during a scan and mapping the position of the shadow. This may be accomplished by mounting the pin on a rotating platform in the scan field  32 , with the pin positioned far enough away from the platform&#39;s center of rotation so that the pin&#39;s shadow will pass across the entire detector array during a rotation of the platform. A calibration algorithm executed on the computer system  28  determines twelve geometrical parameters (three for each arm of the array). These parameters include the angular rotation and translation in two directions of each arm in the scan plane. The calibration mapping equation is expressed as: 
             L   =       rD   ⁢           ⁢   sin   ⁢           ⁢   θ         r   ⁢           ⁢     sin   ⁡     (     θ   -   ψ     )         -     d   ⁢           ⁢   sin   ⁢           ⁢   ψ           ,         
where L, r, D, d, θ and ψ are depicted in  FIG. 3 .
 
   If it is assumed for simplicity that each detector cell  24  yields a rectangular aperture response whose width equals the projected cell spacing, the minimum target field and maximum target resolution may be determined. For an embodiment of the system  10  having a spacing of 150 cm between the source  12  and the array vertex where the inner arms  22   a - 22   b  meet, a spacing of 102 cm between the source  12  to the center of the zone  32 , and a detector arm length of 14.4 cm, the minimum target diameter is shown in  FIG. 4  and the maximum cut-off frequency is shown in  FIG. 5 . 
   The resolution specified in  FIG. 5  refers to the center cell in the target detector array. These plots indicate only the minimum possible target field and the maximum possible target detector resolution for one embodiment for the geometrical parameters chosen. In an optimal targeting VRX CT scanner, the inner arms  22   a - 22   b  of the array  18  could be closed almost completely (ψ 1a +ψ 1b →zero), such that the maximum geometrical resolution approaches infinity. However, in practice there are limitations imposed on system resolution by the x-ray focal spot size, system magnification, minimum detectable signal, x-ray penetration and scatter in the detector (non-rectangular line spread function) and other factors. 
     FIGS. 6A and 6B  depict reconstructed images of a section of a plasticized human forearm made using an embodiment of the invention as depicted in  FIG. 1 . The target zone  30  is indicated by the dashed circle. Several features of these images are noteworthy: (1) the resolution of the target zone  30  is higher than in the rest of the image, which is apparent from the sharpness and structure present in the lower bone compared with the upper bone and from the sharpness of the micro-cracks or dark lines in the target zone: (2) there is no visible image artifact demarcating the transition from the target zone  30  to the outer zone; and (3) the target imaging has produced no discernable image artifacts. 
   To determine the feasibility of high-resolution target imaging, a simulation was conducted based on experimental data. One purpose of the simulation was to determine whether significant resolution improvement could be obtained in the target zone  30  without introducing significant image artifacts. In conducting the simulation, a first scan was made of an anatomical specimen of a human thigh preserved in formalin using a single-arm storage phosphor CT scanner. An image reconstruction from this first scan is shown in  FIG. 7 . Anatomical features of note are the femur, calcified femoral artery, muscles outlined by fat, and subdermal straia. Soft tissue differentiation is inherently poor in the specimen because the formalin-perfused tissues are nearly isodense. (The “scratches” in the image are caused by imperfections in the storage phosphor screen.) The projection data comprise a sinogram having dimensions of 1400 samples by 1350 views with a sampling distance of 140 μm. The data were expanded (interpolated) to 2800 samples (70 μm sampling distance) to avoid loss of resolution in transforming from the spatially-linear high-resolution space to the equiangular space used in the reconstruction algorithm. The central section of  FIG. 7  (within the dashed circle) depicts the high-resolution CT target data. For purposes of this simulation, it is important that high-contrast structure be present outside the target field to properly test for the production or absence of artifacts. 
   The low-resolution full-field data were then simulated by averaging the expanded fill projection data set over groups of two adjacent pre-expansion samples down to 700 samples (280 μm). This corresponds to the image shown in  FIG. 8 . The low-resolution data were obtained from the high-resolution data by averaging rather than by rescanning, to avoid problems with the geometrical fidelity (non-flatness) of the tilted storage phosphor screen and with image registration. Comparing the original high-resolution sampling with the low-resolution sampling, the spatial resolution in  FIG. 8  should be approximately one-half that in  FIG. 7 . 
   The target reconstruction was made by discarding the high-resolution data outside the target region and using the low-resolution data to fill in the sinogram outside the high-resolution target region.  FIG. 9  compares target zones of (a) the high-resolution reconstruction, (b) the target reconstruction and (c) the low-resolution reconstruction. This simulation indicates that the target reconstruction preserves most of the high resolution of the original image. The general image quality and absence of artifacts in the target reconstruction are noteworthy. 
   In principle, embodiments of the system  10  could be used only in a targeting mode, such as depicted in  FIG. 1 . However, it is probably more feasible to first scan a subject in a non-targeting mode, where each inner arm  22   a - b  is aligned in parallel with its adjacent outer arm  20   a - b  so as to simulate a two-arm configuration (ψ 1a =ψ 2a , ψ 1b =ψ 2b ). This first scan could be done at the lowest possible x-ray dose that still reveals the morphology upon which the target region will be chosen. Then, a technician could observe the reconstructed image from the first scan and identify the target region-of-interest on a display screen of the computer system  28 , such as by using a screen cursor. Once the target region is identified, the subject  16  can be automatically moved laterally so that the target region is centered in the target zone  30 . Then the system  10  can be adjusted to the high-resolution configuration ( FIG. 1 ) and the target scan performed. 
   Another possibility is not to reposition the subject  16 , but to have the inner arms  22   a - b  constantly sliding sideways during the scan so that they always remain directly behind the target zone. Advantages of this technique are that the patient does not have to be repositioned and there is reduced danger of the subject being pressed into the arms of the scanner array  18 . The latter problem should be avoided anyway because the perimeter of the subject  16  is known from the first CT scan. 
   While preferred embodiments of the invention provide four arms (as shown in  FIGS. 1 and 2 ), it is also possible to obtain high-resolution targeting functionality with a dual-arm VRX scanner by substantially increasing the angulation of one arm (the “target” arm), while the angulation of the other arm (the “outer” arm) remains substantially the same. This is shown in  FIGS. 10 and 11 . One disadvantage of this scheme as compared to four-arm embodiments is that the anatomy in the outer zone is sampled only once, not twice (from opposite directions) as is done in normal CT and VRX-CT scanning. This may create some shading artifacts in the outer zone, but not in the target zone. In the embodiment depicted in  FIG. 10 , the full double sampling requirement (at opposite directions) is met everywhere inside the target zone. Also, there is no central gap in this embodiment. However the target resolution is approximately two times lower than in the embodiment of  FIG. 1 . In the  FIG. 11  embodiment, the entire situation is reversed. Hence, it appears that the preferred mode may depend on the particular application. In yet another embodiment, a third arm may be added to span the upper part of the outer field depicted in  FIGS. 10 and 11 . 
   As described herein, various embodiments of the invention can image subjects in a target imaging mode using a multi-arm VRX detector system, where the subject ranges in size from that of human patients to small animals and down to microscopy samples. Thus, the invention allows a focal anatomical region to be imaged at even higher resolution than has been previously possible using prior high-resolution CT techniques. 
   Although a preferred embodiment of the system  10  includes a detector array  18  having four arms, it will be appreciated that the invention is not limited by the number of arms provided in the array. For example, the embodiments depicted in  FIGS. 10 and 11  include detector arrays having two arms. In other embodiments, the detector array may include three, five, or more arms positioned at various ψ angles to provide multiple zones having different resolution levels. 
   The foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.