Whole-body dual-energy bone densitometry using a narrow angle fan beam to cover the entire body in successive scans

Abstract

A scanning radiographic densitometer constructs a broad area, two dimensional projection image from a combination of a set of smaller fan beam scans by tilting the axis of each such smaller scan to construct an effective larger fan beam to reduce artifacts caused by height dependant overlap of the multiple fan beams. The data is projected to a non-planar image surface to eliminate local area distortion such as may cause error in density measurements and to permit some overlap without height sensitive effects.

Description

This application is a continuation-in-part of application Ser. No. 07/916,797 filed Nov. 16, 1992 mean means of stepper motor driven belt 41. As before, the stepper motors driving belts 35 and 37 allow a determination of the precise movement of their respective components through a tallying of the steps taken, as will be understood to those of ordinary skill in the art.

The turntable 39 supports a C-arm collar 38. Collar 38 is generally arcuate to enclose and slidably hold a C-arm 40 such that the ends of the C-arm may rotate about an isocenter 42 as the body of the C-arm 40 slides through the collar 38. The C-arm 40 is constructed as described in U.S. Pat. No. 4,955,046 to Aldona A. Siczek and Bernard W. Siczek entitled: "C-Arm for X-ray Dignostic Examination". The C-arm 40 is motorized, as is understood in the art, to allow remote control over the positioning of the C-arm 40 in collar 38.

The radiation source 44, which is an x-ray tube, is mounted at one end of the C-arm 40 via a support beam 46 and is oriented to direct a polychromatic x-ray fan beam 48 along beam axis 49 generally towards the isocenter 42. The fan beam emanates from a focal spot 45 and diverges away from the beam axis 49 within a fan beam plane 57 to define a fan beam angle φ.

The fan beam 48 is received by a linear detector array 50 extending perpendicularly to the fan beam axis 49, within the fan beam plane 57, and generally on the opposite side of the patient 14. The linear detector array 50 is comprised of a number of adjacent detector elements 47 each of which may detect the attenuation of one ray of the fan beam 48. The linear detector array 50 may be a scintillation type detector, as is understood in the art, having scintillation materials which convert x-rays to visible light to be detected by photodetectors which produce a corresponding electrical signal. Each detector element 47 of the detector array 50 incorporates two side-by-side scintillators and photodetectors to measure the x-rays fluence, of the polychromatic fan beam 48, in one of two energy bands and thus to provide, during scanning, a dual energy measurement at each point in the scan. As noted above, such dual energy measurements allow the tissue of the patient 14 being measured at a given point associated with a detector element 47 to be characterized as to its composition, for example, into bone or soft tissue.

The detector array 50 is affixed to a stop plate 52 and mounted to the opposing end of the C-arm 40.

Together, motion of the pallet 34 and slider 36 permit a scanning by the detector 50 and radiation source 44 of the densitometer 10, the scanning translating the beam axis 49 across the patient 14, whereas the motion of the turntable 39 (of FIG. 3(c)) allows for control of the angle of the fan beam plane 57 with respect to the patient 14, as will be described.

The motion of the slider 36 (shown in FIG. 3(c)) is not limited to providing a scanning motion but may be used, in conjunction with rotation of the C-arm 40 in collar 38, to provide improved imaging of specific structures in the body without disturbing the patient 14 from the supine position. For example, imaging of the femur 53 of a supine patient 14 is ideally done at an angle of approximately 20°-25° from vertical. In prior art devices this typically required uncomfortable inward rotation of the leg of the patient 14. The ability, in the present invention, both to rotate the C-arm 40 and to move the slider 36 along the transverse axis 18, and thus to move the isocenter 42, permits this imaging to be done without movement of the patient 14. Specifically, the desired angle of the C-arm 40 is simply selected and the slider 36 moved so that the beam axis 49 aligns with the femur 53. This and other aspects of the architecture of the densitometer 10 are discussed in the parent application Ser. No. 07/944,626 filed Sep. 14, 1992 and entitled: "Method for Analyzing Vertebral Morphology Using Digital Radiography", hereby incorporated by reference.

Combined motion of the C-arm 40, the slider 36, the pallet 34 and the table 12 permit the densitometer 10 to scan images not simply along the anterior/posterior and lateral directions, but at any angle of the C-arm 40. Each of these actions of the C-arm 40, the slider 36, the pallet 34, and the table 12 may be controlled by a computer 56 having a display terminal 58 and a keyboard 60 such as are well known in the art. By providing step commands to the motors associated with the various components above described, the computer 56 may control and locate these components, for example, by adjusting and tracking the height of the table 12, through actuators 30. The computer 56 also turns the radiation source 44 on and off and importantly collects digitized attenuation data from the individual elements of the linear detector array 50 to generate a matrix of measured data elements over the patient 14.

Referring now to FIGS. 2(a) and 4, radiation source 44 and the detector array 50 may be positioned with respect to collar 38 so that the beam axis 49 is substantially vertical. For a whole body scan of a patient 14, the detector array 50 can be oriented transversely as indicated by 50(b) so as to scan longitudinally as indicated generally by the sequence of areas A1, B1 and C1 from the patient's head to the patient's foot. During this scanning, the fan beam axis 49 traces a first path 59. At the end of this scan, a second longitudinal row of data would be taken conforming generally to the sequence of areas A2, B2 and C2 with fan beam axis tracing along second path 61, from the patient's foot to the patient's head. Four to five such longitudinal rows may be required for a full body scan.

Typically, at the conclusion of the scan of the first path 59, following the sequence A1, B1, C1 . . . , both the radiation source 44 and detector army 50 would both be moved transversely so that the fan beam axis 49, still vertical, intercepts the second scan path 61. The fan beam axis 49 as so displaced is designated 49', and is moved transversely by an amount equal to the transverse width (measured within the fan beam plane 57) of the fan beam 48 as it enters the patient 14. This displacement, which is generally smaller than the fan beam width as it exits the patient 14, ensures that all volumes of the patient 14 are illuminated in one of the several longitudinal paths of the whole body scan. This scanning procedure, however, will also produce a triangular overlap area 69 of redundant measurement between fan beams on paths 59 and 61 and will cause certain volume elements of the patient within that area 69 to be illuminated twice and hence measured twice during the scanning. For example, vertically aligned cubic volume elements 66, 67 and 68 within the patient 14 and approximately half-way between scan paths 59 and 61 will be scanned during motion along both scan paths 59 and 61.

Referring now also to FIG. 5, this dual measurement of volume elements 66-68 will in general cause a transverse spatial distortion in the image of these structures. This distortion rises from the different angles of the measuring rays and, in general, the lack of information as to the height of the volume elements 66-68 within the patient 14. When the data of the individual scans along paths 59 and 61 are simply combined, the uncertainty in height of the volume elements 66-68 translates to an uncertainty in transverse position, and the image exhibits a transverse spreading or smearing. For example, if an image is projected to an imaginary plane at the height of the upper surface of the detector array 50 (a default image plane if the raw data from the detector array is otherwise unprocessed), then cubic volume element 66 having true projected outline 70 will project to a rectangular element 72 having wing portions 74 of lower density than a central portion 76.

In addition to the spatial distortion caused by the multiple measurements in area 69, the redundancy of the data will distort the absorption values associated with the points of the projected image. The image's central portion 76, for example, will be the sum of two measurements of volume element 66 whereas the wing areas 74 will be only one measurement of volume element 66. In theory, this error can be corrected by a weighting of the projection data so that the effect of the redundancy is eliminated, however, again because the height of the volume element 66 is not known, an accurate weighting system cannot be derived. In general, height information is not available in a two-dimensional projection.

The present invention recognizes that the distortion of FIG. 5 is not simply due to the overlapping of fan beams 48 along adjacent scans but rather because of the vacation in overlap as a function of height within the patient 14. Accordingly, the print invention provides a method of orienting the fan beam axis 49 for the scanning of multiple longitudinal columns so that the overlap, if any, is constant along the length of the fan beam axis 49.

Referring now to FIGS. 6 and 7, this requirement of constant overlap between fan beams 48 of scans of adjacent columns of the patient 14 requires the edges of the fan beams, opposed about the fan beams axes 49 within the beam plane 57, be parallel, and most simply abut one another. As shown in FIG. 7, if the fan beam 48 associated with scan path 59 is designated 48(a) and its axis 49(a) and the fan beam 48 associated with scan path 61 is designated 48(b) and it axis 49(b), and so forth for the remainder of the fan beams 48 employed in the whole body scan of patient 14, then each of the successive axes 49 will be displaced about the focal spot 45 by exactly φ, the fan beam angle, and the edges of the adjacent fan beams will just abut when viewed from the perspective of the patient 14 and the table 12. In this case the focal spot 45 for each of the fan beams 48(a)-(d) is the same (with respect to the position of the table 12) for each scan, or more precisely, does not move along, the fan beam plane 57 with respect to the table 12.

Alternatively, as seen in FIG. 11, the edges of the fan beams 48(a)-(d) may overlap slightly but by a constant width. Again, each of the successive axes 49 for the fan beams 48(a)-(d) will be displaced angularly by exactly φ the fan beam angle, but the focal spots 45(a)-(d), for each fan beam 48(a)-(d), respectively, will no longer be fixed in the table reference frame. Nevertheless, because the amount of overlap is unvarying as a function of distance along the fan beam axis 49, using the appropriate projection and weighting process, as will be described, image artifacts caused by the overlap may be removed. Although the areas of overlap 69' are of constant thickness, they change in transverse location depending on the height of the beams in the patient 14. This would seem to raise the same problem of height dependence caused by triangular areas of overlap 69 of FIG. 4, however, the height dependence can be eliminated for constant thickness overlap areas 69' by the proper choice of a projection plane, as will be described below.

In both of the cases of FIGS. 7 and 11, the fan beams 48(a)-(d) are combined to realize an effective, larger fan beam. In the case of FIG. 11, the projections in the area of overlap must be weighted to present the redundant data from having a disproportionate effect on the composite projection image. This weighting may be, at a minimum, simply discarding one set of redundant data (a weighting of zero) or by giving the two sets of data a pair of weights that sum to one. At present, the possibility of patient motion, makes no overlap or the discarding of overlap data preferred, because a weighting and combining blurs the image and is less preferred for diagnosis than some mis-registration in the combined image.

Further, it will be recognized that the amount or overlap must be kept small, even if there is no height dependence, because the important condition is that the rays measuring each volume element of the patient be at one angle, and the rays of the overlapping edges of the fan beams will have approximately the same angle only for small amounts of overlap.

Referring now to FIG. 6, although the effective larger fan beam may be assembled from fan beams 48(a-)48(a)-(d) in a straightforward way in the reference frame of the table 12, the actual motion of the C-arm 40, the table 12 and the slider 36 and pallet 34 of the densitometer 10 in the reference frame of the room is more complex. The angle of the fan beam axes 49(a)-(d) may be achieved simply by rotating the C-arm 40 within its collar 38. However, generally, this rotation will change the height of the focal spot 45 with respect to the table 12 and will change the transverse location of the focal spot 45 with respect to the table 12. Accordingly, compensatory motion of the table 12, up or down and transversely, will need to be performed. The proper orientation of the fan beams 48(a)-(d) is thus performed by a set of motions of the various components of the densitometer 10 working together under the control of computer 56.

It should be noted that the effective wide area fan beam might be expected to produce considerable spatial distortion if used with a single linear detector array spanning the entire effective fan beam (or if the detector array 50 were simply translated along a line beneath the effective fan beam). Such distortion would be caused by the increasing distance between the focal spot 45 and the elements of the detector array 50 for the edgemost rays of the effective fan beam. An increase in distance causes an increased magnification of the image received by the detector array 50 which can also affect quantitative measurements such as bone density to be described below. Nevertheless, the present invention avoids this extreme distortion by piecewise approximating a curved detector (of constant distance from the focal spot for the entire effective fan beam) by means of the short segments of the actual detector array 50.

Nevertheless, each short segment 50 still deviates from a true curved detector and thus, the detector elements of each detector 50 have varying distances from the effective focal spot 45 of the composite fan beam. This deviation can be corrected in the projection process of the present invention, as will be described.

Referring to FIG. 8, a fan beam 48(a) of the effective fan beam includes a number of rays 82 comprising adjacent triangular zones of equal angle about the focal spot 45. To a first approximation, each ray 82 measures a an equal area of the patient 14. Ideally, then, each ray 82 should map to a single picture element (pixel) of a two-dimensional projection image constructed of the data collected in the scan. This mapping of rays 82 to pixels, preserves the local spatial fidelity of the image and prevents distortion in the quantitative values assigned to each pixel such as may be area sensitive. For example, if the attenuation of the energy of the fan beam 48 by the patient 14 indicates bone mineral content (BMC) in grams, the diagnostically useful quantity of bone mineral density (BMC) in g/cm² requires an accurate preservation of area information. This equal area pixel mapping is advantageous in the measurement of BMD.

Nevertheless, the spatial periodicity of the rays 82 will not in general match that of the detector elements 47 of the detector array 50. For example, if the outermost ray 82 of a fan beam 48(a) exactly subtends the outermost detector element 47(a) of the detector array 50, a more centrally located ray 82 will subtends subtend less than the area of a more centrally located detector element 47(e). If the raw data from the detector elements 47 is directly mapped to pixels of an image, area distortion will occur. Further, the distance of the outermost detector elements 47(a) from the focal spot 45 will typically be greater than that of the more centrally located detector elements 47(e). This distance variation will cause magnification distortion, as generally discussed above.

Accordingly, referring also to FIGS. 9 and 10, the data obtained from each detector element 47 is adjusted by a projection process to pixels in a non-planar image surface. During the scanning process, the data from each detector element 47 of the detector array 50 is collected in a matrix 75 having elements 77 associated with a given coordinate in the scan (with respect to the table 12) and a row and column in the matrix 75. Generally the rows of the matrix 75 will correspond to variations in the transverse coordinate of the data of the scan, and the columns will correspond to variations in the longitudinal coordinate of the data of the scan. A single row 81 represents the data for one position of the effective fan beam and the values of the data of that row 81 provide a projection signal 83. The value of the projection signal 83 is a stepwise continuous function of the number of the detector element.

Referring to FIGS. 8 and 10, the projection signal 83 may be projected to a curved image surface 90 having pixels 80 exactly subtending one ray 82 each. This mapping 92 is accomplished by partitioning the projection signal 83 according to the geometric relationship between the pixels 80 of the curved image surface 90 and the detector elements 47. For example, pixel 80(a) spans the projection signals produced by detector elements 47(c) and 47(b). Accordingly the value of pixel 80(a) is simply the average value of the detector signals within the span or a weighted average of the values of the projection signals 83 for detector elements 47(c) and (d) in proportion to how much they are overlapped. This projection process is repeated for each pixels pixel 80 of the image surface 90 until all the data has been projected.

If a curved image surface 90 is adopted equal to the radius of curvature focal spot 45 for that image surface 90, then moving the image surface 90 up and down along the fan beam axes 49 is simply a uniform scaling of the image. Preferably, the absolute height of the image surface 90 will be selected to approximately bisect the height of the patient 14. This will reduce the magnitude of the magnification error in the image caused by the diverging rays 82 of the fan beams 48 by reducing the absolute value of the distance between volume elements 66-68 of the patient 14 from the image surface 90. The use of a an image surface 90 curved about the focal spot 45 also eliminates height dependency of the areas of overlap 69' as discussed with respect to FIG. 11, because in the projection geometry the overlap will have constant transverse location in the image surface 90.

Referring now to FIG. 10, in an anterior/posterior scan of the patient 14, where the fan beam axis 49 is oriented vertically, the data of a rectilinear matrix 75 of data elements 77 is acquired. Each element 77 of the matrix 75 has a location corresponding to a particular path of a ray of the fan beam 48 through the patient 14, and to one detector element 47 of the detector array 50, and each data element 77 has a value related to the attenuation of that ray as it passes through the patient 14. As is understood in the art, the computer 56 stores the pixel values and their relative spatial locations so that each data element 77 may be readily identified to the particular area of the patient 14 at which the data of the data element 77 was collected.

According to well understood dual energy imaging techniques, the value of each data element 77 is derived from measurements of the patient at two energy levels and thus provides information indicating the composition of the material causing that attenuation. In particular, the data element value indicates the bone mineral content of the volume of the patient corresponding to the data element location.

The above description has been that of a preferred embodiment of the present invention. It will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.

Claims

1. An imaging system for obtaining diagnostic images of a patient comprising:

a radiation source for directing a fan beam of radiation toward the patient, the fan beam diverging about a radiation axis but substantially within a beam plane from a focal spot;

a radiation detector opposing the radiation source along the radiation axis for receiving the diverging beam of radiation after passage through the patient to produce a projection signal indicating the attenuation of the beam of radiation for multiple rays within the beam;

a translating means for translating the radiation axis along a first and second path across the patient, the first and second paths being spaced apart and substantially perpendicular to the beam plane;

a repositioning means for rotating the radiation axis about the focal spot by a displacement angle, within the beam plane so as to move the radiation axis from the first path to the second path; and

means for combining the projection signal obtained along the first and second path to produce a two dimensional projection image.

2. The imaging system of claim 1 wherein the repositioning means rotates the displacement radiation axis without displacement of the focal spot within the beam plane with respect to the patient as measured when the radiation axis is aligned with one of the first and second paths.

3. The imaging system of claim 1 wherein the fan beam has a fan beam angle measured within the beam plane and the repositioning means rotates the displacement radiation axis by the fan beam angle.

4. The imaging system of claim 1 wherein the radiation detector is a linear array of detector elements, each subtending a first width of the fan beam along the linear array, and wherein the projections projection signals includes a plurality of elements element signals from each element, the imaging system including:

a projector for mapping the element signals to pixels of a non-planar image surface generally normal to the radiation axis, each pixel subtending second widths of the fan beam varying from the first widths.

5. The imaging system of claim 4 wherein the non-planar image surface is a section of a cylinder having a constant radius equal to the distance between the surface and the radiation source to substantially bisect the patient.

6. An imaging system for obtaining diagnostic images of a patient comprising:

a radiation source for directing a fan beam of radiation toward the patient, the fan beam diverging about a radiation axis but substantially within a beam plane from a focal spot;

a linear array of detector elements opposing the radiation source along the radiation axis, each detector element subtending a first width of the fan beam along the linear array, the linear array for receiving the diverging beam of radiation after passage through the patient to produce a projection signal which includes a plurality of element signals corresponding to the detector elements and indicating the attenuation of the beam of radiation for given rays within the beam; and

a projector for mapping the element signals to pixels of a non-planar image surface generally normal to the radiation axis, each pixel subtending second widths of the fan beam varying from the first widths.

7. The imaging system of claim 6 wherein the non-planar image surface is a section of a cylinder having a constant radius equal to the distance between the surface and the radiation source the radius selected to substantially bisect the patient.

8. The imaging system of claim 1 wherein the means for combining provides a weighting of the projection signal of areas of overlap between areas of the fan beam when the radiation axis scanning of the first and second paths. 9. The imaging system of claim 1 wherein the imaging system is a bone densitometer. 10. The imaging system of claim 9 wherein the bone densitometer is a dual energy bone densitometer. 11. The imaging system of claim 9 wherein the radiation source is a polychromatic x-ray source. 12. The imaging system of claim 9 wherein the fan beam is scanned in a raster pattern over the patient, the raster scan formed of the first and additional paths wherein the number of paths and the separation of the paths is chosen to ensure complete illumination of the total body of the patient.

13. The imaging system of claim 1 wherein the repositioning means rotates the radiation axis about the focal spot by a displacement distance within the beam plane while translating the focal spot by displacement distance within the beam plane so as to move the radiation axis from the first path to the second path.