Method and apparatus for piece-wise radiographic scanning

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.

FIELD OF THE INVENTION 
The present invention relates generally to radiographic instruments and 
more particularly to an apparatus for assembling broad area images from 
narrow beam radiographic scans. 
BACKGROUND OF THE INVENTION 
Scanning radiographic equipment differs from conventional radiography in 
that it employs a narrowly collimated beam of radiation, typically x-rays 
formed into, for example, a fan beam, rather than a broad area cone beam. 
The small beam size used in scanning radiographic equipment allows 
replacement of an image forming sheet of radiographic film, used with 
conventional radiographic equipment, with a small area array of detector 
elements. 
The detector elements receiving the transmitted radiation produce 
electrical signals which may be converted to digital values by an analog 
to digital converter for the later development of an image or for other 
processing by computer equipment. The ability to quantify the measurement 
of the transmitted radiation, implicit in the digitization by the analog 
to digital converter, allows not only the formation of a radiographic 
"attenuation" image but also the mathematical analysis of the composition 
of the attenuating material by dual energy techniques. See generally, 
"Generalized Image Combinations in Dual KVP Digital Radiography", by 
Lehmann et al. Med. Phys. 8(5) September/October 1981. Such dual energy 
techniques quantitatively compare the attenuation of radiation at two 
energies to distinguish, for example, between bone and soft tissue. Dual 
energy techniques allow the measurement of bone mass, such measurement 
being important in the treatment of osteoporosis and other bone diseases. 
The limited area of the beam of radiation used in scanning radiographic 
systems allows the use of limited area detectors permitting high 
resolution with relatively lower cost. The limited area of the detectors, 
however requires that the beam be scanned along several adjacent paths if 
large area images are to be constructed. Typically, a fan beam will be 
scanned in a raster pattern over the area to be measured, each line of the 
scan separated by somewhat less than the width of fan beam, to ensure 
complete illumination of the entire volume of the imaged object, with the 
directions of scanning being generally perpendicular to the direction of 
the radiation and the plane of the fan beam. 
Images formed by a scanning radiographic system are potentially more 
accurate than those produced by a typical broad beam radiograph system. 
This accuracy arises from the limited divergence, in the scanning 
direction, of the rays of the fan beam, as compared to a broad area cone 
beam. This narrow collimation of the fan beam reduces "parallax" in the 
projected image, particularly of anatomical planar surfaces that are 
nearly parallel with the plane of the fan beam--such as the superior and 
inferior borders of the vertebrae in the spine when the scanning 
directions is along the superior-inferior axis of the body. 
Morphological measurements of the vertebrae, and other structures, which 
benefit from reduced parallax are used to evaluate various dimensions of a 
vertebra to detect crushing or other deformation that are one element of 
certain bone diseases such as osteoporosis. See e.g. Minne et al., "A 
Newly Developed Spine Deformity Index (SDI) to Quantitate Vertebral Crush 
Factors in Patients with Osteoporosis," Bone and Mineral, 3:335-349 
(1988); J. C. Gallagher et al, "Vertebral Morphometry: Normative Data," 
Bone and Mineral, 4:189-196 (1988); Hedlund et al, "Vertebral Morphometry 
in Diagnosis of Spinal Fractures," Bone and Mineral, 5:59-67 (1988); and 
Hedlund et al, "Change in Vertebral Shape in Spinal Osteoporosis," 
Calcified Tissue International, 44:168-172 (1989). Automatic techniques 
for morphological measurements of bone are described in U.S. patent 
application Ser. No. 07/944,626 filed Sep. 14, 1992 and entitled: "Method 
for Analyzing Vertebral Morphology Using Digital Radiography" assigned to 
the same assignee as the present application and hereby incorporated by 
reference. 
Nevertheless, images developed with scanning fan beam equipment can include 
certain distortions or artifacts. In particular, it has been noted that 
objects at the interface between two adjacent scan paths contain a 
blurring or distortion in a direction perpendicular to the scan path. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for constructing 
broad area images from a sequence of narrow fan beam scans. The invention 
recognizes that a source of image artifacts in combining narrow, fan beam 
scans is the varying amount of overlap between the fan beams when the axes 
of the fan beams are held parallel. This overlap causes some volume 
elements of the patient to be measured with rays at two different angles. 
The amount of overlap depends on the height of the structure being imaged, 
as measured along the path of the fan beams, and thus cannot, in general, 
be determined or corrected in a two dimensional image. 
The present invention varies the angle of the axis of each fan beam so as 
to create a larger, effective fan beam of arbitrary width and to eliminate 
any height dependant overlap. The elimination of height dependent overlap 
ensures that each volume element of the patient is measured by rays at 
only one angle. Specifically, the invention employs an imaging system 
having a radiation source directing a fan beam of radiation toward the 
patient, where the fan beam diverges about a radiation axis, substantially 
within a beam plane, from a focal spot. A radiation detector opposing the 
radiation source along the radiation axis receives 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. 
The radiation axis may be moved along a first and second path across the 
patient, the first and second paths being spaced apart and substantially 
perpendicular to the beam plane. In moving between the first and second 
paths of the scan, the radiation axis is rotated about the focal spot by a 
displacement angle, within the beam plane. The signals obtained along the 
first and second path are then combined to produce a two dimensional 
projection image. 
It is thus one object of the invention to reduce image artifacts, caused by 
combining image data obtained from multiple scannings of a narrow fan 
beam. Creating a larger, effective fan beam eliminates areas of overlap or 
produces areas of overlap that, with appropriate projections, are constant 
regardless of the height of the imaged structure, and which therefore can 
be eliminated by a constant weighting factor applied to the data of the 
overlapping area. 
The radiation detector may be a linear array of detector elements, each 
subtending a first width of the fan beam along the linear array, where the 
projections signals include a plurality of elements signals from each 
detector element. A projector may be employed to map 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. The non-planar image surface may be positioned midway along 
the height of the patient as measured along the radiation access. 
It is thus another object of the invention to reduce the distortion caused 
by the divergence of rays in both the narrow measuring fan beams and the 
larger, effective fan beam by mapping the element signals to pixels of a 
non-planar surface so that each such pixel represents rays of the fan beam 
passing through equal areas of the patient. This reduces variations, for 
example, in bone mineral density measurements, which are sensitive to 
distortion in the measured area. 
It is another object of the invention to reduce the magnitude of 
magnifications induced errors on the projected image. By positioning the 
non planar image surface to approximately bisect the body, distance 
between the imaging plane and any particular structure in the body, such 
as affects magnifications, is reduced to a minimum. 
The foregoing and other objects and advantages of the invention will appear 
from the following description. In the description, reference is made to 
the accompanying drawings which form a part hereof and in which there is 
shown by way of illustration, a preferred embodiment of the invention. 
Such embodiment does not necessarily represent the full scope of the 
invention, however, and reference must be made therefore to the claims 
herein for interpreting the scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a bone densitometer 10 constructed according to the 
present invention includes a table 12 for supporting a patient 14 in a 
sitting position prior to and after an examination (as shown) or in a 
supine position along the table's longitudinal axis 16 during an 
examination. The table 12 is constructed of epoxy impregnated carbon fiber 
laminated over a foamed plastic core. This combination of materials is 
extremely light, and generally radiolucent and stiff. Further, the 
attenuation is extremely uniform so as to prevent the introduction of 
artifacts into the radiographic images. The table 12 has a generally 
linear cross-section along the longitudinal axis 16 and an upwardly 
concave cross-section along a transverse axis 18 perpendicular to the 
longitudinal axis 16. Thus, the table 12 is a trough-shaped sheet whose 
transverse curvature provides additional resistance to longitudinal 
bending. 
Support pillars 20 hold either longitudinal end of the table 12. The 
support pillars 20 are separated by a distance greater than the typical 
height of the patients to be examined so that the support pillars 20 do 
not obstruct the scanning process nor attenuate the measuring radiation. 
The longitudinal stiffness of the table 12 allows it to bridge the 
distance between the pillars 20 as an unsupported horizontal span thereby 
eliminating additional radiation attenuating structure. 
In one embodiment shown in FIG. 2(b) the transverse width of the table 12 
varies along its longitudinal extent being widest near the support pillars 
20, and thus near the head and feet of the patient 14 when the patient 14 
is in the supine position on the table 12, and narrowest at the 
mid-portion of the table 12 corresponding generally to the area of the 
patient's vertebrae. This narrowing of the table 12 is in the form of two 
rounded notches 24 extending inward toward the center of the table from 
either transverse edge and imparting to the table an hourglass shape as 
viewed along a vertical axis 22 perpendicular to the longitudinal and 
transverse axes 16 and 18 respectively. 
Referring now to FIGS. 1, 3(a) and 3(b), support pillars 20 extend 
vertically downward around upward extending posts 26, the latter which are 
attached, at their bottom ends, to a bed 28 supporting the densitometer 
10. The support pillars 20 each include a horizontal architrave 21, 
extending the width of the table 12 and attached to a respective end of 
the table 12, and vertical channel shaped casing 23 surrounding the posts 
26 to vertically slide in engagement with the posts 26 guided by a set of 
rollers 17 attached to the casing 23. The casings 23, and hence the 
support pillars 20, may be positioned vertically as driven by actuators 30 
each comprising a nut 27 attached to an outer casing wall and a lead screw 
29 received at one end by the nut 27 and at the other end by a right 
angled drive 25 attached beneath the nut to the bed 28. A common drive 
shaft 31 connects each right angled drive 25 to a single stepper motor 
(not shown) so that rotation of the drive shaft 31 turns the right angled 
drives 25 and hence the lead screws 29 in tandem so as to raise and lower 
the table 12 on pillars 20 while maintaining the table's horizontal 
attitude. The number of steps made by the stepper motor is simply related 
to the change in table height. 
Referring to FIG. 1 and 3(c) the bed 28 includes two longitudinal rails 32 
which form a track for supporting a transversely extending gantry pallet 
34, and which allow the gantry pallet 34 to be positioned longitudinally 
along substantially the entire length of the densitometer 10 (as indicated 
by longitudinal axis 16). 
The gantry pallet 34 includes transverse rails 33 carried by rollers (not 
visible) fitting within the rails 32 and motivated by a stepper motor 
driven flexible belt 35. Riding on the rails 33 of the gantry pallet 34 is 
a slider 36 moved transversely by stepper motor driven belt 37. The slider 
36 supports a turntable 39 having a vertically oriented axis of rotation 
19 and rotated by mean 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 Diagnostic 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 .phi.. 
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.degree.-25.degree. 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 
array 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 arises 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 variation in overlap as a function of height within 
the patient 14. Accordingly, the present 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 beam 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 .phi., 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 
.phi. 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 dependance caused by triangular areas of overlap 69 
of FIG. 4, however, the height dependance 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 prevent 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 of 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)-(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 
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/cm.sup.2 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 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 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 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.