U.S. patent application number 11/438053 was filed with the patent office on 2006-09-21 for computed tomography with increased field of view.
This patent application is currently assigned to The Board of Trustees of the Leland Stanford Junior University. Invention is credited to Rebecca Fahrig, Norbert J. Pelc, Edward G. Solomon.
Application Number | 20060210015 11/438053 |
Document ID | / |
Family ID | 36576552 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060210015 |
Kind Code |
A1 |
Pelc; Norbert J. ; et
al. |
September 21, 2006 |
Computed tomography with increased field of view
Abstract
A volumetric computed tomography system with a large field of
view has, in a forward geometry implementation, multiple x-ray
point sources emitting corresponding fan beams at a single detector
array. The central ray of at least one of the fan beams is radially
offset from the axis of rotation of the system by an offset
distance D. Consequently, the diameter of the in-plane field of
view provided by the fan beams may be larger than in a conventional
CT scanner. Any number of point sources may be used. Analogous
systems may be implemented with an inverse geometry so that a
single source array emits multiple fan beams that converge upon
corresponding detectors.
Inventors: |
Pelc; Norbert J.; (Los
Altos, CA) ; Fahrig; Rebecca; (Palo Alto, CA)
; Solomon; Edward G.; (Menlo Park, CA) |
Correspondence
Address: |
LUMEN INTELLECTUAL PROPERTY SERVICES, INC.
2345 YALE STREET, 2ND FLOOR
PALO ALTO
CA
94306
US
|
Assignee: |
The Board of Trustees of the Leland
Stanford Junior University
NovaRay
|
Family ID: |
36576552 |
Appl. No.: |
11/438053 |
Filed: |
May 18, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11039716 |
Jan 19, 2005 |
7062006 |
|
|
11438053 |
May 18, 2006 |
|
|
|
Current U.S.
Class: |
378/9 |
Current CPC
Class: |
A61B 6/032 20130101;
G01N 2223/419 20130101; A61B 6/4014 20130101; G01N 23/046 20130101;
G01T 1/2985 20130101 |
Class at
Publication: |
378/009 |
International
Class: |
A61B 6/00 20060101
A61B006/00; G01N 23/00 20060101 G01N023/00; G21K 1/12 20060101
G21K001/12; H05G 1/60 20060101 H05G001/60 |
Claims
1. A method of computed tomography comprising: providing multiple
x-ray point sources and a detector array; emitting from the x-ray
point sources corresponding fan beams directed at the detector
array; acquiring x-ray data at the detector array; and rotating the
x-ray point sources and the detector array around a rotational
axis; wherein the fan beams have corresponding central rays
connecting the corresponding point sources to a midpoint of the
detector array, wherein the central ray of at least one of the fan
beams is offset from the rotational axis by a substantial offset
distance D.
2. The method of claim 1 wherein the fan beams have corresponding
sets of rays, each ray having a radial offset distance from the
rotational axis, wherein the radial offset distances of the rays in
the sets are selected such that there is an overlap of radial
offset distances between the sets.
3. The method of claim 1 wherein at least one of the fan beams has
a central ray that is radially offset from the rotational axis by a
distance D approximately equal to or greater than a width of the
corresponding fan beam near the rotational axis.
4. The method of claim 1 wherein a diameter of a field of view
provided by the fan beams is larger than an extent of the detector
array.
5. The method of claim 1 wherein none of the central rays passes
through the rotational axis.
6. The method of claim 1 wherein the central rays all intersect at
a midpoint of the detector array.
7. The method of claim 1 wherein rays in one of the fan beams have
offset distances from the rotational axis that differ from offset
distances of rays in an adjacent one of the fan beams by
approximately 2R/N, where R is a radius of a field of view provided
by the fan beams and N is the number of point sources.
8. A computed tomography system comprising: a detector array;
multiple x-ray point sources for generating corresponding fan beams
directed at the detector array; wherein the x-ray point sources and
the detector array are capable of being rotated together around a
rotational axis; and wherein the fan beams have corresponding
central rays connecting the corresponding point sources to a
midpoint of the detector array, wherein the central ray of at least
one of the fan beams is offset from the rotational axis by a
substantial offset distance D.
9. The system of claim 8 wherein the fan beams have corresponding
sets of rays, each ray having a radial offset distance from the
rotational axis, wherein the radial offset distances of the rays in
the sets are selected such that there is an overlap of radial
offset distances between the sets.
10. The system of claim 8 wherein at least one of the fan beams has
a central ray that is radially offset from the rotational axis by a
distance D approximately equal to or greater than a width of the
corresponding fan beam near the rotational axis.
11. The system of claim 8 wherein a diameter of a field of view
provided by the fan beams is larger than an extent of the detector
array.
12. The system of claim 8 wherein none of the central rays passes
through the rotational axis.
13. The system of claim 8 wherein the central rays all intersect at
a midpoint of the detector array.
14. The system of claim 8 wherein rays in one of the fan beams have
offset distances from the rotational axis that differ from offset
distances of rays in an adjacent one of the fan beams by
approximately 2R/N, where R is a radius of a field of view provided
by the fan beams and N is the number of point sources.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of copending U.S. patent
application No. Ser. 11/039716 filed Jan. 19, 2005, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to systems and
methods for computed tomography. More specifically, it relates to
improved techniques for increasing the field of view in computed
tomography.
BACKGROUND OF THE INVENTION
[0003] In a conventional third-generation computed tomography (CT)
system a single x-ray source 100 generates a fan beam 102 directed
at an extended detector array 104, as shown in the cross-sectional
view of FIG. 1. Fan beam 102 has a collection of rays diverging
from source 100 at a divergence angle .alpha., as shown. A system
of this type, where the fan beam diverges from a single point
source to a large array of detectors, is said to have a forward
geometry. In an inverse geometry system, the, point source is
exchanged for a small array of detectors (or a single detector) and
detector array is exchanged for a source array, so that the set of
measurement rays converge at the detectors. In the context of the
present invention, forward and inverse geometry systems have
similar geometrical properties. Thus, the common geometrical
properties of both forward and inverse geometries can be described
by considering just the forward geometry case.
[0004] The rays of the fan beam 102 include a central ray 108 which
is defined to be the ray from the point source 100 that intersects
a midpoint 110 of the detector array 104. (In the corresponding
inverse geometry, the central ray is the ray from the midpoint of
the source array to the mid-point of the small detector.) Note that
in this conventional system the central ray 108 passes through (or
very close to) a rotational axis 106 of the system. During
operation of the system, source 100 and detector 104 are rotated
around rotational axis 106 to various rotated positions. For
example, FIG. 1 shows a rotated position corresponding to a
rotation of the central ray 108 by an angle .theta.. As the source
100 and detector 104 rotate, fan beam 102 also rotates, providing
the system with the capability to acquire x-ray transmission data
at various angles from which an image is reconstructed. The
rotational angles .theta.must cover a sufficient range so as to
allow objects to be properly reconstructed. In this case, the range
of .theta. values must be at least .alpha. plus 180 degrees. A
field of view (FOV) 114 of the system is the region that is always
exposed to the fan beam. Thus, for example, any portion of an
object that is positioned within FOV 114 will be viewed from all
rotational angles of the system. Outside of FOV 114, however, image
data is not available at some rotational angles. As a result, CT
systems are designed to reconstruct three-dimensional
representations of objects within the FOV of the system. (Here the
FOV is the in-plane FOV, i.e., the FOV within the cross-sectional
plane of the fan beam which is perpendicular to the rotational
axis.)
[0005] In the conventional CT system shown in FIG. 1 the size of
FOV 114 is limited by the size of the detector array 104. In
particular, the diameter of FOV 114 is always significantly less
than the extent of the detector array. An increased FOV can be
provided by increasing the size of the detector array, as shown in
FIG. 2. A source 200 emits a fan beam 202 toward a larger detector
array 204. Fan beam 202 has a central ray 208 which passes through
(or very close to) rotational axis 206 and intersects a midpoint
210 of detector array 204. Due to the increased size of the
detector array 204, the system has an increased FOV 214 as compared
to the smaller FOV 212 provided by the smaller detector.
(Similarly, an inverse geometry system also has an increased FOV if
it has an increased source array size.) Although the FOV of a CT
system can be increased using a larger detector array, increasing
the size of the array often introduces significant technical
difficulty and expense.
[0006] Another drawback of this CT system design is that the source
and detector must rotate through a large angle to acquire images
from a sufficiently large range of angles. If a patient moves
during the rotation, the image data from different angles will not
be consistent, resulting in artifacts and errors in the
reconstructed three-dimensional representation. Alternative CT
system designs (such as U.S. Pat. No. 5,966,422 to Dafni et al. and
U.S. Pat. No. 4,196,352 to Berninger et al., which are incorporated
herein by reference) have been proposed in an attempt to overcome
this disadvantage. For example, FIG. 3 shows a CT system with
multiple sources 300, 302, 304 and multiple corresponding detector
arrays 306, 308, 310. The sources 300, 302, 304 emit corresponding
fan beams 312, 314, 316 having respective central rays 318, 320,
322 all intersecting at a point coincident with (or very close to)
an axis of rotation 324. Because the three source and detector
pairs simultaneously provide image data at different angles, the
required rotational angle is reduced by three, helping to mitigate
problems caused by patient movement during scanning. However, the
field of view 326 of this system suffers from the same problem as
the conventional single source-detector system of FIG. 1. To
increase the FOV of this system, the detector array sizes must be
increased. In any case, the FOV is always less than the detector
size. Moreover, despite the use of three detector arrays and
sources, there is no FOV increase compared to the single
source-detector system of FIG. 1. (The same disadvantages apply to
the analogous inverse geometry system.)
[0007] An alternative CT system that provides a slight increase in
FOV is shown in FIG. 4 (see also U.S. Pat. No. 5,430,297 to Hawman,
which is incorporated herein by reference). A single source 400
emits a fan beam 402 directed at a detector array 404. A central
ray 406 of fan beam 402 intersects a midpoint 410 of detector 404.
In contrast to the conventional system of FIG. 1, however, the fan
beam 402 of source 400 is offset from centerline 412 so that the
central ray 406 is offset from the rotational axis 408 of the
system. Consequently, line 418 from source 400 passing through axis
408 intersects the detector 404 at a point 420 that is far from
midpoint 410. As the source 400 and detector 404 rotate around
rotational axis 408, the single fan beam also rotates around axis
408. Due to the offset of the fan beam, the FOV 414 of this system
is larger than the FOV 416 of a comparable system with no offset,
provided the system rotates through at least 360 degrees. The FOV
414, however, while larger than FOV 416, is still substantially
limited unless the detector array is quite large. In particular,
the diameter of the FOV of this system is always less than twice
that of the system of FIG. 1, and generally less than the extent of
the detector array. Moreover, the asymmetry of the system geometry
requires a rotation of at least 360 degrees, introduces
complexities to the data processing required to reconstruct a
representation of the object from the data collected at various
angles, and in general has non-uniform noise behavior. (The
analogous inverted system has similar limitations.)
SUMMARY OF THE INVENTION
[0008] The present invention provides improved CT systems and
methods that enjoy substantially increased FOV. The diameter of the
in-plane FOV of CT systems according to the present invention can
be larger than the in-plane extent of the detector (or source)
array. Thus, the invention provides CT systems with increased FOV
without the expense and complication of larger detector (or source)
array sizes required in the past.
[0009] According to one aspect of the invention, a method is
provided for volumetric computed tomography. In a forward-geometry
implementation, multiple x-ray point sources emit corresponding fan
beams at a single detector array at different corresponding times.
X-ray image data is acquired at the detector array as the x-ray
point sources and the detector are both rotated together around a
rotational axis. Each of the fan beams has a central ray passing
from the source to the midpoint of the detector. Thus, the central
rays of at least two fan beams intersect at the detector midpoint,
and the central ray of at least one fan beam is offset from the
rotational axis by an offset distance. The diameter of the in-plane
field of view provided by the combination of the fan beams is
preferably larger than an in-plane extent of the detector
array.
[0010] In some embodiments there are two sources, where at least
one source has a fan beam whose central ray is offset from the
rotational axis. The other source has a fan beam whose central ray
may pass through the rotational axis (i.e., have no offset) or may
be offset from the rotational axis.
[0011] In other embodiments, there are three or more sources, where
at least one source has a fan beam whose central ray is offset from
the rotational axis. The other sources have fan beams whose central
rays may pass through the rotational axis or may be offset from the
rotational axis. Additional embodiments include inverse geometry
analogues and generalizations of the principles to 3D systems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a cross-sectional illustration of a conventional
CT system having an x-ray point source emitting a fan beam toward a
detector array.
[0013] FIG. 2 is a cross-sectional illustration of a conventional
CT system similar to the system of FIG. 1 except with a larger
detector array to provide an increased field of view.
[0014] FIG. 3 is a cross-sectional illustration of a known CT
system similar to that of FIG. 1 except with multiple sources and
multiple corresponding detector arrays providing more efficient
scanning, but no increase in field of view.
[0015] FIG. 4 is a cross-sectional illustration of a known CT
system similar to that of FIG. 1 except with the single source
offset a small distance off axis, providing up to a factor of two
increase in field of view.
[0016] FIG. 5 is a cross-sectional illustration of a CT system
having two sources whose fan beams are both offset from the
system's rotational axis according to an embodiment of the present
invention.
[0017] FIG. 6 is a cross-sectional illustration of a CT system
having three sources, where two of the three fan beams are offset
from the system's rotational axis according to an embodiment of the
present invention.
[0018] FIG. 7 is a cross-sectional illustration of a CT system
having four sources whose fan beams are all offset from the
system's rotational axis according to an embodiment of the present
invention.
[0019] FIG. 8 is a cross-sectional illustration of an inverse
geometry CT system having one source array and two detectors,
wherein the two corresponding fan beams are offset from the
system's rotational axis according to an embodiment of the present
invention.
DETAILED DESCRIPTION
[0020] A volumetric CT system according to one embodiment of the
invention is illustrated in FIG. 5. Two x-ray point sources 500 and
502 emit corresponding fan beams 504 and 506 at a single detector
array 508 at different corresponding times. X-ray image data is
acquired from the detector array 508 to reconstruct a
representation of an object of interest. The x-ray point sources
500 and 502, as well as the detector 508 are rotated together
around a rotational axis 520 of the system. Consequently, fan beams
504 and 506 also rotate about axis 520. The fan beams 504 and 506
have corresponding central rays 510 and 512 that bisect the
detector array 508 at a midpoint 514. Because the multiple fan
beams are directed toward a common detector array from sources
having different locations, the central rays 510 and 512 have
different angular orientations and are radially offset from the
rotational axis 520 by a significant offset distance D, resulting
in a FOV 516 for the system which is significantly larger than the
limited FOV 518 of prior systems. There are prior art systems that
employ a technique called "focal-spot wobbling" to improve in-plane
sampling and reduce certain artifacts. In these systems, the focal
spot is rapidly moved a short distance, causing a displacement of
the central ray of less than 1% of the FOV of a single centered fan
beam. With respect to the present invention, the displacement
caused by focal spot wobbling is not significant. The offset
distance D of a fan beam is considered significant when it is
approximately equal to or larger than 25%, and preferably on the
order of 50% for the system of FIG. 5, of the width of the fan beam
near the rotational axis. Moreover, the diameter of the in-plane
field of view provided by the fan beams of this system can be
larger than an in-plane extent of the detector array 508. In prior
systems, the FOV is smaller than the size of the detector
array.
[0021] Another embodiment of the invention is illustrated in FIG.
6. Three x-ray point sources 600, 602, 604 emit three respective
fan beams 606, 608, 610 directed at a single detector array 618.
Because a single detector is used with multiple sources, the
sources emit their corresponding beams at different corresponding
times. Fan beams 606, 608, 610 have three respective central rays
612, 614, 616 with different angular orientations. Although central
ray 614 of beam 608 passes through rotational axis 622, central
rays 612 and 616 of beams 606 and 610, respectively, are offset
from rotational axis 622 by an offset distance D. Central rays 612,
614, 616 intersect the detector array 618 at a midpoint 620. Due to
the novel design, FOV 624 can be significantly larger even than the
extent of the detector array 618. As evident from the figure, the
FOV 624 is nearly three times larger than the FOV 626 of a prior
art CT system. To obtain a FOV of comparable size, the prior art CT
system would require a significantly larger detector array,
increasing the expense and complexity of the system.
[0022] In another embodiment of the invention, the system of FIG. 6
is modified by eliminating source 604, which yields a system in
which fan beam 608 has a central ray that passes through the
rotational axis 622, while fan beam 606 has a central ray that is
offset from the rotational axis 622. Thus, at least one of the two
fan beams (i.e., in this case beam 606) has a central ray that is
offset. The FOV 624 for this embodiment is larger than that of the
embodiment described above in relation to FIG. 5 even though they
both use the same size detector array and fan beam. In addition,
the edges of the fan beams in this system do not intersect at the
rotational axis as they do in the system of FIG. 5, avoiding
problems with discontinuities there. Note, however, that this
embodiment with only two fan beams requires a 360 degree rotation
to obtain the maximum FOV.
[0023] In yet another embodiment of the invention, four sources
700, 702, 704, 706 emit four respective fan beams 708, 710, 712,
714 directed at a single detector array 724, as shown in FIG. 7.
Fan beams 708, 710, 712, 714 have respective central rays 716, 718,
720, 722 which intersect detector 724 at a midpoint 726 and are
offset from the rotational axis 728 by offset distances D.sub.1
(for central rays 718 and 720) and D.sub.2 (for central rays 716
and 722). Note that the diameter of FOV 730 is almost twice as
large as the extent of detector array 724. In contrast, prior art
systems such as that shown in FIG. 1 have a FOV diameter on the
order of half the size of the detector array.
[0024] In an alternate embodiment, the system of FIG. 7 is modified
by eliminating either one or both of the sources 704 and 706. The
system still will have multiple sources, at least one of which has
a fan beam whose central ray is offset from the axis of rotation.
Elimination of sources, however, may require increased rotation of
the system to acquire sufficient data.
[0025] The system of FIG. 7 can also be modified by adding still
more sources, providing a further increase in the FOV of the
system. Although providing multiple offset sources increases the
FOV, the greatest FOV increase per additional source is obtained
when there are fewer sources. Thus, it is preferred that the number
of sources is an integer from two to ten. Although not necessary,
it is most preferred to have an odd number of sources, where one of
the sources has a fan beam whose central ray passes through the
rotational axis and all the other sources have fan beams whose
central rays are offset from the rotational axis. An odd number of
sources is preferred over an even number of sources in order to
avoid sampling discontinuities at the center of rotation where
edges of two innermost fan beams may intersect. It is possible,
however, for a system with an even number of sources to avoid this
problem by increasing the overlap between the two innermost fan
beams, i.e., slightly decreasing the displacements of their central
rays from the rotational axis.
[0026] The offsets of the central rays of the fan beams provide the
system with a diversity of radial samples. In embodiments where N
fan beams are symmetrically placed about the center of rotation and
are uniformly spaced, the rays in one fan preferably have offset
distances from the rotational axis that differ from the offset
distances of the rays in an adjacent fan beam by approximately
2R/N, where R is the radius of the FOV. In other embodiments,
however, the rays in the fan beams are not necessarily offset
uniformly.
[0027] It should also be noted that in alternate embodiments the
distances from the sources to the detector array may be different
from each other. In addition, the distances from the sources to the
axis of rotation may be different from each other.
[0028] In view of the above description, those skilled in the art
will appreciate that various inverse geometry systems analogous to
the systems described above may be provided by replacing the
multiple point sources with multiple small detectors (e.g., small
detector arrays) and replacing the detector array with a source
array whose collimators provide x-rays directed at the multiple
detectors. (Examples of inverse geometry systems can be seen in US
Patent Application Publication 20030043957 to Pelc and US Patent
Application Publication 20030043958 to Mihara et al., which are
incorporated herein by reference.) For example, a preferred
embodiment of the present invention having an inverse geometry is
shown in FIG. 8. A single source array 818 is comprised of a large
number of source locations, each with its own collimator within the
collimator array 828. The collimators in 828 are designed to limit
the x-rays so that they are directed at the three detectors 800,
802, 804. For example, each collimator may be designed to
simultaneously illuminate all three detector arrays. An alternative
collimator design is to dedicate each source position and
corresponding collimator to direct x-rays to just one of the
detector arrays, alternating adjacent collimators between the three
detectors. In one implementation, the source array has 100 source
positions in the lateral direction, separated from each other by
2.5 mm, and the detector has 50 detector elements in the lateral
dimension, separated from each other by about 1 mm. The net effect
of the source array 818 is to produce three fan beams 806, 808, 810
directing x-rays to converge upon three corresponding detectors
800, 802, 804. As in the forward geometry system, each fan beam
806, 808, 810 has a corresponding central ray 812, 814, 816 that is
defined as the line connecting the midpoint 820 of the source array
to the center of the corresponding detector array 800, 802, 804. A
conventional inverted fan beam system has a single detector 802 and
corresponding inverted fan 808 which determines the limited FOV
826. In contrast, the present embodiment has additional detectors
800, 804 and corresponding fan beams 806, 810 providing increased
FOV 824. Fan beams 806, 810 have respective central rays 812 and
816 which are both offset from the axis of rotation 822 by radial
distance D. More generally, note that each of the three inverted
fan beams has a set of rays, each having a radial offset distance
from the axis of rotation, the central ray providing one example of
such a ray and its offset distance D. Ideally, the radial distances
for the rays in all the beams are selected so as to have these
three sets of radial distances overlapping slightly. Moreover,
ideally the distribution of radial distances from all the fan beams
should be relatively smooth and relatively uniform. The set of
radial distances sampled by the set of three detector arrays
produces FOV 824.
[0029] In the forward as well as the inverse geometry embodiments
described above, the FOV has been described as a two dimensional
field of view. As will be clear to one of skill in the art, the
present invention is also useful in volumetric or 3D systems.
(Examples of various known 3D CT systems are disclosed in US Patent
Application Publication 20030043957 to Pelc, U.S. Pat. No.
6,229,870 to Morgan, U.S. Pat. No. 6,654,440 to Hsieh and U.S. Pat.
No. 5,966,422 to Dafni et al., which are incorporated herein by
reference.) For example, the systems of FIGS. 5, 6, and 7 could be
modified so that the detector is a 2-dimensional detector and the
fan beams are cone beams. In one rotation of the gantry, the
systems would be able to collect data to reconstruct a 3D volume,
wherein the present invention is being used to increase the field
of view in the trans-axial direction (i.e., in the plane of the
drawings). Similarly, the system of FIG. 8 could be a volumetric CT
scanner if the source array is a 2D array, having an extent into
the plane of the drawing, and each detector array is a 2D array
having an extent into the plane of the drawing. In a preferred
embodiment of this latter system the extents of the source and
detector arrays into the plane of the drawing are approximately the
same.
[0030] In operation, the systems described above are used in a
manner similar to conventional CT systems. Thus, an object of
interest is placed within the FOV of the system and x-ray
projection data are acquired at various rotational angles. The
projection data is then processed by a computer to produce
representations (e.g., images) of the object which may be displayed
for viewing by a radiologist in the case of medical diagnostic
applications. The systems could also be used for other
applications, such as non-destructive testing or baggage
inspection.
[0031] The reconstruction algorithms used in CT systems for
processing projection data (e.g., see U.S. Pat. No. 5,825,842 to
Taguchi, which is incorporated herein by reference) may be adapted
to operate with systems employing the principles of the present
invention. For the inverse geometry system, one possible
reconstruction algorithm re-bins the data into parallel ray
projections, with the data from all the detector arrays being used
together in the re-binning. Forward geometry systems would process
data analogously, re-binning the data into parallel ray
projections.
[0032] The present invention also provides the possibility for
other modified reconstruction techniques. For example, in a system
such as shown in FIGS. 5, 6, or 7, consider the case where two fan
beams are produced by two sources that are positioned at the same
distance from the axis of rotation (e.g., fan beam pair 504 and 506
in FIG. 5) and are mounted with an angle .delta. between them, so
that after the gantry rotates by an angle .delta. the second source
is at the same location that the first source was in prior to the
rotation. As a result, the fan beam data produced by the first
source at gantry angle .theta. can be combined with the data
produced by the second source at gantry angle .theta.+.delta. to
produce a larger fan beam for reconstruction. Extensions and
variations of this approach for the systems of FIGS. 6 or 7, or for
inverse geometry systems, will be evident to those skilled in the
art.
* * * * *