Digital imaging using a scanning mirror apparatus

Method and apparatus for generating a large-field, high-resolution digital image of an object by sequentially generating multiple optical scenes representative of different portions of the object, and then sequentially directing each optical scene onto an optical detector to generate multiple sub-images of the different portions of the object. Each scene is induced using a separate X-ray sub-beam, each of which is generated by spatially filtering a portion of an incident X-ray field with a spatial filter moving in concert with the scene-directing device. Once generated, the sub-images are combined to form the large-field, high-resolution image.

BACKGROUND 
This invention relates to imaging systems using electronic detectors, such 
as charge-coupled devices (CCDs). 
CCDs containing multi-pixel arrays are typically used in imaging systems to 
detect optical radiation and generate electronic images. When exposed to 
an optical field, each pixel in the array generates a light-induced 
electronic charge related to the intensity of the field. The charges of 
the array are then digitized and processed to generate the resultant 
digital image. Image resolution is dictated by the parameters of the CCD's 
pixel array; current high-end CCDs typically include 2048.times.2048-pixel 
arrays having dimensions of about 5 cm.times.5 cm, with each pixel having 
a dimension of about 25 .mu.m.times.25 .mu.m. CCDs having high spatial 
resolution are particularly useful, for example, in fields such as 
mammography and radiography, where sizes of the smallest lesions are 
typically between about 0.2 and 0.4 mm.sup.2. 
While CCDs represent effective means for generating digital electronic 
images, they are expensive and have limited spatial resolution and 
detector area. In particular, CCDs fabricated to detect large-area images 
with high spatial resolution are prohibitively expensive as the cost of a 
CCD scales non-linearly with the size of the pixel array. Moreover, the 
probability of defects within the pixel array is greatly increased for 
large-area CCDs. Lower-cost CCDs with adequate spatial resolution are 
commercially available, although the detector areas of these devices are 
often too small to effectively image a region of interest. 
There is a need, therefore, to generate detection methods and devices for 
producing large-area, high-resolution images at reasonable costs. Imaging 
techniques involving the combination of images detected with multiple 
CCDs, each having high spatial resolution but a small effective area, have 
been taught in the prior art. For instance, U.S. Pat. No. 5,138,642 
describes an X-ray system that generates multiple segments of an optical 
image; each segment is delivered to a separate CCD detector. The 
light-induced signals from each detector are then combined to produce an 
image containing each of the segments. 
SUMMARY OF THE INVENTION 
In general, in one aspect, the invention features a method for generating a 
large-field, high-resolution digital image of an object. The method 
includes the steps of: (a) generating a first optical scene representative 
of a first portion of the object, and then directing the first optical 
scene onto an optical detector to generate a sub-image of the first 
portion of the object; (b) generating a second optical scene 
representative of a second portion of the object, and then directing the 
second optical scene onto the optical detector to generate a sub-image of 
the second portion of the object; and, (c) combining all sub-images to 
form a large-field, high-resolution image of the object. 
The method may further include, prior to step (c), the steps of generating 
third and fourth optical scenes representative of, respectively, third and 
fourth portions of the object, and then sequentially directing these 
scenes onto the optical detector to generate sub-images of the third and 
fourth portions of the object. In this case, for example, step (c) 
includes combining the first, second, third, and fourth sub-images to form 
the large-field, high-resolution image. 
In preferred embodiments, the optical scene is directed onto an optical 
detector by a reflecting assembly. The reflecting assembly may be 
"scanned" (i.e., linearly translated or radially rotated) once the 
sub-image corresponding to the first optical scene is generated; this 
allows an additional optical scene to be directed onto the same optical 
detector. During scanning, the reflecting assembly is preferably rotated 
radially about a central axis, or longitudinally translated, to 
sequentially direct each optical scene onto the detector. Preferably, the 
reflecting assembly includes a first and second mirror, with the first 
mirror configured to receive and reflect the optical scene off the second 
mirror and onto the optical detector. In another embodiment, the 
reflecting assembly has a single mirror positioned to reflect the optical 
scene onto the optical detector. In this case, the mirror assembly is 
surrounded by other mirrors configured to receive and reflect the scene 
onto the rotatable (or translatable) mirror. In still other embodiments, a 
prism may be used in place of any of the mirrors. Preferably, in all 
cases, the directing step further includes the step of focussing the 
optical scene onto the optical detector with an imaging system (e.g., a 
lens or series of lenses). 
In other preferred embodiments, prior to the generating step, each of steps 
(a) and (b) includes spatially filtering first and second portions of an 
incident X-ray field to generate first and second X-ray sub-fields, and 
then projecting the first and second X-ray sub-fields (i) through the 
first and second portions of the object, and then (ii) through an 
X-ray-to-optical conversion screen to produce the first and second optical 
scenes. The step of spatial filtering includes positioning an opening of 
an X-ray-attenuating filter along the portion of the incident X-ray field 
in order to produce an X-ray field having a reduced spatial profile. 
Once the sub-image corresponding to the optical scene is generated, the 
spatial filter is scanned and re-positioned to filter a second portion of 
the incident X-ray field. During the scanning step, the spatial filter may 
be rotated about a central axis to sequentially spatially filter portions 
of the incident X-ray field. Alternatively, the filter may include 
openings spaced in a linear fashion. In this case, the filter is 
longitudinally translated to sequentially spatially filter portions of the 
incident X-ray field. Preferably, in all cases, scanning of the spatial 
filter is coordinated with the scanning of the reflecting assembly, 
allowing these two devices to move in concert. For instance, in a 
preferred embodiment, the scanning of the spatial filter and the 
reflecting assembly are coordinated so that following step (a), the 
spatial filter and mirror assembly are simultaneously rotated radially 
along the same central axis, with the opening of the spatial filter 
positioned to filter a second portion of the incident X-ray field, and the 
reflecting assembly positioned to direct a second optical scene onto the 
optical detector. In this case, between steps (a) and (b), the spatial 
filter and mirror assembly may be simultaneously rotated about 90.degree. 
along the same central axis. 
In other preferred embodiments, the optical detector is a CCD including a 
512.times.512, 1024.times.1024, or a 2048.times.2048-pixel array. CCDs 
having larger or smaller pixel arrays may also be used. 
In still other preferred embodiments, step (c), i.e., the "combining" step, 
is performed on a computer using a computer algorithm. The algorithm 
allows the computer to perform the steps of (i) adjusting the 
two-dimensional array of points of each sub-image so that, when combined, 
each sub-image is representative of a separate portion of the object; and 
(ii) combining the adjusted two-dimensional arrays of points to form the 
resultant complete image of the object. 
In another aspect, the invention features an apparatus for generating a 
large-field, high-resolution image of an object. The apparatus includes a 
means for generating optical scenes representative of at least two 
spatially separate portions of the object; a scanning and reflecting 
assembly configured to sequentially receive and then direct the optical 
scenes onto an optical detector to generate a sub-image for each optical 
scene; and, electronics (e.g., a computer) for combining the sub-images 
together to at least partially form the large-field, high-resolution image 
of the object. 
The inventions have many advantages. For example, the method allows rapid 
generation of high-quality images, such as those images taken during 
radiography or mammography, which can be used to detect with high accuracy 
lesions and tumors in patients. By generating and then combining multiple 
electronic sub-images to form a complete image, the method allows 
production of large-area, high-resolution images using a single, 
commercially available, small-area detector. Because the cost of CCD 
detectors scales disproportionately with the detector area, this method 
allows generation of high-quality images at a relatively low cost. 
Additionally, by using two detectors to form two sub-images, and then 
recombining the sub-images to form a single image, the method allows 
generation of very high-resolution images with relatively low-end, 
low-cost detectors. This allows, for example, detection of small-scale 
lesions which may otherwise not be resolved using conventional detecting 
methods.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring first to FIGS. 1 and 2A-2D, a digital imaging system 10 includes 
a reflecting assembly 12 which collects and steers an X-ray-induced 
optical scene 14 through an imaging system 42 and onto a detector 16. The 
scene is generated by passing an X-ray beam 22 from an X-ray source 24 
through an open portion 26 of a rotating filter wheel 28 composed 
primarily of an X-ray-attenuating material (e.g., lead or a lead-based 
alloy) to generate an X-ray sub-beam 30. This sub-beam 30 has a spatial 
profile that represents one portion, e.g., one quadrant of X-ray beam 22. 
The sub-beam is then passed through a portion 18 of an object 20, which 
may be, for example, a human tissue. The incident X-ray sub-beam 30 is 
modulated as it passes through the object to produce a modulated X-ray 
sub-beam 30' which strikes a portion of an X-ray-to-optical conversion 
device 32, such as a luminescing (i.e., phosphoresing or fluorescing) 
screen, to generate the optical scene 14. Other image intensifiers known 
in the art, such as flat-panel or electrostatic intensifiers, may be used 
in place of the luminescing screen to generate the optical scene. 
A pair of mirrors 34, 36 positioned on a surface 37 of a rotating mount 38 
steer and collect the scene 14. To collect the optical scene, the first 
mirror 34 is positioned and angled with respect to the irradiated portion 
of the X-ray-to-optical conversion device 32 to reflect the scene 14 onto 
the surface of the second mirror 36. This optic is aligned along the 
central axis of the system, indicated in FIG. 1 by the arrow 35, and is 
positioned and angled above an opening 40 in the rotating mount 38. 
Optical scene 14 is reflected by the second mirror 36 through the opening 
40 onto an imaging optical system 42, shown in FIG. 1 as a lens, where it 
is focussed onto the surface of the detector 16. The imaging system may be 
aligned and configured to magnify or demagnify the scene so that the 
optically active region of the detector is fully exposed. Although a 
single lens is shown, it is understood that imaging systems containing 
multiple optical components, such as reflective or refractive optics, may 
also be used. Detector 16 (e.g., a CCD) generates an electronic image 
corresponding to the optical scene 14, which is sent along line 44 to a 
computer 46 for display and analysis. 
Filter wheel 28 rotates about the central axis 35 to generate multiple 
X-ray sub-beams for imaging. Mount 38 rotates about the same axis, 
preferably at the same rotational rate, to sequentially image the 
X-ray-induced optical scenes onto the detector 16, thereby forming 
multiple sub-images. These components can be rotated, for example, using 
standard motor-driven translational stages. 
Because both the filter wheel 28 and the mount 38 may require openings in 
their respective centers to pass X-ray and optical fields, rotation is 
preferably driven by means coupled to the outer perimeter of these 
devices. For example, for filters and mounts having circular cross 
sections, the outer perimeter of these devices may include teeth 
configured to match external gears. In this case, rotation is driven by 
rotating the external gears about a separate axis. Other conventional 
means for rotation, such as belt-driven or magnetically coupled means, may 
also be used. 
Once generated, each sub-image is sequentially digitized and stored in a 
computer 46, such as in a buffer or memory. When all sub-images are 
detected and stored, the complete image of the region of interest of the 
object 20 is determined by recombining the sub-images. 
In addition, in order to increase the spatial resolution of the system, the 
digital imaging system 10 may be translated by a fraction of a pixel after 
one portion 18 of the object is imaged. This generates an additional 
sub-image, having a shifted pixel structure, for the same portion of the 
object. Eight sub-images, for example, would be generated for the four 
portions of the object. Recombination of each sub-image using this method 
results in an image having even higher spatial resolution than the image 
produced using a single sub-image for each portion. 
In general, images are recombined using standard algorithms known in the 
art. Because each sub-image is "pixelized," i e. contains a 
two-dimensional array of points according to the pixel array of the 
detector, recombination may be accomplished by shifting the pixels of each 
sub-image by a predetermined number of pixels, and then adding the 
modified sub-images to form the complete image. 
FIGS. 7A and 7B, for example, show schematically how multiple sub-images 
can be recombined to form a single image. In this case, multiple 
sub-images 25a-25d (shown in FIG. 7A) may be stored in the computer as a 
two-dimensional array of points, e.g., (x.sub.0,0, x.sub.0,1, . . . 
x.sub.n-1,n-1), where n for this case is 2048 and x.sub.a,b represents an 
element of each array. The recombination algorithm may then be used to 
shift each point of each array (i.e., "re-index" the array, as shown in 
FIG. 7B) by the same amount n so that a modified array is generated for 
each sub-image. In the schematic drawing shown, for example, the array of 
sub-image 25b' is modified to have indexing of (x.sub.n-1,0, x.sub.n,0, . 
. . x.sub.2n-1,n-1). The arrays of sub-images 25c' and 25d' are re-indexed 
in a similar manner. Once re-indexed, the arrays are such that combination 
of the sub-images allows a single image to be formed (this is indicated 
schematically by the four adjacent sub-images 25a'-25d' in FIG. 7B). 
Although it is not indicated in the figure, overlap between elements of 
the array may be used to eliminate "seams" in the image. 
FIG. 5, for example, shows how the multiple sub-images 120a-120d are 
recombined and partially overlapped to reconstruct the complete image. In 
this case, four 2048.times.2048-pixel sub-images 120a-120d are generated 
and then recombined in a square fashion to form a complete image 122 
having the same spatial resolution and approximately four times the area. 
To limit errors in the image reconstruction, the pixel structure of all 
four sub-images 120a-120d generated during the imaging process must be 
precisely controlled so that distorted regions, or regions of missing 
information, are avoided in the complete image 122. This problem is 
avoided in two separate ways. First, the mechanical tolerances of all 
mounting and filtering components, and control over the degree of rotation 
of the filter wheel and rotatable mirror mount, must be accurately 
controlled, preferably to within a displacement of better than 0.25 mm. 
Second, the pixel structure of each sub-image is controlled so that the 
inner boundaries of each digitized sub-image overlap those of the 
subsequent sub-image by a few pixels (e.g., 12 pixels, or about 0.58 mm, 
may be used as the region of overlap). In this case, the total image 
intensity of the pixels in the region of overlap represents the average 
intensity of the two overlapping pixels. Alternatively, image processing 
techniques known in the art may be used to eliminate any artifacts (e.g., 
abnormally high or low intensity values) in the regions of overlap. 
Thus, when each quadrant of the object is exposed, a 2048.times.2048 
sub-image is obtained, resulting in the generation of the four sub-images 
120a-120d, each representing a different quadrant of the imaged region. 
During recombination, the left-most 12 pixels of the sub-images 120a and 
120b are overlapped, respectively, with the right-most 12 pixels of the 
sub-images 120d and 120c. Similarly, the bottom-most 12 pixels of 
sub-images 120a and 120d are spatially overlapped, respectively, with the 
topmost 12 pixels of sub-images 120b and 120c. Marker pixels 125a-125d may 
be designated in the 12-pixel regions on the sides of each sub-image to 
allow for accurate overlap during the recombination procedure. 
The image quality is ultimately determined by the optical detector. In 
addition to having high spatial resolution, it is preferred that the 
detector have a low-noise output and high quantum efficiency (i.e., high 
conversion of optical photons into electronic signals). Currently 
available CCDs are the preferred detectors, although other electronic 
imagers, such as photodiode arrays, charge-injection devices, amorphous 
silicon detectors, video cameras, position-sensitive detectors, 
photomultiplier tubes, and image intensifiers may be used as the detector 
of the invention. CCDs are available, for example, from Scientific 
Technologies, Inc. 
Phosphor screens which may be used as the X-ray-to-optical converter of the 
invention are composed of, for example, optically transparent or 
semi-transparent scintillating materials, e.g., glass-based scintillating 
materials, CdWO.sub.4, thallium-activated sodium iodide (i.e., NaI(T1)), 
terbium-doped glass scintillators, transparent plastic scintillators, 
ceramic-based scintillating materials, including Gd.sub.2 O.sub.3, 
Gd.sub.2 O.sub.2 S:Pr,Ce,X, where X is F or Cl, Gd.sub.2 O.sub.2 S:Pr, 
Y.sub.2 O.sub.3 /Gd.sub.2 O.sub.3, and related ceramic-based materials, 
e.g., as described in U.S. Pat. Nos. 4,747,973, 4,518,546, 4,473,513, and 
4,525,628, and in U.S. Ser. No. 08/287,239, the contents of all of which 
are incorporated herein by reference. Any commercially available phosphor 
screen, such as those manufactured by 3M, may be used in accordance with 
the invention. 
In all cases, the methods and apparatus of the invention are used according 
to standard procedures in the imaging arts. For example, in mammography or 
radiography, the digital imaging system may be used to replace a standard 
X-ray imaging system with no effect on the normal imaging procedure. 
With reference now to FIGS. 2A-2D, both the filter wheel 28 and mirror 
mount 38 are rotated in a time-dependent fashion, and are used in 
conjunction with an X-ray source 24 and detector to image a region of the 
object 20. During imaging, generation of the optical scenes 14a-14d is 
accomplished by first spatially filtering the incident X-ray beam 22 to 
produce X-ray sub-beams 30a-30d. As is typical of X-ray sources, the 
incident beam 22 may have a diverging, conical spatial profile. To filter 
the beam, the open portion 26 of the wheel is positioned along one section 
of the beam 22, and the wheel is rotated about an axis centered with 
respect to the beam, thereby allowing transmission of multiple sub-beams 
during a rotational cycle. As shown in FIG. 2A, once transmitted, the 
X-ray sub-beam 30a irradiates a portion 18a of the object 20, resulting in 
a modulated X-ray beam 30a' which then impinges on a portion of the 
X-ray-to-optical conversion means 32 to produce the optical scene 14a. The 
first mounted mirror 34 is positioned to reflect the scene 14a off the 
surface of the second mirror 36, through the opening 40 in the rotating 
mount 38, and onto the imaging and detector systems. 
Once the first sub-image is generated and stored in the buffer or memory of 
the computer, both the filter wheel 28 and mirror mount 38 are rotated 
90.degree. in the same direction, indicated as being clockwise in the 
figures by the arrows 47 and 48; the degree and direction of rotation may 
be varied depending on the size of the region to be imaged, and is the 
same for the filter and the mirror mount. Typically, the rotational 
process takes between 0.1 and 5 seconds. During rotation, the positions of 
the X-ray source 24, X-ray-to-optical conversion means 32, and the 
detector 16 remain fixed. Preferably, during rotation, both the X-ray beam 
and detector are blocked; this reduces exposure of the object and detector 
to, respectively, X-ray and optical radiation during rotation. After 
rotation of the spatial filter and mirror mount, the X-ray beam is 
filtered so that a separate sub-beam is transmitted and used to expose 
another portion of the object. Similarly, by moving in concert with the 
filtering wheel, the pair of mirrors 34, 36 are positioned to collect and 
steer the induced optical scene through the imaging optics and onto the 
detector. 
With reference now to FIG. 2B, following simultaneous rotation of the 
filter wheel 28 and rotating mount 38, a spatially separate sub-beam 30b 
is generated and used to irradiate a new portion 18b of the object 20, 
thereby producing a modulated beam 30b' which impinges a second portion of 
the X-ray-to-optical conversion means 32 to generate a new optical scene 
14b. The scene is then collected by the newly positioned mirror pair 34, 
36, where it is steered onto the imaging and detecting systems to generate 
and store a sub-image representative of the region 18b. Once the sub-image 
is digitized and stored, the process of rotating the filter wheel and 
rotatable mount is repeated, as shown in FIGS. 2C and 2D, to generate and 
store sub-images of separate regions 18c, 18d of the object. Once 
collected and stored, each digitized sub-image is adjusted (i.e., the 
pixels may be shifted) and then combined using image-combining computer 
algorithms to form the complete image of the region. 
Although the embodiments shown in FIGS. 1 and 2A-2D show a scanning mirror 
apparatus wherein both the first 34 and second 36 mirrors are rotated, 
other methods for achieving multiple sub-images are also within the scope 
of the invention. For example, with reference now to FIG. 3, scanning 
mirror apparatus 80 may include a single rotating mirror 84 surrounded by 
four (or more) stationary mirrors 82a-82d mounted on a stage 81. Each of 
the stationary mirrors is positioned to receive the emitted optical scenes 
representative of the portions, e.g., four quadrants, of the region to be 
imaged. The rotating mirror 84 is mounted on a rotating stage 86 and is 
configured to rotate about a central axis aligned with the center of the 
filter wheel and the region to be imaged. In this case, an emitted scene 
is reflected off one of the stationary mirrors and onto the rotating 
mirror 84, where it is then reflected through an opening 88 and steered 
through the imaging system and onto the detector. Once a sub-image is 
collected, the stage 86 is rotated (as indicated by the arrow 87) to 
direct a scene reflected off a neighboring stationary mirror onto the 
detector. This process is repeated until all desired sub-images are 
collected. 
In still other embodiments, all reflecting mirrors are stationary, and the 
detector is mounted on a stage configured to translate and detect each of 
the emitted optical scenes. In addition, it is understood that in all 
cases, more or less than four optical scenes (and corresponding 
sub-images) may be collected and used to form the complete image. In 
general, this is achieved by reducing the degree of rotation of both the 
filter wheel and mirror mount about the central axis. 
In all cases, the mirrors used to reflect the optical scenes are coated 
with materials, such as standard dielectric stack coatings or reflective 
metal coatings, having a high reflectivity matched to the emission 
spectrum of the luminescing screen. Typically, this region is in the range 
of about 500-550 nm. All optics should be chosen to minimize distortion of 
the wavefronts of each optical scene. Reflecting prisms, or partially 
transmitting beam-splitting optics may be used as alternatives to mirrors. 
Referring now to FIG. 4, optical scenes 100a-100d are detected at times 
t=t.sub.s1 through t=t.sub.s4 by the CCD 102 to generate a complete image. 
The CCD is preferably a commercially available, high-end imager containing 
an array of 2048.times.2048 pixels, with each pixel having an area of 
about 24 .mu.m.times.24 .mu.m. The time interval At between detection of 
subsequent scenes, i.e. .DELTA.t=t.sub.si -t.sub.si+1 where i may be, for 
example, between 1 and 3, is limited by the rate at which the CCD can 
transmit data to an analog-to-digital (A/D) converter 104. This rate is 
typically between a few hundred KHz to about 5 MHz. For example, a CCD 
having a 2048.times.2048-pixel array at 2 bytes/pixel and operating near 4 
MHz requires about 1 second to output a sub-image to the A/D converter. 
CCDs operating at higher frequencies can be read out at faster rates, 
thereby expediting the image collecting process. During the data 
collection time period, the CCD 102 is typically left in an integration 
mode, thereby allowing the optical scene to be continuously collected and 
averaged. In addition, the CCD is typically cooled to limit thermal 
effects which may decrease the signal-to-noise ratio of the detected 
sub-image. A non-cooled CCD may be used if the pixel readout rate is 
sufficiently fast to prevent thermal current build-up. 
Once generated, the electronic images representative of the detected scenes 
are sequentially sent to the A/D converter 104 at times t=t.sub.I1 through 
t=t.sub.I4. The amplitude of the analog signal for each pixel corresponds 
to the magnitude of the optical intensity at the pixel, and the collective 
response of the array represents the total sub-image to be digitized. The 
light-induced electronic images are then digitized and sequentially 
transported at times t=t.sub.D1 through t=t.sub.D4 to the CPU 106, where 
they are stored in memory as digitized sub-images 101a-101d. 
The timing of the conversion of the optical scenes 100a-100d to the digital 
sub-images 101a-101d is controlled by a standard data acquisition unit 108 
(available, for example, from National Instruments) contained within the 
computer. The data acquisition unit 108 sends time-dependent control 
signals 108a-108c to the CCD 102, A/D converter 104, and CPU 106, and 
additionally receives status signals from the CPU indicating when a 
digitized sub-image has been registered in memory. At this point, the CPU 
also sends out a control signal 110 which is received by a controller 
driving the rotation of the spatial filter and mirror mount, thereby 
allowing subsequent optical scenes representative of separate portions of 
the object to be generated. Although not shown in FIG. 4, it is understood 
that current amplifiers and pre-amplifiers may be used in combination with 
the CCD in order to amplify the output analog signal. 
The CCD used in accordance with the present invention preferably includes 
(i) a large optically active area; (ii) high spatial resolution (i.e., 
small pixel size); and, (iii) high total quantum gain (i.e., high quantum 
efficiency at the emission peak of the X-ray-to-optical conversion 
screen). Any currently available CCD may be used as the optical detector. 
These devices typically can be read in both serial and parallel modes at 
rates between about 50 KHz and 10 MHz, and have quantum efficiencies 
ranging from approximately 30% to 80%, depending on the device and 
manufacturer. 
During the imaging procedure, the optical scene may be demagnified on the 
face of the CCD in order to increase the field of coverage of the imaging 
system. Preferably, the optical scene is imaged onto the CCD with a size 
ratio in the range of 1:1 (no demagnification) to 2:1 (50% 
demagnification). To maximize the intensity of the image, the imaging lens 
must have a high optical coupling efficiency at the wavelength of the 
emitted scene. In the radiographic application, for example, the F-number 
may be approximately F/1, and is typically between F/0.7 to F/1.4, 
although this is not an absolute requirement. If an image intensifier is 
to be used, the F-number may be significantly higher, thereby allowing for 
an even smaller lens aperture. 
Other criteria which may affect the optical coupling efficiency of the lens 
include optical transparency at the central wavelength of the emitted 
scene, optical properties of the anti-reflection coatings, purity of the 
glass used to fabricate the lens, and the degree of aberration in the 
lens. The lens used in the imaging system is preferably designed for close 
imaging, rather than for infinite object distances. To this effect, 
air-spaced astigmatic lenses may be used in the imaging system. Similarly, 
to reduce vignetting effects, the lens used in the imaging system 
preferably has a large effective diameter, and may be artificially 
flattened on a single side. 
In alternate embodiments, the present invention allows imaging of regions 
other than those which can be divided into four quadrants. For example, by 
modifying the spatial filtering device to translate linearly with respect 
to the spatial profile of the X-ray beam, X-ray sub-beams can be generated 
and used to sequentially irradiate, for example, an elongated region of 
tissue. In this case, the scanning mirror apparatus is configured to 
linearly translate in order to collect optical scenes induced by the 
linearly configured sub-beams, and then steer those beams into the CCD. As 
before, the optical scenes are then detected and digitized, and then 
combined in a linear fashion to form the complete image of the elongated 
region. Regions having unconventional shapes and sizes can be imaged in a 
similar fashion. 
In still other embodiments, a single scene can be optically separated into 
multiple components, with each component being monitored with a separate 
detector. This allows formation of a small-scale image having spatial 
resolution which is twice that of a conventional image. Referring now to 
FIG. 6, in such an embodiment, an X-ray source 150 generating an X-ray 
beam 152 is used to irradiate an object 154. The modulated X-ray beam then 
passes through a conventional image intensifier 156 to generate a single 
optical scene 158, which is split into scenes 160, 160a using a 
beam-splitter 162 (e.g., a pelical beam-splitter). Each scene is then 
imaged using imaging systems 164, 164a (e.g., single lenses) onto 
detectors 166, 166a (e.g., CCDs), resulting in the generation of two 
separate electronic images which can then be digitized using electronic 
means 168, 168a and stored in a computer 170. Each detector 166, 166a is 
spatially offset relative to each other by a distance equal to a fraction 
of a single pixel, resulting in the generation of digital images which are 
correspondingly offset. The offset digital images are then computationally 
recombined to form a small-scale, high-resolution digital image. 
In this embodiment, because the optical scene is split into two or more 
lower-intensity components, it may be necessary to use a high-gain image 
intensifier as shown in the figure, although this device is not essential. 
Alternatively, a fast X-ray screen may be used for X-ray-to-optical 
conversion. 
Other Embodiments 
Other embodiments are within the scope of the following claims. For 
example, the methods of the invention may be used for imaging applications 
not associated with radiography or mammography; other applications include 
large-scale image production in cinematography, or applications relating 
to conventional and confocal microscopy.