IMAGING METHODS USING RADIATION DETECTORS

Disclosed herein is a method, comprising: for i=1, . . . , N, one by one, exposing a radiation detector to a radiation beam (i) thereby causing the radiation detector to capture a partial image (i) of the radiation beam (i), wherein N is an integer greater than 1; for i=1, . . . , N, determining, in the partial image (i), Mi pinpointing picture elements of a boundary image (i) of a boundary (i) of the radiation beam (i), wherein Mi is a positive integer; and stitching the partial images (i), i=1, . . . , N resulting in a combined image based on the Mi (i=1, . . . , N) pinpointing picture elements.

TECHNICAL FIELD

The disclosure herein relates to imaging methods using radiation detectors.

BACKGROUND

A radiation detector is a device that measures a property of a radiation. Examples of the property may include a spatial distribution of the intensity, phase, and polarization of the radiation. The radiation may be one that has interacted with an object. For example, the radiation measured by the radiation detector may be a radiation that has penetrated the object. The radiation may be an electromagnetic radiation such as infrared light, visible light, ultraviolet light, X-ray or γ-ray. The radiation may be of other types such as α-rays and β-rays. An image sensor of an imaging system may include multiple radiation detectors.

SUMMARY

Disclosed herein is a method, comprising: for i=1, . . . , N, one by one, exposing a radiation detector to a radiation beam (i) thereby causing the radiation detector to capture a partial image (i) of the radiation beam (i), wherein N is an integer greater than 1; for i=1, . . . , N, determining, in the partial image (i), Mi pinpointing picture elements of a boundary image (i) of a boundary (i) of the radiation beam (i), wherein Mi is a positive integer; and stitching the partial images (i), i=1, . . . , N resulting in a combined image based on the Mi (i=1, . . . , N) pinpointing picture elements.

In an aspect, for i=1, . . . , N, the boundary image (i) is a closed line.

In an aspect, for i=1, . . . , N, the boundary image (i) is a rectangle.

In an aspect, for i=1, . . . , N, the Mi pinpointing picture elements comprise a pinpointing picture element (i, 1), a pinpointing picture element (i, 2), a pinpointing picture element (i, 3), a pinpointing picture element (i, 4), and a pinpointing corner picture element (i), and wherein for i=1, . . . , N, the pinpointing corner picture element (i) is on both (A) a straight line going through the pinpointing picture element (i, 1) and the pinpointing picture element (i, 2), and (B) a straight line going through the pinpointing picture element (i, 3) and the pinpointing picture element (i, 4).

In an aspect, for i=1, . . . , N, the boundary image (i) is not a closed line.

In an aspect, for i=1, . . . , N, intensity of radiation gradually falls when moving from inside the radiation beam (i) to outside the radiation beam (i) across the boundary (i) of the radiation beam (i).

In an aspect, for i=1, . . . , N−1, a region (i) of the partial image (i) bounded by the boundary image (i) overlaps a region (i+1) of the partial image (i+1) bounded by the boundary image (i+1).

In an aspect, for i=1, . . . , N, values of picture elements of the partial image (i) outside the boundary image (i) as pinpointed by the Mi pinpointing picture elements are not used in determining values of picture elements of the combined image.

In an aspect, for i=1, . . . , N, values of some picture elements of the partial image (i) outside the boundary image (i) as pinpointed by the Mi pinpointing picture elements are used in determining values of picture elements of the combined image.

Disclosed herein is a method, comprising: exposing a first radiation detector to a radiation beam thereby causing the first radiation detector to capture a first beam image of the radiation beam; and determining, in the first beam image, M1 pinpointing picture elements of a first boundary image of a boundary of the radiation beam, wherein M1 is a positive integer.

In an aspect, the first boundary image is a closed line.

In an aspect, the first boundary image is a rectangle.

In an aspect, the M1 pinpointing picture elements comprise a first pinpointing picture element, a second pinpointing picture element, a third pinpointing picture element, a fourth pinpointing picture element, and a pinpointing corner picture element, and wherein the pinpointing corner picture element is on both (A) a first straight line going through the first and second pinpointing picture elements, and (B) a second straight line going through the third and fourth pinpointing picture elements.

In an aspect, the first boundary image is not a closed line.

In an aspect, intensity of radiation gradually falls when moving from inside the radiation beam to outside the radiation beam across the boundary of the radiation beam.

In an aspect, the method further comprises: exposing a second radiation detector to the radiation beam thereby causing the second radiation detector to capture a second beam image of the radiation beam; and determining, in the second beam image, M2 pinpointing picture elements of a second boundary image of the boundary of the radiation beam, wherein M2 is a positive integer.

Disclose herein is an apparatus, comprising a first radiation detector configured to (A) capture a first beam image of a radiation beam in response to the first radiation detector being exposed to the radiation beam and (B) determine, in the first beam image, M1 pinpointing picture elements of a first boundary image of a boundary of the radiation beam, wherein M1 is a positive integer.

In an aspect, the first boundary image is a closed line.

In an aspect, intensity of radiation gradually falls when moving from inside the radiation beam to outside the radiation beam across the boundary of the radiation beam.

In an aspect, the apparatus further comprises a second radiation detector configured to (A) capture a second image of the radiation beam in response to the second radiation detector being exposed to the radiation beam and (B) determine, in the second beam image, M2 pinpointing picture elements of a second boundary image of the boundary of the radiation beam, wherein M2 is a positive integer.

DETAILED DESCRIPTION

FIG.1schematically shows a radiation detector100, as an example. The radiation detector100may include an array of pixels150(also referred to as sensing elements150). The array may be a rectangular array (as shown inFIG.1), a honeycomb array, a hexagonal array, or any other suitable array. The array of pixels150in the example ofFIG.1has 21 pixels150arranged in 3 rows and 7 columns. In general, the array of pixels150may have any number of pixels150arranged in any way.

A radiation may include particles such as photons (electromagnetic waves) and subatomic particles (e.g., neutrons, protons, electrons, alpha particles, etc.) Each pixel150may be configured to detect radiation incident thereon and may be configured to measure a characteristic (e.g., the energy of the particles, the wavelength, and the frequency) of the incident radiation. The measurement results for the pixels150of the radiation detector100constitute an image of the radiation incident on the pixels. It may be said that the image is of an object or a scene which the incident radiation come from.

Each pixel150may be configured to count numbers of particles of radiation incident thereon whose energy falls in a plurality of bins of energy, within a period of time. All the pixels150may be configured to count the numbers of particles of radiation incident thereon within a plurality of bins of energy within the same period of time. When the incident particles of radiation have similar energy, the pixels150may be simply configured to count numbers of particles of radiation incident thereon within a period of time, without measuring the energy of the individual particles of radiation.

Each pixel150may have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident particle of radiation into a digital signal, or to digitize an analog signal representing the total energy of a plurality of incident particles of radiation into a digital signal. The pixels150may be configured to operate in parallel. For example, when one pixel150measures an incident particle of radiation, another pixel150may be waiting for a particle of radiation to arrive. The pixels150may not have to be individually addressable.

The radiation detector100described here may have applications such as in an X-ray telescope, X-ray mammography, industrial X-ray defect detection, X-ray microscopy or microradiography, X-ray casting inspection, X-ray non-destructive testing, X-ray weld inspection, X-ray digital subtraction angiography, etc. It may be suitable to use this radiation detector100in place of a photographic plate, a photographic film, a PSP plate, an X-ray image intensifier, a scintillator, or another semiconductor X-ray detector.

An image sensor of an imaging system (not shown) may include multiple radiation detectors100. In an embodiment, all the pixels150of the radiation detectors100of the image sensor may be coplanar (i.e., a plane intersects all the pixels150of all the radiation detectors100. In an alternative embodiment, for each radiation detector100of the image sensor, the pixels150of the radiation detector100may be coplanar, but all the pixels150of all the radiation detectors100of the image sensor may be not coplanar. For example, the pixels150of a first radiation detector100of the image sensor may be on a first plane, but the pixels150of a second radiation detector100of the image sensor may be on a second plane different from the first plane. The first plane and the second plane may be parallel to each other, or may be not parallel to each other. For example, the radiation detectors100of the image sensor may be arranged on an inner surface (i.e., concave surface) of a parabola.

FIG.2Aschematically shows a simplified cross-sectional view of the radiation detector100ofFIG.1along a line2A-2A, according to an embodiment. More specifically, the radiation detector100may include a radiation absorption layer110and an electronics layer120. The electronics layer120may include one or more application-specific integrated circuit (ASIC) chips for processing or analyzing electrical signals which incident radiation generates in the radiation absorption layer110. The radiation detector100may or may not include a scintillator (not shown). The radiation absorption layer110may comprise a semiconductor material such as silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. The semiconductor material may have a high mass attenuation coefficient for the radiation of interest.

FIG.2Bschematically shows a detailed cross-sectional view of the radiation detector100ofFIG.1along the line2A-2A, as an example. More specifically, the radiation absorption layer110may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region111and one or more discrete regions114of a second doped region113. The second doped region113may be separated from the first doped region111by an optional intrinsic region112. The discrete regions114are separated from one another by the first doped region111or the intrinsic region112. The first doped region111and the second doped region113have opposite types of doping (e.g., region111is p-type and region113is n-type, or region111is n-type and region113is p-type). In the example ofFIG.2B, each of the discrete regions114of the second doped region113forms a diode with the first doped region111and the optional intrinsic region112. Namely, in the example inFIG.2B, the radiation absorption layer110has a plurality of diodes (more specifically,FIG.2Bshows 7 diodes corresponding to 7 pixels150of one row in the array ofFIG.1, of which only 2 pixels150are labeled inFIG.2Bfor simplicity). The plurality of diodes have an electrode119A as a shared (common) electrode. The first doped region111may also have discrete portions.

The electronics layer120may include an electronic system121suitable for processing or interpreting signals generated by the radiation incident on the radiation absorption layer110. The electronic system121may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuitry such as a microprocessor, and memory. The electronic system121may include one or more ADCs. The electronic system121may include components shared by the pixels150or components dedicated to a single pixel150. For example, the electronic system121may include an amplifier dedicated to each pixel150and a microprocessor shared among all the pixels150. The electronic system121may be electrically connected to the pixels150by vias131. Space among the vias may be filled with a filler material130, which may increase the mechanical stability of the connection of the electronics layer120to the radiation absorption layer110. Other bonding techniques are possible to connect the electronic system121to the pixels150without using the vias131.

When radiation from the radiation source (not shown) hits the radiation absorption layer110including diodes, particles of the radiation may be absorbed and generate one or more charge carriers (e.g., electrons, holes) by a number of mechanisms. The charge carriers may drift to the electrodes of one of the diodes under an electric field. The field may be an external electric field. The electrical contact119B may include discrete portions each of which is in electrical contact with the discrete regions114. The term “electrical contact” may be used interchangeably with the word “electrode.” In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different discrete regions114(“not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow to a different one of the discrete regions114than the rest of the charge carriers). Charge carriers generated by a particle of the radiation incident around the footprint of one of these discrete regions114are not substantially shared with another of these discrete regions114. A pixel150associated with a discrete region114may be a space around the discrete region114in which substantially all (more than 98%, more than 99.5%, more than 99.9%, or more than 99.99% of) charge carriers generated by a particle of the radiation incident therein flow to the discrete region114. Namely, less than 2%, less than 1%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the pixel150.

FIG.2Cschematically shows a detailed cross-sectional view of the radiation detector100ofFIG.1along the line2A-2A, as another example. More specifically, the radiation absorption layer110may include a resistor of a semiconductor material such as silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does not include a diode. The semiconductor material may have a high mass attenuation coefficient for the radiation of interest. In an embodiment, the electronics layer120ofFIG.2Cmay be similar to the electronics layer120ofFIG.2Bin terms of structure and function.

When the radiation hits the radiation absorption layer110including the resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of the radiation may generate10to100,000charge carriers. The charge carriers may drift to the electrical contacts119A and119B under an electric field. The electric field may be an external electric field. The electrical contact119B includes discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of the radiation are not substantially shared by two different discrete portions of the electrical contact119B (“not substantially shared” here means less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). Charge carriers generated by a particle of the radiation incident around the footprint of one of these discrete portions of the electrical contact119B are not substantially shared with another of these discrete portions of the electrical contact119B. A pixel150associated with a discrete portion of the electrical contact119B may be a space around the discrete portion in which substantially all (more than 98%, more than 99.5%, more than 99.9% or more than 99.99% of) charge carriers generated by a particle of the radiation incident therein flow to the discrete portion of the electrical contact119B. Namely, less than 2%, less than 0.5%, less than 0.1%, or less than 0.01% of these charge carriers flow beyond the pixel associated with the one discrete portion of the electrical contact119B.

FIG.3Aschematically shows an imaging system300, according to an embodiment. In an embodiment, the imaging system300may include the radiation detector100, a radiation source310, and a mask320. In an embodiment, the absorption layer110(FIG.2A) of the radiation detector100may face the radiation source310and the mask320(i.e., the absorption layer110is between the mask320and the electronics layer120of the radiation detector100).

In an embodiment, the operation of the imaging system300may be as follows. An object330may be positioned between the mask320and the radiation detector100. The radiation source310may generate radiation toward the mask320. In an embodiment, the portion of the radiation from the radiation source310incident on a mask window322of the mask320may be allowed to pass through the mask320(for example, the mask window322may be not opaque to the radiation), while the portion of the radiation from the radiation source310incident on other parts of the mask320may be blocked. As a result, after passing through the mask window322of the mask320, the radiation from the radiation source310becomes a radiation beam represented by an arrow340(hence thereafter the radiation beam may be referred to as the radiation beam340).

In an embodiment, radiation particles of the radiation beam340some of which have penetrated the object330may hit the absorption layer110(FIG.2A) of the radiation detector100causing the radiation detector100to capture a beam image360(FIG.3B) of the radiation beam340. In an embodiment, the mask window322of the mask320may have a rectangular shape. As a result, the radiation beam340may have the shape of a truncated pyramid having 4 sides which form a boundary342of the radiation beam340.

In an embodiment, with reference toFIG.3A-FIG.3B, an image362ein the beam image360of the edge (perimeter)322eof the mask window322may be a rectangle having four sides362e1,362e2,362e3, and362e4. The image362emay be considered the image of the boundary342of the radiation beam340. As a result, the image362emay also be called the boundary image362e.

FIG.3Cshows contents of a portion364of the beam image360in terms of picture elements and their values as an example. Each picture element of the beam image360corresponds to a pixel150(FIG.1) and may be represented by a rectangular box. The value in a box indicates the intensity of radiation of the radiation beam340incident on the corresponding pixel150. For example, a value of zero in a box ofFIG.3Cindicates that the pixel150corresponding to the picture element represented by the box receives no incident radiation particles from the radiation beam340.

In an embodiment, with reference toFIG.3A-FIG.3C, the determination of a pinpointing corner picture element E in the beam image360where the north east corner362e12of the boundary image362eis supposed to be may start with determining in the beam image360a pinpointing picture element A through which the side362e1of the boundary image362eis supposed to pass. In an embodiment, the determination of the pinpointing picture element A may be as follows. Firstly, a row366of picture elements in the beam image360intersecting the side362e1of the boundary image362emay be chosen.

In an embodiment, the radiation source310and the edge322eof the mask window322(FIG.3A) may be such that intensity of radiation gradually falls when moving from inside the radiation beam340to outside the radiation beam340across the boundary342of the radiation beam340. As a result, when moving from left to right in the row366across the side362e1of the boundary image362e(FIG.3C), the values of picture elements gradually fall from 12 to 0. The specific picture element values of 0, 2, . . . , and 12 are chosen for illustration only.

In an embodiment, the pinpointing picture element A may be determined to be a picture element of the row366having a value which is the average value of (A) the maximum picture element value before the picture element value drop (i.e., 12) and (B) the minimum picture element value after the picture element value drop (i.e., 0). So, the average value is (12+0)/2=6. As a result, the pinpointing picture element A of the boundary image362emay be determined to be the picture element represented by the grayed-out box as shown inFIG.3C.

In an embodiment, the determination of the pinpointing corner picture element E may further include determining in the beam image360(1) a pinpointing picture element B through which the side362e1of the boundary image362eis supposed to pass, and (2) picture elements C and D through both of which the side362e2of the boundary image362eis supposed to pass. In an embodiment, the determinations of the pinpointing picture elements B, C, and D may be similar to the determination of the pinpointing picture element A described above. Next, in an embodiment, the pinpointing corner picture element E may be determined to be a picture element in the beam image360which is on both (1) a first straight line going through the pinpointing picture elements A and B, and (2) a second straight line going through the pinpointing picture elements C and D.

The pinpointing corner picture element E (where the north east corner362e12of the boundary image362eis supposed to be), the pinpointing picture elements A and B (through both of which the side362e1of the boundary image362eis supposed to pass), and the pinpointing picture elements C and D (through both of which the side362e2of the boundary image362eis supposed to pass) each helps determine the position of the radiation detector100with respect to the radiation beam340. In general, the more pinpointing picture elements of the boundary image362eare determined, the more accurately the position of the radiation detector100with respect to the radiation beam340is determined.

FIG.3Dis a flowchart380summarizing and generalizing the determination of the position of the radiation detector100with respect to the radiation beam340by determining one or more pinpointing picture elements of the boundary image362e, according to an embodiment. Specifically, in step382, a radiation detector (e.g., the radiation detector100ofFIG.3A) may be exposed to a radiation beam (e.g., the radiation beam340ofFIG.3A) thereby causing the radiation detector to capture a beam image (e.g., the beam image360ofFIG.3B) of the radiation beam. In step384, in the beam image, M pinpointing picture elements (e.g., the pinpointing picture elements A, B, C, D, and E ofFIG.3B) of a boundary image (e.g., the boundary image362eofFIG.3B) of a boundary (e.g., the boundary342ofFIG.3A) of the radiation beam may be determined, wherein M is a positive integer (e.g., M=5 inFIG.3B).

In an embodiment, the determinations of the pinpointing picture elements A, B, C, D, and E as described above may be performed by the radiation detector100. In an embodiment, the boundary image362emay be a closed line (i.e., having no end point) as shown inFIG.3B. This happens when the entire radiation beam340falls on the radiation detector100(FIG.3A). In an alternative embodiment, a portion of the radiation beam340may fall outside the radiation detector100as shown inFIG.3E. As a result, with reference toFIG.3F, the resulting boundary image362e(which includes straight line segments PQ QR, and RS) is not a closed line and has 2 end points P and S.

In an embodiment, with reference toFIG.3G, the imaging system300may further include another radiation detector100′ similar to the radiation detector100. In an embodiment, the radiation detector100′ may also be exposed the radiation beam340thereby causing the radiation detector100′ to capture a beam image (not shown, but similar to the beam image360ofFIG.3B) of the radiation beam340. In an embodiment, one or more pinpointing picture element determinations similar to the pinpointing picture element determinations described above with respect to the radiation detector100may also be performed for the radiation detector100′, thereby providing the position of the radiation detector100′ with respect to the radiation beam340.

FIG.4A-FIG.4Gschematically show an operation of the imaging system300ofFIG.3A, according to an alternative embodiment. An object430to be imaged may be a sword inside a carton box (not shown) for example; and the radiation used for imaging may be X-ray. For simplicity, only the radiation detector100and the radiation beams for imaging are shown inFIG.4A,FIG.4C, andFIG.4E(i.e., the other parts of the imaging system300such as the radiation source310and the mask322are not shown). Moreover, the radiation detector100and the radiation beams are shown in top views inFIG.4A,FIG.4C, andFIG.4E.

In an embodiment, the operation of the imaging system300in capturing an image of the object430using multiple exposures may be as follows. For the first exposure, the radiation detector100may be exposed to a radiation beam440(FIG.4A) causing the radiation detector100to capture a beam image460which may also be called a first partial image460(FIG.4B).

Next, in an embodiment, for the second exposure, the object430may remain stationary and the imaging system300(FIG.3A) including the radiation detector100, the radiation source310, and the mask320may be moved to the right from the position as shown inFIG.4Ato the next position as shown inFIG.4C. Then, the radiation detector100may be exposed to a radiation beam440′ (FIG.4C) causing the radiation detector100to capture a beam image460′ which may also be called a second partial image460′ (FIG.4D).

Next, in an embodiment, for the third exposure, the object430may remain stationary and the imaging system300(FIG.3A) including the radiation detector100, the radiation source310, and the mask320may be moved to the right from the position as shown inFIG.4Cto the next position as shown inFIG.4E. Then, the radiation detector100may be exposed to a radiation beam440″ (FIG.4E) causing the radiation detector100to capture a beam image460″ which may also be called a third partial image460″ (FIG.4F).

In an embodiment, with reference toFIG.4A-FIG.4B, during the first exposure, the position of the radiation detector100with respect to the radiation beam440may be determined by determining, in the first partial image460, one or more pinpointing picture elements (not shown) of the boundary image462eof the boundary442of the radiation beam440. Similarly, in an embodiment, with reference toFIG.4C-FIG.4D, during the second exposure, the position of the radiation detector100with respect to the radiation beam440′ may be determined by determining, in the second partial image460′, one or more pinpointing picture elements (not shown) of the boundary image462e′ of the boundary442′ of the radiation beam440′. Similarly, in an embodiment, with reference toFIG.4E-FIG.4F, during the third exposure, the position of the radiation detector100with respect to the radiation beam440″ may be determined by determining, in the beam image460″, one or more pinpointing picture elements (not shown) of the boundary image462e″ of the boundary442″ of the radiation beam440″.

In an embodiment, the first partial image460, the second partial image460′, and the third partial image460″ may be stitched resulting in a combined image470(FIG.4G) of the object430based on (A) the position of the radiation detector100with respect to the radiation beam440in the first exposure, (B) the position of the radiation detector100with respect to the radiation beam440′ in the second exposure, and (C) the position of the radiation detector100with respect to the radiation beam440″ in the third exposure. The shapes and positions of the radiation beams440,440′ and440″ are known and stitching the partial images460,460′ and460″ may be further based on them. In other words, the first partial image460, the second partial image460′, and the third partial image460″ may be stitched resulting in the combined image470(FIG.4G) of the object430based on (A) the one or more pinpointing picture elements in the beam image460of the boundary image462eof the boundary442of the radiation beam440in the first exposure, (B) the one or more pinpointing picture elements in the beam image460′ of the boundary image462e′ of the boundary442′ of the radiation beam440′ in the second exposure, and (C) the one or more pinpointing picture elements in the beam image460″ of the boundary image462e″ of the boundary442″ of the radiation beam440″ in the third exposure.

FIG.5shows a flowchart500summarizing and generalizing the operation of the imaging system300described above for obtaining an image of the object430using multiple exposures, according to an embodiment. Specifically, in step510, for i=1, . . . , N, one by one, a same radiation detector (e.g., the radiation detector100ofFIG.4A) may be exposed to a radiation beam (i) (e.g., the radiation beam440ofFIG.4A) thereby causing the radiation detector to capture a partial image (i) (e.g., the first partial image460ofFIG.4B) of the radiation beam (i), wherein N is an integer greater than 1 (e.g., N=3 inFIG.4A-FIG.4G).

In step520, for i=1, . . . , N, in the partial image (i) (e.g., the first partial image460inFIG.4B), Mi pinpointing picture elements of a boundary image (i) (e.g., the boundary image462eofFIG.4B) of a boundary (i) (e.g., the boundary442ofFIG.4A) of the radiation beam (i) (e.g., the radiation beam440ofFIG.4A) may be determined, wherein Mi is a positive integer. In step530, the partial images (i), i=1, . . . , N (e.g., the partial images460,460′, and460″) may be stitched resulting in a combined image (e.g., the combined image470ofFIG.4G) based on the Mi (i=1, . . . , N) pinpointing picture elements.

In an embodiment, with reference toFIG.4A-FIG.4G, the region463(FIG.4B) of the first partial image460bounded by the boundary image462emay overlap the region463′ (FIG.4D) of the second partial image460′ bounded by the boundary image462e′. This may happen when the radiation beam440′ (FIG. C) illuminates some part of the object430(or the scene) illuminated earlier by the radiation beam440(FIG.4A).

Similarly, in an embodiment, the region463′ (FIG.4D) of the partial image460′ bounded by the boundary image462e′ may overlap the region463″ (FIG.4F) of the partial image460″ bounded by the boundary image462e″. This may happen when the radiation beam440″ (FIG. E) illuminates some part of the object430(or the scene) illuminated earlier by the radiation beam440′ (FIG.4C).

In an embodiment, with reference toFIG.4B, the values of some picture elements of the first partial image460outside the boundary image462eas pinpointed by the one or more pinpointing picture elements of the boundary image462e(like the picture element365ofFIG.3Cwhich is outside the boundary image362eas pinpointed by the pinpointing picture elements A, B, C, D, and E) may be used in determining the values of some picture elements of the combined image470(FIG.4G). Similarly, in an embodiment, with reference toFIG.4D, the values of some picture elements of the second partial image460′ outside the boundary image462e′ as pinpointed by the one or more pinpointing picture elements of the boundary image462e′ may be used in determining the values of some picture elements of the combined image470(FIG.4G). Similarly, in an embodiment, with reference toFIG.4F, the values of some picture elements of the third partial image460″ outside the boundary image462e″ as pinpointed by the one or more pinpointing picture elements of the boundary image462e″ may be used in determining the values of some picture elements of the combined image470(FIG.4G).

In an alternative embodiment, with reference toFIG.4B, the values of the picture elements of the first partial image460outside the boundary image462eas pinpointed by the one or more pinpointing picture elements of the boundary image462eare not used in determining the values of picture elements of the combined image470(FIG.4G). Similarly, in an embodiment, with reference toFIG.4D, the values of the picture elements of the second partial image460′ outside the boundary image462e′ as pinpointed by the one or more pinpointing picture elements of the boundary image462e′ are not used in determining the values of picture elements of the combined image470(FIG.4G). Similarly, in an embodiment, with reference toFIG.4F, the values of the picture elements of the third partial image460″ outside the boundary image462e″ as pinpointed by the one or more pinpointing picture elements of the boundary image462e″ are not used in determining the values of picture elements of the combined image470(FIG.4G).

In the embodiments described above, the mask window322of the mask320(FIG.3A) has a rectangular shape. In general, the mask window322may have any shape (e.g., trapezoid, etc).