Patent Publication Number: US-7593510-B2

Title: X-ray imaging with continuously variable zoom and lateral relative displacement of the source

Description:
The present application claims priority from U.S. Provisional Patent Application Ser. No. 60/982,099, filed Oct. 23, 2007, which is incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates to methods and systems for controlling the spatial resolution of imaging systems, and specifically to controlling the spatial resolution of such imaging systems by moving a source of radiation relative to an aperture. 
   BACKGROUND OF THE INVENTION 
   The present application contains subject matter related to that of US Published Patent Application US-2006-0245547, filed Mar. 21, 2006, which is incorporated herein by reference. 
   Current x-ray imaging systems typically make use of penetrating radiation characterized by a relatively wide-angle pattern that emerges from an x-ray generator such as an x-ray tube. Referring to the prior art configuration depicted in  FIG. 1 , the angular field of view A of the x-ray beam is conventionally determined by the angular extent P of an x-ray beam  14  emergent from x-ray source  10 , in combination with any subsequent collimating structure  12 . For example, in the situation depicted in  FIG. 1 , a wide-angle radiation pattern P emitted by x-ray source  10  and propagating toward the object under inspection  16  is blocked by a highly attenuating material  13  with a stationary collimating aperture  12  that transmits a fraction of the incident radiation in the form of a small fan beam  18 . The term “opaque” refers herein to matter that does not effectively transmit the incident radiation. Here, the field-of-view A of x-ray radiation reaching the object  16  is determined by the angular size of the stationary aperture  12  viewed from the x-ray source  10 . Referring to  FIG. 2 , in some cases, x-ray imaging systems may shape the emitted radiation into a scanning pencil beam by means of a chopper wheel  20 , or otherwise. In such systems, a continuously moving collimator (or spatial modulator)  20 , usually in the form of an opaque rotating wheel with appropriately placed aperture(s)  22 , sequentially selects small portions from the wide-angle radiation pattern P emitted by x-ray source  10 , positioned at a fixed distance L away from the collimator, and scans the object under inspection (OUI)  16  with a beam B, the transitory position  23  of which on the OUI  16  is accurately knows as a function of time. As used herein and in any appended claims, the term “quasi-collimation” refers to limiting the spatial extent of radiation by means of a single aperture, and, in that sense, beam B is quasi-collimated. As a result of such scan, a backscatter image may be created point-by-point by collecting backscattered radiation from each irradiated pixel for each collimator scan cycle. 
   For purposes of the current description, a field-of-view (FOV) is defined as the angular extent of an aggregate image comprised by a sequence of transitory illuminating spots formed by an aperture traversing the pattern of penetrating radiation, as viewed from the source. “Imaging” generally refers to generation of a multidimensional representation of values characterizing an aspect of an object or a scene, whether as a stored array or as a displayed representation. “Penetrating radiation” refers to probe radiation, such as in the x-ray portion of the electromagnetic spectrum, which passes into an object, not necessarily traversing the object, and which allows interrogation of various features of the object by virtue of interaction of the probe radiation with the object. “Scanning” a radiation pattern refers to moving a beam of the radiation in a systematic fashion. 
   “Pencil-shaped,” as used herein, refers to a beam having any cross-sectional shape, the extent of each dimension of the cross-section, transverse to the beam propagation direction, being comparable, though not necessarily equal. “Flux,” as used herein and in any appended claims, refers to either the number, or total power, of x-ray photons crossing a unit cross-sectional area per unit of time. 
   In prior art scanning x-ray inspection systems of  FIGS. 1 and 2 , the overall field-of-view, as defined by the span of the radiation-traversing motion of the aperture(s)  22 , the angular field-of-view A, is fixed, since it is provided by an x-ray tube&#39;s focal spot  11  (shown in  FIG. 1 ), beam forming aperture(s)  12  and  22 , and predetermined distance L, all designed to suit a specialized objective. The fixed FOV limits such system to a narrow range of uses, and typically precludes imaging objects outside of a particular design distance, or range of distances, to the OUI  16 . An object at a distance shorter than the design distance is “cut-off”, while an object more distant that the design distance suffers resolution loss. 
   SUMMARY OF THE INVENTION 
   In accordance with preferred embodiments of the present invention, methods and apparatus are provided for varying the field-of-view of imaging systems that have a source of penetrating radiation and a first and second aperture disposed in the path of the penetrating radiation. The field of view is varied, in accordance with preferred embodiments of the invention, by repositioning the source of radiation with respect to the apertures shaping the beam. As a result of varying the FOV, the areal resolution of x-ray imaging can be controlled. In particular, a translator is provided for repositioning the source relative to the first aperture transversely with respect to the path of emitted radiation. 
   In further embodiments, methods and apparatus are provided for varying the flux of penetrating radiation incident on a target for any instant FOV. This is achieved by changing the spectral, temporal, or spatial characteristics of the beam. According to yet other preferred embodiments of the invention, methods and apparatus are provided for scanning a target in a raster fashion. This may be achieved by repositioning the relative positions of the source of radiation and the aperture in a plane transverse to the optical axis of the system. 
   In various embodiments, the source of penetrating radiation may be an x-ray tube or, alternatively, it may be a radioactive source, or an accelerator. The spatial modulator may include one or more rotating chopper wheels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of the invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings: 
       FIG. 1  is a schematic illustration of a prior art stationary x-ray imaging system. 
       FIG. 2  shows a perspective view of a prior art scanning x-ray imaging system and illustrates a general definition of a FOV. 
       FIGS. 3A and 3B  schematically illustrate principles of changing a FOV according to an embodiment of the current invention. 
       FIG. 4  provides a perspective view of the embodiment of  FIG. 3  containing a rotating spatial modulator and limiting a field-of-view in two dimensions. 
       FIG. 5  shows front and top views of a spatial modulator with adjustable apertures according to the invention. 
       FIG. 6  shows a spatial modulator of the invention having two concentric sets of differently sized radially disposed apertures. 
       FIG. 7  provides a top view of the embodiment employing the spatial modulator of  FIG. 6 . 
       FIG. 8  demonstrates an embodiment of a raster-scanned x-ray imaging system in accordance with an embodiment of the invention. 
       FIG. 9  illustrates an alternative embodiment of the invention with a spatial modulator in a cylindrical form. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENT OF THE INVENTION 
   For the purposes of the current invention, the term “zoom” refers to user-defined control of an imaging system&#39;s FOV, concurrently implicating control of the areal resolution of the imaging system. “Areal resolution” refers to the resolution corresponding to the inspection of an object as projected onto a plane. A “normal” to an aperture is defined as a direction perpendicular to a plane containing the aperture. 
   The angular FOV of a system comprising a source of radiation and governed by ray optics is determined by the dimensions and any scanning limits of a field stop of the system in conjunction with the separation between the source and the field stop. With reference to  FIGS. 3A and 3B , embodiments of the current invention allow the FOV of an x-ray imaging system to be varied continuously, either automatically or by an operator, by moving x-ray source  10  toward, or away, from a field stop (i.e., a beam forming aperture) by use of an actuator (designated generally by numeral  24  in  FIG. 4 ). Source  10  provides penetrating radiation, and may be an x-ray tube, or a radioactive source, or any other source of penetrating radiation, including, for example, an accelerator, either electrostatic or linear. Actuator  24  may be a motor in conjunction with a worm drive, for example, or any other mechanism for translating the relative displacement between source  10  and a field stop. When source  10  and a beam-forming aperture  12  are separated by a short distance L 1  the angle of radiation emanating from the x-ray source and transmitted through the aperture, which functions as a field stop of the system, defines a wide field-of-view A 1  shown in xz-plane in  FIG. 3A . Source  10  may be characterized by a focal spot  11  of energetic particles impinging upon a target to generate x-rays P. In a distant imaging set-up depicted in  FIG. 3B , when the source  10  is positioned farther away from the aperture  12  at a distance L 2 &gt;L 1 , aperture  12  subtends a smaller angle A 2  as viewed from the focal point of the source thus defining a correspondingly narrower FOV A 2 &lt;A 1 . The ability to control the separation between the source and the beam-forming aperture allows controlling the spatial extent of the beam of radiation passing through the aperture toward the OUI and, thereby, managing the cross-section of a pencil-shaped beam scanned across the OUI. Consequently, the separation between the source  10  and the aperture  12  efficiently governs zooming, in or out, of x-ray imaging system of the OUI, allowing the smaller or the bigger portion of the OUI to be irradiated as a function of the source-to-aperture separation. It is understood that, in practice, the range of source motion and, therefore, zoom are limited, on one side to the maximum output angle allowed by the x-ray tube&#39;s construction, and on the other side to space limitations in the system. Flux constraints may also impose practical limitations. 
   While an x-ray beam B is scanning the object, either the object under inspection or the x-ray source and collimator may also be moved in a direction substantially orthogonal to the beam propagation direction. A two dimensional image of the object may be created by a combination of collimator scanning and real or virtual motion of the source and/or object. 
     FIG. 4  depicts a variable-zoom scanning system  40 , where apertures  12 , forming successive field stops and shaping a beam by scanning a wide-angle pattern  14  of penetrating radiation emanating from source  10 , are disposed on a spatial modulator in the form of a chopper wheel  20  rotating in the xy-plane about an axis  200 . To constrain the spatial extent of the beam additionally in a transverse direction, a second collimating aperture stop  42  may be provided in the path of penetrating radiation. Source  10  is coupled to a translator  24 . Translator  24  repositions the source  10  with respect to chopper  20  and, particularly, along and/or transverse to the normal  210  to apertures  12  of scanning system  40  using motor  25  or any other mechanical, electrical, pneumatic or other suitable means, optionally computerized. 
   Field-of-view A (defined by the view, from source  10 , of the angular extent of the image  28  that is comprised by the transitory illuminating spots  30  of the scanning apertures  12 ) is reduced by moving the source  10  away from the wheel  20  as shown in  FIG. 4  (and, therefore, increasing the separation between the source and the wheel from L 1  to L 2 ), the output flux of penetrating radiation in a scanning beam  32  (which may have any specified cross-sectional shape, within the scope of the present invention), incident on the object under inspection OUI  34  at any instant of time, decreases as well. This is because a progressively smaller portion of wide-angle radiation pattern of the source  10  is being subtended by the one of the apertures  12 . To improve grainy and statistically poor images that may result from reduced flux leading to insufficient irradiation of the object, or, otherwise, to adjust resolution, an embodiment  50  of the device of the invention, shown in  FIG. 5  in front and side views, provides for ancillary variation of the flux of beam  32  of  FIG. 4  by altering the transverse cross-section of the beam  32 . As illustrated in  FIG. 5 , chopper wheel  20  may be equipped with a cam mechanism  42  having several degrees of operative freedom  43  that provide for user-defined adjustments  44  of the dimensions of the apertures  12 . When the source  10  is positioned farther away from the wheel  20 , and the FOV is reduced, the apertures  12  may be enlarged to allow more x-ray photons to traverse apertures  12 . On the other hand, when the source  10  is moved closer to the wheel  20  and the FOV of the system is increased, the apertures  12  may be appropriately closed down to reduce the flux. Furthermore, the spatial extent of the beam in a transverse direction may be adjusted by providing suitable means  46  for varying the extent of the aperture stop  12  of  FIG. 4 , thereby improving spatial, or areal, resolution. As a result, the flux of penetrating radiation reaching the object and, therefore, the quality of the x-ray imaging, may be maintained across the zooming range of the system of the invention. The adjustments of the spatial extent of radiation according to the embodiment of  FIG. 5  can be carried out at any instant of time and do not depend on instantaneous separation between the source and the chopper wheel. 
   Alternatively, maintaining a throughput flux substantially unchanged across the zooming range of the system can be achieved with an embodiment  60 , schematically depicted in  FIG. 6 . Here, wheel  20  contains a set of apertures  12  and is additionally furnished with a second set of apertures  52 . The two sets of apertures are disposed concentrically and circularly at different radii with respect to the axle  200  defining the rotational axis of wheel  20 , with the apertures  52  being appropriately smaller in extent than the apertures  12 . As shown in  FIG. 6 , the rotating wheel  20  creates, therefore, two complementary zones of apertures for scanning the radiation incident upon the wheel. In operation, source  10  (not shown) of embodiment  60  is typically adapted for repositioning not only along the local optical axis of the system but also in the transverse direction, parallel to x-axis as shown in  FIG. 6 . For example, solely repositioning of the source  10 , which is initially aligned for operation with the apertures  52 , away from the wheel  20  (in −z direction of  FIG. 6 ) reduces the FOV of the system and the flux captured by the apertures  52 , as was discussed in reference to  FIGS. 3 and 4 . However, a simultaneous relative displacement of the source transversely to the axis  200  would suitably align the source with the set of apertures  12  having larger dimensions and capable of accepting more x-ray photons, thus compensating for the reduction of flux due to increased source-to-field-stop distance L of the system. Functionally, therefore, the embodiment  60  accommodates scanning of the incident radiation closer to the axis of rotation for a distant imaging (or small FOV use) and toward the edge of the wheel for near-field imaging (or wide FOV use). A complex displacement of the source  10  of embodiment  60  is indicated in  FIG. 6  in projection on the plane of the wheel  20  with an arrow  54  and foot-prints  56  and  58  of the radiation pattern that correspond to the positions of the source  10  at shorter and longer distances l,L from the wheel, respectively. In  FIG. 7 , showing the embodiment  60  in top view, the initial and the final positions of the source  10  are respectively designated as i and ii. It is understood that having multiple sets of apertures at different radii on the spatial modulator  20  also provides additional flexibility in that, if space constraints do not allow the source  10  to be moved sufficiently far away from the modulator to cover the designed range of FOV, multiple sets of apertures help to recover a full range of zoom. 
   Embodiments of the current invention may provide advantages over the prior art by moving an x-ray source in the direction transverse to the optical axis of the system. In the embodiment  80  of  FIG. 8 , for example, the source  10  is displaced perpendicularly to the z-axis from the position j to another position jj, as indicated by an arrow  62 . A beam formed by the aperture(s)  12  of the wheel  20  and the collimator  22 , tracks the motion of the source, as represented by the respective change in the orientation of the marginal ray from  64 , j  to  64 , jj , and appropriately scans the target  66  in −x direction. Combined with scanning the radiation pattern in xy-plane due to rotation of the wheel  20  about axle  200 , such transverse repositioning  62  of the source  10  generates a raster scan of the target  66 . Although particularly suited for distant imaging, the use of this embodiment is not limited to that application. 
   In alternative embodiments of the present invention, the integration time of the detector of the imaging system may be synchronized with operator-modifiable speed of rotation of the wheel  20 . Such simultaneous adjustment of the scanning speed and detection time helps maintaining both the image size and the flux reaching the detector substantially unchanged across full zooming range of the imaging system. 
   All of the heretofore described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. For example, a chopper  20  performing spatial modulation of penetrating radiation and forming it into a scanning beam may be in the form of cylindrical chamber, as shown in  FIG. 9 . The orientation of apertures of the spatial modulator and that of the collimator, as well as mutual positioning of the modulator and collimator with respect to source  10  can be varied as dictated by the experimental use of the system. The order, in which the apertures of the spatial modulator and the collimator are disposed in the path of penetrating radiation with respect to the source of penetrating radiation, can be varied. In this regard it should be understood that for the purposes of this disclosure the designations “first aperture” and “second aperture” are reciprocal. An additional aperture, functioning as a field stop of the system, either variable or fixed, can be disposed in the path of radiation prior to or after the modulator. Change of rotational speed of the spatial modulator, synchronization of the speed of rotation of the spatial modulator with the integration time of the detector, or motor driving the translator for repositioning the source may be computerized or otherwise user-defined. Also, to effect relative motion of the source with respect to beam-forming apertures, the source may remain stationary and the spatial modulator and the collimator can be moved with respect to the source. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.