Patent Application
20080008372
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention.
In various embodiments, the method according to the invention comprising the steps of:(a) acquiring a plurality of projection images from different projection angles; (b) defining an area of interest of each projection image based on at least one predefined area; and (c) back-projecting the area of interest of each projection image to reconstruct at least one three dimensional image.
While the present technique is described herein with reference to medical imaging applications, it should be noted that the invention is not limited to this or any particular application or environment. Rather, the technique may be employed in a range of applications, such as baggage and parcel handling and inspection, part inspection and quality control, and so forth, to mention but a few.
The present invention also provides a system to construct a three-dimensional image of an object using tomosynthesis with significantly improved collimator artifacts reduction and improved image quality.
A stream of radiation 12 is emitted by the source 10 and impinges an object 20, for example, a patient in medical applications. A portion of the radiation 14 passes through or around the object 20 and impacts a detector array, represented generally at reference numeral 30. Detector elements of the array produce electrical signals that represent the intensity of the incident X-ray beam. These signals are acquired and processed to reconstruct a volumetric image or 3D image of the features within the object.
A collimator 40 is a device used in medical imaging applications to limit the field of an X-ray beam to a shape and size just sufficient to expose the area requiring diagnosis in a patient's body, and prevent unnecessary exposure of the surrounding area to X-rays. A collimator 40 may be placed before or after the patient or object on need basis. Generally in digital tomosynthesis pre-patient collimation is used The collimator 40 may define the size and shape of the X-ray beam 12 that emerges from the X-ray source 10. Apparently, the collimator 40 defines the field of view (FOV) in the projection images so that unnecessary radiation to patient anatomy outside the regions of clinical interest can be avoided or minimized.
Source 10 is controlled by a controlling device 50 which furnishes both power and control signals for tomosynthesis examination sequences, including positioning of the source 10 relative to the object 20 and the detector 30. Moreover, detector 30 is coupled to the controlling device 50, which commands acquisition of the signals generated in the detector 30. The controlling device 50 may also execute various signal processing and filtration functions, such as for initial adjustment of dynamic ranges, interleaving of digital image data, and so forth. In general, controlling device 50 commands operation of the imaging system 100 to execute examination protocols and to process acquired data. In the present context, controlling device 50 also includes signal processing circuitry, typically based upon a general purpose or application-specific digital computer, associated memory circuitry for storing programs and routines executed by the computer, as well as configuration parameters and image data, interface circuits, and so forth. The controlling device 50 coordinates with the imaging device 100 identifying the edges of the collimation. This is achieved by the conventionally used techniques including “accurate device feed back”, “image based detection method” or a combination of two or any other similar techniques present in the industry.
In the embodiment illustrated in
Additionally, as will be appreciated by those skilled in the art, the source of radiation may be controlled by an X-ray controller 52 disposed within controlling device 50. Particularly, the X-ray controller 52 is configured to provide power and timing signals to the X-ray source 10. A motor controller 54, also disposed within controlling device 50, may be utilized to control the movement of the positional subsystem 26.
Further, the controlling device 50 is also illustrated comprising a data acquisition system 56. The detector 30 is typically coupled to the controlling device 50, and more particularly to the data acquisition system 56. The data acquisition system 56 receives data collected by readout electronics of the detector 30. The data acquisition system 56 typically receives sampled analog signals from the detector 30 and converts the data to digital signals for subsequent processing by a computer 70. In another embodiment, the sampled signals are converted to digital signals within the detector 30, and the digital signals are communicated by a wired, optical or wireless interface to the data acquisition system 56.
The computer 70 is typically coupled to the controlling device 50. The data collected by the data acquisition system 56 may be transmitted to the computer 70 and moreover, to a memory 60. It should be understood that any type of memory adapted to store a large amount of data may be utilized by such an exemplary system 100. The memory 60 stores the vertices of the collimation device, which may be used for further processing in defining the area of interest of the image. The computer 70 is also configured to receive commands and scanning parameters from an operator via an operator workstation 80, typically equipped with a keyboard and other input devices. Computer 70 also performs the reconstruction of a volumetric image from the projection image data set. The projection images or the volumetric images may be transmitted to the display 90 for review and moreover, to a memory 60 for storage. An operator may control the system 100 via the input devices. Thus, the operator may observe the projection images or the reconstructed volumetric image and other data relevant to the system from computer 70, initiate imaging, and so forth. All these functions may be carried out by a single computer, or they may be distributed across several computers, maybe comprising specific hardware, for example for fast reconstruction.
A display 90 coupled to the operator workstation 80 may be utilized to observe the reconstructed volumetric image, or a suitably processed version thereof, and to control imaging. It should be further noted that the computer 70 and the operator workstation 80 may be coupled to other output devices, which may include standard or special purpose computer monitors and associated processing circuitry. One or more of the operator workstations 80 may be further linked in the system for outputting system parameters, requesting examinations, viewing images, and so forth. In general, displays, printers, workstations, and similar devices supplied within the system may be local to the data acquisition components, or may be remote from these components, such as elsewhere within an institution or hospital, or in an entirely different location, linked to the image acquisition system via one or more configurable networks, such as the Internet, virtual private networks, and so forth.
In the tomosynthesis imaging system 100, then source 10 emits an X-ray of radiation from a focal point. In an embodiment the collimator 30 is placed between the source 10 and the object 20. The stream of radiation is directed towards a particular region of the object. The particular region of the object is typically chosen by an operator so that the most useful scan of a region may be made. In a typical operation of the system 100, X-ray source 10 is positioned opposite the detector 40, with the object 20 (patient or other subject or object of interest) and the collimator disposed between, the X-ray source 10 may then project an X-ray beam from the focal point towards the detector 30, through the object 20. The collimator defines the field of view in the image and limits the excess exposure of radiation to the object. Since the initial beam is passed through the collimator, the beam impinging with the object will have the field of view of the collimator. Once the beam interacts with the object being imaged the intensities of the beam will be modulated by the characteristics of the object.
The computer is programmed to define an area of interest of the plurality of projection image acquired by the data acquisition. The area of interest is defined based on the field of view of the collimator from the projected images. The computer is also programmed to back project the area of interest of each projection image for reconstructing at least one 3D image. The processed data, the data of the projection image falling within the area of interest, are then typically input to a reconstruction algorithm to formulate a volumetric image of the scanned volume. In tomosynthesis, a limited number of projection images are acquired, typically thirty or less, each at a different angle relative to the object and detector. Reconstruction algorithms are typically employed to perform the reconstruction on this projection image data to produce the volumetric image. Reconstructed volumetric images may be displayed to show the three-dimensional characteristics of these features and their spatial relationships. The reconstructed volumetric image is typically arranged in slices. In some embodiments, a single slice may correspond to features of the imaged object located in a plane that is essentially parallel to the detector plane. Though the reconstructed volumetric image may comprise a single reconstructed slice representative of structures at the corresponding location within the imaged volume, more than one slice image is typically computed.
In one embodiment the distance between the source 210 and the detector 230 is approximately 180 cm and the total range of motion of the source 210 is between 31.5 cm and 131 cm, which translates to ±5° to ±20° where 0° is a centered position. In this embodiment, typically at least eleven projections are acquired, covering the fall angular range.
The detector 230 is generally formed by a plurality of detector elements, generally corresponding to pixels, which sense the intensity of X-rays that pass through and around a region of interest. Depending upon the X-ray attenuation and absorption for the intervening structures, the radiation impacting each pixel region will vary. In one embodiment, the detector 230 consists of a 2,048×2,048 rectangular array of elements, with a pixel size of 200 μm×200 μm, though other configurations and sizes of both detector 230 and its pixels are, of course, possible. Each detector element produces an electrical signal that represents the intensity of the X-ray beam at the position of the element on the detector.
In one embodiment, detector 230 is an amorphous silicon flat panel digital X-ray detector. However, detector 230 may be any X-ray detector that provides a digital projection image including, but not limited to, a charge-coupled device (CCD), a digitized film screen, or another digital detector such as a direct conversion detector. The low electronic noise and fast read-out times of such detectors enable acquisitions with many projections at low overall patient dose compared to competing detector technologies.
Once the projection radiographs have been obtained, they are then spatially translated with respect to each other and superimposed in such a manner that the images of structures in the tomosynthesis plane overlap exactly. The images of structures outside the tomosynthesis plane do not overlap exactly, resulting in a depth dependent blurring of these structures. By varying the amount of relative translation of the projection radiographs, the location of the tomosynthesis plane can be varied within the object. Each time the tomosynthesis plane is varied, the image data corresponding to the overlapping structures is superimposed and a 2-D image of the structure in the tomosynthesis plane is obtained. Once a complete set of 2-D images of the object has been obtained, a 3-D image of the object is generated from the set of 2-D images.
At block 540, the vertices defining the field of view of the collimator is identified. This could be strategically achieved in two steps: First, most of commercial X-ray collimators have built-in positioning feedback with an accuracy of 1 cm. Although the accuracy is far from enough, the information can be used as a starting point. Second, based on the collimator feedback, an image processing algorithm is typically used to refine the coordinates of the vertices by means of image search. At block 550, an area of interest of each projection image is identified. In an embodiment, the area of interest is identified based on the field of view of the collimator. Due to perspective projection, the vertices of the collimator will be projected to slightly different locations on the detector in relative to its origin, resulting the size and shape of the field of view defined by the collimator are different across projection images. Hence the vertices of the collimator are identified for each projection image. At block 560, part of the projection image falling within the field of view of the collimator is identified and is back-projected to reconstruct the image. Various back-projection techniques mention may be used in reconstructing the image.
At block 570, cropping of the reconstructed image is performed. This step is used if multiple slice images are reconstructed. The application of collimation and back projection of the projection image can result in an inadvertent effect: the image size of the reconstructed slice images is different (depending on the height). The reason for doing cropping is the cone-beam geometry. That is, the X-ray beam from a point source has a cone shape. Most of commercial x-ray equipment today utilizes point source. Because of the cone-beam geometry, when back-project projection data to reconstruct slice images, the slice image that is closer to the source will have smaller field of view than any slice further from the source. This effect results in the slice images with different image sizes. Hence all slice images are cropped to the same size irrespective of their distance to the source. In an embodiment the cropping is done based on a predefined area. The predefined area includes areas defined by the vertices of the field of view of the collimator in one or more projection images, or the field of view of the collimator defined in one or more reconstructed images. However there are many ways to do so, such as: crop all slice to a predefined size, crop all slice to the field of view of the bottom slice image, crop all slices to the field of view of the middle slice, crop all slices to the field of view of the top slice, crop all slices to the filed of view of the zer0-degree projection image, etc. Or the cropping may be done using P1˜P4 information from one or more projection images, P1˜P4 information from one or more slice images, or a mixture of both
While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made to the embodiments without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only, and should not limit the scope of the invention as set forth in the following claims.
