X-ray or gamma ray systems or devices – Specific application – Tomography
Reexamination Certificate
2002-12-03
2004-06-15
Bruce, David V (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Tomography
C378S021000, C378S901000
Reexamination Certificate
active
06751284
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of medical imaging, and more specifically to the field of tomosynthesis. In particular, the present invention relates to minimizing contrast variations in tomosynthetic reconstructions.
Tomographic imaging technologies are of increasing importance in medical diagnosis, allowing physicians and radiologists non-invasively to obtain three-dimensional representations of selected organs or tissues of a patient. Tomosynthesis is a variation of conventional planar tomography in which a limited number of radiographic projections are digitally acquired at different angles relative to the patient. In tomosynthesis, an X-ray source produces a fan or cone-shaped X-ray beam that is collimated and passes through the patient to then be detected by a set of detector elements. The detector elements produce a signal based on the attenuation of the X-ray beams. The signals may be processed to produce a radiographic projection, comprising generally the line integrals of the attenuation coefficients of the object along the ray path. The source, the patient, or the detector are then moved relative to one another for the next exposure, typically by moving the X-ray source, so that each projection is acquired at a different angle.
By using reconstruction techniques, such as filtered backprojection, the set of acquired projections may then be reconstructed to produce diagnostically useful three-dimensional images. Because the three-dimensional information is obtained digitally during tomosynthesis, the image can be reconstructed in whatever viewing plane the operator selects. Typically, a set of slices representative of some volume of interest of the imaged object is reconstructed, where each slice is a reconstructed image representative of structures in a plane that is parallel to the detector plane, and each slice corresponds to a different distance of the plane from the detector plane.
In addition, because tomosynthesis reconstructs three-dimensional data from projections, it provides a fast and cost-effective technique for removing superimposed anatomic structures and for enhancing contrast in in-focus planes as compared to the use of a single X-ray radiograph. Further, because the tomosynthesis data consists of relatively few projection radiographs that are acquired quickly, often in a single sweep of the X-ray source over the patient, the total X-ray dose received by the patient is comparable to the dose of a single conventional X-ray exposure and is typically less than the dose received from a computed tomography (CT) examination. In addition, the resolution of the detector employed in tomosynthesis is typically greater than the resolution of detectors used in CT examinations. These qualities make tomosynthesis useful for such radiological tasks as detecting pulmonary nodules or other difficult to image pathologies.
Though tomosynthesis provides these considerable benefits, the techniques associated with tomosynthesis also have disadvantages. In particular, the reconstruction problem is difficult to solve because only incomplete information is available due to the nature of the technique. That is, the radiographic projections may be acquired from only a few angles within a relatively narrow angular range and are not densely spaced over the full angular range, limiting the amount of information acquired, Advanced reconstruction algorithms are employed to solve these reconstruction problems. A good reconstruction algorithm provides efficient separation of overlying tissue, minimizes artifacts, and enhances contrast, particularly of small structures.
Reconstructed data sets in tomosynthesis often exhibit a blurring of structures in the direction of the projections that were used to acquire the tomosynthesis data. These artifacts associated with an imaged structure will vary depending on the orientation of the structure with respect to the acquisition geometry. Therefore, the blurring of structures may create undesirable image artifacts and inhibit the separation of structures located at different heights in the reconstruction of the imaged volume.
Systems employing advanced reconstruction algorithms utilizing a re-projection consistency constraint, either directly or indirectly, to obtain high-quality reconstructions attempt to recover the contrast of the imaged structures and to minimize the aforementioned blurring of structures. In algorithms incorporating a re-projection consistency constraint, the degree of contrast which can be recovered and the degree of blurring will vary depending on the algorithm used, the acquisition geometry of the imaging system, and the geometry, position, and orientation of the imaged object or structure. Examples of algorithms incorporating the re-projection consistency constraints include linear/additive ART, matrix inversion tomosynthesis (MITS), volumetric non-linear reconstruction, and generalized filtered backprojection. As can be observed in these algorithms, the contrast recovered and the remaining blur are interdependent. In particular, the re-projection consistency constraint has the effect of keeping constant the total amount of the contrast of the reconstructed structure and the contrast of the blurring artifacts associated with that structure. Hence, the better the reconstruction algorithm is at suppressing blurring, the better the contrast of the reconstructed structure.
However, the shape and extent of the remaining blur is strongly dependent on the shape and orientation of the structure in relation to the specific system geometry used for image acquisition, as discussed above. In particular, if the X-ray source travels along a generally linear trajectory during the imaging: process, a structure that is “long” in a direction generally parallel to the linear path of the source will produce a widespread blur, while a structure that is “short” in a direction generally parallel to the linear path of the source will produce only a localized blur. For example, an elongated structure will produce a widespread blur if it is oriented generally parallel to the linear path of the source, and only a localized blur if it is oriented generally perpendicular to the source trajectory. Due to the aforementioned interdependence between contrast of the reconstructed structure and the remaining blur due to that structure, the contrast of the reconstruction of that same elongated structure is higher if it is oriented generally perpendicular to the source trajectory, and lower if it is oriented generally parallel to the source trajectory.
One manner in which this problem has been indirectly addressed has been to utilize symmetric system geometries, such as in circular tomosythesis, which acquire projections at a number of different orientations relative to each structure orientation. For example, in circular tomosynthesis, the X-ray source is not moved in a linear trajectory, but is instead moved in a circular trajectory in a plane substantially parallel to the plane of the detector. However, in many instances, a less symmetric acquisition geometry may be preferred, for example for reasons of system complexity or scanning speed. For instance, in pulmonary tomosynthesis, a generally linear or an elongated two-dimensional geometry (such as elliptical) may be preferred. An effective method of minimizing contrast variability in reconstructed images while allowing the use of non-symmetric source trajectories, for example linear or elongated acquisition geometries, is therefore needed.
BRIEF DESCRIPTION OF THE INVENTION
The present technique provides a novel approach to correcting contrast asymmetry in three-dimensional images derived from radiographic projections. Particularly, the technique applies a filter which provides contrast symmetry in the reconstructed image. The technique thereby compensates for contrast variations attributable to acquisition system geometry and subject orientation.
In accordance with one aspect of the technique, a method is provided for processing radiographic image data
Claus Bernhard Erich Hermann
Eberhard Jeffrey Wayne
Bruce David V
Fletcher Yodar, P.C.
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