Conventional C-arm X-ray imaging systems acquire images along planar paths having a 3-dimensional acquisition axis. The 3-dimensional acquisition axis is the axis about which the x-ray source and radiation detector, held in fixed geometry by the C-arm, rotate. An examination subject being x-rayed by a conventional imaging system may have highly dense, radio-opaque objects (e.g. dental fillings, aneurysm clips or stents, screws, plates, etc.) disposed at various points through his/her body. These radio-opaque objects may include metals or other dense materials. When attempting to capture an image of an area of an examination subject's anatomy proximate the radio-opaque object(s), high x-ray absorption and deflection or scatter of the x-rays is directed at these objects. The deflected and scattered x-rays are picked up by the radiation detector at various locations other than their anticipated path from the source to the radiation detector. When the x-rays are deflected and strike the radiation detector, added noise is introduced into the x-ray image data. While some absorption and scatter is expected, the increased or complete absorption due to the presence of highly dense objects in the subject being imaged will result in artifact(s) obscuring the surrounding anatomy that is of interest. These artifact(s) include at least one of beam hardening artifacts and scatter artifacts. Beam hardening artifacts arise when x-rays are completely blocked from reaching the radiation detector due to the presence of highly dense objects. The presence of beam hardening artifacts produces streaking in the x-ray images. Scatter artifact occurs when dense objects deflect the x-rays and redirect them in different directions. When the x-rays strike the radiation detector, artifacts will result which effect image quality of the x-ray images. These artifact(s) created by the presence of metal or similarly dense radio opaque objects within the examination subject being x-rayed are hereinafter referred to as “metal artifacts.” These metal artifacts degrade the quality of the 3-dimensional image to be constructed. These metal artifacts are manifested in the 3-dimensional image as lines emanating from and extending radially away from the object. The metal artifacts raise the intensity value of voxels (a combination of the words volumetric and pixels) along these lines with a maximum increase in intensity proximal to the object and decreasing intensity moving away from the object. The term voxel refers to a volume element in 3-dimensional space. A voxel in 3-dimensional space is analogous to a pixel in 2-dimensional space. The metal artifacts are worst along the path of the x-ray beams as they pass through the radio-opaque object(s). By nature of conventional scanning geometry (rotation of the source and detector within a single plane) the metal artifacts are most prominent and create the most effects adjacent to the radio-opaque object and in the plane of rotation—within the acquisition plane. This means that metal artifacts in the 3-dimensional (CT) image are fixed and are largely constrained to the planes containing the radio-opaque object generating the metal artifacts and perpendicular to the 3-dimensional acquisition axis. The 3-dimensional acquisition axis is the axis about which the x-ray source and radiation detector rotate.
If the region of interest in the examination subject happens to lie adjacent to the radio-opaque object and in a direction perpendicular to the 3-dimensional acquisition axis, the metal artifacts in the image significantly degrade and even preclude an accurate diagnosis of the region of interest when the images are acquired along a planar path. A path, as used hereinafter, refers to the route along which the image acquisition system travels. A planar path refers to a path along a single plane.
FIGS. 2 and 3 illustrate the planar 3-dimensional acquisition problem caused by metal artifacts introduced by a radio-opaque object inside the examination subject along a single plane. The planar 3-dimensional path is defined by the rotation of the x-ray source and radiation detector about a single, fixed rotation axis 204. Metal artifacts 201 are formed by a previously-implanted platinum coil, which has been implanted in the examination subject being imaged in order to treat an aneurysm. The platinum coil mass 202 is located in close proximity to a vessel, which contains a stenosis 205. It is desirable to obtain a 3-dimensional constructed image of the examination subject that accurately depicts the vessel containing the stenosis 205 along with its location with respect to the coil mass 202 and other anatomy. This image is used to quantify the stenosis 205 and evaluate treatment options (e.g. angioplasty, stenting or stenting with angioplasty). However, metal artifacts 201 produced in a constructed 3-dimensional image taken along a planar path obscures the stenosis 205. The effects of the metal artifacts are worst and degrade the image most in the path of the x-ray beam and adjacent to the radio-opaque object(s) causing the metal artifacts.
FIG. 3 further shows the effects of metal artifacts 301 each time the x-ray imaging system is rotated and image data is acquired. FIGS. 2 and 3 show a situation where the axis of rotation is perpendicular to the image (e.g. coming out of the image toward the reader). In this arrangement, the metal artifacts branch out from a fan beam 203 in FIGS. 2 and 3. In the planar acquisition situation, as depicted in FIGS. 2 and 3, the number of projections that contain metal artifacts for areas around the radio-opaque object greatly outnumbers the number of projections that do not contain metal artifacts. When the ratio of the number of projections that contain metal artifacts to those that do not is high, more metal artifacts are present in the constructed image.
Metal artifact is sometimes moved away from a region of interest by changing the anatomical geometry of the examination subject with respect to the acquisition axis. For example, if an examination subject has dental fillings the examination subject is moved with relation to the scanner by tilting the head to shift metal artifacts created from dental fillings away from a particular region of interest (e.g. skull base or carotid arteries). However, moving the examination subject may not always be viable as it is not always possible to reorient the examination subject's anatomy with respect to the table support. Tilting the examination subject's head is hindered by the presence of a breathing tube or is precluded by a need to maintain examination subject's current positioning.
In computed tomography (CT) scanners the rotation plane is titled with respect to the examination subject (by applying a gantry tilt in a cranial or caudal direction), allowing the metal artifacts to be shifted without moving the examination subject. A “cranial” direction of movement is movement from the patient's middle toward the patient's head, while a “caudal” direction of movement is a movement from the patient's middle toward the patient's feet. Known C-arm X-ray imaging systems acquire 3-dimensional images by rotating the source and detector (fixed in geometric relationship by the C-arm) of the imaging system about a system specified acquisition axis. This rotation about a single acquisition axis forces a beam axis to be contained in a single plane, throughout the movement of the C-arm. This acquisition geometry places the majority of metal artifact adjacent to the artifact generating object and within planes in the reconstructed image that are parallel to the acquisition plane (perpendicular the acquisition axis). As seen in FIGS. 2 and 3, metal artifact can negatively affect the readability and diagnostic value of the acquired images and, by virtue of the mechanism by which the imaging data is acquired, the metal artifact is focused in the acquisition plane.
Additionally, a great deal of effort has been spent improving the quality of 3-dimensional images that contain metal artifacts, by attempting to improve the quality of the data within the signals acquired. However, solutions that shift metal artifacts within the image do not reduce the overall effect of the metal artifacts in the image because the image data is acquired along a planar path. Image data acquired along a planar path will still contain the effects of metal artifacts. Several post processing techniques to reduce the overall effect of metal artifacts have been made. The main disadvantage of post processing techniques to reduce overall effects of the metal artifacts is that the metal artifacts exist in the image data acquired. There is a need to reduce the overall effects of the metal artifacts present in the image data acquired. A system according to invention principles addresses these deficiencies and related problems.