As is known in the art, two techniques for acquiring images of body sections are tomosynthesis and tomography.
In tomosynthesis, a plurality of images are acquired as a radiation source traverses a path relative to the object being imaged. The radiation source emits a radiation signal (typically collimated) which impinges upon the object being imaged. At least portions of the emitted radiation signal propagate through the object and are sensed or detected by a fixed detector located on an opposite side of the object from the source.
The images acquired by the detector are combined (reconstructed) by a system of back-projection, error calculation and correction, and forward projection repeated one or more iterations until a stopping criteria (including one of iteration count) is reached. It is possible to reconstruct any plane in the object being imaged that is parallel to the detector. Generally, features of structures outside of the tomosynthesis plane of interest are of reduced intensity do not align precisely when they are so combined, which results in a depth-dependent reinforcement of in-plane, and diminution of out-of-plane structures. These out-of-plane structures are superimposed into the reconstructed plane, which can degrade the overall reconstructed image quality and results in an image having relatively low depth resolution.
While the above-described tomosynthesis technique has proven useful for detecting early forms of breast cancer, it is still possible for the detection of breast cancer to be missed by a physician or radiologist reviewing the data. For example, breast cancer may be missed by being obscured by radiographically dense, fibroglandular breast tissue or positioned inadequately outside the imaging volume for example.
The tomosynthesis technique of imaging tissue also generally requires that the detector and the object being imaged (e.g. a breast) be maintained in a stationary or fixed position while the radiation source is moved and positioned for obtaining the plurality of images. This limits the range over which the source can be moved and thus limits the number of different angles of images available to provide the final reconstructed image of the object
Similar to tomosynthesis systems described above, tomography systems also incorporate a radiation source and a detector. In contrast to tomosynthesis systems, tomography systems move both the radiation source and the detector mutually about the object being imaged.
In a tomography system, a plurality of images are acquired as a collimated radiation source traverses a path relative to the object being imaged. The radiation source emits a radiation signal which impinges upon the object being imaged. At least portions of the emitted radiation signal propagate through the object and are sensed or by a detector or detectors located on an opposite side of the object from the source. As mentioned above, in a tomography system, both the radiation source and the detector traverse a path about the object being imaged while the object itself is maintained in a stationary position. In one prior art scheme, the source and detector are translated back and forth and at the end of each translation the source and detector are incremented rotationally to maintain a fixed spatial relationship. Signals corresponding to detected density variations are fed into a computer which, when information from the whole scan is complete, produces data representative of density variations in a plane which is transverse to the translation direction through which the source and detector are moved. An image can then be constructed from the data.
One drawback associated with such tomography systems when applied to certain imaging problems such as breast imaging is that the most desirable source and/or detector path can physically intersect with the body or organ being imaged (e.g. a breast). This is due, in part, because the location of the point or axis about which the radiation source and detector move is typically located proximate the object being imaged. Another drawback associated with tomography systems is that to compensate for the limited movement of the system about a path surrounding the object being imaged, the source provides a larger area of radiation which can result in radiating structures that are not of interest. For example, when it is desired to perform breast imaging, it may be necessary to also expose the chest, shoulder and heart/lungs of the person to radiation in order to provide sufficient data to accomplish the breast tomography.
It would, therefore, be desirable to provide a tomography system that allows imaging of an entire object without having source, detector or support structures intersect with the object or body of the subject being imaged. It would also be desirable to provide a system which reduces the amount of radiation directed toward structures not being imaged. It would be further desirable to provide a breast imaging system in which a source and detector are able to move over a relatively wide range of angles to allow a clinically useful set of measurements to be made.