Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
2001-09-28
2004-05-18
Luu, Thanh X. (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S23700G
Reexamination Certificate
active
06737652
ABSTRACT:
BACKGROUND OF THE INVENTION
Coded aperture imaging has been proposed in the past as a means for improving the spatial resolution, sensitivity, and signal-to-noise ratio (SNR) of images formed by x-ray or gamma ray radiation. For many imaging applications, coded aperture cameras have proven advantageous relative to other candidate systems, including the single pinhole camera and multihole collimator systems. In contrast to these other systems, for instance, the coded aperture camera is characterized by high sensitivity, while simultaneously achieving exceptional spatial resolution in the reconstructed image.
In contrast to the single pinhole camera, coded aperture systems utilize multiple, specially-arranged pinholes to increase the overall photon transmission, and hence the sensitivity, of the imaging camera. In operation, radiation from the object to be imaged is projected through the coded aperture mask and onto a position-sensitive detector. The coded aperture mask contains a number of discrete, specially arranged elements that are either opaque or transparent to the incident photons. The raw signal from the detector does not reflect a directly recognizable image, but instead represents the signal from the object that has been modulated or encoded by the particular aperture pattern. This recorded signal can then be digitally or optically processed to extract a reconstructed image of the object.
SUMMARY OF THE INVENTION
Most commonly, coded aperture imaging techniques are employed in the field of astronomy. For instance, coded aperture techniques have been used for many years in the imaging of distant x- or gamma-ray astronomical sources. These applications are classic “far field” (i.e. small object angle) imaging applications, where, under the proper conditions, conventional deconvolution techniques may be employed to produce ideal images of the source.
However, for near field applications, including nuclear medicine, molecular imaging, materials analysis, and contraband detection, among others, these same deconvolution methods do not produce ideal object images. Rather, in near field imaging applications, the reconstructed object images are corrupted by artifacts, including unsightly lines and/or bows through the object image. The presence of these artifacts render coded aperture systems far less attractive for near field imaging applications, particularly for applications such as nuclear medicine imaging, which rely heavily on visual inspection of the reconstructed object image. In the particular case of nuclear medicine imaging, for example, the presence of these artifacts may render it difficult or impossible to distinguish actual objects from artifacts in the reconstructed images, and may be a prohibitive factor weighing against the widespread adoption of coded aperture techniques.
The present invention relates to improved systems and methods for near-field coded aperture imaging of radiation-emitting sources. According to one aspect, the present invention is directed to reducing and/or eliminating the artifacts that are inherent in previous near field coded aperture imaging systems such that the improved sensitivity and resolution of these systems can be practically utilized.
In one aspect, the present invention is based upon the recognition that near field artifacts result primarily from the physical process of projecting radiation from the object through the coded aperture and onto the detector. It has heretofore been assumed that, for coded aperture arrays having perfect cross-correlation properties, the projected image obtained at the detector is the convolution of the object with the aperture mask. In fact, however, this projection contains a non-linear cos
3
(&thgr;) term affecting the image. For far field applications, including astronomical applications where the object-to-detector distance is many orders of magnitude greater than the detector size, this cos
3
(&thgr;) term can be approximated as equal to one, and the projected image on the detector is the straightforward convolution of the object and aperture mask.
For the near field applications of the present invention, however, this cos
3
(&thgr;) term does not approximate as one, and therefore it gives rise to visible artifacts which corrupt the reconstructed object image. The present invention relates in one aspect to systems and methods for reducing or removing these artifacts from the reconstructed image. More specifically, the invention comprises providing a radiation-emitting object within the field of view of a coded aperture camera; generating a signal corresponding to a first image of radiation from the object projected through a first coded aperture mask pattern; generating a signal corresponding to a second image of radiation from the object projected through a second coded aperture mask pattern, where the second pattern is the “negative” of the first; and then processing the combined data from these projections to obtain a reconstructed image of the object.
In the context of the present invention, the second mask pattern is related to the first mask pattern because the second pattern is the one associated to the negative of the decoding pattern associated with the first physical mask pattern. For each coded aperture mask pattern, there is an associated decoding function, G, that is used to decode the recorded image and produce the source reconstruction. One may change the sign of G to produce −G, i.e. the negative of the decoding array. The second projection is then taken with the physical mask associated with −G, which according to this invention is referred to as the “negative” mask. As near-field artifact effects are dependent on G and −G, when the data sets from both projections are combined, the near-field artifacts cancel from the reconstructed image.
Some mask families, including the URA and m-sequence families, have the particular property that the “negative” mask may be produced by simply interchanging the position of the opaque and transparent elements of the physical mask. This is because, for these families, the array, A, and decoding array, G, are identical. Thus, when signs change for the transparent and opaque elements of the decoding array, the elements simply interchange with one another. Also, because the array and decoding array are identical, the elements similarly change for the associated “negative” physical mask. The resultant mask, with the elements interchanged, is sometimes referred to as the “antimask” of the first mask.
For some near-field applications, two physical masks may be used to provide the two masks: the first being the mask pattern, and the second being the negative mask pattern. Depending on the particular mask family chosen, the second mask could be the antimask of the first. In this example, the physical masks can be interchanged between projections, either by operator intervention, or by an automated process.
In other cases, depending on the particular mask pattern used, a single physical mask may comprise both the mask and its negative mask. For some mask families, for instance, the negative of the original mask pattern (i.e. the mask associated with −G) may be obtained by simply rotating the physical mask by a given angle. For certain antisymmetric mask patterns, including a No-Two-Holes-Touching MURA pattern, for instance, a first mask pattern will produce its negative pattern when rotated by 90°.
The use of a mask and its antimask has been reported previously in connection with far-field astronomical applications for the much different goal of reducing non-uniform background and compensating for systematic irregularities. For instance, in the context of a balloon or satellite-borne gamma-ray telescope, the mask-antimask technique may be useful to “average out” irregularities that may develop in the detector apparatus, particularly given the practical difficulties in accessing and actively maintaining the equipment.
On the other hand, near field coded aperture imaging applications uniquely suffer from the problem of near fiel
Accorsi Roberto
Gasparini Francesca
Lanza Richard C.
Hamilton Brook Smith & Reynolds P.C.
Luu Thanh X.
Massachusetts Institute of Technology
Yam Stephen
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