Patent Number: 
Section: claims

1. A method of reconstructing a tomographic image of a subject imaged with a nuclear imaging detector having a non-parallel hole collimator comprising:acquiring first projection data of a radiation field within the subject at a first view angle of said detector;acquiring second projection data of said radiation field within the subject at a second view angle of said detector different than said first view angle;combining the first and second projection data to form a difference image; anddetermining a location of the radiation field relative to the collimator surface using the difference image based on Point Spread Function (PSF) which is non-stationary and dependent on a location of a gamma event with respect to the collimator surface of the non-parallel hole collimator. 2. The method as recited in claim 1, wherein the non-parallel hole collimator is one of a group consisting of: a multi-focal collimator, a varying-focal length collimator, a fan beam collimator, and an astigmatic collimator. 3. The method as recited in claim 1, wherein the location determined using the difference image is used in a SPECT image reconstruction algorithm. 4. The method as recited in claim 1, wherein the location determined using the difference image is used in a nuclear planar image reconstruction algorithm. 5. The method as recited in claim 1, wherein said detector is swiveled about its central axis at said second view angle prior to acquiring said second projection data. 6. The method as recited in claim 1, wherein said detector at said second view is translated with respect to a center of a gantry on which said detector is mounted prior to acquiring said second projection data. 7. The method as recited in claim 1, wherein said detector at said second view angle is swiveled about its central axis and is translated with respect to a center of a gantry on which said detector is mounted prior to acquiring said second projection data. 8. A system for reconstructing images including:a nuclear imaging detector having a non-parallel hole collimator;a gantry supporting said detector; anda computer for combining two images of a radiation field obtained from said detector at different positions relative to said gantry, to form a difference image, and computing a location of said radiation field using the difference image based on Point Spread Function (PSF) which is non-stationary and dependent on a location of a gamma event with respect to a collimator surface of the non-parallel hole collimator. 9. The system as recited in claim 8, wherein the non-parallel hole collimator is one of a group consisting of a multi-focal collimator, a varying-focal length collimator, a fan beam collimator, and an astigmatic collimator. 10. The system as recited in claim 8, wherein said computed location is used in an image reconstruction algorithm. 11. A computer program embodied as computer-executable instructions stored on a computer-readable medium, the program comprising instructions for:combining at least two nuclear images of a radiation field within an imaging subject acquired by a nuclear detector having a non-parallel hole collimator, taken from different locations relative to said non-parallel hole collimator, to form a difference image; andcalculating a location of the radiation field relative to the surface of the non-parallel hole collimator using the difference image based on Point Spread Function (PSF) which is non-stationary and dependent on a location of a gamma event with respect to the surface of the non-parallel hole collimator. 12. The computer program as recited in claim 11, wherein said location is calculated using a chi-squared algorithm. 13. The computer program as recited in claim 11, wherein the program further comprises determining the distance between the radiation field and a surface of the collimator. 14. The computer program as recited in claim 11, wherein the program further comprises instructions to reconstruct a SPECT image. 15. The computer program as recited in claim 11, wherein the program further comprises instructions to reconstruct a nuclear planar image. 16. A method of estimating depth information of a radiation event from different planar images acquired using a detector having a non-parallel hole collimator, comprisingobtaining a difference image between said different planar images; andestimating depth information based on Point Spread Function (PSF) which is non-stationary and dependent on a location of a gamma event with respect to a collimator surface of the non-parallel hole collimator. 17. The method as recited in claim 16, wherein the non-parallel hole collimator is one of a group consisting of: a multi-focal collimator, a varying-focal length collimator, a fan beam collimator, and an astigmatic collimator.