Apparatus and method for image reconstruction for a synthetic aperture gamma ray imager

1. An imaging system, comprising:

a platform having mounted thereon a coded-aperture imaging device and positioned to receive radiation over a baseline; and

a computer configured to:

acquire a plurality of far-field datasets over the baseline, the plurality of far-field datasets comprising data received via the coded-aperture imaging device;

form a first image from the plurality of far-field datasets; and

form a second image if the first image indicates presence of a source, the second image formed from the plurality of far-field datasets using an estimated source location from the first image and thereby having a higher contrast than the first image.

2. The imaging system of claim 1 wherein the computer is configured to use the estimated source location from the first image and enhance contrast of the second image via weighting factors that are computed using the far-field datasets as a function of platform position.

3. The imaging system of claim 2 wherein the computer is configured to form the second image via one of back-projection and an Ordered Subsets Expectation Maximization (OSEM) algorithm.

4. The imaging system of claim 1 wherein the computer is configured to back-project the acquired plurality of far-field datasets to form the first image.

5. The imaging system of claim 4 wherein the computer is configured to back-project each far-field dataset to form the first image in one of a list mode as a single-photon operation and an aggregated mode using subsets of the plurality of far-field datasets as a single spatial response.

6. The imaging system of claim 1 further comprising a mobile vehicle having the computer and the platform mounted thereon.

7. The imaging system of claim 1 wherein the coded-aperture imaging device comprises:

a position-sensitive detector; and

a mask having a defined pattern of apertures therein such that each acquired far-field dataset is a shadow of the mask formed at positions on the position-sensitive detector.

8. The imaging system of claim 7 wherein the position-sensitive detector includes a two-dimensional array of elements configured to be stimulated in one dimension as a function of azimuthal angle and in a second dimension as a function of elevation angle.

9. A method of imaging comprising:

acquiring, via a position-sensitive detector, a plurality of far-field images obtained over a plurality of sub-apertures, each far-field image comprising a measured distribution of photons on the position-sensitive detector acquired along an imaging baseline;

reconstructing an initial near-field image using the plurality of far-field images;

assessing the initial near-field image to determine whether a source is indicated to be present in the initial near-field image; and

reconstructing a second near-field image using data that comprises the plurality of far-field images if a source is indicated to be present in the first near-field image, the second near-field image having a higher contrast than the initial near-field image.

10. The method of imaging of claim 9 comprising, if a source is indicated as present in the initial near-field image:

determining a location of the source in the initial near-field image;

calculating a weighting vector that is a function of a location of the position-sensitive detector with respect to the location of the source; and

reconstructing the second near-field image having enhanced contrast using the weighting vector.

11. The method of imaging of claim 9 wherein reconstructing the second near-field image comprises reconstructing the second near-field image via one of back-projection and an Ordered Subsets Expectation Maximization (OSEM) algorithm.

12. The method of imaging of claim 11 wherein, if the second near-field image is reconstructed via an OSEM algorithm, then reconstructing the second near-field image comprises reconstructing the second near-field image via at least three iterations of the OSEM algorithm.

13. The method of imaging of claim 9 comprising transporting the position-sensitive detector to a plurality of imaging locations over the imaging baseline; and

wherein the imaging baseline is defined by motion of a vehicle by which the position-sensitive detector is transported.

14. The method of imaging of claim 9 wherein acquiring the plurality of far-field images comprises acquiring the measured distribution of photons on the position-sensitive detector in both an azimuthal direction and an elevational direction.

15. The method of imaging of claim 9 comprising reconstructing the initial near-field image by back-projecting each far-field image in one of a list mode and an aggregated mode.

16. A non-transitory computer readable storage medium having stored thereon a program, which, when executed by a processor, causes the processor to:

acquire a plurality of far-field images from a coded-aperture imaging device over sub-apertures of a synthetic aperture;

reconstruct a first near-field image of the source using the far-field images, the first near-field image having a first contrast; and

if a source is present in the first near-field image, the processor is further caused to reconstruct a second near-field image using the far-field images, the second near-field image having a second contrast that is higher than the first contrast.

17. The non-transitory computer readable storage medium of claim 16 wherein, if the processor is caused to reconstruct a second image, the processor is caused to calculate a plurality of weighting values that are a function of an estimated location of the coded-aperture imaging device in the first near-field image and to reconstruct and enhance contrast of the second near-field image using the plurality of weighting values.

18. The non-transitory computer readable storage medium of claim 17 wherein the plurality of weighting values, when expressed as a variable W, are calculated as:

W i =|X−X| −2 ×cos( Z );

where Z is an angle between a normal of the synthetic aperture at an i th location and the location of the coded-aperture imaging device in the first near-field image;

X i is a location of a center of the i th sub-aperture; and

X is a location of the source in the first near-field image.

19. The non-transitory computer readable storage medium of claim 16 wherein the program causes the processor to reconstruct the first near-field image in one of a list-mode operation and an aggregated operation.

20. The non-transitory computer readable storage medium of claim 19 wherein, for C measured counts, the list-mode operation is expressed as a sum:

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wherein:

(x i ,y i ) is a location of the imaging device at the time of measurement of the i th count;

θ i is the angular orientation of the platform at that moment; and

d i is the detector that recorded the i th count.

21. The non-transitory computer readable storage medium of claim 19 wherein, for an aggregated operation, the aggregated back-projection is expressed as a sum:

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wherein

S is the number of baseline segments defined for the aggregation scheme;

C s is the number of counts in each segment s;

(x s ,y s ) is the nominal location of the segment, defined by:

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and

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;

and

θ s is the orientation angle of segment “s.”

22. The non-transitory computer readable storage medium of claim 16 wherein, if a source is present in the first near-field image, the processor is further caused to reconstruct the second near-field image using one of back-projection and an Ordered Subsets Expectation Maximization (OSEM) algorithm.

23. An imaging system, comprising:

a platform having mounted thereon a coded-aperture imaging device and positioned to receive radiation over a baseline; and

a computer configured to:

acquire a plurality of far-field datasets over the baseline, the plurality of far-field datasets comprising data received via the coded-aperture imaging device;

acquire first image data from a previous field of view;

calculate weighting factors from the first image data; and

form a second image from the plurality of far-field datasets and enhance contrast thereof using the calculated weighting factors.