Coded aperture imaging

• US 20020075990A1
• Filed: 09/28/2001
• Published: 06/20/2002
• Est. Priority Date: 09/29/2000
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Gasparini, Francesca, Lanza, Richard C., Accorsi, Roberto
• Granted Patent: US 6,737,652 B2
• Time in Patent Office: Days
• US Class Current: 378/2
• CPC Class Codes: G01T 1/295 using coded aperture devices, e.g. Fresnel zone plates handling of radiation of particles, e.g. using diaphragms, collimators, diffraction G21K1/00

Abstract

Improved systems and methods for 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.

A system and method of the present invention utilizes two projections of radiation from an object, the first through a first coded aperture mask pattern, and the second through a second coded aperture mask pattern, where a decoding array associated with the second mask pattern is the negative of a decoding array associated with the first mask pattern. Data from both projections is combined to produce a reconstructed object image that is substantially free of near-field artifacts.

The present invention additionally relates to further improvements in the sensitivity and spatial resolution of coded aperture imaging applications. More particularly, the present invention relates to improvements in the design and fabrication of coded aperture masks for use in coded aperture imaging devices. Improvements can be made to the resolution, for instance, by selecting smaller pixel sizes for the opaque and transparent elements of the coded aperture mask. Also, it is possible to improve the signal-to-noise ratio for near-field applications by selecting an appropriate thickness for the mask. The signal-to-noise ratio may be further improved by selecting the appropriate mask pattern based on the particular characteristics of the near-field imaging problem at hand.

Claims

1. A method of imaging radiation using coded apertures comprising:

generating a first signal at a detector as a result of near-field radiation from an object being projected through a first coded aperture mask pattern;

generating a second signal at the detector as a result of near-field radiation from the object being projected through a second coded aperture mask pattern, where the second mask pattern is associated with a decoding array that is the negative of a decoding array associated with a first pattern; and

combining the first and second signals to obtain a reconstructed image of the object.

2. The method of claim 1 wherein the object is located less than about 10 meters from the detector.

3. The method of claim 1 wherein an image of the object projected through a coded aperture mask pattern is the convolution of the coded aperture mask pattern with the object that is modulated by a cos³ (θ

) term, where cos³ (θ

) is not approximately equal to one.

4. The method of claim 1 wherein the emitted radiation is in the form of x-rays or gamma rays.

5. The method of claim 1 wherein the emitted radiation is in the form of infrared radiation.

6. The method of claim 1 further comprising interrogating the object with an energy source to produce the emitted radiation.

7. The method of claim 1 wherein the object is anatomical.

8. The method of claim 7 further comprising evaluating the reconstructed image to determine the presence of tumors.

9. The method of claim 7 wherein the object contains a radioactive material.

10. The method of claim 1 further comprising evaluating the reconstructed image to determine the existence of contraband articles.

11. The method of claim 1 further comprising:

after radiation from the object is projected through the first coded aperture mask pattern, replacing a first coded aperture mask with a second coded aperture mask.

12. The method of claim 1 wherein a single coded aperture mask comprises both the first mask pattern and the second mask pattern.

13. The method of claim 12 further comprising:

after radiation from the object is projected through the first coded aperture mask pattern, rotating the coded aperture mask by a predetermined angle to provide the second mask pattern.

14. The method of claim 13 wherein the pre-determined angle is 90 degrees.

15. The method of claim 1 wherein radiation from the object is projected through the first coded aperture mask pattern and radiation from the object is projected through the second coded aperture mask pattern for approximately equal time durations.

16. The method of claim 1 further comprising positioning the camera and/or object such that the center of mass of the object is at about the center of the field of view of the camera.

17. The method of claim 1 wherein the generated signals are processed using a decoding function associated with each mask pattern.

18. The method of claim 17 wherein the decoding function is scaled to obtain a reconstructed image of a two-dimensional slice of a three dimensional object.

19. The method of claim 1 wherein the generated signals from the first and second projections are added to cancel near-field artifacts that would otherwise appear in the reconstructed image.

20. The method of claim 1 wherein processing data from the projections includes dividing the recorded data by a pre-factor to correct for zero-order artifacts.

21. The method of claim 1 further comprising:

while obtaining an image projected through a mask pattern, changing the position of the object relative to the coded aperture mask and the detector to obtain an image of a cross-sectional slice of a three-dimensional object.

22. The method of claim 1 wherein the mask patterns are mosaicked.

24. The apparatus of claim 23 wherein the detector array receives radiation from an object that is less than about 10 meters from the detector array.

25. The apparatus of claim 23 further comprising display means for displaying a visible image of the object.

26. The apparatus of claim 23 wherein the emitted radiation is in the form of x-rays or gamma rays.

27. The apparatus of claim 23 wherein the emitted radiation is in the form of infrared radiation.

28. The apparatus of claim 23 wherein the at least one coded aperture mask comprises a first coded aperture mask having a first mask pattern and a second coded aperture mask having a second mask pattern.

29. The apparatus of claim 28 additionally comprising means for interchanging the position of the first and second masks.

30. The apparatus of claim 23 wherein a single coded aperture mask comprises the first mask pattern and the second mask pattern.

31. The apparatus of claim 30 additionally comprising means for rotating a coded aperture mask having a first mask pattern by a pre-determined angle to provide the second mask pattern.

32. The apparatus of claim 23 further comprising means for changing the position of the object relative to the coded aperture mask and the detector to obtain an image of a cross-sectional slice of a three-dimensional object.

33. The apparatus of claim 23 wherein the mask patterns are mosaicked.

35. The method of claim 34 wherein the elements are less than 2 mm wide.

36. The method of claim 34 wherein the elements are approximately 1.5 mm wide.

38. The method of claim 37 wherein the mask thickness is approximately 1.5 mm.

39. The method of claim 37 wherein the figure of merit is the inverse of the standard deviation of the reconstructed image of a point source of radiation projected through a selected mask pattern, the point source located at the center of the field of view of the mask pattern.

40. The method of claim 37 wherein the figure of merit is the signal-to-noise ratio of the reconstructed image of a point source of radiation projected through a selected mask pattern, the located at a given position within the field of view of the mask pattern.

23. An apparatus for imaging an object emitting near-field radiation comprising:

a detector array for detecting at least one of the energy or position of radiation emitted by the object and generating a signal in response thereto;

at least one coded aperture mask, including a first mask pattern and a second mask pattern, disposed between the detector and the object, such that a first portion of the radiation is detected by the detector array after passage through the first mask pattern, and a second portion of the radiation is detected by the detector array after passage through the second mask pattern, where the second mask pattern is associated with a decoding array that is the negative of a decoding array associated with a first pattern; and

a data processor for processing signals, the signals generated in response to radiation passed through the first mask pattern and radiation passed through the second pattern, to produce a reconstructed image of the object that is substantially free of near-field artifacts.

34. A method for fabricating a coded aperture mask for use in a coded aperture imaging device, the coded aperture imaging device including an array of detector elements and having a selected field-of-view, the method comprising:

selecting a pixel size for elements of a coded aperture mask, such that the size of the pixels approximately matches the smallest available pixel size to maintain the field-of-view of the coded aperture imaging device;

selecting a coded aperture mask pattern; and

on an aperture plate, defining a plurality of opaque and transparent elements, the opaque elements being substantially opaque to radiation emitted by an object to be imaged and the transparent elements being substantially transparent to radiation emitted by an object to be imaged, where the plurality of opaque and transparent elements is defined according to the selected coded aperture mask pattern, and each element has a size that is equal to the selected pixel size.

37. A method for fabricating a coded aperture mask for use in a coded aperture imaging device, where the device includes a detector for detecting radiation emitted from objects located at a pre-determined distance from the detector and a processor for reconstructing an image of the object, the method comprising:

selecting a coded aperture mask pattern;

defining a figure of merit;

through simulation, graphing the figure of merit as a function of mask thickness for the pre-determined object-to-detector distance;

based upon the graphing, selecting a mask thickness that approximately matches the maximum value of the figure of merit for the pre-determined object-to-detector distance; and

on an aperture plate having the selected mask thickness, defining a plurality of opaque and transparent elements, the opaque elements being substantially opaque to radiation emitted by an object to be imaged and the transparent elements being substantially transparent to radiation emitted by an object to be imaged, where the plurality of opaque and transparent elements is defined according to the selected coded aperture mask pattern.

41. A method for fabricating a coded aperture mask for use in a coded aperture imaging device, comprising:

selecting a material for fabricating the coded aperture mask, the material having a thickness;

calculating the signal-to-noise ratio (SNR) for at least one coded aperture mask pattern using the equation;

${\mathrm{SNR}}_{\mathrm{ij}}=\frac{\left(1-t\right)<br><br>{\mathrm{N\psi <br><br>}}_{\mathrm{ij}}}{\sqrt{\left(1-t\right)<br><br>\underset{k,l}{\sum <br><br>}<br><br>{\mathrm{\psi <br><br>}}_{\mathrm{kl}}<br><br>\underset{u,v}{\sum <br><br>}<br><br>A_{\mathrm{kl}}<br><br>G_{\mathrm{ij}}^2+\left(t+\mathrm{\xi <br><br>}\right)<br><br>\underset{u,v}{\sum <br><br>}<br><br>G_{\mathrm{ij}}^2}}$

where t is an estimate of an amount of radiation from an object to be imaged that is transmitted through opaque elements of the mask, N is a number of open holes in the mask, ψ

is an estimate of the concentration of the object to be imaged, A is a mask array function, G is a decoding array function associated with the mask array function, and ξ

is an estimate of background radiation;

based upon said calculation, selecting a coded aperture mask pattern; and

on the selected material, defining a plurality of opaque and transparent elements, the opaque elements being substantially opaque to radiation emitted by an object to be imaged and the transparent elements being substantially transparent to radiation emitted by an object to be imaged, where the plurality of opaque and transparent elements is defined according to the selected coded aperture mask pattern.