PMT-based pattern matching calibration for gamma camera using non-uniform pinhole aperture grid mask
Point source responses of pinhole apertures in a non-uniform grid mask used to spatially calibrate a gamma camera can be modeled as a two-dimensional Gaussian model function. Pinhole data from each pinhole location are added together to generate a complete Gaussian model of the flood image from the mask. The Gaussian model then is subjected to global and PMT-based pattern matching with an actual input flood image obtained using the mask, to obtain a transformed Gaussian model that is more accurately aligned with actual pinhole locations of the mask. The transformed Gaussian model then can be used in a peak detection process for calibration images, which are used to develop LC coefficients for the camera.
1. Apparatus for spatial calibration of a gamma camera including a scintillation crystal and an array of photodetectors optically coupled to the scintillation crystal, comprising:
a) a mask having a plurality of pinhole apertures arrayed in a non-rectangular distribution;
b) a point source of radiation capable of being scanned over the entirety of said plurality of pinhole apertures; and
c) a processor for collecting individual point source response data from each of said plurality of pinhole apertures, modeling said point source response data as a Gaussian function, generating at least one Gaussian model patch for the entirety of said mask, transforming said Gaussian model patch using a set of alignment transformation parameters, and using said transformed Gaussian model patch to correct input image data for spatial distortions.
2. The apparatus of claim 1 , wherein said array of photodetectors comprises an array of photomultiplier tubes.
3. The apparatus of claim 1 , wherein said set of alignment transformation parameters are obtained from pattern matching said Gaussian model patch with an actual input image obtained using said mask.
4. The apparatus of claim 3 , wherein said pattern matching is performed on a global basis over the entire input image.
5. The apparatus of claim 3 , wherein said array of photodetectors comprises an array of photomultiplier tubes (PMTs), and said pattern matching is performed on a PMT basis for all PMTs in said input image.
6. A method for spatial calibration of a gamma camera including a scintillation crystal and an array of photodetectors optically coupled to the scintillation crystal, comprising the steps of:
a) mounting a mask having a plurality of pinhole apertures arrayed in a non-rectangular distribution adjacent to said scintillation crystal;
b) scanning a point source of radiation over the entirety of said plurality of pinhole apertures;
c) collecting individual point source response data from each of said plurality of pinhole apertures;
d) modeling said point source response data as a Gaussian function;
e) generating at least one model patch from the collection of Gaussian model data;
f) pattern matching said simulated uniform flood image with an actual input image obtained using said mask to obtain a set of transformation factors; and
g) transforming said simulated uniform flood image using said transformation factors, to obtain a transformed uniform flood image that is used to correct input image data for spatial distortions.
7. The method of claim 6 , wherein the step of pattern matching comprises the step of:
optimizing the minimization function Ψ(X)=∫∫[ƒ(x,y)−ℑ x {g(x,y)}] 2 dxdy.
8. The method of claim 7 , wherein the step of transforming comprises the step of applying a transformation function
T
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g
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where g(x,y) represents said Gaussian function data, x o , y o represent displacement factors between g(x,y) and input image function data f(x,y), and l x , l y represent magnification displacement factors between g(x,y) and f(x,y).
9. The method of claim 6 , wherein the steps of pattern matching and transforming are applied to a global image patch.
10. The method of claim 6 , wherein the steps of pattern matching and transforming are applied to PMT-based image patches.
11. The method of claim 6 , wherein the steps of pattern matching and transforming are applied to a global image patch and to PMT-based image patches.
12. A method for spatial calibration of a gamma camera, comprising the steps of:
pattern matching an input image of a uniform flood radiation field imaged through a pinhole aperture mask, with a Gaussian model estimation of said uniform flood field, to obtain a set of transformation factors;
transforming said Gaussian model using said obtained set of transformation factors to obtain a transformed Gaussian model; and
using said transformed Gaussian model for spatial calibration of said gamma camera.
13. The method of claim 12 , wherein the step of pattern matching comprises the step of: optimizing the minimization function
Ψ( X )=∫∫[ƒ( x, y )−ℑ x {g ( x, y )}] 2 dxdy (6).
14. The method of claim 13 , wherein the step of transforming comprises the step of applying a transformation function
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where g(x,y) represents said Gaussian function data, x 0 , Y 0 represent displacement factors between g(x,y) and input image function data f(x,y), and l x , l y represent magnification displacement factors between g(x,y) and f(x,y).