Characterization of a pinhole tomograph with 180° acquisition

Pinhole single photon emission computed tomography (SPECT) offers attractions for thyroid imaging, but acquisition must be limited to 180°. The authors have mounted a 3.2 mm pinhole collimator on a rotating gamma camera, magnification 2.1, and characterized its response in the 10 cm field-of-view (FOV) by imaging points, lines, and a Picker thyroid phantom. The authors compared both full and half-circle data acquisition. Image reconstruction was done with a new Discrete Vertex Set (DVS) algorithm and the Feldkamp (FDK) algorithm. For full-circle acquisition, spatial resolution is quite uniform across the FOV, averaging 5.3 mm FWHM, and the reconstructed images show an intensity variation less than 5% in all directions; the FDK and DVS algorithms perform similarly. For half-circle reconstruction, spatial resolution shows more variation across the FOV, averaging 5.4 mm FWHM, and the maximum intensity variation is ±20%. Some differences between the FDK and DVS algorithms are apparent. The point response at non-centered positions along the horizontal transverse axis shows tails in the horizontal transverse direction for the FDK-half reconstruction, whereas for the DVS-half reconstruction there is blurring in the vertical direction at these positions. The thyroid phantom with its 6, 9, and 12 mm cold spots and 12 mm hot spot can be visualized in both full and half-circle reconstructions, allowing size determination