SPECT using a specially designed cone beam collimator.

A specially designed high resolution converging collimator having a focal length of 50 cm has been evaluated for cone beam single photon emission computed tomography (SPECT). The focal region was investigated by imaging a point source placed at the expected focal point and along the central ray of the collimator in front of and behind the focal point. Technetium-99m point source sensitivities measured in air at 5, 10, 15, and 20 cm from the collimator surface are 4.2, 5.5, 7.3, and 10.5 cts.sec-1.microCi-1 when used with a single camera SPECT system. A commercially available parallel hole collimator, with similar resolution characteristics has a measured sensitivity of 3.3 cts.sec-1.microCi-1. Volume sensitivities of 9,780 and 4,945 (cts.sec-1)/(microCi.ml-1) were measured for the cone beam and parallel hole collimators, respectively, using a 17-cm-diameter spherical source. Reconstructed spatial resolution (FWHM) on the axis-of-rotation ranged between 10 and 11 mm for both collimators when the radius of rotation was equal to 15 cm. Using equal acquisition times SPECT images of phantoms scanned with the cone beam collimator were visually improved compared with images acquired using the parallel hole collimator. These results demonstrate that a factor of 2 improvement in volume sensitivity can be demonstrated with a cone beam collimator compared with a commercially available parallel hole collimator. Further improvements are possible using shorter focal lengths, astigmatic focusing, and larger field of view cameras.