Abstract
Pinhole collimation provides ultra-high-resolution SPECT images since a large magnification factor is attained. Due to the heavy weight of the pinhole, the center of rotation (COR) displacement introduces serious artifacts into the reconstructed image. The study proposes to replace pinhole collimator by a beam stopper (BS) collimator. In pinhole, the direction of detected photon is collimated by hole. The design of BS is to reverse the pinhole, i.e. the hole is now filled by tungsten and the original collimator is replaced by air. As a result, the photon is collimated in a reverse order and the projection data obtained from BS is opposite to that from the pinhole. Thus for imaging, the BS required the projection data from the scanning: (1) without BS and (2) with BS. The difference between the two scans yields similar projection data from pinhole. The study used noiseless data to calculate sensitivity and resolution. The results show that BS characteristic is comparable to that with pinhole collimator. After sampling of full circle, the subtracted sinograms can be reconstructed a high resolution tomographic image similar to that obtained from the pinhole collimator. Simulations on point sources and Utah phantom filled with 99mTc were performed using a SimGATE Monte Carlo code developed by us. The data were reconstructed using filtered back projection (FBP) and Ordered Subset Expectation Maximization (OSEM) algorithm. BS reconstructed images were smoothed by the SHINE (Statistical and heuristic image noise extraction) method. The results showed that BS optimized height is 1.2 mm,and that BS and pinhole projections were matched in shape and validate the feasibility of BS in high resolution imaging. In point sources FBP image, pinhole FWHM is 1.17±0.0283 mm and BS FWHM result is 1.15±0.122 mm. It indicated that BS image were close to pinhole image but more noise. In Utah phantom test, we found the noise introduces contrast difference between pinhole and BS, especially using FBP reconstruction. Image noise of BS was reduced after SHINE method and the MSE (mean square error) was lower to 40~50%. It showed that improvement in image contrast is evident. Then BS-SPECT image quality is comparable to that with pinhole SPECT. The study validated the feasibility of BS in imaging. The advantages of the propose collimator design over pinhole are much lighter, cheaper and more convenient to use.