在小動物的分子藥物研究上，需要超高解析度的造影設備來輔助，而針孔式準直儀具有放大效用可增加解析度，因此針孔式準直儀(pinhole)成為高解析度影像最主要使用的器材。但偵檢器與準直儀的重量，容易造成旋轉臂彎曲與變形，使得旋轉中心(center of rotation)產生偏移。些微的旋轉中心偏移會在影像中造成嚴重的假影。本實驗提出射束擋塊(beam stopper)取代傳統的針孔式準直儀，大幅減少準直儀的重量和成本，並能降低產生假影的機率。
針孔式準直儀利用孔徑侷限光子的行走路徑，而射束擋塊是將傳統針孔式準直儀作物質上的反轉，即孔徑位置由鎢取代。因此所收到的投影與傳統針孔式準直儀的投影是相反的。根據此特性，射束檔塊成像所需之投影資訊包含下列兩部分：(1)無射束擋塊與(2)有射束擋塊之投影資料。在相同掃描時間下，將這兩部份的投影值相減即可得到與傳統針孔式準直儀收到之相似投影。
本研究利用解析方法比較傳統針孔式準直儀與射束擋塊特性，及評估射束擋塊的可行性，並且對重建影像進行評估。實驗使用SimGATE蒙地卡羅軟體，模擬點射源與Utah假體，模擬核種為99mTc。影像重建使用濾器反投影法(filtered back projection, FBP)與子集序列最大期望值法(Ordered Subset Expectation Maximisation, OSEM)。本研究也利用SHINE(Statistical and heuristic image noise extraction)降噪方法提升射束擋塊重建影像之影像品質。
實驗結果可知射束檔塊最佳化厚度為1.2 mm，隨著錐角越大，照野以及系統的靈敏度皆會隨之增加；但錐角的增加會造成解析度下降，這些特性與針孔式準直儀相同。解析方法結果發現兩者投影的形狀極為相似，從點射源影像重建結果也可獲得相似的重建影像，因此驗證了射束檔塊方法的可行性。經由FBP重建可得針孔準直儀的解析度為1.17±0.0283 mm，而射束擋塊解析度為1.15±0.122 mm；從點射源重建的影像中可以觀察到使用射束擋塊方法所產生的背景雜訊較多。Utah假體重建影像結果發現兩者在影像對比度上差異甚大，尤其是使用了FBP方法。透過SHINE降噪方法後，射束擋塊影像的均方差(mean square error)可下降至40~50%，大幅提升影像對比度及影像均勻效果，有效改善重建影像品質。
本研究證明射束檔塊概念在核醫造影上的可行性，提供更輕便的準直儀用來取代傳統的針孔式準直儀，使成本花費降低也保留了高解析度的優勢。

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.

Accorsi R and Metzler SD 2004 Analytic determination of the resolution-equivalent effective diameter of a pinhole collimator IEEE Trans. Med. Imaging 23 750–63

Beekman FJ, van der Have F, Vastenhouw B, et al. 2005 U-SPECT-I: a novel system for sub-millimeter resolution tomography of radiolabeled molecules in mice. J Nucl Med;46:1194–1200

Beekman FJ and Vastenhouw B 2003 Design and simulation of U-SPECT, an ultra-high resolution molecular imaging system. IEEE Nuclear Science Symp. Conf. Rec.,

Chuang et al. 2005 Novel scatter correction for three-dimensional positron emission tomography by use of a beam stopper device. Nucl. Instr. and Meth. A 551: 540-552 SCI

Chang Lee-Tzuu 1979 Attenuation correction and incomplete projection single photon emission computed tomography. IEEE Transactions on Nucl. Sci., vol. NS-26, No. 2

Gullberg GT 1979 The Reconstruction of Fan-Beam Data by Filtering the Back-Projection. Computer graphics and image processing 10, pp. 30-47

Horn B. K. P. 1979 Fan beam reconstruction method .Proceedings of the IEEE, VOL. 67, NO. 12: 1616-1623

Hudson HM, Larkin RS 1994 Accelerated image reconstruction
using ordered subsets of projection data. IEEE Trans Med
Imag 13: 601–609.

Ishizu K et al. 1995 Ultra-high-resolution SPECT system using four pinhole collimators for small animal studies. J. Nucl. Med., vol. 36, pp.
2282–2287.

Li Jianying, Ronald J, Jaszczak A et al. 1994 filtered backprojection algorithm for pinhole SPECT with a displaced center of rotation Phys. Med. Biol. 39 165-176.

Jaszczak RJ, Li J, Wang H, Zalutsky MR, and Coleman RE 1994 Pinhole collimation for ultra-high-resolution, small-field-of-view SPECT. Phys. Med. Biol., vol. 39, pp. 425–437.

Jaszczak RJ, Li J, Wang H, Zalutsky MR, and Coleman RE 1995 A filtered-backprojection algorithm for fan-beam SPECT which corrects for patient motion Phys. Med. Biol. 40 283

Kupinski MA, Barrett HH, eds. 2005 Small-Animal SPECT Imaging. New York, NY: Springer Science1Business Media Inc.

Li Jianying, Ronald J, Jaszczak A et al. 1994 filtered backprojection algorithm for pinhole SPECT with a displaced center of rotation Phys. Med. Biol. 39 165-176.

Meikle SR, Kench P, Kassiou M, Banati RB 2005 Small animal SPECT and its place in the matrix of molecular imaging technologies. Phys Med Biol;50:R45–R61.

Metzler SD, Bowsher JE, Smith RF, and Jaszczak RJ 2001 Analytic determination of pinhole collimator sensitivity with penetration. IEEE Trans. Med. Imag., vol. 20, p. 730.

Ogawa K, Kawade T, Nakamura K, Kubo K, and Ichihara T 1998 Ultra high resolution pinhole SPECT for small animal study. IEEE Trans.
Nucl. Sci., vol. 45, pp. 3122–3126

Pascal H. and Jacky M. 2002 Statistical and heuristic image noise extraction (SHINE): a new method for processing Poisson noise
in scintigraphic imagesPhys. Med. Biol. 47 pp. 4329–4344

Smith MF and Jaszczak RJ An analytic model of pinhole aperture
penetration for 3D pinhole SPECT image reconstruction. Phys. Med.
Biol., vol. 43, p. 761, 1998.
van der Have F, Beekman FJ 2004 Photon penetration and scatter in micro-pinhole imaging: a Monte Carlo investigation. Phys Med Biol.;49:1369 –1386.

Yokoi T. 2000 Implementation of reconstruction program using maximum likelihood-expectation maximization (ML-EM) algorithm. Jpn J Nucl Med Tech 20: 331–340

Mehmet Yavuz and Fessler J A, 1998 Statistical image reconstruction methods for randoms-precorrected PET scans Medical Image Analysis volume 2, number 4, pp 369–378

Wang H, Smith MF, Stone CD, and Jaszczak RJ 1998 Astigmatic
single photon emission computed tomography imaging with a displaced
center of rotation. Med. Phys., vol. 25, no. 8, pp. 1493–1501

Weber DA, Ivanovic DM, Franceschi D, Strand SE, Erlandsson K,
Franceschi M, Atkins HL, Coderre JA, Susskind H, Button T, and
Ljunggren K 1994 Pinhole SPECT—An approach to in-vivo high-resolution SPECT imaging in small laboratory animals. J. Nucl. Med., vol. 35,pp. 342–348.