使用三維正子斷層掃描可增加同符事件的數量，但也會造成偵測到三重同符事件的機率提高，因此三重同符事件目前已漸漸成為一個需要重視的議題。三重同符事件是在同符時間窗內偵測到三個光子，在傳統的正子斷層掃描系統中若是偵測到三重同符事件時往往都會直接丟棄。然而三重同符事件內通常都含有真實同符事件的成分，因此本研究的目的是提出一個新的方法，回收三重同符事件內的可用資訊，進而提升系統的靈敏度。在此我們提出利用時間、距離、能量和活度的多重機率組合為概念，以進行三重同符事件中真實同符事件的回收。利用蒙地卡羅模擬三維正子斷層掃描對均勻假體與NEMA-like假體進行掃描的結果，藉此來對我們提出的方法進行驗證。模擬結果顯示，當活度越高時三重同符事件的數量越多，而在1~20mCi之間若能完全回收真實三重同符事件，則可增加3%~48%的真實同符事件。使用本研究針對三重同符事件所提出的回收方法，在均勻假體與NEMA-like假體中皆可回收近92%以上的真實三重同符事件。此外，在活度為1~15mCi時可增加3%~57%的NECR。本研究提出的方法可以提升同符事件的計數率，並且能輕易的使用在臨床研究中，且不需外加任何硬體設備。

With the increase in 3D acquisition of positron emission tomography (PET) data, triple event is becoming a more and more relevant issue. Triple event may occur when three photons are detected within a coincidence time window. For traditional PET system, the triple coincidence is discarded. However, the triple coincidence usually contains a valid true coincidence and an unrelated gamma. Thus, the purpose of the study is to propose a novel method to enhance the sensitivity of PET imaging via recycling the triple coincidence. A combined likelihoods model involving time, geometry, energy, and activity information is proposed to recover the true coincidence form the triple coincidence. Monte Carlo simulations of 3D PET on an NEMA phantom and a uniform phantom were conducted to validate the proposed approach. Results showed that the amounts of triple event increase with the increase of the activities. The ratios between the true triple and true double coincidence can achieved about 3% to 48% at the activity levels from 1 mCi to 20 mCi. By using the proposed method, more than 92% true coincidence can be recovered from triple coincidence on both phantoms. Furthermore, the NECR gains can be achieved by 3% ~ 57% with the activity levels from 1 to 15mCi. We conclude that ours proposed methods can improve the counting statistics for positron emitters and are readily applicable to clinical studies as no hardware modification is needed.

Andreyev A and Celler A 2011 Dual-isotope PET using positron-gamma emitters Physics in medicine and biology 56 4539
Badawi R, Miller M, Bailey D and Marsden P 1999 Randoms variance reduction in 3D PET Physics in medicine and biology 44 941
Brasse D, Kinahan P E, Lartizien C, Comtat C, Casey M and Michel C 2005 Correction methods for random coincidences in fully 3D whole-body PET: impact on data and image quality Journal of nuclear medicine 46 859-67
Buvat I and Castiglioni I 2002 Monte Carlo simulations in SPET and PET QJ Nucl. Med 46 48-61
Cherry S R, Sorenson J A and Phelps M E 2012 Physics in nuclear medicine: Saunders)
Conti M 2007 Tailoring PET time coincidence window using CT morphological information Nuclear Science, IEEE Transactions on 54 1599-605
Couturier O, Luxen A, Chatal J-F, Vuillez J-P, Rigo P and Hustinx R 2004 Fluorinated tracers for imaging cancer with positron emission tomography European journal of nuclear medicine and molecular imaging 31 1182-206
Daube-Witherspoon M E, Karp J S, Casey M E, DiFilippo F P, Hines H, Muehllehner G, Simcic V, Stearns C W, Adam L-E and Kohlmyer S 2002 PET performance measurements using the NEMA NU 2-2001 standard Journal of Nuclear Medicine 43 1398-409
Delso G, Martinez M-J, Torres I, Ladebeck R, Michel C, Nekolla S and Ziegler S I 2009 Monte Carlo simulations of the count rate performance of a clinical whole-body MR/PET scanner Medical physics 36 4126
Gambhir S S 2002 Molecular imaging of cancer with positron emission tomography Nature Reviews Cancer 2 683-93
Gonias P, Bertsekas N, Karakatsanis N, Saatsakis G, Gaitanis A, Nikolopoulos D, Loudos G, Papaspyrou L, Sakellios N and Tsantilas X 2007 Validation of a GATE model for the simulation of the Siemens biograph™ 6 PET scanner Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 571 263-6
Ikoma Y, Toyama H, Uemura K and Uchiyama A 2001 Evaluation of the reliability in kinetic analysis for dual tracer injection of FDG and flumazenil PET study. In: Nuclear Science Symposium Conference Record, 2001 IEEE: IEEE) pp 2054-7
Jan S, Santin G, Strul D, Staelens S, Assie K, Autret D, Avner S, Barbier R, Bardies M and Bloomfield P 2004 GATE: a simulation toolkit for PET and SPECT Physics in medicine and biology 49 4543
Miller P W, Long N J, Vilar R and Gee A D 2008 Synthesis of 11C, 18F, 15O, and 13N radiolabels for positron emission tomography Angewandte Chemie International Edition 47 8998-9033
Rust T and Kadrmas D 2006 Rapid dual-tracer PTSM+ ATSM PET imaging of tumour blood flow and hypoxia: a simulation study Physics in medicine and biology 51 61
Strother S, Casey M and Hoffman E 1990 Measuring PET scanner sensitivity: relating countrates to image signal-to-noise ratios using noise equivalents counts Nuclear Science, IEEE Transactions on 37 783-8
Thielemans K, Mustafovic S and Tsoumpas C 2006 STIR: software for tomographic image reconstruction release 2. In: Nuclear Science Symposium Conference Record, 2006. IEEE: IEEE) pp 2174-6
Townsend D 2004 Physical principles and technology of clinical PET imaging Annals-Academy of Medicine Singapore 33 133-45
Watson C, Newport D, Casey M, DeKemp R, Beanlands R and Schmand M 1997 Evaluation of simulation-based scatter correction for 3-D PET cardiac imaging Nuclear Science, IEEE Transactions on 44 90-7
Young H, Baum R, Cremerius U, Herhloz K, Hoekstra O and Lammertsma A 1999 Position paper. Measurement of clinical and subclinical tumour response using [18 F]-fluorodeoxyglucose and positron emission tomography: review and 1999 EORTC recommendations Eur J Cancer 35 1773-82
Zanzonico P 2004 Positron emission tomography: a review of basic principles, scanner design and performance, and current systems. In: Seminars in nuclear medicine: [Orlando, FL, etc.] Grune & Stratton [etc.]) pp 87-111