aesign of a Depth of Interaction (DOI) PET Detector Module

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1 aesign of a Dept of Interaction (DOI) PET Detector Module R.S. Miyaoka, T.K. Lewellen, H. Yu', and D.L. McDanie1 University of Wasington and "General Electric Medical Systems 'University of Wasington Medical Center, Seattle, WA General Electric Medical Systems, Waukesa, WI tat by controlling ow ligt is 1s (A and B) DO1 information can of ligt collected using simple t amount of ligt is sared wen front face of a crystal and very atment (e.g., polised, g compounds are investigated. units ave been evaluated. A ources distant from is non-symmetric rce profile is a result of assigning ong line of response (LOR), see, dased-misplaced). Te spatial FOV. DO1 inform n also enance te performance of a PET detector by improving te sampling detector system. to extract DO1 information from a e been proposed [l-81; owever, no metods is te requirement of (i.e., potodiodes or additional s)). Wit te discovery of LSO block detectors optimize te way Figure 1. Line of response (dased line) misplace due to DO1 effect. [ll]. Using simple Anger logic [1] modules wit up to 64 crystals ave been decoded using 4 PMTs [ 111. Instead of using te ligt to decode as many crystals as possible, tis work investigates metods to sare ligt to extract DO1 information. Te ypotesis is tat by controlling ow ligt is sared between neigboring crystals (A and B) DO1 information can be extracted from te ratio of ligt collected using simple Anger logic [(A-B)/(A+B)]. Te interface between crystals will be designed so tat a significant amount of ligt is sared wen a poton interacts near te front face of a crystal and very little ligt is sared wen an interaction occurs near te back of a crystal (see Figure ). Furtermore, te interface will be designed so te front section of te detector (were most of te interactions will occur) is more sensitive to DO1 effects tan te back section of te detector unit. For tis investigation a detector unit consists of two optically coupled crystals. 11. EXPERIMENTAL METHODS BGO, GSO and LSO detector units were built and evaluated. Te dimensions and surface finises of te crystals used are listed in Table 1. Eac of te four detector units tested are illustrated in Figures 3-5. Eac detector unit was coupled to a 16 cannel (4x4) metal dynode PMT (R6568, Hamamatsu Corp., Japan) for testing. Te LSO crystals were /98/$1.~ 1998 IEEE 939

2 paint optical coupler opaque reflector Crystal BGO GSO #1 GSO # A B Dimensions (mm) 3.9 x 4.1 x x 4.1 x 3.6 x 3.5 x 4* LSO. x. x * 1L I PMT I Figure. DO1 decoding strategy. Surface Finis polised unpolised 3.5 x 4 unpolised.6 x 4 polised polised coupled to te PMT via mm diameter by 7.5cm long double clad optic fibers (Kurraray, Japan). Te BGO and GSO crystals were directly coupled to te PMT. A ig index of refraction, 1.74, (Cargille, New Jersey) was used to facilitate ligt saring between crystals. Te same was used to glue te individual sections of GSO detector unit # and te LSO detector unit. For te crystals wit polised surfaces, te section of te interface coupled wit te was rougened to enance ligt saring. Detector units were evaluated using bot wite latex paint and WE Teflon as te opaque reflective material along te crystal interface. After te crystal interface was completed te detector units were wrapped in WE Teflon and coupled to te PMT. 4mm + 3.9" x 3" e:e : ~ 3.5" x 4" 1" l-tz7-l Figure 4. Detector interface for GSO detector unit #. Two.6 x 3.5 x 1" crystals are glued togeter to form crystals A and B.." x " 17" paint [[ optical fiber n n mm by 7.5cm l n n n n i I H6865PMT I Figure 5. Detector interface for LSO detector unit. Two. x. x lomm crystals are glued togeter to form crystals A and B. A block diagram of te acquisition setup is illustrated in Figure 6. Te detector unit was exposed to a narrow flux (-3") of 511 kev potons (flux perpendicular to long axis of crystals) using a sielded line source. Te poton flux was stepped along te lengt of te detector unit in -3" increments. Only signals from te two cannels directly coupled to te detector unit were acquired. Te FERA ADC (LeCroy, Cestnut Ridge, NY) integration time was ns for LSO, 3 ns for GSO and 775 ns for BGO. A long background acquisition was taken for te LSO detector unit to correct for its natural background activity. oa line source I black box wit DO1 detector unit 6" paint n n n n l I H6865PMT I Figure 3. Detector design for GSO detector unit #1 and BGO detector unit. Mac PowerPC LabView Figure 6. Experimental setup. 94

3 5 detector unit #1 (DO1 sown in Figure d points associated wi FWHM ratio values were eac fit ave not been corrected for te 1 3 dept (mm) Figure 8. Ratio peak and FWHM values of ratio plot versus DO1 for GSO detector unit #1 (crystal B). Horizontal line is estimate of DO1 uncertainty for dept position. unit are sown in Fi tere was movement E 5 x Y.9 i Figure 9. Uncertainty in estimate of DO1 for GSO detector unit #l. 15 r GSO detector unit #1 at DO1 positions (c)and 3mm(d). 1 E E5 c) G '3 5 c Figure 1. Uncertainty in estimate of DO1 for GSO detector unit #. 941

4 15 1 E s 5 x v ei '- : r: -5 =I Figure 11. Uncertainty in estimate of DO1 for LSO detector unit. 1.o.s.9. i; a 4 ;.8 c-' 8 c: a N.e + m Figure 1. Plot of normalization potopeak position (ADC cannel) versus DOL Te position of te potopeak (energy spectra) also varied wit DOI. A plot of te potopeak position versus DOI, individually normalized for eac detector unit, is sown in Figure 1. IV. DISCUSSION For tree of te detector units evaluated a DO1 accuracy of -5" was attained for te front section of te crystals and better tan lomm accuracy was attained for te front alf of te detector units. Wile tere was some movement in te ratio peaks for te BGO detector unit, almost no DO1 information was provided. Wile GSO crystals produce more ligt tan BGO crystals, te amount of ligt collected from te front section of te GSO detector unit #1 was only sligtly more tan te ligt collected from te BGO module. Additionally, te ligt collected from te LSO detector unit was approximately equal to te ligt collected from te BGO detector unit. We believe tat te DO1 decoding tecnique worked well for GSO detector unit #1 because ligt lost at te unpolised surfaces of te GSO crystals makes te collection of ligt a strong function of te initial direction of te ligt potons (a requirement to make tis tecnique work). It is unlikely tat ligt potons produced near te rear of te detector unit will survive enoug surface interactions (reflections) to make it across te optically coupled region of te crystal. For polised crystals (4x4x3mm), our results indicate tat te amount of ligt sared across te optically coupled interface is not very correlated wit were te ligt originated. Wile te LSO crystals were polised, te collection of ligt was still a strong function of te initial direction of te ligt potons. Tis is because for very narrow crystals it takes many more reflections for ligt originating near te rear of te crystal to make it to te optical interface. Wile we glued crystals togeter because we did not ave crystals tat were long enoug to evaluate tis tecnique, it (gluing) may ave serendipitously improved te DO1 decoding performance of te LSO and te GSO # detector units. V. MODULE DESIGN Te decoding strategy for a detector module consisting of 3 detector units (total of 64 crystals); eac crystal aving a mm by mm front face is sown in Figure 14 (see next page). Fiber optic connectors are used to route te ligt from te crystals to te 16 cannel metal dynode PMT (H6568-1, Hamamatsu, Japan). Eac anode (labeled 1-16) receives ligt from 4 crystals. A discrete decoding sceme (Table ) will be used to determine te detector unit ( crystals - labeled a-#) of interest. Once a detector unit is selected, Anger logic is used to determine wic crystal te event occurred in and to estimate DOL Table. Decoding strategy for 64 crystal detector module (3 - detector units). detector anode detector anode detector anode detector anode unit pair unit pair unit pair unit pair U 1-5 i y 13-1 b 1-6 j 6-9 r 9-14 z 14-1 c -6 k 6-1 s 1-14 uu 14- d f 1-15 bb 15- e 3-7 m 7-11 U cc 15-3 f 3-8 n 8-11 v dd 16-3 g 4-8 o 8-1 w 1-16 ee p 5-1 x 1-13 ff 13-4 VI. CONCLUSION AND FUTURE DIRECTIONS Some DO1 information can be extracted from a two crystal detector unit. Te amount of DO1 information is a strong function of surface finis and te crystal dimensions. A DO1 uncertainty of -5" was attained for te front section of te GSO and LSO detector units evaluated. Te uncertainty increased to -1" in te center section of te detector units. Te uncertainty in te rear of te detector units was limited by te end of te crystal. 94

5 Figure 14. Decoding strategy for 64 crystal detector module (3 - detector units). Te numbered squares represent dynode cannels of a 16 cannel multi-anode PMT (Hamamatsu R6568-1). Te rectangles (italic letters) represent detector units. Eac detector unit is represented by a unique anode pair. Decoding of te module is illustrated in ].e Table. Te module design described in tis paper is proposed for specialized imaging jystems (e.g., small animal, breast, dedicated ead) and i>, not practical for wole-body PET systems. On te otker and, a 6x6 module comprised of 4x4~3" GSO or LSO crystals wit DO1 capability would very attractive for clinical PET imaging systems. Terefore, we ave started to extend tis tecnique to larger detector arrays (e.g., 1x4 and 1x6). Our preliminary findings are promising. Since te 31 determination only as to be done once, extending a 1x6 module into a 6x6 module sould be somewat straigtforward. Yamasita T, Watanabe M, Simizu K, Ucda H, "Hig Resolution Block Detectors for PET," IEEE Trans Nuc Sci., vo1.37(1), February, 199. Karp JS, Daube-Witerspoon ME, "Dept-of- Interaction Determination in NaI(TI) and BGO Scintillation Crystals Using a Temperature Gradient, " Nucl. Instr. Met. vol.a6 pp.59-17, October, Bartzakos P, Tompson CJ, "A Dept-Encoded PET Detector", IEEE Trans Nuc Sci., vo1.38(), pp.73-8, April, Rogers J, "A Metod for Correcting te Dept-of- Interaction Blurring in PET Cameras", IEEE Trans Med Imag. vo1.14(1), pp.146-5, Marc, Melcer CL, Scweitzer JS, "Cerium-doped Lutetium Oxyortosilicate: A Fast, Efficient New Scintillator," IEEE Trans Nuc Sci., vo1.39(4), pp.5-5, August, 199. Takagi K, Fukazawa T, "Cerium-activated Gd/sub /SiO/sub 51 single crystal scintillator," Appl. Pys. Lett. vo1.4( 1), pp.43-5, January, Tornai MP, German G, Hoffman EJ, "Positioning and Energy Response of PET Block Detectors wit Different Ligt Saring Scemes", IEEE Trans NUC SC~. ~1.41(4) pp Aug Anger HO. Radioisotope Cameras, in Instrumentation in Nuclear Medicine (vol 1) cap 19. Academic Press, VII. ACKNOWLEDGMENTS Tis work was sup3orted in part by PHS grant CA4593 and General Electric N.edica1 Systems. Te autors would like to tank Dr. Simon Cerry for loaning us te LSO crystals and providing us wit some mm double clad optical fibers. Te autors would also like to tank Mr. Scott Dolson for providing some GSO crystals during te early stages of tis work. VIII. REFERENCES Derenzo SE, Moses WW, et. al., "Initial Caracterization of a Position-Sensitive Potodiode/BGO Detector for PET," IEEE Trans Nuc Sci., vol. 36( 1), pp , Felxuary Simizu K, Yamasita T, Omura T, Watanabe M, Ucida H, 'Development of 3-D Detector System for Positron C:?," IEEE Trans Nuc Sci., vo1.35( 1), pp.717-, Feb-uary, Rogers J, et. al., "Design of a Volume-imaging Positron Emission Tomograp", IEEE Trans Nuc Sci., vo1.36( l), pp.993-7, February, Wong WH, "Designing a Stratified Detection System for PET Cameras," IEEE Trans Nuc Sci., vo1.33( l), pp.591-6, February,