- Email: [email protected]
Phoswich solutions for the PET DOI problem
Nuclear Instruments and Methods in Physics Research A 648 (2011) S288–S292
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Phoswich solutions for the PET DOI problem L. Eriksson a,b,c,d,n, C.L. Melcher d, M. Zhuravleva d, M. Eriksson b, H. Rothfuss a,d, M Conti a a
Siemens Medical Solutions, Molecular Imaging, Knoxville, TN, USA Karolinska Institute, Stockholm, Sweden University of Stockholm, Stockholm, Sweden d Scintillation Materials Research Center, University of Tennessee, Knoxville, TN, USA b c
a r t i c l e i n f o
a b s t r a c t
Available online 19 November 2010
A high spatial resolution in PET can be achieved by using small detector elements. To maintain good sensitivity these elements have to be quite long, thus introducing parallax error and making the spatial resolution non-uniform over the image volume. Uniformity of spatial resolution can be improved by utilizing depth-of-interaction (DOI) information to reduce the parallax error. In the present study we have focused on phoswich approaches based on interacting scintillators, that is, a phoswich combination in which one scintillator emits light in the excitation band of the other. We have looked at LaBr3:Ce and LaCl3:Ce and the interactions of those two scintillators with LSO:Ce, GSO:Ce and YSO:Ce. The reasons to use the two Lanthanum scintillators are twofold: light output is high and the two different emission wavelengths, 350 nm (LaCl3:Ce) and 380 nm (LaBr3:Ce) may produce different interactions with the three oxyorthosilicate scintillators. In addition a possible DOI detector comprising LuAG:Pr pixels with a thin LSO:Ce layer at one end has been evaluated. A Bollinger–Thomas set-up was used to measure luminescence rise and luminescence decay time characteristics in all cases. When using LaCl3:Ce, the phoswich combinations with YSO:Ce and GSO:Ce showed phoswich decay time characteristics as expected for a simple convolution of the decay times of the two phoswich components. A correction was needed, however, for the LaCl3:Ce–LSO:Ce phoswich due to the LSO:Ce intrinsic activity. For the LaBr3:Ce–LSO:Ce phoswich, corrections were needed for noninteracting LaBr3:Ce light in addition to the expected phoswich interaction. & 2010 Elsevier B.V. All rights reserved.
Keywords: High resolution PET Phoswich detectors Lutetium oxyorthosilicate LSO Gadolinium oxyorthosilicate GSO Yttrium oxyorthosilicate YSO Lanthanum Bromide LaBr3 Lanthanum chloride LaCl3 Scintillator interactions
1. Introduction Positron Emission Tomography (PET) has two main missions, spatial resolution and sensitivity. The present work concerns spatial resolution. The spatial resolution is related to the pixel size, but since long pixels are needed for sensitivity, many detection element combinations will have a substantial radial elongation. The elongation problem can be solved by registering the depth-of-interaction (DOI) of gamma rays in the detection elements. A relatively easy way to achieve this is to use the phoswich concept, that is, to construct the pixel from two or more sections of different scintillators and then identify the gamma ray absorbing section based on pulse shape discrimination techniques. This has been done for the Siemens HRRT systems having a phoswich combination based on LSO:Ce and LYSO:Ce. Separation is based on scintillation decay time with LSO:Ce having a decay time around 40 ns and LYSO:Ce (30%Lu) having a decay time of 55 ns [1–3]. This is an example of a classical phoswich with non-interacting scintillators.
n Corresponding author at: Siemens Healthcare Molecular Imaging, 810 Innovation Dr, Knoxville, TN 37932, USA. Tel.: + 1 865 218 2246; fax: +1 865 218 3000. E-mail address: [email protected] (L. Eriksson).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.049
Recently other combinations have been successfully tried such as LuYAP:Ce and LSO:Ce [4–6]. A potential problem is that the LuYAP:Ce emission occurs in the excitation band of LSO:Ce changing the emission characteristics making it important in what order, relative to the PMT, the scintillators are arranged. By placing the LSO:Ce in the front with LuYAP:Ce behind, close to the PMT, the interactions can be minimized and good phoswich functionality can be obtained. When light from one scintillator is absorbed by a second scintillator, the resulting photo luminescence may have a decay time that is quite different from the decay time from gamma excitation. Photo luminescence of GSO:Ce excited at 345 nm gives a decay time of around 25 ns, while excitations based on gamma rays may give a decay time of around 70 ns [7]. Furthermore, the interaction and the scintillation emission can be described as a convolution of the two decay times of the two scintillators. Part of our efforts have been devoted to phoswich combinations based on LaCl3:Ce and LaBr3:Ce combined with LSO:Ce, GSO:Ce and YSO:Ce. The reason for this are the differences in emission wavelengths of the two lanthanide crystals, with emission around 350 nm for LaCl3:Ce and around 380 nm for LaBr3:Ce, which may create some interesting differences for the phoswich performance. Fig. 1 shows the LaBr3:Ce and LaCl3:Ce emissions in the LSO:Ce excitation band. The expected phoswich performance is a convolution
L. Eriksson et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S288–S292
Fig. 1. The figure shows the emissions of the LaCl3 and LaBr3 and the excitation bands of LSO. The excitation bands for GSO and YSO are similar to LSO. It seems reasonable that a correction has to be applied for the LaBr3–LSO phoswich, since a fraction of the LaBr3 light emission falls in the emission band of LSO. The PMTs, recording the light from the phoswich, may see (1) re-emitted LSO light due to interactions with LaBr3, (2) direct non-interacting LaBr3 light and (3) intrinsic LSO light due to Lu-176 activity in the LSO scintillator. The LaCl3 emission is entirely inside the excitation range of LSO (YSO and GSO) effectively converting its light into re-emitted LSO (YSO and GSO) light. For the LSO case we still will have a fraction of light generated by the Lu-176 impurity.
Fig. 2. Overview of the Bollinger–Thomas set up for measuring luminescence decay of the phoswich detectors. The PMTs are only seeing the back scintillator layer of the phoswich.
between the decay times of the two phoswich components. The phoswich decay time characteristics have been measured using a Bollinger–Thomas set up [8] as shown in Fig. 2. An elegant way to solve the DOI problem may be to place a thin scintillator layer on top of a scintillator pixel where the scintillator pixel light interacts with the scintillating top layer adding a mixture of different scintillation times that can be used for DOI determination [9]. By understanding the physics behind the interacting scintillators the phoswich concept can be extended and may provide much more elegant solutions to the DOI problem.
2. Methods 2.1. Experimental details—initial measurements The scintillator crystals used in this work are reasonable candidates for interacting phoswich detectors in PET combining high light output, relatively high detection efficiency for gamma rays, and fast scintillation decay. The selected scintillators were LaBr3:Ce (Saint-Gobain, BrilLance-380) and LaCl3:Ce (Saint-Gobain,
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BrilLance-350) as front layer scintillators; and LSO:Ce, GSO:Ce and YSO:Ce as the back layer in the phoswich configurations. Photoluminescence decay (PL) profiles of the back layer scintillator crystals were recorded at room temperature using the timecorrelated single photon counting technique (TCSPC) with a Fluorolog 3 lifetime spectrofluorometer (Horiba Jobin Yvon). The instrument is equipped with interchangeable pulsed UV-NanoLED light sources that are selected to match the excitation bands of the scintillators. The width of the excitation pulses are 1 ns. In this work, we used 345, 360 and 371 nm LEDs as UV excitation sources to get the PL response during activation of the excitation band of the three back layer scintillators. A double monochromator with a 3 nm band-pass was used to select the emission wavelength. Scintillation decay times of the individual scintillators and of the phoswich detectors were measured under gamma-ray excitation with 137Cs (622 keV) and 241Am (59.5 keV) sources. When using the 241 Am source, the alpha particles were stopped with a plastic filter. A Bollinger–Thomas set-up [8], operating in TCSPC mode with Photonis XP2020Q photomultiplier tubes and NIM electronics was used (Fig. 2). As an example of the differences between PL luminescence and gamma ray luminescence we show results from a GSO:Ce scintillator in Fig. 3a and b. 2.2. Experimental details—phoswich measurements The scintillators LSO:Ce, YSO:Ce and GSO:Ce were all combined with LaBr3 and LaCl3 in phoswich arrangements with the lanthanum scintillators in the front. Both lanthanum scintillators were obtained from Bicron with the sizes 13 mm diameter, 13 mm long. The 241Am source was placed on top of the front scintillator. Since very few 59.5 keV photons penetrate the 13 mm Lanthanum scintillators, direct 59.5 keV activation of the LSO:Ce, YSO:Ce and GSO:Ce crystals was ignored. The phoswich combinations were optically combined by using optical grease. Reflector foil was applied covering the sides of the back scintillator and finally black tape was used to block direct light from the lanthanum scintillators to enter the PMTs in the Bollinger–Thomas set up. The gamma ray luminescence decay kinetics were measured for the LaBr3/LSO/ YSO/GSO and the LaCl3/LSO/YSO/GSO combinations. 2.3. Experimental details—Measurements with a LuAG:Pr pixel with a LSO backing Du et al [9] presented a phoswich detector in with LSO:Ce pixels with a thin front layer of Ce activated YGG (Y3 (Al,Ga)5O12:Ce). Following a gamma ray absorption, the emission of light from LSO:Ce activates the YGG layer and the sum between pure LSO:Ce light and convolved YGG:Ce – LSO:Ce light is seen as a change in decay time, which depends on the depth-of-interaction in the LSO:Ce pixel. We tried a very similar approach, but instead using a LuAG:Pr pixel, 10 10 20 mm3 unpolished with a thin LSO:Ce layer in the front (10 10 1 mm3). The LuAG:Pr pixel with the LSO:Ce backing was covered with black tape to make the response more nonuniform. The approach looked promising theoretically, looking at the sum of the convolution between LSO:Ce and LuAG:Pr and the direct LuAG:Pr light. We used a modified Bollinger–Thomas system with a collimation detector added, used to identify scintillations from the front and the back part of the LuAG:Pr pixel.
3. Results There is a difference between the photoluminescence measurements and the gamma ray luminescence for the three ‘‘back’’ layer scintillators, LSO:Ce, YSO:Ce and GSO:Ce. We showed the GSO:Ce case
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Table 1 Decay times for LSO, YSO and GSO in photoluminescence and in gamma ray luminescence. Photo luminescence for LaBr3 and LaCl3 were not measured. Scintillator
Decay time for photo luminescence (ns)
Decay time for gamma ray luminescence (ns)
LSO YSO GSO (LaBr3) (LaCl3)
31.2 35.8 24.3 N/A N/A
37.6 62.2 74.0 (55%) 430 (45%) 17.2 20.3 (75%) 170 (25%)
Fig. 3. (a) Photoluminescence in GSO with 345 nm activation. (b) Am241 gamma activation of GSO.
in Fig. 3 with around 25 ns in photoluminescence decay and 73 ns in gamma ray luminescence decay. The results for the gamma ray luminescence and the photoluminescence measurements are summarized in Table 1. Photoluminescence is the effect we have to consider in the case of scintillator interactions. A direct hit in the back layer of 511 keV photons result in a response based on gamma ray luminescence while a hit in the top layer, in for example the LaBr3:Ce layer, results in a convolution between LaBr3:Ce decay time and back scintillator PL decay time, a phoswich luminescence with a rise time given by the LaBr3:Ce luminescence and a decay time given by the PL decay of the back layer. However, as seen from Fig. 1, a fraction of LaBr3:Ce light may be transmitted across the back-layer without interactions and the data as recorded by the Bollinger–Thomas set up is now a sum between LaBr3-back-layer phoswich data and a fraction of pure LaBr3 data. When the back layer is LSO:Ce there is also a contribution of gamma activated LSO:Ce light from the intrinsic 176Lu activity. The Bollinger–Thomas data S are compared to model equations described in Eqs. (1) and (2) below.
Fig. 4. (a) Am241 activated LaBr3_GSO phoswich. A direct Labr3 light fraction has been fit to the data. The fitted direct fraction is 17%. This would imply that 17% of all the LaBr3 events generated by the Am sources are passing through the GSO without interaction and entering the Bollinger–Thomas set up. The fit indicates a 10 ns rise time and a 28 ns decay time for the LaBr3_GSO phoswich. (b). Am241 activated LaCl3_GSO phoswich. No corrections done. The fit gives a 19 ns rise time and a 28 ns decay time for the LaCl3_GSO phoswich and a second decay time of 219 ns.
The plots and fits of the Lanthanum phoswich decay time data measurements are shown in Fig. 4 for the case of GSO. All phoswich data are summarized in Table 2.
L. Eriksson et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S288–S292
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Table 2 Phoswich rise and decay times for LaBr3/LaCl3 combined with GSO, YSO and LSO. Back layer scintillator
Front layer: LaBr3 Phoswich scintillation response
Notes on LaBr3 phoswich data
Measured Rise time (ns)
Expected Rise time (ns)
Measured Decay time
Expected Decay time (ns)
Corrections
GSO YSO LSO
10 18 17
17 17 17
28 ns 35 ns 31
24 36 31
17% LaBr3 5% LaBr3 5%(LaBr3 + LSO)
Back layer scintillator
Front layer: LaCl3 Phoswich scintillation response
GSO YSO LSO
Notes on LaCl3 phoswich data
Measured Rise time (ns)
Expected Rise time (ns)
Measured Decay time (ns)
Expected Decay time (ns)
Corrections
19 18 18
20 20 20
28 37 30
24 36 31
No corrections No corrections 20 % (LSO)
Fig. 5. A modified Bollinger–Thomas set up with addition of a collimation detector, used in coincidence with the regular luminescence event registration. A Na22 point source is used. The collimation detector has a 2 2 2 mm3 LSO scintillator. Together with the point source the collimation detector can select either the front or the back of the TEST pixel, a 10n10n20 mm3 LuAG:Pr scintillator with an LSO backing of 10 10 1 mm3 with a 33 ns PL decay time.
The LaCl3:Ce phoswich results are based on uncorrected data (Eq. (1)), except for the LaCl3:Ce/LSO:Ce phoswich with a small correction for LSO:Ce intrinsic scintillations. S ¼ bck þ act½frac et=decay1 þ ð1fracÞet=decay2 et=rise
ð1Þ
The constant act is the start number of counts found by the fit, frac is the fraction of the phoswich luminescence decay channeled via decay1 and bck is the background. The LaBr3:Ce phoswich data are analyzed according to Eq. (2). S ¼ bck þ actðet=decay et=rise Þ þfraction LaBr3 corr
ð2Þ
where LaBr3corr is the direct contribution of the LaBr3:Ce luminescence data added to the phoswich information and modulated by the constant fraction. The fits of the LaBr3:Ce data in phoswich combinations with GSO/YSO/LSO to Eq. (2) are ill-conditioned and sensitive to start values and starting points. In Fig. 4a results from a successful fit is shown. Only in this case (LaBr3:Ce/GSO:Ce data) both the rise and decay times and the assumed LaBr3:Ce direct contribution (the constant fraction) gave reasonable values. For the other LaBr3:Ce phoswich combinations, the one with YSO:Ce and LSO:Ce, data were analyzed by adding small amounts of the fraction to the direct LaBr3:Ce light to keep the ill-conditioned least squares fit under control. For these cases we can only indicate that direct LaBr3:Ce
light is present and that reasonable phoswich rise and decay time data can be obtained by assuming a certain fraction. This was not the case for the LaCl3:Ce phoswich combinations. A direct fit based on Eq. (1) gave reasonable rise time and decay time data. We conclude that the LaCl3/GSO/YSO phoswich data as analyzed by the Bollinger–Thomas technique directly provide correct rise time and decay time data. The experimental data do support the assumption of different phoswich interactions for LaBr3:Ce and LaCl3:Ce when combined with GSO/YSO LSO. Concerning the experiment with the Du approach we first looked at the situation described in the Du paper, a LSO:Ce pixel with YGG:Ce backing. The photo detector sees the sum of LSO:Ce light and light due to the YGG:Ce–LSO:Ce interactions. A 100% LSO:Ce signal gives 40 ns decay time and a 50% LSO:Ce 50% LSO:Ce–YGG:Ce light gives 54 ns assuming a photo detector quantum efficiency of the YGG:Ce light of 30% relative to LSO:Ce light. The situation is of course very much improved for photo sensors with a better response in the green wavelength region like an APD or SiPM. For a photo sensor with the same quantum efficiency for both emissions the 50% admixture would now give 76 ns instead of 54 ns. A similar calculation for LuAG:Pr with LSO:Ce backing, assuming the same quantum efficiency for both emissions (LuAG:Pr light and
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LSO:Ce light) gives 20 ns for 100% LuAG:Pr light and 41 ns for a 50% mixture of both types. Experimentally with the 10 10 20 mm3 LuAG:Pr pixel with a 10 10 1 mm3 LSO:Ce backing, wrapped with black tape to break up the uniformity response, we found 27 ns activating the front of the crystal and 33 ns activating the back of the crystal using the modified Bollinger–Thomas set up with the added collimation detector (Fig. 5). This decay time difference needs to be improved in order to obtain useful DOI information from this scintillator information (Fig. 6a and b).
4. Conclusions The Bollinger–Thomas set up was used to study phoswich detectors based on LaBr3 or LaCl3 as the front detectors combined with GSO/YSO/LSO as the back detector. The phoswich detectors were shielded in such a way that the set up only registered events from the back scintillator. The LaBr3 or LaCl3 detectors only were activated with 59.5 keV gamma rays from an Am-241 source. Scintillation decay time spectra were recorded. The LaBr3/GSO/ YSO/LSO data showed the presence of the expected interaction with the back scintillator and, in addition, the presence of direct LaBr3 scintillation information. The LaCl3/GSO/YSO/LSO data showed only the interaction between the front and the back scintillator. An attempt to design a phoswich detector based on a LuAG:Pr pixels with a thin LSO backing showed a DOI sensitive admixture between LuAG:Pr light and re-emitted LSO light. In the present detector design, however, the DOI sensitivity was too low. References [1] M. Schmand, L. Eriksson, M.E. Casey, et al., IEEE Trans. Nucl. Sci. NS-46 (4)(1999) 985. [2] K. Wienhard, M. Schmand, M.E. Casey, et al., IEEE Trans. Nucl. Sci. NS-49 (1)(2002) 104. ¨ [3] A. Varrone, N. Sjoholm, L. Eriksson, et al. Eur. J. Nucl. Med. Mol. Imag., doi:10.1007/s00259-009-1156-3. [4] PET scanner. USPTO patent # 7,102,135 B2 (CERN, P. Lecoq). [5] P. Sempere Roldan, E. Chereul, O. Dietzel, et al., Nucl. Instr. and Meth. A 571 (2007) 498. [6] J. Ho Jung, Y. Choi, Y.Hyun Chung, et al., Nucl. Instr. and Meth. A 571 (2007) 669. [7] M. Zhuravleva, C.L. Melcher, L. Eriksson, IEEE Nucl. Sci. Symp. Conf. Rec. (2009) Session J04 oral contribution number 6. [8] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instrum.. 32 (9)(1961) 1044. [9] H. Du, Y. Yang, J. Glodo, Y. Wu, et al., Phys. Med. Biol. 54 (2009) 1757.
Fig. 6. (a) LuAG:Pr 10 10 20 mm3 pixels with LSO backing (10 10 1 mm3). Front gate applied. (b) LuAG:Pr 10 10 20 mm3 pixels with LSO backing (10 10 1 mm3). Back gate applied.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Phoswich solutions for the PET DOI problem L. Eriksson a,b,c,d,n, C.L. Melcher d, M. Zhuravleva d, M. Eriksson b, H. Rothfuss a,d, M Conti a a
Siemens Medical Solutions, Molecular Imaging, Knoxville, TN, USA Karolinska Institute, Stockholm, Sweden University of Stockholm, Stockholm, Sweden d Scintillation Materials Research Center, University of Tennessee, Knoxville, TN, USA b c
a r t i c l e i n f o
a b s t r a c t
Available online 19 November 2010
A high spatial resolution in PET can be achieved by using small detector elements. To maintain good sensitivity these elements have to be quite long, thus introducing parallax error and making the spatial resolution non-uniform over the image volume. Uniformity of spatial resolution can be improved by utilizing depth-of-interaction (DOI) information to reduce the parallax error. In the present study we have focused on phoswich approaches based on interacting scintillators, that is, a phoswich combination in which one scintillator emits light in the excitation band of the other. We have looked at LaBr3:Ce and LaCl3:Ce and the interactions of those two scintillators with LSO:Ce, GSO:Ce and YSO:Ce. The reasons to use the two Lanthanum scintillators are twofold: light output is high and the two different emission wavelengths, 350 nm (LaCl3:Ce) and 380 nm (LaBr3:Ce) may produce different interactions with the three oxyorthosilicate scintillators. In addition a possible DOI detector comprising LuAG:Pr pixels with a thin LSO:Ce layer at one end has been evaluated. A Bollinger–Thomas set-up was used to measure luminescence rise and luminescence decay time characteristics in all cases. When using LaCl3:Ce, the phoswich combinations with YSO:Ce and GSO:Ce showed phoswich decay time characteristics as expected for a simple convolution of the decay times of the two phoswich components. A correction was needed, however, for the LaCl3:Ce–LSO:Ce phoswich due to the LSO:Ce intrinsic activity. For the LaBr3:Ce–LSO:Ce phoswich, corrections were needed for noninteracting LaBr3:Ce light in addition to the expected phoswich interaction. & 2010 Elsevier B.V. All rights reserved.
Keywords: High resolution PET Phoswich detectors Lutetium oxyorthosilicate LSO Gadolinium oxyorthosilicate GSO Yttrium oxyorthosilicate YSO Lanthanum Bromide LaBr3 Lanthanum chloride LaCl3 Scintillator interactions
1. Introduction Positron Emission Tomography (PET) has two main missions, spatial resolution and sensitivity. The present work concerns spatial resolution. The spatial resolution is related to the pixel size, but since long pixels are needed for sensitivity, many detection element combinations will have a substantial radial elongation. The elongation problem can be solved by registering the depth-of-interaction (DOI) of gamma rays in the detection elements. A relatively easy way to achieve this is to use the phoswich concept, that is, to construct the pixel from two or more sections of different scintillators and then identify the gamma ray absorbing section based on pulse shape discrimination techniques. This has been done for the Siemens HRRT systems having a phoswich combination based on LSO:Ce and LYSO:Ce. Separation is based on scintillation decay time with LSO:Ce having a decay time around 40 ns and LYSO:Ce (30%Lu) having a decay time of 55 ns [1–3]. This is an example of a classical phoswich with non-interacting scintillators.
n Corresponding author at: Siemens Healthcare Molecular Imaging, 810 Innovation Dr, Knoxville, TN 37932, USA. Tel.: + 1 865 218 2246; fax: +1 865 218 3000. E-mail address: [email protected] (L. Eriksson).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.11.049
Recently other combinations have been successfully tried such as LuYAP:Ce and LSO:Ce [4–6]. A potential problem is that the LuYAP:Ce emission occurs in the excitation band of LSO:Ce changing the emission characteristics making it important in what order, relative to the PMT, the scintillators are arranged. By placing the LSO:Ce in the front with LuYAP:Ce behind, close to the PMT, the interactions can be minimized and good phoswich functionality can be obtained. When light from one scintillator is absorbed by a second scintillator, the resulting photo luminescence may have a decay time that is quite different from the decay time from gamma excitation. Photo luminescence of GSO:Ce excited at 345 nm gives a decay time of around 25 ns, while excitations based on gamma rays may give a decay time of around 70 ns [7]. Furthermore, the interaction and the scintillation emission can be described as a convolution of the two decay times of the two scintillators. Part of our efforts have been devoted to phoswich combinations based on LaCl3:Ce and LaBr3:Ce combined with LSO:Ce, GSO:Ce and YSO:Ce. The reason for this are the differences in emission wavelengths of the two lanthanide crystals, with emission around 350 nm for LaCl3:Ce and around 380 nm for LaBr3:Ce, which may create some interesting differences for the phoswich performance. Fig. 1 shows the LaBr3:Ce and LaCl3:Ce emissions in the LSO:Ce excitation band. The expected phoswich performance is a convolution
L. Eriksson et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S288–S292
Fig. 1. The figure shows the emissions of the LaCl3 and LaBr3 and the excitation bands of LSO. The excitation bands for GSO and YSO are similar to LSO. It seems reasonable that a correction has to be applied for the LaBr3–LSO phoswich, since a fraction of the LaBr3 light emission falls in the emission band of LSO. The PMTs, recording the light from the phoswich, may see (1) re-emitted LSO light due to interactions with LaBr3, (2) direct non-interacting LaBr3 light and (3) intrinsic LSO light due to Lu-176 activity in the LSO scintillator. The LaCl3 emission is entirely inside the excitation range of LSO (YSO and GSO) effectively converting its light into re-emitted LSO (YSO and GSO) light. For the LSO case we still will have a fraction of light generated by the Lu-176 impurity.
Fig. 2. Overview of the Bollinger–Thomas set up for measuring luminescence decay of the phoswich detectors. The PMTs are only seeing the back scintillator layer of the phoswich.
between the decay times of the two phoswich components. The phoswich decay time characteristics have been measured using a Bollinger–Thomas set up [8] as shown in Fig. 2. An elegant way to solve the DOI problem may be to place a thin scintillator layer on top of a scintillator pixel where the scintillator pixel light interacts with the scintillating top layer adding a mixture of different scintillation times that can be used for DOI determination [9]. By understanding the physics behind the interacting scintillators the phoswich concept can be extended and may provide much more elegant solutions to the DOI problem.
2. Methods 2.1. Experimental details—initial measurements The scintillator crystals used in this work are reasonable candidates for interacting phoswich detectors in PET combining high light output, relatively high detection efficiency for gamma rays, and fast scintillation decay. The selected scintillators were LaBr3:Ce (Saint-Gobain, BrilLance-380) and LaCl3:Ce (Saint-Gobain,
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BrilLance-350) as front layer scintillators; and LSO:Ce, GSO:Ce and YSO:Ce as the back layer in the phoswich configurations. Photoluminescence decay (PL) profiles of the back layer scintillator crystals were recorded at room temperature using the timecorrelated single photon counting technique (TCSPC) with a Fluorolog 3 lifetime spectrofluorometer (Horiba Jobin Yvon). The instrument is equipped with interchangeable pulsed UV-NanoLED light sources that are selected to match the excitation bands of the scintillators. The width of the excitation pulses are 1 ns. In this work, we used 345, 360 and 371 nm LEDs as UV excitation sources to get the PL response during activation of the excitation band of the three back layer scintillators. A double monochromator with a 3 nm band-pass was used to select the emission wavelength. Scintillation decay times of the individual scintillators and of the phoswich detectors were measured under gamma-ray excitation with 137Cs (622 keV) and 241Am (59.5 keV) sources. When using the 241 Am source, the alpha particles were stopped with a plastic filter. A Bollinger–Thomas set-up [8], operating in TCSPC mode with Photonis XP2020Q photomultiplier tubes and NIM electronics was used (Fig. 2). As an example of the differences between PL luminescence and gamma ray luminescence we show results from a GSO:Ce scintillator in Fig. 3a and b. 2.2. Experimental details—phoswich measurements The scintillators LSO:Ce, YSO:Ce and GSO:Ce were all combined with LaBr3 and LaCl3 in phoswich arrangements with the lanthanum scintillators in the front. Both lanthanum scintillators were obtained from Bicron with the sizes 13 mm diameter, 13 mm long. The 241Am source was placed on top of the front scintillator. Since very few 59.5 keV photons penetrate the 13 mm Lanthanum scintillators, direct 59.5 keV activation of the LSO:Ce, YSO:Ce and GSO:Ce crystals was ignored. The phoswich combinations were optically combined by using optical grease. Reflector foil was applied covering the sides of the back scintillator and finally black tape was used to block direct light from the lanthanum scintillators to enter the PMTs in the Bollinger–Thomas set up. The gamma ray luminescence decay kinetics were measured for the LaBr3/LSO/ YSO/GSO and the LaCl3/LSO/YSO/GSO combinations. 2.3. Experimental details—Measurements with a LuAG:Pr pixel with a LSO backing Du et al [9] presented a phoswich detector in with LSO:Ce pixels with a thin front layer of Ce activated YGG (Y3 (Al,Ga)5O12:Ce). Following a gamma ray absorption, the emission of light from LSO:Ce activates the YGG layer and the sum between pure LSO:Ce light and convolved YGG:Ce – LSO:Ce light is seen as a change in decay time, which depends on the depth-of-interaction in the LSO:Ce pixel. We tried a very similar approach, but instead using a LuAG:Pr pixel, 10 10 20 mm3 unpolished with a thin LSO:Ce layer in the front (10 10 1 mm3). The LuAG:Pr pixel with the LSO:Ce backing was covered with black tape to make the response more nonuniform. The approach looked promising theoretically, looking at the sum of the convolution between LSO:Ce and LuAG:Pr and the direct LuAG:Pr light. We used a modified Bollinger–Thomas system with a collimation detector added, used to identify scintillations from the front and the back part of the LuAG:Pr pixel.
3. Results There is a difference between the photoluminescence measurements and the gamma ray luminescence for the three ‘‘back’’ layer scintillators, LSO:Ce, YSO:Ce and GSO:Ce. We showed the GSO:Ce case
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Table 1 Decay times for LSO, YSO and GSO in photoluminescence and in gamma ray luminescence. Photo luminescence for LaBr3 and LaCl3 were not measured. Scintillator
Decay time for photo luminescence (ns)
Decay time for gamma ray luminescence (ns)
LSO YSO GSO (LaBr3) (LaCl3)
31.2 35.8 24.3 N/A N/A
37.6 62.2 74.0 (55%) 430 (45%) 17.2 20.3 (75%) 170 (25%)
Fig. 3. (a) Photoluminescence in GSO with 345 nm activation. (b) Am241 gamma activation of GSO.
in Fig. 3 with around 25 ns in photoluminescence decay and 73 ns in gamma ray luminescence decay. The results for the gamma ray luminescence and the photoluminescence measurements are summarized in Table 1. Photoluminescence is the effect we have to consider in the case of scintillator interactions. A direct hit in the back layer of 511 keV photons result in a response based on gamma ray luminescence while a hit in the top layer, in for example the LaBr3:Ce layer, results in a convolution between LaBr3:Ce decay time and back scintillator PL decay time, a phoswich luminescence with a rise time given by the LaBr3:Ce luminescence and a decay time given by the PL decay of the back layer. However, as seen from Fig. 1, a fraction of LaBr3:Ce light may be transmitted across the back-layer without interactions and the data as recorded by the Bollinger–Thomas set up is now a sum between LaBr3-back-layer phoswich data and a fraction of pure LaBr3 data. When the back layer is LSO:Ce there is also a contribution of gamma activated LSO:Ce light from the intrinsic 176Lu activity. The Bollinger–Thomas data S are compared to model equations described in Eqs. (1) and (2) below.
Fig. 4. (a) Am241 activated LaBr3_GSO phoswich. A direct Labr3 light fraction has been fit to the data. The fitted direct fraction is 17%. This would imply that 17% of all the LaBr3 events generated by the Am sources are passing through the GSO without interaction and entering the Bollinger–Thomas set up. The fit indicates a 10 ns rise time and a 28 ns decay time for the LaBr3_GSO phoswich. (b). Am241 activated LaCl3_GSO phoswich. No corrections done. The fit gives a 19 ns rise time and a 28 ns decay time for the LaCl3_GSO phoswich and a second decay time of 219 ns.
The plots and fits of the Lanthanum phoswich decay time data measurements are shown in Fig. 4 for the case of GSO. All phoswich data are summarized in Table 2.
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Table 2 Phoswich rise and decay times for LaBr3/LaCl3 combined with GSO, YSO and LSO. Back layer scintillator
Front layer: LaBr3 Phoswich scintillation response
Notes on LaBr3 phoswich data
Measured Rise time (ns)
Expected Rise time (ns)
Measured Decay time
Expected Decay time (ns)
Corrections
GSO YSO LSO
10 18 17
17 17 17
28 ns 35 ns 31
24 36 31
17% LaBr3 5% LaBr3 5%(LaBr3 + LSO)
Back layer scintillator
Front layer: LaCl3 Phoswich scintillation response
GSO YSO LSO
Notes on LaCl3 phoswich data
Measured Rise time (ns)
Expected Rise time (ns)
Measured Decay time (ns)
Expected Decay time (ns)
Corrections
19 18 18
20 20 20
28 37 30
24 36 31
No corrections No corrections 20 % (LSO)
Fig. 5. A modified Bollinger–Thomas set up with addition of a collimation detector, used in coincidence with the regular luminescence event registration. A Na22 point source is used. The collimation detector has a 2 2 2 mm3 LSO scintillator. Together with the point source the collimation detector can select either the front or the back of the TEST pixel, a 10n10n20 mm3 LuAG:Pr scintillator with an LSO backing of 10 10 1 mm3 with a 33 ns PL decay time.
The LaCl3:Ce phoswich results are based on uncorrected data (Eq. (1)), except for the LaCl3:Ce/LSO:Ce phoswich with a small correction for LSO:Ce intrinsic scintillations. S ¼ bck þ act½frac et=decay1 þ ð1fracÞet=decay2 et=rise
ð1Þ
The constant act is the start number of counts found by the fit, frac is the fraction of the phoswich luminescence decay channeled via decay1 and bck is the background. The LaBr3:Ce phoswich data are analyzed according to Eq. (2). S ¼ bck þ actðet=decay et=rise Þ þfraction LaBr3 corr
ð2Þ
where LaBr3corr is the direct contribution of the LaBr3:Ce luminescence data added to the phoswich information and modulated by the constant fraction. The fits of the LaBr3:Ce data in phoswich combinations with GSO/YSO/LSO to Eq. (2) are ill-conditioned and sensitive to start values and starting points. In Fig. 4a results from a successful fit is shown. Only in this case (LaBr3:Ce/GSO:Ce data) both the rise and decay times and the assumed LaBr3:Ce direct contribution (the constant fraction) gave reasonable values. For the other LaBr3:Ce phoswich combinations, the one with YSO:Ce and LSO:Ce, data were analyzed by adding small amounts of the fraction to the direct LaBr3:Ce light to keep the ill-conditioned least squares fit under control. For these cases we can only indicate that direct LaBr3:Ce
light is present and that reasonable phoswich rise and decay time data can be obtained by assuming a certain fraction. This was not the case for the LaCl3:Ce phoswich combinations. A direct fit based on Eq. (1) gave reasonable rise time and decay time data. We conclude that the LaCl3/GSO/YSO phoswich data as analyzed by the Bollinger–Thomas technique directly provide correct rise time and decay time data. The experimental data do support the assumption of different phoswich interactions for LaBr3:Ce and LaCl3:Ce when combined with GSO/YSO LSO. Concerning the experiment with the Du approach we first looked at the situation described in the Du paper, a LSO:Ce pixel with YGG:Ce backing. The photo detector sees the sum of LSO:Ce light and light due to the YGG:Ce–LSO:Ce interactions. A 100% LSO:Ce signal gives 40 ns decay time and a 50% LSO:Ce 50% LSO:Ce–YGG:Ce light gives 54 ns assuming a photo detector quantum efficiency of the YGG:Ce light of 30% relative to LSO:Ce light. The situation is of course very much improved for photo sensors with a better response in the green wavelength region like an APD or SiPM. For a photo sensor with the same quantum efficiency for both emissions the 50% admixture would now give 76 ns instead of 54 ns. A similar calculation for LuAG:Pr with LSO:Ce backing, assuming the same quantum efficiency for both emissions (LuAG:Pr light and
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LSO:Ce light) gives 20 ns for 100% LuAG:Pr light and 41 ns for a 50% mixture of both types. Experimentally with the 10 10 20 mm3 LuAG:Pr pixel with a 10 10 1 mm3 LSO:Ce backing, wrapped with black tape to break up the uniformity response, we found 27 ns activating the front of the crystal and 33 ns activating the back of the crystal using the modified Bollinger–Thomas set up with the added collimation detector (Fig. 5). This decay time difference needs to be improved in order to obtain useful DOI information from this scintillator information (Fig. 6a and b).
4. Conclusions The Bollinger–Thomas set up was used to study phoswich detectors based on LaBr3 or LaCl3 as the front detectors combined with GSO/YSO/LSO as the back detector. The phoswich detectors were shielded in such a way that the set up only registered events from the back scintillator. The LaBr3 or LaCl3 detectors only were activated with 59.5 keV gamma rays from an Am-241 source. Scintillation decay time spectra were recorded. The LaBr3/GSO/ YSO/LSO data showed the presence of the expected interaction with the back scintillator and, in addition, the presence of direct LaBr3 scintillation information. The LaCl3/GSO/YSO/LSO data showed only the interaction between the front and the back scintillator. An attempt to design a phoswich detector based on a LuAG:Pr pixels with a thin LSO backing showed a DOI sensitive admixture between LuAG:Pr light and re-emitted LSO light. In the present detector design, however, the DOI sensitivity was too low. References [1] M. Schmand, L. Eriksson, M.E. Casey, et al., IEEE Trans. Nucl. Sci. NS-46 (4)(1999) 985. [2] K. Wienhard, M. Schmand, M.E. Casey, et al., IEEE Trans. Nucl. Sci. NS-49 (1)(2002) 104. ¨ [3] A. Varrone, N. Sjoholm, L. Eriksson, et al. Eur. J. Nucl. Med. Mol. Imag., doi:10.1007/s00259-009-1156-3. [4] PET scanner. USPTO patent # 7,102,135 B2 (CERN, P. Lecoq). [5] P. Sempere Roldan, E. Chereul, O. Dietzel, et al., Nucl. Instr. and Meth. A 571 (2007) 498. [6] J. Ho Jung, Y. Choi, Y.Hyun Chung, et al., Nucl. Instr. and Meth. A 571 (2007) 669. [7] M. Zhuravleva, C.L. Melcher, L. Eriksson, IEEE Nucl. Sci. Symp. Conf. Rec. (2009) Session J04 oral contribution number 6. [8] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instrum.. 32 (9)(1961) 1044. [9] H. Du, Y. Yang, J. Glodo, Y. Wu, et al., Phys. Med. Biol. 54 (2009) 1757.
Fig. 6. (a) LuAG:Pr 10 10 20 mm3 pixels with LSO backing (10 10 1 mm3). Front gate applied. (b) LuAG:Pr 10 10 20 mm3 pixels with LSO backing (10 10 1 mm3). Back gate applied.