- Email: [email protected]
Revisited position arithmetics for LaBr3:Ce continuous crystals
Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386 www.elsevier.com/locate/npbps
Revisited position arithmetics for LaBr3 :Ce continuous crystals R. Pania∗ , F. Vittorinia , M. N. Cintia , P. Bennatib , R. Pellegrinia , S. Ridolfia , R. Scaf`ec , S. Lo Meod , M. Mattiolie , F. L. Navarriad , G. Moschinif , A. Fabbrib , E. D’Abramob , V. Orsolini Cencellig , D. Saccoh a
INFN Rome and Experimental Medicine Dpt. Sapienza University of Rome, Rome (Italy).
b
INFN Rome and EDEMOM PhD School of Microelectronics, University of Rome III, Rome (Italy).
c
INFN and ENEA, Casaccia, Rome (Italy).
d e f
INFN Bologna and Physics Dpt. Alma Mater Studiorum - University of Bologna, Bologna (Italy)
INFN Rome and Physics Dpt. Sapienza University of Rome, Rome (Italy).
INFN - LNL and Physics Dpt. University of Padua, Padua (Italy).
g
INFN Department of Engineering Electronic “Roma III” University of Rome, Rome (Italy).
h
INFN Rome and ISPESL, Rome (Italy).
The development of the molecular imaging technique based on radiopharmaceuticals is requiring sophisticated devices with sub-millimeter spatial resolution and high detection efficiency. Recently, in the field of scintillation gamma camera, it has been demonstrated that continuous crystals may improve spatial resolution with respect to the use of scintillation arrays. In this work we propose a new algorithm that improves spatial resolution. It calculates the position from a single scintillation event by squaring the charge collected on a multi-anodes photomultiplier tube (MA-PMT). It is able to remove the position linearity distortion due to the light reflections and the truncations at the crystal edge. We present measurements from a compact gamma camera based on a 51 mm × 51 mm × 4.0 mm LaBr3 :Ce continuous scintillation crystal, coupled to a MA-PMT Hamamatsu H8500. The results show a strong improvement in spatial resolution and confirm the high potential in using of LaBr continuous crystal for molecular imaging applications.
1. Introduction New imaging devices with sub-millimeter spatial resolution and high detection efficiency are required in the development of highly sophisticated molecular imaging technique for small animal applications. Continuous scintillation crystals with high light yield could fit such a performance and it could allow to overcome spatial resolution limitation arising from the use of the scintillation arrays. In fact, if a weighted centroid algorithm (Anger logic) is used, the spatial resolution is roughly dependent on the light dis∗ Corresponding
Author: [email protected] “Sapienza” University of Rome Viale Regina Elena 272 00161, Rome Italy (telephone and fax: +390649918277)
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.10.109
tribution spread and on the photoelectron number generated from photocathode [1][2]. Unfortunately position linearity is strongly affected by light reflection and truncation at the crystal edges (light edge effect). Over the last year many researchers have been proposing different continuous crystals to build small Field of View (FoV) detectors, obtaining a spatial resolution never lower than 1.8-2.0 mm at the cost of poor detection efficiency [3]. In this work, we highlight that the “Anger logic” algorithm produces an intrinsic position non-linearity affecting in a non negligible way spatial resolution for continuous crystal (less than 10 × 10 cm2 FoV) . We propose a different approach to calculate the position mean value based on the squared charge collected on each an-
384
R. Pani et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386
ode. This solution could improve spatial resolution by correcting position linearity also for thick scintillation crystals.
light (P SFlight ) coming from a single scintillation event and the corresponding one, which is called Image PSF (P SFimage ) obtained from several scintillation events.
2. Equipment and Method We analyze the response of a compact scintillation gamma camera based on 51 × 51 × 4.0 mm3 LaBr3 :Ce continuous crystal, by Saint Gobain, coupled through 3.0 mm glass window to a Hamamatsu H8500 MA-PMT. This photomultiplier has an external size of 52 × 52 mm2 , the photocathode is bialkali (27% QE) and 12 stages metal channel dynode are used as electron multiplier. A 8 × 8 anode matrix (64 channels) is used for positioning where each individual 6 mm pitch anode works as a single PMT in a standard gamma camera. A single anode electronic readout developed at ”Roma3” University [4] is used to acquire and digitize the charge and reconstruct by the new algorithm the impinging gamma position. LaBr3 :Ce gamma camera shows 70% of intrinsic detection efficiency and 8.5% energy resolution at 140 keV photon energy [5]. Furthermore, we use Monte Carlo simulation (GEANT4 version 4.9.0) with the aim to validate the effect of the new algorithm on intrinsic position nonlinearity response [6]. Experimental and simulated data are referred to detector FoV scanning with a 0.4 mm 99m Tc collimated source at 1.5 mm step. 2.1. Imaging formation processes: Anger centroid algorithm Centroid algorithm was elaborated by Anger in 1958 and it is still now the basic principle of imaging reconstruction in modern scintillation gamma Cameras [7]. They use a phototube matrix to sample the scintillation light distribution and output signals proportional to the collected charge (Point Spread Function of the light - P SFlight ). The centroid algorithm calculates the position (X,Y ) of event by event as a mean value of the measured charge distribution, which represents a point in the imaging plane; many γ-ray interactions then give rise to the image of the emitting source. In Fig.1, we compare both PSFs involved in this process: the PSF of
Figure 1. Comparison between PSF of light scintillation (left) and a PSF image as produced by many scintillation events (right)
In Fig.2, the three principal steps needed to obtain the X and Y position of each scintillation events are shown. The anodic array of MA-PMT operates a sampling of the P SFlight obtaining the charge distribution shown in Fig.2b. The classical Anger algorithm applied on this charge distribution can be written as follows: j n j xj (1) XC = j nj k where nj = k nj is the “projection” of the charge collected along the j-th column, (xj ) is the anode coordinate and (Xc ) is the centroid coordinate along the x-direction (Fig. 2c). The same is applied along the y direction. Spatial resolution is related to the statistic un2 ). certainty of the scintillation event position (σX C By applying its statistical definition we can write: σcharge σX C = √ nphe
(2)
Where σcharge represent the standard deviation of the charge distribution as projected along x
R. Pani et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386
385
fects both σcharge and position linearity and consequently ISR. In conclusion, assuming a poissonian distribution for (nkj ), we define ISR by: ISR =
2.35 σcharge · √ L nphe
(6)
, as its homologue in equation 3 is where σcharge a form factor depending only on the PSF shape, and L is the linearity coefficient. We calculate the FWHM of charge spread distribution, position linearity and spatial resolution by applying the standard and the new algorithm on the photoelectron distribution, then comparing them with simulated data.
3. Results Figure 2. a) Light scintillation spread from a single scintillation event. b) Charge distribution as sampled by the anode array. c) Charge projection along one direction.
direction, and (nphe ) the average number of photoelectrons. So we can define the Intrinsic Spatial Resolution (ISR) of the detector with respect to the Full Width of Half Maximum, as: ISR = F W HMP SFimage =
F W HMP SFlight (3) √ nphe
2.2. New algorithm for SMALL FoV devices The centroid algorithm is modified by the following: j n j xj (4) XC = j nj with nj
=
(nkj
− t)
2
(5)
k
where (t) represents a threshold useful to remove the light background into the crystal, which af-
In Fig.3, two different charge spreads are shown. The worsening of position linearity due to the reflection and truncation of charge spread is the main drawback, as it is confirmed by the Monte Carlo simulation. Position linearity coefficient values obtained from experimental results (0.67 ± 0.03) and simulation (0.65 ± 0.03) are in good agreement. In Fig.4, the new procedure of position determination is described in three steps. The new algorithm allows to narrow the original charge spread with respect to the original distribution as qualitatively shown in Fig.4. In Fig.5, the scanning of the central FoV obtained from 0.4 mm 99m Tc collimated source at 1.5 mm step is shown. The improvement in position linearity 5 is clearly visible from the correspondence of measured peak distance with the scanning step. According to equation (6) the spatial resolution improve from (1.6 ± 0.1) mm to (1.1 ± 0.1) mm proportionally to the L value. 4. Conclusions We present a new procedure to calculate the scintillation event position in a continuous crystal. The new algorithm consists on squaring the charge collected by multi-anodes photodetectors utilized in a compact LaBr3 :Ce gamma camera. The new algorithm is able to strongly improve
386
R. Pani et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386
Figure 3. Two different experimental charge spreads measured at the center (0 mm) and at the edge (25 mm) are superimposed to the position linearity response. The worsening of linearity response is related to the truncation of light distribution.
Figure 4. Three step procedure shows the effect on the scintillation light spread distribution. The final distribution (c) is narrower and with less noise than the original one.
spatial resolution of continuous crystals by correction of position distortion mainly occurring when light distributions are largely truncated at the edges. Applying this procedure we can get full advantage from the high light yield properties of LaBr3 :Ce crystal in continuous shape for gamma ray imaging applications. Figure 5. Image profiles of a scanning of the detector
REFERENCES 1. R.G. Baker and J.W. Scrimger, Phys Med Biol, 12:51-63, 1967. 2. J. G. Rogers, et al., Phys. Med. Biol., 1986 Vol. 31, No 10, pp.1061-1090. 3. P. Despr`es et al., IEEE Transactions On Nuclear Science, Vol. 53, No. 3, June 2006. 4. E. D’Abramo et al., Nuclear Science Symposium Conference Record, 2006 IEEE, Volume 5, Page(s):3072 - 3075 5. R. Pani et al., Nuclear Science Symposium Conference Record, 2005 IEEE, Volume 4,Page(s):2061 - 2065 6. S. LoMeo, et al., Nuclear Science Sympo-
central FoV, with a 0.4 mm 99m Tc point source at 1.5 mm step, a) with the standard algorithm and b) with the new algorithm.
sium Conference Record, NSS-IEEE 2008, Page(s):4964 - 4968 7. H. O. Anger, Rev. Sci. Instrum. 29, 27-33, 1958.
Revisited position arithmetics for LaBr3 :Ce continuous crystals R. Pania∗ , F. Vittorinia , M. N. Cintia , P. Bennatib , R. Pellegrinia , S. Ridolfia , R. Scaf`ec , S. Lo Meod , M. Mattiolie , F. L. Navarriad , G. Moschinif , A. Fabbrib , E. D’Abramob , V. Orsolini Cencellig , D. Saccoh a
INFN Rome and Experimental Medicine Dpt. Sapienza University of Rome, Rome (Italy).
b
INFN Rome and EDEMOM PhD School of Microelectronics, University of Rome III, Rome (Italy).
c
INFN and ENEA, Casaccia, Rome (Italy).
d e f
INFN Bologna and Physics Dpt. Alma Mater Studiorum - University of Bologna, Bologna (Italy)
INFN Rome and Physics Dpt. Sapienza University of Rome, Rome (Italy).
INFN - LNL and Physics Dpt. University of Padua, Padua (Italy).
g
INFN Department of Engineering Electronic “Roma III” University of Rome, Rome (Italy).
h
INFN Rome and ISPESL, Rome (Italy).
The development of the molecular imaging technique based on radiopharmaceuticals is requiring sophisticated devices with sub-millimeter spatial resolution and high detection efficiency. Recently, in the field of scintillation gamma camera, it has been demonstrated that continuous crystals may improve spatial resolution with respect to the use of scintillation arrays. In this work we propose a new algorithm that improves spatial resolution. It calculates the position from a single scintillation event by squaring the charge collected on a multi-anodes photomultiplier tube (MA-PMT). It is able to remove the position linearity distortion due to the light reflections and the truncations at the crystal edge. We present measurements from a compact gamma camera based on a 51 mm × 51 mm × 4.0 mm LaBr3 :Ce continuous scintillation crystal, coupled to a MA-PMT Hamamatsu H8500. The results show a strong improvement in spatial resolution and confirm the high potential in using of LaBr continuous crystal for molecular imaging applications.
1. Introduction New imaging devices with sub-millimeter spatial resolution and high detection efficiency are required in the development of highly sophisticated molecular imaging technique for small animal applications. Continuous scintillation crystals with high light yield could fit such a performance and it could allow to overcome spatial resolution limitation arising from the use of the scintillation arrays. In fact, if a weighted centroid algorithm (Anger logic) is used, the spatial resolution is roughly dependent on the light dis∗ Corresponding
Author: [email protected] “Sapienza” University of Rome Viale Regina Elena 272 00161, Rome Italy (telephone and fax: +390649918277)
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.10.109
tribution spread and on the photoelectron number generated from photocathode [1][2]. Unfortunately position linearity is strongly affected by light reflection and truncation at the crystal edges (light edge effect). Over the last year many researchers have been proposing different continuous crystals to build small Field of View (FoV) detectors, obtaining a spatial resolution never lower than 1.8-2.0 mm at the cost of poor detection efficiency [3]. In this work, we highlight that the “Anger logic” algorithm produces an intrinsic position non-linearity affecting in a non negligible way spatial resolution for continuous crystal (less than 10 × 10 cm2 FoV) . We propose a different approach to calculate the position mean value based on the squared charge collected on each an-
384
R. Pani et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386
ode. This solution could improve spatial resolution by correcting position linearity also for thick scintillation crystals.
light (P SFlight ) coming from a single scintillation event and the corresponding one, which is called Image PSF (P SFimage ) obtained from several scintillation events.
2. Equipment and Method We analyze the response of a compact scintillation gamma camera based on 51 × 51 × 4.0 mm3 LaBr3 :Ce continuous crystal, by Saint Gobain, coupled through 3.0 mm glass window to a Hamamatsu H8500 MA-PMT. This photomultiplier has an external size of 52 × 52 mm2 , the photocathode is bialkali (27% QE) and 12 stages metal channel dynode are used as electron multiplier. A 8 × 8 anode matrix (64 channels) is used for positioning where each individual 6 mm pitch anode works as a single PMT in a standard gamma camera. A single anode electronic readout developed at ”Roma3” University [4] is used to acquire and digitize the charge and reconstruct by the new algorithm the impinging gamma position. LaBr3 :Ce gamma camera shows 70% of intrinsic detection efficiency and 8.5% energy resolution at 140 keV photon energy [5]. Furthermore, we use Monte Carlo simulation (GEANT4 version 4.9.0) with the aim to validate the effect of the new algorithm on intrinsic position nonlinearity response [6]. Experimental and simulated data are referred to detector FoV scanning with a 0.4 mm 99m Tc collimated source at 1.5 mm step. 2.1. Imaging formation processes: Anger centroid algorithm Centroid algorithm was elaborated by Anger in 1958 and it is still now the basic principle of imaging reconstruction in modern scintillation gamma Cameras [7]. They use a phototube matrix to sample the scintillation light distribution and output signals proportional to the collected charge (Point Spread Function of the light - P SFlight ). The centroid algorithm calculates the position (X,Y ) of event by event as a mean value of the measured charge distribution, which represents a point in the imaging plane; many γ-ray interactions then give rise to the image of the emitting source. In Fig.1, we compare both PSFs involved in this process: the PSF of
Figure 1. Comparison between PSF of light scintillation (left) and a PSF image as produced by many scintillation events (right)
In Fig.2, the three principal steps needed to obtain the X and Y position of each scintillation events are shown. The anodic array of MA-PMT operates a sampling of the P SFlight obtaining the charge distribution shown in Fig.2b. The classical Anger algorithm applied on this charge distribution can be written as follows: j n j xj (1) XC = j nj k where nj = k nj is the “projection” of the charge collected along the j-th column, (xj ) is the anode coordinate and (Xc ) is the centroid coordinate along the x-direction (Fig. 2c). The same is applied along the y direction. Spatial resolution is related to the statistic un2 ). certainty of the scintillation event position (σX C By applying its statistical definition we can write: σcharge σX C = √ nphe
(2)
Where σcharge represent the standard deviation of the charge distribution as projected along x
R. Pani et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386
385
fects both σcharge and position linearity and consequently ISR. In conclusion, assuming a poissonian distribution for (nkj ), we define ISR by: ISR =
2.35 σcharge · √ L nphe
(6)
, as its homologue in equation 3 is where σcharge a form factor depending only on the PSF shape, and L is the linearity coefficient. We calculate the FWHM of charge spread distribution, position linearity and spatial resolution by applying the standard and the new algorithm on the photoelectron distribution, then comparing them with simulated data.
3. Results Figure 2. a) Light scintillation spread from a single scintillation event. b) Charge distribution as sampled by the anode array. c) Charge projection along one direction.
direction, and (nphe ) the average number of photoelectrons. So we can define the Intrinsic Spatial Resolution (ISR) of the detector with respect to the Full Width of Half Maximum, as: ISR = F W HMP SFimage =
F W HMP SFlight (3) √ nphe
2.2. New algorithm for SMALL FoV devices The centroid algorithm is modified by the following: j n j xj (4) XC = j nj with nj
=
(nkj
− t)
2
(5)
k
where (t) represents a threshold useful to remove the light background into the crystal, which af-
In Fig.3, two different charge spreads are shown. The worsening of position linearity due to the reflection and truncation of charge spread is the main drawback, as it is confirmed by the Monte Carlo simulation. Position linearity coefficient values obtained from experimental results (0.67 ± 0.03) and simulation (0.65 ± 0.03) are in good agreement. In Fig.4, the new procedure of position determination is described in three steps. The new algorithm allows to narrow the original charge spread with respect to the original distribution as qualitatively shown in Fig.4. In Fig.5, the scanning of the central FoV obtained from 0.4 mm 99m Tc collimated source at 1.5 mm step is shown. The improvement in position linearity 5 is clearly visible from the correspondence of measured peak distance with the scanning step. According to equation (6) the spatial resolution improve from (1.6 ± 0.1) mm to (1.1 ± 0.1) mm proportionally to the L value. 4. Conclusions We present a new procedure to calculate the scintillation event position in a continuous crystal. The new algorithm consists on squaring the charge collected by multi-anodes photodetectors utilized in a compact LaBr3 :Ce gamma camera. The new algorithm is able to strongly improve
386
R. Pani et al. / Nuclear Physics B (Proc. Suppl.) 197 (2009) 383–386
Figure 3. Two different experimental charge spreads measured at the center (0 mm) and at the edge (25 mm) are superimposed to the position linearity response. The worsening of linearity response is related to the truncation of light distribution.
Figure 4. Three step procedure shows the effect on the scintillation light spread distribution. The final distribution (c) is narrower and with less noise than the original one.
spatial resolution of continuous crystals by correction of position distortion mainly occurring when light distributions are largely truncated at the edges. Applying this procedure we can get full advantage from the high light yield properties of LaBr3 :Ce crystal in continuous shape for gamma ray imaging applications. Figure 5. Image profiles of a scanning of the detector
REFERENCES 1. R.G. Baker and J.W. Scrimger, Phys Med Biol, 12:51-63, 1967. 2. J. G. Rogers, et al., Phys. Med. Biol., 1986 Vol. 31, No 10, pp.1061-1090. 3. P. Despr`es et al., IEEE Transactions On Nuclear Science, Vol. 53, No. 3, June 2006. 4. E. D’Abramo et al., Nuclear Science Symposium Conference Record, 2006 IEEE, Volume 5, Page(s):3072 - 3075 5. R. Pani et al., Nuclear Science Symposium Conference Record, 2005 IEEE, Volume 4,Page(s):2061 - 2065 6. S. LoMeo, et al., Nuclear Science Sympo-
central FoV, with a 0.4 mm 99m Tc point source at 1.5 mm step, a) with the standard algorithm and b) with the new algorithm.
sium Conference Record, NSS-IEEE 2008, Page(s):4964 - 4968 7. H. O. Anger, Rev. Sci. Instrum. 29, 27-33, 1958.