Crystals and light collection in nuclear medicine

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ELSEVIER

PROCEEDINGS SUPPLEMENTS

Nuclear Physics B (Proc. Suppl.) 78 (1999) 616-621

www.elsevier.nl/locate/npe

Crystals

and light collection

in N u c l e a r

Medicine

F. Vittori ~'*, F. de Notaristefani a and T. Malatesta b a Department of Physics "E. Amaldi" - University of Rome III - INFN - Section of Rome 1 b INFN Section of Rome 1 - Medical Physics Laboratory of Regina Elena Institute of Rome - Recently, particular attention has been paid to the development of small Field of View (FOV) breast imagers for Nuclear Medicine applications provided with YAP :Ce and CsI, both thallium and sodium doped, multipillar matrices. In order to improve their energy and spatial resolution performances, it is necessary to increase the light output of such scintillating matrices. We have developed a MonteCarlo code with the purpose of studying the detection efficiency as well as the light collection efficiency of crystal pillars as a function of their geometry and surfaces treatment. By analysing both the experimental and the simulations results, we have found out that the best choice for this gamma detector is represented by a CsI(Na) matrix made up of orthogonally arranged pillars whose dimensions are 2x2x3 mm 3. For this geometry, our simulations have showed a detection efficiency of 62 %, a collection efficiency of 80 % and an expected light output of 68 %.

Abstract

1. INTRODUCTION Scintimammography with Tc99m-Sestamibi represents a sensitive, specific and non invasive method to define the nature of radiologically described breast masses and would be very useful as a complement to X-ray mammography in screening programs for breast cancer. It has a specificity of about 87-89 % and a sensitivity of about 92 % [1,2] that decreases dramatically to about 50 % when considering lesions smaller than 1 c m [3,4].

Recent works have focused on the development of compact gamma cameras designed specifically for breast imaging and characterised by an optimal geometry and an improved spatial resolution [5,6,7]. Indeed, an accurate small Field of View gamma camera placed in the same geometry of conventional X-ray mammography, i.e. with breast compression, can improve the detection of subcentimetre cancer and can allow integration imaging between functional and morphological diagnostics, thus providing the basis of a multimodality system for breast imaging.

A recent work [8] presented both the physical and the clinical trials results of the HIRESPET Collaboration scintimammography detector now under clinical experimentation at the Institute of tumours Regina Elena of Rome (Italy). For the time being, this new breast imager for scintigraphy diagnosis represents the best compromise between FOV (10xl0 cm2), energy resolution over the entire FOV (20 %), spatial resolution (2.3 mm) and low cost. However, a better energy resolution could further improve the overall detector performance allowing the reduction of the Compton background coming mainly from the heart. In order to improve energy resolution it is necessary to increase the light output of the scintillating crystal. As a consequence, the scintillating crystal as well as the matrix and the pillars dimensions play a fundamental role in achieving the optimal performance of the detector. In this work we investigate and discuss the optimal choice of the scintillating crystal and the matrix-pillars geometry focusing our attention on two parameters: (1) collection efficiency of the scintillating photons and (2) detection efficiency of the primary gamma photons.

* Intemet address " [email protected]. HIRESPET Web Home Page : www.romal .infn.it/-hirespet This work was supported by CNR, INFN and MURST. 0920-5632/99/$ - see front matter © 1999 ElsevierScience B.V. All fights reserved.

PII S0920-5632(99)00614-3

E lqttori et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 616-621

2. SCINTILLATING CRYSTALS Table 1 displays the main characteristics of the most widely used scintillating crystals in Nuclear Medicine applications. NaI(T1) has been extensively used on Anger cameras thanks to its excellent light yield. Because of its hygroscopicity, NaI has to be enveloped in an hermetically sealed housing provided with a glass window for the scintillating photons detection. Moreover, it has a decay time of 230 ns and its emission is peaked at 415 nm where NaI shows a refraction index of 1.85. BGO instead is mainly used in multipillar configuration on PET detectors thanks to its short attenuation length at 511 keV. It is quite hard but its main drawback is represented by the poor light yield equal to about 8 %. It shows a refraction index of 2.15 and a decay time of about 300 ns. Recently, particular attention has been paid to YAP :Ce and CsI, both thallium and sodium doped, when arranged in multipillar configuration [9,10] because they have good mechanical and scintillating properties. YAP :Ce has a light yield of about 50-55% compared to NaI(T1), an average Z of 39, a density of 5.37 gr/cm3 and a scintillation decay time of 27 ns. Moreover, it has high hardness, is not hygroscopic and lacks a cleavage plane. CsI instead is quite soft and slightly hygroscopic. When doped with thallium it shows a light yield, integrated on the bialkali photocathode qauntum efficiency response, of about 50 % and a scintillation decay time of 900 ns. CsI(TI) light yield increases considerably when used in

617

conjunction to a photodiode thanks to its emission spectrum peaked at 550 nm. When doped with sodium instead, CsI shows a light yield of about 85 % because its emission spectrum (peaked at 420 nm) well matches the bialkaliphotocathode quantum efficiency response. It has a decay time of 630 ns and a refraction index of 1.83. Finally, LSO :Ce probably shows the best compromise between light yield and attenuation length at 140 keV but is naturally radioactive and then is inappropriate for a low count rate application.

3.EQUIPMENT AND METHODS 3.1 Experimental set-up Light yield measurements on crystals were accomplished by arranging the experimental set-up shown in figure 1. A planar NaI(TI) crystal and several YAP :Ce and CsI(T1) matrices were coupled to a traditional phototube (Philips X2020Q) and the output signals were integrated with a charge ADC (LeCroy 2249W). The integration time was set with gate pulses of 200 ns width for YAP :Ce and 4~ts width for both NaI(T1) and CsI(T1). The crystals were irradiated by means of a Co57 collimated source (1 mm collimating diameter). The YAP:Ce matrices consist of several orthogonally arranged pillars with thicknesses which range between 3 and 28 mm and sizes of 0.6x0.6 and lxl mm 2. Each pillar is covered by a 5 ~tm thick reflective diffractive layer with a total

Table 1 - Main characteristics of the most widely used scintillating crystals in Nuclear Medicine Scintillafin8 Crystals Properties YAP:Ce LSO:Ce NaI(TI) CsI(T1) CsI(Na) BGO CsI CsI NaI YA103 Lu2(SiO4)O Chemical formula Be4Ge3012 5.37 7.40 7.13 4.51 4.51 3.67 Density (gr/cm3) no slight slisht no no yes Hygroscopicity 1.93 1.82 2.15 1.78 1.83 1.85 Refraction index 420 480 550 420 370 415 ~-max(nm) 28000 18000 8200 59000 39000 38000 Light ph/MeV 45-50 85 50 75 100 yield %* 230 300 900 630 27 40 Decay time (ns) 0.408 0.086 0.277 0.277 0.697 0.107 Attenuation 140 keV 1.23 length (cm) 511 keV 3.05 1.11 2.43 2.43 2.24 * with respect to NaI(TI) and integrated on the spectral response of a bialkali photocathode

E Vittori et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 616-621

618

CRYSTAL MATRIX

X2020Q

FAN OUT

D LAY LINE

ADC

MCA

GATE PULSE GENERATOR

Figure 1- Block diagram of the system used for the light yield measurements on scintillating crystals

dead zone of 10 ~m and a light transmission of less than 5%. The CsI(T1) matrices instead are made up of orthogonally arranged pillars with a size of lxl mm 2 and a thickness of 3 and 10 mm. Each pillar is covered by a diffusive white layer (epoxy) with a total dead zone between two adjacent elements of about 250 p.m The planar NaI(T1) crystal has a 25 mm diameter, a 1 mm thick and is covered by an aluminium housing, 2 mm of glass window and a 0. i mm aluminium entrance window The experimental collection efficiencies of the crystal matrices were evaluated by means of the following formula OL pillar CEpiltar = OLptanar

CE planar LYpillars crystal

(1)

where CE is the collection efficiency of the scintillating photons, OL is the measured output light and LY is the intrinsic light yield of crystals with respect to that of NaI(T1). It has to be noted that we assumed a collection efficiency of the NaI(T1) planar crystal equal to 0.9. 3.2 Monte Carlo code We have developed a Monte Carlo code in order to reproduce the main phenomena which occur when a gamma photon with an energy lower than 1.02 MeV enters into a crystal, that is energy release and scintillating photons propagation, with the final goal of indicating the optimal choice of the scintillating crystal and of the pillars and matrix dimensions which allow to improve spatial and energy resolution of the detector. Each gamma photon entering the crystal is assumed to undergo photoelectric absorption or Compton scattering

event so that it releases part of all of its energy into the crystal bulk. A previous work [11] demonstrated that a 140 keV primary gamma photon releases almost all its energy in a 0.2 mm diameter cylinder of NaI, CsI or YAP :Ce. This is why we assumed that photoelectrons produced by gamma photon interactions release all their kinetic energy in the position where are generated. Then, for each energy release event a number of scintillating photons, given by the amount of released energy and the light yield of crystals, are emitted according to an isotropic distribution. All optical photons are tracked by the program on an individual basis until they are detected at the output surface (or at the photocathode surface if the crystal is assumed to be coupled to a PMT) or absorbed into the crystal bulk or onto the crystal surfaces. Two types of optical finishing have been considered for the crystal surfaces: paint and reflective. The former assumes a diffusive reflecting material deposited onto the surfaces characterised by a reflection coefficient. If random sampling shows that reflection occurs, it is assumed to be Lambertian. In this case the angle of reflection is independent from the angle of incidence and is sampled from a distribution given by I(0)=I(0)cos(0) where 0 represents the angle with respect to the surface normal. The behaviour of the reflective surface is given by the measurements carried out on the reflective multi-layer deposited on the YAP :Ce crystals surfaces. A recent work [12] demonstrated that: (1) the reflectance of the cited multi-layer is about 90 % for incidence angles lower than 50 degrees and is about 100 % for higher angles; (2) the absorption length of YAP :Ce crystal is equal to about 14 cm.

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E Vittori et al./Nuclear Physics B (Proc. SuppL) 78 (1999) 616-621

The output surface of the pillar can be coupled to a light photons detector whose entrance window is characterised by its geometry and refraction index. Photons that impinge the output surface are first tested for the possibility of Fresnel reflection if a change in refractive index occurs at this surface. If the reflection is selected, the angle of reflection is set equal to the incidence angle ; if instead the reflection doesn't occur, the photon is assumed to follow Snell's law refraction. Depending on the incidence angle and refractive indexes of the crystal and of the entrance window of the light photon detector, total internal reflection may take place.

4. RESULTS Figure 2 shows the dependence of photon collection efficiency on length and cross-sectional area of YAP :Ce pillars coupled to a quartz window.

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The solid lines represent the Monte Carlo results of pillars with 2x2, lxl, 0.6x0.6 and 0.3x0.3 mm 2 cross-sectional area and with a thickness which ranges between 1 and 30 m m The black dots represent the experimental collection efficiency values measured on YAP :Ce pillars with 0.6x0.6 and lxl mm 2 cross-sectional area. There is a good agreement between Monte Carlo results and experimental ones. In the upper part of the graphic, it is visible the collection efficiency theoretical limit due to the critical angle given by the refraction indexes of YAP :Ce and quartz. As a matter of facts, if we hypothesise perfectly reflective lateral surfaces of the pillars and an infinitive absorption length of the crystal, this limit can be calculated by means of the following formula oc

- - 0 - - - 0 . 3 mm - - 0 - - - 0 . 6 mm 1 mm x - 4 3 - - 2 mm x • 0.6 mm • 1 mm x

x x 1 2 x 1

0.3 mm montecarlo 0.6 mm montecarlo mm montecarlo mm montecarlo 0.6 mm experimental mm experimental

Figure 2 Dependence of photon collection efficiency on length and cross-sectional area of YAP :Ce pillars coupled to a quarz window

o 4re

= 1 - COS0c =

(2)

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where 0c is the critical angle given by the refraction indexes at the crystal-optical photons detector interface. The formula (2) yields a maximum collection efficiency of 0.37, 0.46, 0.43, 0.41 and 0.28 for a quartz entrance window coupled to YAP :Ce, CsI(T1), CsI(Na), NaI(T1) and BGO respectively. Taking into account the absorption effect given by the lateral surfaces and the crystal bulk, the collection efficiency is even lesser and, as visible in figure 2, it decreases significantly while increasing the pillars length. This effect is mainly due to the absorption effect given by the lateral surfaces. Indeed, in spite of the high reflectance shown by the reflective multi-layer deposited onto the pillars lateral surfaces, our simulation showed that in a 2x2x5 mm 3 YAP :Ce pillar about 75 % of the scintillating photons are absorbed onto the lateral surfaces, about 25 % are detected and only about 5 % are absorbed into the crystal bulk. However, it has to be noted that the high percentage of absorbed photons is also due to the large number of photons that impinge the pillar output surface with an incidence angle greater than the critical angle (51 degrees for a YAP-quartz

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E Vittori et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 616-621

interface) so that they are reflected back, trapped into the pillar and, after many internal reflections, absorbed onto the lateral surfaces or into the bulk. Our simulation showed that the highest collection efficiency of scintillating photons occurred when the distal surface from the PMT was diffusive and the lateral surfaces of the pillars were reflective. Figure 3 displays the collection efficiency comparison between pillars having a 0.3x0.3 and 2x2 mm 2 size, with diffusive and reflective top surface. Taking into account the difference in collection efficiency relative to the two above mentioned conditions, it is clearly visible that the longer is the pillar the lesser is this difference. Moreover, the smaller is the crosssectional area the lesser is the length at which this difference disappears. 7O ~R v >_ 6 0 o

~ 50 IT 40u_ LU

z 30 0 ~ 20,

% L\.

of the high number of internal reflections on the lateral surfaces that scintillating photons undergo before arriving to the PMT interface. On the contrary, the positive effect given by the diffusive top surface is well visible with a 1 mm thick pillar (fig. 4b). As a matter of facts, with this geometry the number of photons that arrive to the PMT interface with an incidence angle lower than 40 degrees is considerably increased when using a pillar with a diffusive top surface. As regards CsI(T1) pillars with all diffusive surfaces, our simulation (with a surfaces reflectance equal to 0.95) showed a collection efficiency equal to 67 % and 25 % for lxlx3 and l x l x l 0 mm 3 dimensions pillars, respectively. These results can be well compared with the experimental ones equal to 74+7 % and 23_+2 % obtained with CsI(TI) pillars having the same dimensions. In the case of 2x2x3 mm 3 CsI(Na) pillars instead, our simulations showed a collection efficiency of about 80 %. 1

0,8

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This behaviour can be well understood by analysing the incidence angle distribution of the detected scintillating photons reported in figure 4 concerning a lxl mm 2 size YAP :Ce pillar. The diffusive top surface of the pillar randomises the trajectories of the photons promoting the direction of PMT. Nevertheless, this effect is not clearly visible with a 20 mm thick (fig. 4a) pillar because

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Figure 4 - Incidence angle distribution of the detected scintillating photons Vertical a x i s number of detected photons (arbitrary units) Horizontal axis' Incidence angle (degrees) - l x l mm2 size YAP :Ce pillar with reflective (white bars) and diffusive (gray bars) top surface" a) 20 mm thick pillar, b) 1 mm thick pillar.

E Vittori et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 616-621

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this configuration is about 68 % whereas the detection efficiency at 140 keV is about 60 %. With the assumed light output, we should expect an energy resolution increase from 20 % to 15 % FWHM when using a R3292 PSPMT by Hamamatsu, which is the optical photons detector actually used on the HIRESPET breast imager for scintimammography. However, in order to further improve energy resolution, we have planned an attentive study on the performance of CsI(Na) pillars coupled to a more accurate position sensitive optical photons detector such as the ISPA tube [13].

, THICKNESS (mm) 1

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Csl2mmx2mmmm " " Y A P : C e 2 mm x 2

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Figure 5 - Full Energy Peak efficiency at 140 keV for YAP and CsI pillar and matrices.

The last parameter we have to take into account in order to make the proper choice of the scintillating crystal and the pillars dimensions, is the Full Energy Peak (FEP) efficiency. Figure 5 shows the Monte Carlo FEP efficiency at 140 keV for YAP and CsI pillars having a 2x2 mm 2 area and a thickness which ranges between 1 and 15 mm. As a reference we see that a 2x2x3 mm 3 CsI pillar and a 2x2x10 mm 3 YAP pillar show a FEP efficiency equal to about 60 % and 48 %, respectively.

5. CONCLUSIONS We have intensively studied collection efficiency and detection efficiency of scintillating crystal multipillar arrays, with the final goal of improving the energy resolution of the HIRESPET detector dedicated to scintimammography without compromising its overall performances. According to our study, the best compromise between light output and detection efficiency is represented by a CsI(Na) matrix made up of 2x2x3 mm 3 pillars. As a matter of facts, taking into account intrinsic light yield of the crystal (85%) and collection efficiency of the pillars (80%), the expected light output for

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