Figure 5: Reflectance versus layers, density and power of microcracks

Figure 9: Image of acquisition and profile of pixels of the actual detector using an engraved crystal- same profile acquired from simulation

Figure 8: : Image of acquisition and profile of pixels of the actual detector using a 13x13 pixelated array of LYSO crystals- same profile acquired from simulation

Figure 7: Examples of histogram fitting

Simulations, testing and results for the pixelation of LYSO crystals for gamma detectors using the sub-surface laser engraving techniques

Konstantinou G.1, Chil R.1, Ripoll J.1, Udias J.M.3, Desco M.1,2 and Vaquero J.J.1,2

1 Departamento de Bioingeniería e Ingeniería Aeroespacial, Universidad Carlos III de Madrid. Spain2 Instituto de Investigación Sanitaria Gregorio Marañón, Madrid. Spain

3 Universidad Complutense de Madrid, CEI Moncloa, Madrid. Spain

• One of the most common topologies for the scintillator crystals used in gamma radiation detectors for PET scanners is pixelation. The size of these pixels, the crystal surface treatment and the reflector thickness inserted between crystals directly affects the energy and the spatial resolution as well as the sensitivity of the scanner.

• Since the fabrication of pixels is laborious, complex and expensive. we focused our effort in researching the feasibility of fabricating pixels within monolithic LYSO scintillator crystals using the proven technique of sub-surface laser engraving (SSLE) with a Nd:YAG laser.

• To evaluate possible designs considering the limitations of the engraving procedure, we used the GEANT4 based GAMOS macro environment to analyze the light propagation inside the engraved crystals, using information about the relative reflective quality and optical behavior of the engraved surfaces.

Email: gkonstantinou@hggm.es Web: http://image.hggm.es

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Introduction

Results-Analysis

Discussion

GAMOS Simulation The possibilities of surface configuration are endless, while the outcome of each configuration can’t be easily calculated with analytical methods. To avoid extensive engraving efforts that would yield no results, a simulation platform has been developed using the GEANT4 based GAMOS macro interface. In this way, the results from the simulations can be used to select the best candidates for engraving.

Materials-Methods

Figure 6: Engraved surfaces for different combinations of energy and density.

Table 1: Calculated coefficients according to the UNIFIED model

CaseDiffuse

lobe Specular lobeSpecular

Spike Total Reflection

Lumirror 0.999 3E-6 1E-11 0.99

Engraving 1 0.688 0.295 0.017 0.54

Engraving 2 0.761 0.198 0.041 0.42

Engraving 3 0.725 0.248 0.027 0.35

Engraving 4 0.677 0.301 0.022 0.40

Figure 2: SPIM setting for the measurements of surface characteristics

Figure 1: SSLE engraving procedure using a 532 nm Nd:YAG laser

Focusing unit xyz

laser

Motor xy

Motor z

• The SSLE reflective patterns collimate light in a similar manner as the reference reflector, although up to now the best reflectivity renders only a 60% of the total reflection of the reference

• Since the transparency of the reflective surfaces can be controlled, we can compensate for the light sharing needed for Anger logic readouts integrating light-guides in the scintillation crystal block.

Figure 4: a) Model used for the GAMOS simulation; b) Pixelated array; and c) engraved crystal

SSLE system We used a commercial SSLE Nd:YAG laser with pulses of 6.8 to 7.2 ns and 1.2 to 2 W. Since the configuration of the engraved grid designed plays a crucial role in the overall reflectivity of the resulting surfaces, we need to characterize the accuracy and repeatability of the engraving system. Initial tests were done on standard K9 glass blocks before using LYSO crystals.

Image analysis algorithm The histograms retrieved from the images present the expected predominant characteristics. The total reflectance histogram can be successfully modeled using three Gaussian functions (figure 7). These correspond to the possibility of reflection according to the UNIFIED model, allowing us to calculate the required coefficients (table 1). The total reflectivity is normalized to the Lumirror reference.

Reflectance measurement resultsFigure 5 depicts the optical behavior of different grids of microcracks, tuning pulse power, distance between cracks and number of layers. Figure 6 shows microscopy images of the different surfaces illuminated with a blue incoherent light.

Measurements with microscopes To measure the resulting surfaces, we illuminate the surfaces with either a 405 or a 480nm laser (close to the wavelength of the light emitted by the LYSO crystal), and record the reflected light using a high resolution scientific CMOS camera. A white reflector strip (Lumirror) was used as the normalization reference. A second setting using incoherent illumination of the expected wavelength range was also tried and provided similar results, thus was preferred for the bulk of our measurements.

Surface Model used for simulations The characterization of the optical properties and reflectance of the engraved surfaces followed the UNIFIED model proposed by Nayar and al. In this the surface is comprised of microfacets, with orientation distributed around the average with a standard deviation of σα, while a number of factors describe a statistical model, dictating the possibility a photon has to be reflected or not, in a lambertian or specular way, perfectly or imperfectly.

A

B C

Increasing energy

Incr

easi

ng d

ensi

ty

Conclusions, Future plans• The SSLE technique can be used to create reflective patterns inside monolithic scintillator crystals. • Special care has been taken to avoid destruction of the crystal due to heat deposition during the engraving process; in our case we timed the engraving sequence in such a way

that we allowed heat dissipation, but this introduces the penalty of a longer engraving process.• DOI patterns can be created either by manipulating the transparency of the surfaces layering different topologies of microcracks or by introducing distinct geometries.• Such modifications of the engraving pattern take place at practically no cost and constitute a highly promising solution for industrial fabrication, reducing substantially the

cost of the detector.• The simulations will be improved by adding the electronic stage and its noise and inhomogeneities, as well as imperfections and edge efects in the optical components.

Figure 3: Components of the UNFIED model for the reflectance according to Nayar and al.

Comparison of simulations and readoutIn order to evaluate the parameters used on our simulation we used results from our detectors and compared to the simulation results. In both cases (acquisition and simulations) the number of counts is low. The simulation doesn’t include the electronics behavior. Figure 8 is for the original pixelated array, and figure 9 is a single layer engraved crystal (images are zoomed).

This work is supported by the projects TEC2011-28972-C02-01, PI11/00616 from the Spanish Ministerio de Ciencia e Innovación, P20137MIT-3024 TOPUS from Comunidad de Madrid, INFIERI 2012-FP7-ITN-317446 from EU. The authors acknowledge the support for the simulations of Pedro Arce from CIEMAT, Madrid.