The Temporal PET Camera: A New Concept With High Spatial and Timing Resolution for PET Imaging

This contribution proposes a new temporal PET camera concept yielding a precise spatio-temporal localization of a scintillation event within a monolithic scintillator. This concept is promising for PET imaging. The key idea behind this concept is the ability of the system to accurately localize the region of detected un-scattered photons on the Si-PMT detector plane. Then, by ray tracing, an accurate estimate of the depth and timing of the scintillation event is provided. An estimation of the potential performance of such a system, based on extensive Monte Carlo simulations, is also presented. Keywords: Gamma camera; Si-PMT, PET imaging; TOF; CRT; Spatial resolution; monolithic scintillators 1. Introduction The use of monolithic scintillator-based detectors is promising for designing time of flight (TOF) positron electron tomography (PET) devices. Monolithic scintillators exhibit a number of interesting properties such as excellent energy resolution, high γ photon capture efficiency and relatively simple detector assembly. This detector concept is all the more interesting with the availability of Si-PMT which offers high gain, fast response, insensitivity to magnetic fields and potential cost effectiveness in a compact package. J. Imaging 2015, 1 46 Yet the concepts so far proposed to recover the localization in space (3D position X, Y, Z) and time T0 of the scintillation event occurring in a monolithic scintillator suffer from a number of limitations, such as: • If an Anger logic is used to recover the spatial localization of the scintillation event (projection of the 3D position on the (X,Y)-plane), the crystal must be thin (around 10 mm) to have good spatial resolution. However, in that case, many events could be lost due to insufficient thickness to absorb the 511 keV γ ray. • Uncertainties on scintillation light ray tracing related to the difference of propagation speed between UV photons (n =1,9) and γ photons (n=1) do not allow very precise timing of the scintillation event (estimation of the initial time T0). • Obtaining information about the interaction depth requires either two layers of crystals or two layers of photo-detectors, which raises the cost of the system. Recently, very promising results have been obtained applying an Si-PMT array coupled to a bare 18 mm × 16 mm × 10 mm monolithic LaBr3 : 5%Ce [1]. A spatial resolution of 1.6 mm FWHM was obtained. The timing resolution was CRT = 198ps. The front side read-out necessary for this geometry was described as tricky. In this work, we propose a new concept yielding a precise localization of a scintillation event in space and time with a monolithic scintillator. This concept will be analyzed here for the case of PET (511 keV γ), but it can also be used at other energies as long as the incidence angle of the γ ray can be constrained (Lead collimators for SPECT, Compton camera, calorimeters, ....). Our objective, in this paper, is to describe the proposed concept and estimate its ultimate precision performances in an idealized case for PET applications. The paper is organized as follows. Section 2 introduces the concept of a Gamma camera and some recent works aimed at enhancing its performances. Section 3 is dedicated to the major contribution of this paper which is the new concept of temporal Gamma cameras. Synthetic simulations as well as performance evaluation of the spatio-temporal localization of the temporal Gamma camera are introduced in this section. Section 4 concludes the paper. 2. The Gamma Camera Concept In the Anger γ camera, the γ ray incidence direction is controlled through absorption on a lead collimator, so that the photons’ incidence is close to normal on the scintillator plane. Absorbtion of the γ ray creates a scintillation event. The UV scintillation light is emitted isotropically. The light is then channelled through the plane by reflection on the interfaces. The number of photons detected on the scintillation plane roughly decreases by 1/R, whereR is the distance to the projection of the scintillation point on the detector plane. This property is used to find the center of the distribution of photons and thus gives the X,Y coordinates of the scintillation event [2]. However, there are still a number of drawbacks with this method: • The photon distribution is spread out, so the images are noisy. • The reconstruction does not work well close to the edges of the plates, so it is not possible to tile many plates to make a bigger one. The crystals must be big enough, restricting the choice of available scintillators to NaI:Tl and CsI:Tl. J. Imaging 2015, 1 47 • This method has a severe limitation taking into account events which are close in space and time: the pile up effect [6]. • The scintillator energy resolution is a key item in obtaining good images with an Anger camera, so experiments have been done using LaBr3 : 5%Ce [3]. Sub-millimetric spatial resolution has been obtained on a special geometry at 140 keV. However, the conditions of the experiment were far from the actual setup in radiology. • NaI(Tl) Gamma cameras have been used in the past for PET [7,10]. However, the localization of the events was too coarse. In addition, the stopping power of NaI(Tl) was too low and the Compton effect was an issue. The concept we propose below uses a denser crystal than NaI(Tl) and should overcome most of the drawbacks of previous Gamma camera systems. It also should be significantly cheaper to produce than conventional lutetium oxyorthosilicate (LSO)-based PET. 3. Proposed Temporal Gamma Camera The availability of fast, efficient, segmented detectors like Si-PMT, fast scintillators like LaBr3:Ce and very fast mixed ASICs [5], able to give a read-out and to trigger 16 channels in less than 100 ps could open the way to using temporal information rather than photon counts to reconstruct the scintillation event. But is there any temporal information that could be used for image reconstruction? We consider a monolithic scintillator crystal of thickness h, with a similar design to a state-of-the-art Gamma camera. The upper surface would be roughened and covered by a white wrapping so as to ensure a diffusive surface. The lower surface would be polished with a layer of segmented Si-PMT glued to the crystal. In this paper, we will not consider the impact on resolution of Si-PMT segmentation pitch. No correction will be done for the photon detection efficiency of the Si-PMT. Once a photoelectric event E(X, Y, Z, T ) takes place inside the crystal at position (X, Y, Z) and time T , the scintillation light is emitted isotropically following the scintillator emission light curve. Below, for the sake of clarity, we will discuss only the case of a pure photo-electric event. A future paper will discuss the impact of the Compton effect, but as forward scattering is dominant at 511 keV, Compton diffusion does not significantly affect the result. The photons directed towards the base of the crystal (see Figure 1) are not scattered and are detected directly. Their path is a straight line to the detector. All the other photons will be subjected to at least one scatter and thus have a longer light path in the crystal, subsequently impacting the photo-detectors. Between the crystal and the detector, an optical interface with an index step is deployed. Thus, there is a critical angle θc above which the photon will be reflected and thus not detected. The unscattered photons will thus be located in a cone whereby the summit is the event position and the opening angle is θc. If nc is the crystal index and ng is the index of the glue between the crystal and the Si-PMT, the critical angle is θc = arcsin(ng/nc). Hence the image of the un-scattered photons on the detector plane will be a disc centered on the location of the interaction (X, Y ) and whose diameter will be (L−Z)∗ tan(θc). We can label those un-scattered photons by order of detection on the detector plane, where Pn will denote the n photon detected with spatio-temporal coordinates on the detector plane (xn, yn, tn). Figure 1 illustrates the behavior of emitted photons after the scintillation event: (a) photons emitted inside the cone are detected directly by the photodetector, (b) photons emitted outside the cone in the downside J. Imaging 2015, 1 48 direction are reflected before being scattered by the upper plane of the crystal, and (c) photons emitted in the upper direction will be scattered in a random direction. Scattering of scintillation photons could also occur in some inclusions inside the crystal. This would create secondary centers of emission in the crystal, mimicking a Compton event. However, both CeBr3 and LYSO are available on the market and have the advantageous property of a very low concentration of scattering imperfections (inclusion is of a size less than 1 mm with a density less than 0.1 cm−3). Hence, the number of scattered photons, after the first scintillation event, will be below the detection limits in actual systems. Version July 7, 2015 submitted to Sensors 5 of 17 c Diffusing surface