Application of WLS strips for position determination in strip PET tomograph based on plastic scintillators

A method of determination of a gamma quantum absorption point in a plastic scintillator block using a matrix of wavelength-shifting (WLS) strips is proposed. Application of this method for improvement of position resolution in newly proposed PET detectors based on plastic scintillators is presented. The method enables to reduce parallax errors in reconstruction of images which occurs in the presently used Positron Emission Tomography scanners. Section 1: Introduction Plastic scintillators are characterized by relatively short light pulses with decay time on the order of a nanosecond and, therefore, they are widely used in nuclear and particle physics experiments for fast timing measurements. Typically, they have a form of a strip with rectangular cross-section and are read out at both ends by photomultipliers (see e.g. Ref.[1]). Also other solutions are used such as for example scintillator plates read out by arrays of photomultipliers [2,3]. The high timing resolution offered by the plastic scintillators is exploited in a newly invented type of positron emission tomograph (PET) using such scintillators for detection of the 511 keV gamma quanta originating from positron annihilation. Two alternative solutions of the tomograph were proposed [4]. One solution, referred to as the strip PET [5], contains scintillator strips read out by pairs of photomultipliers and arranged around a cylindrical surface forming a tomograph tunnel. Position of the gamma quantum interaction point in the strip further on we call it shortly "the interaction point" is determined on the basis of a time difference in propagation of light pulses registered by the pair of photomultipliers. The second solution, referred to as the matrix PET [6], uses plastic scintillator plates read out by arrays of photomultipliers. Registered amplitude and time of propagation of light pulses allow for localization of the interaction point in the plate. A key feature of both solutions is a high precision in measurement of difference in a time-of-flight (TOF) of the annihilation quanta allowing for determination of position of the positron annihilation along a line of response (LOR). This feature allows for substantial suppression of background in the reconstructed PET images and is one of the main advantages of the plastic scintillators compared to essentially slower inorganic crystals which are used in the contemporary commercial PET scanners. A disadvantage of the plastic scintillators compared with the inorganic crystals is substantially lower detection efficiency for the gamma quanta. It can be compensated by increasing a length and a thickness of the scintillator segments. However, it leads to worsening of the timing resolution and thus also of the position resolution. Even if a very high precision for measurement of the time difference of 100 ps (FWHM) is assumed, the resulting position resolution is only moderate and equals 7.5 mm (FWHM). This estimation was done taking into account that a speed of propagation of light in a scintillator strip is roughly two times smaller than in vacuum. We propose to improve the position resolution by additional detection of scintillation light escaping the scintillator segments with matrixes of WLS strips. Read out of plastic scintillators by means of WLS elements is a well established techniques which is applied in many particle physics detectors [7]. Usage of the WLS strips was also proposed for read out of arrays of inorganic crystals in PET detectors [8]. Section 2: Position determination by means of WLS strips For determination of a position of the interaction point in a plastic scintillator we propose to use a set of parallel WLS strips which register scintillation photons escaping the scintillator. This concept is illustrated in Fig. 1 showing a side view of a scintillator strip and a set of parallel WLS strips placed above the scintillator. In the figure, we introduced y-z coordinate system with the origin (y=z=0) located in a geometrical center of the scintillator strip and the z-axis oriented along the strip. Fig. 1. Schematic drawing illustrating application of WLS strips for determination of coordinates of the interaction point in a scintillator strip. For illustration of propagation of the scintillation photons in the strip, indicated are trajectories of photons emitted from the geometrical center at every 10o with respect to the y-axis. For emission angles larger than the critical angle, which for plastic scintillator material with refractive index n=1.58 equals to 39.2o, the emitted photons undergo total internal reflections from the walls of the scintillator strip and propagate towards the photomultipliers (PMs). Their trajectories are indicated with thin, black lines. For emission angles smaller than the critical angle, scintillation photons can escape the scintillator strip through a side wall and are absorbed in the WLS strips. Trajectories of such photons are indicated as thick, blue lines and the WLS strips which absorb these photons are marked with yellow color. Applied WLS material is selected in such a way, that it absorbs photons with wavelength range characteristic for emission spectrum of the scintillator (see Fig.2). Secondary photons emitted isotropically by the WLS strips propagate towards photomultipliers attached at their both ends. The isotropic emission of photons with the wavelength corresponding to small absorption in the WLS constitutes the crucial feature of the presented solution, and it causes that some of the secondary photons are trapped in the WLS fiber and propagates via internal reflections