A novel method for the line-of-response and time-of-flight reconstruction in TOF-PET detectors based on a library of synchronized model signals
Introduction
Positron Emission Tomography (PET) is at present one of the most technologically advanced diagnostics methods that allows for non-invasive imaging of physiological processes occurring in a patient׳s body. In the PET tomography the information about the distribution of annihilation points, and hence about the density distribution of the administered radiopharmaceuticals inside the patient׳s body, is carried out by pairs of gamma quanta which are registered in detectors surrounding the patient. All commercial PET devices use inorganic scintillator materials as radiation detectors – usually these are the LBS (BGO) (GE Healthcare), LSO (Siemens) or LYSO (Philips) crystals [1], [2], [3], [4]. Determination of the interaction point of gamma quanta in PET detectors is based on the measurement of charge of signals generated by photomultipliers or avalanche photodiodes (APD) connected optically to inorganic crystal blocks cut into array of smaller elements. The spatial resolution achievable with this method is equal approximately to the dimensions of the small elements of the crystal block. Determination of interaction points for both annihilation quanta enables reconstruction of the line-of-response (LOR). In turn, the measurement of the difference between the arrival times of gamma quanta to the detectors, referred to as time-of-flight (TOF) difference, allows us to calculate position of the annihilation point along the LOR. The TOF resolution of about of 400 ps achievable with LSO crystals [5], allows for a substantial improvement of a signal to noise ratio in the reconstruction of PET images [1], [3], [6].
Although detectors used in Positron Emission Tomography are presently at the highly advanced stage of development there is still a large room for improvement, and there is an ongoing research especially aiming at (i) refinement of time resolution by search and adaptation of new inorganic crystals [7], [8], [9], [10], [11], (ii) reduction of parallax errors due to the unknown depth of interaction (DOI) e.g. by application of new geometrical configurations of crystals and APD and photomultipliers [12], [13], [14], [15], [16], (iii) finding cost-effective solutions which would allow for construction of large detectors enabling single-bed whole-body PET imaging as e.g. straw tubes drift chambers [17], [18] or large area resistive plate chambers (RPC) [19], [20], and (iv) adaptation of PET detectors for their simultaneous usage together with MRI and CT modalities [12], [21], [22], [23], [24], [25], [26].
Recently a new concept of large acceptance Jagiellonian PET (J-PET) system (see Fig. 1) based on strips of polymer scintillators arranged in a large acceptance detectors was proposed [27], [28], [29], [30], [31]. The J-PET detector allows to solve the challenges discussed above in an utterly new way. It offers improvement of TOF resolution due to the usage of fast plastic scintillators, it enables a fusion with MRI and CT modalities in a way allowing for simultaneous morphological and functional imaging [32], [33], it permits to determine the depth of interaction [34], and constitutes a promising solution for single-bed whole-body PET imaging. At present it is however in its early stage of development and requires elaborations of new hit-position [35] and TOF reconstruction methods which would allow to make use of the potential it offers. This paper is devoted to the presentation of a reconstruction method that allows to exploit the advantages of the J-PET detector but it may also be applied to other types of scintillator detectors.
In scintillator detectors, amplitude and shape of signals change strongly with distance of the hit position to the converter, leading to a deterioration of the spatial and time resolution. The proposed method of position and time reconstruction turns this disadvantage into an advantage, and makes use of the signal shape variation in hit position reconstruction. The method is based on determination of the degree of similarity between measured signals and standard signals stored in the data base and on a novel concept of signals׳ synchronization.
In the following, for the sake of completeness, the J-PET concept is briefly described. Next, in order to facilitate a clear explanation of the reconstruction method we introduce a way of representing signals and describe an example of the creation of the library of model signals. Further on we describe the invented method of signals׳ synchronization, which is crucial for the reconstruction of LOR and TOF. Finally, the experimental results are presented in the last section of this paper.
Section snippets
The J-PET detector system
The J-PET test chamber is built out of strips of organic scintillator, forming a cylinder. One of the possible arrangements of strips is visualized schematically in Fig. 1. Light signals from each strip are converted to electrical signals by two photomultipliers placed at opposite ends of the strip. The position and time of reaction of gamma quanta in the detector material can be determined based on the time of arrival of light signals to the ends of the scintillator strips. In Ref. [30] we
Signal representation
In the current TOF-PET detectors the reconstruction of line-of-response and of TOF values is based on the charge and time distributions measured for each annihilation event without referring to the external sets of model signals. In this paper we present a novel method for the reconstruction of the interaction point in PET detectors and for the reconstruction of time differences between the arrival of the annihilation quanta to PET detectors.
The description of the proposed method is based on
Generation of the library of model signals
The reconstruction method described in this paper requires generation of a data base of synchronized model signals for various interaction points. The library of model signals is generated by scanning the scintillator strip with a collimated beam of annihilation quanta with profile smaller than the spatial resolution required for the hit position reconstruction. For example, a beam with the profile width of FWHM equal to 1 mm can be used. Scanning may be performed using a source of annihilation
Method of the model signals synchronization
A raw data base determined as described in the previous section constitutes a set of points P in a space Ωm. Coordinates of points P correspond to moments of time in which signals pass the discriminator thresholds with respect to the time of the triggering signal. Few exemplary signals from the raw data base are shown in Fig. 2B. The signals may be synchronized with respect to the signal of the reference detector, which allows for the precise determination of the calibration offsets (tdelay),
The reconstruction method of LOR and TOF
The reconstruction of the time and position of the interaction of gamma quanta in the detector is based on comparison of measurement signals for a given event with synchronized model signals stored in the library. The hit position is reconstructed as this which corresponds to the signal from the library which is most similar to the measurement signal, and hit time of the interaction is reconstructed as a relative time between the measured signal and the most similar one in the library. A degree
Double-strip J-PET prototype
The J-PET detector system shown in Fig. 1 is axially symmetric and its performance may be tested using a double strip prototype which allows for simultaneous registration of two annihilation quanta and reconstruction of both LOR and TOF. Therefore, the functioning of the J-PET detector and validation of the reconstruction method proposed in this paper was verified using the double strip prototype outlined in Fig. 5. The prototype is built out from BC-420 scintillator strips [40] with dimensions
Summary
A method enabling reconstruction of hit time and hit position of gamma quanta in scintillator detectors was described and validated based on the experimental data collected with the double-module prototype of the J-PET detector. The method is based on a comparison of measured signal probed in the voltage or time domains with synchronized model signals from the library. The hit time and hit position are reconstructed as these correspond to the signal from the library which is most similar to the
Acknowledgments
We acknowledge technical and administrative support by T. Gucwa-Ryś, A. Heczko, M. Kajetanowicz, G. Konopka-Cupiał, W. Migdał, K. Wołek and the financial support by the Polish National Center for Development and Research through Grant no. INNOTECH-K1/IN1/64/159174/NCBR/12, the Foundation for Polish Science through MPD program, the EU, MSHE Grant no. POIG.02.03.00-161 00-013/09, and Doctus – the Lesser Poland PhD Scholarship Fund.
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