Abstract
In the past years, the gamma-ray detector designs based on the monolithic crystals have demonstrated to be excellent candidates for the design of high-performance PET systems. The monolithic crystals allow to achieve the intrinsic detector resolutions well below state-of-the-art; to increase packing fraction thus, increasing the system sensitivity; and to improve lesion detectability at the edges of the scanner field of view (FOV) because of their intrinsic depth of interaction (DOI) capabilities. The bottleneck to translate to the clinical PET systems based on a large number of monolithic detectors is eventually the requirement of mechanically complex and time-consuming calibration processes. To mitigate this drawback, several methods have been already proposed, such as using non-physically collimated radioactive sources or implementing the neuronal networks (NN) algorithms trained with simulated data. In this work, we aimed to simplify and fasten a calibration process of the monolithic based systems. The Normal procedure consists of individually acquiring a 11 × 11 22Na source array for all the detectors composing the PET system and obtaining the calibration map for each module using a method based on the Voronoi diagrams. Two reducing time methodologies are presented: (i) TEST1, where the calibration map of one detector is estimated and shared among all others, and (ii) TEST2, where the calibration map is slightly modified for each module as a function of their detector uniformity map. The experimental data from a dedicated prostate PET system was used to compare the standard calibration procedure with both the proposed methods. A greater similarity was exhibited between the TEST2 methodology and the Normal procedure; obtaining spatial resolution variances within 0.1 mm error bars and count rate deviations as small as 0.2%. Moreover, the negligible reconstructed image differences (13% deviation at most in the contrast-to-noise ratio) and almost identical contrast values were reported. Therefore, this proposed method allows us to calibrate the PET systems based on the monolithic crystals reducing the calibration time by approximately 80% compared with the Normal procedure.
Highlights
In the PET detectors, two main types of scintillator crystals are usually employed namely, pixelated and monolithic
We propose an approach to apply the detector calibration process based on the Voronoi diagrams [10] in the PET scanners based on a large number of monolithic detectors
The detector output signals are fed into a data acquisition (DAQ) system based on the 12-bit analog-to-digital converters (ADCs) with 1 GB ethernet connection, and the summed signal of either all silicon photomultipliers (SiPMs) rows or columns, was fed into a trigger board that allows coincidences within a 5 ns coincidence window
Summary
In the PET detectors, two main types of scintillator crystals are usually employed namely, pixelated and monolithic. The advantages and disadvantages of each one are extensively described elsewhere [1] They offer intrinsic resolutions that are well below the state-of-the-art and an improvement of the system sensitivity, as they do not contain zero detection zones, unlike the pixelated crystals. The DOI information permits to correct for the parallax errors, which strongly affect the systems with small apertures (i.e., small animal and organ dedicated scanners), and at the edges of the field of view (FOV) in the human size scanners. Both width and position of the source profile improve when applying the DOI correction independently of the system diameter [4, 5]. Regarding cost, analyzing the different providers for scintillator crystals and studying the price differences between the several pixel arrays and monolithic crystals with similar volumes, it can be concluded that they are cheaper than the traditional pixelated scintillators for the pixel sizes smaller than 1.5 mm × 1.5 mm, as the ones used in the pre-clinical PET imaging
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