Two-crossed-polarizers based optical property modulation method for ionizing radiation detection for positron emission tomography

Abstract

Recent work shows that Pockels effect and optics pump-probe measurement could be utilized as a novel method for 511 keV ionizing radiation photon detection for positron emission tomography (PET) which could potentially overcome the inherent physical limitation for coincidence time resolution of around 100 ps (Tao et al 2016 Phys. Med. Biol. 61 7600-22). In this paper, we embrace this observation and introduce a two-crossed-polarizers based setup to achieve similar detection concept, which is a simpler and more compact setup with comparable ionizing radiation detection capability as the setup used in the previously proposed work. We evaluated the performance of our experimental setup with Lithium Niobate (LiNbO 3 ) and Cadmium Telluride (CdTe) detector crystals, and the desired properties of an ideal detector crystal were discussed. The modulation signal induced by 511 keV photons in both LiNbO 3 and CdTe can be detected with repeatable signal amplitude using two-crossed-polarizers based method, while CdTe could provide eight times higher detection sensitivity to 511 keV photons than LiNbO 3 under the same bias voltage, suggesting high effective Z number and high density properties of CdTe, as well as a shorter carrier lifetime and lower carrier mobility of LiNbO 3 . In addition, the strength of modulation signal increased linearly with bias voltage before saturation. The modulation signal strength in LiNbO 3 continued to increase after 2000 V due to its high resistivity which could reduce the dark current in the detector, while the modulation signal of CdTe with low resistivity tended to be saturated at a bias voltage higher than 1200 V. Therefore, further increasing the bias voltage for detector crystals (especially for LiNbO 3 ) may enhance the modulation strength and improve the detection sensitivity for annihilation photons.

State-of-the-art challenges and emerging technologies in radiation detection for nuclear medicine imaging: A review

“…There is an intrinsic limitation of about 10 ps time resolution for scintillation due to the statistical fluctuations in the generation of scintillation photons [95,96]. One proposed method to improve the time resolution of PET systems is via optical property modulation.…”

“…The experimental set up for utilizing the Pockels effect involves a polarized probe laser pumped through a crystal with an applied bias to another polarizer. The light is then focused onto a photodiode via a lens [96,97]. The refractive index of a Pockels cell is found from Eq.…”

“…where n(E) is the refractive index of the Pockels cell, E is the applied electric field, n 0 is the refractive index without applying an electric field, and γ is the Pockels effect coefficient [96,97]. In addition to the modification of the refractive index, the transmitted light intensity is also dependent on the applied electric field.…”

“…The photodiode can then measure the change in transmitted light to measure the modulation of the optical properties of the detector crystal [95][96][97]. This method has been completed with various materials, such as cadmium telluride (CdTe) [96][97][98], bismuth silicate [98], yttrium aluminum garnet (YAG) [99], lead bismuth gallium (PbBiGa) [98], and lithium niobate (LiNbO 3 ) [96]. Empirical analysis has predicted improvement in the timing resolution on the order of 1 ps [99].…”

Optical Simulation and Experimental Assessment with Time–Walk Correction of TOF–PET Detectors with Multi-Ended Readouts

“…The coincidence timing resolution (CTR) of the PET detector is influenced by various factors such as scintillator properties, structure, size, surface roughness, rise time, decay time, photosensor device, readout circuit, and threshold voltage [ 8 , 9 , 10 , 11 , 12 , 13 ]. Much research has been carried out on detector structure, timing measurement, and compensation algorithms [ 11 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. J.…”