Speaker
Description
Introduction
Indium Gallium Zinc Oxide (IGZO) thin-film transistor (TFT) flat panel detectors (FPDs) have the potential to overcome limitation of a-Si:H TFT detectors by avoiding the cost increase of CMOS technology [1-3]. Main advantages of IGZO-TFT FPDs when compared to a-Si:H TFT FPDs reside in the higher electrons (≥ 10cm2∕Vs vs. ≤ 1cm2∕Vs) and holes (~0.1 cm2∕Vs vs. ~0.0005 cm2∕Vs) mobility, higher pixel infill factor, smaller pixel size and improved noise and spatial resolutions [2]. These characteristics determine improvements in image quality, especially in those applications requiring low exposure dose, high spatial resolution and high frame rate. On the other hand, the technology for manufacturing IGZO-TFT FPDs is quite mature and allows to reduce the detector cost up to 4 times lower than CMOS FPDs one.
The use of IGZO-TFT FPDs is spreading in x-ray medical applications, but there is still lack of literature related to characterization at exposure technique factors typical in cone-beam computed tomography (CBCT) scanners. The aim of this work is to characterize the AXIOS-3030 IGZO-TFT FPD produced by Teledyne Dalsa (Teledyne Technology Inc, CA – USA) for spectra and beam intensity of common use in CBCT for image-guided radiotherapy treatments.
Material and methods
The characterized AXIOS-3030 IGZO-TFT FPD consists of an active area of 299 mm × 299 mm with native pixel pitch of 146 µm, that can operate either in 1×1 or 2×2 binning mode with maximum frame rate in continuous mode of 44 fps and 88 fps, respectively. It is coupled with a CsI scintillator layer and can operate in two gain modes: a low gain mode with a saturation dose of 55 µGy and a high gain one with a saturation dose of 3 µGy (RQA5 beam quality).
The detector was characterized for spectra used in the CBCT mounted on the TrueBeam Radiotherapy system (Varian Medical System Inc) at three different tube voltages: 80 kV, 100 kV and 120 kV. The Ti flat-filter was used (no-bow tie) with resulting beam half value layers (HVL) of 3.034 mmAl, 3.804 mmAl and 4.562 mmAl measured at the central axis of the emitted cone beam. The detector was placed with the upper surface at 93 cm from the focal spot and 15 cm spacers were adopted for reducing the impact of the backscatter from the patient support. It was operated in low gain mode both for 1×1 and 2×2 pixel binning and an acquisition period of 50 ms was set. Characterizations included the evaluation of the detector response curve (i.e. the pixel value as function of the incident air kerma at the detector surface), the noise, evaluated as the pixel standard deviation by varying of the expected pixel value, the modulation transfer function (MTF) and the detector lag. For the detector-response curve evaluation, the incident air kerma at the detector surface was measured with a solid-state multiparameter back-shielded sensor (AGMS-D+) connected to an Accu-gold+ digitizer module (Radcal Corp). Average pixel size and standard deviation were evaluated over a region of interest (ROI) of 156 × 156 pixels for binning 1×1, and 88 × 88 pixel for binning 2 × 2. The same ROIs have been used for the evaluation of the detector lag, investigated using the behavior of the average pixel value for continuous detector irradiation. The presampled detector MTF was measured by using a 25 µm slit test object, by sampling profiles both in vertical and horizontal directions.
Results
Figure 1 shows the detector response curve for 1 × 1 (fig. 1a) and 2 × 2 (fig. 1b) pixel binning. In both cases and for the used spectra, the detector saturated at about 45 µGy. Red lines represent linear fits of the curves, obtained with the exclusion of measurements beyond the saturation points. The R2 fitting coefficients are > 0.9995 in all cases, demonstrating a linear behavior of the detector. The pixel standard deviation as function of the incident air kerma and for 1 × 1 pixel binning is reported in fig. 2.
The study, conducted at 10 mA, showed that the pixel value reaches a plateau after a few seconds of continuous exposition, with an increase lower than 1% (fig. 3).
The MTF curves reached 10% value (MTF10%) for spatial frequency of 3.1 mm-1 for 1 × 1 pixel binning (fig. 4a) and of 2.3 mm-1 for 2 × 2 pixel binning (fig. 4b). No impact of the spectra on the detector spatial resolution can be outlined by the MTF analysis, nor on the direction of the evaluation.
Conclusions
We characterized an IGZO TFT flat panel detector for spectra and operation mode typically adopted in CBCT for image guided radiotherapy. The detector showed a linear response curve, with a saturation level at about 45 µGy of incident air kerma. The detector presented a signal lag that increases the pixel value, which growths up to a plateau of no more than 1% after 3 s of continuous exposition. The spatial resolution of the detector, evaluated as the MTF10%, resulted 3.1 mm-1 for 1 × 1 pixel binning and of 2.3 mm-1 for 2 × 2 pixel binning.
References
[1] Zhao C and Kanickiv J 2014 Med Phys 41.9: 091902. DOI: 10.1118/1.4892382
[2] Sheth NM et al 2022 Medi Phys 49(5):3053-3066. DOI: 10.1002/mp.15605
[3] Oh S et al 2023 JINST 18(10), C10016. DOI: 10.1088/1748-0221/18/10/C10016
| Workshop topics | Detector systems |
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