It has been shown that the very high spatial resolution and simpler manufacturing (no need for photodiode) give certain advantages for photoconductor-based imagers. The a-Se had been a well-studied material for a long time and became the first photoconductor material used commercially for X-ray imaging. Today, several a-Se-based imagers exist on the market, but their main application is mammography. The major drawbacks are the low X-ray conversion efficiency and the very high electrical bias requirements.
Single-crystalline CdTe and CZT are excellent X-ray detectors with energy resolution, but at present, their homogeneous deposition in polycrystalline forms on large areas is difficult, and they cannot be directly deposited onto TFT arrays because of their high substrate temperature requirements. Deposition onto a separate substrate and then bump bonding to the TFT array has not been very successful at large sizes. Pixellated single crystalline detectors are getting cheaper, but their size is limited by the crystal size as currently configured. Tiling is possible, but at present, this technology is too expensive for large-area X-ray imaging.
PbI2 and HgI2 are newer materials that do not have as long an R & D history nor as extensive a learning curve development as a – Se. Both have high X-ray sensitivity and excellent resolution. At present, the polycrystalline PbI2 has longer lag, but it can be significantly decreased by optimizing the deposition parameters as shown in Figure 25.11. Another issue is that it is difficult to deposit a thick – layer PbI2 of good quality because, as the layer grows thicker, the morphology becomes porous. At present, mammography seems to be the best application area for PbI2.
PVD HgI2 has already proven many good properties including low lag and high DQE. However, it still has a problem with the pixel-.o-pixel homogeneity and reproducibility. If perfected, this material could be used for all X-ray imaging modalities. PIB HgI2 has further advantages that it can be easily deposited onto large areas and in thick layers. This gives opportunities to use it as a highly efficient imager in radiography and radiotherapy as well as in industrial imaging. The only drawback is the relative high image lag, which limits its application in high frame rate (fluoroscopy, CT, and tomosynthesis) imaging.
New directions are X-ray imagers on flexible substrates and using polycrystalline Si and organic semiconductors for the readout instead of a – Si, but these technologies are still in their infancy.