ABSTRACT
The most heavily investigated detector concept for medical photon-counting X-ray imaging, in particular photon-counting X-ray computed tomography (CT), is based on the semiconductors CdTe and CdZnTe (CZT). Alternatively, a detector design based on Si is under investigation. Both approaches have advantages and disadvantages, which leaves room for the development of other types of detector. In this chapter, we investigate the feasibility of scintillation detectors based on silicon photomultipliers (SiPMs) for this application.
We first summarize the potential clinical advantages of energy-resolving photon-counting X-ray detectors, the associated challenges, and the working principles and drawbacks of detectors based on CdTe/CZT and Si. We then introduce the concept of SiPM-based scintillation detectors and discuss the scintillator and SiPM properties that determine two fundamental performance metrics, viz. the rate capability and the energy resolution. We have previously developed a pulse shape model that takes all of these properties into account. We use the model to estimate the expected detector performance as a function of pixel size, for LaBr3:Ce and YAP:Ce scintillators coupled to currently available SiPM technology and to a more advanced SiPM prototype.
The model calculations show that the short decay time constant of the scintillators, combined with the reduced recharge time constant of the new SiPM prototype, results in pulses short enough that a pixel size of 400 µm (LaBr3:Ce) or 333 µm (YAP:Ce) yields comparable rate capability as a CdTe/CZT detector with a pixel size of 500 µm. The high light yield and nearly proportional response of the investigated scintillators results in good energy resolution, provided that the SiPM does not suffer from excessive saturation when the pixel size is reduced to such small dimensions. We show that the level of saturation remains limited with the new SiPM prototype and that energy resolutions of 12% (LaBr3:Ce) and 15% (YAP:Ce) at 60 keV may be achieved.
Our work indicates that it may be feasible to develop SiPM-based scintillation detectors for medical photon-counting X-ray imaging with comparable rate capability and energy resolution as CdTe/CZT detectors. Remaining challenges include a dose-efficient optical isolation between pixels. However, the fact that the detector is based on light transport rather than charge transport means that issues typically associated with CdTe/CZT detectors, such as charge sharing and polarization, do not play a role.
