Silicon photomultiplier-based scintillation detectors for photon-counting CT: A feasibility study

Framework for evaluating photon-counting detectors under pile-up conditions

Purpose

While X-ray photon-counting detectors (PCDs) promise to revolutionize medical imaging, theoretical frameworks to evaluate them are commonly limited to incident fluence rates sufficiently low that the detector response can be considered linear. However, typical clinical operating conditions lead to a significant level of pile-up, invalidating this assumption of a linear response. Here, we present a framework that aims to evaluate PCDs, taking into account their non-linear behavior.

Approach

We employ small-signal analysis to study the behavior of PCDs under pile-up conditions. The response is approximated as linear around a given operating point, determined by the incident spectrum and fluence rate. The detector response is subsequently described by the proposed perturbation point spread function (pPSF). We demonstrate this approach using Monte-Carlo simulations of idealized direct- and indirect-conversion PCDs.

Results

The pPSFs of two PCDs are calculated. It is then shown how the pPSF allows to determine the sensitivity of the detector signal to an arbitrary lesion. This example illustrates the detrimental influence of pile-up, which may cause non-intuitive effects such as contrast/contrast-to-noise ratio inversion or cancellation between/within energy bins.

Conclusions

The proposed framework permits quantifying the spectral and spatial performance of PCDs under clinically realistic conditions at a given operating point. The presented example illustrates why PCDs should not be analyzed assuming that they are linear systems. The framework can, for example, be used to guide the development of PCDs and PCD-based systems. Furthermore, it can be applied to adapt commonly used measures, such as the modulation transfer function, to non-linear PCDs.

Time-of-flight scatter rejection in x-ray radiography

Objective.Time-of-flight (TOF) scatter rejection allows for identifying and discarding scattered photons without the use of an anti-scatter grid (ASG). Although TOF scatter rejection was initially presented for cone-beam computed tomography, we propose, herein, to extend this approach to x-ray radiography. This work aims to evaluate with simulations if TOF scatter rejection can outperform ASGs for radiography.Approach.GATE was used to simulate the radiography of a head and a torso and a water cylinder with bone inserts in a system with total timing jitters from 0 ps up to 500 ps full-width-at-half-maximum. The transmission factor of TOF scatter rejection for primary and scattered photons was evaluated as if it were a virtual ASG.Main results.With a total timing jitter of 50 ps, TOF scatter rejection can reach a selectivity of 4.93 with a primary photons transmission of 99%. Reducing the timing jitter close to 0 ps increases the selectivity up to 15.85 for a head and torso radiography, outperforming typical ASGs which usually have a selectivity from 2.5 to 10 with a primary photons transmission from 50% to 70%.Significance.This suggests that TOF scatter rejection may be suitable to replace ASGs in applications requiring lower radiation exposure if sufficiently low timing jitter is achieved.

The Nanoplasmonic Purcell Effect in Ultrafast and High-Light-Yield Perovskite Scintillators

The development of X-ray scintillators with ultrahigh light yields and ultrafast response times is a long sought-after goal. In this work, a fundamental mechanism that pushes the frontiers of ultrafast X-ray scintillator performance is theoretically predicted and experimentally demonstrated: the use of nanoscale-confined surface plasmon polariton modes to tailor the scintillator response time via the Purcell effect. By incorporating nanoplasmonic materials in scintillator devices, this work predicts over tenfold enhancement in decay rate and 38% reduction in time resolution even with only a simple planar design. The nanoplasmonic Purcell effect is experimentally demonstrated using perovskite scintillators, enhancing the light yield by over 120% to 88 ± 11 ph/keV, and the decay rate by over 60% to 2.0 ± 0.2 ns for the average decay time, and 0.7 ± 0.1 ns for the ultrafast decay component, in good agreement with the predictions of our theoretical framework. Proof-of-concept X-ray imaging experiments are performed using nanoplasmonic scintillators, demonstrating 182% enhancement in the modulation transfer function at four line pairs per millimeter spatial frequency. This work highlights the enormous potential of nanoplasmonics in optimizing ultrafast scintillator devices for applications including time-of-flight X-ray imaging and photon-counting computed tomography.

Detection performance of pixelated lutetium-yttrium oxyorthosilicate (LYSO) scintillators for high-resolution photon-counting CT imaging

High-resolution photon-counting detector (PCD) computed tomography (CT) imaging is increasingly used for several applications. Recent technological advances in CT instrumentation have introduced various types of radiation detectors. Therefore, this work aims at evaluating the lutetium-yttrium oxyorthosilicate (LYSO) scintillator for use in PCD CT from a detector point of view. To do so, a mini-CT prototype was designed and constructed based on the pixelated LYSO blocks. The detector comprises four 10 × 10 linearly arranged LYSO blocks coupled with four position-sensitive photomultiplier tubes. The prototype utilizes a point gamma-ray source along with a cone-beam collimator. An in-home MATLAB-based data processing software package was also developed for storing the list-mode data, event positioning, and energy windowing. A set of experiments were conducted to assess the performance of the constructed energy-resolved LYSO:Ce detector for mini-CT imaging. The results show good crystal identification for all blocks with a maximum peak-to-valley ratio of 3.48. In addition, the findings confirm that the developed detector is position-sensitive. The 20% energy window provides an optimal performance by simultaneously providing good crystal identification and a scatter removal factor of 0.71. A 96% uniformity was also observed when the detector was irradiated with a uniform flood. The spatial resolution of the mini-CT prototype in the x- and y-directions was calculated to be 0.9 and 0.93 mm, respectively, corrected for a magnification factor of 2.5. It is concluded that the pixelated LYSO crystal is a promising alternative to the current detectors and would be the scintillator of choice for high-resolution PCD CT imaging tasks.