Demonstration of enhanced iodine K-edge imaging using an energy-dispersive X-ray computed tomography system with a 25 mm/s-scan linear cadmium telluride detector and a single comparator

Assessment of FLUKA, PENELOPE and MCNP6 Monte Carlo codes for estimating gold fluorescence applied to the detection of gold-infused tumoral volumes

Different kinds of nanoparticles have been widely studied for biomedical purposes, including applications like dose enhancement in radiotherapy treatments and contrast agent in radiological studies. Recent work suggests that gold nanoparticles can be used as contrast agents in K-edge imaging and X-ray Fluorescence Computed Tomography, mainly due to their high K-edge energy value and good biocompatibility. However, the gold X-ray fluorescence (XRF) signal obtained in these procedures is relatively week when compared with Compton or bremsstrahlung radiation emitted in the surrounding tissues, mainly because it is not possible to achieve large gold nanoparticles concentrations within biological tissues added to the XRF is attenuated by other tissues when leaving the patient body. This work presents a feasibility study on implementation of FLUKA, PENELOPE and MCNP6 Monte Carlo codes to model the detection of gold XRF emitted by a small volume containing different gold concentrations and located at different depths in a tissue-equivalent phantom. Results indicate that there is good agreement between PENELOPE and FLUKA for gold Kα and Kβ lines estimations when highly symmetric simulation scenario and kilovoltage X-ray beam were used, achieving differences lower than 2%; however, differences up to 6 times were observed between FLUKA and MCNP6 under the same conditions. In addition, remarkable differences were obtained when megavoltage X-ray beam was used, being up to 11 times between PENELOPE and FLUKA and up to 4 times between FLUKA and MCNP6 for gold Kα and Kβ lines estimations. In this regard, a suitable normalization method was proposed and implemented to perform cross-comparisons of XRF estimations obtained from the Monte Carlo codes. By means of the proposed method, FLUKA, PENELOPE and MCNP6 can be successfully implemented to assess which configuration (gold concentration and target volume depth) leads to a better detection of gold XRF, despite differences in XRF estimation between the codes.

Impact of anti-charge sharing on the zero-frequency detective quantum efficiency of CdTe-based photon counting detector system: cascaded systems analysis and experimental validation

Inter-pixel communication and anti-charge sharing (ACS) technologies have been introduced to photon counting detector (PCD) systems to address the undesirable charge sharing problem. In addition to improving the energy resolution of PCD, ACS may also influence other aspects of PCD performance such as detector multiplicity (i.e. the number of pixels triggered by each interacted photon) and detective quantum efficiency (DQE). In this work, a theoretical model was developed to address how ACS impacts the multiplicity and zero-frequency DQE [DQE(0)] of PCD systems. The work focused on cadmium telluride (CdTe)-based PCD that often involves the generation and transport of K-fluorescence photons. Under the parallel cascaded systems analysis framework, the theory takes both photoelectric and scattering effects into account, and it also considers both the reabsorption and escape of photons. In a new theoretical treatment of ACS, it was considered as a modified version of the conventional single pixel (i.e. non-ACS) mode, but with reduced charge spreading distance and K-fluorescence travel distance. The proposed theoretical model does not require prior knowledge of the detailed ACS implementation method for each specific PCD, and its parameters can be experimentally determined using a radioisotope without invoking any Monte-Carlo simulation. After determining the model parameters, independent validation experiments were performed using a diagnostic x-ray tube and four different polychromatic beams (from 50 to 120 kVp). Both the theoretical and experimental results demonstrate that ACS increased the first and second moments of multiplicity for a majority of the x-ray energy and threshold levels tested, except when the threshold level was much lower than the x-ray energy level. However, ACS always improved DQE(0) at all energy and threshold levels tested.

K-edge energy-based calibration method for photon counting detectors

In recent years, potential applications of energy-resolved photon counting detectors (PCDs) in the x-ray medical imaging field have been actively investigated. Unlike conventional x-ray energy integration detectors, PCDs count the number of incident x-ray photons within certain energy windows. For PCDs, the interactions between x-ray photons and photoconductor generate electronic voltage pulse signals. The pulse height of each signal is proportional to the energy of the incident photons. By comparing the pulse height with the preset energy threshold values, x-ray photons with specific energies are recorded and sorted into different energy bins. To quantitatively understand the meaning of the energy threshold values, and thus to assign an absolute energy value to each energy bin, energy calibration is needed to establish the quantitative relationship between the threshold values and the corresponding effective photon energies. In practice, the energy calibration is not always easy, due to the lack of well-calibrated energy references for the working energy range of the PCDs. In this paper, a new method was developed to use the precise knowledge of the characteristic K-edge energy of materials to perform energy calibration. The proposed method was demonstrated using experimental data acquired from three K-edge materials (viz., iodine, gadolinium, and gold) on two different PCDs (Hydra and Flite, XCounter, Sweden). Finally, the proposed energy calibration method was further validated using a radioactive isotope (Am-241) with a known decay energy spectrum.

Measurement of X-ray spectra using a Lu2(SiO4)O-multipixel-photon detector with changes in the pixel number

To measure X-ray spectra with high count rates, we developed a detector consisting of a Lu2(SiO4)O [LSO] crystal with a decay time of 40 ns and a multipixel photon counter (MPPC). The photocurrents flowing through the MPPC are converted into voltages and amplified by a high-speed current-voltage amplifier, and event pulses from the amplifier are sent to a multichannel analyzer to measure spectra. We used three MPPCs of 100, 400 and 1600 pixels/mm(2), and the MPPCs were driven under pre-Geiger mode at a temperature of 20 °C. At a tube voltage of 100 kV and a tube current of 5.0 μA, the maximum count rate was 12.8 kilo-counts per second. The event-pulse widths were 200 ns, and the energy resolution was 53% at 59.5 keV using a 100-pixel MPPC.

Optimization of K-edge imaging with spectral CT

PURPOSE

Spectral∕multienergy CT has the potential to distinguish different materials by K-edge characteristics. K-edge imaging involves the two energy bins on both sides of a K-edge. The authors propose a K-edge imaging optimization model to determine these two energy bins.

METHODS

K-edge image contrast with spectral CT depends on the specifications of the two energy bins on both sides of a K-edge in the attenuation profile of a relatively high atomic number material. The wider the energy bin width is, the lower the noise level is, and the poorer the reconstructed image contrast is. Here the authors introduce the signal difference to noise ratio (SDNR) criterion to optimize the energy bin widths on both sides of the K-edge for the maximum SDNR.

RESULTS

The authors study K-edge imaging with spectral CT, analyze the effect of K-edge energy bins on the resultant image quality, and establish guidelines for the optimization of energy thresholds. In simulation, the authors demonstrate that our K-edge imaging optimization approach maximizes SDNR in reconstructed images.

CONCLUSIONS

This proposed approach can be readily generalized to deal with more general settings and determine the best energy bins for K-edge imaging.