Perovskite-based X-ray detectors have a lot to offer the field of diagnostic imaging: low production costs, direct conversion, high absorption efficiency and superior spatial resolution to existing detectors. But while these advantages have been demonstrated in previous studies, researchers have not yet determined whether they translate to improvements in clinical applications.
To investigate this potential in more depth, researchers in the X-ray Cancer Imaging and Therapy Experimental (XCITE) Lab at the University of Victoria in Canada have performed virtual clinical trials on next-generation perovskite detectors integrated into common X-ray imaging devices, reporting their findings in Physics in Medicine & Biology.
The team investigated the perovskite crystal methylammonium lead bromide (MAPbBr3), which combines high charge carrier mobility and long carrier lifetimes, making it extremely sensitive to incident X-ray photons. Indeed, some MAPbBr3 crystals show equivalent performance to that of cadmium zinc telluride (CZT), a promising material used in cutting edge medical imaging techniques such as photon-counting CT.
To determine which imaging applications may suit perovskite detectors, the researchers used TOPAS Monte Carlo (MC) simulations to calculate the energy deposition efficiency (EDE, the fraction of absorbed energy relative to the incident energy) of MAPbBr3, for crystal thicknesses between 40 and 15 mm and beam energies from 20 keV to 6 MeV.
They compared the results with four other detector materials: amorphous selenium (a-Se), commonly used for mammography; caesium iodide (CsI), the standard detector material for kilovoltage (kV) CT; gadolinium oxysulphide (GOS), as used in kV and megavoltage (MV) imaging; and CZT.
Due to the lead content in MAPbBr3, the perovskite exhibited the highest energy absorption of all the detectors in the mammographic energy range. For MV imaging, only CZT had superior EDE, while for kV imaging, perovskite did not generally perform as well as the others. Based on these findings, the team chose three imaging systems to study: the Koning dedicated breast CT scanner, and Varian’s Truebeam kV and MV cone-beam CT (CBCT) systems.
“The EDE simulations motivated the inclusion of breast CT, a more niche imaging system that we would not have simulated otherwise,” explains first author Jericho O’Connell. “The kV- and MV-CBCT systems would have been included regardless, as they are key parts of the radiotherapy workflow.”
Virtual clinical trials
O’Connell and colleagues used Fastcat hybrid MC simulations to optimize the perovskite detector design for each application. By maximizing the detective quantum efficiency (DQE, the efficiency of converting an input signal to an output image), they calculated the optimal thicknesses for the perovskite crystals as 0.30, 0.86 and 1.99 mm, for breast CT, kV- and MV-CBCT, respectively. They then used these device-specific detectors in a series of virtual clinical trials.
For the breast CT trial, the researchers simulated a breast phantom with microcalcifications imaged using the default CsI detector and a perovskite detector with the same pixel pitch (0.194 mm). The perovskite detector increased contrast in the microcalcifications by 87%, clearly visualizing a calcified lesion that was poorly defined using the CsI detector. This could enable more accurate identification of such structures in breast cancer screening when using a perovskite detector, which can be manufactured at lower cost than CsI.
In the kV and MV CBCT virtual trials, the researchers imaged an XCAT head phantom. In both cases, the perovskite detector dramatically improved image quality compared with the default detectors. In the kV images, spatial resolution in fine bone features and tissue contrast was improved dramatically using the perovskite detector, increasing the CNR in brain and skull by 8% and 13%, respectively, compared with the CsI detector.
The MV image focused on a skull region containing silver fillings that would generally produce large streaking artefacts in kV images. The high efficiency of the perovskite detector compared with a GOS detector resulted in dramatic improvement in CNR and enabled a metal artefact-free image of the jaw. The researchers point out that the improved contrast in MV-CBCT images with a perovskite detector could enable imaging of patients on radiotherapy machines without a kV on-board imager, as is the case for most systems in low- and middle-income countries.
Replacing the current detectors on the breast CT, kV-CBCT and MV-CBCT machines with optimized perovskite detectors improved the DQE of these systems by 12.1%, 9.5% and 86.1%, respectively. “Perovskite detectors perform better than current detectors in breast CT and kV-CBCT applications, and are far superior to current MV-CBCT detectors in terms of CNR and DQE,” the researchers conclude.
Next, the team plans to create prototype perovskite-based flat-panel detectors to experimentally verify the virtual trials. “We are excited to report that we have sent off crystals to get a prototype pixelated detector made through AY Sensors,” O’Connell tells Physics World. “Stay tuned for the experimental characterization of the prototype detector.”
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