Since their discovery in the late 19th century, the use of X-rays for medical imaging has steadily increased into the modern era, resulting in higher levels of radiation exposure for patients. Larger doses and repeated exposures to radiation are associated with an increased risk of cancer.1 Additionally, surgeons who utilize intraoperative X-ray imaging are also subject to potentially increased risk, particularly when they exceed the occupational limit of ionizing radiation exposure.1,2 Concerns are growing regarding cumulative radiation exposure, particularly among spinal and endovascular neurosurgeons who utilize minimally invasive techniques. These procedures require significantly more fluoroscopy, and consequently involve greater doses of radiation to the thyroid, hands, and torso.3 We have previously expressed concerns over the development of cataracts and cancer in surgeons attributable to radiation exposure, hoping to foster awareness among providers and promote improvement of current imaging modalities.4 Flat digital detectors presently used in diagnostic and interventional procedures utilize 1 of 2 technologies. The most widely used method involves indirect X-ray conversion via thallium-doped cesium iodide scintillators with a silicon-based photodetector array. The second method relies on direct conversion involving selenium-based photoconductors, which is especially useful in high-resolution mammography. Although direct conversion provides a greater sensitivity than the indirect method, the X-ray absorptivity is poor, partly due to much greater electrical field requirements, thus limiting its applications.5 Fortunately, a new solution has been hypothesized to function as an improved photoconductor, an organic-inorganic hybrid perovskite (MAPbX3; MA = CH3NH3 and X = Cl, Br, or I). Perovskites are a class of chemical compounds that share this basic crystal architecture, which allows these compounds to absorb sunlight and conduct electricity. Based on these characteristics, perovskites could serve as an ideal photoconductive layer for highly sensitive detectors, but a method for generating thick (about 800 μm) perovskite films has eluded researchers, until recently. A South Korean team of engineers developed an all-solution-based synthesis of printable polycrystalline perovskites that may be used as a photoconductor array.5 To create the layered film, a bottom polyimide-perovskite composite (incorporating iodine as the halogen) forms the hole-transporting layer (HTL). The 830-μm-thick polycrystalline perovskite (MPC) photoconductor layer is printed onto the HTL, and covered by the hole-blocking layer, which instead uses bromine as the composite halogen (Figure). The interlayers are then spin-casted, which reduces the “dark current,” an intrinsic background leakage in photoconductor materials. Dampening of this background current is essential to limit the noise level of the X-ray detector and thereby enable use in medical imaging devices. Furthermore, throughout the medical X-ray energy domain of 10 to 200 keV, the film's stable 3-dimensional crystalline structure and elemental incorporation of heavy lead and iodine atoms enable mass attenuation coefficients comparable to other commonly used metals. Figure. View largeDownload slide Illustration of an all-solution-processed digital X-ray detector. Indium tin oxide (ITO) serves as the electrode. TFT = thin film transistor. Reprinted by permission from Macmillan Publishers Ltd: Nature,5 copyright 2017. Figure. View largeDownload slide Illustration of an all-solution-processed digital X-ray detector. Indium tin oxide (ITO) serves as the electrode. TFT = thin film transistor. Reprinted by permission from Macmillan Publishers Ltd: Nature,5 copyright 2017. The radiation dose during a 5 ms exposure time was 25 μGyair. Observed X-ray sensitivities of the perovskite composite film were up to 11 μC mGyair−1cm−2, a 100-fold increase over those of the current direct (0.3 μC mGyair−1cm−2) and indirect (0.4 μC mGyair−1cm−2) conversion methods. Thick (830 μm) films demonstrated more extensive wavelength absorption and increased photoluminescence than previously synthesized thin (0.5 μm) films. In contrast to selenium photoconductors, the perovskite film has a reduced spatial resolution. This is likely attributable to the size of the individual crystallite structures. The authors posit that their low-cost preparation of thick perovskite films may enable future applications in low-dose X-ray imaging, and even energy harvesting, due to the enhanced photoconductive properties of this materal.5 Use in solar powered energy cells6 may improve modern devices powering spinal cord stimulators,7 but low-dose X-ray detectors represent a wider-reaching utility for perovskite films. Although frameless navigation, image guidance, or intraoperative 3-dimensional imaging currently serve to reduce surgeons’ X-ray exposure, they have little to no effect in limiting patients’ total radiation dose.4 Perovskite photoconductors could remedy this problem through application in all radiation-based imaging devices. Perioperatively, patients and surgeons would jointly benefit, particularly for a broad scope of spinal and interventional procedures. Applications to cranial procedures may be on the horizon as well; further reduction of the dark current and an increased decay time may optimize future use in computed tomography. Refinement of the spatial resolution remains a current limitation. However, perovskite-based X-ray detectors appear promising, and in combination with established radiation-limiting recommendations2 would likely improve medical imaging techniques while abating the hazardous effects of radiation for patient and surgeon. REFERENCES 1. Ahn Y, Kim CH, Lee JH, Lee SH, Kim JS. Radiation exposure to the surgeon during percutaneous endoscopic lumbar discectomy. Spine . 2013; 38( 7): 617- 625. Google Scholar CrossRef Search ADS PubMed 2. Srinivasan D, Than KD, Wang AC et al. Radiation safety and spine surgery: systematic review of exposure limits and methods to minimize radiation exposure. World Neurosurg . 2014; 82( 6): 1337- 1343. Google Scholar CrossRef Search ADS PubMed 3. Bindal RK, Glaze S, Ognoskie M, Tunner V, Malone R, Ghosh S. Surgeon and patient radiation exposure in minimally invasive transforaminal lumbar interbody fusion. J Neurosurg Spine . 2008; 9( 6): 570- 573. Google Scholar CrossRef Search ADS PubMed 4. Wang MY. Physician protect thyself. World Neurosurg . 2015; 83( 2): 154. Google Scholar CrossRef Search ADS PubMed 5. Kim YC, Kim KH, Son DY et al. Printable organometallic perovskite enables large-area, low-dose X-ray imaging. Nature . 2017; 550( 7674): 87- 91. Google Scholar CrossRef Search ADS PubMed 6. Bereuter L, Williner S, Pianezzi F et al. Energy harvesting by subcutaneous solar cells: a long-term study on achievable energy output. Ann Biomed Eng . 2017; 45( 5): 1172- 1180. Google Scholar CrossRef Search ADS PubMed 7. McAuley J, Farah N, van Gröningen R, Green C. A questionnaire-based study on patients' experiences with rechargeable implanted programmable generators for spinal cord stimulation to treat chronic lumbar spondylosis pain. Neuromodulation . 2013; 16( 2): 142- 146. Google Scholar CrossRef Search ADS PubMed Copyright © 2017 by the Congress of Neurological Surgeons
Neurosurgery – Oxford University Press
Published: Mar 1, 2018
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