Artur Omar

Background: Multi‐energy computed tomography (MECT) and electron density (ED) phantoms are designed for evaluating the performance of CT scanners. They include substitute materials representing the energy‐ and material‐dependent CT numbers of different organs, tissues, or contrast agents. While MECT phantoms are intended for dual‐energy CT (DECT) and photon‐counting CT (PCCT) applications, ED phantoms are meant for the calibration of CT numbers to a metric relevant for radiation therapy treatment planning. Purpose: To compare three standard commercial phantoms in terms of iodine quantification for diagnostic imaging and tissue substitute characterization for radiation therapy. The phantoms investigated include the Multi‐Energy CT/Advanced Electron Density Phantom (AED) by Sun Nuclear Corporation (Melbourne, FL, USA), the CIRS Model 662/062M phantom (Norfolk, VA, USA), and the newly introduced Multi‐Energy QA Phantom by QRM (Möhrendorf, Germany), for which we report the first evaluation results. Methods: The phantoms were imaged using a GE Revolution Apex DECT scanner (GE Healthcare, Waukesha, WI, USA), a Siemens SOMATOM X.ceed DECT scanner (Siemens Healthineers, Forchheim, Germany), and a Siemens NAEOTOM Alpha PCCT scanner. Iodine‐in‐water inserts were evaluated using virtual monochromatic images and iodine maps, whereas the tissue equivalence of four insert sets (AED, CIRS‐ED, CIRS‐EPTN, and QRM) was theoretically assessed. For this purpose, vendor‐specified elemental compositions and physical mass densities were used to derive relative electron density (RED), effective atomic number (EAN), mean excitation energy (I‐value), and mass density (MD), which were then compared with corresponding human tissues listed in ICRU Report 44. CT scans of the phantoms containing these tissue substitutes were subsequently used for CT number calibration to proton stopping power‐ratio (SPR), RED, and MD. Results: Fitted slopes for reconstructed iodine concentrations were accurate to within 8% for the QRM and AED phantom inserts. For the best performing scanner (PCCT), the slope was within 3%. The CIRS iodine‐in‐water inserts showed substantially higher deviations of up to 22%. Linear attenuation coefficients derived from 70 keV virtual monochromatic images generally agreed with theoretically expected values to within about 10% for the QRM and AED iodine inserts (for concentrations of 1 mg/mL or higher). Regarding the QRM, AED and CIRS‐EPTN tissue substitute sets: they were closely tissue‐equivalent in terms of RED, EAN, I‐value (and hence SPR), but not in terms of MD. The CIRS‐ED inserts were not, however, closely tissue‐equivalent in terms of EAN. Consequently, the QRM, AED and CIRS‐EPTN substitutes satisfy the consensus guide criteria of the European proton therapy community for CT calibration, whereas the CIRS‐ED set does not. Conclusion: The commercially available phantoms of Sun Nuclear (AED) and QRM are suitable for both MECT performance assessment and CT‐calibration for radiation therapy. However, the batch of CIRS iodine‐in‐water inserts used in this study was unsuitable for MECT assessment. Also, the tissue substitute set provided with the CIRS 062M ED phantom was less tissue‐equivalent than the other sets evaluated.

Objectives: To assess occupational exposure in a neurointensive care unit (NICU) equipped with a mobile head CT system. Methods: The exposure was investigated in a NICU equipped with the recently introduced Siemens Healthineers SOMATOM On.site mobile head CT system, which features integrated radiation shielding consisting of a foldable 0.5-mm lead curtain covering the front of the gantry opening and a detachable 1-mm lead radiation shield covering its back. Ambient dose (H∗(10)) and scattered radiation (air kerma) were measured at selected locations. Moreover, the radiology nurse operating the scanner and the primary clinician attending the examinations wore passive dosimeters on their protective glasses to estimate the eye lens dose (Hp(3)) and active dosimeters on the chest pocket of their protective aprons to estimate the effective dose (Hp(10)). Results: The integrated radiation shielding typically achieves a 91%-99% dose reduction such that, over a 3-month period, neither the ambient dose nor the eye lens doses measured exceeded the minimum detectable level of the dosimeters. The average doses measured by active personal dosimeters across 31 adult patient examinations were 0.07 ± 0.03 µSv for the operating nurse and 0.03 ± 0.02 µSv for the attending clinician. Conclusions: Mobile head CT with integrated shielding can substantially reduce occupational exposure and, with appropriate radiation safety training, obviate the need for additional personal protective equipment, especially when personnel remain behind the operator console. Advances in knowledge: This work provides practical guidance to ensure occupational radiation safety when using a mobile head CT system in the NICU.

Background: X‐ray radiation protective equipment is essential for ensuring the safety of medical staff. It is therefore important to verify its effectiveness, including confirming the specified lead equivalence (), as it is a recognized standard protective value. Current methods require multiple comparative measurements with reference lead sheets, rendering the process laborious, susceptible to errors, and challenging to apply across a large medical facility with diverse protective equipment. Purpose: To introduce an efficient method for evaluating lead equivalence based on a computational model involving analytical spectrum modeling. Methods: The method consists of measuring the transmission of the protective equipment and then translating it into a lead‐equivalent thickness using a computational model. In this work, an example implementation is presented utilizing the SpekPy toolkit for spectrum modeling. To validate the method, it was used to estimate the thickness of high‐purity lead sheets with known thicknesses (0.1–1.0‐mm Pb). Furthermore, its application is demonstrated for two lead‐free aprons (0.25‐ and 0.35‐mm ), a lead‐vinyl apron (0.5‐mm ), a lead–acrylic and a lead–plywood mobile screen (0.5‐ and 1.0‐mm ). Because the approach is based on measuring the transmission utilizing the primary x‐ray tube beam (rather than scatter from a phantom), Monte Carlo (MC) simulations were performed to identify x‐ray tube settings that reproduce clinically relevant scatter beams. Scatter spectra were simulated for different scatter angles (45, 90, 135), tube voltages (60–120 kV), and filtration (0.1–1 mm added copper). Analytical primary spectra were then matched to scatter spectra in terms of first and second tenth‐value layer (TVL) thicknesses in lead. Results: The method is accurate to within approximately 3% and is suitable for both narrow and broad beams. For broad beams, it is necessary to scale the measured transmission by the buildup factor for lead, as the analytical spectrum model does not account for scatter. This factor, which transfers broad‐beam air kerma to narrow‐beam air kerma, ranges from 1.0 to 1.5 for 50–120‐kV beams incident upon lead sheets with thicknesses of 0.1–1.0 mm. Without this factor, the lead equivalence can be underestimated by up to 28%. Using the method developed, it was found that the effectiveness of lead‐free aprons decreases by up to 20% for high‐kV and high‐filtration beams, while other equipment investigated agreed more closely with specifications. The MC simulations of scatter spectra indicated that scatter beams are generally softer than primary beams, with a reduction in TVL by up to 54% (average of 25%). The entire range of scatter‐mimicking primary beams can be realized with tube voltages 50–100 kV and less than 0.3 mm added copper filtration. Conclusions: The method developed can accurately convert measured transmission into lead equivalence using a computational model, which eliminates the need to handle physical lead sheets. The transmission can be measured using recommended scatter‐mimicking x‐ray tube beams, derived here for a broader range of scatter angles and clinical beams with higher filtration than has previously been considered.

Purpose: To develop an analytical model for bremsstrahlung production in a thick x‐ray target (i.e., the x‐ray tube anode) that takes into account the intrinsic bremsstrahlung angular distribution.Methods: X‐ray spectrum models developed from theoretical principles have traditionally treated the angular distribution of the bremsstrahlung production as spherically uniform. This assumption stems from the rationale that electrons promptly attain a diffuse directional distribution in an x‐ray target due to multiple scattering, thereby effectively masking the intrinsic bremsstrahlung angular distribution. In this work, a model that explicitly accounts for the angular distribution of the bremsstrahlung production is presented. The model combines Monte Carlo‐calculated depth, energy, and angular distributions of electrons penetrating the x‐ray target, and incorporates theoretical results for the differential bremsstrahlung cross section. The effects of using different simplified model assumptions for the electron penetration and the intrinsic bremsstrahlung angular distribution are analyzed for tungsten and molybdenum targets in the energy range 20–300 keV.Results: Typical assumptions of previous models are shown to introduce errors in calculated spectra. Particularly, it is shown that predictions of fluence and air kerma free‐in‐air can be overestimated by 15–30% (2–3% in aluminum half‐value layer thickness) for clinically relevant beam qualities. The present model is able to reproduce comprehensive Monte Carlo calculations of the bremsstrahlung production generally to within 1%.Conclusions: The bremsstrahlung model developed in this work is an improvement over previous models in that the main features of the electron penetration and the resulting bremsstrahlung are considered in detail. The model can be used for more accurate predictions of the energy and angular distribution of x rays emitted from an x‐ray tube.

Purpose: To present and validate a complete x‐ray emission model (bremsstrahlung and characteristic x‐ray emission) for the energy range 20–300 kV.Methods: An analytical x‐ray spectrum model that combines the bremsstrahlung emission model developed in Part I with a previously developed characteristic x‐ray emission model is validated by comparison with Monte Carlo calculations, published measured spectra, and models developed by other authors. Furthermore, the assumptions and limitations of previous spectrum models are summarized, and their predictions are compared with results obtained by Monte Carlo simulations of x rays emitted from tungsten and molybdenum targets.Results: The model is able to reproduce narrow‐beam Monte Carlo calculations to within 0.5% in terms of the first and second aluminum half‐value layer thickness (HVL). Compared with measured spectra, the difference in HVL is < 2% for typical diagnostic and therapeutic beam qualities available at primary standard laboratories. Compared with previous spectrum models, the present model performs especially well for low kilovoltage x‐ray beams (below 50 kV), and is reliable for a wider range of take‐off angles, that is, the angle between the target surface and the direction of emission. The difference in model and Monte Carlo predictions of the energy‐fluence weighted air kerma (i.e., the photon energy absorption in air) is < 0.5% using the present model, while previous spectrum models can differ by more than 10%.Conclusions: The x‐ray emission model developed in this work has been validated against Monte Carlo calculations and measured results. The model provides an efficient alternative to comprehensive Monte Carlo simulations and is an improvement over previous models. The model can be used to predict both central‐ and off‐axis spectra, as well as off‐axis effects such as the (anode) heel effect.

Study Design.: Prospective observational study. Objective.: To assess staff and patient radiation exposure during augmented reality surgical navigation in spine surgery. Summary of Background Data.: Surgical navigation in combination with intraoperative three-dimensional imaging has been shown to significantly increase the clinical accuracy of pedicle screw placement. Although this technique may increase the total radiation exposure compared with fluoroscopy, the occupational exposure can be minimized, as navigation is radiation free and staff can be positioned behind protective shielding during three-dimensional imaging. The patient radiation exposure during treatment and verification of pedicle screw positions can also be reduced. Methods.: Twenty patients undergoing spine surgery with pedicle screw placement were included in the study. The staff radiation exposure was measured using real-time active personnel dosimeters and was further compared with measurements using a reference dosimeter attached to the C-arm (i.e., a worst-case staff exposure situation). The patient radiation exposures were recorded, and effective doses (ED) were determined. Results.: The average staff exposure per procedure was 0.21 ± 0.06 μSv. The average staff-to-reference dose ratio per procedure was 0.05% and decreased to less than 0.01% after a few procedures had been performed. The average patient ED was 15.8 ± 1.8 mSv which mainly correlated with the number of vertebrae treated and the number of cone-beam computed tomography acquisitions performed. A low-dose protocol used for the final 10 procedures yielded a 32% ED reduction per spinal level treated. Conclusion.: This study demonstrated significantly lower occupational doses compared with values reported in the literature. Real-time active personnel dosimeters contributed to a fast optimization and adoption of protective measures throughout the study. Even though our data include both cone-beam computed tomography for navigation planning and intraoperative screw placement verification, we find low patient radiation exposure levels compared with published data. Level of Evidence: 3