Adult Phantom Research Articles

Radiation dosimetry analyses of radiographic imaging systems used for orthodontic treatment: comparison among child, adolescent, and adult patients.

The aim of this study was to compare the effective doses of orthodontic radiographs in children, adolescents, and adults. We exposed a child, an adolescent (simulated by an adult female phantom), and adult male phantoms using common scanning protocols for panoramic radiography, cephalography, and cone-beam computed tomography (CBCT). Glass dosimeters were placed in the organs of the phantom to measure the absorbed doses. The effective doses were deduced using tissue weighting factors as defined in the ICRP Publication 103. For panoramic imaging, the parotid gland had the highest absorbed dose in the child and the submandibular glands had the highest absorbed dose in both the adolescent and adult phantoms. For cephalography, the organs and tissues located closest to the X-ray tube had the highest absorbed dose values. For CBCT, the lenses of the eyes received the highest absorbed dose. Effective doses with CBCT were the greatest in the adolescent phantom, followed by in the adult and child phantoms. Dental practitioners should be aware of patient age, as younger patients will incur greater risks from radiation.

Preclinical Incorporation Dosimetry of [18F]FACH-A Novel 18F-Labeled MCT1/MCT4 Lactate Transporter Inhibitor for Imaging Cancer Metabolism with PET.

Overexpression of monocarboxylate transporters (MCTs) has been shown for a variety of human cancers (e.g., colon, brain, breast, and kidney) and inhibition resulted in intracellular lactate accumulation, acidosis, and cell death. Thus, MCTs are promising targets to investigate tumor cancer metabolism with positron emission tomography (PET). Here, the organ doses (ODs) and the effective dose (ED) of the first 18F-labeled MCT1/MCT4 inhibitor were estimated in juvenile pigs. Whole-body dosimetry was performed in three piglets (age: ~6 weeks, weight: ~13–15 kg). The animals were anesthetized and subjected to sequential hybrid Positron Emission Tomography and Computed Tomography (PET/CT) up to 5 h after an intravenous (iv) injection of 156 ± 54 MBq [18F]FACH. All relevant organs were defined by volumes of interest. Exponential curves were fitted to the time–activity data. Time and mass scales were adapted to the human order of magnitude and the ODs calculated using the ICRP 89 adult male phantom with OLINDA 2.1. The ED was calculated using tissue weighting factors as published in Publication 103 of the International Commission of Radiation Protection (ICRP103). The highest organ dose was received by the urinary bladder (62.6 ± 28.9 µSv/MBq), followed by the gall bladder (50.4 ± 37.5 µSv/MBq) and the pancreas (30.5 ± 27.3 µSv/MBq). The highest contribution to the ED was by the urinary bladder (2.5 ± 1.1 µSv/MBq), followed by the red marrow (1.7 ± 0.3 µSv/MBq) and the stomach (1.3 ± 0.4 µSv/MBq). According to this preclinical analysis, the ED to humans is 12.4 µSv/MBq when applying the ICRP103 tissue weighting factors. Taking into account that preclinical dosimetry underestimates the dose to humans by up to 40%, the conversion factor applied for estimation of the ED to humans would rise to 20.6 µSv/MBq. In this case, the ED to humans upon an iv application of ~300 MBq [18F]FACH would be about 6.2 mSv. This risk assessment encourages the translation of [18F]FACH into clinical study phases and the further investigation of its potential as a clinical tool for cancer imaging with PET.

INVESTIGATION OF THE INFLUENCE OF THYROID LOCATION ON IODINE-131S VALUES.

The use of iodine-131S values based on reference computational phantoms with fixed thyroid model may lead to significant dosimetric errors in patients who may have different thyroid location from the reference phantoms. In the present study, we investigated individual thyroid location variation by examining the computed tomography image sets of 40 adult male and female patients. Subsequently, the thyroid location of the adult male and female mesh-type reference phantoms of the International Commission on Radiological Protection (ICRP) was adjusted to match each the highest, mean and the lowest locations of the thyroid observed in this dataset. The thyroid-adjusted phantoms were implemented into the Geant4 Monte Carlo code to calculate thyroid location-dependent iodine-131S values (rT←thyroid) for a total of 30 target regions. The maximum variation among the observed thyroid locations was 39mm and 36mm for male and female patients, respectively. The mean thyroid locations of both male and female patients showed a good agreement with the ICRP reference phantoms. The thyroid location-dependent Iodine-131S values were significantly different from the reference phantoms for most target regions by up to a factor of 3. The use of thyroid location-dependent S values in dose reconstructions should help quantify the dosimetric uncertainty in epidemiologic investigations of patients receiving iodine-131 therapy for hyperthyroidism and thyroid cancer.

Dosimetric comparison between different radiotherapy protocols for prostate cancer using Geant4 Monte Carlo simulation

We aimed to evaluate absorbed doses received by organs at risk (OARs) following prostate treatment with external beam radiotherapy (EBRT), using different techniques (3D-CRT and box), number of fields (4 and 6 fields) and photon energies (6 and 10 MV). The MIRD5 adult male anthropomorphic phantom in GEANT4 package was used. However, the prostate, rectum and both femoral heads were not available, hence, were created within the phantom. A dose of 75 Gy was prescribed to the prostate, in all simulated treatments. Field size of 6.4 × 5.9 cm2 was used in all techniques. For 3D-CRT technique, beams with similar shape to the prostate were used, while for box technique, square-shaped beams were used. Absorbed doses to OARs (rectum, urinary bladder and both femoral heads) were then evaluated. The 3D-CRT technique resulted in lower dose to OARs compared to box technique. Also, the findings show an inverse relationship between number of fields and the OARs doses. There was no dose difference between the OARs with different beam energies. In conclusion, the 6-field 3D-CRT technique with 6 MV photon beam is an ideal treatment option for prostate cancer.

Dose distributions in adult and child head phantoms for panoramic and cone beam computed tomography imaging of the temporomandibular joint.

The aim of this study was to map and compare the distributions of absorbed doses with Gafchromic film for panoramic radiography and cone beam computed tomography (CBCT) examinations of the temporomandibular joint (TMJ) by using adult and child phantoms. Gafchromic films were placed at 5 selected levels of anthropomorphic head phantoms of an adult and a child. Clinical protocols for panoramic and CBCT imaging of the TMJ were used for three 2-dimensional or 3-dimensional dental x-ray units. Mean absorbed doses in a set of radiosensitive tissues within the oral and maxillofacial regions were estimated. The absorbed doses varied considerably among and within radiosensitive tissues. The bone surface and the salivary glands received the highest absorbed doses compared with other tissues, in both panoramic and CBCT examinations of the TMJ. The radiation burden to the adult phantom was generally higher than that to the child phantom. Small right and left fields of view were associated with lower amounts of radiation, in contrast to a single larger field of view. The absorbed dose within all radiosensitive tissues varied considerably in relation to examination type, x-ray unit, clinical settings, and patient age. The mean doses were smaller when using 2 (bilateral) 4×4 cm volumes than with use of one 14×5 cm volume.

A Universal Protocol for Abdominal CT Examinations Performed on a Photon-Counting Detector CT System

The aims of this study were to investigate the feasibility of using a universal abdominal acquisition protocol on a photon-counting detector computed tomography (PCD-CT) system and to compare its performance to that of single-energy (SE) and dual-energy (DE) CT using energy-integrating detectors (EIDs). Iodine inserts of various concentrations and sizes were embedded into different sizes of adult abdominal phantoms. Phantoms were scanned on a research PCD-CT and a clinical EID-CT with SE and DE modes. Virtual monoenergetic images (VMIs) were generated from PCD-CT and DE mode of EID-CT. For each image type and phantom size, contrast-to-noise ratio (CNR) was measured for each iodine insert and the area under the receiver operating characteristic curve (AUC) for iodine detectability was calculated using a channelized Hotelling observer. The optimal energy (in kiloelectrovolt) of VMIs was determined separately as the one with highest CNR and the one with the highest AUC. The PCD-CT VMIs at the optimal energy were then compared with DE VMIs and SE images in terms of CNR and AUC. Virtual monoenergetic image at 50 keV had both the highest CNR and highest AUC for PCD-CT and DECT. For 1.0 mg I/mL iodine and 35 cm phantom, the CNRs of 50 keV VMIs from PCD-CT (2.01 ± 0.67) and DE (1.96 ± 0.52) were significantly higher (P < 0.001, Wilcoxon signed-rank test) than SE images (1.11 ± 0.35). The AUC of PCD-CT (0.98 ± 0.01) was comparable to SE (0.98 ± 0.01), and both were slightly lower than DE (0.99 ± 0.01, P < 0.01, Wilcoxon signed-rank test). A similar trend was observed for other phantom sizes and iodine concentrations. Virtual monoenergetic images at a fixed energy from a universal acquisition protocol on PCD-CT demonstrated higher iodine CNR and comparable iodine detectability than SECT images, and similar performance compared with DE VMIs.

Dosimetric assessment in different tumour phenotypes with auger electron emitting radionuclides: 99mTc, 125I, 161Tb, and 177Lu

Internal radiotherapy using Auger-emitting radionuclides is a relatively new technique that presents interesting advantages with respect to external radiotherapy, such as localized tumor efficacy. The aim of this study was to assess the dosimetric effectiveness in irradiating a tumor partitioned in different phenotypes, with different radionuclides directed at each tumor phenotype: 99mTc, 125I, 161Tb, and 177Lu.State of the art Monte Carlo PENELOPE code and ICRP adult female reference voxel phantom (AFP) were used in order to mimic a lung tumor volume composed by four different phenotypes. For each radionuclide above mentioned, the decay modes (accessed through ICRP-107 data files) considered encompassed Auger electrons and β, X and ϒ radiation. Two main radiation therapy scenarios were simulated: i) the entire tumor was irradiated homogenously with each of the radionuclides; ii) each tumor phenotype was filled with a different radionuclide. The optimal dosimetric configuration was studied in terms of Dose Efficiency (DE), defined as tumor-to-healthy dose ratio.The Monte Carlo model was validated by comparing the results of SAF values in AFP and cellular S-values for 125I with the ones present in the bibliography. In the first scenario, calculations showed that the highest DE is reached by 125I. Namely, with 125I a gain dose factor (GDF) of about 2.8, 2.7 and 122.5 could be achieved, with respect to 177Lu, 161Tb, and 99mTc, respectively. In the second scenario, a combination of radionuclides directed to each of the phenotypes can act as enhancement for DE or dose in the tumor tissues, with respect to using only one radionuclide in the 4 tumor phenotypes.According to this study, the hypothetical use of different electron beam qualities directed to different tumor phenotypes of the same tumor could act as a radio-sensitizer and, at the same time, minimize dose to the surrounding healthy tissues.

Monte Carlo simulation of non-target organ doses and radiation-induced secondary cancer risk in Tanzania from radiotherapy of nasopharyngeal by using Co-60 source

A few studies have reported the risks of secondary cancer in patients after radiation therapy in Tanzanian hospitals. This study was designed to assess non-target organ doses from Co-60 teletherapy for patients with nasopharyngeal cancer and to estimate the risk of radiation-induced secondary cancer. Specific organ-absorbed doses were evaluated in anatomically realistic 15-year-old and adult male phantoms based on Monte Carlo simulation. The assessed organ doses were used to calculate the lifetime secondary cancer risks in non-target organs according to the BEIR VII risk models. Survival information and the baseline cancer risks were associated with statistical data for the Tanzanian population. The average absorbed doses in non-target organs depended heavily on the distance from the target isocentre and patient's age. Organs near the tumour target, such as the salivary glands, brain, thyroid glands, eye bulb and oesophagus, received very high absorbed doses. The absorbed doses decreased exponentially towards the testis and prostate which are far from the tumour target. The highest received absorbed doses in non-target organs were due to the leakage in the gantry head and scattered radiation from collimator and inside the patient body. The absorbed doses were higher for young patients than in adults because of the larger body area exposed to high-dose radiation. The lifetime attributed risks for all organs considered were relatively lower than the baseline cancer risks. Results can help provide future database about radiation-induced second primary cancer incidence after Co-60 external beam radiation therapy for nasopharyngeal cancer in Tanzania and in many other developing countries.

Organ dose measurements using an adult anthropomorphic phantom and risk estimation of cancer incidence from CBCT exposures

Cone-beam CT (CBCT) has become an essential tool for pre-treatment verification of the patient's position and for targeting the tumor volume localization in Image Guided Radiotherapy (IGRT). CBCT imaging is employed generally on a daily-basis, for each treatment fraction, and several times per patient, to ensure the correct patient's set-up. This leads to cumulative imaging doses to the tissues surrounding the exposed target-organs. The objective of this work is to determine the patient organ doses from a thorax CBCT scan in order to estimate risk of cancer incidence due to CBCT exposures.A thorax CBCT scan was performed in an anthropomorphic phantom of an adult male (CIRS ATOM) and the organ doses were assessed from point measurements using Thermo Luminescent Detectors (TLDs). The measurements were performed using a CBCT imaging system mounted on a LINAC (EdgeTM, Varian Medical Systems). The lifetime attributable risk (LAR) of cancer incidence was determined using the BEIR risk models. Considering a single thorax CBCT scan, the highest organ doses were calculated for heart and left lung which registered values of 5.84 ± 0.99 mGy and 4.90 ± 0.83 mGy, respectively. In contrast, the lowest organ dose was determined for right lung, with an absorbed dose of 3.07 ± 0.52 mGy. Regarding risk estimation, after a complete course of IGRT treatment for lung cancer (24 fractions) and assuming that, at least, one CBCT scan is performed per fraction, the LAR of cancer incidence varies between 27 to 309 cases per 100.000 exposed persons, depending on the organ evaluated. This work highlights the need to determine radiation induced cancer risks arising from CBCT repeated exposures in order to optimize the selected scanning protocols and consequently the radiological protection of patient. Furthermore, accurate organ dose calculation is fundamental to reduce the uncertainties associated with radiological risk estimation in imaging procedures.

MODELLING AND SIMULATION FOR RADIOLOGICAL DOSE ASSESSMENT OF PSAMMOTHERAPY AND CLIMATOTHERPY.

Patient and occupational dose rates due to psammotherapy (sand therapy) and climatotherapy treatments in high natural background areas in Egypt have been evaluated. Monte Carlo mathematical simulations using adult human phantoms were applied to consider the effect of elevated 238U, 232Th and 40K concentrations and the nonhomogeneous distribution of natural radionuclides in beach sand. Three situations: phantom covered by sand or lying on the beach and points in air at several heights above sand level, were considered. The gamma-ray doses per treatment were calculated at a reference point located on the phantom surface centrally above the genital area. The thus calculated patient-absorbed-dose ranges at this reference point were 0.006-0.018mGy and 0.004-0.023mGy per climatotherapy and psammotherapy treatments, respectively.

Estimation of External Contamination and Exposure Rates Due to Fission Product Release

In the event of a radiological incident, the release of fission products into the surrounding environment and the ensuing external contamination present a challenge for triage assessment by emergency response personnel. Reference exposure rate and skin dose rate calibration data for emergency response personnel are currently lacking for cases where receptors are externally contaminated with fission products. Simulations were conducted to compute reference exposure rate coefficients and skin dose rate coefficients from photon-emitting fission products of radiological concern. To accomplish this task, simplified mathematical skin phantoms were created using surface area and height specifications from International Commission on Radiological Protection Publication 89. Simulations were conducted using Monte Carlo radiation transport code using newborn, 1-y-old, 5-y-old, 10-y-old, 15-y-old, and adult phantoms for 22 photon-emitting radionuclides. Exposure rate coefficient data were employed in a case study simulating the radionuclide inventory for a 17 × 17 Westinghouse pressurized water reactor, following three burn-up cycles at 14,600 MWd per metric ton of uranium. The decay times following the final cycle represent the relative activity fractions over a period of 0.5-30 d. The resulting data can be used as calibration standards for triage efforts in emergency response protocols.

Assessment of dental radiation dose reduction utilizing mathematical pediatric phantom models: applications in clinical practice.

Replacing conventional round intraoral collimators with rectangular collimators provides a considerable radiation dose reduction in adult patients. This study aimed to determine the radiation dose reduction via mathematical phantom when converting from round to appropriately sized rectangular collimation in children ages 5 to 15 years. Virtual full mouth series (FMX) were simulated using a commercially available radiation dose software. This software is designed to calculate patient radiation doses from x-ray exams for various age pediatric and adult mathematical phantoms. For this pediatric study an 18-image FMX was simulated for the 15-year-old and a 12-image FMX was simulated for the 5-year-old and 10-year-old pediatric phantoms. An area of 12.0 to 16.8 cm2 represented rectangular collimation, while a 20.4 to 31.7 cm2 area represented typical round collimation. Effective doses decreased in all ages by nearly 60% when switching from 31.7 cm2 round to 12.0 cm2 rectangular collimation. Reduction in absorbed doses to the thyroid (70% to 73%), salivary glands (62% to 78%), and active bone marrow (60% to 62%) were also noted when switching from the largest to smallest collimation. This study suggests the use of rectangular collimators provides clinically relevant dose reduction for pediatric patients, even when altering from smaller round to rectangular collimation with equivalent beam quality, and this information can be utilized in all dental practices.

Calibration of a SPECT-CT gamma camera with child and adult thyroid-neck phantoms for in vivo monitoring of radioiodine in the exposed population in case of nuclear emergency

The need to quantify the uptake of 131I in the thyroid of the exposed population or workers is one of the main concerns to take into account in case of nuclear or radiological emergencies. In such scenarios, due to the high volatility and rapid intake by inhalation of 131I, it is very important a rapid identification of the exposed individuals to know their level of internal contamination in order to establish action protocols and countermeasures. In vivo measurements of 131I in the thyroid by gamma spectrometry at Whole Body Counters (WBC) is the recommended technique, but it is difficult to manage in case of a large number of individuals potentially exposed. It is known that gamma cameras located in the nuclear medicine services are available at most hospitals and could be used as an alternative method in order to provide support in the emergency response. Thus, this work describes a methodology to calibrate this kind of equipment with anthropometric phantoms to carry out direct measurement of 131I in thyroid for the quantification of internal contamination of the exposed population. A Gamma Camera (GC) of a public hospital from Madrid, H.U. La Paz, was calibrated for such purpose taking into account that a realistic geometry (10 cm distance from GC to phantom) and a rapid screening (counting time of 300 s with collimators removed) of internally contaminated individuals is necessary at early stage response. The calibration factors obtained for 131I vary with thyroid size being in the range of 0.0459 to 0.0541 cps·Bq-1. The minimum detectable activity (MDA) also varies with the thyroid size of the phantoms being in the range of 67 Bq to 79 Bq. An estimation of minimum detectable effective dose for children (E(70)) and adults (E(50)) has been carried out, taking into account such MDA values. Results show that, assuming a scenario of acute inhalation of 131I by members of the public, this methodology allows estimating doses far below 1 mSv three days after the intake. Moreover, the validation of the methodology has been carried out by participating in an international intercomparison exercise (CATHYMARA project, EC-OPERRA 2016–2017) for the in vivo measurement of 131I in thyroid to the exposed population in emergencies.

Large-scale individual monitoring of internal contamination by gamma-emitting radionuclides in nuclear accident scenarios

The results obtained in a measurement campaign concerning internal contamination by the gamma-emitting radionuclides of a large number of individuals are presented in this work. The aim is to assess the effectiveness of the spectrometric method in an emergency response following a nuclear power plant accident or a spread of radionuclides in the atmosphere due to an act of terrorism. An HPGe portable spectrometer, deployed in a collective protection apparatus, was used for both whole-body and thyroid measurements. An adult bottle mannequin absorption (BOMAB) and thyroid phantoms were used to evaluate the detector performance. The BOMAB phantom was provided by the Italian Institute of Ionizing Radiation Metrology (INMRI) for the ENEA intercomparison exercise. Thyroid phantoms were provided by the Belgian Nuclear Research Centre for the ‘Child and Adult Thyroid Monitoring After Reactor Accident’ European intercomparison exercise. The instrument performance was further evaluated by collecting spectral data from healthy volunteers, using acquisition times of 180 s and 100 s, respectively, for the whole-body and thyroid measurements. The detector showed good accuracy in quantifying radionuclide activities in the adult BOMAB and in the thyroids of persons of all ages. The proposed method allows us to detect in vivo activity leading to a committed effective dose E(50) and committed thyroid equivalent doses HT greater than 2 mSv due to all gamma-emitting fission products, if the scan is performed within five days after intake. Assuming, for instance, an acute inhalation of 137Cs and 131I, the obtained detection limit values for adults lead to a E(50) value equal to 0.08 mSv and an HT value of 0.27 mSv. The E(50) and HT values show that the proposed method can be successfully used when the dose assessment must be rapidly performed for a large number of individuals in the eventuality of the scenarios previously mentioned.

Staff Doses and Patient Skin Doses in Hybrid Operating Room Procedures – An Anthropomorphic Phantom Study with Active Electronic Dosimeters

Introduction - Fluoroscopically guided hybrid procedures have rapidly increased among vascular surgeons. During these interventions, the personnel are at risk of stochastic or deterministic effect of ionizing radiation. Minimizing the absorbed radiation dose is essential to reduce its harmful effects for the patients and personnel. The aim of our study was to quantify the effects of different imaging settings on the staff dose and patient skin dose with the active electronic dosimeters (AED). Methods - Measurements to determine radiation dose in different imaging and geometry settings were performed in a hybrid operation room (OR) equipped with a robotic C-arm interventional angiography system (Artis Zeego, Siemens Healthcare) with an anthropomorphic adult female phantom (ATOM 702-D, CIRS). Scattered radiation doses were measured with nine calibrated Philips DoseAware AEDs and a Raysafe Xi survey detector (Unfors Raysafe) that were placed at different locations in the OR mimicking the positions of surgeons and nurses during the procedure (Figure 1). Furthermore, a Raysafe R/F detector was placed under the phantom in the primary beam to measure the entrance surface air kerma. The evaluated imaging set-ups contained low-dose, medium-dose, and high-dose fluoroscopy for abdomen, low-dose DSA for abdomen, fluorofade, roadmap, and dyna-CT. The effect of magnification was evaluated with five magnification levels, tube angulation with ±30° settings, and field size, and source-to-skin distance with a few set-ups. We also determined the effects of RADPAD radiation protection shields (Worldwide Innovations and Technologies) on staff doses. Radiation was on for 20 seconds in all the imaging set-ups, except for Dyna-CT. The results are expressed as relative doses (RD) compared to the cumulative dose (1.05 μSv) measured with the AED B (“operating surgeon”) in the reference imaging setting with abdomen low-dose fluoroscopy program (4 pps) and C-arm in posterior-anterior position without magnification. Results - The use of fluorofade did not increase the cumulative dose (RD=1.1) whereas the roadmap increased it threefold (RD=3.1). The RD was 4.5 with normal-fluoro 7.5 pps instead of 4 pps and 7.0 when high-dose-fluoro was used 7.5 pps. The RDs for the magnifications 2,3,4 and 5 were 0.9, 1.0, 2.0, and 2.3 respectively. Decreasing the field-of-view to 25% of the reference field-of-view halved the radiation dose (RD=0.52). The RDs for the LAO 30° and RAO 30° were 1.6 and 2.1, respectively. The RADPAD shield decreased the cumulative dose to fifth (RD=0.23). DSA increased the cumulative dose 33-fold but the RADPAD shield decreased the DSA RD to 6.6. The RD for Dyna CT was 79. The patient skin dose in the reference setting was 229 μGy whereas 58-fold with the DSA. The doses in the AED D (“assisting nurse) were unmeasurable due to low dose in most of the set-ups. The AED F gained 4-fold doses compared to the AED E as the protection shields were on the right side of the table. Conclusion - Radiation exposure to personnel, as well as to the patients, can significantly be reduced in the hybrid OR procedures with optimization. For this purpose, the AEDs offer excellent tools.

Dose reduction in head and neck organs through shielding and application of different scanning parameters in cone beam computed tomography: an effective dose study using an adult male anthropomorphic phantom.

The aim of this study was to determine the effect of shielding and scanning parameters on radiation dose reduction to the organs in the head and neck region in cone beam computed tomography (CBCT). An anthropomorphic phantom and optically stimulated luminescent dosimeters were used to calculate the changes in effective or equivalent doses to 9 anatomic structures through the addition of a thyroid collar, radiation safety glasses, and a radiation safety cap and by using different scanning protocols on a CS 9300 CBCT unit. The thyroid collar alone yielded dose reductions of 46% to the thyroid gland and at least 38% to the esophagus, but no more than 12% to the salivary glands. The radiation safety cap significantly reduced doses to the brain and the pituitary gland. Full shielding resulted in dose reductions of at least 50% to the thyroid gland, at least 47% to the esophagus, and approximately 35% to the brain and the pituitary gland. Significant dose reductions were recorded for all tissues with the "low dose" setting compared with the standard setting. Increased protection of the organs in the head and neck regions can be achieved by using various forms of shielding in CBCT imaging, with selection of the most appropriate scanning parameters based on the purpose of the examination.

A real-time Monte Carlo tool for individualized dose estimations in clinical CT

The increasing awareness of the adverse effects associated with radiation exposure in computed tomography (CT) has necessesitated the quantification of dose delivered to patients for better risk assessment in the clinic. The current methods for dose quantification used in the clinic are approximations, lacking realistic models for the irradiation conditions utilized in the scan and the anatomy of the patient being imaged, which limits their relevance for a particular patient. The established gold-standard technique for individualized dose quantification uses Monte Carlo (MC) simulations to obtain patient-specific estimates of organ dose in anatomically realistic computational phantoms to provide patient-specific estimates of organ dose. Although accurate, MC simulations are computationally expensive, which limits their utility for time-constrained applications in the clinic. To overcome these shortcomings, a real-time GPU-based MC tool based on FDA’s MC-GPU framework was developed for patient and scanner-specific dosimetry in the clinic. The tool was validated against (1) AAPM’s TG-195 reference datasets and (2) physical measurements of dose acquired using TLD chips in adult and pediatric anthropomorphic phantoms. To demonstrate its utility towards providing individualized dose estimates, it was integrated with an automatic segmentation method for generating patient-specific models, which were then used to estimate patient- and scanner-specific organ doses for a select population of 50 adult patients using a clinically relevant CT protocol. The organ dose estimates were compared to corresponding dose estimates from a previously validated MC method based on Penelope. The dose estimates from our MC tool agreed within 5% for all organs (except thyroid) tabulated by TG-195 and within 10% for all TLD locations in the adult and pediactric phantoms, across all validation cases. Compared against Penelope, the organ dose estimates agreed within 3% on average for all organs in the patient population study. The average run duration for each patient was estimated at 23.79 s, representing a significant speedup (~700×) over existing non-parallelized MC methods. The accuracy of dose estimates combined with a significant improvement in execution times suggests a feasible solution utilizing the proposed MC tool for real-time individualized dosimetry in the clinic.

Determination of iodine detectability in different types of multiple-energy images for a photon-counting detector computed tomography system.

In addition to low-energy-threshold images (TLIs), photon-counting detector (PCD) computed tomography (CT) can generate virtual monoenergetic images (VMIs) and iodine maps. Our study sought to determine the image type that maximizes iodine detectability. Adult abdominal phantoms with iodine inserts of various concentrations and lesion sizes were scanned on a PCD-CT system. TLIs, VMIs at 50keV, and iodine maps were generated, and iodine contrast-to-noise ratio (CNR) was measured. A channelized Hotelling observer was used to determine the area under the receiver-operating-characteristic curve (AUC) for iodine detectability. Iodine map CNR ( ) was significantly higher ( ) than for TLIs ( ) and lower ( ) than for VMIs at 50keV ( ) for 0.5mgI/cc and a 35-cm phantom. For the same condition and an 8-mm lesion, iodine detectability from iodine maps ( ) was significantly lower ( ) than both TLIs ( ) and VMIs ( ). VMIs at 50keV had similar detectability to TLIs and both outperformed iodine maps. The lowest detectable iodine concentration was 0.5mgI/cc for an 8-mm lesion and 1.0mgI/cc for a 4-mm lesion.

High correlation between radiation dose estimates for 256‐slice CT obtained by highly parallelized hybrid Monte Carlo computation and solid‐state metal‐oxide semiconductor field‐effect transistor measurements in physical anthropomorphic phantoms

Accurate, patient-specific radiation dosimetry for CT scanning is critical to optimize radiation doses and balance dose against image quality. While Monte Carlo (MC) simulation is often used to estimate doses from CT, comparison of estimates to experimentally measured values is lacking for advanced CT scanners incorporating novel design features. We aimed to compare radiation dose estimates from MC simulation to doses measured in physical anthropomorphic phantoms using metal-oxide semiconductor field-effect transistors (MOSFETs) in a 256-slice CT scanner. Fifty MOSFETs were placed in organs within tissue-equivalent anthropomorphic adult and pediatric radiographic phantoms, which were scanned using a variety of chest, cardiac, abdomen, brain, and whole-body protocols on a 256-slice system. MC computations were performed on voxelized CT reconstructions of the phantoms using a highly parallel MC tool developed specifically for diagnostic X-ray energies and rapid computation. Doses were compared between MC estimates and physical measurements. The average ratio of MOSFET to MC dose in the in-field region was close to 1 (range, 0.96-1.12; mean±SD, 1.01±0.04), indicating outstanding agreement between measured and simulated doses. The difference between measured and simulated doses tended to increase with distance from the in-field region. The error in the MC simulations due to the limited number of simulated photons was less than 1%. The errors in the MOSFET dose determinations in the in-field region for a single scan were mainly due to the calibration method and were typically about 6% (8% if the error in the reading of the ionization chamber that was used for the MOSFET calibration was included). Radiation dose estimation using a highly parallelized MC method is strongly correlated with experimental measurements in physical adult and infant anthropomorphic phantoms for a wide range of scans performed on a 256-slice CT scanner. Incorporation into CT scanners of radiation-dose distribution estimation, employing the scanner's reconstructed images of the patient, may offer the potential for accurate patient-specific CT dosimetry.

Organ doses from CT localizer radiographs: Development, validation, and application of a Monte Carlo estimation technique.

The purpose of this study was to simulate and validate organ doses from different computed tomography (CT) localizer radiograph geometries using Monte Carlo methods for a population of patients. A Monte Carlo method was developed to estimate organ doses from CT localizer radiographs using PENELOPE. The method was validated by comparing dosimetry estimates with measurements using an anthropomorphic phantom imbedded with thermoluminescent dosimeters (TLDs) scanned on a commercial CT system (Siemens SOMATOM Flash). The Monte Carlo simulation platform was then applied to conduct a population study with 57 adult computational phantoms (XCAT). In the population study, clinically relevant chest localizer protocols were simulated with the x-ray tube in anterior-posterior (AP), right lateral, and PA positions. Mean organ doses and associated standard deviations (in mGy) were then estimated for all simulations. The obtained organ doses were studied as a function of patient chest diameter. Organ doses for breast and lung were compared across different views and represented as a percentage of organ doses from rotational CT scans. The validation study showed an agreement between the Monte Carlo and physical TLD measurements with a maximum percent difference of 15.5% and a mean difference of 3.5% across all organs. The XCAT population study showed that breast dose from AP localizers was the highest with a mean value of 0.24mGy across patients, while the lung dose was relatively consistent across different localizer geometries. The organ dose estimates were found to vary across the patient population, partially explained by the changes in the patient chest diameter. The average effective dose was 0.18mGy for AP, 0.09mGy for lateral, and 0.08mGy for PA localizer. A platform to estimate organ doses in CT localizer scans using Monte Carlo methods was implemented and validated based on comparison with physical dose measurements. The simulation platform was applied to a virtual patient population, where the localizer organ doses were found to range within 0.4%-8.6% of corresponding organ doses for a typical CT scan, 0.2%-3.3% of organ doses for a CT pulmonary angiography scan, and 1.1%-20.8% of organ doses for a low-dose lung cancer screening scan.