Photoluminescence of negatively charged excitons in high magnetic fields

We have studied the low-temperature photoluminescence of the two-dimensional electron gas in a single GaAs quantum well in magnetic fields up to 50 T over four orders of magnitude of illumination intensity. At the very highest illumination powers, where the recombination is excitonic at zero field, we find that the binding energy of both the singlet and triplet states of the negatively charged exciton (X-) increase monotonically with the applied field above 15 T. This contradicts recent calculations for X-, but is in agreement with adapted calculations for the binding energy of negative-donor centers. At low-laser powers we observe a strong transfer of luminescence intensity from the singlet (ground) state to the tripler (excited) state as the temperature is reduced below 1 K. This is attributed to the spin polarization of the two-dimensional electron gas by the applied magnetic field. [S0163-1829(99)01104-2]

Determination of size-specific exposure settings in dental cone-beam CT

OBJECTIVES: To estimate the possible reduction of tube output as a function of head size in dental cone-beam computed tomography (CBCT). METHODS: A 16 cm PMMA phantom, containing a central and six peripheral columns filled with PMMA, was used to represent an average adult male head. The phantom was scanned using CBCT, with 0-6 peripheral columns having been removed in order to simulate varying head sizes. For five kV settings (70-90 kV), the mAs required to reach a predetermined image noise level was determined, and corresponding radiation doses were derived. Results were expressed as a function of head size, age, and gender, based on growth reference charts. RESULTS: The use of 90 kV consistently resulted in the largest relative dose reduction. A potential mAs reduction ranging from 7 % to 50 % was seen for the different simulated head sizes, showing an exponential relation between head size and mAs. An optimized exposure protocol based on head circumference or age/gender is proposed. CONCLUSIONS: A considerable dose reduction, through reduction of the mAs rather than the kV, is possible for small-sized patients in CBCT, including children and females. Size-specific exposure protocols should be clinically implemented. KEY POINTS: • Fixed exposure settings in CBCT results in overexposure for smaller patients • For children, considerable dose reduction is possible without compromising image quality • A reduction in mAs is more dose-efficient than a kV reduction • An optimized exposure protocol was proposed based on phantom measurements • This protocol should be validated in a clinical setting.status: publishe

The growing concern of radiation dose in paediatric dental and maxillofacial CBCT: an easy guide for daily practice

OBJECTIVES: To provide an indication-based and scanner-specific radiation dose and risk guide for paediatric patients undergoing dental and maxillofacial cone beam computed tomography (CBCT) examinations. METHODS: Five commercially available scanners were simulated in EGSnrc Monte Carlo (MC) code. Dedicated, in-house built, head and neck voxel models, each consisting of 22 segmented organs, were used in the study. Organ doses and life attributable risk (LAR) for cancer incidence were assessed for males and females, aged 5 to 14 years old, for every clinically available protocol: central upper and lower incisors, upper and lower premolars, upper and lower jaws, cleft palate, temporal bone, sinus, dentomaxillofacial complex, and face and skull imaging. Dose results were normalised to the x-ray tube load (mAs) and logarithmic curves were fit to organ dose and risk versus age data. RESULTS: Females demonstrated higher LAR values in all cases. A well-established dose decreasing pattern with increasing age-at-exposure was observed. Central upper incisor protocols were those with the lowest risk, contrary to skull protocols which provided the highest LAR values. Salivary glands and oral mucosa were the highest irradiated organs in all cases, followed by extrathoracic tissue (ET) in protocols where the entire nasal cavity was inside the primary field. The dose to thyroid was considerably high for younger patients. CONCLUSIONS: This work provides an extensive dose assessment guide for 5 dental CBCTs, enabling detailed dose assessment for every paediatric patient. KEY POINTS: • Radiation dose concerns due to the growing use of paediatric dental and maxillofacial CBCT underline the need for justification that should in part be based on radiation exposure in radiology. • Patient-specific dose calculations based on Monte Carlo simulations and head-neck paediatric voxel models overcome the limitations of conventional thermoluminescent dosimeter (TLD) dosimetry and provide proper guidance for justification of CBCT exposures. • Monte Carlo simulations with head-neck models reveal an organ dose and radiation risk decreasing pattern with increasing age at exposure, and with decreasing size of the scanning volume of interest (field of view).status: publishe

Reduction of scatter-induced image noise in cone beam computed tomography: effect of field of view size and position

To measure the effect of field of view (FOV) size and position on scatter-induced image noise in cone beam computed tomography (CBCT).publisher: Elsevier articletitle: Reduction of scatter-induced image noise in cone beam computed tomography: effect of field of view size and position journaltitle: Oral Surgery, Oral Medicine, Oral Pathology and Oral Radiology articlelink: http://dx.doi.org/10.1016/j.oooo.2015.10.017 content_type: article copyright: Copyright © 2016 Elsevier Inc. All rights reserved.status: publishe

Rotating and translating anthropomorphic head voxel models to establish an horizontal Frankfort plane for dental CBCT Monte Carlo simulations: a dose comparison study

In order to carry out Monte Carlo (MC) dosimetry studies, voxel phantoms, modeling human anatomy, and organ-based segmentation of CT image data sets are applied to simulation frameworks. The resulting voxel phantoms preserve patient CT acquisition geometry; in the case of head voxel models built upon head CT images, the head support with which CT scanners are equipped introduces an inclination to the head, and hence to the head voxel model. In dental cone beam CT (CBCT) imaging, patients are always positioned in such a way that the Frankfort line is horizontal, implying that there is no head inclination. The orientation of the head is important, as it influences the distance of critical radiosensitive organs like the thyroid and the esophagus from the x-ray tube. This work aims to propose a procedure to adjust head voxel phantom orientation, and to investigate the impact of head inclination on organ doses in dental CBCT MC dosimetry studies. The female adult ICRP, and three in-house-built paediatric voxel phantoms were in this study. An EGSnrc MC framework was employed to simulate two commonly used protocols; a Morita Accuitomo 170 dental CBCT scanner (FOVs: 60  ×  60 mm(2) and 80  ×  80 mm(2), standard resolution), and a 3D Teeth protocol (FOV: 100  ×  90 mm(2)) in a Planmeca Promax 3D MAX scanner. Result analysis revealed large absorbed organ dose differences in radiosensitive organs between the original and the geometrically corrected voxel models of this study, ranging from  -45.6% to 39.3%. Therefore, accurate dental CBCT MC dose calculations require geometrical adjustments to be applied to head voxel models.journal_title: Physics in Medicine and Biology article_type: note article_title: Rotating and translating anthropomorphic head voxel models to establish an horizontal Frankfort plane for dental CBCT Monte Carlo simulations: a dose comparison study copyright_information: © 2016 Institute of Physics and Engineering in Medicine date_received: 2016-04-19 date_accepted: 2016-10-19 date_epub: 2016-11-28status: publishe

Development of a paediatric head voxel model database for dosimetric applications

To develop a database of paediatric head voxel models intended for Monte Carlo (MC) dosimetric applications.status: publishe

CUSTOMISATION OF A MONTE CARLO DOSIMETRY TOOL FOR DENTAL CONE-BEAM CT SYSTEMS

A versatile EGSnrc Monte Carlo (MC) framework, initially designed to explicitly simulate X-ray tubes and record the output data into phase space data files, was modified towards dental cone-beam computed tomography (CBCT) dosimetric applications by introducing equivalent sources. Half value layer (HVL) measurements were conducted to specify protocol-specific energy spectra. Air kerma measurements were carried out with an ionisation chamber positioned against the X-ray tube to obtain the total filtration attenuation characteristics. The framework is applicable to bowtie and non-bowtie inherent filtrations, and it accounts for the anode heel effect and the total filtration of the tube housing. The code was adjusted to the Promax 3D Max (Planmeca, Helsinki, Finland) dental CBCT scanner. For each clinical protocol, calibration factors were produced to allow absolute MC dose calculations. The framework was validated by comparing MC calculated doses and measured doses in a cylindrical water phantom. Validation results demonstrate the reliability of the framework for dental CBCT dosimetry purposes.status: publishe

ARTICLE Development of a 3D human body library based on polygonal mesh surface for whole body counter set-up calibration

A Whole Body Counter (WBC) is a common facility to routinely assess the internal contamination of exposed workers and especially in the case of radiation release accidents. The calibration of the counting device is usually done by using anthropomorphic physical phantoms representing the human body. Due to such a challenge of having representative physical phantoms a virtual calibration has been introduced. The use of computational phantoms and Monte Carlo methods to simulate radiation transport have been demonstrated to be a worthy alternative, not only as a complementary calibration method but also as a powerful tool for the design and optimization of counting geometries. In this study we introduce a methodology for building a 3D human body library that spans variation in both subject shape and size. Open source codes as MakeHuman and Blender software packages have been used for the creation and modelling of 3D humanoid characters based on polygonal mesh surfaces. Also, a home-made software was developed whose main goal is to convert the binary 3D voxel grid into a MCNPX input file. This paper summarizes (1) the development of the MaMP and FeMP -Male and Female Mesh Phantoms, the two first mesh phantoms generated from a series of 3D human bodies library created at SCK-CEN and (2) presents a comparison between the counting efficiency results obtained from the simulations of both mesh phantoms using MCNPX code

Dose distribution for dental cone beam CT and its implication for defining a dose index

OBJECTIVES: To characterize the dose distribution for a range of cone beam CT (CBCT) units, investigating different field of view sizes, central and off-axis geometries, full or partial rotations of the X-ray tube and different clinically applied beam qualities. The implications of the dose distributions on the definition and practicality of a CBCT dose index were assessed. METHODS: Dose measurements on CBCT devices were performed by scanning cylindrical head-size water and polymethyl methacrylate phantoms, using thermoluminescent dosemeters, a small-volume ion chamber and radiochromic films. RESULTS: It was found that the dose distribution can be asymmetrical for dental CBCT exposures throughout a homogeneous phantom, owing to an asymmetrical positioning of the isocentre and/or partial rotation of the X-ray source. Furthermore, the scatter tail along the z-axis was found to have a distinct shape, generally resulting in a strong drop (90%) in absorbed dose outside the primary beam. CONCLUSIONS: There is no optimal dose index available owing to the complicated exposure geometry of CBCT and the practical aspects of quality control measurements. Practical validation of different possible dose indices is needed, as well as the definition of conversion factors to patient dose.status: publishe