Dose optimisation in computed radiography

After the installation of computed radiography (CR) systems in three hospitals in Luxembourg a patient dose survey was carried out for three radiographic examinations, thorax, pelvis and lumbar spine. It was found that the patient doses had changed in comparison with the patient doses measured for conventional radiography in the same three hospitals. A close collaboration between the manufacturers of the X-ray installations, the CR imaging systems and the medical physicists led to the discovery that the speed class with which each radiographic examination was to be performed, had been ignored, during installation of the digital imaging systems. A number of procedures were carried out in order to calibrate and program the X-ray installations in conjunction with the CR systems. Following this optimisation procedure, a new patient dose survey was carried out for the three radiographic examinations. It was found that patient doses for the three hospitals were reduced.     

A conceptual optimisation strategy for radiography in a digital environment

Using a completely digital environment for the entire imaging process leads to new possibilities for optimisation of radiography since many restrictions of screen/film systems, such as the small dynamic range and the lack of possibilities for image processing, do not apply any longer. However, at the same time these new possibilities lead to a more complicated optimisation process, since more freedom is given to alter parameters. This paper focuses on describing an optimisation strategy that concentrates on taking advantage of the conceptual differences between digital systems and screen/film systems. The strategy can be summarised as: (a) always include the anatomical background during the optimisation, (b) perform all comparisons at a constant effective dose and (c) separate the image display stage from the image collection stage. A three-step process is proposed where the optimal setting of the technique parameters is determined at first, followed by an optimisation of the image processing. In the final step the optimal dose level-given the optimal settings of the image collection and image display stages-is determined.     

Image quality and dose management in digital radiography: a new paradigm for optimisation

The advent of digital imaging in radiology, combined with the explosive growth of technology, has dramatically improved imaging techniques. This has led to the expansion of diagnostic capabilities, both in terms of the number of procedures and their scope. Throughout the world, film/screen radiography systems are being rapidly replaced with digital systems. Many progressive medical institutions have acquired, or are considering the purchase of computed radiography systems with storage phosphor plates or direct digital radiography systems with flat panel detectors. However, unknown to some users, these devices offer a new paradigm of opportunity and challenges. Images can be obtained at a lower dose owing to the higher detective quantum efficiency (DQE). These fundamental differences in comparison to conventional film/screens necessitate the development of new strategies for dose and quality optimizations. A set of referral criteria based upon three dose levels is proposed.     

A dose index as a tool to estimate paediatric patient doses in digital projection radiography

Radiation exposure during childhood is estimated to have a lifetime risk up to seven times greater than exposures at adult age. Therefore, paediatric patient dose monitoring is a major concern in radiology. The aim of this study is to evaluate the validity of a dose index, displayed at the digital imaging and communication in medicine (DICOM) header of the images, as a dosimetric quantity to estimate the entrance surface dose (ESD) for each procedure. It was carried out over a sample of 156 paediatric patients who underwent a chest examination. National Radiation Protection Board Report 318 model was used to categorise patients by age and to estimate their thickness. Corrected dose index values were compared with calculated ESD, estimated from tube output and radiographic technique. The deviation between both values remained within 2% for every age group, except for patients up to 1 y. Therefore, the index could be used to estimate ESD, allowing to manage greater patient dose databases.     

Assessment of paediatric CT exposure in a Portuguese hospital

In this study, the characterisation of radiation exposure of paediatric patients in computerised tomography (CT) procedures was performed for a Portuguese hospital. Dosimetric data and technical parameters used for CT examinations were retrieved, compiled and analysed over a period of 1 y. Five paediatric age groups were considered, covering the age interval from 0 (newborn) to 18 y old and, for each age group, the relative frequency of the most frequent CT examinations (head, ears, sinuses, chest and abdomen examinations) is analysed. The exposure settings used (kilovolt and milliampire) were compared with the values established in the local (hospital) clinical protocols for consistency analysis. Average CT dose index(vol) and dose length product values, per age group, are presented as well as the corresponding estimated mean effective dose values. Results showed an evident need for a protocol review, in order to adjust practices to international guidelines for performing optimised paediatric CT examinations. Also, an increased awareness of staff to Radiological Protection principles in CT in particular, these of utmost importance, seems necessary.     

Effect of scatter on reconstructed image quality in cone beam computed tomography: evaluation of a scatter-reduction optimisation function

The effect of scatter on reconstructed image quality in cone beam computed tomography was investigated and a function which can be used in scatter-reduction optimisation tasks was tested. Projections were calculated using the Monte Carlo method in an axially symmetric cone beam geometry consisting of a point source, water phantom and a single row of detector elements. Image reconstruction was performed using the filtered backprojection method. Image quality was assessed by the L2-norm-based difference relative to a reference image derived from (1) weighted linear attenuation coefficients and (2) projections by primary photons. It was found that the former function was strongly affected by the beam hardening artefact and did not properly reflect the amount of scatter but the latter function increased with increasing beam width, was higher for the larger phantom and exhibited properties which made it a good candidate for scatter-reduction optimisation tasks using polyenergetic beams.     

Optimising automatic exposure control in computed radiography and the impact on patient dose

Automatic exposure controls (AECs) used with computed radiography (CR) equipment need to be set for a constant signal level in the resultant images. The response varies with the energy of the X-ray beam in a different way from conventional film screen combinations. Dose to the imaging receptor has been employed in adjustment of the AECs for varying exposure conditions for CR systems installed in hospitals in the west of Scotland. However, other parameters could potentially be applied. In this study, three quantities have been investigated for use in setting the AEC function: the exposure indicator defined by the CR manufacturer, dose to the image receptor and image noise. Experiences gained in setting up the systems are described and results of a patient dose survey are reported.     

Patient doses from screen-film and full-field digital mammography in a population-based screening programme

The aim of this study was to compare mean glandular dose (MGD) in all full-field digital mammography (FFDM) and screen film mammography (SFM) systems used in a national mammography screening program. MGD from 31 screening units (7 FFDM and 24 SFM), based on an average of 50 women at each screening unit, representing 12 X-ray models (6 FFDM and 6 SFM) from five different manufacturers were calculated. The MGD was significantly lower for FFDM compared with SFM (craniocaudal): 1.19 versus 1.27 mGy, respectively, mediolateral oblique: 1.33 versus 1.45 mGy, respectively), but not all of the FFDM units provided lower doses than the SFM units. Comparing FFDMs, the photon counting scanning-slit technology provides significantly lower MGDs than direct and indirect conversion digital technology. The choice of target/filter combination influences the MGD, and has to be optimised with regard to breast thickness.     

Correlation of image quality with exposure index and processing protocol in a computed radiography system

The correlation of image quality with the exposure index (EI)and the processing protocol was investigated in a Kodak computedradiography (CR) system using clinical radiographs and a waterphantom containing an aluminium and a copper stepwedge. Thephantom was exposed to different dose levels and the acquiredimages were processed using four clinical protocols. The qualityof these images was evaluated in terms of image brightness,contrast and noise. In clinical radiographs, there was no straightforwardcorrelation of image quality with EI. In phantom images, higherEI values improved contrast and reduced noise but after a pointthis improvement does not justify the implied increase in patientdose. Image brightness, contrast and noise were also stronglydependent on the processing protocol. To obtain the images ofsatisfactory quality with the Kodak CR system, a dose slightlyhigher than those used in 400 relative speed screen–filmsystems and a processing protocol designated for the specificradiographic examination are required.     

A method of predicting the image noise in paediatric multi-slice computed tomography images

The purpose of this study was to develop an equation with which to determine the tube current to be used in order to obtain a certain image noise level for differently sized children undergoing multi-slice computed tomography examination. The relationship between image noise and detector dose for different examination protocols was established for a LightSpeed Ultra, an eight slice CT from GEMS, using homogeneous water phantoms of different sizes. Three different anatomical areas (head, thorax and abdomen) were studied in 111 patients between 0 and 17 y of age. The mean ratio between the calculated and the measured noise in patient images was established for the different areas. Head examinations showed the best correlation (measured-to-calculated noise ratio = 1.01). In the thorax, the calculated noise was generally higher than the measured noise (ratio = 0.74), and in the abdomen, the opposite result was found (ratio = 1.20).     

The Entry-Level Engineer: Problems in Transition from Student to Professional

When students enter the workforce, they must make a transition from an academic to a professional role. This study, based on interviews with professional engineers in supervisory roles, suggests that many basic skills required in the workplace, including the ability to work on a team and to communicate with one's peers and supervisors, are missing or insufficiently developed in recent college graduates. Many employers have programs to overcome these deficiencies, but we should also consider what we as educators can do to better prepare students for their future roles, and what students themselves can do to ease the transition.     

Assessment of radiographic screen-film systems: a comparison between the use of a microdensitometer and a drum film digitiser

A high-end drum film digitiser (Tango, Germany) and a calibrated linear microdensitometer developed by PTB were used to assess the modulation transfer function (MTF) and the noise power spectra (NPS) of 3 mammographic screen film systems at optical density levels of 0.8, 1.5 and 2.5. The use of a drum scanner to assess MTF and NPS data appears to be adequate but requires an appropriate characterisation of the scanner to verify its internal noise level and its MTF. It is further necessary to calibrate the scanner output in terms of visual diffuse optical densities. Processing of two-dimensional digital data of grating images need to be more strictly defined for accurate MTF measurements of screen-film systems. Nevertheless, even now it seems to be feasible to use commercially available high-end and well calibrated scanners to assess screen film systems. This is especially important for quality assurance purposes because important parameters of screen film systems such like MTF and NPS can now be determined without using sophisticated microdensitometers which are not commercially available.     

Electrical Conductivity of Tungsten in a Continuous Transition from Condensed to Gaseous State

A pulse heating technique is developed that makes it possible to investigate the transition of a metal from a condensed to a gaseous state while maintaining almost uniform temperature and pressure distributions in a sample. By means of the technique, the electrical conductivity of tungsten was measured in a process during which a pressure in the range of 30–100 kbar was applied to the sample and its density decreased from the standard solid density to a density 15–20 times less. Since the pressures are substantially higher than the critical pressure, the transition from a condensed to a gaseous state was continuous. Earlier results have shown that along isobars in the range of 30–60 kbar the density dependence of the electrical conductivity changes radically at a certain density value (at which it has a pronounced knee). At the knee, the density is approximately 10 times less than the standard solid density, and the internal energy is about two times the sublimation energy. The dependence of the electrical conductivity near the knee becomes smoother as the pressure increases. In this paper new results on the conductivity of tungsten at the pressures up to 100 kbar are presented. It is shown that the knee becomes remarkably flatter and smoother than the corresponding low pressure dependence. Nevertheless, the main features of the electrical conductivity dependence observed at low pressures persist at the maximum applied pressure.     

Controlling the Cassie-to-Wenzel Transition: an Easy Route towards the Realization of Tridimensional Arrays of Biological Objects

Nano-Micro Letters - In this paper we provide evidence that the Cassie-to-Wenzel transition, despite its detrimental effects on the wetting properties of superhydrophobic surfaces, can be exploited...     

An optimisation strategy in a digital environment applied to neonatal chest imaging

The aim of this study was to find the optimum tube voltage for neonatal chest imaging in computed radiography. The study was designed to take full advantage of the benefits of digital imaging, for example, by comparing the tube voltages at constant effective dose. A phantom study using a living rabbit was first conducted. Images were collected at tube voltages ranging from 40 to 90 kV(p). The reproduction of four structures (central vessels, peripheral vessels, carina and thoracic vertebrae) was rated by 10 radiologists. The reproduction of both central and peripheral vessels was relatively independent of tube voltage. The carina was better reproduced at higher tube voltages whereas the opposite was true for the thoracic vertebrae. Based on the higher importance of the reproduction of the carina it was decided that 90 kV(p) was the optimal tube voltage. To validate the result from the phantom study, a follow-up study was conducted in which images of neonates collected at the tube voltage regularly used at Sahlgrenska University Hospital (70 kV(p)) were compared with images collected at the tube voltage proposed by the phantom study. The follow-up study confirmed the results from the phantom study that the reproduction of the carina was better at 90 than at 70 kV(p). In conclusion, for neonatal chest imaging-given the same effective dose-90 kVp gives better reproduction of important structures than the regularly used 70 kV(p).     

Effective dose calculations in conventional diagnostic X-ray examinations for adult and paediatric patients in a large Italian hospital

The effective dose E is an efficient and powerful parameter to study the radioprotection of the patient. In our hospital, eight radiological departments and more than 100 radiological X-ray tubes are present. The effective doses were calculated for adults and paediatric patients in 10 standard projections. To calculate E, first the entrance skin dose (ESD) was evaluated by a mathematical model that was validated by >400 direct measurements taken with an ionisation chamber on four different phantoms: the overall accuracy of the model was better than 12%. Second, to relate ESD to E, conversion coefficients calculated by Monte Carlo techniques were used. The E-values obtained were of the same order as those presented in the literature. Finally, we analysed how the study of E distributions among the various radiological departments can help to optimise the procedures, by identifying the most critical examinations or sub-optimal clinical protocols.     

A multilevel approach to the optimisation of a composite light rail vehicle bodyshell

The optimisation of design parameters associated with composite sandwich bodyshell walls of light rail vehicles (LRV) is investigated. The main objective is to evaluate a multi-level computational procedure that leads to optimum wall ply thickness and geometric shapes for areas such as door and window openings. The optimisation procedures are extensively automated whereby iterative finite element solutions are executed under the control of a software suite containing information on the optimisation parameters, objective functions and constraints. The steps in the process include a global finite element analysis of the complete LRV bodyshell subjected to an industry standard loading, identification of a critically loaded panel and its division into domains, and optimisation of a window panel fillet radius within these domains. Numerical results are obtained in order to demonstrate the viability of the method as a design tool.     

Price Cap Policies in the Transition from Monopoly to Competitive Markets1

Price cap regulation is replacing traditional rate of returnregulation in a number of jurisdictions. This development canbe viewed as a regulatory mechanism facilitating the transitionfrom monopoly to competitive markets. However, multiple goalscannot be achieved via a single instrument. Pure price capshave been modified to better meet the mix of regulatory objectives.These modifications further constrain incumbent telcos.