Patient dose measurements in radiological practices.

Internationally agreed standards for radiation measurements applied to medicine underpin the application of scientific methods to both therapeutic and diagnostic radiological practices. Equally it is recommended that these standards should underpin radiation measurements within the fields of nuclear energy and industrial applications of ionizing radiations. Such measurements should also apply to all exposed groups: patients, workers and members of the general public. It would appear that the underlying philosophy as well as measurement methods, including units, employed in the therapeutic and diagnostic domains have developed separately and independently over the past 30 years. Similarly, although radiological imaging methods are fundamental to both domains, a similar situation appears to apply to the assessment of image quality. This letter attempts to highlight the present situation regarding the role and relevance of dosimetric methods applied in both therapeutic and diagnostic radiological practices. In particular the present situation is discussed in relation to the primary objectives of the International Commission on Radiological Units and Measurements (ICRU). Scope for harmonization and unification of scientific methods applied in therapy and diagnosis is highlighted.

Central dose data management and analysis in IT-driven radiation protection strategies.

The applications of information technology in health care are now widespread and continue to grow. Medical imaging is at the forefront of this revolution and the introduction of digital detection methods to replace film is now addressing the diagnostic X-ray market, the most routinely employed imaging modality. The introduction of picture archiving and communication systems, hospital and radiology information systems is well underway, and the integration of radiation protection initiatives into these developments is desirable. In north-west UK, a project aimed at developing and implementing IT-driven radiation protection strategies has been underway for the past 10 y. Such strategies are geared towards the support of European Commission patient dose directive 97/43 EURATOM, in particular the need to implement clinical audit, patient dose audit and to establish dose reference levels. This paper demonstrates the national and local requirements for establishing a central dose data management system for use in radiation protection strategies. In particular, such a system can help develop and support the role of a medical physics expert in optimisation. The scientific requirements for such an approach are presented in this paper, and a prototype system is described. Preliminary results obtained with the central data management facility are also presented and the implication for analysing multiple site dose data in optimisation strategies for digital radiographic technology is highlighted.

Evaluation and testing of computed radiography systems.

The implementation of film replacement digital radiographic imaging systems throughout Europe is now gathering momentum. Such systems create the foundations for totally digital departments of radiology, since radiographic examinations constitute the most prevalent modality. Although this type of development will lead to improvements in the delivery and management of radiological service, such widespread implementation of new technology must be carefully monitored. The implementation of effective QA tests on installation, at periodic intervals and as part of a routine programme will aid this process. This paper presents the results of commissioning tests undertaken on a number of computed radiography imaging systems provided by different manufacturers. The aim of these tests was not only to provide baseline performance measurements against which subsequent measurements can be compared but also to explore any differences in performance, which might exist between different units. Results of measurements will be presented for (1) monitor and laser printer set-up; (2) imaging plates, including sensitivity, consistency and uniformity; (3) resolution and contrast detectability; and (4) signal and noise performance. Results from the latter are analysed in relationship with both system and quantum noise components.

RADIUS--closing the circle on the assessment of imaging performance.

The RADIUS (Radiological Imaging Unification Strategy) project addresses the assessment of image quality in terms of both physical and clinically relevant measures. The aim is to unify our understanding of both types of measure as well as the numerous underlying factors that play a key role in the assessments of imaging performance. In this way it is expected to provide a solid basis for the improvement in radiological safety management, where not only radiation risks are considered but also diagnostic risks of incorrect clinical outcomes (i.e. false positive/false negative). The project has applied a variety of relevant experimental and theoretical methods to this problem, which is generic to medical imaging as a whole. Digital radiography of the chest and the breast has been employed as the clinical imaging domain vehicles for the study. The project addressed the problem from the following directions: role and relevance of pathology, human observer studies including receiver operating characteristics, image quality criteria analysis, structural noise analysis, physical measurements on clinical images, physical measurements on imaging system, modelling of imaging system, modelling of visual processes, modelling of doses delivered and IT-based scientific support strategies. This paper presents an overview of the main outcomes from this project and highlights how the research outcomes actually apply to the real world. In particular, attention will be focused on new and original findings and methods and techniques that have been developed within the framework of the project. The relevance of the project's outcomes to future European research will also be presented.

Establishment of a comprehensive set of regional DRLs for CT by means of electronic X-ray examination records.

The International Commission on Radiological Protection (ICRP) has indicated that the diagnostic reference level (DRL) has the optimisation of protection as its objective for diagnostic and interventional procedures [International Commission on Radiological Protection. Protection against ionising radiation from external sources: ICRP Report 105 (2007)]. An important aim of this paper was to demonstrate a straightforward and cost-effective mechanism for undertaking patient dose audits that can be employed in the production of local and regional DRLs for use by medical physics experts in the provision of scientific support services to diagnostic radiology. The process developed employs electronic X-ray examination records obtained from multiple hospital sites transferred to a central processing and reporting facility. Results of a large-scale audit of patient doses resulting from CT examinations are presented. Doses are expressed in terms of dose length product (DLP) and were collected by remotely accessing electronic examination records held in hospital radiology information systems. Data were collected from 18 hospital sites involving up to 123 different types of examinations covering an ∼18-month period from July 2011 to December 2012. In total, 177 000 CT examination records were collected. Values have been validated against equivalent records obtained from digital imaging and communications in medicine (DICOM) header data and found to be in excellent statistical agreement. Extremely large variations in DLP values were noted for many examinations when data for all scanners were pooled. Results are discussed in relation to other surveys and differences highlighted in terms of the variations in methodologies employed and the numbers of examination records investigated. A mechanism for establishing DRLs is proposed, which could help to unify mechanisms for establishing DRLs for CT examinations.

Analysis of x-ray computed tomography images using the noise power spectrum and autocorrelation function.

A discrete representation of the reconstruction process, consistent with the method of data collection, has been used to derive expressions for the noise power spectrum, autocorrelation function and noise equivalent quanta (NEQ) of a computed tomography (CT) image. These parameters have been expressed in terms of basic scanning factors such as tube current, exposure time, slice width and number of detectors. Each of these factors affects the overall magnitude of the noise power spectrum, but the spatial frequency dependence is also determined by the type of reconstruction filter used in the computer algorithm. The noise power spectrum has been calculated for scanners employing either a ramp or Hanning weighted ramp filter. Predictions made from this theoretical analysis have been compared with experimental measurements made on various CT scanners. Measurements were made of the modulation transfer function (MTF) by techniques which permitted us to deduce the contributions of the algorithmic and non-algorithmic components to the overall MTF. NEQ values have been calculated for a number of CT scanners.

A suggested theory of the conventional tomographic imaging process.

An attempt is made to re-examine the well established theoretical basis of conventional tomography in the light of more detailed techniques being applied elsewhere in image analysis. Transfer function theory has been used to quantify the amount of edge detail reproduced by this process and the information in a tomogram is investigated in terms of edge detail associated with well resolved (unblurred) detail and also that associated with particular anatomical layers within the tomographed object, the theory has been used to define a cut plane thickness. A variety of different tube movements have been considered and calculated values of cut plane thickness are compared with those predicted by a simple geometric model. Besides the blurring associated with the tube movement the effect of x-ray focal spot, film-screen combination and also visual response of the observer have been included and their importance in the definition of cut plane thickness highlighted. One advantage of this approach appears to be the facility to quantify the performance of different tomographic tube movements and highlight the role of tomographic detail reproduced from body sections well removed from the cut plane.

Noise transfer in screen-film subtraction radiography.

The transfer of noise through the stages of a screen-film subtraction process was investigated. In particular the effect of measuring aperture size and film density on the measured noise granularity was studied. The relative contributions of quantum noise and film grain noise were found to depend on both aperture and density. Noise measurements are presented for a medium speed screen-film combination and also for a low noise alpha 16-Kodak Industrex C system. The effect of grain mottle and film blur on noise transfer through the multiple print process employed in subtraction radiography is demonstrated.

Noise and contrast detection in computed tomography images.

A discrete representation of the reconstruction process is used in an analysis of noise in computed tomography (CT) images. This model is consistent with the method of data collection in actual machines. An expression is derived which predicts the variance on the measured linear attenuation coefficient of a single pixel in an image. The dependence of the variance on various CT scanner design parameters such as pixel size, slice width, scan time, number of detectors, etc., is then described. The variation of noise with sampling area is theoretically explained. These predictions are in good agreement with a set of experimental measurements made on a range of CT scanners. The equivalent sampling aperture of the CT process is determined and the effect of the reconstruction filter on the variance of the linear attenuation coefficient is also noted, in particular, the choice and its consequences for reconstructed images and noise behaviour. The theory has been extended to include contrast detail behaviour, and these predictions compare favourably with experimental measurements. The theory predicts that image smoothing will have little effect on the contrast-detail detectability behaviour of reconstructed images.

Contrast-detail assessment of computed tomography scanners.

The design and construction of contrast-detail detectability phantoms for the assessment of the imaging performance of CT scanners are described. These phantoms have been employed to undertake a series of measurements on a number of different machines. The results are expressed in terms of object contrast which is consistent with the conventional definition of radiological contrast. For one particular scanner, the imaging performance of a number of image reconstruction and post-processing filters has been investigated. The results indicate that there was no observed energy dependence of contrast-detail detectability once differences in technique factors and attenuation were taken into account. A similar conclusion was drawn from the results of a set of measurements made to examine the effect on detectability of additional filtration.

An atmospheric pressure ionography system suitable for mammography.

Ionography is a possible replacement for silver halide based imaging systems in diagnostic radiology. This type of imaging depends upon the ionisation produced in a gas by incident x-radiation. The ions formed are collected on a plastic foil and the image is made visible by a suitable development process. The physical design properties which can affect sensitivity and image quality of an ionography system are outlined in this paper. Based upon the requirements of adequate sensitivity and resolution an atmospheric pressure ionography system is described which has been designed primarily for use in mammography. Freon 13B1 gas was chosen as the x-ray absorbing medium because it contains a relatively high atomic number atom (bromine, Z = 35) and also has a high density. It is a relatively cheap gas, has a high breakdown potential and has satisfactory resolving capability, particularly when employed at atmospheric pressure with low energy x-ray beams. The performance of the system is discussed in terms of sensitivity and resolution. Problems associated with an atmospheric pressure system are outlined and possible solutions described. Preliminary clinical results obtained with the system are also presented.

A prototype digital ionographic imaging system.

A prototype digital ionographic imaging system is described. The process involves direct electrical reading of a latent electrostatic image which has been produced by an ionographic technique. The main design features of the system are outlined and their effect on its imaging performance highlighted. A preliminary assessment of the performance of the system in terms of resolution, noise, contrast and sensitivity is presented and compared with that of a conventional screen-film combination.

Image processing of digital chest ionograms.

A number of image-processing techniques have been applied to a digital ionographic chest image in order to evaluate their possible effects on this type of image. In order to quantify any effect, a simulated lesion was superimposed on the image at a variety of locations representing different types of structural detail. Visualization of these lesions was evaluated by a number of observers both pre- and post-processing operations. The operations employed included grey-scale transformations, histogram operations, edge-enhancement and smoothing functions. The resulting effects of these operations on the visualization of the simulated lesions are discussed.

An assessment of the radiation dose received by staff using fluoroscopic equipment.

Measurements of scattered radiation were made on two fluoroscopic installations, one with an overcouch tube and one with an undercouch tube. The annual doses to staff involved in a fluoroscopic procedure undertaken on both types of unit were estimated using measured dose rates and weighting factors referred to in ICRP 26. Environmental monitoring at the protective barrier was performed for various room layouts to determine the effect of both equipment and room design on the annual dose received by staff at the control panel. In all circumstances the annual dose to staff inside the control cubicle was negligible.

Eye dose measurements using conventional and rare-earth screens during tomography of the para-nasal sinuses.

Eye dose measurements have been performed when using medium speed conventional and rare-earth screen-film combinations during tomography of the para-nasal sinuses. The measurements show that using conventional intensifying screens with the A.P. view a total eye dose of about 20 rad may be given during an examination. This eye dose can be reduced by 98% using the P.A. position. If rare-earth screen/film combinations are employed the eye dose measured in the A.P. view was reduced by 75% of that obtained with conventional screens without detectable loss of image quality. A total eye dose reduction of about 99.5% was measured in the P.A. view with the rare-earth systems.

Radiation exposure to children during cardiac catheterization.

The purpose of this study was to assess the suitability of different radiation dosimetry methods and record radiation exposures during paediatric catheterization. Three methods of dosimetry were employed: thermoluminescent dosimetry (TLD), dose-area product and calculation of entrance surface dose from calibrated exposure factors. Examinations included bi-plane fluoroscopy, and cineangiography for diagnosis and treatment of congenital heart disease. The most suitable method of radiation dosimetry for cardiac catheterization is the use of calculated entrance dose or a dose-area product meter. Children were exposed to high levels of radiation during cardiac catheterization but there was a wide variation in radiation dosage. Careful consideration should be given to the suitability of radiation dosimetry for cardiac catheterization.

An investigation into the levels of radiation exposure in diagnostic examinations involving fluoroscopy.

In order to investigate the levels of radiation exposure resulting from fluoroscopic examinations, area-exposure product measurements were performed on 6532 patients whilst undergoing a variety of examinations at a large district general hospital. Results for both the same and different types of examinations, performed in two different X-ray rooms by a number of different radiologists, are compared in order to highlight some of the factors which influence the wide variations in patient exposure which frequently occur in radiological examinations. Variations in exposure of patients of different weights are also presented.

A quantitative evaluation of film and film/screen combinations for mammographic examination.

A test phantom containing simulated micro-calcifications was constructed. This was imaged by seven systems suitable for use in mammography, firstly in scatter-free conditions and secondly with the addition of tissue-equivalent scattering material. The ability of the systems to resolve micro-particles of known size was assessed using the techniques of signal detection theory. The resultant receiver operating characteristic curves separated the systems into three major groups. Firstly, high resolution systems requiring high radiation dose techniques; secondly at the other extreme, a mediocre imaging system producing its results at low radiation levels; and thirdly a larger middle group producing intermediate resolution of fine particles at similar, acceptable dose levels. The ability to image small particles does not increase linearly with the radiation dose. Scatter did not affect all of the systems to the same extent.

Diagnostic radiology better images--lower dose compromise or correlation? A European strategy with historical overview.

Quality Criteria for diagnostic radiographic images include the definition of necessary image details, performance of the imaging system and appropriate choice of radiographic parameters. Actions have established a framework for the optimisation of these three groups of Quality Criteria. Their evolution, practice and efficacy will be demonstrated for conventional and digital radiology. The link between the Quality Criteria and exposure of the patient and staff will be discussed. The historical follow up of these aspects will appreciate the efforts done all over Europe since the twenties and will reveal the potential for continuously optimising diagnostic radiology in many respects: in day-to-day practice, new technical design and radiological concepts.

Establishing local and regional DRLs by means of electronic radiographical X-ray examination records.

The objective of the paper is to demonstrate that patient dose audits may be undertaken at the local and regional levels by employing electronic examination records contained in Radiology Information Systems (RISs) that have been collected, analysed and managed by modern IT systems. The resulting mean and third quartile values obtained may then be used to establish local and regional dose reference levels (DRLs) as part of an optimisation strategy. The method involved the collection of roughly 1.3 million radiographical examination records stored in hospital RIS over a 3-y period from 10 hospital sites in the north of England. These were analysed according to the process employed in the national patient dose (NPD) audits undertaken every 5 y in the UK. Data processing and analysis methods are described that are suitable for handling very large data sets quickly and efficiently. Because RIS data involve manual data entry it may be susceptible to data entry errors. Therefore, a comparison of results obtained from both RIS and DICOM generated data was first of all undertaken in order to 'calibrate' the RIS-based method and demonstrate its accuracy. The results obtained from this comparison indicate that the RIS-based examination records provide patient dose distributions with an equivalent statistical accuracy compared with those employing DICOM data and, therefore, may be employed in patient dose audits in order to establish both local and regional DRLs for use in patient dose management and optimisation strategies.