Assessment and optimisation of digital radiography systems for clinical use

Digital imaging has long been available in radiology in the form of computed tomography (CT), magnetic resonance imaging (MRI) and ultrasound. Initially the transition to general radiography was slow and fragmented but in the last 10-15 years in particular, huge investment by the manufacturers, greater and cheaper computing power, inexpensive digital storage and high bandwidth data transfer networks have lead to an enormous increase in the number of digital radiography systems in the UK. There are a number of competing digital radiography (DR) technologies, the most common are computer radiography (CR) systems followed by indirect digital radiography (IDR) systems. To ensure and maintain diagnostic quality and effectiveness in the radiology department appropriate methods are required to evaluate and optimise the performance of DR systems. Current semi-quantitative test object based methods routinely used to examine DR performance suffer known short comings, mainly due to the subjective nature of the test results and difficulty in maintaining a constant decision threshold among observers with time. Objective image quality based measurements of noise power spectra (NPS) and modulation transfer function (MTF) are the ‘gold standard’ for assessing image quality. Advantages these metrics afford are due to their objective nature, the comprehensive noise analysis they permit and in the fact that they have been reported to be relatively more sensitive to changes in detector performance. The advent of DR systems and access to digital image data has opened up new opportunities in applying such measurements to routine quality control and this project initially focuses on obtaining NPS and MTF results for 12 IDR systems in routine clinical use. Appropriate automatic exposure control (AEC) device calibration and a reproducible measurement method are key to optimising X-ray equipment for digital radiography. The uses of various parameters to calibrate AEC devices specifically for DR were explored in the next part of the project and calibration methods recommended. Practical advice on dosemeter selection, measurement technique and phantoms were also given. A model was developed as part of the project to simulate CNR to optimise beam quality for chest radiography with an IDR system. The values were simulated for a chest phantom and adjusted to describe the performance of the system by inputting data on phosphor sensitivity, the signal transfer function (STF), the scatter removal method and the automatic exposure control (AEC) responses. The simulated values showed good agreement with empirical data measured from images of the phantom and so provide validation of the calculation methodology. It was then possible to apply the calculation technique to imaging of tissues to investigate optimisation of exposure parameters. The behaviour of a range of imaging phosphors in terms of energy response and variation in CNR with tube potential and various filtration options were investigated. Optimum exposure factors were presented in terms of kV-mAs regulation curves and the large dose savings achieved using additional metal filters were emphasised. Optimum tube potentials for imaging a simulated lesion in patient equivalent thicknesses of water ranging from 5-40 cm thick for example were: 90-110kVp for CsI (IDR); 80-100kVp for Gd2O2S (screen /film); and 65-85kVp for BaFBrI. Plots of CNR values allowed useful conclusions regarding the expected clinical operation of the various DR phosphors. For example 80-90 kVp was appropriate for maintaining image quality over an entire chest radiograph in CR whereas higher tube potentials of 100-110 kVp were indicated for the CsI IDR system. Better image quality is achievable for pelvic radiographs at lower tube potentials for the majority of detectors however, for gadolinium oxysulphide 70-80 kVp gives the best image quality. The relative phosphor sensitivity and energy response with tube potential were also calculated for a range of DR phosphors. Caesium iodide image receptors were significantly more sensitive than the other systems. The percentage relative sensitivities of the image receptors averaged over the diagnostic kV range were used to provide a method of indicating what the likely clinically operational dose levels would be, for example results suggested 1.8 µGy for CsI (IDR); 2.8 µGy for Gd2O2S (Screen/film); and 3.8 µGy for BaFBrI (CR). The efficiency of scatter reduction methods for DR using a range of grids and air gaps were also reviewed. The performance of various scatter reduction methods: 17/70; 15/80; 8/40 Pb grids and 15 cm and 20 cm air gaps were evaluated in terms of the improvement in CNR they afford, using two different models. The first, simpler model assumed quantum noise only and a photon counting detector. The second model incorporated quantum noise and system noise for a specific CsI detector and assumed the detector was energy integrating. Both models allowed the same general conclusions and suggest improved performance for air gaps over grids for medium to low scatter factors and both models suggest the best choice of grid for digital systems is the 15/80 grid, achieving comparable or better performance than air gaps for high scatter factors. The development, analysis and discussion of AEC calibration, CNR value, phosphor energy response, and scatter reduction methods are then brought together to form a practical step by step recipe that may be followed to optimise digital technology for clinical use. Finally, CNR results suggest the addition of 0.2 mm of copper filtration will have a negligible effect on image quality in DR. A comprehensive study examining the effect of copper filtration on image quality was performed using receiver operator characteristic (ROC) methodology to include observer performance in the analysis. A total of 3,600 observations from 80 radiographs and 3 observers were analysed to provide a confidence interval of 95% in detecting differences in image quality. There was no statistical difference found when 0.2 mm copper filtration was used and the benefit of the dose saving promote it as a valuable optimisation tool

A novel method for comparing radiation dose and image quality, between and within different X-ray units in a series of hospitals

Objectives: To develop a novel method for comparing radiation dose and image quality (IQ) to evaluate adult chest X-ray (CXR) imaging among several hospitals. Methods: CDRAD 2.0 phantom was used to acquire images in eight hospitals (17 digital X-ray units) using local adult CXR protocols. IQ was represented by image quality figure inverse (IQFinv), measured using CDRAD analyser software. Signal to noise ratio (SNR), contrast to noise ratio (CNR) and conspicuity index (CI) were calculated as additional measures of IQ. Incident air kerma (IAK) was calculated using a solid-state dosimeter for each acquisition. Figure of merit (FOM) was calculated to provide a single estimation of IQ and radiation dose. Results: IQ, radiation dose and FOM varied considerably between hospitals and X-ray units. For IQFinv, the mean (range) between and within the hospitals were 1.42 (0.83-2.18) and 1.87 (1.52-2.18), respectively. For IAK, the mean (range) between and within the hospitals were 93.56 (17.26 to 239.15) µGy and 180.85 (122.58-239.15) µGy, respectively. For FOM, the mean (range) between and within hospitals were 0.05 (0.01 to 0.14) and 0.03 (0.02-0.05), respectively. Conclusions: The suggested method for comparing IQ and dose using FOM concept along with the new proposed FOM, is a valid, reliable and effective approach for monitoring and comparing IQ and dose between and within hospitals. It is also can be beneficial for the optimisation of X-ray units in clinical practice. Further optimisation for the hospitals /X-ray units with low FOM are required to minimise radiation dose without degrading IQ

Effect of technical parameters on dose and image quality in a computed radiography system

The discovery of X-rays was undoubtedly one of the greatest stimulus for improving the efficiency in the provision of healthcare services. The ability to view, non-invasively, inside the human body has greatly facilitated the work of professionals in diagnosis of diseases. The exclusive focus on image quality (IQ), without understanding how they are obtained, affect negatively the efficiency in diagnostic radiology. The equilibrium between the benefits and the risks are often forgotten. It is necessary to adopt optimization strategies to maximize the benefits (image quality) and minimize risk (dose to the patient) in radiological facilities. In radiology, the implementation of optimization strategies involves an understanding of images acquisition process. When a radiographer adopts a certain value of a parameter (tube potential [kVp], tube current-exposure time product [mAs] or additional filtration), it is essential to know its meaning and impact of their variation in dose and image quality. Without this, any optimization strategy will be a failure. Worldwide, data show that use of x-rays has been increasingly frequent. In Cabo Verde, we note an effort by healthcare institutions (e.g. Ministry of Health) in equipping radiological facilities and the recent installation of a telemedicine system requires purchase of new radiological equipment. In addition, the transition from screen-films to digital systems is characterized by a raise in patient exposure. Given that this transition is slower in less developed countries, as is the case of Cabo Verde, the need to adopt optimization strategies becomes increasingly necessary. This study was conducted as an attempt to answer that need. Although this work is about objective evaluation of image quality, and in medical practice the evaluation is usually subjective (visual evaluation of images by radiographer / radiologist), studies reported a correlation between these two types of evaluation (objective and subjective) [5-7] which accredits for conducting such studies. The purpose of this study is to evaluate the effect of exposure parameters (kVp and mAs) when using additional Cooper (Cu) filtration in dose and image quality in a Computed Radiography system

Digital chest radiography: an update on modern technology, dose containment and control of image quality

The introduction of digital radiography not only has revolutionized communication between radiologists and clinicians, but also has improved image quality and allowed for further reduction of patient exposure. However, digital radiography also poses risks, such as unnoticed increases in patient dose and suboptimum image processing that may lead to suppression of diagnostic information. Advanced processing techniques, such as temporal subtraction, dual-energy subtraction and computer-aided detection (CAD) will play an increasing role in the future and are all targeted to decrease the influence of distracting anatomic background structures and to ease the detection of focal and subtle lesions. This review summarizes the most recent technical developments with regard to new detector techniques, options for dose reduction and optimized image processing. It explains the meaning of the exposure indicator or the dose reference level as tools for the radiologist to control the dose. It also provides an overview over the multitude of studies conducted in recent years to evaluate the options of these new developments to realize the principle of ALARA. The focus of the review is hereby on adult applications, the relationship between dose and image quality and the differences between the various detector systems

Comparative analysis of radiation dose and low contrast detail detectability using routine paediatric chest radiography protocols

Objectives: To compare low contrast detail (LCD) detectability and radiation dose for routine paediatric chest X-ray (CXR) imaging protocols among various hospitals. Methods: CDRAD 2.0 phantom and medical grade polymethyl methacrylate (PMMA) slabs were used to simulate the chest region of four different paediatric age groups. Radiographic acquisitions were undertaken on 17 X-ray machines located in eight hospitals using their existing CXR protocols. LCD detectability represented by image quality figure inverse (IQFinv) was measured physically using the CDRAD analyser software. Incident air kerma (IAK) measurements were obtained using a solid-state dosimeter. Results: The range of IQFinv, between and within the hospitals, was 1.40-4.44 and 1.52-2.18, respectively for neonates; 0.96-4.73 and 2.33-4.73 for a 1-year old; 0.87-1.81 and 0.98-1.46 for a 5-year old and 0.90-2.39 and 1.27-2.39 for a 10-year old. The range of IAK, between and within the hospitals, was 8.56-52.62 µGy and 21.79-52.62 µGy, respectively for neonates; 5.44-82.82 µGy and 36.78-82.82 µGy for a 1-year old; 10.97-59.22 µGy and 11.75-52.94 µGy for a 5-year old and 13.97-100.77 µGy and 35.72-100.77 µGy for a 10-year old. Conclusions: Results show considerable variation, between and within hospitals, in the LCD detectability and IAK. Further radiation dose optimisation for the four paediatric age groups, especially in hospitals /X-ray rooms with low LCD detectability and high IAK, are required. Keywords: Paediatric chest radiography, CDRAD phantom, low contrast detail detectability and radiation dose

Impact of body part thickness on AP pelvis radiographic image quality and effective dose

Introduction: Within medical imaging variations in patient size can generate challenges, especially when selecting appropriate acquisition parameters. This experiment sought to evaluate the impact of increasing body part thickness on image quality (IQ) and effective dose (E) and identify optimum exposure parameters. Methods: An anthropomorphic pelvis phantom was imaged with additional layers (1 to 15 cm) of animal fat as a proxy for increasing body thickness. Acquisitions used the automatic exposure control (AEC), 100 cm source to image distance (SID) and a range of tube potentials (70 to 110 kVp). IQ was evaluated physically and perceptually. E was estimated using PCXMC software. Results: For all tube potentials, signal to noise ratio (SNR) and contrast to noise ratio (CNR) deceased as body part thickness increased. 70 kVp produced the highest SNR (46.6 to 22.6); CNR (42.8 to 17.6). Visual grading showed that the highest IQ scores were achieved using 70 and 75 kVp. As thickness increases, E increased exponentially (r=0.96; p<0.001). Correlations were found between visual and physical IQ (SNR r= 0.97, p<0.001; CNR r=0.98, p<0.001). Conclusion: To achieve an optimal IQ across the range of thicknesses, lower kVp settings were most effective. This is at variance with professional practice as there is a tendency for radiographers to increase kVp as thickness increases. Dose reductions were experienced at higher kVp settings and are a valid method for optimisation when imaging larger patients

Optimisation of paediatrics computed radiography for full spine curvature measurements using a phantom: a pilot study

Aim: Optimise a set of exposure factors, with the lowest effective dose, to delineate spinal curvature with the modified Cobb method in a full spine using computed radiography (CR) for a 5-year-old paediatric anthropomorphic phantom. Methods: Images were acquired by varying a set of parameters: positions (antero-posterior (AP), posteroanterior (PA) and lateral), kilo-voltage peak (kVp) (66-90), source-to-image distance (SID) (150 to 200cm), broad focus and the use of a grid (grid in/out) to analyse the impact on E and image quality (IQ). IQ was analysed applying two approaches: objective [contrast-to-noise-ratio/(CNR] and perceptual, using 5 observers. Monte-Carlo modelling was used for dose estimation. Cohen’s Kappa coefficient was used to calculate inter-observer-variability. The angle was measured using Cobb’s method on lateral projections under different imaging conditions. Results: PA promoted the lowest effective dose (0.013 mSv) compared to AP (0.048 mSv) and lateral (0.025 mSv). The exposure parameters that allowed lower dose were 200cm SID, 90 kVp, broad focus and grid out for paediatrics using an Agfa CR system. Thirty-seven images were assessed for IQ and thirty-two were classified adequate. Cobb angle measurements varied between 16°±2.9 and 19.9°±0.9. Conclusion: Cobb angle measurements can be performed using the lowest dose with a low contrast-tonoise ratio. The variation on measurements for this was ±2.9° and this is within the range of acceptable clinical error without impact on clinical diagnosis. Further work is recommended on improvement to the sample size and a more robust perceptual IQ assessment protocol for observers

Feasibility of using Lodox to perform digital subtraction angiography

Bibliography: leaves 150-157.Many cases in trauma involve vessel imaging to determine integrity and the origin of lesions or blockages. Digital subtraction angiography (DSA) is a tool used to improve the clarity of the vessels being imaged for better and easier decision making in diagnostics and planning. Lodox, a low dose x-ray system developed by Debex (Pty) Ltd, a subsidiary of de Beers, was designed specifically for the trauma environment. It therefore follows that, if possible, a function so readily used in trauma, such as DSA, should be added to the imaging repertoire of an x-ray system designed for use in this environment. In this dissertation the feasibility of using Lodox to perform DSA is therefore explored. In doing so, the requirements of a trauma unit and the theory behind DSA were researched so as to obtain a better understanding into what would be required

Optimising image quality for medical imaging

OPTIMAX 2016 was held at the University of Salford in Greater Manchester. It is the fourth summer school of OPTIMAX with other renditions having been organized at the University of Salford (2013), ESTeSL, Lisbon (2014) and Hanze UAS, Groningen (2015). For OPTIMAX 2016, 72 people participated from eleven countries, comprising PhD, MSc and BSc students as well as tutors from the seven European partner universities. Professional mix was drawn from engineering, medical physics/ physics and radiography. OPTIMAX 2016 was partly funded by the partner universities and partly by the participants. Two students from South Africa and two from Brazil were invited by Hanze UAS (Groningen) and ESTeSL (Lisbon). One student from the United Kingdom was funded by the Nuffield Foundation. The summer school included lectures and group projects in which experimental research was conducted in five teams. Each team project focus varied and included: optimization of full spine curvature radiography in paediatrics; ultrasound assessment of muscle thickness and muscle cross-sectional area: a reliability study; the Influence of Source-to-Image Distance on Effective Dose and Image Quality for Mobile Chest X-rays; Impact of the anode heel effect on image quality and effective dose for AP Pelvis: A pilot study; and the impact of pitch values on Image Quality and radiation dose in an abdominal adult phantom using CT. OPTIMAX 2016 culminated in a poster session and a conference, in which the research teams presented their posters and oral presentations. This book comprises of two sections, the first four chapters concern generic background information which has value to summer school organization and also theory on which the research projects were built. The second section contains the research papers in written format. The research papers have been accepted for the ECR conference, Vienna, 2017 as either oral presentations or posters