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

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

Comparative linear accuracy and reliability of cone beam CT derived 2-dimensional and 3-dimensional images constructed using an orthodontic volumetric rendering program.

The purpose of this project was to compare the accuracy and reliability of linear measurements made on 2D projections and 3D reconstructions using Dolphin 3D software (Chatsworth, CA) as compared to direct measurements made on human skulls. The linear dimensions between 6 bilateral and 8 mid-sagittal anatomical landmarks on 23 dentate dry human skulls were measured three times by multiple observers using a digital caliper to provide twenty orthodontic linear measurements. The skulls were stabilized and imaged via PSP digital cephalometry as well as CBCT. The PSP cephalograms were imported into Dolphin (Chatsworth, CA, USA) and the 3D volumetric data set was imported into Dolphin 3D (Version 2.3, Chatsworth, CA, USA). Using Dolphin 3D, planar cephalograms as well as 3D volumetric surface reconstructions were (3D CBCT) generated. The linear measurements between landmarks of each three modalities were then computed by a single observer three times. For 2D measurements, a one way ANOVA for each measurement dimension was calculated as well as a post hoc Scheffe multiple comparison test with the anatomic distance as the control group. 3D measurements were compared to anatomic truth using Student\u27s t test (PiÜ50.05). The intraclass correlation coefficient (ICC) and absolute linear and percentage error were determined as indices of intraobserver reliability. Our results show that for 2D mid sagittal measurements that Simulated LC images are accurate and similar to those from PSP images (except for Ba-Na), and for bilateral measurements simulated LC measurements were similar to PSP but less accurate, underestimating dimensions by between 4.7% to 17%.For 3D volumetric renderings, 2/3 rd of CBCT measurements are statistically different from actual measurements, however this possibly is not clinically relevant

Relative Merits of 3D Visualization for the Detection of Subtle Lung Nodules

A new imaging modality called bi-plane correlation imaging (BCI) was examined to determine the merits of using BCI with stereoscopic visualization to detect subtle lung nodules. In the first aim of this project, the optimal geometry for conventional projection imaging applications was assessed using a theoretical model to develop generic results for MTF, NNPS, eDQE. The theoretical model was tested with a clinical system using two magnifications and two anthropomorphic chest phantoms to assess the modalities of single view CXR and stereo/BCI. Results indicated that magnification can potentially improve the signal and noise performance of digital images. Results also demonstrated that a cross over point occurs in the spatial frequency above and below which the effects of magnification differ indicating that there are task dependent tradeoffs associated with magnification. Results indicated that magnification can potentially improve the detection performance primarily due to the air gap which reduced scatter by 30-40%. For both anthropomorphic phantoms, at iso-dose, eDQE(0) for stereo/BCI was ~100 times higher than that for CXR. Magnification at iso-dose improved eDQE(0) by ~10 times for BCI. Increasing the dose did not improve results. The findings indicated that stereo/BCI with magnification may improve detection of subtle lung nodules compared to single view CXR. With quantitative results in place, a pilot clinical trial was constructed. Human subject data was acquired with a BCI acquisition system. Subjects were imaged in the PA position as well as two oblique angles. Realistic simulated lesions were added to a subset of subjects determined to be nodule free. A BCI CAD algorithm was also applied. In randomized readings, radiologists read the cases according to viewing protocol. For the radiologist trainees, the AUC of lesion detection was seen to improve by 2.8% (p < 0.05) for stereoscopic viewing after monoscopic viewing compared to monoscopic viewing only. A 13% decrease in false positives was observed. Stereo/BCI as an adjunct modality was beneficial. However, the full potential of stereo/BCI as a replacement modality for single view chest x-ray may be realized with improved observer training, clinically relevant stereoscopic displays, and more challenging detection tasks.Doctor of Philosoph

Comparative linear accuracy of cone beam CT derived 3D images in orthodontic analysis.

Objective . To compare the in vitro reliability and accuracy of linear measurements between cephalometric landmarks on CBCT 3D images with varying basis projection images to direct measurements on human skulls. Methods . Sixteen linear dimensions between anatomical sites marked on 19 human skulls were directly measured. Skulls were imaged with CBCT at three settings: 153, 306, and 612 basis projections. The mean absolute error and modality mean of linear measurements between landmarks on 3D images were compared to the anatomic truth. Results . No difference in mean absolute error between the scan settings was found. The average skull absolute error between marked reference points were less than the distances between unmarked reference sites. Conclusion . CBCT measurements were consistent between scan sequences and for direct measurements between marked reference points. Reducing the number of projections for 3D reconstruction did not lead to reduced dimensional accuracy and potentially provides reduced patient radiation exposure

Optically Active Rare-Earth Doped Films Synthesized by Pulsed Laser Deposition for Biomedical Applications

Optically active materials are used in many biomedical applications ranging from medical imaging to light therapies. Investigating the effects of differing nanostructure configurations on the optical performance of these materials can improve tunability, efficiency, and practicality for their respective applications. This work utilizes pulsed laser deposition (PLD) to develop nanostructured thin films and determines their optical performance for applications in computed radiography for medical imaging and in LEDs which can be used in biomedical applications such as photobiomodulation. In computed radiography, scattering of the stimulation light by the storage phosphor crystal grain boundaries in imaging plates negatively impacts spatial resolution. Storage phosphor plates with thinner phosphor layers have been developed to reduce scattering distance and increase spatial resolution, although at the expense of reduced x-ray absorption. A transparent or translucent nanostructured film, containing a much higher percentage of storage phosphor crystals achievable in bulk glass-ceramic materials made by conventional methods, may have acceptable photostimulated luminescence efficiency and imaging performance characteristics greater than commercial imaging plates. In an attempt to achieve a nanostructured film with superior performance in x-ray imaging, a glass-ceramic imaging plate for computed radiography was synthesized via PLD for the first time. The imaging plate was comprised of Eu-doped BaCl2 crystallites and an amorphous matrix. Nanolayered films comprising of BaF2, Eu2O3, and Al2O3 were synthesized via PLD with differing layered configurations to manipulate the coordinate surrounds of the europium dopant and determine its effects on optical properties. TEM cross-section analysis was conducted to verify the desired nano-layering. Different post-deposition heat treatments were investigated, and the films were evaluated for applications as a phosphor layer for UV-pumped white light LEDs which can be used for solid-state lighting and biomedical light therapies. A Mn dopant was added to europium to discover the threshold for the amount of manganese necessary to optically influence the nanolayered films. Although Mn/Eu co-doping did not prove advantageous for white light LEDs, all nanostructures of Eu-doped films have the potential for the desired application. Nanoscale control of optically-active thin films was demonstrated using pulsed laser deposition. Determining the effects of differing nanostructures on optical properties can lead to improvements in certain biomedical applications

A comparative study evaluating the performance of diagnostic radiography units and protocols for paediatric and adult chest radiography examinations

Purpose: Little is known about the variations in image quality (IQ) and radiation dose for paediatric and adult chest radiography (CXR), between and within hospitals. Large variations in IQ could influence the diagnostic accuracy, and variations in radiation dose could affect the risk to patients. This thesis aims to develop, validate and then use a novel method for comparing IQ and radiation dose for paediatric and adult CXR imaging examinations and report variation between a series of public hospitals. Method: A Figure of Merit (FOM) concept was used for the purposes of comparing IQ and radiation dose, between and within hospitals. Low contrast detail (LCD) detectability, using the CDRAD 2.0 phantom, was utilised as the main method for IQ evaluation. The validity of utilising LCD detectability, using CDRAD 2.0 phantom, for evaluating visual IQ, simulated lesion visibility (LV) and CXR optimisation studies, was investigated. This was done by determining the correlation between the LCD detectability and visual measures of IQ and LV for two lesions with different locations and visibility in the Lungman chest phantom.The CDRAD 2.0 phantom and two anthropomorphic phantoms (adult Lungman and the neonatal Gammex phantom) were used to simulate the chest region. Radiographic acquisitions were conducted on 17 X-ray units located in eight United Kingdom (UK) public hospitals within the North-west of England using their existing CXR protocols. The CDRAD 2.0 phantom was combined with different thicknesses of Polymethyl methacrylate (PMMA) slabs to simulate the chest regions of 5 different age groups: neonate, 1, 5, 10 years and adults. A Lungman phantom, with and without the fat jacket, was used to simulate average and larger sized patients. IQ was evaluated using a number of methods, including: 1) physically, by calculating LCD detectability as represented by an image quality figure inverse (IQFinv) using the CDRAD analyser software; 2) using images acquired from the anthropomorphic phantoms – for this, a relative visual grading analysis (VGA) method was used. Additionally, signal to noise ratios (SNR), contrast to noise ratios (CNR) and conspicuity indices (CI) were calculated for all phantom image data in this study. Incident air karma (IAK) was measured using a solid-state dosimeter. Results: Regarding the validation of utilising LCD detectability for evaluating visual IQ and LV, and CXR optimisation studies, a strong positive correlation (r = 0.91; p < 0.001) was observed between IQFinv and the visual IQ scores from the Lungman phantom. A good correlation was observed between IQFinv and visual LV from the Lungman phantom for both lesions (lesion 1 (with low visibility) (r = 0.79; p < 0.001); lesion 2 (with high visibility) (r =0.68; p < 0.001), respectively). Considerable variation in standard imaging protocols/techniques, radiation dose, IQ and FOM were observed between the hospitals, while within hospital variation was lower. A weak correlation between IQ and radiation dose was observed across most of the age groups studied. Conclusion: A novel method has been established to evaluate and compare IQ and radiation dose between and within hospitals based on an FOM concept. This combines IQ and radiation dose into a single factor and is the first of its kind to reported within the field of medical imaging. It can be confirmed that LCD detectability using the CDRAD 2.0 phantom is valid for evaluating visual IQ and LV and can be of use within routine quality assurance and optimisation studies in digital radiography. Further radiation dose optimisation for the paediatric age groups and adult group, especially in hospitals /X-ray machines with low IQ and high IAK, are required

Comparative TMJ imaging accuracy using iCAT cone beam computerized tomography.

A blinded observational cross-sectional in vitro study was conducted to compare the diagnostic accuracy of observers viewing images made using cone beam computerized tomography (CBCT), panoramic radiography and linear tomography. The sample consisted of 37 TMJ articulations from 30 human skulls demonstrating either normal condylar morphology (n=19) or erosion of the lateral pole (n=18). The articulations were imaged using corrected angle linear tomography, normal and TMJ specific panoramic radiography and CBCT. Images and 10 re-reads were presented to 10 observers. Multiple CBCT multi-planar images were presented both statically and interactively. Intra-observer reliability was determined by weighted kappa (Kw) and diagnostic accuracy by the fitted area under the ROC curve (Az). Means were compared using ANOVA (piÜ.05). Our results show CBCT images provide superior reliability and greater accuracy than corrected angle linear tomography and TMJ panoramic projections in the detection of condylar cortical erosion

Portal Imaging Using a CSI (TL) Scintillator Coupled to a Cooled CCD Camera

The purpose of this research was to design a high performance digital portal imaging system, using a transparent x-ray scintillator coupled to a cooled CCD camera. Theoretical analysis using Monte Carlo simulation was performed to calculate the QDE, SNR and DQE of the system. A prototype electronic portal imaging device (EPID) was built, using a 12.7 mm thick, 20.32 cm diameter, CsI (Tl) scintillator, coupled to an Astromed ® liquid nitrogen cooled CCD TV camera. The system geometry of the prototype EPID was optimized to achieve high spatial resolution. Experimental evaluation of the prototype EPID was performed, by determining its spatial resolution, contrast resolution, depth of focus and light scatter. Images of phantoms, animals and human subjects were acquired using the prototype EPID and were compared with those obtained using conventional and high contrast portal film and a commercial EPID. An image processing protocol was developed. The protocol was comprised of preprocessing, noise removal and image enhancement algorithms. An adaptive median filter algorithm for the removal of impulse noise was developed, analyzed and incorporated into the image processing protocol. Results from the theoretical analysis and experimental evaluation have indicated that the performance of the CsI (Tl) - CCD system is comparable or superior to that of current commercial and experimental portal imaging technologies, such as high contrast portal film, commercial TV camera based EPIDs, and amorphous silicon based flat panel EPIDs