A review of image quality and dose issues in digital fluorography and digital subtraction angiography

This paper addresses image quality and dose issues in Digital Fluorography (DF) and Digital Subtraction Angiography (DSA), commencing with a brief review of methods and protocols for image quality assessment in DF and DSA. The relative scarcity of standards and test protocols for unsubtracted DF is highlighted. Pooled results from a large number of quality assurance tests are then used to illustrate trends in the choice of radiation dose per image currently employed in typical DF and DSA work. It is concluded that, although relatively high doses per image may be justified in terms of image quality improvement for DSA, there is little justification for the large range of exposures used in DF for nominally identical examinations. It is argued that the use of high doses per image in DF would not be expected to offer an advantage in terms of image signal-to-noise ratio.     

Method of simulating dose reduction for digital radiographic systems

The optimisation of image quality vs. radiation dose is an important task in medical imaging. To obtain maximum validity of the optimisation, it must be based on clinical images. Images at different dose levels can then either be obtained by collecting patient images at the different dose levels sought to investigate-including additional exposures and permission from an ethical committee-or by manipulating images to simulate different dose levels. The aim of the present work was to develop a method of simulating dose reduction for digital radiographic systems. The method uses information about the detective quantum efficiency and noise power spectrum at the original and simulated dose levels to create an image containing filtered noise. When added to the original image this results in an image with noise which, in terms of frequency content, agrees with the noise present in an image collected at the simulated dose level. To increase the validity, the method takes local dose variations in the original image into account. The method was tested on a computed radiography system and was shown to produce images with noise behaviour similar to that of images actually collected at the simulated dose levels. The method can, therefore, be used to modify an image collected at one dose level so that it simulates an image of the same object collected at any lower dose level.     

Patient dose in digital projection radiography

In projection radiography, two types of digital imaging systems are currently available, computed radiography (CR) and direct radiography (DR): a difference between them can be stated in terms of dose and image quality. In the Radiology Emergency Department of our hospital, a flat-panel DR equipment (Siemens Axiom Aristos FX) and two CR systems (Kodak CR-850) are employed. In 2006, five standard radiographic examinations (abdomen, chest, lumbar spine, pelvis, skull) were considered: doses delivered to patients in terms of both entrance skin dose (ESD) and effective dose (E) were calculated and compared in order to study the dosimetric discrepancies between CR and DR. Assessment of image quality is undertaken by Consultant Radiologists to ensure that the quality criteria for diagnostic radiographic images of the European guidelines were met. Results showed that both ESD and E in DR are lower than that in CR; all images met the criteria in the European Guidelines for both modalities and were used for reporting by the radiologists. Since the operators are the same and the image quality is comparable in both modalities, this study shows that in the considered examinations, DR can perform better than CR from a dosimetric point of view.     

Optimisation of dose per image in digital imaging

Three current digital radiography modalities are briefly described: digital fluorography, storage phosphor radiography and amorphous selenium radiography. For all of these modalities, the dose used to form an image can be varied considerably. Threshold contrast detail techniques were used to investigate the consequence of changing dose per image at the image receptor. At low doses, contrast resolution is limited by X ray quantum noise while system noise limits contrast resolution at high dose per image values.     

Border processing in digital holography by extension of the digital hologram and reduction of the higher spatial frequencies

When a digital holographic reconstruction is performed, digital diffraction effects occur at the borders when the hologram amplitudes at the two opposite border points are different on each vertical or horizontal line. We propose a method of digital hologram extension to reduce such diffraction effects. The method consists of extending the size of the digital hologram and of filling the extended part by complex values that minimize, according to a numerical criterion, the highest spatial frequencies. The theoretical aspects of the method are given and the results from a demonstration are provided.     

Nodule detection in digital chest radiography: introduction to the RADIUS chest trial

Most digital radiographic systems of today have wide latitude and are hence able to provide images with a small constraint on dose level. This opens up for an unprejudiced dose optimisation. However, in order to succeed in the optimisation task, good knowledge of the imaging and detection processes is needed. As a part of the European-wide research project 'unification of physical and clinical requirements for medical X-ray imaging'-governed by the Radiological Imaging Unification Strategies (RADIUS) Group-a major image quality trial was conducted by members of the group. The RADIUS chest trial was focused on the detection of lung nodules in digital chest radiography with the aims of determining to what extent (1) the detection of a nodule is dependent on its location, (2) the system noise disturbs the detection of lung nodules, (3) the anatomical noise disturbs the detection of lung nodules and (4) the image background and anatomical background act as pure noise for the detection of lung nodules. The purpose of the present paper is to give an introduction to the trial and describe the framework and set-up of the investigation.     

ICRP recommendations on 'Managing patient dose in digital radiology'

The International Commission on Radiological Protection (ICRP) approved the publication of a document on 'Managing patient dose in digital radiology' in 2003. The paper describes the content of the report and some of its key points, together with the formal recommendations of the Commission on this topic. With digital techniques exists not only the potential to improve the practice of radiology but also the risk to overuse radiation. The main advantages of digital imaging: wide dynamic range, post-processing, multiple viewing options, electronic transfer and archiving possibilities are clear but overexposures can occur without an adverse impact on image quality. It is expected that the ICRP report helps to profit from the benefits of this important technological advance in medical imaging with the best management of radiation doses to the patients. It is also expected to promote training actions before the digital techniques are introduced in the radiology departments and to foster the industry to offer enough technical and dosimetric information to radiologists, radiographers and medical physicists to help in the optimisation of the imaging.     

Planning of occupational dose reduction at BWR power plant by past dose record analysis combined with on-site workers' idea analysis

In order to establish a plan for occupational dose reduction at operating plants, outage inspection works that involve high-dose exposure were selected and a determination of the major causes of high-dose exposure made by plant-by-plant comparison of doses received during inspection works. The comparison was made to investigate the relationship between exposure and the volume of objects to be inspected, working time and man-hour of each work process and ambient dose rates at work areas. In parallel with this, an analysis has also been carried out on 400 data items in a questionnaire survey conducted on relevant individuals, including foremen, radiation safety personnel, on-site workers and plant designers regarding ideas for dose reduction methods. With combination of these two analyses, matters that require improvement will be highlighted, then modification of equipment or revision of work procedures necessary for occupational dose reduction will be planned by plant designers through review.     

银纳米颗粒的化学还原法制备简介及其应用

银纳米颗粒的光学性能,如局域表面等离子体共振(Localized Surface Plasmon Resonance,LSPR)特性可通过其形貌、尺寸、外部介电环境的调控而实现变化。不同形貌的银纳米颗粒具有强弱不同的局域表面等离子体共振效应,从而表现出独特的光学性质。综述了利用化学还原法制备不同形貌的银纳米颗粒,主要包括柠檬酸钠还原法、多元醇法以及种子介导生长法,分析了这3种合成方法的机理和特点,将近年来不同形貌银纳米颗粒的研究进展进行了综述。最后介绍了不同形貌银纳米颗粒在表面增强拉曼散射(Surface-Enhanced Roman scattering,SERS)基底、催化、抗菌领域上的应用研究,并总结和展望了银纳米颗粒在合成和相关应用领域的发展前景。     

Relevant training issues for introduction of digital radiology: results of a survey

Council Directive 97/43/Euratom establishes the need for adequate training of radiology staff. The transition to digital radiology implies changes in various imaging aspects, which are not sufficiently covered by current institutional training programmes. This work aimed to assess how professionals, experienced in digital imaging, acquired their expertise and hence, what form institutional training should take. Within DIMOND III, a survey on training and resources was performed among radiology professionals. A lack of institutional education for digital radiology was found. In the transition to digital radiology, 30-35% train on the job and another 23-28% receive training from digital equipment vendors. A general agreement exists on the need for new quality criteria and strategies for dose management. Issues relevant for conventional/digital transition are only sparsely covered in EC training programmes. Based on these results, a set of training issues was produced, to be included in future European guidelines.     

Phantom studies for possible dose reduction in CT head procedures

This paper presents the results of phantom studies to investigate possible dose reduction in relation to image quality in head examinations. The studies were performed using five single-slice computed tomography (CT) scanners. Beginning from the manufacturer's protocols (i.e. default protocols in the scanner software) for routine head (adult) examinations, the values of kV(p), anode current and time were modified. Low-contrast resolution and spatial resolution were controlled using a Catphan 424 phantom. Radiation dose was checked using a polymethylmethacrylate phantom and a pencil ionisation chamber. It was found that CT dose index may be reduced up to three times with practically no loss of image quality.     

Quantization noise and its reduction in lensless Fourier digital holography

Digital holography is an imaging technique that enables recovery of topographic 3D information about an object under investigation. In digital holography, an interference pattern is recorded on a digital camera. Therefore, quantization of the recorded hologram is an integral part of the imaging process. We study the influence of quantization error in the recorded holograms on the fidelity of both the intensity and phase of the reconstructed image. We limit our analysis to the case of lensless Fourier off-axis digital holograms. We derive a theoretical model to predict the effect of quantization noise and we validate this model using experimental results. Based on this, we also show how the resultant noise in the reconstructed image, as well as the speckle that is inherent in digital holography, can be conveniently suppressed by standard speckle reduction techniques. We show that high-quality images can be obtained from binary holograms when speckle reduction is performed.     

Patient dose reduction in CT examinations by optimising scanogram acquisition

The objective was to determine the optimum settings of the scanogram performed in computed tomography (CT) examinations for scan localisation. Head, abdomen and thorax scanograms were performed on a Rando anthropomorphic phantom using various selectable combinations of tube voltage and tube current values. Thermoluminescence dosemeters were used to obtain entrance skin dose data. Effective dose was estimated using normalised organ dose data provided by the National Radiological Protection Board. One hundred and twelve head, 114 thoracic and 111 abdominal patient scanograms were obtained with lower settings than those recommended by the operator's manual. Scanogram sufficiency was assessed by four observers. Head and thoracic scanograms obtained with 80 kV/50 mA and abdominal scanograms obtained with 80 kV/75 mA were found to be acceptable, even though the operator's manual recommendation was 120 kV/100 mA. Thus, the scanogram effective dose was reduced by 72%, 84% and 88% for head, thorax and abdomen examination respectively. Effective dose from a complete CT examination may be reduced by up to 3.5% without any subsequent image quality degradation of the diagnostically important sectional images.     

一种数字前馈式有源降噪耳罩系统研究

传统耳罩利用隔声原理降低噪声,一般在高频段有着良好的降噪性能,但由于耳罩体积和重量的限制,针对700 Hz以下低频段噪声的降噪性能较差。引入基于滤波-X最小均方(Filter-X Least Mean Square,FXLMS)算法的前馈式有源降噪技术,将有效克服该缺点,使耳罩在低频范围取得良好的降噪性能。实验结果与分析表明,设计的基于STM32控制器的数字前馈式有源降噪耳罩对1 000 Hz以下窄带噪声有15~25 d B的降噪效果。     

Speckle reduction in optical coherence tomography images using digital filtering

Speckle noise is a ubiquitous artifact that limits the interpretation of optical coherence tomography images. Here we apply various speckle-reduction digital filters to optical coherence tomography images and compare their performance. Our results indicate that shift-invariant, nonorthogonal wavelet-transform-based filters together with enhanced Lee and adaptive Wiener filters can significantly reduce speckle and increase the signal-to-noise ratio, while preserving strong edges. The speckle reduction capabilities of these filters are also compared with speckle reduction from incoherent angular compounding. Our results suggest that by using these digital filters, the number of individual angles required to attain a certain level of speckle reduction can be decreased.     

Varied tube potential with constant effective dose at lumbar spine radiography using a flat-panel digital detector

The purpose of the study was to evaluate the image quality at different tube potential (kV) settings using anteroposterior lumbar spine radiography as a model. An Alderson phantom was used with a flat-panel detector. The tube potential varied between 48 and 125 kV while the tube charge (mAs) was adjusted to keep an effective dose of 0.11 mSv. Image quality was assessed with a visual grading analysis and with a CDRAD contrast-detail phantom together with a computer program. The VGA showed inferior image quality for the higher kV settings, > or =96 kVwith similar results for the contrast-detail phantom. When keeping the effective dose fixed, it seems beneficial to reduce kV to get the best image quality despite the fact that the mAs is not as high as with automatic exposure. However, this cannot be done with automatic exposure, which is set for a constant detector dose.     

Retrospective patient dose analysis of a digital radiography system in routine clinical use

The log files generated in the flat-panel detector based digital radiography systems (Philips Digital Diagnost) at 11 different hospitals were used to acquire data regarding the dose-area product (DAP) and the entrance air kerma (EAK) for all X-ray examinations performed in routine clinical use. A retrospective statistical analysis of the data with respect to the dose levels was performed for several examination types. The resulting average dose levels were compared with the diagnostic reference levels (DRLs). While for the same type of examination the median dose level at different hospitals could differ by up to a factor of 5, almost all the dose levels were found to be much below the published DRL. Only in one hospital for one type of examination (Thorax PA) the DRL for the DAP was slightly exceeded, while for the same site and examination the EAK was still below the DRL. In conclusion, examination log files can be used for retrospective dose analysis and correlation with DRLs.     

The quality assurance and constancy checking of fluoroscopy and fluorography systems

X ray procedures which involve fluoroscopy often give significant radiation doses to patients. A valuable radiation protection procedure is the adoption of a quality assurance programme which will ensure the continual production of optimal quality images with the minimum necessary dose to the patient. Various publications exist which provide information on quality assurance (QA) and constancy checking of fluoroscopy and fluorography systems. These publications were reviewed and tests recommended for QA and constancy checking of fluoroscopy and fluorography systems are presented. It was found that not much information exists on QA and constancy checking for digital subtraction angiography and digital fluorography systems. More research is required in this field. Information on protocols used by various countries of the European Community for QA and constancy checking of fluoroscopy and fluorography systems was gathered. From this information it is apparent that there is a need for harmonisation within Europe.     

Integrated analysis and design of analog and digital processing in imaging systems: introduction to the feature issue

This feature presents themes arising from the Integrated Computational Imaging Systems topical meeting sponsored by the OSA in fall 2001. These themes revolve around joint design of digital and analog image processing to improve the specificity and efficiency of optical image sensors.