Overview of two years of clinical experience of chest tomosynthesis at Sahlgrenska University Hospital.

Since December 2006, approximately 3800 clinical chest tomosynthesis examinations have been performed at our department at Sahlgrenska University Hospital. A subset of the examinations has been included in studies of the detectability of pulmonary nodules, using computed tomography (CT) as the gold standard. Visibility studies, in which chest tomosynthesis and CT have been compared side-by side, have been used to determine the depiction potential of chest tomosynthesis. Comparisons with conventional chest radiography have been made. In the clinical setting, chest tomosynthesis has mostly been used as an additional examination. The most frequent indication for chest tomosynthesis has been suspicion of a nodule or tumour. In visibility studies, tomosynthesis has depicted over 90 % of the nodules seen on the CT scan. The corresponding figure for chest radiography has been <30 %. In the detection studies, the lesion-level sensitivity has been approximately 60 % for tomosynthesis and 20 % for chest radiography. In one of the detection studies, an analysis of all false-positive nodules was performed. This analysis showed that all findings had morphological correlates on the CT examinations. The majority of the false-positive nodules were localised in the immediate subpleural region. In conclusion, chest tomosynthesis is an improved chest radiography method, which can be used to optimise the use of CT resources, thereby reducing the radiation dose to the patient population. However, there are some limitations with chest tomosynthesis. For example, patients undergoing tomosynthesis have to be able to stand still and hold their breath firmly for 10 s. Also, chest tomosynthesis has a limited depth resolution, which may explain why pathology in the subpleural region is more difficult to interpret and artefacts from medical devices may occur.

Effect of clinical experience of chest tomosynthesis on detection of pulmonary nodules.

BACKGROUND: The new technique chest tomosynthesis refers to the principle of collecting low-dose projections of the chest at different angles and using these projections to reconstruct section images of the chest at a radiation dose comparable to that of chest radiography. PURPOSE: To investigate if, for experienced thoracic radiologists, the detectability of pulmonary nodules obtained after only a short initial learning period of chest tomosynthesis improves with additional clinical experience of the new technique. MATERIAL AND METHODS: Two readings of the same clinical chest tomosynthesis cases, the first performed after 6 months of clinical experience and the second after an additional period of 1 year, were conducted. Three senior thoracic radiologists, with more than 20 years of experience of chest radiography, acted as observers, with the task of detecting pulmonary nodules in a jackknife free-response receiver operating characteristics (JAFROC1) study. The image material consisted of 42 patients with and 47 patients without pulmonary nodules examined with chest tomosynthesis. Multidetector computed tomography (MDCT) was used as a reference. The total number of nodules was 131. The JAFROC1 figure of merit (FOM) was used as the principal measure of detectability. RESULTS: The difference in the observer-averaged JAFROC1 FOM of the two readings was 0.004 (95% confidence interval: -0.11, 0.12; F-statistic: 0.01 on 1 and 2.65 df; P=0.91). Thus, no significant improvement in detectability was found after the additional clinical experience of tomosynthesis. CONCLUSION: The study indicates that experienced thoracic radiologists already within the first months of clinical use of chest tomosynthesis are able to take advantage of the new technique in the task of detecting pulmonary nodules.

Visual grading characteristics (VGC) analysis: a non-parametric rank-invariant statistical method for image quality evaluation.

Visual grading of the reproduction of important anatomical structures is often used to determine clinical image quality in radiography. However, many visual grading methods incorrectly use statistical methods that require data belonging to an interval scale. The rating data from the observers in a visual grading study with multiple ratings is ordinal, meaning that non-parametric rank-invariant statistical methods are required. This paper describes such a method for determining the difference in image quality between two modalities called visual grading characteristics (VGC) analysis. In a VGC study, the task of the observer is to rate his confidence about the fulfilment of image quality criteria. The rating data for the two modalities are then analysed in a manner similar to that used in receiver operating characteristics (ROC) analysis. The resulting measure of image quality is the VGC curve, which--for all possible thresholds of the observer for a fulfilled criterion--describes the relationship between the proportions of fulfilled image criteria for the two compared modalities. The area under the VGC curve is proposed as a single measure of the difference in image quality between two compared modalities. It is also described how VGC analysis can be applied to data from an absolute visual grading analysis study.

Can the average glandular dose in routine digital mammography screening be reduced? A pilot study using revised image quality criteria.

There is a need for tools that in a simple way can be used for the evaluation of image quality related to clinical requirements in mammography. The aim of this work was to adjust the present European image quality criteria to be relevant also for digital mammography images, and to use as simple and as few criteria as possible. A pilot evaluation of the new set of criteria was made with mammograms of 28 women from a General Electric Senographe 2000D full-field digital mammography system. One breast was exposed using the standard automatic exposure mode, the other using about half of that absorbed dose. Three experienced radiologists evaluated the images using visual grading analysis technique. The results indicate that the new quality criteria can be used for the evaluation of image quality related to clinical requirements in digital mammography in a simple way. The results also suggest that absorbed doses for the mammography system used may be substantially reduced.

Clinical evaluation of a new set of image quality criteria for mammography.

The European Commission (EC) quality criteria for screen-film mammography are used as a tool to assess image quality. A new set of criteria was developed and initially tested in a previous study. In the present study, these criteria are further evaluated using screen-film mammograms that have been digitised, manipulated to simulate different image quality levels and reprinted on film. Expert radiologists have evaluated these manipulated images using both the original (EC) and the new criteria. A comparison of three different simulated dose levels reveals that the new criteria yield a larger separation of image criteria scores than the old ones. These results indicate that the new set of image quality criteria has a higher discriminative power than the old set and thus seems to be more suitable for evaluation of image quality in mammography.

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.

Influence of the characteristic curve on the clinical image quality of lumbar spine and chest radiographs.

The "European Guidelines on Quality Criteria for Diagnostic Radiographic Images" do not address the choice of the film characteristic (H&D) curve, which is an important parameter for the description of a radiographic screen-film system. The image contrast of clinical lumbar spine and chest radiographs was altered by digital image processing techniques, simulating images with different H&D curves, both steeper and flatter than the original. The manipulated images were printed on film for evaluation. Seven experienced radiologists evaluated the clinical image quality by analysing the fulfilment of the European Image Criteria (ICS) and by visual grading analysis (VGA) of in total 224 lumbar spine and 360 chest images. A parallel study of the effect of the H&D curve has also been made using a theoretical model. The contrast (DeltaOD) of relevant anatomical details was calculated, using a Monte Carlo simulation-model of the complete imaging system including a 3D voxel phantom of a patient. Correlations between the calculated contrast and the radiologists' assessment by VGA were sought. The results of the radiologists' assessment show that the quality in selected regions of lumbar spine and chest images can be significantly improved by the use of films with a steeper H&D curve compared with the standard latitude film. Significant (p<0.05) correlations were found between the VGA results and the calculations of the contrast of transverse processes and trabecular details in the lumbar spine vertebrae, and with the contrast of blood vessels in the retrocardiac area of the chest.

The influence of different technique factors on image quality of chest radiographs as evaluated by modified CEC image quality criteria.

The Commission of the European Communities (CEC) research project "Predictivity and optimisation in medical radiation protection" addressed fundamental operational limitations in existing radiation protection mechanisms. The first part of the project aimed at investigating (1) whether the CEC image quality criteria could be used for optimization of a radiographic process and (2) whether significant differences in image quality based on these criteria could be detected in a controlled project with well known physical and technical parameters. In the present study, chest radiographs on film were produced using healthy volunteers. Four physical/technical parameters were varied in a carefully controlled manner: tube voltage (102 kVp and 141 kVp), nominal speed class (160 and 320), maximum film density (1.3 and 1.8) and method of scatter reduction (grid (R=12) and air gap). The air kerma at the entrance surface was measured for all patients and the risk-related dose H(Golem), based on calculated organ-equivalent dose conversion coefficients and the measured entrance air kerma values, was calculated. Image quality was evaluated by a group of European expert radiologists using a modified version of the CEC quality criteria. For the two density levels, density level 1.8 was significantly better than 1.3 but at the cost of a higher patient radiation exposure. The correlation between the number of fulfilled quality criteria and H(Golem) was generally poor. An air gap technique resulted in lower doses than scatter reduction with a grid but provided comparable image quality. The criteria can be used to highlight optimum radiographic technique in terms of image quality and patient dose, although not unambiguously. A recommendation for good radiographic technique based on a compromise between image quality and risk-related radiation dose to the patient is to use 141 kVp, an air gap, a screen-film system with speed 320 and an optical density of 1.8.

Demonstration of correlations between clinical and physical image quality measures in chest and lumbar spine screen-film radiography.

The ability to predict clinical image quality from physical measures is useful for optimization in diagnostic radiology. In this work, clinical and physical assessments of image quality are compared and correlations between the two are derived. Clinical assessment has been made by a group of expert radiologists who evaluated fulfillment of the European image criteria for chest and lumbar spine radiography using two scoring methods: image criteria score (ICS) and visual grading analysis score (VGAS). Physical image quality measures were calculated using a Monte Carlo simulation model of the complete imaging system. This model includes a voxelized male anatomy and was used to calculate contrast and signal-to-noise ratio of various important anatomical details and measures of dynamic range. Correlations between the physical image quality measures on the one hand and the ICS and VGAS on the other were sought. 16 chest and 4 lumbar spine imaging system configurations were compared in frontal projection. A statistically significant correlation with clinical image quality was found in chest posteroanterior radiography for the contrast of blood vessels in the retrocardiac area and a measure of useful dynamic range. In lumbar spine anteroposterior radiography, a similar significant correlation with clinical image quality was found between the contrast and signal-to-noise ratio of the trabecular structures in the L1-L5 vertebrae. The significant correlation shows that clinical image quality can, at least in some cases, be predicted from appropriate measures of physical image quality.

The influence of different technique factors on image quality of lumbar spine radiographs as evaluated by established CEC image criteria.

In this study we have investigated the image quality of lumbar spine radiographs taken after recording technical and physical parameters. Two technical parameters were altered, tube voltage (70 kV and 90 kV for the anteroposterior (AP) projection and 77 kV and 95 kV for the lateral projection) and sensitivity of the film-screen system (sensitivity class 400 and 600). In total, 85 images were included in the study. Entrance surface dose (ESD) was measured using thermoluminescent dosemeters. The mean value of ESD for the different technique groups varied between 1.9 mGy (90 kV, sensitivity class 400) and 4.6 mGy (70 kV, sensitivity class 400) for the AP projection, and between 6.4 mGy (95 kV, sensitivity class 600) and 20.4 mGy (70 kV, sensitivity class 400) for the lateral projection. Image criteria given in the "European Guidelines on Quality Criteria for Radiographic Images" were used to assess image quality. Two evaluation methods have been employed. A straightforward scoring of fulfilled image criteria, and visual grading analysis using the structures defined in the image criteria. The latter method provided a sharper distinction between groups of images taken using different radiographic techniques. The average number of fulfilled image criteria for the AP projections varied between 0.74 (90 kV, sensitivity class 400) and 0.87 (70 kV, sensitivity class 400). For the lateral projection this number varied between 0.79 (95 kV, sensitivity class 600) and 0.84 (77 kV, sensitivity class 600). This study shows that image criteria are useful tools in clinical studies of image quality.

Image quality for five modern chest radiography techniques: a modified FROC study with an anthropomorphic chest phantom.

The purpose of the study was to compare the image quality for one conventional and four digital chest radiography techniques. Three storage phosphor systems, one selenium drum system, and one film-screen system were compared using a modified receiver-operating-characteristics method. Simulated pathology was randomly positioned over the parenchymal regions and the mediastinum of an anthropomorphic phantom. Eight observers (four chest radiologists, one specialist in general radiology, one hospital physicist, and two radiographers) evaluated 60 images for each technique. The selenium drum system (Philips, Eindhoven, The Netherlands) rated best for the detection of parenchymal nodules. Together with the storage phosphor system of generation IIIN (Philips/Fuji), the selenium drum system also rated best for detection of thin linear structures. The storage phosphor system of generation V (Fuji) rated best for the detection of mediastinal nodules. The first generation of the storage phosphor system from Agfa (Mortsel, Belgium) rated worst for the detection of parenchymal nodules and thin linear structures. These differences were significant (p < 0.0001). Averaging the results for all test objects, the selenium drum system and the storage phosphor system of generation V were significantly better than the other systems tested. The film/screen system performed significantly better than the first-generation storage phosphor system from Agfa, equal to the generation IIIN storage phosphor system (Philips/Fuji) and significantly worse than the selenium drum system (Philips) and the generation-V storage phosphor system (Fuji). The conclusion is therefore that the image quality of selenium-based digital technique and of the more recent generations of storage phosphor systems is superior to both conventional technique and storage phosphor systems using image plates of older types.

Storage phosphor and film-screen mammography: performance with different mammographic techniques.

The aim of this study was to compare the image quality of storage phosphor plates with that in screen-film radiograms in mammography. Two anode/filter combinations were also compared--Mo/Mo and W/Rh. S Storage phosphor plates, generation IIIN (Fuji, Tokyo, Japan) and a conventional screen-film system (Kodak, Rochester, N. Y.) were evaluated using two mammographic units. One unit had a 0.6-mm focal spot, an anode/filter combination of Mo/Mo and no grid (AMo); the other had a 0.3-mm focal spot, a grid, and two possible combinations of anode/filter Mo/Mo (BMo) and W/Rh (BW). Simulated tumours and microcalcifications were randomly positioned in an anthropomorphic breast phantom (RMI model 165, no. 210-009, Radiation Measurements Inc., Middleton, Wisconsin). The image quality was evaluated using a modified version of receiver operating characteristics analysis. Five observers evaluated 300 films and 300 hard copy images each. Radiation doses were also determined. The image quality of the conventional screen-film images was significantly better than that for the storage phosphor plate mammograms. The BMo system rated best, for the detection of both tumours and microcalcifications, although it was not significantly different from the BW system. Systems BMo and BW rated significantly better than the AMo system for both image receptors studied. The mean absorbed dose was twice as high for the BMo system as for the AMo and BW systems for both conventional and digital technique. The mammograms produced with the screen-film combination gave a significantly better detectability than the storage phosphor plates used in this study. Substantial dose reduction could be achieved using an anode/filter combination of W/Rh instead of Mo/Mo with no significant loss of information in the images.

Influence of anode-filter combinations on image quality and radiation dose in 965 women undergoing mammography.

PURPOSE: To evaluate how anode-filter combinations influence image quality in and mean glandular dose to breasts of different thicknesses and compositions. MATERIALS AND METHODS: Mammograms were obtained with a molybdenum (Mo) anode and a Mo filter at 26 kVp, a Mo anode and a rhodium (Rh) filter at 27 kVp, or a tungsten (W) anode and a Rh filter at 26 kVp in 965 women. One anode-filter-tube voltage combination was used in the right breast and another in the left. The mean glandular dose to each breast was calculated. RESULTS: Image contrast was highest in the Mo-Mo mammograms. However, depiction of the glandular tissue, pectoral muscle, and skin and subcutis was significantly (P < .001) better with the Mo-Rh and the W-Rh than with the Mo-Mo combination. The average mean absorbed doses to the glandular tissue for W-Rh and Mo-Rh were 50% and 75%, respectively, of that for Mo-Mo. CONCLUSION: Breast thickness is the most important parameter in selection of an anode-filter-tube voltage combination. Compared with Mo-Mo, both Mo-Rh and W-Rh gave good image quality of the mammary gland and a considerably lower absorbed dose. Mo-Rh-27 kVp is recommended for breast thicknesses of 60 mm or less; W-Rh-26 kVp, for breast thicknesses of greater than 60 mm.

Digital chest radiography--comparing two types of CRT monitors in a ROC experiment with a chest phantom.

Two principally different 1000-line CRT (cathode ray tube) monitors connected to a digital image intensifier system for chest radiography were compared in terms of image quality and viewing comfort. Image quality was assessed by means of ROC analysis. Five observers tried to find simulated pathology positioned over the lungs and the mediastinum of an anthropomorphic phantom. The observers also graded the viewing comfort of the two CRTs in terms of flicker, light levels, and general comfort. No significant difference in the detection of simulated pathology was demonstrated between the two monitors. A slight preference for the newer CRT (Imlogix) was seen as compared to the older standard CRT (Siemens) regarding flicker and general comfort.

In search of optimum chest radiography techniques.

A nation-wide study of chest radiography in Sweden had previously revealed a large variation in the physical and technical factors involved, in particular the radiation dose. In this study the image quality could not be assessed unambiguously. The aim of the present study was to try to establish a correlation between visual grading of radiographs and physical and technical factors in order to find the optimum chest X-ray system. The physical and technical performance of 24 chest units was evaluated. Radiographs were taken of an anthropomorphic chest phantom supplied with test structures simulating various pathologies. Image quality was assessed by visual grading analysis of the radiographs. The physical and technical parameters of the units rated best were used to exemplify good radiographic practice. The results were in agreement with the recommendations issued by the Commission of the European Communities (CEC). It was also shown that low radiation dose is compatible with high-quality radiographic imaging of the chest.

Digital luminescence radiography using a chest phantom. Comparison between radiographs displayed on monitor and hard-copy.

Ninety-eight digital radiographs of a chest phantom with simulated tumors in the mediastinum and left lung and a pneumothorax-simulation in the right hemithorax were compared with the corresponding examinations saved on optical disk and viewed on a 1,000-line monitor. The examinations were reviewed by 7 radiologists with different experience, and receiver operating characteristic (ROC) curves were constructed. There was no significant difference between the hard-copy and the monitor results. A significant interobserver difference was seen only with the low attenuating 6 mm "tumor" and then only between the observer with the highest and the one with the lowest scores.

Digital chest radiography: should images be presented in negative or positive mode?

The observer performance for video monitor images presented in positive mode ('bones black') was compared to that of images presented in negative mode ('bones white') in a digital system for chest radiography based on a large image intensifier. In the first part of the study (I), the original, non-linear grey-scale reversal performed in the computer of the digital image intensifier system (DS1000) was studied together with fixed and variable settings of light and contrast. In the second part of the study (II), true grey-scale reversal performed in a minicomputer (Micro VAX II) where the images were also processed with edge and contrast enhancement, was studied. The time spent viewing the images was also recorded. Before the second part of the study, the image intensifier system was optimized with regard to spatial and contrast resolution and dose settings. Simulated pathology was randomly positioned over the lungs and the mediastinum of an anthropomorphic phantom. Observer performance was evaluated with Receiver Operating Characteristic (ROC) analysis on the digital images. In the first part of the study, a significant advantage for the positive images was seen, but no significant difference between fixed and variable settings of light and contrast. In the second part of the study, no significant difference in the detectability of the test structures was seen between positive and negative images. A significant improvement in the detectability was seen between the first and the second part of the study for the nodules over the mediastinum in both negative and positive images. No significant difference in the time spent observing positive and negative images, was seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of optimization and image processing in digital chest radiography: an ROC study with an anthropomorphic phantom.

A digital system for chest radiography based on a large image intensifier was compared to a conventional film-screen system. The digital system was optimized with regard to spatial and contrast resolution and dose. The images were digitally processed for contrast and edge enhancement. A simulated pneumothorax and two simulated nodules were positioned over the lungs and the mediastinum of an anthropomorphic phantom. Observer performance was evaluated with ROC analysis. Five observers assessed the processed digital images and the conventional full-size radiographs. The time spent viewing the full-size radiographs and the digital images was recorded. For the simulated pneumothorax, the results showed perfect performance for the full-size radiographs and detectability was high also for the processed digital images. No significant difference in the detectability of the simulated nodules was seen between the two imaging systems. The results for the digital images showed a significantly improved detectability for the nodules in the mediastinum as compared to a previous ROC study where no optimization and image processing was available. No significant difference in detectability was seen between the former and the present ROC study for small nodules in the lung. No difference was seen in the time spent assessing the conventional full-size radiographs and the digital images. The study indicates that processed digital images produced by a large image intensifier are equal in image quality to conventional full-size radiographs for low-contrast objects such as nodules.

Dual-photon absorptiometry for determination of bone mineral content in the calcaneus with correction for fat.

A portable dual-photon absorptiometry system for the determination of the bone mineral content BMC in vivo in the calcaneal bone is described. With knowledge of the total thickness of the heel at the measuring point the BMC can be corrected for fat and lean soft tissue. The system is capable of measuring the BMC in the heel bone with a precision of 1.2% in vitro and 1.8% in vivo at an absorbed dose to the skin of 0.2 mSv.