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.     

Image quality measurements and metrics in full field digital mammography: an overview

This paper gives an overview of test procedures developed to assess the performance of full field digital mammography systems. We make a distinction between tests of the individual components of the imaging chain and global system tests. Most tests are not yet fully standardised. Where possible, we illustrate the test methodologies on a selenium flat-panel system.     

Current challenges of full field digital mammography

Full field digital mammography (FFDM) has advantages over screen-film mammography (SFM), but some important challenges remain. The first challenge is related to the specific characteristics of FFDM. It remains unclear, which shape and limiting values of the modulation transfer function have the most influence on the performance of a detector, such as the effect of the image display on the overall image quality and the effect of processing on cancer detection. In order to assess the image quality of FFDM, we have set up a scoring system. The second challenge is related to screening mammography: is the quality of an image the same when it is viewed on different monitors and with different processing algorithms? Is Computer Aided Diagnosis necessary in a screening environment? In FFDM, the effect of different detectors, processing and display possibilities on the image and on cancer detection are not clearly investigated.     

The application of image quality measurements for digital angiography

The image quality (IQ) evaluation of a charge-coupled device (CCD)-based digital angiography system was assessed with respect to modulation transfer function and noise power spectrum. These values were used to calculate the system's frequency-dependent detective quantum efficiency (DQE). The X-ray image detector was an image intensifier (II) lens coupled to a CCD camera. Two measurement setups were used. Setup A is standard IQ assessment, while Setup B more closely represented clinical conditions (polymethyl methacrylate (PMMA) of varying thickness placed between the X-ray tube and II, with test object positioned between PMMA slices 30 cm from the II). Exposure parameters varied according to automatic brightness control settings. Setup B included X-ray radiation scattered by the patient-PMMA. A clinical DQE, describing the transmission of the input signal-to-noise ratio associated with both primary and secondary X-ray spectra, was defined.     

Adaptive image interpolation for full-field digital x-ray mammography

Some full-field digital x-ray mammography systems use multiple abutted two-dimensional detectors. Gaps between adjacent detectors produce seams in the composite image. We develop an adaptive linear interpolation method for estimation of missing data in the seams. In this method, the interpolation path is determined on the basis of the similarity between adjacent subimage boundary profiles. In experiments with phantom images and clinical mammograms, the adaptive method performed significantly better than the conventional linear interpolation.     

Clinical aspects of quality criteria in digital radiography

In clinical radiological practice two main questions regarding image quality have to be looked at: Which degree of image quality is needed? and: How can this image quality be objective? Image quality requirements depend on size, density and contrast of the objects of interest and on the clinical question which has to be answered. In comparison to conventional radiography, the digital radiographic techniques offer additional features concerning optimisation of image quality and dose, like a wider dynamic range, digital fluoroscopic techniques and post-processing. Therefore it is necessary to define new quality criteria for these techniques. In the following, typical examples demonstrate the development of the 'Diagnostic Requirements for Digital Radiographic Procedures'. These examinations include projection radiographs with digital luminiscent radiography, fluoroscopic procedures with digital image intensifier radiography and angiographic procedures with digital subtraction angiography. The clinical question of a radiological examination also forms the background to optimisation strategies considering image quality and radiation dose.     

Reference levels for image quality in mammography

Diagnostic reference levels are a well-established tool for optimisation in diagnostic radiology. A dose-reference level for certain diagnostic procedures, meant to identify practices with unusually high doses, is usually set at the third quartile of the distribution of doses in different diagnostic centres. If image quality is somehow quantified, the same 'worst quarter' principle can also be used to identify practices with less than optimal performance in terms of image quality. In Slovenia, the performance of mammographic centres is evaluated annually. Technical testing includes average glandular dose determination and evaluation of technical image quality using the image of a mammographic phantom. From the phantom image, simple image quality parameters are derived and for some of them reference levels can be established. In this study, the results from ten years of testing (1996-2006) at mammography centres are presented and the usefulness of reference levels is evaluated.     

Equivalence of digital image correlation criteria for pattern matching

In digital image correlation (DIC), to obtain the displacements of each point of interest, a correlation criterion must be predefined to evaluate the similarity between the reference subset and the target subset. The correlation criterion is of fundamental importance in DIC, and various correlation criteria have been designed and used in literature. However, little research has been carried out to investigate their relations. In this paper, we first provide a comprehensive overview of various correlation criteria used in DIC. Then we focus on three robust and most widely used correlation criteria, i.e., a zero-mean normalized cross-correlation (ZNCC) criterion, a zero-mean normalized sum of squared difference (ZNSSD) criterion, and a parametric sum of squared difference (PSSD(ab)) criterion with two additional unknown parameters, since they are insensitive to the scale and offset changes of the target subset intensity and have been highly recommended for practical use in literature. The three correlation criteria are analyzed to establish their transversal relationships, and the theoretical analyses clearly indicate that the three correlation criteria are actually equivalent, which elegantly unifies these correlation criteria for pattern matching. Finally, the equivalence of these correlation criteria is further validated by numerical simulation and actual experiment.     

Objective assessment of image quality in conventional and digital mammography taking into account dynamic range

The goal of this work is to develop a method to objectively compare the performance of a digital and a screen-film mammography system in terms of image quality. The method takes into account the dynamic range of the image detector, the detection of high and low contrast structures, the visualisation of the images and the observer response. A test object, designed to represent a compressed breast, was constructed from various tissue equivalent materials ranging from purely adipose to purely glandular composition. Different areas within the test object permitted the evaluation of low and high contrast detection, spatial resolution and image noise. All the images (digital and conventional) were captured using a CCD camera to include the visualisation process in the image quality assessment. A mathematical model observer (non-prewhitening matched filter), that calculates the detectability of high and low contrast structures using spatial resolution, noise and contrast, was used to compare the two technologies. Our results show that for a given patient dose, the detection of high and low contrast structures is significantly better for the digital system than for the conventional screen-film system studied. The method of using a test object with a large tissue composition range combined with a camera to compare conventional and digital imaging modalities can be applied to other radiological imaging techniques. In particular it could be used to optimise the process of radiographic reading of soft copy images.     

Comparison of full field digital (FFD) and computed radiography (CR) mammography systems in Greece

The purpose of this study is to evaluate and compare the performance of 52 full field digital (FFD) and computed radiography (CR) mammography systems checked by the Greek Atomic Energy Commission with respect to dose and image quality. Entrance surface air kerma (ESAK) was measured and average glandular dose (AGD) was calculated according to the European protocol on dosimetry in mammography. The exposures were performed using the clinical protocol of each laboratory. The image quality was assessed by the total score of resolved phantom structures incorporated in an American College of Radiology accreditation phantom. The mean ESAK values for FFD and CR systems were 4.59 ± 1.93 and 5.0 ± 1.78 mGy, respectively, whereas the AGD yielded a mean value of 1.06 ± 0.36 mGy for the FFD and 1.04 ± 0.35 mGy for the CR systems. Considering image quality, FFD systems indicated a mean total score of 13.04 ± 0.89, whereas CR systems a mean total score of 11.54 ± 1.06.     

Organisational aspects of mammography screening in digital settings: first experiences of Luxembourg

Luxembourg has been conducting a breast cancer screening programme since 1992, like a large number of other European countries, as early detection and treatment of breast cancer have been proven to reduce mortality. The majority of these screening programmes are based on analogue X-ray technology and have optimised their organisation of transporting, archiving and reading with respect to films. Last decade is marked by enormous developments in digital mammography. Different technologies such as flat panel-, computed radiography- and scanning systems became available. Digital mammography is expected to have a major impact on quality and organisation of breast cancer screening programmes. Screening programmes are now faced with a huge challenge of incorporating the digital technology, including implementation of electronic image exchange, conception of new electronic workflow, establishing adapted quality assurance programmes and training of radiologists and technical personnel. Initial experiences of the Luxembourg approach in organising digital mammography screening and its quality assurance are reported.     

A survey on performance status of mammography machines: image quality and dosimetry studies using a standard mammography imaging phantom

It is essential to perform quality control (QC) tests on mammography equipment in order to produce an appropriate image quality at a lower radiation dose to patients. Imaging and dosimetric measurements on 15 mammography machines located at the busiest radiology centres of Mumbai, India were carried out using a standard CIRS breast imaging phantom in order to see the level of image quality and breast doses. The QC tests include evaluations of image quality and the mean glandular doses (MGD), which is derived from the breast entrance exposure, half-value layer (HVL), compressed breast thickness (CBT) and breast tissue compositions. At the majority of the centres, film-processing and darkroom conditions were not found to be maintained, which is required to meet the technical development specifications for the mammography film in use as recommended by the American College of Radiology (ACR). In most of the surveyed centres, the viewbox luminance and room illuminance conditions were not found to be in line with the mammography requirements recommended by the ACR. The measured HVL values of the machines were in the range of 0.27-0.39 mm aluminium (Al) with a mean value of 0.33±0.04 mm Al at 28 kV(p) following the recommendation provided by ACR. The measured MGDs were in the range of 0.14-3.80 mGy with a mean value of 1.34 mGy. The measured MGDs vary between centre to centre by a factor of 27.14. Referring to patient doses and image quality, it was observed that only one mammography centre has exceeded the recommended MGD, i.e. 3.0 mGy per view with the value of 3.80 mGy and at eight mammography centres the measured central background density (CBD) values for mammography phantom image are found to be less than the recommended CBD limit value of 1.2-2.0 optical density.     

A web-based digital analysis interface for image quality assessment

Image processing and transmission systems may introduce some amount of distortion or artefacts in a digital image. This fact usually leads to a visual or statistical image quality assessment (IQA) required in many applications and research studies in order to analyse a product image in terms of deteriorations as well as effects of the processing. There are numerous IQA criteria presented in the literature separately that makes it really difficult both for use in comparative analyses and for educational purposes. In this presented work, a user friendly web-based digital educational interface for full-reference or no-reference image quality assessment using MATLAB builder NE has been developed. In addition to this, developed system performs user-defined optional deteriorations (median noise, Gaussian blur, motion blur, high-pass filter and jpeg compression) on the image and assesses deterioration of the image. It is an extremely easy, fast and economical way of analysing digital images, especially designed for researchers, graduate and postgraduate students who work on digital image processing. Using this web-based tool well contributes to teaching all of the IQA methods and quality effects of systematic distortions on the image as well as establishes a scientific benchmark for researchers.     

Survey of mammography practice in Croatia: equipment performance, image quality and dose

A national audit of mammography equipment performance, image quality and dose has been conducted in Croatia. Film-processing parameters, optical density (OD), average glandular dose (AGD) to the standard breast, viewing conditions and image quality were examined using TOR(MAM) test object. Average film gradient ranged from 2.6 to 3.7, with a mean of 3.1. Tube voltage used for imaging of the standard 45 mm polymethylmethacrylate phantom ranged from 24 to 34 kV, and OD ranged from 0.75 to 1.94 with a mean of 1.26. AGD to the standard breast ranged from 0.4 to 2.3 mGy with a mean of 1.1 mGy. Besides clinical conditions, the authors have imaged the standard phantom in the referent conditions with 28 kV and OD as close as possible to 1.5. Then, AGD ranged from 0.5 to 2.6 mGy with a mean of 1.3 mGy. Image viewing conditions were generally unsatisfying with ambient light up to 500 lx and most of the viewing boxes with luminance between 1000 and 2000 cd per m(2). TOR(MAM) scoring of images taken in clinical and referent conditions was done by local radiologists in local image viewing conditions and by the referent radiologist in good image viewing conditions. Importance of OD and image viewing conditions for diagnostic information were analysed. The survey showed that the main problem in Croatia is the lack of written quality assurance/quality control (QA/QC) procedures. Consequently, equipment performance, image quality and dose are unstable and activities to improve image quality or to reduce the dose are not evidence-based. This survey also had an educational purpose, introducing in Croatia the QC based on European Commission Guidelines.     

Displacement field denoising for high-temperature digital image correlation using principal component analysis

Principal component analysis (PCA) was extended to minimize the noise effect in digital image correlation (DIC) measurement under a high-temperature atmosphere environment. First, the principle of PCA was introduced, and the singular vectors and singular values for each component of the displacement fields from DIC were obtained. Then, the simulated high-temperature speckle images were developed to investigate the influences of noise on the DIC method under a high-temperature environment. Finally, the displacement fields of several special conditions were extracted from the simulated speckle images and experimental images; the effects of noise on the PCA were also analyzed.     

Chest radiography: a comparison of image quality and effective dose using four digital systems

Image quality (IQ) and effective dose for chest radiography was compared for four digital imaging systems that used three different detector technologies: a-Si/TFT flat-panel detector (FPD), scanning-slot/charge-coupled device (CCD) and photostimuable phosphor (PSP). On each system a phantom was exposed at 125 kV(p) for automatic exposure control (AEC) and 2, 4 and 8 Gy receptor dose using identical geometrical conditions. All images were scored as softcopy images by three observers. The effective dose was calculated for each exposure condition. For AEC, superior IQ was observed for the GE FPD compared with all the other systems, which showed similar IQ performance. For all systems the entrance surface dose associated with AEC was within the European recommendations but variations in the effective dose were observed between the four systems. For identical receptor dose levels superior IQ was observed with the FPDs. Thorascan was noted for its low effective dose and Agfa CR was associated with the highest effective dose. FPD systems showed a better overall performance, followed by the CCD and PSP systems.     

Threshold pixel size for shape determination of microcalcifications in digital mammography: a pilot study

The effect of pixel size on shape determination in screening digital mammography systems was studied using a shape identification task as the measured outcome. Ten microcalcifications on screen-films were digitised to a range of pixel sizes (2.5-200 microm) and extracted from computed radiography (CR) images (50 microm) acquired under equivalent imaging conditions. Fifteen observers attempted to identify the shape of each microcalcification at each pixel size. The results were collated to provide a fraction of correct responses vs. pixel size curve for each microcalcification. Averaging over all shapes, pixel values >100 microm lead to a significant decrease in shape determination ability (p < 0.01) for digitised screen-film. For CR images, half the shapes were not properly identified. Hence, although 20-100 microm was sufficient for microcalcification shape determination for digitised screen-film images, 50 microm was only borderline sufficient for the CR digital images.     

The use of a figure-of-merit (FOM) for optimisation in digital mammography: a literature review

The use of image quality parameters in digital mammography such as contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) has been widespread, with the intention of detector evaluation and/or quantitative evaluation of the system performance. These parameters are useful in ensuring adequate system performance when tests are done against international standards or guidelines. Parameters like CNR are relative quantities that lie within a range that is manufacturer and system dependent. The use of a figure-of-merit (FOM) is a relatively new concept as a tool in digital mammography permitting quantitative assessment in terms of image quality and patient dose. This review summarises the available evidence for the use and applicability of an FOM in digital mammography.     

Physical image quality comparison of four types of digital detector for chest radiology

Image quality for similar exposure conditions has been compared for two computed radiography (CR) systems (needle-based and conventional storage phosphor) and two flat-panel (DR) systems from different manufacturers mainly devoted to chest radiology. Image quality was assessed with a contrast-detail object and acrylic material to simulate clinical conditions. Specific image evaluation software was used to measure the contrast and obtain an image quality figure. Phantom and detector incident air kerma were measured for all images. Image quality differences were significant, and in the range of 100-300 microGy (phantom incident air kerma) the needle-based CR system and one of the DR systems show similar image quality and they are quite superior when compared with the conventional CR system.     

Finite element formulation for a digital image correlation method

A finite element formulation for a digital image correlation method is presented that will determine directly the complete, two-dimensional displacement field during the image correlation process on digital images. The entire interested image area is discretized into finite elements that are involved in the common image correlation process by use of our algorithms. This image correlation method with finite element formulation has an advantage over subset-based image correlation methods because it satisfies the requirements of displacement continuity and derivative continuity among elements on images. Numerical studies and a real experiment are used to verify the proposed formulation. Results have shown that the image correlation with the finite element formulation is computationally efficient, accurate, and robust.