Low-contrast Detail Detectability Research Articles

Relationship between the visual evaluation of pathology visibility and the physical measure of low contrast detail detectability in neonatal chest radiography

IntroductionThe detectability of low contrast detail (LCD) is a method used to assess image quality (IQ) in neonatal radiography; however, there is a lack of data on the relationship between LCD detectability and visual IQ. The study aims at investigating the relationship between the LCD detectability and visual IQ and pathology visibility (PV). MethodsSeveral acquisition parameters were employed to obtain a group of images from a neonatal Gammex chest phantom. Three observers applied relative visual grading analysis (VGA) for assessing the IQ and PV. A simulated pneumothorax visibility (PNV) and simulated hyaline membrane disease visibility (HMV) represented PV. Next, a CDRAD 2.0 phantom was radiographed utilising the same acquisition protocols, and several paired images were obtained. With the use of CDRAD analyser software, the detectability of LCD was assessed and expressed by an image quality figure inverse (IQFiinv) metric. The correlation between the IQFinv and each of IQ, PNV and HMV was examined. ResultsThe physical measure (IQFinv) and the visual assessment of IQ were shown to be strongly correlated (r = 0.95; p < 0.001). Using Pearson's correlation, the IQFinv, PNV, and HMV were found to be strongly correlated (r = 0.94; p < 0.001) and (r = 0.92; p < 0.001), correspondingly. ConclusionResults of the study show that physical measures of LCD detectability utilising the CDRAD 2.0 phantom is strongly corelated with visual IQ and PV (PNV and HMV) and can be used to evaluate IQ when undertaking neonatal chest radiography (CXR). Implications for practiceThis study establishes the feasibility of utilising the physical measure (IQFinv) and the CDRAD 2.0 phantom in routine quality assurance and neonatal CXR optimisation studies.

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

ObjectivesTo compare low contrast detail (LCD) detectability and radiation dose for routine paediatric chest X-ray (CXR) imaging protocols among various hospitals. MethodsCDRAD 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. ResultsThe 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. ConclusionsResults 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.

MO-DE-207A-01: Impact of Statistical Weights On Detection of Low-Contrast Details in Model-Based Iterative CT Reconstruction

Purpose: Penalized-weighted least-square reconstruction has become an important research topic in CT, to reduce dose without affecting image quality. Two components impact image quality in this reconstruction: the statistical weights and the use of an edge-preserving penalty term. We are interested in assessing the influence of statistical weights on their own, without the edge-preserving feature. Methods: The influence of statistical weights on image quality was assessed in terms of low-contrast detail detection using LROC analysis. The task amounted to detect and localize a 6-mm lesion with random contrast inside the FORBILD head phantom. A two-alternative forced-choice experiment was used with two human observers performing the task. Reconstructions without and with statistical weights were compared, both using the same quadratic penalty term. The beam energy was set to 30keV to amplify spatial differences in attenuation and thereby the role of statistical weights. A fan-beam data acquisition geometry was used. Results: Visual inspection of images clearly showed a difference in noise between the two reconstructions methods. As expected, the reconstruction without statistical weights exhibited noise streaks. The other reconstruction appeared better in this aspect, but presented other disturbing noise patterns and artifacts induced by the weights. The LROC analysis yield the following 95-percent confidence interval for the difference in reader-averaged AUC (reconstruction without weights minus reconstruction with weights): [0.0026,0.0599]. The mean AUC value was 0.9094. Conclusion: We have investigated the impact of statistical weights without the use of edge-preserving penalty in penalized weighted least-square reconstruction. A decrease rather than increase in image quality was observed when using statistical weights. Thus, the observers were better able to cope with the noise streaks than the noise patterns and artifacts induced by the statistical weights. It may be that different results would be obtained if the penalty term was used with a pixel-dependent weight. F Noo receives research support from Siemens Healthcare GmbH

Factors Affecting Contrast-Detail Performance in Computed Tomography: A Review

This article reports on recent research findings into the factors that influence the detectability performance of different systems of computed tomography (CT) scanners. These systems include multidetector CT (MDCT) of different slice numbers, dual-source CT (DSCT), and cone-beam CT (CBCT). The introduction of more slices for MDCT, DSCT, and the new technology of CBCT increases the need to optimize the image quality and to examine the potential reduction of radiation doses to the patient. Low-contrast detail detectability is a method that has proven to be an appropriate evaluation method for this purpose. However, it is essential to recognize factors that affect detectability performance and understand how these factors influence image quality and radiation dose. It is argued that deep understanding of the influences of these factors is the key to image quality optimization in terms of contrast-detail detectability and radiation dose reduction. The purpose of this article is, therefore, to specify these factors and to explain their influence on detectability performance and hence on CT image quality. Further low-contrast detail studies are required to optimize imaging performance of different CT systems and scanners.

Optimisation of computed radiography systems for chest imaging

The main thrust of this study is to investigate methods of optimising the radiation dose–image quality relationship in computed radiography (CR) systems for chest imaging. Specifically, this study investigates the possibility of reducing the patient radiation exposure through an optimal selection of tube filtration, exposure parameters and air gap technique, in parallel with a study of the image quality, particularly low contrast detail detectability, signal-to-noise ratio (SNR) and scatter fraction (SF). The CDRAD phantom was used to assess the quality of the CR images. Tissue equivalent Polystyrene blocks were placed in the front of the phantom as scattering material with thicknesses of 5 and 15 cm to simulate an adult chest and heart/diaphragm regions, respectively. A series of exposure techniques were used, including Cu filtration with various thicknesses of Cu in the presence and absence of an air gap, whilst the exposure was kept as constant as possible throughout. The estimated patient effective dose and skin entrance dose were calculated using the NRPB-SR262 X-ray dose calculation software. The results have shown that the low contrast-detail detectability in the lung and the heart/diaphragm regions improves when using an air gap and no Cu filtration, particularly at low kilovoltage (kVp). However, there is no significant difference in low contrast-detail in the absence or presence of a 0.2 mm Cu filtration. SF values for the lung and heart regions decrease when using both, the air gap technique and a 0.2 mm Cu filtration, particularly at low kVp. SNR values for the lung and heart regions improve when using a small Cu thickness. In conclusion, this investigation has shown that the quality of chest CR images could be improved by using an air gap technique and a 0.2 mm Cu filtration at low kVp, particularly at 99 kVp.

Performance evaluation and testing of digital intra-oral radiographic systems

Four digital intra-oral radiographic systems were tested and evaluated; three charge-coupled device (CCD) based systems from RVG, Visualix and Sidexis and a photostimulable phosphor (PSP) system from DenOptix. Image quality was assessed using three purpose-built phantoms to measure uniformity, low contrast detail detectability and signal-to-noise ratio (SNR). Limiting resolution was measured using a 20 lp mm(-1) bar pattern. Radiation output inaccuracies caused difficulties in achieving optimum doses for CCD systems. However, the accuracy was improved by using K-edge filters. SNR measurements proved to be a useful tool in assessing system performance. Each system has specific attributes: resolution was highest for the RVG system, the Visualix system measured the highest SNR and the lowest exposure settings were on the Sidexis system. Test methods and phantoms developed are suitable for acceptance testing and commissioning digital dental X-ray systems and for programming each system to produce an optimum level of image quality.

Methods and materials for the measurement of subjective and objective measurements of image quality.

The evaluation of medical imaging systems may be performed using both subjective (e.g. high contrast limiting resolution, and low contrast detail detectability), and objective/physical (e.g. characteristic curve, signal-to-noise ratio, modulation transfer function, and Wiener spectra) indices of image quality. For the purposes of Quality Assurance subjective measurements currently have a wider usage in the clinical environment due to the inherent simplicity of the measurements; however, more rigorous objective approaches are gaining acceptability, and represent a paradigm for possible future QA measurements. Both approaches have their advantages and disadvantages. There is currently wide debate as to the interpretation of these measurements. However, there appears to be a lack of standardisation among scientists in the various approaches to making these measurements, and especially so in the case of objective indices; furthermore, the measurement protocol used must have a bearing on the interpretation of subsequent results. The purpose of this paper is to (i) present the most fundamental indices in the characterisation of 'image quality', and (ii) recommend approaches to the measurement of these indices.

A search for improved technique factors in paediatric fluoroscopy

A Monte Carlo computational model of a fluoroscopic imaging chain was used for deriving optimal technique factors for paediatric fluoroscopy. The optimal technique was defined as the one that minimizes the absorbed dose (or dose rate) in the patient with a constraint of constant image quality. Image quality was assessed for the task of detecting a detail in the image of a patient-simulating phantom, and was expressed in terms of the ideal observer's signal-to-noise ratio (SNR) for static images and in terms of the accumulating rate of the square of SNR for dynamic imaging. The entrance air kerma (or air kerma rate) and the mean absorbed dose (or dose rate) in the phantom quantified radiation detriment. The calculations were made for homogeneous phantoms simulating newborn, 3-, 10- and 15-year-old patients, barium and iodine contrast material details, several x-ray spectra, and for imaging with or without an antiscatter grid. The image receptor was modelled as a CsI x-ray image intensifier (XRII). For the task of detecting low- or moderate-contrast iodine details, the optimal spectrum can be obtained by using an x-ray tube potential near 50 kV and filtering the x-ray beam heavily. The optimal tube potential is near 60 kV for low- or moderate-contrast barium details, and 80-100 kV for high-contrast details. The low-potential spectra above require a high tube load, but this should be acceptable in paediatric fluoroscopy. A reasonable choice of filtration is the use of an additional 0.25 mm Cu, or a suitable K-edge filter. No increase in the optimal tube potential was found as phantom thickness increased. With the constraint of constant low-contrast detail detectability, the mean absorbed doses obtained with the above spectra are approximately 50% lower than those obtained with the reference conditions of 70 kV and 2.7 mm Al filter. For the smallest patient and x-ray field size, not using a grid was slightly more dose-efficient than using a grid, but when the patient size and field size were increased a fibre interspaced grid resulted in lower doses than imaging without a grid. For a 15-year-old patient the mean absorbed doses were up to 40% lower with this grid than without the grid.

Low-dose phase contrast x-ray medical imaging

Phase contrast x-ray imaging is a powerful technique for the detection of low-contrast details in weakly absorbing objects. This method is of possible relevance in the field of diagnostic radiology. In fact, imaging low-contrast details within soft tissue does not give satisfactory results in conventional x-ray absorption radiology, mammography being a typical example. Nevertheless, up to now all applications of the phase contrast technique, carried out on thin samples, have required radiation doses substantially higher than those delivered in conventional radiological examinations. To demonstrate the applicability of the method to mammography we produced phase contrast images of objects a few centimetres thick while delivering radiation doses lower than or comparable to doses needed in standard mammographic examinations (typically mean glandular dose (MGD)). We show images of a custom mammographic phantom and of two specimens of human breast tissue obtained at the SYRMEP bending magnet beamline at Elettra, the Trieste synchrotron radiation facility. The introduction of an intensifier screen enabled us to obtain phase contrast images of these thick samples with radiation doses comparable to those used in mammography. Low absorbing details such as thick nylon wires or thin calcium deposits within breast tissue, invisible with conventional techniques, are detected by means of the proposed method. We also find that the use of a bending magnet radiation source relaxes the previously reported requirements on source size for phase contrast imaging. Finally, the consistency of the results has been checked by theoretical simulations carried out for the purposes of this experiment.