Investigation of optimum X-ray beam tube voltage and filtration for chest radiography with a computed radiography system

The purpose of this study was to determine the optimum tube voltage and amount of added copper (Cu) filtration for processed chest radiographs obtained with an Agfa 75.0 Computed Radiography (CR) system. The contrast-to-noise ratio (CNR) was measured in the lung, heart/spine and diaphragm compartments of a validated chest phantom using various tube voltages and amounts of Cu filtration. The CNR was derived as a function of air kerma at the CR plate and with the effective dose. As rib contrast can interfere with detection of nodules in chest radiography, a tissue-to-rib ratio (TRR) was derived to investigate which tube voltages suppress the contrast of rib. Although processing algorithms affect the signal and noise in a way that is hard to predict, we found that, for a given set of processing parameters, the CNR was related to the plate air kerma and effective dose in a logarithmic manner (all R(2) >or=0.97). For imaging of the lung region, a low voltage (60 kVp) produced the highest CNR, whereas a high voltage (125 kVp) produced the highest TRR. In the heart/spine region, 80-125 kVp produced the highest CNR, while in the diaphragm region 60-90 kVp produced the highest CNR. For chest radiography with this CR system, the optimal tube voltage depends upon the region of interest. Of the filters tested, a 0.1 mm Cu thickness was found to provide a statistically significant increase in the CNR in the diaphragm region with tube potentials of 60 kVp and 80 kVp, without affecting the CNR in the other anatomical compartments.     

Digital chest radiography with a large image intensifier. Evaluation of diagnostic performance and patient exposure

A digital system for chest radiography based on a large image intensifier was compared with a conventional film-screen system. The diagnostic performance was evaluated with special reference to the digital monitor images with a modified version of receiver operating characteristic (ROC) analysis--free response ROC (FROC) analysis--on a chest equivalent phantom. Measurements of spatial resolution and energy imparted were also performed. The detectability of low-contrast objects as well as spatial resolution was better for the full-size film-screen radiographs than for both the digital monitor images and the 100 mm photofluorograms. The image-intensifier system has a potential for considerable dose savings in relation to the conventional technique provided that fluoroscopy is excluded in the positioning of the patients.     

Correlation of the clinical and physical image quality in chest radiography for average adults with a computed radiography imaging system

Objective:

The purpose of this study was to examine the correlation between the quality of visually graded patient (clinical) chest images and a quantitative assessment of chest phantom (physical) images acquired with a computed radiography (CR) imaging system.

Methods:

The results of a previously published study, in which four experienced image evaluators graded computer-simulated postero-anterior chest images using a visual grading analysis scoring (VGAS) scheme, were used for the clinical image quality measurement. Contrast-to-noise ratio (CNR) and effective dose efficiency (eDE) were used as physical image quality metrics measured in a uniform chest phantom. Although optimal values of these physical metrics for chest radiography were not derived in this work, their correlation with VGAS in images acquired without an antiscatter grid across the diagnostic range of X-ray tube voltages was determined using Pearson’s correlation coefficient.

Results:

Clinical and physical image quality metrics increased with decreasing tube voltage. Statistically significant correlations between VGAS and CNR (R=0.87, p<0.033) and eDE (R=0.77, p<0.008) were observed.

Conclusion:

Medical physics experts may use the physical image quality metrics described here in quality assurance programmes and optimisation studies with a degree of confidence that they reflect the clinical image quality in chest CR images acquired without an antiscatter grid.

Advances in knowledge:

A statistically significant correlation has been found between the clinical and physical image quality in CR chest imaging. The results support the value of using CNR and eDE in the evaluation of quality in clinical thorax radiography.Chest radiography is one of the most frequently performed diagnostic radiographic examinations in the UK. A Health Protection Agency report [1] in 2010 showed that chest radiographs represented 19.6% of all radiographic examinations (albeit the contribution to collective dose was small at about 0.5%), so optimisation of radiation dose and image quality in chest radiography is an important research area, especially in the era of digital imaging. It is also a legal requirement in the UK under the Ionising Radiation (Medical Exposure) Regulations 2000 [2] to optimise all medical exposures.Many investigators [3–12] have shown that it is not the system (including quantum) noise that is the limiting factor in chest radiography, but rather the projected patient anatomy or “anatomical noise”. It therefore follows that any images used to optimise a digital radiographic system for chest radiography must contain clinically realistic anatomical features and noise. The assessment of image quality of digital systems is typically undertaken with physical quality metrics, such as modulation transfer function (MTF), noise power spectra (NPS), detective quantum efficiency (DQE), contrast-to-noise ratio (CNR) and threshold contrast measurements [13–19]. Although these parameters describe the inherent performance of the imaging detector extremely well, it is difficult to link them to clinical image quality (i.e. the adequacy of patient images) [20] and therefore it is difficult to use them in any optimisation exercise. Furthermore, recent work by Samei et al [21,22] has shown that these metrics are not only detector centric but also not measured under typical clinical conditions. An alternative metric, the effective DQE (eDQE), was therefore proposed by the authors of that work. This new metric was designed to provide a measure of the signal-to-noise transfer characteristics of a digital imaging system measured under clinical conditions, using a phantom designed for a specific examination type, e.g. chest radiography. More recently, the same group argued that, although the eDQE provides a more clinically realistic measure of DQE, it does not take into account the radiation risk to the patient; hence, the concept of effective dose efficiency (eDE) was proposed [23], which is the effective noise equivalent quanta (eNEQ) normalised to the effective dose. However, a link between eDE and clinical image quality had not been established.Although there is a lack of work demonstrating a link between the physical and clinical image quality, De Crop et al [24] have recently established a correlation between a contrast detail phantom and clinical chest image quality by grading radiographs of embalmed cadavers and comparing the results with those derived from the phantom. However, only three cadavers were used, limiting the statistical significance of their findings, and no pathology of interest, such as lung nodules, was available.In this study, the results of a previous optimisation study performed by our group [25] using computer-simulated postero-anterior (PA) chest images have been used to investigate their correlation with the physical image quality metrics, eDE and CNR, across the diagnostic energy range (50–125 kV). The simulated images of the previous study contained clinically realistic projected anatomy and lung nodules (simulated images were used to avoid the obvious ethical issues associated with experimenting on real patients). We have chosen CNR, as this is a simple measure of image quality that is easy to use and understand, and it is often used to obtain practical measures of object detectability. However, CNR can only really be used to assess large area contrast sensitivity, as it does not include the system MTF or any noise variations with spatial frequency. Conversely, the eDE does include system resolution and noise properties as a function of spatial frequency, so this alternative more complex metric has also been investigated. It should be noted that the physical image quality metrics described in this work are not being optimised in themselves (i.e. optimal values of CNR and eDE for chest imaging are not being investigated) but are being used to predict, using a simple phantom, how the radiographic technique affects the clinical image quality.     

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.     

Digital radiography in general dental practice: a field study

OBJECTIVES: The aim of this study was to conduct a field study to survey the performance of digital radiography and how it was used by dentists in general dental practice. METHODS: 19 general dental practitioners were visited at their clinics. Ambient light (illuminance) was measured in the rooms where the monitors were placed. Different technical display parameters were noted. Test images and two phantoms--one low-contrast phantom and one line-pair resolution phantom--were used to evaluate the digital system. How the dentists used the enhancement program was investigated by noting which functions were used. RESULTS: Average illuminance in the operating room was 668 lux (range 190-1250 lux). On radiographs of the low-contrast phantom taken at the clinic, the ability to observe the holes decreased as illuminance increased. On average, the "light percentage" initially set on the monitor had to be decreased by 17% and contrast by 10% to optimize the display of the test images. The general dental practitioners used the enhancement programs most often to alter brightness and contrast to obtain the subjectively best image. Large differences between the clinics were noted. CONCLUSION: Knowledge of how to handle digital equipment in general dental practice should be improved. A calibrated monitor of good quality should be a given priority, as should proper ambient light conditions. There is a need to develop standardized quality controls for digital dental radiography.     

A phantom for dose-image quality optimization in chest radiography

Optimization in chest radiography requires evaluation of patient dose and image quality. This study is aimed at proposing a simple geometrical phantom that realistically simulates the important anatomical regions of the thorax. For this purpose, the standard LucAl chest phantom is modified by adding an "anthropomorphic" insert and image quality test plate. Different test objects are arranged on the plate in three important anatomical areas; lung, cardiac, and subdiaphragmal regions. The aim is to simultaneously find two types of image quality index, objective and subjective, which can be used to compare different images in order to select the better image. Two objective indices are proposed, areal contrast index DeltaC(a) and scatter fraction P(s) and two subjectively estimated, a low contrast visualization index P(low) and a high contrast visualization index P(high). To demonstrate the potential of this phantom method it was applied to an X-ray unit to find the optical film density that ensures optimal visualization in different anatomical areas. It was found for the X-ray system under investigation that the automatic exposure control could be set to produce an optical density of about 1.8 in the lung field. The reported method is easily implemented in any clinical situation where optimization of chest radiography is needed.     

Effect of high- and low-energy entrance surface dose allocation ratio for two-shot dual-energy subtraction imaging on low-contrast resolution

IntroductionDual-energy subtraction (DES) imaging can obtain chest radiographs with high contrast between nodules and healthy lung tissue, and evaluating of chest radiography and evaluating exposure conditions is crucial to obtain a high-quality diagnostic image. This study aimed to investigate the effect of the dose allocation ratio of entrance surface dose (ESD) between high- and low-energy projection in low-contrast resolution of soft-tissue images for two-shot DES imaging in digital radiography using a contrast-detail phantom (CD phantom).MethodsA custom-made phantom mimicking a human chest that combined a CD phantom, polymethylmethacrylate square plate, and an aluminum plate (1–3 mm) was used. The tube voltage was 120 kVp (high-energy) and 60 kVp (low-energy). The ESD was changed from 0.1 to 0.5 mGy in 0.1 mGy increments. Dose allocation ratio of ESD between 120 kVp and 60 kVp projection was set at 1:1, 1:2, 1:3, and 2:1. Inverse image quality figure (IQF_inv) was calculated from the custom-made phantom images.ResultsWhen the total ESD and aluminum thickness were constant, no significant difference in IQF_inv was observed under most conditions of varied dose allocation ratio. Similarly, when the total ESD and the dose allocation ratio were constant, there was no significant difference in IQF_inv based on the aluminum plate thickness.ConclusionUsing IQF_inv to evaluate the quality of the two-shot DES image suggested that dose allocation ratio did not have a significant effect on low-contrast resolution of soft-tissue images.Implications for practiceThe present results provide useful information for determining exposure conditions for two-shot DES imaging.     

Improved pulmonary nodule detection with scanning equalization radiography

The potential for improved pulmonary nodule detection with scanning equalization radiography (SER) was evaluated by means of observer performance testing during the interpretation of posteroanterior conventional radiographs and SER images of an anthropomorphic chest phantom with simulated nodules. A test set of 200 conventional and 200 SER radiographs of phantoms containing either one nodule or none was interpreted by four radiologists attempting to detect a nodule and indicate a confidence value. Their ability to detect nodules positioned over the lung was slightly improved with SER compared with conventional radiography (sensitivity, .56 vs .70); for nodules over the mediastinum or diaphragmatic areas, it was much improved (sensitivity, .29 vs .64). The results were also analyzed with receiver-operating characteristic methods, which revealed a significant improvement in lesion detect-ability over the thicker body parts with SER images. The capability of equalized chest radiographs to provide improved lesion detectability suggests that SER may set a new standard for film-based chest radiography and have a large clinical application.     

Image quality and dose comparison among screen-film, computed, and CT scanned projection radiography: applications to CT urography.

PURPOSE: To evaluate image quality and dose for abdominal imaging techniques that could be used as part of a computed tomographic (CT) urographic examination: screen-film (S-F) radiography or computed radiography (CR), performed with moving and stationary grids, and CT scanned projection radiography (CT SPR). MATERIALS AND METHODS: An image quality phantom underwent imaging with moving and stationary grids with both a clinical S-F combination and CR plate. CT SPR was performed with six CT scanners at various milliampere second and kilovolt peak settings. Entrance skin exposure (ESE); spatial, contrast, and temporal resolutions; geometric accuracy; and artifacts were assessed. RESULTS: S-F or CR images, with either grid, provided image quality equivalent to that with the clinical standard, S-F with a moving grid. ESE values for both S-F and CR were 435 mR (112.2 microC/kg [1 mR = 0.258 microC/kg]) with a moving grid and 226 mR (58.3 microC/kg) with a stationary grid. All CT SPR images provided inferior spatial resolution compared with S-F or CR images. High-contrast objects generated substantial artifacts on CT SPR images. Compared with S-F, CR and CT SPR provided improved resolution of small low-contrast objects. The contrast between iodine and soft-tissue-mimicking structures on CT SPR images acquired at 80 kVp was twice that at 120 kVp. CT SPR images with acceptable noise levels required a midline ESE value of approximately 300 mR (77.4 microC/kg) at 80 kVp. CONCLUSION: S-F and CR provided better spatial resolution than did CT SPR. However, CT SPR provided improved low-contrast resolution compared with S-F, at exposures comparable to those used for S-F or CR.     

Digital slot-scan charge-coupled device radiography versus AMBER and Bucky screen-film radiography: comparison of image quality in a phantom study

PURPOSE: To evaluate the image quality and performance of a chest digital radiography system and to compare this with the image quality and performance of advanced multiple-beam equalization radiography (AMBER) and Bucky screen-film radiography systems. MATERIALS AND METHODS: The chest digital radiography system is a digital charge-coupled device (CCD) chest imaging unit that uses slot-scan technology. A contrast-detail test object was used in combination with a phantom that simulates the primary and scatter transmission for the lungs and mediastinum. Twelve phantom images were obtained with each modality (ie, CCD digital radiography and AMBER and Bucky screen-film radiography) and were judged by six observers. CCD digital radiography soft-copy reading was compared with AMBER hard-copy reading. To measure image quality, contrast-detail curves were constructed from the observer data. The Wilcoxon signed rank test was used for statistical analysis. RESULTS: For the lung configuration, contrast-detail curves showed lower threshold depth for hard-copy images obtained with CCD digital radiography than for those obtained with Bucky screen-film radiography. For hard-copy images, the difference between contrast-detail curves for CCD digital radiography and those for Bucky screen-film radiography was statistically significant (P < .006). No significant difference was found between CCD digital radiography and AMBER for hard-copy images obtained in either the lung or mediastinum configuration. For the lung configuration, a lower threshold depth was observed for CCD digital radiography soft-copy reading than for AMBER hard-copy reading, with significantly different contrast-detail curves for CCD digital radiography soft copy and AMBER hard copy (P < .006). No significant difference was found between either system for the mediastinum configuration. CONCLUSION: Contrast-detail curves indicate that image quality for the CCD chest system provides a digital alternative to AMBER and Bucky screen-film radiography.     

A comparison of low contrast performance for amorphous Silicon/caesium iodide direct radiography with a computed radiography: A contrast detail phantom study

A range of digital image acquisition devices exists in diagnostic radiology. This study compares contrast performance of two such systems: an amorphous Silicon/caesium iodide (a-Si:CsI) based flat panel (DR) digital chest radiography system and a computed radiography (CR) system. Images of a contrast detail resolution phantom were acquired at a range of radiation doses. Three observers assessed all hardcopy images using a four-alternative forced choice observer perception technique. Contrast detail performance was calculated and low contrast performance quantified.The DR system demonstrated significantly better low contrast performance and potential dose savings of up to 75% compared to the CR system. Threshold levels of contrast detail resolution were defined and levels of under- and over-exposure, compared to the threshold level, were highlighted. Both systems were noise limited at lower exposures and latitude limited at higher exposures. The results demonstrate that the DR system should perform better than the CR system under typical clinical conditions relevant to chest radiography particularly for the detection of low contrast details such as lung metastases or pneumothoraces.     

New method of evaluating edge-preserving adaptive filters for computed tomography (CT): digital phantom method

To evaluate the characteristics of edge-preserving adaptive filters for selectively eliminating noise without affecting resolution in low-dose scanning, we have developed a digital phantom image and evaluated noise statistical values, noise characteristics, and resolution characteristics. The results confirmed that edge-preserving adaptive filters function as smoothing filters in low-contrast regions containing noise, permitting the density resolution to be improved, while the strength of the smoothing filter is reduced to maintain spatial resolution in high-contrast regions containing small structures. It has therefore been confirmed that edge-preserving adaptive filters function as filters for selectively eliminating only the noise elements that are increased when the exposure dose is reduced and that such filters are effective for improving image quality. Using such digital phantom images, images acquired using conditions that are difficult to set in actual CT scanning can be obtained and images specifically for the evaluation target can easily be generated. In addition, the noise level, frequency distribution of the noise, and resolution characteristics of the objects present in the input image can be freely set. It is concluded that evaluation of processing using a digital phantom image is effective for evaluating image processing.     

Use of a digitally reconstructed radiograph-based computer simulation for the optimisation of chest radiographic techniques for computed radiography imaging systems

Objectives

The purpose of this study was to derive an optimum radiographic technique for computed radiography (CR) chest imaging using a digitally reconstructed radiograph computer simulator. The simulator is capable of producing CR chest radiographs of adults with various tube potentials, receptor doses and scatter rejection.

Methods

Four experienced image evaluators graded images of average and obese adult patients at different potentials (average-sized, n=50; obese, n=20), receptor doses (n=10) and scatter rejection techniques (average-sized, n=20; obese, n=20). The quality of the images was evaluated using visually graded analysis. The influence of rib contrast was also assessed.

Results

For average-sized patients, image quality improved when tube potential was reduced compared with the reference (102 kVp). No scatter rejection was indicated. For obese patients, it has been shown that an antiscatter grid is indicated, and should be used in conjunction with as low a tube potential as possible (while allowing exposure times <20 ms). It is also possible to reduce receptor air kerma by 50% without adversely influencing image quality. Rib contrast did not interfere at any tube potential.

Conclusions

A virtual clinical trial has been performed with simulated chest CR images. Results indicate that low tube potentials (<102 kVp) are optimal for average and obese adults, the former acquired without scatter rejection, the latter with an anti-scatter grid. Lower receptor (and therefore patient doses) than those used clinically are possible while maintaining adequate image quality.Chest radiography is one of the most frequently performed diagnostic radiographic examinations in the UK owing to its value in the management of numerous clinical problems, such as the diagnosis of pulmonary diseases. A recent Health Protection Agency (HPA) report [1] presented the frequency and collective dose for medical and dental examinations in the UK in 2008, and demonstrated that chest radiographs contributed approximately 19.6% of all radiographic examinations (the second largest behind dental), although the contribution to collective dose was small at about 0.5%. It is a legal requirement in the UK under the Ionising Radiation (Medical Exposure) Regulations 2000 [2] to optimise all medical exposures. Because chest radiography is performed so frequently, optimisation of radiographic technique is an important research area, and has been investigated intensively in the literature.Many investigators [3-12] have shown that projected patient anatomy is the limiting factor in chest radiography rather than system (including quantum) noise, and the term “anatomical noise” was derived from their work. It therefore follows that any images used to optimise a digital X-ray system for chest radiography must contain clinically realistic anatomical noise. Typically, work reported in the literature examining phosphor plate computed radiography (CR) chest optimisation has used relatively simple physical phantoms, which enable optimisation of parameters such as signal and noise but do not necessarily contain all the anatomical features (noise) required [13-17]. Our group has also investigated optimising a CR imaging system for chest radiography with a phantom containing only coarse anatomical detail [18-20], but how our conclusions related to the diagnostic quality of the clinical image was undetermined. More recently, several groups have used a computerised voxel phantom in Monte Carlo studies [21-25] in an attempt to model anatomical features, but the resolution of this voxel phantom [26] is relatively coarse (approximately 4 mm long×3 mm wide×3 mm thick) and is therefore likely to produce images of much lower spatial resolution than a real CR image (typical pixel pitch 0.1×0.1 mm). Another disadvantage of this Monte Carlo phantom is that it simulates only four tissue types (soft tissue, bone, bone marrow and lung tissue) and air, thereby limiting the contribution of anatomical noise.In this study we have used computer-simulated chest images that contain clinically realistic projected anatomy (i.e. anatomical noise), produced at various tube potentials, receptor air kerma and scatter rejection methods, to optimise CR chest radiographic techniques. The computer model is based on a digitally reconstructed radiograph (DRR) simulation system—a computer simulation of a conventional two-dimensional (2D) radiograph created from CT data—and has been produced and thoroughly validated by our group [27]. The evaluation of simulated images was carried out by experienced image evaluators, and so this work presents the results of a virtual clinical trial. A brief synopsis of the computer model and how the images are produced is described below:

1. The virtual phantom is derived from the chest portion of real patient CT data sets. The voxel resolution of the phantom is 0.34×0.80×0.34 mm (width×height×depth). This is superior to the resolution of the computerised voxel phantom used in Monte Carlo studies [26].
2. CT number is converted into linear attenuation coefficient (LAC) using formulae derived from the Gammex-RMI model 467 (Gammex-RMI, Nottingham, UK) tissue equivalent phantom. This is a solid water cylinder that contains 17 inserts, the attenuation properties of which mimic the range of attenuations of the various tissues found in vivo. This demonstrates an improvement over the computerised phantom used for Monte Carlo studies, which contains only four tissue types.
3. X-ray spectra are generated using the techniques of Birch and Marshall as described in IPEM report 78 [28].
4. X-ray pencil beams are projected through the CT data set using a ray-casting method of DRR production. A ray-casting method was used as this has been shown to provide superior image quality to other methods such as splatting [29-31]. X-rays are attenuated as they move through the CT data in an exponential manner. The intensity of photons at each energy emerging from the virtual phantom is calculated.
5. Energy absorbed in the virtual CR phosphor is then calculated and converted to CR pixel value. This is the raw DRR.
6. Frequency-dependent noise is added to the raw DRR using a slightly adapted method described by Bath et al [32].
7. Scatter measured experimentally (“scatter masks”) on a clinical CR system is added to the raw DRR. The following separate scatter masks were acquired:
8. 1. With no clinical scatter rejection (i.e. non-grid). The ratio of scattered radiation absorbed in the CR phosphor to that of total radiation at 60 kVp (the scatter factor, SF) ranged from 0.33 to 0.47 in the lung region and from 0.66 to 0.85 in the spine/diaphragm region. SFs at 150 kVp ranged from 0.39 to 0.53 in the lung and from 0.69 to 0.88 in the spine/diaphragm. These values are in general agreement with SFs measured by Floyd et al [33] in humans. There is little change in SF in the spine/diaphragm regions with change in tube potential, but the effect is slightly more pronounced in the lung. This is similar to that reported by Bowenkamp and Boldingh [34].
   2. With an antiscatter grid (strips per mm=4; grid ratio=12). SFs were (on average) 40% lower in the lung (i.e. SF=0.13–0.19) and 48% lower in the spine/diaphragm (i.e. SF=0.26–0.34) when compared with SFs derived without scatter rejection (non-grid). This correlates with the scatter transmission factor of 0.14 derived using a 10-cm-thick solid water phantom reported by Fetterly and Schueler [35].
   3. With a clinical air-gap technique. The size of the air gap used was 20 cm based on the advice of an expert radiographer (Jo Cook, 2010, personal communication). SFs ranged from 0.29 to 0.39 in the lung region and from 0.65 to 0.68 in the spine/diaphragm region.

The resulting DRR images were validated quantitatively with a chest phantom and real patient CR images. Signal-to-noise ratio (SNR) values of the DRR images in the lung, spine and diaphragm regions agreed to within 15% (mean=5%) across the diagnostic energy range when compared with the CR images. Histograms was similar in shape and the dynamic range of the DRR images (minimum and maximum pixel values) were within two standard deviations of the mean of the corresponding values in the CR images. Qualitative validation was carried out by expert image evaluators and they all agreed that the DRRs adequately simulated real CR images, and that they were acceptable to use for optimisation studies. As well as normal chest anatomy, the model includes artificially added lung nodules. Lung nodules were chosen as they are indicative of common malignant disease, such as cancer, and non-malignant diseases such as tuberculosis, pneumonia and sarcoidosis. Expert image evaluators were also asked to score out of 10 (1=definitely not, 10=definitely) whether the position and appearance of the nodules were realistic. The mean (±1 standard deviation) score was 7.8±1.2, thus validating the appearance and positioning of the nodules.     

Analysis of CT acquisition parameters suitable for use in SPECT/CT: A free-response receiver operating characteristic study

PurposeThis dose optimisation free-response receiver operating characteristic (FROC) study compares the diagnostic performance of 8 CT acquisition parameter settings suitable for use with diagnostic quality SPECT/CT systems. Observers were required to localise simulated pulmonary lesions within an anthropomorphic chest phantom.MethodNine observers of varying CT experience (0–10+ years) reviewed 64 cases from each acquisition protocol. FROC methods were used to provide the area under the ROC curve (AUC) figures of merit for each CT acquisition parameter setting. The American College of Radiologists (ACR) CT accreditation phantom was used to acquire quality control (QC) images at each acquisition parameter setting. Measures of contrast to noise ratio (CNR), spatial (contrast) resolution and uniformity could then be compared to the AUC data. Effective dose (E) was estimated using the ImpaCT tool.ResultsAUC analysis revealed no significant difference (p = 0.866) between CT acquisitions for the detection of simulated pulmonary lesions within a chest phantom. The best observer averaged AUC performance was 0.853 (95% CI 0.803–0.903) at an estimated E of 0.82 mSv. The worst observer averaged AUC was recorded at the lowest estimated E (0.28 mSv) was 0.825 (95% CI 0.766–0.885). No correlation was evident between AUC and measures of CNR, spatial resolution or uniformity.ConclusionOur findings suggest that the lowest dose CT acquisition protocol can be used to accurately localise simulated pulmonary lesions within a chest phantom; suggesting that similar protocols could be used on diagnostic quality SPECT/CT systems to allow low-dose evaluations of lung lesions.     

Variable compensation technique for digital radiography of the chest

The authors describe a new technique, variable compensation (VC) radiography, for digital radiography of the chest. It permits retrospective adjustment of image display while maintaining improved mediastinal signal-to-noise ratio (S/N) from aggressive x-ray equalization. A fraction of a logarithmic image representing the profile of the beam intensity incident on the patient is subtracted from a logarithmic equalized image. VC images of a chest phantom were generated with various weightings of the beam-profile image. Edge artifacts were substantially reduced with a weighting of greater than 0.5 and eliminated with a weighting of 1.0. The S/N properties of VC images were measured with a series of plastic squares placed over various regions of the chest phantom. The S/N of the squares in the dense sub-diaphragm were improved twofold compared with the S/N on unequalized radiographs, whereas the S/N in the lung was reduced by 30%. Studies of a volunteer revealed the ability to render images with aggressive equalization (for improved mediastinal visualization) and images with the appearance of traditional chest radiographs.     

64层螺旋CT肺部高分辨率图像质量影响因素的体模研究

目的 分析影响肺部高分辨率CT(HRCT)图像质量的主要因素,探讨常规肺部容积扫描条件下获得HRCT图像质量的可行性.方法 应用64层CT对Catphan500模具进行连续、重复扫描.对比内容:扫描方式(轴面扫描、螺旋扫描)、kV值(140、120 kV)、层厚(1.25、5.00 mm,其中5.00 mm图像折分为1.25 mm)、重建算法(骨+算法、肺算法).评价内容:空间分辨率、密度分辨率、图像噪声.统计分析采用析因设计方差分析.结果 扫描方式在密度分辨率(轴面扫描:11.44±0.04;螺旋扫描:12.61±0.04)、图像噪声(轴面扫描:5.89±0.05;螺旋扫描:6.92±0.05)上差异均有统计学意义(F值分别为539.61、179.02,P值均＜0.01),轴面扫描密度分辨率高于螺旋扫描,图像噪声低于螺旋扫描;重建算法间在空间分辨率(骨+:9.90±0.09;肺:7.40±0.09)、密度分辨率(骨+:11.39±0.04;肺:12.65±0.04)、图像噪声(骨+:6.55±0.05;肺:6.28±0.05)上差异均具有统计学意义(F值分别为375.00、627.95、13.97,P值均＜0.05),骨+算法优于肺算法但噪声值略高.结论 容积扫描在密度分辨率、图像噪声控制上较传统HRCT稍差,但在空间分辨率上容积扫描骨+算法重建与HRCT图像质量相当,用于显示肺内细节时容积高分辨率CT可代替常规HRCT.     

Noise reduction in MR angiography with nonlinear anisotropic filtering

PURPOSE: To evaluate three-dimensional nonlinear anisotropic filtering in suppressing image noise in high spatial resolution magnetic resonance angiograms (MRA) acquired with hybrid undersampled projection reconstruction and phase contrast vastly undersampled isotropic projection reconstruction (PC-VIPR). MATERIALS AND METHODS: Three-dimensional nonlinear anisotropic filtering was quantitatively analyzed and evaluated through the measurement of contrast to noise ratio (CNR) in PC-VIPR images and contrast enhanced peripheral MRA images. To filter MRA images with ultra-high spatial resolution and poor CNR, a spatial frequency dependent nonlinear anisotropic filtering algorithm was proposed that uses two-step processing to filter the whole spatial frequency data. RESULTS: Three-dimensional nonlinear anisotropic filtering was shown to be effective in suppressing noise and improving CNR in MRA with isotropic spatial resolution. Higher CNR was achieved using spatial frequency dependent nonlinear anisotropic filtering. A typical CNR gain of between 50-100% was shown in our studies. CONCLUSION: Three-dimensional nonlinear anisotropic filtering significantly improved CNR in MRA images with isotropic spatial resolution. Spatial frequency dependent nonlinear anisotropic filtering further improved CNR for MRA images with ultra-high spatial resolution and low CNR.     

Low contrast detectability and spatial resolution with model-based Iterative reconstructions of MDCT images: a phantom and cadaveric study

Objectives

To compare image quality [low contrast (LC) detectability, noise, contrast-to-noise (CNR) and spatial resolution (SR)] of MDCT images reconstructed with an iterative reconstruction (IR) algorithm and a filtered back projection (FBP) algorithm.

Methods

The experimental study was performed on a 256-slice MDCT. LC detectability, noise, CNR and SR were measured on a Catphan phantom scanned with decreasing doses (48.8 down to 0.7 mGy) and parameters typical of a chest CT examination. Images were reconstructed with FBP and a model-based IR algorithm. Additionally, human chest cadavers were scanned and reconstructed using the same technical parameters. Images were analyzed to illustrate the phantom results.

Results

LC detectability and noise were statistically significantly different between the techniques, supporting model-based IR algorithm (p?<?0.0001). At low doses, the noise in FBP images only enabled SR measurements of high contrast objects. The superior CNR of model-based IR algorithm enabled lower dose measurements, which showed that SR was dose and contrast dependent. Cadaver images reconstructed with model-based IR illustrated that visibility and delineation of anatomical structure edges could be deteriorated at low doses.

Conclusion

Model-based IR improved LC detectability and enabled dose reduction. At low dose, SR became dose and contrast dependent.

Key Points

? Model- based Iterative Reconstruction improves detectability of low contrast object. ? With model- based Iterative Reconstruction, spatial resolution is dose and contrast dependent. ? Model-based Iterative Reconstruction algorithms enable improved IQ combined with dose-reduction possibilities. ? Improvement of SR and LC detectability on the same IMR data set would reduce reconstructions.

     

Dual-energy digital subtraction chest radiography: technical considerations

In the evaluation of asbestos-related pulmonary and pleural abnormalities, conventional chest radiography has been shown to have a low sensitivity for the detection of lung nodules and subtle interstitial disease. Pleural plaques may simulate pulmonary nodules, and interstitial processes can be masked by adjacent pleural abnormalities. Dual-energy digital subtraction chest radiography may enable investigators to characterize asbestos-related pulmonary and pleural abnormalities with greater accuracy. "Soft-tissue" images, designed to remove pleural calcifications, may allow for better evaluation of the lung parenchyma. "Bone" images, designed to remove soft-tissue structures, may enhance the detection of pleural calcifications. In this pictorial essay we illustrate the methods, technical considerations, and limitations of dual-energy digital subtraction chest radiography performed with global subtraction weighting factors.     

Advanced multiple-beam equalization radiography in chest radiology: a simulated nodule detection study

To evaluate the efficacy of AMBER, a multiple-beam equalization system for chest radiography, the authors performed a nodule detection study using an anthropomorphic chest phantom. AMBER and conventional images were compared. The images were read by four observers, and analysis was done by means of modified receiver-operating characteristic (ROC) curves (free ROC curves [FROC]). The results of the FROC analysis show a significant increase in the detectability of nodules (P less than .001) projected over the mediastinum with the use of AMBER. No significant difference between AMBER and conventional images was noted in detectability of nodules projected over the lung.