Investigating the use of an antiscatter grid in chest radiography for average adults with a computed radiography imaging system

Objective:

The aim of this study was to investigate via simulation a proposed change to clinical practice for chest radiography. The validity of using a scatter rejection grid across the diagnostic energy range (60–125 kVp), in conjunction with appropriate tube current–time product (mAs) for imaging with a computed radiography (CR) system was investigated.

Methods:

A digitally reconstructed radiograph algorithm was used, which was capable of simulating CR chest radiographs with various tube voltages, receptor doses and scatter rejection methods. Four experienced image evaluators graded images with a grid (n = 80) at tube voltages across the diagnostic energy range and varying detector air kermas. These were scored against corresponding images reconstructed without a grid, as per current clinical protocol.

Results:

For all patients, diagnostic image quality improved with the use of a grid, without the need to increase tube mAs (and therefore patient dose), irrespective of the tube voltage used. Increasing tube mAs by an amount determined by the Bucky factor made little difference to image quality.

Conclusion:

A virtual clinical trial has been performed with simulated chest CR images. Results indicate that the use of a grid improves diagnostic image quality for average adults, without the need to increase tube mAs, even at low tube voltages.

Advances in knowledge:

Validated with images containing realistic anatomical noise, it is possible to improve image quality by utilizing grids for chest radiography with CR systems without increasing patient exposure. Increasing tube mAs by an amount determined by the Bucky factor is not justified.Radiography of the chest is one of the most frequently performed diagnostic radiographic examinations in the UK. In 2010, the Health Protection Agency (now Public Health England) reported¹ that chest radiographs represented 19.6% of all radiographic examinations in 2008 (although the contribution to collective dose was small at about 0.5%), so optimization of radiation dose (i.e. ensuring dose is as low as reasonably practicable) and image quality (i.e. ensuring all required clinical structures are visible to the reporting healthcare professional so that an acceptable diagnosis is possible) in chest radiography is an important research area, especially since digital imaging has all but replaced its film-screen counterpart. It is also a legal requirement in the UK under the Ionising Radiation (Medical Exposure) Regulations 2000² to optimize all medical exposures, consistent with the intended purpose.One such technique to optimize image quality in chest radiography is to use a scatter rejection grid. They work by preferentially removing radiation scattered by the body prior to reaching the detector, and their improvement of image quality in film-screen imaging has been recognized for decades.^3–6 This improvement was described by the contrast improvement factor⁷ but came with a cost. Film requires a given level of incident exposure to ensure adequate optical density (OD), and because a grid attenuates most of the scattered radiation (as well as some primary radiation), this necessitates an increase in tube current–time product (mAs). The Bucky factor describes the necessary multiplication by which exposure parameters must be increased, and for film-screen can be anything between 2–6 times the “non-gridded” exposure.^8,9 Regardless of this, there are clear guidelines recommending the use of scatter rejection grids with film-screen systems for adult chest radiography,¹⁰ but none for digital imaging modalities, although Fritz and Jones¹¹ have recently published guidelines for scatter rejection techniques in paediatric digital radiology.Digital image detectors, such as computed radiography (CR) photostimulable powder phosphors, have a larger dynamic range than does film,¹² and grey levels in the resulting image are usually adjusted, irrespective of incident detector dose, to match the output of the display monitor. Therefore, unlike film, digital imaging is not contrast (OD) limited. At doses used clinically (e.g. air kerma of approximately 2–15 µGy at the receptor), digital images are dominated by quantum noise (i.e. other noise sources such as electronic and structural are typically ≤2% of the total noise as consistently demonstrated through in-house routine quality assurance testing of this CR system), which depends on the level of air kerma incident on the detector (signal) and the detector''s detective quantum efficiency. Therefore, when a scatter rejection grid is used, appropriate exposure factors (tube''s peak kilo-voltage and/or mAs) are required to maintain a level of image signal-to-noise ratio (SNR)^13,14 acceptable to the image evaluator, although there is no agreement as to what these appropriate exposure factors should be. A common school of thought, described in a considerable resource by Carlton and Adler,¹⁵ suggests that the lower limit on the increase in mAs should be the reciprocal of the primary transmission (T_p) of the grid and the upper limit, the Bucky factor. The reciprocal of T_p for modern grids is typically 1.2–1.4, which suggests that an increase in mAs of at least 20% is required. However, Tanaka et al¹⁶ have recently demonstrated that the use of grids (grid ratios 5 : 1 to 14 : 1) without increasing exposure factors (compared with “non-gridded” exposures), actually improved the effective noise equivalent quanta (eNEQ) when acquiring images of 20 cm of polymethylmethacrylate (PMMA). They concluded that the improvement to image quality owing to removal of scatter outweighs the increase in quantum noise when a grid is used, although they acknowledged that their work did not use any images with anatomical structure. Similarly, Fetterly and Schueler¹⁷ studied numerous grids with different thicknesses of uniform solid water and suggested that a scatter rejection grid can provide improvement in SNR without increasing exposure for large patients.Given the scarcity of evidence/guidelines in the literature, the aim of this study was two-fold: firstly, to investigate the validity of using a scatter rejection grid for chest radiography of average adults with an Agfa CR imaging system (Agfa, Peissenberg, Germany) across the diagnostic energy range (60–125 kVp); and secondly, to investigate appropriate tube mAs to identify what increase in patient dose (if any) there needs to be. Many recent studies have demonstrated that anatomical noise (the influence of projected anatomy on the image evaluator’s ability to detect potential abnormalities and provide an accurate diagnosis) is the limiting factor in chest radiography,^18–27 so it was felt that the inclusion of realistic anatomy in the images used in this study was of particular importance, rather than using uniform PMMA or solid water only. Therefore, computer-simulated chest radiographs (each containing realistic projected anatomical noise and lung abnormalities/nodules), reconstructed with a scatter rejection grid, were compared by expert image evaluators with images reconstructed without a grid (as per current clinical protocol in our radiology department).     

Trial for improvement of image quality of chest tomography with computed radiography

In conventional X-ray tomography, the presence of blurring which is caused by the object outside the focal plane reduces the image quality of the focal plane. With the application of computed radiography for the conventional chest tomography, two kinds of image processing techniques for the elimination of the blurring were examined. As a result, each image processing technique fairly succeeded in improving the image quality.     

对比细节分析与临床图像质量评估在尸体胸部影像研究上的关系

目的通过使用锡尔技术(Thiel technique)和对比细节模型防腐的尸体,确定临床和胸片物理图像质量的关系。材料与方法使用满足机构伦理委员会要求的人类尸体。在3具使用锡尔技术防腐的尸体上来评估临床图像质量,该     

床旁DR与CR胸部摄影质量的对比研究

目的通过床旁数字化X线摄影(digitalradiography,DR)系统与计算机X线摄影(computedradiography,CR)系统胸部摄影图像质量的对比分析,探讨床旁DR摄影的优势。方法随机抽取2010年拍摄的DR床旁X线胸片1000份和既往CR床旁X线胸片1000份进行对比分析。结果床旁DR摄影的应用提供了更加优良的图像质量,明显减少了放射技师的工作量,同时还降低了患者接受的辐射剂量。结论床旁DR胸部摄影比CR胸部摄影有着明显的优势,对手术患者和危重患者的诊治起到了积极作用。     

Clinical evaluation of irreversible image compression: analysis of chest imaging with computed radiography

To implement a picture archiving and communication system, clinical evaluation of irreversible image compression with a newly developed modified two-dimensional discrete cosine transform (DCT) and bit-allocation technique was performed for chest images with computed radiography (CR). CR images were observed on a cathode-ray-tube monitor in a 1,024 X 1,536 matrix. One original and five reconstructed versions of the same images with compression ratios of 3:1, 6:1, 13:1, 19:1, and 31:1 were ranked according to quality. Test images with higher spatial frequency were ranked better than those with lower spatial frequency and the acceptable upper limit of the compression ratio was 19:1. In studies of receiver operating characteristics for scoring the presence or absence of nodules and linear shadows, the images with a compression ratio of 25:1 showed a statistical difference as compared with the other images with a compression ratio of 20:1 or less. Both studies show that plain CR chest images with a compression ratio of 10:1 are acceptable and, with use of an improved DCT technique, the upper limit of the compression ratio is 20:1.     

Low-dose digital computed radiography in pediatric chest imaging

A prototype digital computed radiographic imaging system that uses laser-stimulated luminescence was evaluated for its ability to achieve reproducible, high-detail, low-dose pediatric chest radiographs. Using this system, we performed a total of 401 examinations in infants and children, and achieved an 85% reduction in radiation dose, as compared with that delivered when film-screen techniques were used. We also achieved satisfactory image resolution, and the images obtained were of acceptable diagnostic quality. A direct comparison of analog and digital radiographs showed that comparable quality and clinical acceptability could be readily maintained between the two techniques. This study shows that high-quality images can be produced by this system at radiation doses reduced by 85% when compared with doses from standard radiographic techniques.     

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.     

Comparing image quality of flat-panel chest radiography with storage phosphor radiography and film-screen radiography

OBJECTIVE: To evaluate image quality of a large-area direct-readout flat-panel detector system in chest radiography, we conducted an observer preference study. A clinical comparative study was conducted of the flat-panel system versus the storage phosphor and standard film-screen systems. MATERIALS AND METHODS: Routine chest radiographs (posteroanterior) of 30 patients that were obtained using flat-panel, storage phosphor, and film screen systems were compared. The visibility of 10 anatomic regions and the overall image quality criteria were rated independently by three radiologists using a 5-point scale. The significance of the differences in diagnostic performance was tested with a Wilcoxon's signed rank test. Dose measurements for the three modalities were performed. RESULTS: The flat-panel radiography system showed an improved visibility in most anatomic structures when compared with a state-of-the-art conventional film-screen system and an equal visibility when compared with a storage phosphor system. The flat-panel system showed the greatest enhancement in the depiction of small detailed structures (p < 0.05) and achieved this with a reduction in overall radiation dose of more than 50%. CONCLUSION: The visibility of anatomic structures provided by this flat-panel detector system is as good as if not better than that provided by conventional or storage phosphor systems while emitting a reduced radiation dose.     

One-shot dual-energy subtraction chest imaging with computed radiography: clinical evaluation of film images

A clinical evaluation of one-shot dual-energy subtraction chest imaging by means of computed radiography (CR) with imaging plates was carried out in a comparison with the original plain CR images. In analyses of chest images of 140 patients, new information, not detected on the original plain CR images, was obtained on subtraction images in 21 patients (15%). Receiver operating characteristic curve studies also verified the superiority of CR subtraction over the original plain CR images for the detection of pulmonary nodules, calcification in a nodule, and rib lesions. Subtraction images complemented the original plain images.     

Dose and perceived image quality in chest radiography

Chest radiography is the most commonly performed diagnostic X-ray examination. The radiation dose to the patient for this examination is relatively low but because of its frequent use, the contribution to the collective dose is considerable. Consequently, optimization of dose and image quality offers a challenging area of research. In this article studies on dose reduction, different detector technologies, optimization of image acquisition and new technical developments in image acquisition and post processing will be reviewed.Studies indicate that dose reduction in PA chest images to at least 50% of commonly applied dose levels does not affect diagnosis in the lung fields; however, dose reduction in the mediastinum, upper abdomen and retrocardiac areas appears to directly deteriorate diagnosis. In addition to patient dose, also the design of the various digital detectors seems to have an effect on image quality. With respect to image acquisition, studies showed that using a lower tube voltage improves visibility of anatomical structures and lesions in digital chest radiographs but also increases the disturbing appearance of ribs.New techniques that are currently being evaluated are dual energy, tomosynthesis, temporal subtraction and rib suppression. These technologies may improve diagnostic chest X-ray further. They may for example reduce the negative influence of over projection of ribs, referred to as anatomic noise. In chest X-ray this type of noise may be the dominating factor in the detection of nodules. In conclusion, optimization and new developments will enlarge the value of chest X-ray as a mainstay in the diagnosis of chest diseases.     

Neonatal chest computed radiography: image processing and optimal image display

OBJECTIVE: The purpose of this study was to determine soft-copy image display preferences of brightness, latitude, and detail contrast for neonatal chest computed radiography to establish a baseline for future work on low-dose imaging. CONCLUSION: Observers preferred brighter images with higher detail contrast and narrow to middle latitude for soft-copy display compared with the typical screen-film hard-copy appearance. Future research on low-dose neonatal chest imaging will be facilitated by an understanding of optimal soft-copy image display.     

A method to optimize the processing algorithm of a computed radiography system for chest radiography

A test methodology using an anthropomorphic-equivalent chest phantom is described for the optimization of the Agfa computed radiography "MUSICA" processing algorithm for chest radiography. The contrast-to-noise ratio (CNR) in the lung, heart and diaphragm regions of the phantom, and the "system modulation transfer function" (sMTF) in the lung region, were measured using test tools embedded in the phantom. Using these parameters the MUSICA processing algorithm was optimized with respect to low-contrast detectability and spatial resolution. Two optimum "MUSICA parameter sets" were derived respectively for maximizing the CNR and sMTF in each region of the phantom. Further work is required to find the relative importance of low-contrast detectability and spatial resolution in chest images, from which the definitive optimum MUSICA parameter set can then be derived. Prior to this further work, a compromised optimum MUSICA parameter set was applied to a range of clinical images. A group of experienced image evaluators scored these images alongside images produced from the same radiographs using the MUSICA parameter set in clinical use at the time. The compromised optimum MUSICA parameter set was shown to produce measurably better images.     

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.     

Effect of dose on image quality in a detector-based dual-exposure, dual-energy system for chest radiography

PURPOSE: To assess the image quality of subtracted soft tissue and bone images of a CsI-detector-based dual-energy system for chest radiography at varying dose levels. MATERIAL AND METHODS: We evaluated a CsI-detector-based, dual-exposure, dual-energy prototype system; 126 patients were categorized into groups of small, medium, and large. Fixed values were applied for mAs and mA. The patients were randomized into two groups with intended higher and lower speed pairs of approximately 400/1000 (high and low energy shot) and 200/500, respectively. True speed equivalents were calculated retrospectively using the detector dose. Image quality was evaluated by two highly experienced radiologists in consensus applying a rating scale of 1 to 5 for quality indicators such as image noise, residual bone structures, motion artifacts, and others. RESULTS: Significantly decreased noise and a significant improvement for display of bone details in the bone image were noted with the higher dose, whereas a significant increase in motion artifacts reduced image quality at the higher dose. CONCLUSION: Radiation dose did not significantly influence the perception of dual-energy image quality. Dual-energy subtraction, as described, has the potential to become a future routine application in chest radiography.     

数字X线摄影与双面阅读计算机X线摄影系统辐射剂量和影像质量的比较

目的 对比研究DR与CR双面阅读成像系统辐射剂量与成像质量的关系.方法 在相同的辐射剂量下用DR和CR双面阅读成像系统对ALVIM统计学摄影模体TRG进行曝光,记录摄影条件和模体表面入射剂量,然后固定此摄影条件kV值,选用不同mAs值(分别为3.2、4.0、4.3、4.8和5.2 mAs)用CR双面阅读成像系统对模体进行曝光,记录表面入射剂量,并将所获取的影像在图像诊断工作站显示器上由3名影像医师进行视读打分,按照5分值判断法评判,绘制ROC曲线,计算每种信号的判断概率值(Pdet),对不同摄影系统及条件下3名观察者对模体影像信息的判断概率平均值采用双因素方差分析(ANOVA)进行比较.结果 在辐射剂量为137.5 μGy时,DR所摄取模体影像内容物中0.5 ～ 1.0 nun直径骨代替物和0.9 ～ 2.0 mm直径肌肉代替物的Pdet值分别是0.742～0.923和0.635 ～0.900,CR双面阅读成像系统所得模体影像内容物中上述直径的骨代替物和肌肉代替物的Pdet值分别是0.526 ～0.586和0.473 ～0.560;DR和CR双面阅读成像系统对不同直径的骨替代物ROC曲线Pdet相差为0.216 ～0.337,以直径1.0mm为著,相差0.337;二者对直径0.9～2.0mm的肌肉替代物相差为0.075 ～0.342,以直径2.0mm为著,相差0.342,差异有统计学意义(F =35.30,P＜0.01).当CR双面阅读系统的摄影条件增为75 kV、4.8 mAs时,辐射剂量为180.4μGy,两种摄影系统所摄取影像ROC曲线的Pdet值基本相同,二者对直径0.5～1.0 mm骨代替物ROC曲线的Pdet相差为-0.003 ～0.009;二者对直径0.9 ～2.0 mm肌肉代替物ROC曲线的Pdet 相差为～0.005 ～0.008,差异无统计学意义(F=5.23,P＞0.05).在相同的判断概率值Pdet下,CR双面阅读成像系统所用表面入射剂量为180.4 μGy,DR所用的表面入射剂量为137.5 μGy.结论 在相同曝光条件下,DR对模体内容物的检出率高于CR双面成像系统;在获得相似图像质量时,DR的辐射剂量低于CR双面阅读成像系统.     

Optimization of chest radiographic imaging parameters: a comparison of image quality and entrance skin dose for digital chest radiography systems

We studied the performance of three computed radiography and three direct radiography systems with regard to the image noise and entrance skin dose based on a chest phantom. Images were obtained with kVp of 100, 110, and 120 and mA settings of 1, 2, 4, 8, and 10. Significant differences of image noise were found in these digital chest radiography systems (P<.0001). Standard deviation was significantly different when the mAs were changed (P<.001), but it was independent of the kVp values (P=.08-.85). Up to 44% of radiation dose could be saved when kVp was reduced from 120 to 100 kVp without compromising image quality.     

Investigation of optimum energies for chest imaging using film-screen and computed radiography

The purpose of the study was to compare the image quality of film-screen (FS) and computed radiography (CR) for adult chest examinations across a range of beam energies. A series of images of the CDRAD threshold contrast detail detection phantom were acquired for a range of tube potential and exposure levels with both CR and FS. The phantom was placed within 9 cm of Perspex to provide attenuation and realistic levels of scatter in the image. Hardcopy images of the phantom were scored from a masked light-box by two scorers. Threshold contrast indices were used to calculate a visibility index (VI). The relationships between dose and image quality for CR and for FS are fundamentally different. The improvements in VIs obtained using CR at 75 kVp and 90 kVp were found to be statistically significant compared with 125 kVp at matched effective dose levels. The relative performance of FS and CR varies as a function of energy owing to the different k-edges of each system. When changing from FS to CR, the use of lower tube potentials may allow image quality to be maintained whilst reducing effective dose. A tube voltage of 90 kVp is indicated by this work, but may require clinical verification.