خدمة تلخيص النصوص العربية أونلاين،قم بتلخيص نصوصك بضغطة واحدة من خلال هذه الخدمة
European Journal of Radiology 72 (2009) 209-217 Contents lists available atScienceDirect
European Journal of Radiology journal homepage:www.elsevier.com/locate/ejrad
Review Dose and perceived image quality in chest radiography Wouter J.H. Veldkamp * , Lucia J.M. Kroft, Jacob Geleijns Department of Radiology, C2S, Leiden University Medical Center, Albinusreef 2, 2333 ZA Leiden, The Netherlands
article info Article history: Received 15 February 2009 Received in revised form 22 May 2009 Accepted 22 May 2009 Keywords: Chest radiography Dose Image quality abstract Chest radiography is the most commonly performed diagnostic X-ray examination.Recent advances in CR are more efficient collection of light by reading both sides of the screen (dual-sided read CR) which results in an increased signal- to-noise ratio, line scan read out yields improved speed and the use of needle-like phosphor allows for improved X-ray absorption efficiency (a thicker phosphor) without loss of spatial resolution [10].For indirect conversion detectors, apart from to the high atomic number and high density, the needle-like structure of CsI reduces the spreading of light in the scintillator that allows the use of a thicker layer with higher efficiency compared with unstructured scintillators like GOS and regular CR systems[16-18].The circles represent the uncertainty in the results: the acquisition technique may vary in practice, differ- ent research methods are used in literature (each giving inherent limited results), differences found between systems are not nec- essary unequivocal and finally, differences between manufacturers may exist per detector type.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.The circles represent the uncertainty in the results: the acquisition technique may vary in practice, the research methods dif- fer in literature and give an inherent limited view and finally, differences between manufacturers may exist per detector type.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.In FPD's the conversion of the latent X-ray image into a measurable signal most often occurs in a layer of Cesium Iodide (CsI-FPD), Gadolinium Oxisulphide (GOS-
Fig.FPD) or Selenium (Se-FPD) in combination with a matrix of thin film transistors (TFTs) from which the light (CsI, GOS) or electrical charge (Se), provoked in the layer, is read out and transformed into a digital image.Chest radiography remains the mainstay for diagnosis of many pulmonary diseases, even despite recent developments in cross sectional imaging of the thorax, particularly computed tomography (CT).doi:10.1016/j.ejrad.2009.05.039 210W.J.H. Veldkamp et al. / European Journal of Radiology 72 (2009) 209-217
imprecision of the recording of an image, i.e. unwanted stochas- tic fluctuations in the image.One of these studies used an anthropomorphic phantom with simulated lesions to investigate detection of lesions in the chest for a digital CCD slot-scan system (Fig.Due to the superior anti scatter properties of the small scanning detector (most scattered photons will go along it) a grid can be omitted, therefore this technique is associated with excellent image quality at relatively low doses[13,15].It was demonstrated however that in radiographic chest images for detec- tion of lung nodules with sizes in the order of 10 mm, the projected anatomy (anatomic noise) is far more disturbing than quantum noise and system noise[8].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.The effective dose related to the lateral chest image is approximately
* Corresponding author at: Department of Radiology, C2S, Leiden University Med- ical Center, PO Box 9600, 2300 RC Leiden, The Netherlands.The digital chest X-ray acquisition technique will be reviewed and recent chest radiography technologies will be assessed, both with respect to diagnostic accuracy and radiation dose to the patient.Anatomic noise can be referred to as overlaying anatomic fea- tures such as ribs, lung vessels, heart, mediastinum, and diaphragm in a chest radiograph.Low exposures will still create images with clear appearance of gross anatomical structures but increased quantum noise will possibly hamper visu- alization of subtle anatomic and pathologic structures.Solutions for pro- jecting the chest on a CCD chip are the use of lenses or tapered optical fibers, at the cost of reduced dose efficiency and degraded image quality.An extensive overview of studies that investigated dose require- ments and image quality of various digital systems for chest radiography is given in a recent publication[19].Fig.New techniques that are currently being evaluated are dual energy, tomosynthesis, temporal subtrac- tion and rib suppression.In a European Directive, the need for optimization of acquisi- tion techniques for X-ray imaging and limitation of patient dose is established[1].The radiation level received at each point of X-ray photons reaching the storage phosphor screen, is stored in the local electron configu- ration by elevating the energy level of electrons (excitation) in the phosphor.The following systems were regarded: four different flat-panel detector systems, two different charge-coupled device systems, one selenium-coated drum, and one storage phosphor sys- tem.Better performance at low spatial frequencies for CsI-FPD systems compared to Se-FPD systems is found by stud- ies that use physical parameters to investigate image quality as a function of spatial frequency.Introduction Chest radiography is the most frequently performed diagnos- tic X-ray examination; it is of value for solving a wide range of clinical problems.Although individual patient dose in chest radiography is rela- tively low, its contribution to the collective dose is significant due to the frequent use of this examination.Chest radiography may be implemented also in screening pro- grammes in some countries, this would have a substantial impact on the collective dose from chest X-rays.The degree of the fluctuations is related to the exposure level: quan- tum noise in a detector element is proportional to the square root of the exposure level (noise expressed by the standard deviation of the signal).Apart from quantum noise other additional noise sources must be considered in digital radiography, i.e. detec- tor noise (for instance electronic noise) and anatomical noise[6-8].Detector noise becomes more significant at low exposure levels whereas for higher exposure levels quantum noise and anatomic noise will dominate in medical radiographs.In the past decade in most western European hospitals radiological film-screen (FS) radiography has been replaced by dig- ital radiography systems.In a uniform digital image, pixel values (that are associ- ated to the individual detector elements) will vary around their expected value.For example, improving the SNR by a factor 2 can only be obtained by increasing the dose by a factor 4 (assuming that quantum noise is the predominant source of noise).This aspect of digital radiographic systems forms an extra challenge and oppor- tunity for optimizing patient dose and (perceived) image quality.After exposure, the photon energy stored in the phosphor plate is read out by a laser scanner and a digital image is obtained.In the nineties, so-called direct radiography (DR) digital sys- tems with a flat-panel detector (FPD) became available for chest radiography.However, increased quantum noise at low doses possibly hamperes visualization of subtle anatomic and pathologic structures.Several studies have shown the advantages of digital systems compared to FS[11-13], for instance the improved visibility of the mediastinal areas in the image.Diagnostic perception
Quantum noise and detector noise hamper detection of objects with low contrast.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.In conclusion, opti- mization and new developments will enlarge the value of chest X-ray as a mainstay in the diagnosis of chest diseases.Advantages of chest radiography over cross sectional imaging are lower cost, lower dose and speed of acquisition and diagnosis.The resulting improvement of image quality will obviously have to be assessed clinically against the increased dose absorbed by the patient.Because of the small dynamic range, film-screen radiogra- phy images appear underexposed at low dose and overexposed at higher dose.An additional aspect of digital radiography is that differences in digital detector technology lead to differences in image quality and dose.This technique uses a linear array of small CCD detectors in combination with a narrow X-ray beam that scans the chest.Differences in diagnostic performance were found among the eight different digital chest systems in the configurations under which they were routinely applied in clinical practice.The DR systems significantly outperformed the single-sided read CR system with respect to image quality whereas the dose levels used with the DR systems were lower.Furthermore, the results suggested that the scanning CCD or slot-scan technique gave best detection results.3.Different digital systems with respect to dose and lesion detection were com- pared in a phantom study (Fig.Detectabil- ity of objects can be improved by increasing radiation dose.The frequent use and diagnostic importance of chest X-ray make that optimization of image quality and patient dose is an impor- tant area of research.Dose and image quality in digital radiography Dose in digital chest radiography mainly affects the noise in the images.Noise in radiography can be defined as uncertainty or 0720-048X/$ - see front matter (C) 2009 Elsevier Ireland Ltd.Here it should be noted that the signal in the detector element is proportional to the number of photons imparting on it [5].Therefore signal-to-noise ratio (SNR) and thereby image quality will improve with higher exposure levels.Detector technology
With the former film-screen systems, the range of patient dose in clinical practice was inherently limited by its sensitivity (speed class).In the observer study the radiologist identified the lesions and indicated the location of the detected lesions.The light is actu- ally transformed by photodiodes in the TFT-layer into an electrical signal.A different technique uses charge-coupled devices (CCD) in combination with a scintillation layer.The dose levels were roughly comparable between the digital and the FS system (speed class 400).Interestingly, differences in detection rates could not be explained by dose (Fig.This is explained by the high atomic number and high density of CsI resulting in good capture of the latent X-ray image.4givesanover- 212W.J.H. Veldkamp et al. / European Journal of Radiology 72 (2009) 209-217
Fig.Consequently, optimization of dose and image quality offers a challenging area of research.They may for example reduce the negative influence of over projection of ribs, referred to as anatomic noise.The effective dose related to a posterior-anterior (PA) radiographic chest image is about 0.02 mSv[2].The associated estimated contribution to the collective dose is about 18%[2].Studies that investigated dose reduction and perceived image quality will also be discussed.The most disturbing effect on image quality (and thereby on diagnosis) is that noise can cover or reduce the visibility of certain structures.This occurs since chest radiography involves the projection of a three-dimensional structure onto a two- dimensional image[3].It has been shown that anatomic noise can have an important negative effect on observer performance in detecting abnormalities especially in chest radiography[3,7,8].With digital radiography underexposure or overexpo- sure is less likely to occur.However, as explained above, the selected dose level used will influence the quantum noise level in the image and thereby the diagnostic potential.Already in the early eighties, computed radiography (CR) with phosphor plate systems was introduced.At first, the quality of these systems was moderate and the dose needed for recording chest images was higher compared to the FS systems.Or, in other words,irrespective of the dose, digital images are presented in similar gray values.W.J.H. Veldkamp et al. / European Journal of Radiology 72 (2009) 209-217211 Fig.Detectors that use a scintillator that converts X-ray into light, are called indirect conversion systems.The detector material in direct conversion systems directly converts X-ray photons into an electrical charge.It was found that with the digital system, the number of lesions observed in the mediastinum was almost twice the number found with the FS system.For the lung lesions no significant differ- ence was found.The systems were assessed for detection of simulated chest disease under clini- cal conditions.Differences in lesion detection (lower bar plot) cannot be explained by differences in dose but by detector design.In addition to patient dose, also the design of the various digital detectors seems to have an effect on image quality.In chest X-ray this type of noise may be the dominating factor in the detection of nodules.X-ray images of the chest provide important information for deciding upon further steps in the establishment of a diagnosis, treatment and follow-up procedure.The loss of visibility is especially significant for low contrast objects.Over the last 20 years the CR systems improved in both dose requirements as in image quality.1.Digital versus conventional chest radiography.For film-screen systems, the optical density is directly related to the dose (see upper row of images).(A) simulated interstitial linear disease; (B) simulated nodule; (C) simulated nodule.On the contrary, at the higher spatial frequencies the Se-based systems show better performance since these systems show less blurring of the image signal[16,17].Dose for various digital systems varies (upper bar plot).4.Common systems are given as a function of dose and image quality in clin- ical practice according to literature.These technologies may improve diagnostic chest X-ray further.(C) 2009 Elsevier Ireland Ltd.For com- parison, this is about 0.5% of a CT scan of the chest.E-mail address:[email protected](W.J.H. Veldkamp).In the Netherlands, about a third of all diagnostic X-ray examinations are a chest X-ray[2].Similar figures are reported in other western countries [3,4].An important source of noise in X-ray images is related to the random manner in which the photons are distributed within the image.This can be explained by wide dynamic range of these detectors[9](Fig.In a short period, different digital radiography chest systems were introduced for clinical use.This is not the case with digital techniques (lower row of images).2.A radiograph of an anthropomorphic phantom (left image).Different lesions are simulated and attached on the phantom.The chest is much larger than currently available CCD chips.A better solution is the uses of the so-called slot-scan technique.In another study the diagnostic performance was compared for eight different digital radiography chest systems[14].Several other researchers investigated or compared different digital systems.Detectability of objects on a uniform background is a function of object contrast and object size (detail).a two times higher compared to the dose of a PA projection [2].Therefore, the relation between these aspects is herewith discussed.Due to the better dynamic range at very low and very high doses, a clear image is shown.A selenium layer is used for this purpose.The differences in detector design were given as explanation here.all impression of the results found in literature as partly discussed above.Common systems are given as a function of dose and image quality in clinical practice.All rights reserved.Tel.: +31 71 5263689; fax: +31 71 5248256.All rights reserved.This type of noise is known as quantum noise.Fig.1.2.2.1.1).2). 3).2).2.2.
European Journal of Radiology 72 (2009) 209–217 Contents lists available atScienceDirect
European Journal of Radiology journal homepage:www.elsevier.com/locate/ejrad
Review Dose and perceived image quality in chest radiography Wouter J.H. Veldkamp ∗ , Lucia J.M. Kroft, Jacob Geleijns Department of Radiology, C2S, Leiden University Medical Center, Albinusreef 2, 2333 ZA Leiden, The Netherlands
article info Article history: Received 15 February 2009 Received in revised form 22 May 2009 Accepted 22 May 2009 Keywords: Chest radiography Dose Image quality abstract 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 subtrac- tion 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, opti- mization and new developments will enlarge the value of chest X-ray as a mainstay in the diagnosis of chest diseases. © 2009 Elsevier Ireland Ltd. All rights reserved.
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