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Recent Advances in Chest Radiography¹

¹ From the Department of Radiology, Duke Advanced Imaging Laboratories, Duke University Medical Center, Box 3808, Durham, NC 27710. Received September 14, 2005; revision requested October 24; revision received November 15; accepted January 2, 2006; final version accepted January 6. Supported by National Institutes of Health grant R01 CA80490 and a research agreement with from GE Healthcare. Address correspondence to H.P.M. (e-mail: Page.mcadams{at}duke.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
There have been many remarkable advances in conventional thoracic imaging over the past decade. Perhaps the most remarkable is the rapid conversion from film-based to digital radiographic systems. Computed radiography is now the preferred imaging modality for bedside chest imaging. Direct radiography is rapidly replacing film-based chest units for in-department posteroanterior and lateral examinations. An exciting aspect of the conversion to digital radiography is the ability to enhance the diagnostic capabilities and influence of chest radiography. Opportunities for direct computer-aided detection of various lesions may enhance the radiologist's accuracy and improve efficiency. Newer techniques such as dual-energy and temporal subtraction radiography show promise for improved detection of subtle and often obscured or overlooked lung lesions. Digital tomosynthesis is a particularly promising technique that allows reconstruction of multisection images from a short acquisition at very low patient dose. Preliminary data suggest that, compared with conventional radiography, tomosynthesis may also improve detection of subtle lung lesions. The ultimate influence of these new technologies will, of course, depend on the outcome of rigorous scientific validation.

© RSNA, 2006

	INTRODUCTION

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
Despite recent advances in cross-sectional imaging of the thorax, chest radiography remains the mainstay for diagnosis of many pulmonary diseases. In most instances, it is the first—and frequently the only—diagnostic imaging test performed in patients known to have or suspected of having a thoracic abnormality. In the United States, and very likely in the world, chest radiography remains the most commonly performed diagnostic imaging test overall.

In the more than 100 years since the discovery of the x-ray, technologic advances have resulted in many improvements in chest radiography (1). Progress in film-based imaging led to the development of excellent screen-film systems designed specifically for chest radiography (1). More recently, advances in electronics and computer technology have resulted in rapid development in digital image receptors and displays. Further, rapid development of image-processing techniques and of advanced applications such as dual-energy and temporal subtraction radiography, digital tomosynthesis, and computer-aided detection (CAD) and diagnosis (CADx) promise to substantially improve the way chest radiography is practiced in the future.

In this article, we will discuss the inherent challenges of chest radiography and the specific advances made to address these challenges, including new digital detector and image display technologies, developments in image-processing techniques and CAD and CADx applications for chest radiography, advanced applications such as dual-energy and temporal subtraction radiography, and chest tomosynthesis.

	CHALLENGES

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
To better appreciate the recent advances in chest radiography, we must first review some of the inherent challenges of the technique, because these challenges have been the prime motivators behind most of the developments. These challenges include but are not limited to issues related to image area and patient body habitus, latitude and dynamic range of x-ray transmission through the chest, scattered radiation, overlap of anatomic structures, and perceptual limitations.

Image Area and Body Habitus
The chest is one of largest and thickest body parts that is imaged on a routine basis. The typical imaging field in an adult readily exceeds 40-cm, particularly for patients with a large body habitus. This large field of view imposes challenging constraints on the size of the image receptor, given that the receptor must also provide consistent and uniform response over the entire field. This large field of view also increases the contribution of scattered radiation to the image, degrading the image's inherent quality. Given the marked increase in obesity in the United States over the past 20 years (2) among both children and adults, these issues will continue to pose considerable challenges to chest radiography in the future (Fig 1). For example, the authors of a recent study (3) found that the number of chest radiographs that were considered limited because of body habitus had more than doubled during a 14-year period at one hospital.

View larger version (127K): [in this window] [in a new window] [Download PPT slide]   Figure 1a: Chest radiographs obtained with computed radiography (CR) technique and grid in morbidly obese patient with signs and symptoms of pulmonary edema. (a) Anteroposterior image is limited by body habitus and was interpreted as showing possible edema. (b) Posteroanterior image obtained 30 minutes later shows no evidence of edema.

View larger version (122K): [in this window] [in a new window] [Download PPT slide]   Figure 1b: Chest radiographs obtained with computed radiography (CR) technique and grid in morbidly obese patient with signs and symptoms of pulmonary edema. (a) Anteroposterior image is limited by body habitus and was interpreted as showing possible edema. (b) Posteroanterior image obtained 30 minutes later shows no evidence of edema.

View larger version (20K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Graph shows dynamic range of an image as function of thickness of soft tissue penetrated and peak voltage (5). Max = maximum, Min = minimum.

View larger version (19K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Graph shows scatter fractions in lung, mediastinum, and diaphragm regions without a grid (black bars), with a grid (white bars), and with a slot-scan device (gray bars). Note marked reduction in scatter with slot-scan device.

The appearance of lung nodules on chest radiographs is of particular concern. The similarity of a lesion's appearance to that of the background anatomy in which it is located causes the conspicuity of the lesion to be poor. This phenomenon can result in up to 30% of pulmonary nodules being missed on initial chest radiographs (10), even though the nodules could be observed retrospectively. Purely on the basis of inherent contrast, a nodule as small as 3 mm in diameter should be visible on chest radiographs, even in the presence of scattered radiation; however, it is rare to detect nodules smaller than about 8 mm on chest radiographs, owing to the influence of anatomic noise. A great deal of work has been performed on the effect of anatomic noise on nodule detectability, beginning with the work of Kundel and colleagues (11,12) several decades ago. More recently, Samei et al (13) demonstrated that anatomic background is far more important than quantum noise in limiting the detectability of lung nodules (Fig 4).

View larger version (101K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: Images in middle-aged woman with history of right partial mastectomy for breast cancer who presented for routine follow-up. (a) Posteroanterior chest radiograph was interpreted as normal. (b) Transverse computed tomographic (CT) scan shows 8-mm nodule (arrowhead) in right lower lobe that was obscured by overlying breast implant, diaphragm, and rib shadows on a.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: Images in middle-aged woman with history of right partial mastectomy for breast cancer who presented for routine follow-up. (a) Posteroanterior chest radiograph was interpreted as normal. (b) Transverse computed tomographic (CT) scan shows 8-mm nodule (arrowhead) in right lower lobe that was obscured by overlying breast implant, diaphragm, and rib shadows on a.

Perceptual errors can occur at both the visual and the cognitive level. Incompleteness of the search task may lead to about 55% of the missed lesions. These errors occur when the observer fails to look at the territory of the lesion (30%) (11,16) or when he or she does not fix his or her eyes on the territory for a dwell time of at least 0.3 second (25%) (17). Cognitive errors (accounting for 45% of missed lesions) can occur when the fixation time on an abnormality candidate exceeds the above limit (eg, a nodule is recognized), but the observer commits a decision-making error by calling the case negative (11).

Results of prior studies suggest that perceptual errors can be reduced by providing radiologists with fixation feedback (18) and by using systematic search strategies, coning devices, and double reading of chest radiographs (19). CAD (further discussed later), with its ability to offer a complete search of the image data, has the potential to reduce certain types of perceptual error. The most effective method by far, however, is the reduction or elimination of anatomic noise, which has been shown to be the main factor limiting the detection of subtle lung nodules (13).

	DETECTOR DEVELOPMENTS

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
The imaging receptor (or detector) is a key component of chest radiography. In the past few decades, changes in receptor technology have brought about one of the most important advances in chest radiography, leading to improved image quality and new image acquisition techniques. This section will summarize the most important developments in this area. Because most advances have been focused on digital technologies, they will be the main focus of this section. The Table provides a summary of the current commercially available digital receptor technologies for chest radiography.

While screen-film receptors have been in continual use for many years, their inherent characteristics have imposed certain limitations on the practice of chest radiography. Owing to the limited light range sensitivity of film, represented by the Hurter and Driffield response of the receptor (Fig 5), the range of x-ray exposures that can be recorded by a screen-film detector is limited. Only a narrow range of exposures can be reliably represented on the film with acceptable contrast; exposures above or below this range will be represented suboptimally. Given the large variability in radiopacity of organs in the thoracic cavity, this characteristic leads to two limitations of analog chest radiography: (a) suboptimal contrast renditions at the extremes of the attenuation range of the thorax and (b) susceptibility to suboptimal image acquisition due to under- or overexposure.

View larger version (13K): [in this window] [in a new window] [Download PPT slide]   Figure 5: Hurter and Driffield curves of analog screen-film (InSight; Eastman Kodak) and digital (CR) systems. Detector signal is optical density for screen-film system and relative digital value for the digital system.

Figure 5 shows the Hurter and Driffield curves for a typical chest radiography screen-film system in comparison with a typical digital detector.

Digital Chest Radiography
While screen-film receptors have been the main technology for acquiring chest radiographs until recently, the use of digital radiography has been on the rise and is expected to replace analog technology in the near future. Digital radiography has also enabled the implementation of picture archiving and communication systems, which have their own associated advantages. There have been six main forces driving the transition to digital:

1. Decoupling of acquisition and display functions of the acquisition device in digital radiography makes it possible to optimize either of those functions independently—for example, by optimizing display contrast independently of the exposure level.

2. Availability of the image data in an electronic form makes it possible to postprocess the image for optimal display and to display the images on the more flexible soft-copy viewing workstations.

3. Availability of the image data in an electronic form makes it possible to archive the data electronically, which uses less space and requires less labor for storage.

4. Availability of the image data in an electronic form makes it possible to distribute images widely and to make copies of images available concurrently to multiple users.

5. Most digital radiography systems have integrated acquisition and processing units in the same physical system, eliminating the need to handle the cassettes and thus improving the workflow for image acquisition in the department.

6. Most digital radiography systems are able to acquire multiple images in a rapid sequence, enabling new advanced imaging applications that require the acquisition of multiple images from the chest, as will be discussed in a later section of this article.

Because of these advantages, digital radiography has been a main focus of new developments in chest radiography. Differing technologies have been the basis of the development of various commercial products, which are summarized below.

In comparing various chest radiography systems, a common metric, detective quantum efficiency, has been used as a basis of comparison. Detective quantum efficiency describes the inherent signal-to-noise performance of an imaging system per unit incident exposure to the detector (22). Most commonly reduced to its value at zero spatial frequency, various chest radiography systems offer a detective quantum efficiency in the range of 20%–70% (23,24). These values are reflective of the system performance in the absence of scattered radiation. Most recently, a new metric, termed effective detective quantum efficiency, has been introduced that further accounts for the presence of scattered radiation (25), reflecting the inherent signal-to-noise performance of an imaging system in actual clinical use.

CR Systems
CR was the first commercial digital imaging modality widely used in chest imaging (26) and currently is still the most common technology for acquiring digital chest radiographs, especially for bedside applications. The technology is based on photostimulable properties of barium halide phosphors. With a cassette similar to that used in screen-film radiography, the phosphor screen is exposed to x-rays. After exposure, the cassette is transported to a computed radiography reader device, which subjects the screen to a scanning laser beam. The laser releases the energy locally deposited by x-rays on the screen, causing the screen to fluoresce. The released light is used to form the image after it is collected by a light guide, digitized, and associated with the geometric location of the laser beam at the time of stimulation. The digital image data are then processed for presentation.

Since its commercial introduction more than 2 decades ago, CR has been under continuous improvement. The most recent advances include the collection of photostimulated light from both sides of the screen, leading to improved detective quantum efficiency (currently available only for mammography) (27), the use of line scanning and light collection technology for improved speed (28), and the use of structured phosphor for improved detection efficiency without associated loss in resolution (29).

In the most common implementation, CR devices produce image quality that tends to be lower than that for other digital radiography systems (23), but CR systems possess certain operational and economic advantages that cause them to remain competitive with other digital modalities. Those advantages can be summarized as follows: (a) In the most common implementation, CR cassettes are identical in size to screen-film cassettes, making it possible to retrofit existing analog radiography rooms; (b) the fact that the CR reader and cassettes are separate entities makes the technology extremely convenient for bedside applications; and (c) the separation of CR reader and cassette makes it further possible to have one (higher-cost) CR reader used in reading multiple (lower-cost) CR cassettes, which allows one CR reader to serve multiple radiography rooms and reduces the up-front cost of the transition to digital radiography.

Flat-Panel Detectors
While CR technology currently has the largest number of installations in the digital radiography market, with its notable advantages in terms of cost and utility for bedside applications, its disadvantages in terms of image quality per unit dose and suboptimal workflow (owing to the physical separation of the acquisition and processing functions) provide opportunities for another noteworthy technology, the flat-panel detector.

Made possible by advances in the fabrication of flat-panel displays for the computer industry, flat-panel detectors are made of thin layers of amorphous silicon thin-film transistors (TFTs) deposited on a piece of glass. The TFT layer is coupled with an x-ray absorptive layer. Depending on the material, there are two types of flat-panel detector. Indirect flat-panel detectors use a phosphor screen, most commonly cesium iodide, to convert the x-rays to light photons, which are subsequently detected by the photodiode array associated with the TFT layer and are converted to charge deposited in the capacitors associated with each TFT (24,30). Direct flat-panel detectors use instead a photoconductor layer, most commonly amorphous selenium that converts the x-ray energy directly to charge, which is subsequently directed to the collecting TFT-capacitor array through the application of a strong electric field (31,32). After exposure, the charge on the capacitors is collected line by line and pixel by pixel by using the associated gate and data lines, forming the raw digital image data for processing and display.

Flat-panel detectors are relatively new, and there continue to be a number of new developments in the technology. Most noteworthy are the use of x-ray capture materials with higher inherent absorption efficiency, improvement in the electronics to increase the frame rate and bit-depth resolution of the images, and use of flexible substrates in place of glass to enable more rugged and damage-resistant detectors, which can perhaps make these detectors practical for bedside applications (33).

Flat-panel detectors have three noteworthy advantages compared with other digital radiographic technologies. These advantages can be summarized as follows:

1. With the use of structured phosphor in indirect detectors and the application of an electric potential in direct detectors, the x-ray–sensitive layer of these detectors can be notably thicker than that of competing technologies. This makes it possible to have increased x-ray detection efficiency with minimal loss of resolution. Consequently, patient dose can be reduced without degradation of image quality (34).

2. Flat-panel detectors can acquire multiple images in a short time. Current frame rates of 30 frames per second are available in the most recent models. This makes it possible to acquire multiple images of the patient with different techniques or from different directions with minimal motion blur. This feature facilitates the use of these detectors for a host of advanced applications such as tomosynthesis and dual-energy imaging.

3. With integration of the acquisition and processing units, flat-panel detectors offer improved workflow compared with that of CR.

CCD- and CMOS-based Detectors
In the past few years, CCD and CMOS cameras have provided an alternative technology for the acquisition of digital chest radiographs. With these detectors, the x-ray energy is first converted to light within a phosphor layer. The light is then directed to a single or a multitude of CCD or CMOS cameras that detect the light image and form the radiograph (35). An important component of these detectors is the coupling of the phosphor layer and the camera. Since most CCD and CMOS sensors are limited in size, it is necessary to demagnify the original light image generated on the phosphor screen so that it can be entirely captured by the camera. This is accomplished by using either a fiberoptic coupler or a lens system. In either case there is a loss of efficiency, since only a small fraction of the light photons generated by the phosphor are detected by the camera(s). Consequently, the inherent efficiency of these detectors is limited (34).

New developments in these types of detectors have been focused on more efficient coupling between the phosphor and the camera, the use of larger sensors to minimize demagnification, and improvements in phosphor efficiency. These advances have led to the development of detectors with detection efficiencies notably higher than those of previous-generation CCD or CMOS devices. These detectors are generally less expensive than other digital technologies.

Slot-Scan Technology
As noted earlier, scattered radiation is of notable concern in chest radiography. Over the years, various techniques have been developed to reduce the contribution of scattered photons to the x-ray image, including the use of antiscatter grids (36,37), which are used in most current chest radiography systems, and air gaps (38). While these techniques can reduce scatter substantially (36), they also lead to increased patient dose, as well as a reduced field of view in cases of an air gap. These fundamental limitations can be overcome with the use of scanning beam and slit devices (37,39–42).

A recent commercial product has taken advantage of slot-scan technology to reduce the amount of scattered radiation on digital chest radiographs (43). The detector consists of a cesium iodide scintillation layer fiberoptically coupled to a series of linear CCDs. With no antiscatter grid in place, a narrow-fan x-ray beam synchronized with the movement of the detector assembly scans the patient. The image data are continuously read from the CCDs as the patient is scanned by using the time-integration method (44,45). After scanning, the image data are processed for optimal display.

The main advantage of this technology is superior scatter rejection with little effect on the detection of primary radiation. This can notably enhance the effective detection efficiency of the imaging system. Results from a recent study (25) show that this enhancement can more than compensate for the inherent efficiency limitation of CCD-based detectors, leading to improved image quality at reduced dose.

The technologies described in the preceding paragraphs represent the bulk of commercial offerings for digital chest radiography. There are a number of other technologies that are currently under development, however, including those related to new sensor materials and new detector designs such as photon-counting devices. Commercial implementation of these technologies awaits further development.

	IMAGE-PROCESSING DEVELOPMENTS

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
Image data acquired with radiographic detectors cannot be viewed without additional processing and proper display. In the case of screen-film images, the film must be chemically processed and is eventually displayed on a light box. In the case of digital images, digital processing is required before a clinician can view the images. Since most of the recent developments in chest radiography have focused on digital modalities, this section will outline the current state of digital image-processing methods for optimal presentation of digital images, as well as hardware components of soft-copy display devices. Readers interested in the current state of film-processing techniques are advised to consult two publications focused on that topic (20,46).

In recent years, the increasingly digital nature of chest radiography, as well as advances in computer technology, have facilitated the application of computerized image analysis for improving the clinical efficacy of chest radiographs. As a general rule, computerized medical image analysis aims to reduce both the perceptual and the cognitive errors that burden the diagnostic interpretation process. Consequently, the latest image-processing developments in chest radiography fall into two general application areas: (a) techniques to improve visual presentation and soft-copy reading of radiographs and (b) techniques to automate diagnostic interpretation of such images. For the most part, these techniques are application dependent.

Prior to display, each digital image commonly undergoes a series of processing steps (Fig 6). Broadly speaking, these processes can be divided into two parts: preprocessing and postprocessing.

View larger version (121K): [in this window] [in a new window] [Download PPT slide]   Figure 6a: Effects of various (a–c) gray-scale and (d–f) equalization postprocessing schemes on appearance of a posteroanterior chest radiograph. Gray-scale processing improves contrast of image features with a corresponding reduction in latitude (ie, range of data that is properly displayed). Equalization, on the other hand, provides improved visualization of details without a corresponding reduction in latitude. (Image courtesy of M. J. Flynn, PhD, Henry Ford Hospital, Detroit, Mich.)

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 6b: Effects of various (a–c) gray-scale and (d–f) equalization postprocessing schemes on appearance of a posteroanterior chest radiograph. Gray-scale processing improves contrast of image features with a corresponding reduction in latitude (ie, range of data that is properly displayed). Equalization, on the other hand, provides improved visualization of details without a corresponding reduction in latitude. (Image courtesy of M. J. Flynn, PhD, Henry Ford Hospital, Detroit, Mich.)

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 6c: Effects of various (a–c) gray-scale and (d–f) equalization postprocessing schemes on appearance of a posteroanterior chest radiograph. Gray-scale processing improves contrast of image features with a corresponding reduction in latitude (ie, range of data that is properly displayed). Equalization, on the other hand, provides improved visualization of details without a corresponding reduction in latitude. (Image courtesy of M. J. Flynn, PhD, Henry Ford Hospital, Detroit, Mich.)

View larger version (165K): [in this window] [in a new window] [Download PPT slide]   Figure 6d: Effects of various (a–c) gray-scale and (d–f) equalization postprocessing schemes on appearance of a posteroanterior chest radiograph. Gray-scale processing improves contrast of image features with a corresponding reduction in latitude (ie, range of data that is properly displayed). Equalization, on the other hand, provides improved visualization of details without a corresponding reduction in latitude. (Image courtesy of M. J. Flynn, PhD, Henry Ford Hospital, Detroit, Mich.)

View larger version (143K): [in this window] [in a new window] [Download PPT slide]   Figure 6e: Effects of various (a–c) gray-scale and (d–f) equalization postprocessing schemes on appearance of a posteroanterior chest radiograph. Gray-scale processing improves contrast of image features with a corresponding reduction in latitude (ie, range of data that is properly displayed). Equalization, on the other hand, provides improved visualization of details without a corresponding reduction in latitude. (Image courtesy of M. J. Flynn, PhD, Henry Ford Hospital, Detroit, Mich.)

View larger version (123K): [in this window] [in a new window] [Download PPT slide]   Figure 6f: Effects of various (a–c) gray-scale and (d–f) equalization postprocessing schemes on appearance of a posteroanterior chest radiograph. Gray-scale processing improves contrast of image features with a corresponding reduction in latitude (ie, range of data that is properly displayed). Equalization, on the other hand, provides improved visualization of details without a corresponding reduction in latitude. (Image courtesy of M. J. Flynn, PhD, Henry Ford Hospital, Detroit, Mich.)

The second type of preprocessing is scaling. One of the advantages of digital radiography is the wide range of exposures within which the detector is able to provide a consistent response. This enables the acquisition of high-latitude chest radiographs with less susceptibility to over- and underexposure. However, the dynamic range of the detector is beyond the perception capability of the human visual system, and full-data presentation leads to considerable contrast reduction. Identification of the anatomically relevant range of exposures is essential for optimal display of the image.

This task is typically achieved through two steps. First, the collimated area is segmented to identify the exposed area and thus exclude unexposed areas from further analysis. Second, the anatomically relevant range of exposures in the exposed area is identified. The most common technique for this task is histogram analysis of the image data. On the basis of the expected general distribution of pixel values in the anatomic area of interest, the image system marks the range of the detector signal of interest to be used for postprocessing.

Image Postprocessing
Once the raw image data are corrected for inherent "flaws" and the useful range of image data is identified, the data undergo image postprocessing. Digital radiography has utilized various digital postprocessing algorithms for enhanced image display of chest radiographs. Broadly speaking, these algorithms can be categorized into three types: gray-scale processing, edge enhancement, and multifrequency processing (48,49).

Gray-scale processing.—This process involves the conversion of detector signal values to display values. In this process, the display intensities of an image are changed by means of either a look-up table or windowing and leveling. Most systems employ a response look-up table similar to the Hurter and Driffield response of screen-film systems, so that digital chest radiographs look similar to conventional film images.

Edge enhancement.—This process aims to enhance fine details within the image by manipulating the high-frequency content of the radiograph, most commonly by using a variant of the unsharp masking technique in which a blurred version of the image is formed, and a fraction of the resultant image is subtracted from to the original image. The process is commonly used to compensate for the lower inherent resolution performance of CR images. However, its use is often balanced against the implied enhancement of quantum mottle within the image and the unusually textured appearance of the lungs.

Multifrequency processing.—While edge enhancement offers only a simplistic modification of spatial frequencies on the radiograph, multifrequency processing involves a more flexible manipulation of multiple portions of the frequency spectrum. The image is initially decomposed into multiple frequency components. The component images are then weighted and added back together. If the processing parameters are set optimally, the resultant image can compress the overall dynamic range of the image while at the same time enhance local contrast. This is of particular utility in chest radiography, where the details of opaque regions of the image are made visible with no compromise in the contrast in the lung regions. The widely used MUSICA multiscale image-processing package offered by Agfa (Greenville, SC) is an example of this type of processing (50).

While these algorithms have enabled flexibility in the presentation quality of chest radiographs, most chest radiography systems are set up to provide digital chest radiographs that mimic the appearance of screen-film images, with varying degrees of success. While it is possible to use the appearance flexibility of digital radiographs to optimize the system in terms of diagnostic performance, that task has remained largely untackled. One reason, perhaps, has been the large number of abnormalities of interest on chest radiographs, each of which might need to be optimized independently. Furthermore, there is uncertainty as to the extent that image postprocessing algorithms can improve the presentation of subtle lesions without similar enhancement of anatomic patterns and increased false-positive rates.

	DISPLAY DEVELOPMENTS

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
Once processed, the digital chest radiograph must be viewed by a radiologist. Soft-copy display is an essential element of contemporary digital chest radiography, because many advantages of digital imaging cannot be realized without soft-copy display. The conventional method to display digital radiographs has been on cathode-ray tubes, which currently still dominate the market (51). More recently, however, active-matrix liquid-crystal displays are rapidly replacing cathode-ray tubes in many facilities (52–54). The advantages of liquid-crystal displays include improved resolution, reduced weight, smaller form factor, reduced reflection, improved bit depth, and improved luminance range (55), although their disadvantages in terms of angular response and structured noise have not yet been fully addressed (56–58).

Another recent trend in soft-copy display has been the increased acceptance of color monitors, some of which have shown acceptable technical performance (59) for radiographic applications. With cathode-ray tubes, a monochrome monitor offers important advantages over a color monitor in terms of image quality due to improved brightness, reduced glare, reduced reflection, and improved resolution (55). Therefore, color cathode-ray tubes have not demonstrated adequate performance for clinical use. Current color liquid-crystal displays, however, do not have the same drawbacks as do color cathode-ray tubes, other than reduced brightness, which can be tolerated given the fact that most current liquid-crystal displays have better luminance response. The use of color monitors offers the advantage of being able to accommodate applications other than image viewing on the same device, with workflow and multitasking advantages. Color monitors would also make it possible to take advantage of color for viewing multidimensional chest images on the same display. Current commercial offerings also have enabled the luminance calibration of color displays to the gray-scale standard display function (60). It is thus expected that color liquid-crystal displays will gradually replace the monochrome monitors in clinical practice.

	APPLICATION DEVELOPMENTS

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
As noted earlier, discerning subtle lesions in chest radiographs is made difficult by the confluence of anatomy that overlies the lesion. An early attempt at overcoming poor lesion conspicuity was equalization radiography (61,62). Equalization was developed to improve the visibility of lesions in dense regions of the chest such as the mediastinum and the retrocardiac and retrodiaphragmatic areas. With screen-film radiography, these areas of the chest were typically underexposed and showed poor lesion contrast owing to the shape of the Hurter and Driffield curve of the film. Equalization modulated the incident intensity onto the patient in such a way that a higher exposure was delivered to the dense regions, and a lower exposure was delivered to the unobscured lungs. This process forced all of the thoracic regions into the higher-contrast "linear" portion of the film response curve. Studies indicated significantly improved detection of lesions when equalization was used (62,63). Despite its promise, equalization has largely disappeared from clinical use because of the edge-enhanced appearance imposed on the images, which some found difficult to interpret, and also because of the more recent transition to digital imaging and away from screen-film radiography.

The advent of digital chest radiography in the 1980s enabled the development of new techniques to improve the detection of subtle lesions. These techniques included algorithms, typically coupled with methodological innovations that used some aspect of imaging physics to improve conspicuity, and/or image subtraction strategies. Three notable techniques are dual-energy imaging, temporal subtraction imaging, and digital tomosynthesis. All of these techniques are implemented by using a conventional chest radiography system coupled with a digital imaging receptor.

Dual-Energy Subtraction Imaging
Dual-energy subtraction imaging can be used to generate images of two independent tissue types, most commonly bone and soft tissue. The dual-energy technique distinguishes bone from soft tissue by using the known energy dependence of x-ray attenuation in soft tissue and in bone. Calcified structures attenuate far more heavily, by means of photoelectric absorption, than do soft-tissue structures; thus, the contrast of calcium diminishes with increases in beam energy much faster than does the contrast of soft tissue. Thus, if two images are acquired at different beam energies, the image obtained at the lower energy will show a larger fraction of contrast from bone than from soft tissue. These two images may be combined in such a way that the soft-tissue or calcium components can be exactly isolated. Typically, an image containing only calcified structures and an image containing only soft-tissue structures are generated (Fig 7). Thus, dual-energy subtraction radiography can improve lung nodule conspicuity by eliminating overlying anatomic noise from the bones. The technique can also be used to better demonstrate calcium in lesions (64–68).

View larger version (136K): [in this window] [in a new window] [Download PPT slide]   Figure 7a: Dual-energy subtraction radiography in healthy middle-aged woman. Conventional (a) posteroanterior, (b) bone-subtracted, and (c) soft-tissue–subtracted images are normal except for scoliosis. Note improved depiction of vascular anatomy in b.

View larger version (135K): [in this window] [in a new window] [Download PPT slide]   Figure 7b: Dual-energy subtraction radiography in healthy middle-aged woman. Conventional (a) posteroanterior, (b) bone-subtracted, and (c) soft-tissue–subtracted images are normal except for scoliosis. Note improved depiction of vascular anatomy in b.

View larger version (166K): [in this window] [in a new window] [Download PPT slide]   Figure 7c: Dual-energy subtraction radiography in healthy middle-aged woman. Conventional (a) posteroanterior, (b) bone-subtracted, and (c) soft-tissue–subtracted images are normal except for scoliosis. Note improved depiction of vascular anatomy in b.

The second commercially available dual-energy method uses flat-panel detectors (73). In this embodiment, a sandwich detector configuration is not practical, so two separate x-ray exposures are made of the patient. The first exposure is at a standard voltage for chest radiography (typically, 120 kV), and the second is acquired at a lower voltage (typically, 60 kV). These images are separated in time by up to several hundred milliseconds, so there is the potential for some motion of patient anatomy between exposures, thus giving rise to some potential edge artifacts in the resulting tissue and bone images. However, the image quality of the flat-panel dual-energy images is much higher than that of the CR embodiment, owing to better energy separation between the low- and high-energy beams, higher x-ray flux in the high-energy image, and better detective quantum efficiency of the flat-panel detector, relative to those of CR. Image postprocessing is accomplished with the flat-panel dual-energy images to mitigate anatomic misregistration between the low- and high-energy images, but some slight edge effects still persist (Fig 8).

View larger version (164K): [in this window] [in a new window] [Download PPT slide]   Figure 8: Soft-tissue subtracted image from dual-energy chest radiograph in patient with right aortic arch. Excess cardiac motion results in incomplete subtraction of soft-tissue components and considerable artifact (arrow) at left cardiac border.

Dual-energy methods have been in clinical use for several years, and there are data in the literature that indicate that dual-energy imaging can improve detection and classification of pulmonary nodules. Early reports (65,67,70,81) indicated statistically significant improvements in the detection of pulmonary nodules and patterns of calcification (Fig 9). Recent studies in which a flat-panel dual-exposure system (Revolution XQ/i; GE Healthcare, Milwaukee, Wis) was used showed the clinical efficacy of dual-energy subtraction for the detection of calcified chest abnormalities (82) and noncalcified pulmonary nodules (83). For 37 calcified chest lesions, radiologists' sensitivity significantly increased from 36% to 66%, while specificity remained constant at 73% (82). Statistically significant but smaller improvements in sensitivity, specificity, and confidence were also observed for 59 noncalcified pulmonary nodules ranging from 0.2 to 2.5 cm in diameter (83).

View larger version (145K): [in this window] [in a new window] [Download PPT slide]   Figure 9a: Dual-energy subtraction radiography in a healthy middle-aged man. (a) Conventional posteroanterior radiograph shows possible lung nodule (arrow) overlying right upper lobe. (b) Bone-subtracted image shows small soft-tissue nodule (arrow) in left lung apex, not seen on a. No nodule is seen in right upper lobe. (c) Soft-tissue–subtracted image confirms that nodule seen on a represents calcification at first costochondral junction (arrow).

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 9b: Dual-energy subtraction radiography in a healthy middle-aged man. (a) Conventional posteroanterior radiograph shows possible lung nodule (arrow) overlying right upper lobe. (b) Bone-subtracted image shows small soft-tissue nodule (arrow) in left lung apex, not seen on a. No nodule is seen in right upper lobe. (c) Soft-tissue–subtracted image confirms that nodule seen on a represents calcification at first costochondral junction (arrow).

View larger version (185K): [in this window] [in a new window] [Download PPT slide]   Figure 9c: Dual-energy subtraction radiography in a healthy middle-aged man. (a) Conventional posteroanterior radiograph shows possible lung nodule (arrow) overlying right upper lobe. (b) Bone-subtracted image shows small soft-tissue nodule (arrow) in left lung apex, not seen on a. No nodule is seen in right upper lobe. (c) Soft-tissue–subtracted image confirms that nodule seen on a represents calcification at first costochondral junction (arrow).

View larger version (172K): [in this window] [in a new window] [Download PPT slide]   Figure 10a: Dual-energy subtraction radiography in an elderly man. (a) Soft-tissue–subtracted posteroanterior image shows coronary artery calcification (arrow) not clearly identified on (b) conventional posteroanterior image. (Image courtesy of R. C. Gilkeson, MD, University Hospital, Cleveland, Ohio.)

View larger version (153K): [in this window] [in a new window] [Download PPT slide]   Figure 10b: Dual-energy subtraction radiography in an elderly man. (a) Soft-tissue–subtracted posteroanterior image shows coronary artery calcification (arrow) not clearly identified on (b) conventional posteroanterior image. (Image courtesy of R. C. Gilkeson, MD, University Hospital, Cleveland, Ohio.)

Temporal Subtraction Imaging
Another way to improve the visual assessment of chest radiographs is with temporal image subtraction. Temporal subtraction techniques aim to selectively enhance areas of interval change by subtracting the patient's previous radiograph from the current one (86). The quality of the difference image strongly depends on the success of the two-dimensional registration and warping of the two radiographs so that the effect of patient positioning variation is minimized (87–89). Generally, the difference image appears uniformly gray in areas of no change. Areas that stand out in the uniform gray background indicate interval change (Fig 11). Several studies (91–98) have shown that temporal subtraction improves the visual perception of subtle abnormalities such as pulmonary nodules, infiltrative opacities, and diffuse lung disease. A 20% reduction in the average reading time when temporal subtraction is used was noted as well (91). Currently, temporal subtraction is commercially available in Japan (99).

View larger version (148K): [in this window] [in a new window] [Download PPT slide]   Figure 11a: Temporal subtraction radiography in a middle-aged patient. (a) Initial posteroanterior radiograph was interpreted as normal, even when compared with (b) that from examination performed 1 year previously. (c) Subtraction image suggests new masses in both upper lobes (arrows), confirmed on (d, e) transverse CT scans. Biopsy revealed synchronous lung cancers. (Reprinted, with permission, from reference 90.)

View larger version (148K): [in this window] [in a new window] [Download PPT slide]   Figure 11b: Temporal subtraction radiography in a middle-aged patient. (a) Initial posteroanterior radiograph was interpreted as normal, even when compared with (b) that from examination performed 1 year previously. (c) Subtraction image suggests new masses in both upper lobes (arrows), confirmed on (d, e) transverse CT scans. Biopsy revealed synchronous lung cancers. (Reprinted, with permission, from reference 90.)

View larger version (178K): [in this window] [in a new window] [Download PPT slide]   Figure 11c: Temporal subtraction radiography in a middle-aged patient. (a) Initial posteroanterior radiograph was interpreted as normal, even when compared with (b) that from examination performed 1 year previously. (c) Subtraction image suggests new masses in both upper lobes (arrows), confirmed on (d, e) transverse CT scans. Biopsy revealed synchronous lung cancers. (Reprinted, with permission, from reference 90.)

View larger version (133K): [in this window] [in a new window] [Download PPT slide]   Figure 11d: Temporal subtraction radiography in a middle-aged patient. (a) Initial posteroanterior radiograph was interpreted as normal, even when compared with (b) that from examination performed 1 year previously. (c) Subtraction image suggests new masses in both upper lobes (arrows), confirmed on (d, e) transverse CT scans. Biopsy revealed synchronous lung cancers. (Reprinted, with permission, from reference 90.)

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 11e: Temporal subtraction radiography in a middle-aged patient. (a) Initial posteroanterior radiograph was interpreted as normal, even when compared with (b) that from examination performed 1 year previously. (c) Subtraction image suggests new masses in both upper lobes (arrows), confirmed on (d, e) transverse CT scans. Biopsy revealed synchronous lung cancers. (Reprinted, with permission, from reference 90.)

Digital tomosynthesis can produce an unlimited number of section images at arbitrary depths from a single set of acquisition images. A digital detector, conventional x-ray tube, and computer-controlled apparatus to move the x-ray tube are used; during motion of the tube, a series of projection radiographs are acquired, and the anatomy at different depths in the patient changes orientation in the projection images owing to parallax. These projection images are then shifted and added to bring into focus objects in a given plane. By varying the amount of shift, different plane depths can be reconstructed. Objects outside of the focus plane are rendered with varying amounts of blur.

A variety of reconstruction approaches have been investigated for tomosynthesis, although the simple shift-and-add approach is the most common. It is important to use a deblurring algorithm to eliminate the residual blur from structures overlying the planes of interest. Various approaches for deblurring have been investigated, including matrix-inversion tomosynthesis (101–105), filtered back projection (106,107), and iterative restoration (108). These deblurring methods produce section images with excellent rendition of anatomy and effective elimination of structures outside the section of interest.

Tomosynthesis has been applied to such diverse applications as angiographic, dental, orthopedic, breast, and chest imaging (104); currently, the areas receiving the most clinical and research interest are breast and chest imaging. Investigators in our laboratory are currently conducting an National Institutes of Health–funded clinical trial of the efficacy of tomosynthesis for improving the detection of pulmonary nodules. As part of this effort, a series of optimization studies were conducted to determine the best acquisition and reconstruction parameters for chest imaging applications. It was found that 20° of vertical motion, 71 projection images, and 69 reconstructed planes are best (109). Spacing of reconstructed sections is typically 3–5 mm. Several adjacent sections are averaged to produce a slab image with much better image quality than a single thin section, but the spacing of these sliding-average slabs remains about 5 mm, sufficient to easily depict nodules of several millimeters in diameter. Total x-ray exposure for acquisition of the tomosynthesis projection images is about equal to that for one lateral screen-film radiograph.

In a pilot study of 20 human subjects (110), tomosynthesis provided subjectively far superior visualization of nodules, vasculature, airways, and ribs than did conventional posteroanterior chest radiographs (Figs 12–14). In the pilot study, preliminary evaluation revealed considerable improvement in the detection of CT-confirmed nodules with digital tomosynthesis compared with detection with conventional posteroanterior radiographs, but these findings must be confirmed quantitatively in the larger ongoing National Institutes of Health trial. Early indications are that tomosynthesis may offer an improvement over conventional radiography in the visualization of chest anatomy, particularly pulmonary nodules, at a lower radiation dose than with CT. This technique may also prove to be less expensive than CT because of the anticipated cost of soon-to-be-available commercial equipment. Tomosynthesis is not envisioned as a replacement for CT (owing to its reduced resolution in the depth direction), but it does appear to offer potential for improved diagnosis when used as an adjunct to conventional chest radiography. A commercial chest tomosynthesis product is being readied for market.

View larger version (152K): [in this window] [in a new window] [Download PPT slide]   Figure 12a: Images in middle-aged woman with history of right partial mastectomy for breast cancer who presented for routine follow-up (same patient as in Fig 4). (a) Digital tomosynthesis section image of whole chest and (b) magnified section images of right lower lobe clearly show right lower lobe nodule (arrows). (c) Conventional posteroanterior radiograph is shown for comparison.

View larger version (161K): [in this window] [in a new window] [Download PPT slide]   Figure 12b: Images in middle-aged woman with history of right partial mastectomy for breast cancer who presented for routine follow-up (same patient as in Fig 4). (a) Digital tomosynthesis section image of whole chest and (b) magnified section images of right lower lobe clearly show right lower lobe nodule (arrows). (c) Conventional posteroanterior radiograph is shown for comparison.

View larger version (148K): [in this window] [in a new window] [Download PPT slide]   Figure 12c: Images in middle-aged woman with history of right partial mastectomy for breast cancer who presented for routine follow-up (same patient as in Fig 4). (a) Digital tomosynthesis section image of whole chest and (b) magnified section images of right lower lobe clearly show right lower lobe nodule (arrows). (c) Conventional posteroanterior radiograph is shown for comparison.

View larger version (140K): [in this window] [in a new window] [Download PPT slide]   Figure 13a: Images in middle-aged man. (a) Conventional posteroanterior radiograph, (b) digital tomosynthesis section image of whole chest, and (c) magnified section images of right hilum show 15-mm nodule (arrows) overlying right hilum. Despite relatively large size of this nodule, it cannot be seen on a owing to superimposition of hilar vessels.

View larger version (142K): [in this window] [in a new window] [Download PPT slide]   Figure 13b: Images in middle-aged man. (a) Conventional posteroanterior radiograph, (b) digital tomosynthesis section image of whole chest, and (c) magnified section images of right hilum show 15-mm nodule (arrows) overlying right hilum. Despite relatively large size of this nodule, it cannot be seen on a owing to superimposition of hilar vessels.

View larger version (179K): [in this window] [in a new window] [Download PPT slide]   Figure 13c: Images in middle-aged man. (a) Conventional posteroanterior radiograph, (b) digital tomosynthesis section image of whole chest, and (c) magnified section images of right hilum show 15-mm nodule (arrows) overlying right hilum. Despite relatively large size of this nodule, it cannot be seen on a owing to superimposition of hilar vessels.

View larger version (118K): [in this window] [in a new window] [Download PPT slide]   Figure 14a: Images in middle-aged man. (a) Conventional posteroanterior radiograph shows possible nodule (arrow) overlying the right first rib. (b) Digital tomosynthesis section images clearly show that this opacity is due to calcification (arrows) at first costochondral junction.

View larger version (128K): [in this window] [in a new window] [Download PPT slide]   Figure 14b: Images in middle-aged man. (a) Conventional posteroanterior radiograph shows possible nodule (arrow) overlying the right first rib. (b) Digital tomosynthesis section images clearly show that this opacity is due to calcification (arrows) at first costochondral junction.

	CAD AND CADX SYSTEMS

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

From an engineering point of view, CAD and CADx systems typically rely on a carefully selected combination of elaborate image-processing, pattern-recognition, and artificial intelligence techniques. Thus far, the application of CAD and CADx analysis in chest radiography has followed a traditional hierarchic model of first detecting and then characterizing potential abnormalities (Fig 15) (111). Initially, image-processing algorithms are applied to identify regions of interest that appear suspicious according to predefined clinical expectations. Subsequently, detailed image feature analysis seeks to capture the morphologic and textural characteristics of the candidate regions. Finally, feature-based decision analysis is implemented to provide a definitive assessment of the candidate regions. The most important among the advances in decision-making analysis is the application of artificial intelligence techniques such as artificial neural networks and knowledge-based systems.

View larger version (38K): [in this window] [in a new window] [Download PPT slide]   Figure 15: Flow diagram illustrates operation of CAD system.

Owing to lack of benchmark image databases, the reported results vary substantially, and direct comparison is impossible because of differences in the size and difficulty level of the private image data sets. Regardless, for all techniques it is a struggle to maintain a clinically acceptable sensitivity level while reducing the number of false-positive detections generated because of overlapping ribs or vessels. Several laboratory observer studies have tested the clinical potential of CAD in a complementary role for lung nodule detection. Reported results (97,99,113–118) show that CAD can assist physicians in improving their overall detection rate for lung nodules.

There is only one commercially available CAD system in the United States for the detection of pulmonary nodules in both digitized and digitally acquired chest radiographs. The RapidScreen system (Riverain Medical, Miamisburg, Ohio) was initially approved for clinical use by the Food and Drug Administration (FDA) in July 2001. In evaluation studies presented to the FDA for premarket approval (119), it was shown that the system helped radiologists improve their detection rate for small lung nodules (9–14.5-mm diameter) by 21%. Other commercial CAD systems for lung nodule detection are thus far available only in Japan (120).

Although not clinically available, several investigators have explored the feasibility of computerized image analysis for determining the malignancy status of pulmonary nodules. The overall aim is to develop CADx systems that could potentially reduce the number of unnecessary diagnostic CT studies. Some laboratory observer studies have shown promising results for such an application (96,121).

Another popular application of CAD in chest radiography is in the detection and differential diagnosis of interstitial lung disease (ILD). Both tasks are considered clinically difficult and are often burdened with subjective assessment and lack of quantifiable descriptions. Unlike the case with pulmonary nodules, the radiographic manifestations of ILD are diverse, and its visual perception and differentiation rely more heavily on texture than on morphology. As a consequence, researchers have focused their attention on applying sophisticated texture-analysis techniques as the foundation of CAD and CADx systems targeting ILD (112). Results of limited observer studies suggest the clinical potential of these systems for substantial improvement in radiologists' performance (96,122–124). However, more work is needed to establish the clinical usefulness of these CAD systems. Finally, other less-well-studied CAD applications in chest radiography involve the detection of cardiomegaly (125), pneumothorax (126,127), interval changes (128), and tuberculosis (129).

Given the increasingly digital nature of chest radiography, CAD will most certainly be an integral part of clinical practice. However, the existing paradigm of CAD playing a complementary role for specific clinical tasks is rather limited. Beyond improving diagnostic accuracy, there is high demand regarding general diagnostic tasks (130), interpretive capabilities, interactive nature, and individualized guidance in patient care (eg, optimal timing for follow-up). Consequently, CAD is expected to keep evolving to meet the increasing new challenges of chest radiography.

	CLINICAL PERSPECTIVE

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 
Over the past decade there have been remarkable advances in the technology applied to conventional thoracic imaging. CR has rapidly become the standard for bedside chest imaging in the United States. Direct radiography has rapidly replaced fixed film-based chest units in most U.S. academic radiology departments and is rapidly replacing such units in the community setting as well. The impetus for these changes can be largely attributed to the advantages inherent in digital imaging. Consistently high-quality images and a marked reduction in the number of repeat examinations required because of technical causes have made digital imaging a favorite of physicians and technologists alike. The ability to readily incorporate digital images into a picture archiving and communication system (PACS) allows for much improved communication between radiologists and referring physicians and has eliminated the problem of the "lost film." The improved availability of critical images and enhanced communication provided through PACS has markedly improved the efficiency of both inpatient and outpatient health care operations. These improved efficiencies, combined with the improved satisfaction on the part of both physicians and patients, have more than justified the increased costs associated with these newer technologies.

Perhaps the most exciting feature of these new techniques is the evolving opportunity to further enhance the diagnostic capabilities and impact of digital chest examinations. In particular, the opportunities for direct CAD of various lesions through sophisticated computer programs, as discussed in this article, offer the possibility of enhancing the radiologist's accuracy while at the same time improving efficiency.

The introduction of newer, heretofore unavailable techniques such as digital tomosynthesis again hold the promise of further revolutionizing what has now become conventional digital chest imaging. As discussed in this article, the opportunity to provide tomographic images of the chest easily and routinely, thus removing overlying and frequently confusing background structures, again holds the promise of substantial improvements in the diagnostic accuracy of conventional chest imaging. Combined with other digitally driven techniques such as dual-energy imaging and temporal subtraction imaging, the opportunities for dramatic improvement in the diagnostic capabilities of conventional digital chest imaging seem promising. Obviously, the effect of these new technologies and of those still to come will need rigorous scientific validation to ensure that the reality truly fulfills the promise of these exciting new advancements.

	ESSENTIALS

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 

• Chest radiography remains the mainstay for diagnosis of many thoracic diseases.
  
• Recent developments in chest radiography have been primarily driven by recognized challenges and limitations of the technique.
  
• Computed radiographic, and now full-field flat-panel detector, systems have replaced or are rapidly replacing conventional film-based systems for chest image acquisition.
  
• Selection of image-processing parameters and display devices has a profound and fundamental effect on the appearance of the digital chest image.
  
• The rapid conversion from screen-film to digital chest image acquisition has greatly facilitated advances such as computer-aided detection and diagnosis, dual-energy or temporal subtraction imaging, and tomosynthesis.
  

	FOOTNOTES

 

Abbreviations: CAD = computer-aided detection • CADx = computer-aided diagnosis • CCD = charge-coupled device • CMOS = complementary metal-oxide semiconductor • CR = computed radiography

	References

TOP ABSTRACT INTRODUCTION CHALLENGES DETECTOR DEVELOPMENTS IMAGE-PROCESSING DEVELOPMENTS DISPLAY DEVELOPMENTS APPLICATION DEVELOPMENTS CAD AND CADX SYSTEMS CLINICAL PERSPECTIVE ESSENTIALS References

 

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