Flat-panel detectors: how much better are they?
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Interventional and fluoroscopic imaging procedures for pediatric patients are becoming more prevalent because of the less-invasive nature of these procedures compared to alternatives such as surgery. Flat-panel X-ray detectors (FPD) for fluoroscopy are a new technology alternative to the image intensifier/TV (II/TV) digital system that has been in use for more than two decades. Two major FPD technologies have been implemented, based on indirect conversion of X-rays to light (using an X-ray scintillator) and then to proportional charge (using a photodiode), or direct conversion of X-rays into charge (using a semiconductor material) for signal acquisition and digitization. These detectors have proved very successful for high-exposure interventional procedures but lack the image quality of the II/TV system at the lowest exposure levels common in fluoroscopy. The benefits for FPD image quality include lack of geometric distortion, little or no veiling glare, a uniform response across the field-of-view, and improved ergonomics with better patient access. Better detective quantum efficiency indicates the possibility of reducing the patient dose in accordance with ALARA principles. However, first-generation FPD devices have been implemented with less than adequate acquisition flexibility (e.g., lack of tableside controls/information, inability to easily change protocols) and the presence of residual signals from previous exposures, and additional cost of equipment and long-term maintenance have been serious impediments to purchase and implementation. Technological advances of second generation and future hybrid FPD systems should solve many current issues. The answer to the question ‘how much better are they?–is ‘significantly better– and they are certainly worth consideration for replacement or new implementation of an imaging suite for pediatric fluoroscopy.
KeywordsFlat-panel detectors Fluoroscopy Interventional radiology
Real-time fluoroscopic imaging has been available for medical diagnostic purposes since the beginning of X-ray applications in medicine, before the turn of the 20th century . A light image derived from the conversion gain of X-rays into visible light photons formed the real-time display for the diagnostician. Technical advances through the next 40 years improved the X-ray to light conversion factor of a standard Patterson fluoroscopic screen, a scintillator without any secondary gain, but light levels were still low, requiring dark adaptation of the human visual response and high radiation dose per frame. In this situation, the diagnostician had to render a diagnosis with less visual acuity, using the dark-sensitive rods of the eye as opposed to the higher acuity cones of the eye that function only in higher intensity light. A breakthrough in fluoroscopy technology occurred with the introduction of the X-ray image intensifier (II), introduced in the late 1940s; the II eventually revolutionized real-time X-ray fluoroscopic imaging as we know it. This device provided an output of non-limiting brightness and smaller (‘minified– image size, achieved by conversion of X-rays to light to electrons to light, with the key being the acceleration of electrons and minification of the image for signal amplification. Finally, the radiologist was able to do work without dark adapting to low light levels, thus improving acuity and image information transfer. Subsequent improvements were the optical coupling of the output image to a television (TV) camera and closed-circuit viewing system for real-time video image display in the fluoroscopy room. Even though the image was significantly degraded by the limited resolution and bandwidth of the TV system, the convenience of viewing, adjusting contrast, and storing images on videotape were significant. Incremental improvements in image quality and image size occurred throughout the 1960s and 1970s.
Revolutionary change in fluoroscopy occurred in the mid-1970s with the introduction of high-speed digitization of the analog video signal. Using rapid image processing, real-time subtraction of pre- and postcontrast images led to the implementation of digital subtraction angiography (DSA), certainly a very important event in the history of interventional angiography. Enhanced video memory and computer technology together with low-dose rapid switching X-ray tubes brought the introduction of ‘pulsed fluoroscopy–to the clinical environment in the mid-1980s. Pulsed fluoroscopy demonstrated the ability to trade-off temporal resolution for lower dose. Even today, some 20 years later, there is a need to understand the appropriate configuration settings for the image quality versus reduced dose for continuous versus still frame (last-image hold) . Scientific-grade charge-coupled device (CCD) cameras as a replacement for vidicon-type analog cameras provided another leap in performance, because of greater temporal stability and low electronic noise. With the current state of the art II/TV system, improvements in image quality for the II have largely flattened.
In the late 1990s, the monetary and technological investments in flat-panel displays led to the implementation of the flat-panel detector (FPD) for radiography by many manufacturers. Soon after, FPD fluoroscopy operation soon began, and with several years of refinement and technological advances, is now poised to compete with the II in the clinical marketplace. The questions regarding its capabilities, however, have not been completely answered, and FPD implementations for fluoroscopy thus far have had a mixed review, particularly as it pertains to image quality at low exposure levels. The intent of this article is to review the II and FPD technologies, discuss the advantages and disadvantages of each, and to remark on some of the future detectors that might indicate the end of the II/TV system as we know it.
Image intensifier technology
The need for image intensification to overcome the limitations of the human visual system under conditions of low light levels typical of early fluoroscopy in terms of lack of acuity and dark adaptation was first described by Chamberlain . The technology to meet these needs, the II, was initially developed by Coltman . A single Patterson scintillator screen commonly used for real-time imaging  simply did not have sufficient light output, and it required the radiologist to be dark-adapted (10–0 min sitting in subdued light and/or wearing red goggles throughout the day) for optimal information transfer prior to performing a study. The II was constructed with a three-stage conversion and amplification process to create a light image of much greater intensity instead of relying on one conversion and amplification stage (X-rays to light) as with the simple scintillator. This device revolutionized the use of fluoroscopy in clinical diagnosis.
Flat-panel detectors for X-ray imaging: thin film transistor arrays
Amorphous silicon (a-Si) thin-film-transistor (TFT) arrays represent the technological research and monetary investment in the creation of compact displays for laptop computers, begun in the late 1980s. In the mid-1990s, feasibility studies for producing an X-ray detector demonstrated that the same technology could be used for acquiring a two-dimensional (2D) projection X-ray image, and subsequently a ‘real-time–fluoroscopy sequence. Currently, in 2006, the technology has advanced to clinical mainstream applications in both radiography and fluoroscopy. Clinical radiography results have demonstrated the clear superiority of FPD systems over screen-film radiography and other digital radiography devices [6, 7], but the question of how much better the FPD is for fluoroscopic applications still requires investigation. Some reports of FPD fluoroscopy capabilities are reviewed below.
The ‘fill-factor–is a characteristic of the TFT and represents the fraction of each detector element that efficiently collects charge from the energy deposited by the absorbed X-ray signal in the converter material above it. Dead areas of the element include the gate, drain, TFT, and capacitor electronics. As the detector element gets smaller, the fill factor gets smaller and less efficient, ultimately setting a lower limit on the achievable spatial resolution. Typical fill factors (1 is ideal) range from about 0.5 to 0.8 for indirect detectors and are larger for direct detectors because of the ability to redirect the charge using ‘mushroom–electrodes .
X-ray signal detection: indirect and direct conversion
Until recently, indirect detection methods have been the only flat-panel technology available for clinical fluoroscopy. Direct FPD systems for fluoroscopy are now available because technological advances have reduced the effects of lag caused by charge trapping in the semiconductor and because overcharging protection circuits have been implemented.
Image intensifier/TV versus flat-panel detector for fluoroscopy
Feature comparison of II/TV and FPD systems
Digital flat panel
Wide, about 5,000:1
Limited by TV, about 500:1
Pin-cushion and ‘S–distortion
Detector size (bulk)
Bulky, significant with large FOV
Image area FOV
40 cm diameter (25% less area)
Better at high dose
Better at low dose
Anecdotal reports of poor image quality rendered by FPD fluoroscopy are, in part, a result of the lack of experience, inappropriate configuration (e.g., radiation dose levels, image postprocessing), and inadequate training regarding the optimal use of these detector systems. First generation FPD systems did not have many of the table-side controls found on a corresponding II/TV system, making the transition difficult (personal communication, Phil Rauch, Henry Ford Hospital, Detroit, Mich., July 2005). Often, disconnect between the radiologist needing to achieve low contrast resolution in the fluoroscopy image and the aggressive low-dose configuration of the FPD system by the system installer has resulted in disgruntled users of such systems (personal communication, Paul Brown, Oregon Health Sciences University, Portland, Ore., February 2006). Ineffective or improper setting of pulsed fluoroscopy rates and dose per frame can lead to inadequate image presentation, reduced temporal resolution, poor image quality and needless overexposure rather than a reduced dose. This is particularly true for current generation FPD systems that do not have the gain characteristics of the II/TV systems. Certainly, for pulsed fluoroscopy, proper dose adjustment per frame and appropriate frame rates are essential to ensure that the optimal image quality is achieved , and with careful adjustments, the chief limitation of FPD systems, namely that of electronic noise, can largely be overcome.
Quantitative evaluation of FPD systems compared to II/TV systems for fluoroscopy in the laboratory setting indicates equivalent or superior performance of the flat-panel system at high and intermediate air kerma rates but poorer performance at low air kerma rates in under-penetrated areas of an image with less than 0.1 μGy per frame using qualitative low-contrast sensitivity phantoms . This is corroborated by data from other investigators using quantitative methods to measure the DQE as a function of spatial frequency (object size) for levels of incident exposure from high to extremely low for both indirect and direct FPD systems [13, 14, 15, 16]. Often cited in articles are incident air kerma of 1 μGy per frame (typical fluoro rate) for a uniform exposure, but much of the image will be less than one-tenth of that level (e.g., 0.1 μGy per frame) where current flat panels have problems with electronic and non-quantum noise sources. What strategies are being considered to overcome this limitation? Already mentioned was lower frame rate pulsed fluoroscopy, to allow increased incident exposure per frame while keeping the overall exposure rate to the patient lower than continuous mode operation. Anti-scatter grids, used for both FPD and II/TV fluoro systems, must be easily removable by the operator prior to imaging smaller pediatric patients to eliminate the dose penalty (grid Bucky factor) for fluoroscopic and radiographic procedures. Another possibility is the implementation of a FPD that provides programmable ‘noiseless–gain to signals dependent on incident (low) exposure; one such device is now under investigation and described in the next section.
Flat-panel technology: variable gain hybrid design
Quality assurance and quality control
An imaging system is only as good as its weakest link, and in the case of real-time fluoroscopic imaging, there are many cascading processes during image formation that can affect the overall image quality and ALARA dose efficiency. Additionally, care must be taken to consider installation of FPD peripheral equipment and heating, ventilation, and air-conditioning requirements that are often overlooked and/or assumed to be insignificant compared to older II/TV systems, when in fact they are not . In order to establish baseline operation values and to ensure optimal functionality, acceptance testing by a qualified medical physicist is strongly suggested during installation and initial implementation. Performance metrics that should be evaluated are reviewed by Rauch , though there are no formal recommendations for FPD systems beyond that of II/TV systems at this time. Additionally, periodic quality control testing and verification of proper system functionality at a frequency relevant to discovering possible equipment problems is important, as is plotting test results to reveal trends and intervene before problems are actually manifested. An excellent resource for additional information on acceptance testing, quality control, and operational issues for fluoroscopic imaging is highly recommended as a starting point .
FPDs designed specifically for fluoroscopic use provide image quality and dose efficiency generally superior to the II systems that they replace, except at the lowest fluoro dose levels. Advantages include excellent image uniformity, no geometric distortion, no veiling glare or vignetting, and small, thin physical size for improved patient accessibility, an important consideration for pediatric imaging. The key disadvantage is less efficiency and performance at the lowest exposure levels; however, with judicious use and configuration knowledge of pulsed fluoroscopy and removable anti-scatter grids, this can be mitigated to a large extent with current technology. As with all technological innovations, there is a learning curve for implementation, adjustment, and optimal use of the technology, with significant room for ongoing improvement. Long-term reliability, radiation tolerance, and replacement costs, currently unknown, will take time to determine. On the horizon are technological advances that promise an ability to optimize image quality at all radiation exposure levels and portend the inevitable demise of the venerable II/TV fluoroscopy system, but not just yet. For pediatric fluoroscopic imaging and the ALARA principle, whether using II/TV systems or FPD systems, always important is the vigilance and understanding by the operator of the fluoroscopic procedure and equipment and ways to minimize study times, exposure times and radiation dose for the benefit of our patients.
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