The design and imaging characteristics of dynamic, solid-state, flat-panel x-ray image detectors for digital fluoroscopy and fluorography

Dynamic, flat-panel, solid-state, x-ray image detectors for use in digital fluoroscopy and fluorography emerged at the turn of the millennium. This new generation of dynamic detectors utilize a thin layer of x-ray absorptive material superimposed upon an electronic active matrix array fabricated in a film of hydrogenated amorphous silicon (a-Si:H). Dynamic solid-state detectors come in two basic designs, the indirect-conversion (x-ray scintillator based) and the direct-conversion (x-ray photoconductor based). This review explains the underlying principles and enabling technologies associated with these detector designs, and evaluates their physical imaging characteristics, comparing their performance against the long established x-ray image intensifier television (TV) system. Solid-state detectors afford a number of physical imaging benefits compared with the latter. These include zero geometrical distortion and vignetting, immunity from blooming at exposure highlights and negligible contrast loss (due to internal scatter). They also exhibit a wider dynamic range and maintain higher spatial resolution when imaging over larger fields of view. The detective quantum efficiency of indirect-conversion, dynamic, solid-state detectors is superior to that of both x-ray image intensifier TV systems and direct-conversion detectors. Dynamic solid-state detectors are playing a burgeoning role in fluoroscopy-guided diagnosis and intervention, leading to the displacement of x-ray image intensifier TV-based systems. Future trends in dynamic, solid-state, digital fluoroscopy detectors are also briefly considered. These include the growth in associated three-dimensional (3D) visualization techniques and potential improvements in dynamic detector design.

Introduction

Diagnostic and interventional radiology have a continuing requirement for dose-efficient x-ray-based modes of imaging to visualize moving anatomical structures, organs and/or clinical devices (e.g., guide-wires, catheters, stents, pacemakers, etc).¹ Historically, real-time dynamic imaging using x-rays (for the purpose of procedural guidance) has been referred to as “fluoroscopy”. The serial acquisition of x-ray images of higher quality for use in diagnosis and documentation is known as “fluorography”. Such imaging techniques are commonly associated with contrast medium aided examinations of the gastrointestinal (GI) tract, the cardiovascular system, and various other soft-tissue organs and structures. During the second half of the 20th century fluoroscopy has been supported by the electronic imaging device known as the x-ray image intensifier television (IITV) system. Such a system comprises a chain of electron-optical imaging components including an x-ray image intensifier tube, suitable coupling lenses plus a high specification closed-circuit television channel combined with a suitable electronic display.² The TV image is either recorded by an electronic camera tube (e.g., a Plumbicon, Saticon, Chalnicon, etc) or a semiconductor charge-coupled device (CCD) sensor.³ Modern IITV fluoroscopy systems are capable of producing good-quality, dynamic x-ray images with economical use of radiation dose.

The emergence of digital subtraction angiography (DSA) imaging equipment circa 1980 pioneered the integration of computerized video processors and magnetic storage discs with x-ray IITV systems.⁴ The success of DSA fuelled a seminal era in the development of digital fluoroscopy/fluorography equipment, which continued through the 1990s. The availability of user-friendly, high-performance, digital x-ray IITV systems led to a radical shift in clinical imaging practice. This included the replacement of spot-film-based recording of clinical results by digital (computerized) fluorography in the screening room. This made it possible to access and replay sequences of fluorographic images on-line, and to view them in a digitally enhanced form. At the same time there was an enthusiastic adoption of radiation dose-saving measures, such as digital recursive filtering (to ameliorate noise), last image hold and two-dimensional (2D) road-mapping, further increasing the clinical usefulness of digital fluoroscopy.⁵ Digital x-ray IITV systems proved flexible and effective platforms for expanding the range of dynamic image acquisition protocols. Digital x-ray IITV systems have been a crucial (albeit largely unsung) enabling technology in modern radiology. Notably they have underpinned the growth in x-ray image-guided interventional radiology. At the turn of the new millennium, the digital x-ray IITV system was the dominant image receptor not only for routine fluoroscopy, but also dynamic x-ray image acquisition in general. Around this time, however, a new generation of dynamic image detectors first appeared, which has subsequently gone on to threaten the established role of digital x-ray IITV systems.

Solid-state, flat-panel detectors were originally designed for use in standard projection radiography; the basic physical and technical characteristics of these devices were described in the preceding review.⁶ Solid-state digital radiography (DR) detectors provide on-line access to the electronic signal data, so the radiographic images are available to view in a matter of seconds after the exposure, (rather than after delays of several minutes more typical of conventional and computed radiography). Significantly, researchers found that with suitable technical optimization these solid-state detectors can be used equally well to record and read-out images at rates high enough to support fluoroscopy.7, 8 Prototype clinical dynamic solid-state detector systems started to appear toward the end of the 1990s.9, 10, 11, 12, 13, 14, 15 The first commercial solid-state detector-based digital fluoroscopy products became available in 2001; these detectors were designed specifically for cardiac imaging.16, 17, 18 In recent years most new cardiac catheterization laboratories have utilized solid-state detectors, ousting digital x-ray IITV from one of its most celebrated clinical roles. With the recent introduction of dynamic, solid-state detectors of larger area, a similar shift away from digital x-ray IITV systems is now occurring in radiography and fluoroscopy and vascular imaging.19, 20, 21, 22, 23, 24, 25 The aim of this review is to describe the physical design and imaging characteristics of the dynamic, solid-state, flat-panel x-ray image detectors that are driving this trend.