Solid-state, flat-panel, digital radiography detectors and their physical imaging characteristics.

Solid-state, digital radiography (DR) detectors, designed specifically for standard projection radiography, emerged just before the turn of the millennium. This new generation of digital image detector comprises a thin layer of x-ray absorptive material combined with an electronic active matrix array fabricated in a thin film of hydrogenated amorphous silicon (a-Si:H). DR detectors can offer both efficient (low-dose) x-ray image acquisition plus on-line readout of the latent image as electronic data. To date, solid-state, flat-panel, DR detectors have come in two principal designs, the indirect-conversion (x-ray scintillator-based) and the direct-conversion (x-ray photoconductor-based) types. This review describes the underlying principles and enabling technologies exploited by these designs of detector, and evaluates their physical imaging characteristics, comparing performance both against each other and computed radiography (CR). In standard projection radiography indirect conversion DR detectors currently offer superior physical image quality and dose efficiency compared with direct conversion DR and modern point-scan CR. These conclusions have been confirmed in the findings of clinical evaluations of DR detectors. Future trends in solid-state DR detector technologies are also briefly considered. Salient innovations include WiFi-enabled, portable DR detectors, improvements in x-ray absorber layers and developments in alternative electronic media to a-Si:H.

Advances in computed radiography systems and their physical imaging characteristics.

Radiological imaging is progressing towards an all-digital future, across the spectrum of medical imaging techniques. Computed radiography (CR) has provided a ready pathway from screen film to digital radiography and a convenient entry point to PACS. This review briefly revisits the principles of modern CR systems and their physical imaging characteristics. Wide dynamic range and digital image enhancement are well-established benefits of CR, which lend themselves to improved image presentation and reduced rates of repeat exposures. However, in its original form CR offered limited scope for reducing the radiation dose per radiographic exposure, compared with screen film. Recent innovations in CR, including the use of dual-sided image readout and channelled storage phosphor have eased these concerns. For example, introduction of these technologies has improved detective quantum efficiency (DQE) by approximately 50 and 100%, respectively, compared with standard CR. As a result CR currently affords greater scope for reducing patient dose, and provides a more substantive challenge to the new solid-state, flat-panel, digital radiography detectors.

Threshold contrast detail detectability measurement of the fluoroscopic image quality of a dynamic solid-state digital x-ray image detector.

Solid-state digital x-ray imaging detectors of flat-panel construction will play an increasingly important role in future medical imaging facilities. Solid-state detectors that will support both dynamic (including fluoroscopic) and radiographic image recording are under active development. The image quality of an experimental solid-state digital x-ray image detector operating in a continuous fluoroscopy mode has been investigated. The threshold contrast detail detectability (TCDD) technique was used to compare the fluoroscopic imaging performance of an experimental dynamic solid-state digital x-ray image detector with that of a reference image intensifier television (IITV) fluoroscopy system. The reference system incorporated Plumbicon TV. Results were presented as a threshold detection index, or H(T)(A), curves. Measurements were made over a range of mean entrance air kerma (EAK) rates typically used in conventional IITV fluoroscopy. At the upper and mid EAK rate range (440 and 220 nGy/s) the solid-state detector outperformed the reference IITV fluoroscopy system as measured by TCDD performance. At the lowest measured EAK rate (104 nGy/s), the solid-state detector produces slightly inferior TCDD performance compared with the reference system. Although not statistically significant at this EAK rate, the difference will increase as EAK is lowered further. Overall the TCDD results and early clinical experiences support the proposition that a current design of dynamic solid-state detector produces image quality competitive with that of modern IITV fluoroscopy systems. These findings encourage the development of compact and versatile universal x-ray imaging systems based upon solid-state detector technology to support R & F and vascular/interventional applications.

A comparison of the physical imaging properties of Fuji ST-V, ST-VA, and ST-VN computed radiography image plates.

The physical imaging performance of ST-V, ST-VA, and ST-VN computed radiography image plates, read with a Philips AC3 acquisition system, was investigated at 70 kVp with 0.5 mm of copper beam filtration for four different entrance air kerma values: 0.5, 2.6, 26, and 260 micro Gy. Measurements included characteristic response, presampling-modulation transfer function, and Wiener spectrum analysis for (18 x 24) cm2 image plates, sampled at 10 pixels/mm. These results were used to calculate DQE spectral descriptions of system performance. ST-VN image plates exhibited a slightly superior DQE performance for the two lower entrance air kerma values investigated.

A comprehensive physical image quality evaluation of a selenium based digital x-ray imaging system for thorax radiography.

A selenium based digital x-ray system dedicated to chest radiography has been installed by the UK Department of Health's Medical Devices Agency at Leeds General Infirmary, UK, to undergo a comprehensive evaluation, including the physical image quality. The underlying characteristics which define the overall image quality of a system are the following: sensitometric response, modulation transfer function, and noise power spectrum. These have been measured objectively on preprocessed digital data acquired under relevant radiographic conditions. The image data is further processed prior to hard copy display. The displayed image quality may only be measured subjectively; threshold contrast detail detectability is such a measure which can be related to the objective measures of image quality. The objective imaging characteristics suggest that Thoravision has a significant advantage over conventional radiography imaging systems. However, subjective measures have demonstrated that the image processing can have a significant effect on the perceived image quality. Thoravision has the potential to deliver a significantly improved image quality to clinicians with no increase in radiation exposure to the patient, or image quality may be maintained with a reduction in radiation exposure. Digital image processing is central to the efficiency with which it achieves this.

Physical imaging performance of a compact computed radiography acquisition device.

A comprehensive investigation of the physical imaging performance of a Philips AC3 computed radiography system using fifth-generation image plate technology has been undertaken. Measurements include characteristic response, modulation transfer function (MTF) and Wiener spectra (WS) for standard and high resolution image plates sampled at 10 pixels/mm. These results were used to calculate noise equivalent quanta (NEQ) and detective quantum efficiency (DQE) spectral descriptions of system performance. Luminescence noise and x-ray quantum noise components were separated. From an estimate of the luminescence noise power, the average system gain was calculated and results show a substantial improvement over earlier generations of computed radiography systems for standard image plates.