Arnold R. Cowen

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.

Visibility of microcalcifications in computed and screen-film mammography

Due to the clinically and technically demanding nature of breast x-ray imaging, mammography still remains one of the few essentially film-based radiological imaging techniques in modern medical imaging. There are a range of possible benefits available if a practical and economical direct digital imaging technique can be introduced to routine clinical practice. There has been much debate regarding the minimum specification required for direct digital acquisition. One such direct digital system available is computed radiography (CR), which has a modest specification when compared with modern screen-film mammography (SFM) systems. This paper details two psychophysical studies in which the detection of simulated microcalcifications with CR has been directly compared to that with SFM. The first study found that under scatter-free conditions the minimum detectable size of microcalcification was approximately 130 microns for both SFM and CR. The second study found that SFM had a 4.6% higher probability of observers being able to correctly identify the shape of 350 microns diameter test details; there was no significant difference for-either larger or smaller test details. From the results of these studies it has been demonstrated that the modest specification of CR, in terms of limiting resolution, does not translate into a dramatic difference in the perception of details at the limit of detectability. When judging the imaging performance of a system it is more important to compare the signal-to-noise ratio transfer spectrum characteristics, rather than simply the modulation transfer function.

Towards image quality, beam energy and effective dose optimisation in digital thoracic radiography.

Abstract This paper outlines how objective measurements of both image quality, in terms of signal-to-noise ratio, and effective dose may be used as tools to find the optimum kVp range for a digital chest radiography system. Measurements were made with Thoravision, an amorphous selenium-based digital chest X-ray system. The entrance surface dose and the effective dose to an anthropomorphic chest phantom were determined demonstrating how effective dose is related to beam quality. The image quality was measured using detective quantum efficiency, threshold contrast and a radiologist preference trial involving 100 patients. The results show that, despite the fact that the entrance surface dose decreases as the kVp increases, the effective dose, a better measure of the risk, reaches a minimum value between 90 and 110 kVp; however, the image quality decreases as the kVp increases. In this study the optimum kVp for chest radiography, using a selenium-based radiography system, is in the range 90–110 kVp. This is contrary to the 120- to 150-kVp range that is commonly used. Also, this study shows how objective measurements can be used to optimise radiographic technique without prolonged patient trials.

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.

X-ray dose reduction in fluoroscopically guided electrophysiology procedures

This study assessed the efficacy of a new dose reduction regime in fluoroscopically guided electrophysiology (EP) procedures, which included diagnostic electrophysiological investigations, radiofrequency ablation, and biventricular pacing. A modified dose regime for fluoroscopy was implemented in one of our cardiac electrophysiology laboratories. The x‐ray system was programmed with a hierarchy of three fluoroscopy doses, and therefore image quality and settings. The default (lowest) dose mode was not expected to be suitable for all patient sizes or for the entirety of all procedures. Staff raised the dose level in a stepped manner as and when required to optimize the imaging requirements of the procedure. Phantom studies indicated that the low dose mode provided adequate image quality for visualizing EP catheters, while significantly lowering patient skin dose. In 52 clinical cases, questionnaires were used to assess the subjective clinical image quality. The mean image quality score for the low dose setting was rated between “adequate” and “good.” The fluoroscopy dose level was raised from the lowest level for 6% of the total fluoroscopy time. Procedural Dose Area Product (DAP) meter readings were analyzed for patients prior to (n = 85) and after (n = 150) the implementation of the low dose regime and showed an overall reduction in DAP rate of 74%. The hierarchical dose regime proved to be acceptable in routine clinical practice for EP procedures, leading to significant reductions in patient doses.

Do flat detector cardiac X-ray systems convey advantages over image-intensifier-based systems? Study comparing X-ray dose and image quality

The recent introduction of “flat-panel detector” (FD)-based cardiac catheterisation laboratories should offer improvements in image quality and/or dose efficiency over X-ray systems of conventional design. We compared three X-ray systems, one image-intensifier (II)-based system (system A), and two FD-based designs (systems B and C), assessing their image quality and dose efficiency. Phantom measurements were performed to assess dose rates in fluoroscopy and cine acquisition. Phantom dose rates were broadly similar for all systems, with all systems classified as offering “low” dose rates in fluoroscopy on standard phantoms. Patient X-ray dose rate and subjective image quality was assessed for 90 patients. Dose area product (DAP) rates were similar for all systems, except system C, which had a lower DAP rate in fluoroscopy. In terms of subjective image quality, the order of preference was (best to worst): system C, system A, system B. This study indicates that the use of an FD detector does not infer an automatic improvement in image quality or dose efficiency over II based designs. Specification and configuration of all of the components in the X-ray system contribute to the dose levels used and image quality achieved.

Signal, noise and SNR transfer properties of computed radiography

An investigation of the signal, noise and signal-to-noise transfer properties of a Fuji 7000 series computed radiography (CR) system using third-generation storage plate technology has been undertaken. The results show a significant improvement in signal-to-noise performance of the system compared to an earlier generation of CR technology. Results of the CR evaluation compare favourably to those obtained from a selection of rare-earth screen-film systems evaluated under identical exposure conditions. These results were further substantiated by results from threshold contrast-detail detectability tests.

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 Healths 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.

Direct digital mammography image acquisition

Abstract. Mammography is a branch of radiology which could benefit greatly from the assimilation of digital imaging technologies. Computerized enhancement techniques could be used to ensure optimum presentation of all clinical images. Beyond this it will facilitate powerful new clinical resources such as computer-assisted diagnosis, tele-mammography, plus digital image management and archiving. An essential precursor to all these advances is the availability of appropriate direct digital mammography (DDM) image-acquisition system(s) to capture high-quality breast X-ray image data at the outset. The only practical DDM image-acquisition system currently available is (photo-stimulable phosphor) computed radiography. Modern computed mammography (CM) uses similar radiation doses to the patient and produces equivalent, albeit different, image quality to screen-film mammography. Computed mammography offers superior rendition of the skin edge and sub-cutaneous tissue and dense parenchyma, while ensuring equivalent micro-calcification detectability. Meanwhile, a variety of new technical approaches to DDM are under active investigation and/or development which promise to supercede film-based mammography. These new (second generation) DDM technologies promise the radiologist superior image quality combined with significant dose savings compared with contemporary imaging systems. In this review we describe and compare the physical and clinical characteristics of CM and the various emerging DDM image-acquisition technologies.

Dose optimization in pediatric cardiac x-ray imaging

PURPOSE The aim of this research was to explore x-ray beam parameters with intent to optimize pediatric x-ray settings in the cardiac catheterization laboratory. This study examined the effects of peak x-ray tube voltage (kVp) and of copper (Cu) x-ray beam filtration independently on the image quality to dose balance for pediatric patient sizes. The impact of antiscatter grid removal on the image quality to dose balance was also investigated. METHODS Image sequences of polymethyl methacrylate phantoms approximating chest sizes typical of pediatric patients were captured using a modern flat-panel receptor based x-ray imaging system. Tin was used to simulate iodine-based contrast medium used in clinical procedures. Measurements of tin detail contrast and flat field image noise provided the contrast to noise ratio. Entrance surface dose (ESD) and effective dose (E) measurements were obtained to calculate the figure of merit (FOM), CNR2/dose, which evaluated the dose efficiency of the x-ray parameters investigated. The kVp, tube current (mA), and pulse duration were set manually by overriding the systems automatic dose control mechanisms. Images were captured with 0, 0.1, 0.25, 0.4, and 0.9 mm added Cu filtration, for 50, 55, 60, 65, and 70 kVp with the antiscatter grid in place, and then with it removed. RESULTS For a given phantom thickness, as the Cu filter thickness was increased, lower kVp was favored. Examining kVp alone, lower values were generally favored, more so for thinner phantoms. Considering ESD, the 8.5 cm phantom had the highest FOM at 50 kVp using 0.4 mm of Cu filtration. The 12 cm phantom had the highest FOM at 55 kVp using 0.9 mm Cu, and the 16 cm phantom had highest FOM at 55 kVp using 0.4 mm Cu. With regard to E, the 8.5 and 12 cm phantoms had the highest FOM at 50 kVp using 0.4 mm of Cu filtration, and the 16 cm phantom had the highest FOM at 50 kVp using 0.25 mm Cu. Antiscatter grid removal improved the FOM for a given set of x-ray conditions. Under aforesaid optimal settings, the 8.5 cm phantom FOM improved by 24% and 33% for ESD and E, respectively. Corresponding improvements were 26% and 24% for the 12 cm phantom and 6% and 15% for the 16 cm phantom. CONCLUSIONS For pediatric patients, using 0.25-0.9 mm Cu filtration in the x-ray beam while maintaining 50-55 kVp, depending on patient size, provided optimal x-ray image quality to dose ratios. These settings, adjusted for x-ray tube loading limits and clinically acceptable image quality, should provide a useful strategy for optimizing iodine contrast agent based cardiac x-ray imaging. Removing the antiscatter grid improved the FOM for the 8.5 and 12 cm phantoms, therefore grid removal is recommended for younger children. Improvement for the 16 cm phantom declined into the estimated margin of error for the FOM; the need for grid removal for older children would depend on practical feasibility in the clinical environment.