Computed radiography (CR) is considered a drop-in addition or replacement for traditional screen-film (SF) systems in
digital mammography. Unlike other technologies, CR has the advantage of being compatible with existing
mammography units. One of the challenges, however, is to properly configure the automatic exposure control (AEC) on
existing mammography units for CR use. Unlike analogue systems, the capture and display of digital CR images is
decoupled. The function of AEC is changed from ensuring proper and consistent optical density of the captured image
on film to balancing image quality with patient dose needed for CR.
One of the preferences when acquiring CR images under AEC is to use the same patient dose as SF systems. The
challenge is whether the existing AEC design and calibration
process-most of them proprietary from the X-ray systems
manufacturers and tailored specifically for SF response
properties-can be adapted for CR cassettes, in order to
compensate for their response and attenuation differences. This paper describes the methods for configuring the AEC of
three different mammography units models to match the patient dose used for CR with those that are used for a KODAK
MIN-R 2000 SF System. Based on phantom test results, these methods provide the dose level under AEC for the CR
systems to match with the dose of SF systems. These methods can be used in clinical environments that require the
acquisition of CR images under AEC at the same dose levels as those used for SF systems.
The image quality of three experimental computed radiography (CR) mammography systems was compared through the
measurement of commonly accepted image-quality metrics such as modulation transfer function (MTF) and detective
quantum efficiency (DQE). The design and configuration of the scanners in the three systems were different in that they
had different signal extraction strategies for each storage phosphor screen. Efforts were also made to improve the image
quality through changes in phosphor layers, phosphor particle morphology, particle size distribution, and phosphor
binder ratio. The effects on overall image quality as a result of these improvements were demonstrated on these systems.
It was found that there were significant variations in system MTF and DQE, depending on how the CR system was
configured. Higher system MTF does not always lead to higher DQE. Screen designs as well as scanning strategies
need to be taken into consideration in order to achieve image quality improvements for the application of
mammography.
An x-ray anti-scatter grid alignment aid has been developed and its performance evaluated for portable chest
radiographic exams. The grid alignment aid consists of one line laser and one cross-hair laser mounted on a supporting
frame. During an x-ray exam, the two lasers generate a predetermined light pattern on the x-ray collimator to indicate the
optimal x-ray source position in the three-dimensional (3D) space. Two grids commonly used in portable chest exams
(40 lp/cm resolution, 6:1 and 8:1 ratio, and 130 cm focal distance) were characterized in terms of the signal-to-noise ratio
improvement factor (SIF), as a function of the source image distance (SID), and x-ray tube lateral displacement from
which the optimal grid operation range was determined. Data indicated the grid alignment aid was able to provide
sufficient accuracy for positioning the x-ray tube to achieve optimal grid usage.
As digital radiography (DR) systems are being increasingly adopted for clinical applications, automatic exposure control (AEC) has remained a critically important component. Unlike traditional screen/film systems, however, there does not currently exist a widely accepted AEC calibration criterion for DR systems. This is due mainly to the signal response characteristics and wide dynamic range of a DR detector, which are inherently different from those of a screen/film system. Consequently, the AEC cutoff dose and its dependence on the kVp selection (i.e., kVp compensation) should be calibrated differently for DR systems. In this paper, we have investigated three possible schemes to set up the AEC compensation based on a constant response of the detector in terms of the signal, receptor dose, or signal-to-noise-ratio (SNR) respectively. The results for each of the setup schemes were evaluated on four different DR detectors (Gd2O2S, CsI(Tl), a-Se, or BaFBrI as x-ray absorption material) based on the measured signal and noise response of the detector under the ISO beam conditions (ISO 6236-1). The results showed that all three setup schemes produced similar results for clinical beams above 70 kVp. Significant differences were observed only at lower kVp (≤60) beams. In addition, schemes of constant signal and constant SNR produced similar results with the only exception for the a-Se detector at low kVp (≤60) beam. These results indicate that the choice of the kVp schemes would be important only for low kVp exams.
An observer study was conducted to compare the diagnostic quality of human-subject images obtained using a-Se (amorphous selenium) and CsI(Tl) (thalium-doped cesium iodide) flat-panel detectors. Each detector was attached to an X-ray source and gantry equipment of similar configuration and was installed in a university hospital radiology department in X-ray rooms within close proximity. One hundred image pairs that represent a stratified sampling of exam types were acquired. For a particular subject, image pairs were captured of the same body part and projection, using each of the two detectors. The images comprising a pair were captured within a few minutes of each other. Using manual exposure methods, the images were captured with technique factors that correspond to average exposure levels equivalent to approximately a 400-speed screen-film system. Raw image data from both digital radiography systems was stored to a research workstation. To achieve images having the same appearance, the same image-processing software was used to render the data from both systems, although different parameters were used in the frequency processing to account for the different MTF and noise properties of the CsI(Tl) and a-Se detectors. The processed images were evaluated by radiologists who used a research workstation that was equipped with a 3 MP flat-panel monitor, and software to facilitate the image comparisons. Radiologists used subjective rank-order criteria to evaluate overall diagnostic quality and preference. Radiologists' ratings indicate that both detectors produce images that have comparable satisfactory diagnostic quality for images captured using exposure technique factors that correspond to a 400-speed screen-film system, but the CsI(Tl) detector produces significantly higher preference, especially for larger and denser exam types.
Results of an investigation into the limiting spatial resolution of a flat-panel amorphous silicon (a-Si:H) X-ray imaging system are reported. The system was comprised of a 127 micrometer pixel pitch a-Si:H array used in conjunction with an overlying Gd2O2S:Tb (GOS) phosphor screen. The pre- sampled modulation transfer function (psMTF) of the system was measured at diagnostic X-ray energies and compared to the value predicted from a knowledge of the spatial resolution of the individual system components. A reproducible drop in the measured psMTF is seen at low spatial frequencies. Measurements of the magnitude of X-ray backscatter from the array substrate, along with the results of a theoretical model for K-fluorescence X-ray scatter, indicate that a significant fraction of this low-frequency drop is due to K-fluorescence from heavy elements in the glass substrate of the array. This K-fluorescence may be excited directly by primary X-rays that penetrate the overlying phosphor and interact in the glass, or by gadolinium K-fluorescence X-rays that escape from the phosphor into the glass. The measurements indicate that the spatial resolution of such an X-ray imaging system may be improved by the use of a substrate containing as low a concentration of heavy elements as possible.
The development and initial characterization of an active matrix, flat-panel imager (AMFPI) incorporating a newly designed, indirect-detection array is reported. The array has a 127 micrometers pitch, a 1536 X 1920 pixel format, and incorporates a pixel design comprising a discrete a-Si:H photodiode coupled to an a-Si:H thin-film transistor. The array represents an aggressive redesign of a previously reported array having the same pitch and format. In particular, this new array was designed with the dual goals of maximizing the optical fill factor so as to enhance sensitivity as well as minimizing the data line capacitance so as to reduce additive electronic noise. Although constrained by the sue of discrete photodiodes, the new design nevertheless successfully achieves a fill factor of approximately 56 percent along with a data line capacitance of approximately 50 pF which are significant improvements over the previous design. In this paper, considerations in the design of such arrays are reviewed and performance results of the AMFPI, based on initial empirical results and theoretical considerations, are presented. Finally, possible trends in the future development of indirect and direct detection AMFPIs are described.
Recent advances in a-Si:H fabrication technology have opened the way for the application of flat panel imaging arrays in a number of areas in medical imaging. Their large area (up to approximately 26 X 26 cm), thin profile (< 1 mm) and real time readout capability make them strong candidates for the replacement of more traditional x-ray imaging technologies such as film and image intensifier systems. As a first step towards a device suitable for clinical use we have created a 24.4 X 19.4 cm array with 127 micrometers pitch pixels. This device serves as a testbed for investigating the effects of design changes on array imaging performance. This paper reports on initial measurements of the spatial resolution of this device used in conjunction with an overlaying Lanex Regular screen and 90 kVp x rays. The measured pre-sampled modulation transfer function (p.s. MTF) is found to fall below the predicted value by up to approximately 8%. At least part of this reduction seems to be due to scattering of light photons between the array and the surface of the phosphor screen contacting the array.
Flat-panel x-ray imaging arrays based upon thin-film electronics are increasingly under development and investigation for a variety of applications. Our research has progressed to the point where three large area designs have now been fabricated, including a new 26 X 26 cm2 array. These arrays are the largest self-scanning, solid-state imaging arrays thus far reported. In all probability, they represent only the first examples of an entirely new class of real-time imaging devices whose properties offer significant advantages over current radiographic and fluoroscopic x-ray technologies. A general overview of the current state of this emerging imaging technology is presented. Our large area array designs are described and x-ray images from a high resolution array are presented. Future challenges as well as anticipated trends and developments are discussed.
Progress toward the development of a large area, flat-panel imager for diagnostic x-ray imaging is described. The initial fabrication of a prototype array with a format of 1536 X 1920 pixels and a pixel pitch of 127 micrometers giving an active area of 19.5 X 24.4 cm2 is reported. With a total of approximately 2.9 million transistors, this ambitious array is on a par with modern microprocessors in terms of transistor count. This work builds upon our concurrent research into the development of a very large area, lower spatial resolution, flat-panel imager for radiotherapy. An overview of the anticipated imaging properties of these devices is presented and future prospects discussed.
Hydrogenated amorphous silicon imaging arrays are being developed for numerous applications in medical imaging. Diagnostic and megavoltage images have previously been reported and a number of the intrinsic properties of the arrays have been investigated. This paper reports on the first attempt to characterize the intrinsic spatial resolution of the imaging pixels on a 450 micrometers pitch, n-i-p imaging array fabricated at Xerox P.A.R.C. The pre- sampled modulation transfer function was measured by scanning a approximately 25 micrometers wide slit of visible wavelength light across a pixel in both the DATA and FET directions. The results show that the response of the pixel in these orthogonal directions is well described by a simple model that accounts for asymmetries in the pixel response due to geometric aspects of the pixel design.
The development of a large-area amorphous silicon array for x-ray imaging is described. The array comprises pixels made up of amorphous silicon transistors and photodiode sensors with a pixel-to-pixel pitch of 450 micrometers . With a format of 512 X 560 pixels, the array has an area of 23 by 25 cm2 making it the largest self-scanning, solid-state, pixelated imaging device ever reported. The first diagnostic x-ray images from such a large area device are demonstrated and a general review of the current state of this technology is given. The properties of such arrays are summarized and future anticipated developments discussed.
Since the design and fabrication of the first pixelated, two-dimensional, hydrogenated amorphous silicon image sensor arrays at Xerox, PARC, in 1988, a variety of milestones have been achieved including the first demonstration of high quality radiographic images of low- contrast, anatomical detail. Current array configurations and design rules offer the prospect of 100 micrometers pixel pitches over 30 by 30 cm2 areas in the next few years. Beyond this, present attempts to extend the size of the substrates to 100 cm on the diagonal by 1996 coupled with the possibility of three-dimensional thin-film electronics could eventually result in a revolution in many forms of x-ray imaging. Such arrays will present challenges in the design of the fast, analog, and digital electronic readout systems required to precisely match the characteristics of the arrays to those of the imaging needs. For such arrays, one of the most important parameters is the dynamic range. Early results are reported for the measured limits on this quantity as obtained through measurements from individual sensors and FETs as well as an improved lower limit as obtained by direct measurements of array pixels.
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