John A. Rowlands

X-ray detectors for digital radiography

Digital radiography offers the potential of improved image quality as well as providing opportunities for advances in medical image management, computer-aided diagnosis and teleradiology. Image quality is intimately linked to the precise and accurate acquisition of information from the x-ray beam transmitted by the patient, i.e. to the performance of the x-ray detector. Detectors for digital radiography must meet the needs of the specific radiological procedure where they will be used. Key parameters are spatial resolution, uniformity of response, contrast sensitivity, dynamic range, acquisition speed and frame rate. The underlying physical considerations defining the performance of x-ray detectors for radiography will be reviewed. Some of the more promising existing and experimental detector technologies which may be suitable for digital radiography will be considered. Devices that can be employed in full-area detectors and also those more appropriate for scanning x-ray systems will be discussed. These include various approaches based on phosphor x-ray converters, where light quanta are produced as an intermediate stage, as well as direct x-ray-to-charge conversion materials such as zinc cadmium telluride, amorphous selenium and crystalline silicon.

The physics of computed radiography

Cassette-based computed radiography (CR) systems have continued to evolve in parallel with integrated, instant readout digital radiography (DR) systems. The image quality of present day CR systems is approaching its theoretical limits but is significantly inferior to DR. Further improvements in CR image quality require improved concepts. The aim of this review is to identify the fundamental limitations in CR performance. This will provide a background for the development of new approaches to improve photostimulable phosphor CR systems. It will also guide research in designing better CR systems to possibly compete with DR systems in terms of image quality parameters such as detective quantum efficiency and yet maintain CR convenience in being portable and more economical.

Amorphous Semiconductors Usher in Digital X‐Ray Imaging

Unlike other major medical imaging methods, such as computed tomography, ultrasound, nuclear medicine and magnetic resonance imaging—all of which are digital—conventional x‐ray imaging remains a largely analog technology. Making the transition from analog to digital could bring several advantages to x‐ray imaging: Contrast and other aspects of image quality could be improved by means of image processing; radiological images could be compared more easily with those obtained from other imaging modalities; the electronic distribution of images within hospitals would make remote access and archiving possible; highly qualified personnel could service remote or poorly populated regions from a central facility by means of “teleradiology”; and radiologists could use computers more effectively to help with diagnosis—work that has already been initiated at the University of Chicago by Kunio Doi and his coworkers.

X-ray imaging performance of structured cesium iodide scintillators.

Columnar structured cesium iodide (CsI) scintillators doped with Thallium (Tl) have been used extensively for indirect x-ray imaging detectors. The purpose of this paper is to develop a methodology for systematic investigation of the inherent imaging performance of CsI as a function of thickness and design type. The results will facilitate the optimization of CsI layer design for different x-ray imaging applications, and allow validation of physical models developed for the light channeling process in columnar CsI layers. CsI samples of different types and thicknesses were obtained from the same manufacturer. They were optimized either for light output (HL) or image resolution (HR), and the thickness ranged between 150 and 600 microns. During experimental measurements, the CsI samples were placed in direct contact with a high resolution CMOS optical sensor with a pixel pitch of 48 microns. The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the detector with different CsI configurations were measured experimentally. The aperture function of the CMOS sensor was determined separately in order to estimate the MTF of CsI alone. We also measured the pulse height distribution of the light output from both the HL and HR CsI at different x-ray energies, from which the x-ray quantum efficiency, Swank factor and x-ray conversion gain were determined. Our results showed that the MTF at 5 cycles/mm for the HR type was 50% higher than for the HL. However, the HR layer produces approximately 36% less light output. The Swank factor below K-edge was 0.91 and 0.93 for the HR and HL types, respectively, thus their DQE(0) were essentially identical. The presampling MTF decreased as a function of thickness L. The universal MTF, i.e., MTF plotted as a function of the product of spatial frequency f and CsI thickness L, increased as a function of L. This indicates that the light channeling process in CsI improved the MTF of thicker layers more significantly than for the thinner ones.

Direct-conversion flat-panel X-ray image sensors for digital radiography

Advances in active-matrix array flat panels for displays over the last decade have led to the development of flat-panel X-ray image detectors. Recent flat-panel detectors have shown image quality exceeding that of X-ray film/screen cassettes. They can also permit the instantaneous capture, readout, and display of digital X-ray images and, hence, enable the clinical transition to digital radiography. There are two general approaches to flat panel detector technology: 1) direct and 2) indirect conversion. The present paper outlines the operating principles for direct-conversion detectors based on the use of photoconductors. It formulates and reviews the required X-ray photoconductor properties for such applications and examines to what extent potential materials fulfill these requirements. The quantum efficiency, X-ray sensitivity, noise, and detective quantum efficiency factors are discussed with reference to current and potential large area X-ray photoconductors.

Imaging performance of amorphous selenium based flat-panel detectors for digital mammography: Characterization of a small area prototype detector

Our work is to investigate and understand the factors affecting the imaging performance of amorphous selenium (a-Se) flat-panel detectors for digital mammography. Both theoretical and experimental methods were developed to investigate the spatial frequency dependent detective quantum efficiency [DQE(f)] of a-Se flat-panel detectors for digital mammography. Since the K edge of a-Se is 12.66 keV and within the energy range of a mammographic spectrum, a theoretical model was developed based on cascaded linear system analysis with parallel processes to take into account the effect of K fluorescence on the modulation transfer function (MTF), noise power spectrum (NPS), and DQE(f) of the detector. This model was used to understand the performance of a small-area prototype detector with 85 microm pixel size. The presampling MTF, NPS, and DQE(f) of the prototype were measured, and compared to the theoretical calculation of the model. The calculation showed that K fluorescence accounted for a 15% reduction in the MTF at the Nyquist frequency (fNy) of the prototype detector, and the NPS at fNy was reduced to 89% of that at zero spatial frequency. The measurement of presampling MTF of the prototype detector revealed an additional source of blurring, which was attributed to charge trapping in the blocking layer at the interface between a-Se and the active matrix. This introduced a drop in both presampling MTF and NPS at high spatial frequency, and reduced aliasing in the NPS. As a result, the DQE(f) of the prototype detector at fNy approached 40% of that at zero spatial frequency. The measured and calculated DQE(f) using the linear system model have reasonable agreement, indicating that the factors controlling image quality in a-Se based mammographic detectors are fully understood, and the model can be used to further optimize detector imaging performance.

Evaluation of the imaging properties of an amorphous selenium-based flat panel detector for digital fluoroscopy

The imaging performance of an amorphous selenium (a-Se) flat-panel detector for digital fluoroscopy was experimentally evaluated using the spatial frequency dependent modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE). These parameters were investigated at beam qualities and exposures within the range typical of gastrointestinal fluoroscopic imaging (approximately 0.1 - 10 microR, 75 kV). The investigation does not take into consideration the detector cover, which in clinical use will lower the DQE measured here by its percent attenuation. The MTF was found to be less than the expected aperture response and the NPS was not white which together indicate presampling blurring. The cause of this blurring was attributed to charge trapping at the interface between two different layers of the a-Se. The effect on the DQE was also consistent with presampling blur, which reduces the aliasing in the NPS and thereby reduces the spatial frequency dependence of the DQE. (The DQE was independent of spatial frequency from 0.12 to 0.73 mm(-1) due to antialiasing of the NPS.) Moreover, the first zero of the measured MTF and the aperture response appeared at the same spatial frequency (6.66 mm(-1) for a pixel of 150 microm). Hence, the geometric fill factor (77%) was increased to an effective fill factor of 99 +/- 1%. A large scale ( approximately 32 pixels) correlation in the noise due to the configuration of the readout electronics caused increased noise power in the gate line NPS at low spatial frequency (< 0.1 mm(-1)). The DQE (f = 0) was exposure independent over a large range of exposures but became exposure dependent at low exposures due to the electronic noise.

Absorption and noise in cesium iodide x‐ray image intensifiers

The measured and theoretically predicted values of detective quantum efficiency (DQE) for a CsI x-ray image intensifier are compared for nine monoenergetic beams of x rays. The agreement between measurement and theory of better than +/- 5% indicates that we have a sound understanding of the physical parameters controlling the DQE. It is shown that the fraction of K-fluorescent x rays escaping from the input phosphor is independent of incident energy. The number of electrons released within the x-ray image intensifier (XRII) by an incident x ray has been measured. The mechanism for energy broadening within the XRII is shown to be predominantly the limited number of electrons and not light absorption.

X-ray imaging with amorphous selenium: X-ray to charge conversion gain and avalanche multiplication gain.

Fluoroscopy is a low dose imaging technique. As such, a very sensitive detector is required to create images of good quality. Present day flat panel active matrix read out systems introduce an amount of noise that inhibits present direct and indirect methods from producing optimal quality images at fluoroscopic exposure rates (0.1-10 microR per frame). The gain of the direct conversion approach using amorphous selenium (a-Se) was investigated to determine whether by increasing the applied electric field, a gain sufficient to overcome the noise limitations of the active matrix could be achieved. Conversion gain and avalanche multiplication in a-Se were investigated as a function of electric field from 10 to 100 V/microm. Our results show a factor of 4 increase in conversion gain is available by increasing electric field from the current standard of 10 V/microm to 100 V/microm. Furthermore, we show that avalanche multiplication can provide an additional gain of up to 25. This increase in signal is sufficient to overcome the noise level encountered in flat panel detectors and permit fully quantum noise limited operation across the whole fluoroscopic range of exposure.

Large-area solid state detector for radiology using amorphous selenium

A large area self-scanned solid-state detector is being developed for digital radiology. It consists of an x-ray sensitive flat-panel employing amorphous selenium ((alpha) )-Se) as the x- ray transducer and active matrix integrated circuit for readout. In principle such detectors could be used for all the currently applied radiological modalities -- radiography, photofluorography, and fluoroscopy. Layers of (alpha) )-Se up to 500 micrometers thick are readout with an array of thin film field effect transistors. The whole structure is integrated onto a glass plate. For all practical purposes the resolution of the system is dictated by the pixel size and readout could be in real-time (i.e., 30 frames/sec).