Search PubMed⌕ Search

Biomedical subjects

J A Rowlands

Publications and source records attributed to J A Rowlands.

At least 19 recordsLinked to original sources
PubMed1997

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.

Electrons↗
PubMed1997

Feasibility of a large area x-ray sensitive vidicon for medical fluoroscopy: signal and noise factors.

A large area x-ray sensitive vidicon is being investigated as an alternative to the x-ray image intensifier and television camera combination for medical fluoroscopy. Signal generation in an x-ray vidicon involves an x-ray photoconductive layer whose surface is scanned by an electron beam. The single x-ray to electron conversion stage of the x-ray vidicon offers a greatly improved modulation transfer function (MTF) over the x-ray image intensifier. This superior MTF allows signal and x-ray quantum noise at high spatial frequencies to be passed to the preamplifier with less attenuation. In cardiac cine applications this would allow quantum noise limited operation at higher spatial frequencies than is possible with the x-ray image intensifier system.

Electrons↗
PubMed1997

Feasibility of a large area x-ray sensitive vidicon for medical fluoroscopy: resolution and lag factors.

A large area x-ray-sensitive vidicon utilizing amorphous selenium (a-Se) is being investigated as an alternative to the x-ray image intensifier and television camera combination (XRII/TV) for medical fluoroscopy. The x-ray vidicon is, to a first approximation, a scaled up version of the 1" (2.54 cm) light-sensitive vidicon. The modulation transfer function (MTF) of an x-ray vidicon is mainly determined by the number of lines used, the beam spot profile, and the electronic bandwidth. The x-ray vidicon is expected to have roughly the same MTF as the 1" vidicon from a XRII/TV system referenced to the x-ray input and as such will be superior to the MTF of the complete XRII/TV system especially in high resolution applications such as cardiac cine. Beam discharge lag is expected to be approximately the same as in the 1" vidicon since they have comparable photoconductor layer capacitance. Photoconductive lag is also expected to be low since a-Se is known to have a low trap density.

Electrons↗
PubMed1997

X-ray imaging with amorphous selenium: theoretical feasibility of the liquid crystal light valve for radiography.

A novel radiographic imaging system based on a liquid crystal light valve is described. A liquid crystal light valve is a photon addressed spatial light modulator that consists of a high resolution, solid-state electrostatic detector (photoconductor) and an electro-optic light modulator (liquid crystal cell) physically coupled in a sandwich structure. We propose a light valve with a thick, x-ray sensitive photoconductive layer and call the system under study the x-ray light valve (XLV). The image formation in the XLV is based on x-ray exposure controlled modulation of light from an external source; the XLV is essentially an x-ray image intensifier that allows the image brightness to be adjusted independently from the x-ray exposure. Thus the XLV may be coupled to an optical imager, such as a charge coupled device (CCD), for image digitization without a secondary quantum sink. A model of the XLV operation is developed to investigate its sensitivity, speed, noise, and resolution. The imaging properties of the XLV are found to be time dependent, which leads to an unusual transmission versus exposure characteristic. The feasibility of clinical use of the XLV based on amorphous selenium (a-Se) photoconductor and a twisted nematic liquid crystal cell is analyzed, and the device is shown to be adaptable to a variety of radiographic imaging tasks.

Biophysical Phenomena↗
PubMed1996

Design of a laser scanner for a digital mammography system.

We have developed a digital readout system for radiographic images using a scanning laser beam. In this system, electrostatic charge images on amorphous selenium (alpha-Se) plates are read out using photo-induced discharge (PID). We discuss the design requirements of a laser scanner for the PID system and describe its construction from commercially available components. The principles demonstrated can be adapted to a variety of digital imaging systems.

Diagnosis, Computer-Assisted↗
PubMed1996

X-ray imaging with amorphous selenium: optimal spectra for digital mammography.

The optimum x-ray spectra for acquisition of digital mammographic images using an amorphous selenium (a-Se) photoconductor are investigated. The recorded images consist of latent charge distributions on the surface of an a-Se plate, which are then read out using two methods, laser discharge, or flat panel recharge. The investigation is based on a model of the breast previously developed for a phosphor-based digital readout system, and has been extended to include the effects specific to the use of photoconductors. The effects of plate thickness, x-ray scatter, readout noise, dose, and the kind of breast tissue on the nature of the optimum spectrum are explored for the two readout methods. The results indicate that use of a kilovoltage setting in the current mammographic range, and a molybdenum target spectrum is appropriate for digital readout of a-Se detectors. This conclusion contrasts with the appreciably higher kilovoltages traditionally used with the xerographic (toner) readout of latent charge images on a-Se.

Biophysical Phenomena↗
PubMed1996

Measurement of quantum noise in fluoroscopic systems for portal imaging.

In fluoroscopic portal imaging systems, a metal plate is bonded to a phosphor screen and together these act as the primary x-ray sensor. The light from the screen is collected and imaged by a lens on the target of a video camera. The demagnification (M) between the large area of the phosphor being imaged and the small active area of the video camera results in poor optical coupling between the screen and the video camera. Consequently x-ray quantum noise is small compared to other noise sources. By reducing the demagnification, the light from the screen is collected more efficiently, so we were able to increase the x-ray quantum noise relative to other noise sources and thus unambiguously identify it. The noise power spectrum was measured as a function of M to determine the relationship between the x-ray quantum noise. shot noise, and amplifier noise. It was found by extrapolation to clinical demagnifications that the amplifier noise dominates x-ray quantum noise, at all spatial frequencies, but the shot noise was less than the x-ray quantum noise at low spatial frequencies. For low spatial frequencies, this implies that a secondary quantum sink can be avoided. If amplifier noise could be sufficiently reduced, x-ray quantum limited images could be obtained in clinical systems at low spatial frequencies.

Equipment Design↗
PubMed1995

X-ray imaging using amorphous selenium: inherent spatial resolution.

This is a theoretical study of the inherent spatial resolution of the latent image on the surface of an amorphous selenium (a-Se) plate used for diagnostic x-ray imaging. The following effects are considered: (A) ranges of primary photoelectrons; (B) reabsorption of K fluorescence; (C) reabsorption of Compton scattered photons; (D) diffusion; (E) the geometric effect due to oblique incidence of x rays; (F) electrostatic effect; and (G) the space charge effect. The modulation transfer function of a-Se in the diagnostic x-ray energy range has been estimated. In conclusion, (A) and (E) are the main factors limiting the resolution, and for diagnostic x rays, the inherent spatial resolution of a-Se plates is much better than that of CsI layers used in x-ray image intensifiers.

Humans↗
PubMed1995

X-ray imaging with amorphous selenium: detective quantum efficiency of photoconductive receptors for digital mammography.

Factors affecting the zero spatial frequency detective quantum efficiency of photoconductor-based x-ray detectors operating in the mammographic energy range are modeled for monoenergetic incident x rays. The problem is separated into two sections: the calculation of the x-ray absorption and the Swank factor. X-ray absorption in this energy range, for most practical photoconductors, is dominated by the photoelectric effect. The Swank factor has four components: fluorescence escape, stochastic variations in gain, variations of gain due to incomplete coupling of charge from the photoconductive layer to the detector electrode, and the nonlinear discharge arising from the field-dependent x-ray gain, an effect that is unique to photoconductors. Calculations are performed for selenium, which is currently the most technologically advanced photoconductor available for digital x-ray imaging. For thicknesses of selenium exceeding 50 microns and for energies between 12 and 50 keV, the detective quantum efficiency of this photoconductor is found to exceed that of a conventional Gd2O2S-based mammographic phosphor screen.

Efficiency↗
PubMed1995

X-ray imaging using amorphous selenium: feasibility of a flat panel self-scanned detector for digital radiology.

We investigate a concept for making a large area, flat-panel detector for digital radiology. It employs an x-ray sensitive photoconductor to convert incident x-radiation to a charge image which is then electronically read out with a large area integrated circuit. The large area integrated circuit, also called an active matrix, consists of a two-dimensional array of thin film transistors (TFTs). The potential advantages of the flat-panel detector for digital radiography include: instantaneous digital radiographs without operator intervention; compact size approaching that of a screen-film cassette and thus compatibility with existing x-ray equipment; high quantum efficiency combined with high resolution. Its potential advantages over the x-ray image intensifier (XRII)/video systems for fluoroscopy include: compactness; geometric accuracy; high resolution, and absence of veiling glare. The feasibility of the detector for digital radiology was investigated using the properties of a particular photoconductor (amorphous selenium) and active matrix array (with cadmium selenide TFTs). The results showed that it can potentially satisfy the detector design requirements for radiography (e.g., chest radiography and mammography). For fluoroscopy, the images can be obtained in real-time but the detector is not quantum noise limited below the mean exposure rate typically used in fluoroscopy. Possible improvements in x-ray sensitivity and noise performance for the application in fluoroscopy are discussed.

Cobalt Radioisotopes↗
PubMed1995

X-ray imaging using amorphous selenium: photoinduced discharge (PID) readout for digital general radiography.

Digital radiographic systems based on photoconductive layers with the latent charge image readout by photoinduced discharge (PID) are investigated theoretically. Previously, a number of different systems have been proposed using sandwiched photoconductor and insulator layers and readout using a scanning laser beam. These systems are shown to have the general property of being very closely coupled (i.e., optimization of one imaging characteristic usually impacts negatively on others). The presence of a condensed state insulator between the photoconductor surface and the readout electrode does, however, confer a great advantage over systems using air gaps with their relatively low breakdown field. The greater breakdown field of condensed state dielectrics permits the modification of the electric field during the period between image formation and image readout. The trade-off between readout speed and noise makes this system suitable for instant general radiography and even rapid sequence radiography, however, the system is unsuitable for the low exposure rates used in fluoroscopy.

Biophysical Phenomena↗
PubMed1991

Digital videofluorography: a new direction in diagnostic imaging.

Accurate information about digital technology is often difficult to obtain because of unrealistic expectations. The development of the digital department is complex and must not be understated. However, we have attempted to show that if a truly committed hospital immediately accepts DVF, this will result in reduced capital and operational costs, as well as lower radiation doses to the patient and operator. We believe this will happen because every major X-ray manufacturer is demonstrating and offering DVF systems, all of which are based on the technology described in this paper. These systems can either be totally integrated into fluoroscopic facilities or be purchased as add-on components to established units. In any event, a modern fluoroscopic facility can cost several hundred thousand dollars, and must last 10 to 15 years. It seems prudent to acquire quality and digital capability so that these units will be adequate in the year 2000 and beyond.

Attitude of Health Personnel↗
PubMed1989

Clinical comparison of analog and digital 100 mm photofluorography.

Many of the problems associated with digital acquisition of clinical images from x-ray intensifier/television systems have been eliminated by the use of a pulsed progressive readout from a 1024 line television camera into a 1024 x 1024 pixel image store, the whole arrangement triggered by the circuitry of a 100 mm camera. By means of a beam splitter, this study demonstrates a clinical comparability between 100 mm and digital images under identical conditions. In addition, radiation dose levels can be reduced by tailoring exposures to individual patients and their clinical needs. Several clinical cases are presented to illustrate the interchangeability of the new digital modality for fluoroscopic examination with an ordinary sized x-ray image intensifier.

Bone and Bones↗
PubMed1985

Videofluorography and pulsed fluoroscopy using a 512 X 512-pixel digital image system.

The combination of videofluorography and pulsed fluoroscopy using an analog videodisc system has previously been investigated with regard to image quality and potential for dose reduction. The authors found that the system could be improved still further by replacing the analog disc with a 512 X 512-pixel digital image system, thereby increasing fluoroscopic image quality and permitting stored images to be recorded with a multiformat camera. The pulsed method is compared with low-dose-rate fluoroscopy, in which a continuous image is obtained at 1/4 of the normal rate. Whereas image quality using a low dose rate was inadequate for any useful purpose, pulsed fluoroscopy was sufficient for all but the most critical stages of the examination.

Computers↗
PubMed1984

Radiation dose implications of digital angiographic systems.

Digital subtraction angiography (DSA) has been widely accepted and applied. The concentration of iodine in the vessels of interest is low in intravenous DSA. The resultant images can be improved to some extent by increasing the radiation dose. Therefore DSA could become, and possibly could remain, a relatively high-dose procedure. The contributions to dose from the various components of the examination such as fluoroscopy, positioning, test exposures, and final acquisition runs are considered separately. Individual segments of a DSA examination are discussed to show how and where opportunities arise to reduce doses to the lowest levels consistent with satisfactory images.

Angiography↗
PubMedDate not supplied

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.

Cesium↗