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Biomedical subjects

J A Rowlands

Publications and source records attributed to J A Rowlands.

At least 19 recordsLinked to original sources
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↗
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↗
PubMedDate not supplied

Videofluorography: the role of temporal averaging.

Videofluorography is the technique of obtaining radiographic images by photographing with a multiformat camera the television images produced during x-ray image intensified fluoroscopy. This technique of producing images has received considerable clinical use, but the radiation exposure used has been in doubt. Experiments were performed to evaluate this radiation exposure. It was found that the imaging chain was reducing mottle by temporal integration. These results were tested by comparing the image quality of videofluorographs with that of photofluorographs . The blurring effect of motion on images was also evaluated as this is a factor which must be balanced against the improvement of mottle due to temporal integration. It was found that exposures used for videofluorography were of the order of 10 microR (2.58 X 10(-9) C kg-1) to the input of the x-ray image intensifier.

Fluoroscopy↗
PubMedDate not supplied

Detective quantum efficiency of x-ray image intensifiers: comparison of scintillation spectrum and rms methods.

For monoenergetic x rays, the detective quantum efficiency (DQE) of an x-ray image intensifier (XRII) obtained by measuring the ratio of signal to root-mean-square (rms) noise is compared with the DQE obtained by the scintillation spectrum method. To obtain agreement between the two methods, the rms noise must be measured at a very low frequency because of the temporal response RXRII (f) of the XRII. This takes a long time and the measurement of the DQE from the scintillation spectrum is much quicker. Measurement of RXRII (f) shows that it is dependent on x-ray flux rate. RXRII (f) measured at the same flux rate as DQE permits correction of DQE measurements to zero frequency. In order to estimate DQE for polyenergetic beams, we need to know as a function of energy not only the DQE but the amount of energy deposited per absorbed x ray. The latter is also obtained from the rms measurement.

Methods↗
PubMedDate not supplied

Noise in stenosis measurement using digital subtraction angiography.

This paper examines statistical errors in the measurement of arterial stenoses by digital videodensitometry. Images of vessel phantoms were acquired using digital subtraction angiographic techniques with low concentrations of an iodine contrast medium and low levels of x-ray exposure. Effects of the spatial and temporal averaging of image information on signal-to-noise ratios in the stenosis measurement were of primary interest. The influences of iodine concentration, x-ray scatter, veiling glare, x-ray energy spectrum, x-ray exposure, and detective quantum efficiency of the system were also included in the theoretical analysis. The agreement between theoretical calculation and experimental measurement of a simulated vessel was verified using measured values of the imaging system parameters. With a 14.2 mg/ml iodine concentration, using 20 mR per image at the entrance to a 13-cm water phantom, and averaging over a 6-mm length of a vessel 6.2 mm in diameter, the standard deviation in a measurement of a vessel's relative cross-sectional area was about 0.05. The extension of these results to practical applications in vivo is discussed.

Angiography↗
PubMedDate not supplied

Optical factors affecting the detective quantum efficiency of radiographic screens.

Parameters related to the detective quantum efficiency (DQE) of several representative screens of different thicknesses, phosphor grain sizes, and optical properties were measured by the scintillation spectrum method, using monoenergetic x rays produced from x-ray fluorescence. The experimental results, including those for spectral shape and average light energies (EA) emitted, are compared with conventional theories of the operation of screens. It was hoped that this would vindicate the theory of the effect of optical properties and so permit the simple calculation of all parameters related to DQE from standard x-ray attenuation tables. Rather more substantial energy-dependent deviations of EA are found than was previously realized, which preliminary analysis suggests are due to both optical effects and photoelectron escape. We conclude that although DQE for a single energy can be calculated by simplified methods to within +/- 10%, the effective DQE when polyenergetic beams are used is much less accurately estimated and requires a fuller theoretical treatment.

Filtration↗
PubMedDate not supplied

Optimization of readout rate of television cameras in pulsed digital radiography.

Adaptation of existing fluoroscopic television rooms to permit radiographs to be recorded directly by the television system would be a very practical way to introduce digital radiography into general use if the image quality was adequate. The main limitation of current systems is spatial resolution. Although a digital system can change the spatial frequency response by application of spatial frequency enhancement, the amount of enhancement which can be usefully applied is a function both of the spatial frequency response of the whole system and image noise. Slow scan television systems have been proposed as a way to reduce noise; and hence, as a way to potentially improve limiting resolution. We examine all the consequences of changing readout scan rate, both theoretically and with measurements on a commercial camera with a lead oxide image tube. It is found that recent improvements in camera tube design, specifically the increase in current capability of the electron gun, have reduced the advantages theoretically available with slow scan. However, an order of magnitude increase in useful dynamic range is still possible.

Humans↗
PubMedDate not supplied

Measurement of the spatial Wiener spectrum of nonstorage imaging devices.

All previous methods for measuring image noise spectra require a noise realization, a static image, typified as a photograph which can be scanned to create the Wiener spectrum. We wished to analyze the spatial noise power spectrum at the output phosphor of a continuously irradiated imaging device, an x-ray image intensifier (XRII), which is incapable of image storage and thus the image is continually changing as a function of both time and space. Our new method utilizes a pair of slits to measure the relative Wiener spectrum of the temporally changing components of the image (i.e., x-ray quantum and XRII gain noises). By measuring the modulation transfer function and the Wiener spectrum of the same XRII on the same apparatus it was possible to demonstrate the spatial frequency dependence of the detective quantum efficiency. Adaptations of the method should permit the measurement of Wiener spectra of fluoroscopic television systems directly from the TV monitor.

Fourier Analysis↗
PubMedDate not supplied

System for digital acquisition of gastrointestinal images.

Previous theoretical work and clinical experience with digital acquisition of fluoroscopic images have identified several problems which needed to be solved. These are: image resolution; blurring due to patient motion, combined with long exposure times; and excessive x-ray quantum mottle levels. We will show that application of pulsed progressive readout (PPR) methods to the TV camera solves these problems. By permitting a high-intensity x-ray pulse to be delivered, all motion is stopped and quantum mottle is reduced to acceptable levels. It will be shown that 1024 x 1024 digital matrices provide adequate resolution and 8-bit digitization is sufficient to permit the same quality as is used in conventional 100-mm photofluorography. User acceptance can be made easier by incorporation of existing photofluorographic controls (with which the radiologist is already familiar) to acquire the digital images. It is possible to interface PPR video systems using existing 100-mm exposure circuits without much modification and the resulting system can be regarded as a digital 100-mm camera.

Digestive System↗
PubMedDate not supplied

A study of motion in gastrointestinal x-ray fluoroscopy.

Studying motion in the gastrointestinal (G.I.) tract is the first step towards our goal of designing a digital algorithm for real-time noise and motion blurring reduction by temporal and spatial averaging in x-ray fluoroscopy. The present work concerns the types of motion relevant to G.I. fluoroscopy (e.g., motion of the walls of the oesophagus, the stomach, the small intestine, and the colon), and measurement of the range of velocities in different parts of the G.I. tract from tape recorded fluoroscopic sequences. The relationship between image contrast and velocity, the effects of motion on system response, and in particular the influence of these parameters on the evaluation of velocities will also be discussed.

Adolescent↗