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Biomedical subjects

A Fenster

Publications and source records attributed to A Fenster.

At least 19 recordsLinked to original sources
PubMed1992

A velocity evaluation phantom for colour and pulsed Doppler instruments.

We describe a phantom designed to evaluate the velocity measurements made with colour and pulsed Doppler instruments. Using a belt to translate a large volume of semi-rigid material through the entire Doppler sample volume eliminates many of the problems associated with flow and string phantoms. A servo-motor with feedback circuitry ensures accurate control of the belt velocity with an uncertainty in the mean velocity of 0.14%. The phantom provides velocities with typical variations of 0.07 cm/s. We have demonstrated the usefulness of this phantom by evaluating the linearity and accuracy of three pulsed Doppler instruments over belt velocities ranging from 0 to 80 cm/s. In addition, the measurements show the effects of the wall filter at low belt velocities. Using this phantom, we have quantified the accuracy, linearity and precision of the velocity measurements made by three colour Doppler instruments. The results also show regions where the colour instruments are aliased and where the wall filter dominates.

Calibration↗
PubMed1992

Daily monitoring and correction of radiation field placement using a video-based portal imaging system: a pilot study.

We have developed a video-based portal imaging system for radiotherapy localization. The system can acquire high quality portal images automatically using short (1-3 monitor unit) irradiations and immediately display the images. The major advantage of the imaging system is that it can be used routinely to check and correct patient positioning before much of the daily irradiation has been delivered. The portal imaging system has been used in a pilot study to monitor five patients during each of their daily treatments. The study has shown that: (i) image quality is sufficiently high to detect discrepancies in field placement from that prescribed on the simulator film; (ii) discrepancies in field placement occur frequently; and, (iii) routine correction of patient and block positioning can reduce the size of these discrepancies. This is the first time that field placement in radiation therapy has been checked and corrected routinely, before the treatment irradiation. However, limitations in the size of the field of view and in the methods of extracting and presenting the geometric information to the users limits the clinical utility of the imaging system. Solutions to these limitations are currently under development.

Computer Terminals↗
PubMed1991

Computer-controlled positive displacement pump for physiological flow simulation.

A computer-controlled pump for use both in the study of vascular haemodynamics and in the calibration of clinical devices which measure blood flow is designed. The novel design of this pump incorporates two rack-mounted pistons, driven into opposing cylinders by a micro-stepping motor. This approach allows the production of nearly uninterrupted steady flow, as well as a variety of pulsatile waveforms, including waveforms with reverse flow. The capabilities of this pump to produce steady flow from 0.1 to 60 ml s-1, as well as sinusoidal flow and physiological flow, such as that found in the common femoral and common carotid arteries are demonstrated. Cycle-to-cycle reproducibility is very good, with an average variation of 0.1 ml s-1 over thousands of cycles.

Arteries↗
PubMed1990

A digital fluoroscopic imaging device for radiotherapy localization.

We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system has a number of modifications compared to previously reported devices. The detector, which consists of a copper plate with Gd2O2S:Tb phosphor bonded directly to the copper, has been designed to maximize light output from the phosphor by increasing the phosphor thickness. The operation of the T.V. camera has been modified so that the light signal is accumulated on the target of the T.V. camera for periods of 0.2-2.0 seconds. Accumulation of the light increases the video signal relative to the fixed noise current generated by the camera, and thus minimizes the camera noise. The resulting image quality is comparable to film, so the imaging system represents a promising alternative to film as a method of verifying patient positioning in radiotherapy.

Fluoroscopy↗
PubMed1978

Split xenon detector for tomochemistry in computed tomography.

The design of a split high-pressure xenon detector array for tomochemistry in computed tomography (CT) is described. Each detector produces a signal from the front primarily due to low energy photons and a signal from the back primarily due to high energy photons. Two methods are described whereby these signals are used to determine the photoelectric and Compton coefficients. From these, the electron density and average atomic number can be determined for each pixel in the image. These methods were tested by computer simulations of scans of a simple phantom, and the resulting Compton and photoelectric images are presented and compared with a conventional CT image. It was found that electron density and atomic number can be determined to an accuracy of better than 4%. The sensitivity to noise was studied, and it was found that the standard deviation of the mean of a 5 X 5 pixel region in the conventional image is about a factor of 3 lower than in the same region in the Compton image and about a factor of 40 lower than in the photoelectric image.

Humans↗
PubMed1975

New methods of imaging in diagnostic radiology.

The basic ideas of electrostatic imaging with special reference to ionography are reviewed. The concept of edge contrast is explained in terms of calculated powder particle trajectories and methods for controlling edge contrast are presented. Also, methods for reading a foil from outside the ionography chamber using the electric field extending through the foil are described. An example of an image taken with liquid in the chamber (liquid lonography) is presented, and the possible extension of liquid ionography to nuclear medicine is discussed.

Absorptiometry, Photon↗
PubMedDate not supplied

Computer-controlled flow simulator for MR flow studies.

A novel computer-controlled flow simulator for use in magnetic resonance (MR) flow experiments was evaluated. The accuracy in constant-flow mode was better than 1%. The accuracy in pulsatile-flow mode was found to be dependent on the interconnecting tubing. The short-term and long-term reproducibilities of pulsatile waveforms were less than or equal to 0.4 mL/sec (1 standard deviation). Increased response times due to the lengths of tubing required in MR flow experiments were surmounted by using a modified tubing configuration and precompensated waveforms. Piston reversal was found not to cause major difficulties in MR flow experiments.

Blood Circulation↗
PubMedDate not supplied

Closed-system ionography for diagnostic radiology.

Three methods are described whereby radiographic electrostatic images are transferred from inside of a liquid ionography chamber to the outside. One of these methods is implemented showing that multiple-charge images can be transferred from a single original without significant degradation of image quality and each copy may be developed with a different amount of edge contrast. This new method of imaging was applied to a radiograph of a test pattern and a hand phanton. An exposure of about 6 mR to the imaging chamber is needed to produce a useful image with a resolution of about 8 1p/mm.

Ions↗
PubMedDate not supplied

Therapy imaging: a signal-to-noise analysis of a fluoroscopic imaging system for radiotherapy localization.

We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system differs from previously reported devices in the construction of the detector and in the operation of the TV camera. The signal, noise, and signal-to-noise properties of this system have been determined by measuring the modulation transfer function [MTF(f)], the noise power spectra [NPS(f)], and by calculating the detective quantum efficiency [DQE(f)] of the system. The results show: (i) that the spatial resolution of the system is determined largely by the lens of the TV camera and by frame grabber; and (ii) that the noise in the system is dominated by the secondary light quanta, due to the poor light collection efficiency of the optical chain. Despite these physical limitations, a contrast-detail study shows that the fluoroscopic system is better at detecting large, low contrast objects than portal films. Therefore the system is already a reasonable alternative to portal films and modifications to the metal plate/phosphor detector, lens, TV camera, and frame grabber should improve the performance of the system further.

Fluoroscopy↗
PubMedDate not supplied

An accurate method for direct dual-energy calibration and decomposition.

We propose the use of conic and cubic surface equations (surfaces of second and third order) to directly approximate the dual-energy equations (the integral equations for the dual-energy log-signal functions, i.e., the negative logarithms of the relative detector signals, considered as functions of the basis-material component thicknesses of the object) and especially their inverses. These types of surface equations require a minimum number of calibration points, and their solutions are smooth, monotonic functions with the correct linear asymptotic behavior. The accuracy of this method is investigated and compared to that of conventional polynomial approximations, both for simulated and real calibration data, taken from two split-detector systems. These systems provide a more stringent test of our method than comparable dual-kVp systems, due to the greater nonlinearity of their log-signal and inverse functions. For these systems, we show that direct approximation of the inverse dual-energy equations using the simple eight-term rational form of the conic surface equation provides an extremely fast decomposition algorithm, which is accurate, robust in the presence of noise, and which can be calibrated with as few as 9 calibration points, or robustly calibrated, with a built-in accuracy check, using only 16 calibration points. Also, we show that extreme accuracy of approximation (to within less than 10(-6) in log-signal and 1 micron in material thickness) is theoretically attainable using the eighteen-term form of the cubic surface equation, which has a closed-form analytic solution. Finally, we consider the effects of noise on calibration accuracy, and derive simple formulas which relate the true and apparent root-mean-square (rms) accuracies. These formulas then allow the comparison of the true rms calibration accuracies of various surface approximations, considered as functions of the total calibration heat loading of the x-ray tube.

Algorithms↗
PubMedDate not supplied

An in-line optical image translator with applications in x-ray videography.

Many applications in radiography require, or would benefit from, the ability to translate, i.e. move, an optical image in the detector plane. In this paper, we describe the design and characterization of a prism-based optical image translator for insertion into existing XRII-video imaging systems. A pair of prisms rotatable about the optical axis form a very compact in-line optical image translator for installation in the parallel light path between an x-ray image intensifier and its video camera. Rotation of the prisms translates the XRII optical image on the camera target. With the addition of x-ray and light collimators to limit the image to a single video line, x-ray streak images may be acquired. By rotating an object in the x-ray beam during a streak, a complete computed tomography (CT) data set may be acquired. This image translator can translate an image anywhere in the focal plane of a 50-mm-output lens within a 40-mm-diam circle. The prisms have an aperture of 50 mm, permitting an optical speed of F/2 with a 50-mm output lens. The design is insensitive to angular alignment errors. This image translator is achromatic, since the spectral width of the output phosphorus of image intensifiers is sufficient to introduce blurring in a nonacrhomatic design. A prism-based image translator introduces image distortion, since the prisms do not operate at minimum deviation. The distortion is less than 4% over all parts of a typical detector area, and less than 1% in the central region of the image.(ABSTRACT TRUNCATED AT 250 WORDS)

Biophysical Phenomena↗
PubMedDate not supplied

A time-delay integration charge-coupled device camera for slot-scanned digital radiography.

We have developed a low-noise digital camera based on a 512 x 96 element CCD operating in the time-delay integration mode. This camera has been combined with an x-ray image intensifier to record radiographic images produced by a scanning slot beam of radiation. This results in the rejection of a large fraction of scattered radiation, without a significant increase in x-ray tube heat loading or image acquisition time. Here we describe the design of our CCD camera and the results of our investigations of camera resolution, linearity, noise, and quantum efficiency. We have found that both the resolution limit (50 mm-1) and the dynamic range (2100) of this novel camera are greater than reported values for conventional video cameras. Applications of this system in digital angiography and mammography are discussed.

Angiography↗
PubMedDate not supplied

Activity distribution of a cobalt-60 teletherapy source.

In the course of quantifying the effect of radiation source size on the spatial resolution of portal images, a concentric ring structure in the activity distribution of a Cobalt-60 teletherapy source has been observed. The activity distribution was measured using a strip integral technique and confirmed independently by a contact radiograph of an identical but inactive source replica. These two techniques suggested that this concentric ring structure is due to the packing configuration of the small 60Co pellets that constitute the source. The source modulation transfer function (MTF) showed that this ring structure has a negligible influence on the spatial resolution of therapy images when compared to the effect of the large size of the 60Co source.

Cobalt Radioisotopes↗
PubMedDate not supplied

Analytic approximation of the log-signal and log-variance functions of x-ray imaging systems, with application to dual-energy imaging.

In the analysis of x-ray system performance, the log-signal function, or negative logarithm of the relative detector signal, and the analogously defined log-variance function, are of central importance. These are smooth, monotonic functions of object thickness, which are nonlinear for nonmonoenergetic x-ray source spectra. If we assume a dual-energy decomposition of the object into two basis materials, then they can be written as analytic functions f(x,y) and f*(x,y), respectively, of the component thicknesses (x,y) of the object. In this paper, we analytically develop the Taylor series of these functions, prove that they converge everywhere, and parametrize their coefficients via suitable central spectral moments of the basis-material attenuation coefficients. We then show how the lower-order moments can be used to construct, in closed form, smooth, monotonic, second-order (conic) surface functions which closely approximate f(x,y) and f*(x,y) over the entire feasible domain. A simplified construction, based on using appropriate asymptotic values of the basis-material attenuation coefficients to match the asymptotic behavior of these functions, is also given. The inclusion of image components with K-edge absorption spectra, such as iodine, is done without effort. Extension of the results to the construction of similar (virtually exact) third-order (cubic) surface approximations is straightforward. As an illustration of the broad applicability of this approach, we extend our analysis to the construction of similar approximations to the inverse (decomposition) functions for an arbitrary dual-energy system, and investigate their numerical accuracy for a model dual-kVp system. We conclude that this extended analysis provides an accurate description of the system behavior in terms of a small number of physically meaningful parameters. This parametrization permits greater physical insight into the system behavior, while at the same time simplifying its mathematical description, and similarly facilitates the analysis of various measures of imaging performance via either analytic or numerical methods.

Aluminum↗
PubMedDate not supplied

A digital-radiographic technique for in vitro tissue thickness measurement using iodine displacement.

A novel digital radiographic method has been developed for measuring the thickness of a tissue sample by iodine displacement. This simple, accurate method is useful both in medical research and in the comparison of pathological and clinical findings. Radiographic measurements of tissue samples in air suffer from the limitation that the quantity measured depends on the product of both thickness and radiographic attenuation coefficient. This technique allows one to obtain thickness measurements from a digital radiograph of a tissue sample suspended in a bath of radio-opaque contrast agent. The attenuation of the iodinated contrast agent is much higher than that of tissue or calcium. Thus the resulting image is determined largely by the contour of the surrounding bath, and is only slightly influenced by the composition of the tissue. This technique improves both the accuracy and precision of radiographic thickness determination. In this paper the iodine displacement technique is described and the accuracy and precision of thickness measurements in appropriate phantoms are quantified. This technique has been used with both image intensifier and screen-film based imaging systems to obtain thickness maps of calcified human aorta, with precision better than 4% and spatial resolution of 2.5 mm-1.

Aged↗