John M. Herron

Computer Electronic Radiography For Early Detection Of Vascular Disease

A computerized electronic radiography system is being developed for early non-invasive detection, characterization and quantification of atherosclerotic lesions. The method uses a conventional x-ray source combined with a solid state detector system which is coupled to a digital computer for processing and display of the radiographic information. The computer stores and optimizes the image for improved interpretation of the image detail. The computer assisted image enhancement, vessel localization and pattern analysis is an integral part of the instrumentation system. The system has been evaluated in preliminary studies to determine contrast sensitivity, radiation exposure, resolution and diagnostic quality. Even at this early developmental stage of this technique, very low contrasts have been visualized. The arterial images obtained with intravenous injections in dogs have demonstrated the potential of this method for the non-invasive detection and quantification of atherosclerotic disease.

Errors in cerebral blood flow determinations by xenon‐enhanced computed tomography due to estimation of arterial xenon concentrations

Errors in the determination of xenon concentrations in arterial blood during inhalation of xenon-oxygen mixtures are used to assess errors in the derivation of regional cerebral blood flow by the xenon-enhanced computed tomography (CT) method. The results of this study indicate that approximating the arterial buildup by a single exponential introduces relatively small errors in estimated flow values. The most significant systematic error is introduced by errors in estimation of the xenon arrival time to the brain in relationship to sequential (CT) scanning times.

Signal fading, erasure, and rescan in storage phosphor imaging

This paper addresses three problems in storage phosphor imaging: natural fading of latent image signals, proper erasure of the exposed plates, and re-scanning for a second readout. Signal, and signal-to-noise ratios were measured as a function of time, erasure power/time, or number of pre-scans to study these problems. The latent image signals were found to decay very rapidly during the first several minutes and stabilize after several hours. The fading effect results in a variable signal gain (signal per unit exposure) which may affect the system calibration and quantitative use of the image data. Complete erasure of the latent image signals is necessary to ensure that no residual image signals are present when the plate is exposed again. It was found that plates used in high exposure applications (GI, therapeutic imaging) may require an excessively long erasure time to prepare them for use in low exposure applications (chest imaging). Although the latent image is partially erased during the readout process, it may sometimes become necessary to re-scan the plate for a second or third readout. It was found that because a large number of energy traps are generated for each x-ray photon, a significant portion of the x-ray information remains intact for reuse after the first or second scans. Measurement of the signals and signal-to-noise ratios are presented to demonstrate and discuss the aforementioned problems or effects.

X-Ray Imaging With Two-Dimensional Charge-Coupled Device (CCD) Arrays

Two basic approaches to digital radiography have been thoroughly investigated in the last decade. In one, the traditional video chain is modified, and the output of the camera is digitized and analyzed by a computer to produce an image of the area of the body being examined. The other approach uses a linear detector array to generate a sequential series of line images as the array is scanned across the area being observed; the line images are then combined by a computer to form a two-dimensional image. We are investigating the characteristics of a third method, a prototype digital radiography system in which two-dimensional diode arrays (CCD) are fiberoptically coupled directly to either a scintillating fiberoptic plate or to a fluorescent screen. In this paper we describe the- concepts and design configuration of this approach, as well as preliminary results from several phantom studies. Our results indicate that high resolution, high signal-to-noise ratio imaging can be attained with this method.

X-ray imaging with 2048 x 2048 CCD array

We are investigating a prototype x-ray imaging system in which a scintillating fiberoptic glass plate and/or a fluorescent screen is fiberoptically coupled to a 2048 x 2048 CCD array (Tektronix). The imaging system includes a fiberoptic minifier to increase the imaging field of view to a clinically usable one. The system also allows for cooling of the CCD to reduce the effect of dark noise on image quality and the use of single-stage light amplification to act as a shutter and to provide gain control. Images are software corrected for dark current, individual pixel gain, and geometric distortion. Preliminary results indicate that high quality x-ray imaging can be obtained using this methodology. This paper describes design concepts and configuration of the system as well as characterizations of the initial x-ray images acquired with the camera.

Frequency-dependent DQE in dual-screen CR imaging

It has been shown that dual-screen image acquisition technique can be used to improve the image signal-to-noise ratio (SNR) in computed radiography (CR) imaging. In chest imaging situations, acquisition with a high resolution (HR) screen and a standard resolution (ST) screen can also be used to improve the modulation transfer function. Unlike in conventional radiography using two screens, the front and back images in dual-screen CR imaging can be separately read out and superimposed with the weighting factors selected to optimize a specific image quality descriptor. The purpose of this paper is to determine the weighting method which would optimize the frequency dependent detective quantum efficiency (DQE) in dual-screen CR imaging with an HR and a ST screen. A theoretical model is derived to relate the DQE in the superimposed image to those in the front and back images and to determine the optimal weighting factors and the maximum DQE that can be achieved. Using this model and DQEs measured for the HR and ST screens, we could estimate optimal weighting factors and maximum DQEs as a function of frequency. Various screen combinations were studied and compared for the maximum DQE that can be achieved. We have shown that for maximum DQE, the front and back images should be weighted in such a way that their magnitudes are proportional to the DQE divided by the MTF. The maximum DQE in the optimally superimposed image is equal to the sum of the DQEs of the front and back images.

Pixel averaging versus digitization using larger apertures: a comparison of the spatial resolution properties

2K (2048 X 2500) or 1K (1024 X 1250) digitized chest film images can be generated by either direct digitization or converting a 4K (4096 X 5000) digitized film image by pixel averaging. In this paper, these two methods are compared for their implication on the resolution properties of the resulting images. A film image of the lead bar resolution pattern was used as the source of all digital images. Signal profiles of the bar pattern were studied to compare the pixel averaging and direct digitization methods. Based on this comparison, it was found that pixel averaging, when used with proper filtering, can be used to simulate direct digitization using larger (210/420) apertures and result in similar square wave response. Pixel averaging, however, can result in better square wave response when 2 to 1 or 4 to 1 straight averaging is used or a sharper kernel is used in pre-filtering.

Digital Radiography of the Chest by Self-Scanning Linear Diode Arrays

Diode array digital radiography DADR is a method of radiographic imaging that combines the advantages of computer technology with self-scanning linear diode arrays. These digital images are superior to those obtained by film in recording and displaying information in the lightest and the darkest areas of the film, resulting in a balanced image of the entire thorax without compromising detail, and at reduced radiation dose. This is a direct result of the wide dynamic range, high contrast sensitivity, fiber optic coupling, small diode size, short exposure time, and rejection of scattered x-rays of the system coupled with digital post-processing enhancement of the image displayed at 1024 X 1024 pixels.

User-friendly electronic film library for digital imaging modalities

A system that provides a simple and reliable means for archiving image data from various digital modalities has been developed and tested. In this system, image data are frame-grabbed from the acquisition device (e. g., CT, MR, or ultrasound) during the normal process of filming cases, in a manner which is essentially transparent to the technologist. Text information is derived from the frame-grabebd data by applying character recognition techniques. This system is adaptable to many types of image-generating devices manufactured by a variety of vendors. It allows for easy review and/or reprinting of previous cases and will eventually allow previous cases to be viewed at full resolution on soft displays. We have demonstrated that the system is feasible alternative to the use of film or magnetic tape for the permanent storage of CT, MR, or ultrasound image data.

Digital x-ray imaging with two-dimensional charge-coupled device arrays

We are investigating the characteristics of a prototype digital radiography imaging system in which six two-dimensional diode arrays (CCD) are directly coupled through a bonded matrix (3 x 2) of fiberoptic minifiers to either a scintillating fiberoptic glass plate or to a fluorescent screen. Images are digitally acquired at a rate of up to 30 frames/sec and software corrected for pixel gain, dark current, and geometric distortion. This paper describes the concepts and design configuration of this approach, as well as preliminary results from several phantom and animal studies. Our results indicate that high resolution (> 4 lp/rnm) and high signal-to-noise ratio images can be obtained with this method. However, the complexity associated with this concept cannot be discounted.