Active energy selective image detector for dual-energy computed radiography.

A new energy selective detector for dual energy computed radiography has been developed that combines many of the advantages of x-ray tube voltage switching and single exposure double screen detectors. The new active detector utilizes electro-optical modulation of the response of the storage phosphor screens to allow dual exposure acquisition with no motion of the screens. Electro-optical modulation can be done rapidly so the detector can acquire the voltage switched images in a short enough time to minimize patient motion artifacts. Voltage switching produces effective detected energy spectra that result in much lower noise for a given patient dose than the effective spectra of double screen detectors. In this paper the new concept is described, optimal voltages and filter materials are determined by computer simulations, and the active detector performance is compared to other energy selective detectors. The new detector provides over 30 times better signal to noise ratio squared (SNR2) for the same dose and over five times better SNR2 for the same x-ray tube loading than a double screen detector. The effects of incomplete erasure of the x-ray exposures are determined quantitatively. With achievable erasing fractions, the SNR2 is over 20 times better for the same dose and over three times better for constant tube loading than a double screen detector. The active detector is also compared to mechanical screen switching. Mechanical switching provides somewhat better SNR2 for the same dose, approximately 1.1 times the active detector SNR2 at optimal x-ray tube voltages. The performance is compared with highly absorbing back screens. If these are used in both the active and passive detectors, both detectors' quality factors increase, but the advantage of the active detector over the passive detector decreases with large back screen thicknesses.

Filter functions for computing multipole moments from the magnetic field normal to a plane.

A technique for calculating the magnetostatic multipole moments (and therefore complete biomagnetic information) from measurements of the normal component of the magnetic field on a plane is described. The technique involves multiplying the measured values by a set of unique filtering functions and integrating. A new formulation and method for calculating the filtering functions is introduced. The method uses expansions in basis sets and matrix inversion. Proofs are given of the nonsingularity of the matrices. Computer simulations of the application of the technique to a circular current loop and a current dipole in a spherical conductor are described.

Biomagnetic Fourier imaging [current density reconstruction].

A technique for reconstructing a current density distribution from measurements of its magnetic field is described. The technique assumes that the current distribution is confined to a single plane. The data it requires are measurements of the magnetic flux on a plane. These can be provided by an integrated planar array of superconducting quantum interference device magnetometers. The approach is based on the magnetic lead field which is derived in a simple way based on energy concepts. Using the lead field and conservation of charge conditions provides two linear, spatially invariant imaging equations relating the current density and flux measurements. These equations are solved using Fourier techniques. The validity of the resulting reconstruction technique is shown both analytically and with a computer model. The effects of not satisfying the planar assumption are described for the case where the currents are parallel but not in the same plane.

Photographic microdensitometry for evaluation of acid phosphatase activity at the electron microscope level.

The dry mass of reaction products in ultrathin sections was determined using electron micrographs of polystyrene spheres of known weight deposited on Formvar membranes and evaluating the negatives photometrically. This method was applied to the quantification of the final reaction product of the acid phosphatase reaction in a model system in which enzyme was incorporated in gelatin. The enzyme activity was demonstrated by the lead precipitation method and quantified by direct microphotometry at the light microscope level. Models were then embedded and sectioned for electron microscopy. Microphotometric values afforded by the electron negatives were in linear correlation with incubation times and enzyme concentration. Section thickness and its possible variations due to deformation or contamination under the electron beam were also evaluated. Measurements of lysosomal acid phosphatase activity in rat kidney sections served to illustrate the application of the technique.

Digital processing of film radiographs.

Initial clinical experience with a system for the digitization, processing, and display of film radiographs is described. Film is digitized using a high-intensity laser scanner; the recorded image data may then be subjected to a wide variety of processing options, with display of processed images on television monitors. The possibilities of clinical applications to processing and display of chest radiographs and film mammograms are described. A comparison of conventional analog subtraction and digitized film subtraction angiography indicated equivalent diagnostic capability, with the advantage of flexible, interactive image processing with the digital technique. A specially designed, energy-selective cassette permits dual-energy imaging from two films effectively exposed to different x-ray energy spectra. Dual-energy imaging may be capable of the characterization of body materials, including lung nodules, and useful for eliminating obscuring radiographic shadows overlying regions of interest.

Generalized image combinations in dual KVP digital radiography.

Dual energy basis decomposition techniques apply to single projection radiographic imaging. The high and low energy images are non-linearly transformed to generate two energy-independent images characterizing the integrated Compton/photoelectric attenuation components. Characteristic linear combinations of these two basis images identify unknown materials, cancel known materials, and generate synthesized monoenergetic images. The problems of intervening materials and material displacement are solved in general for a wide class of clinical imaging tasks. The basis projection angle identifies one from a family of energy selective imaging tasks, and such performance measures as the contrast enhancement factor (CEF) and signal to noise ratio (SNR) are expressed as functions of this angle. Algorithms for the decomposition of high and low energy measurements are compared and experimental images are included.

Initial results with prereconstruction dual-energy computed tomography (PREDECT).

Prereconstruction dual-energy computed tomography (PREDECT) produces rigorously exact reconstructions that accurately separate the total attenuation coefficient into two values, representing the Compton and photoelectric contributions. The images are obtained without polychromatic distortion and with an acceptable dose. PREDECT was used to scan objects and solutions of varying atomic number and electron density, specimens of normal and pathologic brain and other body tissues, and nine patients. The values for Compton and photoelectric attenuation of the different specimens were distinctive enough to provide "tissue signatures" of potential clinical usefulness. Eight of the nine patients studied provided acceptable images, which produced some tissue characterization. PREDECT appears to represent an advance over the previously used postreconstruction methods; areas of greatest potential are differential diagnosis, improved detection of abnormalities, and elimination of the polychromatic artifact.

Optimal processing of computed tomography images using experimentally measured noise properties.

The spatial resolution and noise properties of a computed tomography (CT) image may be altered by two-dimensional linear filtering of the initial image. In this paper, we derive filters that minimize the noise variance subject to a constraint on the spatial resolution. The resulting filter functions can reduce the noise variance by 17% in comparison with conventional filters. The method for obtaining these filters requires knowledge of the noise and imaging properties of the system. We derive theoretical expressions for these properties and introduce experimental techniques for their measurement. The statistical characteristics are shown to be anisotropic, spatially variant, and object dependent. We discuss the implications of this result both for optimal filtering and for the general problem of CT image noise property measurement.

An inaccuracy in computed tomography: the energy dependence of CT values.

The CT values of a variety of materials were studied in an EMI and a Syntex head scanner. The presence of bone-simulating rings changed the CT values despite the use of constant length water paths and software corrections. Errors due to beam hardening in CT scanning are discussed. These errors could be of significance, particularly in quantitative studies. The changes in CT values with KV setting of the scanner are used to illustrate their energy dependence and the peculiarities of the scaling system introduced by EMI. The difficulty in specifying a standard scale or unit for CT scanners is discussed.

Energy-selective reconstructions in X-ray computerized tomography.

All X-ray computerized tomography systems that are available or proposed base their reconstructions on measurements that integrate over energy. X-ray tubes produce a broad spectrum of photon energies and a great deal of information can be derived by measuring changes in the transmitted spectrum. We show that for any material, complete energy spectral information may be summarized by a few constants which are independent of energy. A technique is presented which uses simple, low-resolution, energy spectrum measurements and conventional computerized tomography techniques to calculate these constants at every point within a cross-section of an object. For comparable accuracy, patient dose is shown to be approximately the same as that produced by conventional systems. Possible uses of energy spectral information for diagnosis are presented.

Selective material x-ray imaging using spatial frequency multiplexing.

A method is presented of encoding the transmission at specific regions of the x-ray energy spectrum onto a radiograph. A grating structure is placed in the x-ray beam that consists of alternate strips of material having different x-ray transmission spectra. The average spectral transmission of the two strips produces an image comparable to a conventional radiograph. The difference between the two transmission spectra produces an amplitude modulation of the grating pattern on the radiograph. When this grating pattern is decoded, through optical or scanning methods, it represents the transmission at the specific x-ray difference spectrum.