R. E. Alvarez

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.

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.

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.

Typically and historically mineral oil has been used as liquid insulation in power transformers. Its functions are to ensure the isolation between active parts, absorb heat transmitting it to the outer surfaces and protect other insulators (such as paper) from moisture. Moreover, the analysis of insulating oil provides a diagnosis of the state of the transformer. In the last 10 years, there has been resurgence in the use of natural ester (vegetable oil) because of their ‘green’ credentials. Their biodegradability and high fire point compared to mineral oils are some of its advantages. However, its high cost and low evaluation in service (performance) still limit its application. For mineral oils there is a large database and knowledge obtained from numerous transformers studied over many years. For this reason, certain gas patterns can be related to a specific fault. Conversely, the limited field data related to natural ester makes the analysis of dissolved gases unreliable. Some recent studies attempt to determine the differences between each type of oil when a fault takes place. This study presents a review of the difference between the generated gases in mineral oil and natural ester as a result of the most common faults in transformers.