Kenneth E. Weaver

Potential artifacts associated with the scanning pattern of the EMI scanner.

Physical and geometrical considerations of the EMI scanner are used to predict that regions of the head can be missed by adopting a scanner procedure based on the nominal EMI slice thickness. Using a nominal slice thickness of 1.3 cm and effective focal spot length of 12 mm, one can predict that in the central portion of the head, up to 8% of the region scanned may not be imaged. Moreover, it is shown that simultaneous monitoring of a single focal spot by two adjacent detectors, as in the EMI system, would lead to significant overlap between the two adjacent EMI slices resulting in dual imaging. Experimental examples are shown to illustrate these two artifacts in the EMI scanner.

Prospects for x-ray exposure reduction using rare earth intensifying screens.

From measurements of screen-film sensitometry, Wiener spectra, and x-ray spectra, the speed, mottle, and quantum efficiency of several rare earth screen-film systems have been characterized. These systems offer exposure reductions of from two- to tenfold over conventional calcium tungstate systems. At the slower end, the noise (mottle) and resolution are comparable to what is available with conventional systems. Moreover, films designed to be used in such systems can be made with significantly lower silver content. As the speed advantage is increased from two- to genfold, however, the mottle also increases.

Phantoms for specifications and quality assurance of mr imaging scanners

The accompanying paper examines the subject of NMR phantoms. The paper reports on initial experience with existing phantoms and reviews proposals from various standards groups and professional organizations. Many image tests are illustrated by existing vendor phantoms. The paper concludes that many phantoms already meet or exceed most of the suggestions for tests of classical imaging parameters, which can be pursued by first order adaptations of CT phantoms. The paper does, however, point out the limitations of existing phantoms and raise the possibility of developing phantoms that more accurately reflect human shapes and cavities, and which present more realistic resistive losses, and T1 and T2 values.

The Measurement Of Diagnostic X-Ray Spectra With A High Purity Germanium Spectrometer

This paper describes a system which approaches the state-of-the-art in energy dispersive x-ray spectrometry. Special methods for obtaining x-ray spectra from machines operating at high current levels and other phenomena which could cause spectral distortion are discussed. Entrance and exit spectra from some typical diagnostic procedures are shown. It is demonstrated that the exit spectra can be closely approximated by replacing the phantom with aluminum filtration. Since the sensitometric performance of imaging systems is influenced by beam quality, this technique can provide x-ray beams similar to those used in diagnostic radiology. The usefulness of the spectrometer for accurately measuring kVp and determining unknown elements of an intensifying screen is also illustrated.

Comparative Image Aspects of Radiography and Computed Tomography

Fundamental image descriptors, including contrast, noise, and resolution, were reviewed for radiography and developed where appropriate for computed tomography. These variables were then used to predict limiting system performance as applied to a model for detection of simple tumor-like signals in a homogeneous surround. The results predict that any intrinsic physical advantages of computed tomography may be restricted to a fairly narrow range of tumor diameters, from a few millimeters to a few centimeters, depending on the relative dose levels and scatter levels present in each imaging system. The results indicate high probable yield for radiographic image manipulation involving independent or, preferably, mutual manipulation of scatter levels, contrast enhancement, kVp techniques, and subtraction techniques, where applied to reduce the highly structured backgrounds that can increase the complexity of diagnosis.

Performance Considerations Of X-Ray Tube Focal Spots

In attempting to specify the performance of an x-ray tube focal spot, a number of factors must be taken into consideration which influence the spatial and intensity distribution of the focal spot: Tube potential, tube current, off-focus radiation, and off-axis variations. In addition, a knowledge of the tube loading capabilities is necessary to adequately predict the focal spots role in the total imaging process. This paper presents techniques that were developed to investigate some of these factors and some typical results of work to date.

Selection of Technique Factors for Mobile Capacitor Energy Storage X-Ray Equipment

The technique factors of capacitor energy storage x-ray equipment influence the x-ray beam quality and quantity differently than those encountered with conventional single-phase or constant potential equipment. This is due to the nature of the high voltage waveform applied to the x-ray tube in capacitor energy storage systems. A lack of understanding of this difference can lead to excess patient exposure through inappropriate selection of technique factors by either the manufacturers of these systems or uninformed technologists using them. From analysis of exposure measurements made with a Masonite phantom, a method has been developed for determining whether a selected technique may result in unnecessary patient exposure. In addition, the distinction between technique factors for conventional high tension transformer and capacitor energy storage systems is reviewed.

Radiographic Imaging Using an Effective Rotationally Symmetrical Focal Spot

A mathematical model is presented for predicting the effect of rotation on certain types os asymmetric focal spot intensity distributions. Experimental rotational techniques are used to obtain rotationally symmetric, gaussian-like focal spot distributions from existing asymmetric distributions. Radiographic examples are shown which illustrate the resulting imaging effect of such rotationally symmetric focal spots compared to existing focal spots. In particular, images of resolution test patterns and simulated blood vessels are used to examine some of the radiographic consequences (contrast transfer and spurious imaging) of the use of differing types of focal spot intensity distributions, and to question the utility of single number characterizations of asymmetric focal spots.