Gopala U. V. Rao

Predicting femoral neck strength from bone mineral data. A structural approach.

An interactive computer program was developed to derive femoral neck geometry from raw bone mineral image data for an estimate of hip strength using single plane engineering stress analysis. The program, which we call Hip Strength Analysis (HSA), was developed as an attempt to improve the predictive value of hip bone mineral data for osteoporosis fracture risk assessment. We report a series of experiments with an aluminum phantom and with cadaver femora, designed to test the accuracy of derived geometric measurements and strength estimates. Using data acquired with both Lunar DP3 (DPA) and Hologic QDR-1000 (x-ray) scanners, HSA computed femoral neck cross-sectional areas (CSA) and cross-sectional moments of inertia (CSMI) on an aluminum phantom were in excellent agreement with actual values (r greater than .99). Using Lunar DP3 data, CSA and CSMI measurements at mid-femoral necks of 22 cadaver specimens were in good general agreement with literature values. HSA computed cross-sectional properties of three of these specimens were compared with measurements derived from sequential CT cross-sectional images. Discrepancy between the two methods averaged less than 10% along the length of the femoral neck. Finally, breaking strengths of 20 of the femora were measured with a materials testing system, showing better agreement with HSA predicted strength (r = .89, percent standard of the estimate (%SEE) = 21%) than femoral neck bone mineral density (r = .79, %SEE = 28%).

Regional cerebral blood flow measurements using stable xenon enhanced computed tomography: a theoretical and experimental evaluation

Several theoretical and practical aspects of regional cerebral blood flow measurements using stable xenon gas and CT are discussed. It is shown that by comparing the enhancement at any time T1 with that at saturation or any other time T2, the need to use arbitrary means to bring the arterial concentration data and the CT enhancement data to the same system of measurement units can be eliminated. If CT is performed continuously during the washin phase, say at intervals of 1 min, least squares analysis of the enhancement data can be used to obtain the best possible estimates for the flow rate constant k and the saturation enhancement. However, if only a limited number of scans can be performed, as may be the case in human studies, it is also possible to get a good estimate of k from a knowledge of the ratio of the enhancement at any time T1 with that at any other time T2. Combinations of T1 = 2.0 min and T2 = 4.0 min, T1 = 1.0 min and T2 = 6.0 min, or T1 = 2.0 min and T2 = 5.0 min were found to be the most convenient. It is also shown that the end-tidal xenon concentration in the exhaled air can be accurately assessed indirectly by measuring the oxygen, CO2, and water vapor concentrations, thereby eliminating the need for more expensive methods involving the use of a mass spectrometer or a thermal conductivity gas analyzer.

Physical Characteristics of Modern Radiographic Screen-film Systems

The most important characteristics that determine the. performance of a screen-film system are system sensitivity (speed), slope or the average gradient of the characteristic curve, latitude of the film, system resolution (MTF) and system noise. In addition to these, one has to consider secondary characteristics such as the base plus fog level, the base tint, safelight sensitivity, reciprocity law failure and film granularity while evaluating performence. A comprehensive study of many of these characteristics has been made for a number of film-screen systems on the market. The data has been tabulated in the form of what are called “characteristic tables of radiographic film-screen systems.” In these tables, characteristics such as average gradient, base plus fog level, etc. which depend on film alone, appear along the horizontal axis and MTF values which essentially depend on screens only appear along the vertical axis. The body of the table contains the absolute speeds in units of mR-1 at 80 KVP (heavy filtration) and a quantum mottle index of the various film-screen combinations tested, since these depend both on the film as well as on the screens.

Evaluation of xeroradiographic image quality.

A theory of edge enhancement has been developed to describe xeroradiographic images of linear step objects. The theory is shown to give an accurate description of the relationship between edge enhancement, subject contrast, radiation exposure, selenium charging potential and developer bias potential. An optimal transmitted x-ray exposure exists which gives the maximum edge enhancement. For low contrast structures, this same optimal exposure was found to yield the most acceptable images as judged subjectively. This finding has led to the successful application of automatic exposure termination in mammographic examinations using xeroradiography.

Evaluation of a new x-ray film with reduced crossover.

The improvement in MTF that can be obtained by the reduction of crossover exposures is evaluated using an experimental film supplied by the 3M Company (3M XUD). It was found that image resolution, as measured by the equivalent passband (2 integral of infinity 0 [M(f)]2df), of 3M Trimax 8 screens when used with the experimental 3M XUD film was 2.9 c/mm as compared to 1.8 c/mm when used with a conventional double coated film such as Kodak Ortho G or 3M XD. On the other hand, system speed with 3M XUD film was only 1.6 mR-1 as compared to 3.2 mR-1 with 3M XD film and 2.4 mR-1 with Kodak Ortho G film. These findings were substantiated by phantom studies as well.

Negative-Mode Soft-Tissue Xeroradiography

Positive-mode xeroradiography is an excellent method of visualizing soft-tissue structures in mammography and other procedures; however, toner deletions over a large area may preclude adequate delineation of the soft tissues near high-density structures such as bone. Use of the negative mode in the development cycle provides excellent demonstration of the soft tissues without toner deletions and requires only about 70% of the exposure needed in the positive mode. Negative-mode images also exhibit greater changes in radiographic contrast than those observed with the positive mode. The extent of achilles tendon lacerations was shown particularly well by this technique.

Probability of causation for cancers potentially induced by ionizing radiation

Prompted by increasing litigation claiming cancer induction from exposure to radiation from nuclear weapons tests, the United States Congress enacted an amendment to the Orphan Drug Act (Public Law 97-414, January 4, 1983), through which the National Institutes of Health was directed to and did produce Probability of Causation (PC) Tables. This review defines PC and discusses the data, limitations, and uses of the concept.

On optimizing the xeromammographic image.

This work undertakes a detailed system-based analysis of the xeromammographic process starting from basic considerations. Both the edge enhancement and wide-recording latitude, the two principal characteristics of xeroradiography, are shown to bear an intimate relationship to the electric-field distribution. Criteria and methods are formulated for optimizing xeromammographic image quality and a procedure is developed for calculating the white gap. Densitometric curves are derived for both positive- and negative-mode xeroradiography and found to be in excellent agreement with experiment. The question of image linearity is examined carefully and a threshold value of the electrostatic contrast is established, which sets a natural criterion for the application of Fourier analysis. Furthermore, it is shown that, in xeromammography with its inherently low-contrast structures, an optimal exposure exists which optimizes simultaneously all low-contrast edges. This last finding, coupled with experimental results, suggest immediately the possibility of an automatic exposure termination in xeromammography. Beam hardening is also investigated and it is shown that increased filtration combined with a lower bias potential leads to substantial dose reduction without significant loss of image quality. The paper concludes with a discussion of scattered radiation and how it affects xeromammographic image quality.

An intercomparison of the modulation transfer functions of square and circular focal spots

In field emission x‐ray tubes, the target is conical in shape and the projection of the focal spot along the central axis a circular disk. To determine the resolution capability of such tubes, a theoretical expression has been derived for the MTF of a circular focal spot. It has been shown that the MTF is given by the equation MTF = 2J 1 [πf 0 δ(m − 1) / m] / [πf 0 δ(m − 1) / m] where m is the magnification factor, f 0 the spatial frequency in the object plane, and δ the diameter of the focal spot. J 1 stands for the Bessel function of the first order. The above expression has been tested experimentally and used to compare circular focal spots with square focal spots.

Contrast perception in imaging systems

Abstract In all imaging systems, four basic factors govern the perception of detail: (1) the magnitude of the input contrast in the object being imaged, (2) the fidelity with which this contrast is displayed the image, (3) the statistical uncertainty associated with the contrast thus displayed, and (4) the observers visual performance. If we consider a sinusoidal test distribution of input contrast C in and spatial frequency f, the output contrast C out is equal to C in M ( f ) where M ( f ) is the system MTF. This paper shows that the standard deviation σC out of this output contrast is given by the equation where I 0 is the average intensity of the test distribution, e is the fraction of the emitted photons that is absorbed in the imaging device, A is the area under the point spread function of the system (related inversely to the square of its spatial bandwidth) and τ is the time of exposure. The interpretation of the above equation has been confirmed experimentally. The quantity C out / σC out , as a measure of the signal-to-noise ratio of the system, takes into account all the basic nonsubjective parameters that govern contrast perception. Hence the term Contrast Perceptibility Function is proposed for its variation with spatial frequency.