Biomedical subjects
Publications and source records attributed to L T Niklason.
A phototimer technique chart for mammography is presented along with the methodology used to design it. The chart is based on accurate measurement of breast thickness and helps overcome the inability of the phototimer to track as breast thickness varies. In clinical practice, the chart results in the consistent attainment of optimally exposed films and decreased number of retakes.
Conventional, unsharp masking, and slit-mask (combining slit radiography and unsharp masking) techniques were compared in a clinical nodule detection study in cancer patients who were at risk for metastatic lung disease. Unsharp masking improved detection rates for nodules located in poorly penetrated areas of the chest (25% vs. 52%), with no difference in detection rates for other areas of the image and no change in false-positive detection rates. Results with the slit-mask technique did not differ significantly from those obtained with conventional techniques. In general, the differences in nodule detection rates noted between different techniques in the clinical study were less than had been observed in earlier phantom-nodule detection studies. Possible reasons for these findings are discussed.
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A simple film subtraction technique has been devised that isolates calcium image contrast and mutes tissue image contrast. Two exposures are required. The first is made on XL film using a 65 kVp beam filtered with 2 mm aluminum. The second is made on OG (high contrast) film using a 130 kVp beam filtered with 2 mm copper and 2 mm aluminum. The effective energies of these two beams are approximately 45 keV and 83 keV, respectively. A subtraction image is made, using the low energy image for the mask. With this technique it is possible to detect concentrations of 125 mg/cm3 of diffuse calcification in a chest nodule 1 cm in diameter. If the presence of diffuse calcification is found to be an indicator of benignancy in solitary pulmonary nodules, this technique may have diagnostic value for the detection of such calcification. Computerized tomographic findings are discussed and related to this technique.
A pair of rapidly rotating disks was assembled with multiple radial slits for obtaining chest radiographs in 1/60-1/20 sec., using automatic exposure timing, without visible images of individual slits. Substantial reduction of scattered radiation levels is achieved (one half to one fourth), especially in the retrocardiac and central mediastinal areas of the chest film. Full realization of the potential contrast improvement resulting from this degree of scatter reduction is impaired by the limited latitude of x-ray films used for radiography. These problems and potential solutions are discussed.
A photographic unsharp masking technique for improving the latitude of chest radiographs without sacrificing image contrast or detail is described. An unsharp mask film, prepared from a scout film of the patient's chest, is placed between the film and the front (entrance) screen in the cassette. A second radiograph then is recorded using technique factors that provide a well-penetrated view of the central mediastinum, etc. The unsharp mask absorbs light from the screen in those areas of the chest that normally are well penetrated, preventing overexposure of these areas and resulting in an improved balance of densities across the chest image. Improvement of contrast by a factor of 2 is demonstrated for mediastinal and retrocardiac structures with no loss of contrast in the central lung fields. Nodule detection studies with a chest phantom and simulated nodules suggest that a single unsharp masked film provides higher nodule detection rates than a pair of films consisting of a normally penetrated and an overpenetrated view, possibly because of facilitation of visual search patterns and contrast/brightness adaptation mechanisms of the visual system. Initial clinical studies indicate that unsharp masking may provide additional useful clinical information.
An analysis of the potential value of antiscatter grids for digital subtraction angiography (DSA) is presented. Using the Rose criterion for signal detection, and assuming quantum limited statistical noise, it is shown that grid selectivity and primary transmission are the two most important parameters of grid performance for DSA. In comparison with conventional grids commonly in current use in radiology departments, a grid having a higher level of primary transmission, (approximately 90%) and a moderate level of selectivity (approximately 6) could provide comparable imaging signal-to-noise ratios, but with 20% to 45% smaller patient dose. Implications of these results are discussed.
The performance characteristics of prototype grids constructed with 0.1-mm-thick Ta strips were investigated. Significant improvement in scatter cleanup was noted in comparison to commercially manufactured grids, which are constructed with 0.04-0.05-mm-thick Pb strips. Highly efficient scatter rejection, approaching that of slit radiography techniques, may be possible using crossed grids constructed with 0.1-mm Ta strips. By constructing a grid from a crossed pair of linear grids and moving these grids at right angles to each other, grid lines can be eliminated from crossed grid images.
Luminance scatter fractions were measured for patients and phantoms in various regions of the chest film. Scatter fractions were found to be highly variable, being quite high in the regions of the chest with a large equivalent tissue thickness (e.g. mediastinum, chest wall, and subdiaphragmatic areas). For example, approximately 90% of the radiation reaching the mediastinum region of the chest film was found to be scattered radiation when a grid was not used. Even with a 12:1 grid, less than 50% of the available primary beam contrast was being displayed in the mediastinum. In regions of the chest film with high scatter fractions, film latitude was also a problem, further reducing image contrast in these regions. Image contrast and scatter fractions in chest radiography were investigated for different scatter rejection techniques including air gaps, antiscatter grids, and slit radiography.
An automated method is developed for the detection and staging of skeletal changes due to hyperparathyroidism on digitized hand radiographs. Subperiosteal bony resorption, particularly along the radial margins of the middle and proximal phalanges, is among the earliest manifestations of secondary hyperparathyroidism. In order to quantify the severity of bone resorption in these regions, the computer method analyzes the roughness of the phalangeal margins, as projected on the radiograph. The regions of interest, which contain the phalanges, are obtained from the digitized hand radiographs by an image preprocessor. The radial margin of each phalanx is detected by a model-guided boundary-tracking scheme. The roughness of these boundaries is then quantified by the mean-square variation and the first moment of the power spectrum. A receiver operating characteristic (ROC) study comparing the computer detection of hyperparathyroidism with the diagnosis by three experienced skeletal radiologists was performed by evaluating 84 hand images from 22 patients. Our present computer method can achieve a true-positive rate of 94% and a true-negative rate of 92%. Such a computer-aided diagnosis system may assist radiologists in their assessment of primary and secondary hyperparathyroidism, since it is both accurate and not subject to either intra- or interobserver variations.
Digital mammography systems allow manipulation of fine differences in image contrast by means of image processing algorithms. Different display algorithms have advantages and disadvantages for the specific tasks required in breast imaging-diagnosis and screening. Manual intensity windowing can produce digital mammograms very similar to standard screen-film mammograms but is limited by its operator dependence. Histogram-based intensity windowing improves the conspicuity of the lesion edge, but there is loss of detail outside the dense parts of the image. Mixture-model intensity windowing enhances the visibility of lesion borders against the fatty background, but the mixed parenchymal densities abutting the lesion may be lost. Contrast-limited adaptive histogram equalization can also provide subtle edge information but might degrade performance in the screening setting by enhancing the visibility of nuisance information. Unsharp masking enhances the sharpness of the borders of mass lesions, but this algorithm may make even an indistinct mass appear more circumscribed. Peripheral equalization displays lesion details well and preserves the peripheral information in the surrounding breast, but there may be flattening of image contrast in the nonperipheral portions of the image. Trex processing allows visualization of both lesion detail and breast edge information but reduces image contrast.
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