Ben A. Arnold

Digital Radiography: An Overview

A new class of diagnostic imaging systems, loosely termed Digital Radiography, has emerged during the past year from the laboratory into the forefront of radiological practice. These systems have in common the acquisition of a two dimensional projection image in digital format but include a variety of detectors, techniques and applications. Digital radiography apparatus are conveniently divided into point scanned, line scanned and area systems. Each system is briefly described and its performance capabilities are reviewed. Digital video radiography is given added emphasis due to its rapid development for intravenous angiography. The key requirements and present limitations of major components of the video system are noted. Gross specifications of the next generation of computers for digital radiography are postulated.

The effect of phosphor K x‐rays on the MTF of rare‐earth screens

The LSFs and MTFs of two high resolution rare-earth screen-film combinations were measured at two beam qualities. The two beam qualities were chosen to provide x-ray spectral distributions either above or below the K-edge of the screen phosphor. The LSFs were found to be photon energy dependent. This energy dependence is attributed to the generation and reabsorption of phosphor K x-rays resulting in a broadening of the LSF.

Measurements of Reciprocity Law Failure in Green-Sensitive X-Ray Films

Reciprocity law failure was measured for four brands of medical x-ray films exposed with intensifying screens. Three of the films are green light-sensitized for use in combination with green light-emitting rare-earth screens. These films showed larger reciprocity failure effects than one conventional blue-sensitive film, Dupont Cronex-2. Development conditions had a small effect on reciprocity failure. As part of the investigation, a detector was constructed with a response that accurately monitors the light emission from the double screen-cassette combination over a wide range of x-ray photon energies.

Magnification Mammography: A Low-Dose Technique

Image quality and radiation exposures of a mammographic technique using direct radiographic magnification at 2 X with a microfocal spot x-ray tube and a fast, double screen-film system were compared to those of conventional contact mammography with a rare-earth screen and molybdenum target tube. The results indicate that the magnification technique yields improved detection of microcalcifications and comparable visualization of soft-tissue details, with a large reduction in radiation exposure. This technique has demonstrated the feasibility of carrying out high-quality mammography with an entrance dose of 1.35 X 10(-3) Gy (135 mrad) for the average breast.

The LSF and MTF of rare-earth oxysulfide intensifying screens.

The line spread function (LSF) and modulation transfer function (MTF) of 9 rare-earth screen/film systems were measured and compared with those of two fast calcium tungstate systems, using double-emulsion films sandwiched between two screens and mounted in regular cassettes. The LSFs were found to fit exponential functions. These results indicate that the increased sensitivity of rare-earth phosphors over calcium tungstate can be used to construct screens with a higher MTF or increased speed. The fast rare-earth systems allow the use of smaller focal spots for increased resolution while reducing the radiation dose to the patient.

Peak SNR in automated coronary calcium scoring: Selecting CT scan parameters and statistically defined scoring thresholdsa)

PURPOSE Development and verification of peak signal-to-noise ratio (SNR(P)) equations for determining optimum CT scanning and scoring parameters for a new automated coronary calcium scoring program (N-vivo). Experimental evaluation of the new program for scoring small calcium hydroxyapatite (CaHA) microspheres with small voxel CT images. METHODS Theoretical SNR(P) equations were developed using measures of noise, resolution, contrast, scatter, and x-ray photon energy. A coronary calcium scoring test phantom containing very small CaHA microspheres was scanned simultaneously with a calibration phantom at three kVps, three voxel sizes, and three phantom sizes. Agatston and calibrated mass scores, noise standard deviations, peak noise, and peak signal voxel intensities were measured by the N-vivo method for individual microspheres and in patient CT scans. RESULTS The SNR(P) equation was predictive of the optimum voxel size, kVp, and phantom size, and allowed automated computation of mass scoring thresholds specific to each patient and CT scan. The smallest microcalcification scored in the full sized phantom with the N-vivo method by calibrated mass score (volume 0.075 mm3, mass 0.17 mg) was approximately four times smaller than that scored with the Agatston method (0.27 mm3, mass 0.63 mg). CONCLUSIONS The SNR(P) equation can be used to model and optimize calcium scoring and CT scan parameters. The common assumptions that noise is too high in thin slice CT or requires high radiation dose for CAC scoring are shown to be misleading. The N-vivo method showed higher detection sensitivity for small microspheres and more consistent mass scores at different beam energies than the conventional Agatston method. Advanced calcium scoring methods with higher sensitivity may allow improved monitoring of plaque changes and provide earlier detection of atherosclerosis.

Evaluation of Mammographic Screen-Film Systems

Four screen-film systems were evaluated for their imaging properties in mammography, Modulation-transfer functions were measured at 40 kVp. Absolute screen-film sensitivities in mR and entrance exposures were measured with tungsten and molybdenum target tubes. Five radiologists viewed radiographs of a phantom containing microgranules of SiC ranging in diameter from 590 to 120 micrometer. The Rarex-B screen--composed of yttrium oxysulfide--performed best, allowing phantom radiographs at 185 mR with image quality sufficient to demonstrate microgranules greater than 330 micrometer in dimension.

A Scatter Correction Algorithm For Digitally Acquired Radiographs (SCADAR)

An algorithm for reducing the scatter component in digitally acquired radiographic images is introduced. Using two images acquired in different geometries, the scatter component is calculated on a pixel by pixel basis and used to correct an original image. This process is expedited by using an array processor for computation. The theoretical basis of the algorithm is developed, and experimental results on simple Lucite phantom images are reported.

Reciprocity Law Failure In Green Sensitive X-Ray Films

Reciprocity law failure was measured for four brands of medical X-ray films exposed with intensifying screens. Three of the films are green light sensitized for use in combination with green light emitting rare-earth screens. These films showed larger reciprocity failure effects than one conventional blue sensitive film, Dupont Cronex-2. The development conditions had a small effect on reciprocity failure. The effect of reciprocity failure on the measurements of physical image quality parameters is discussed.