Determination of radiographic screen-film system characteristic curve and its gradient by use of a curve-smoothing technique

Vessel size measurements in angiograms: a comparison of techniques

As interventional procedures become more complicated, the need for accurate quantitative vascular information increases. In response to this need, many commercial vendors provide techniques for measurement of vessel sizes, usually based on derivative techniques. In this study, we investigate the accuracy of several techniques used in the measurement of vessel size. Simulated images of vessels having circular cross sections were generated and convolved with various focal spot distributions taking into account the magnification. These vessel images were then convolved with Gaussian image detector line spread functions (LSFs). Additionally, images of a phantom containing vessels with a range of diameters were acquired for the 4.5", 6", 9", and 12" modes of an image intensifier-TV (II-TV) system. Vessel sizes in the images were determined using a first-derivative technique, a second-derivative technique, a linear combination of these two measured sizes, a thresholding technique, a densitometric technique, and a model-based technique. For the same focal spot size, the shape of the focal spot distribution does not affect measured vessel sizes except at large magnifications. For vessels with diameters larger than the full-width-at-half-maximum (FWHM) of the LSF, accurate vessel sizes (errors approximately 0.1 mm) could be obtained by using an average of sizes determined by the first and second derivatives. For vessels with diameters smaller than the FWHM of the LSF, the densitometric and model-based techniques can provide accurate vessel sizes when these techniques are properly calibrated.

Computer-aided bootstrap generation of characteristic curves for radiographic imaging systems

Determination of the characteristic curve is essential for quantitative evaluation of digital as well as screen-film radiographic imaging systems. When it is not practical to generate the entire curve through variation of a single exposure parameter, bootstrap methods can be used. For the bootstrap method used here, curve segments are generated by varying one exposure parameter and multiple segments are produced at different exposure levels by varying a second parameter. The segments then are joined to form a single composite characteristic curve. If the second parameter is one for which the sensitivity of the receptor is not constant (such as x-ray tube potential or beam filtration), the shift of each segment along the log relative-exposure axis needed to join the overlapping curve sections is not known and some form of segment matching must be employed. A spline interpolation method and a polynomial fit integral method were developed for automatic segment matching and compared. For both methods, the shifts between successive segment pairs which result in optimal overlap are determined and the cumulative shifts to obtain the complete composite curve are calculated for all segments. The two methods are evaluated for several image receptors and with a known curve generated from an analytic function. Both methods closely agreed in the shifts determined for the image receptors and provided a good visual matching of the curve segments. The spline interpolation method more accurately determined the appropriate shifts for the mathematically generated curve segments. The bootstrap methods can provide complete, accurate characteristic curves, and the segment joining program makes the process fast and precise.

Measurement of x-ray image intensifier sharpness in the x-ray department

A method is presented for determining the modulation transfer function (MTF) of an image intensifier as it is found in the x-ray department. The image of an edge input into the image intensifier tube is photographed at the television camera port with a high quality camera and the photographic image scanned with a microdensitometer. Densitometric conversion enables the determination of the edge spread function and further calculation produces the line spread function and the MTF. A correction is made for the camera/lens/film/developer response function. Results from the use of the technique on over 25 x-ray image intensifiers in New Zealand hospital x-ray departments are presented and discussed. Most of the image intensifiers had been in use for more than 7 years. The low-frequency drop values were similar to reported values with an average value of 22%, but modulation values at 1 cycle/mm (the average value was 29%) were generally lower than values previously reported, indicating that the sharpness performance in the clinical environment is inferior to claims for new or state-of-the-art intensifiers.

Algorithm for the Determination of Projected Urethral Lumen Cross-Sections and Shape Functions

A method used to obtain computer graphic representations of projected urethral lumen cross-sections is described. The cross-sectional area of the lumen is measured accurately at all stages of voiding. Two methods of background subtraction are considered, direct subtraction from nonvoiding x-rays and subtraction from a least squares regression line. The projected cross-sections are described by simple shape functions. These shape descriptors increase the technique's potential application by providing a quantitative measure of the projected cross-sections.

A comparison of physical image quality indices and observer performance in the radiographic detection of nylon beads

Although various models have been proposed in an attempt to predict the usefulness of a radiographic image in terms of its physical characteristics, no previous work has shown whether a single physical image quality index, such as a signal-to-noise ratio, can reliably predict the performance of a human observer over a broad range of image characteristics. We studied the relationship between physical and visual image quality for the task of detecting nylon beads in radiographs. Thirty-seven imaging cases with different combinations of physical image characteristics were considered; these included variations in object size and magnification, X-ray beam quality, screen-film system, screen-film contact, film density and illumination, and viewing distance. For each imaging case, visual image quality was quantified in terms of observer performance in a 2AFC visual detection experiment. Physical image quality indices were calculated according to eight different models of the detection process; these indices combined data regarding object size and attenuation, screen-film system MTF, film gradient, noise Wiener spectrum, and visual system response. The results of this work indicate that, for the conditions studied, human observer detection performance most closely resembles that of a sub-optimal statistical decision process.

The validity of Monte Carlo simulation in studies of scattered radiation in diagnostic radiology

A computer program using Monte Carlo methods has been developed for the simulation of photon scattering in tissue-equivalent phantoms for incident x-rays in the diagnostic energy range. The study verified that the sampling schemes used in the program can produce random samples according to the theoretical probability distribution functions which describe the photon-scattering process. The Monte Carlo program was applied to determinations of the scatter fractions and edge responses for various phantom sizes, X-ray energies, and recording systems. These quantities were also measured experimentally under comparable imaging conditions. Excellent agreement was obtained between the predicted and experimental results. This investigation established the validity of our Monte Carlo calculations for studies of the physical characteristics of scattered radiation in diagnostic radiology.