Kurt Rossmann

Point Spread-Function, Line Spread-Function, and Modulation Transfer Function

IN linear, invariant communication systems has been of increasing usefulness in the study of radiographic imaging systems (1–4). Indeed, its application is not confined to the imaging systems themselves but can be extended to the analysis of the entire radiological process involving exposing, imaging, and visual detection operations (5). This analysis will eventually result in quantitative descriptions of the inherent limitations of present radiological processes and, hopefully, in the development of improved processes yielding increased diagnostic certainty. At present the number of investigators working in this field is very small-too small, in fact, to insure a satisfactory rate of progress toward the important goal. This is partly due to the fact that the physics and mathematics involved are highly specialized and cannot readily be assimilated from the existing literature. Any author in this field is faced with the dilemma of writing lucidly for readers having diverse backgrounds of scientific trainin...

Evaluation of Receiver Operating Characteristic Curve Data in Terms of Information Theory, with Applications in Radiography

A method is presented for quantitative evaluation of observer detection performance data based on elementary principles of information theory. The resulting index of detectability, average information content per observation, is compared with previously proposed measures of observer performance both on theoretical grounds and for the practical problem of evaluating radiographic screen-film systems.

Radiographic applications of receiver operating characteristic (ROC) curves

•The basic concepts underlying the theory and experimental determination of receiver operating characteristic (ROC) curves are discussed. Such curves were used to describe the detectability of the image of 2 min Lucite beads (similar to certain small gallstones) in a noisy background of radiographic mottle. Results are shown for four typical radiographic screen-film combinations with differing optical, sensitometric, and noise properties. In some cases there was qualitative correlation between the detectability described by the ROC curves and the mathematical model describing radiographic mottle. INDEXTERMS: Radiographs. Radiology and Radiologists

Radiographic applications of signal detection theory.

Abstract Receiver operating characteristic (ROC) curves describe observer performance in the detection of small noiselimited signals such as the image of a 2 mm Lucite bead against a background of radiographic mottle. A combination of a fast-speed screen and normal-speed :film yields greater signal detectability than a combination of a medium-speed screen and fast-speed film.

Comparison of Image Quality Obtained with Optical and Radiographic Magnification Techniques in Fine-Detail Skeletal Radiography: Effect of Object Thinkness1

High-resolution radiography may be done using either optical or radiographic magnification. In the former technique, Industrial Type M film was used without screens and the image was viewed with 4–10× optical magnification. In the latter technique, RP film was used with Detail screens and 4× geometric magnification together with a microfocus x-ray tube having a nominal focal spot size of 50 µm. The imaging properties of both techniques were evaluated by means of H & D curves, modulation transfer functions, and Wiener spectra. It was found that for thin objects such as the hand, optical magnification provides better bone images than radiographic magnification; whereas for thicker parts such as the knee, radiographic magnification is superior.

Truncation errors in calculating the MTF of radiographic screen-film systems from the line spread function

The computed modulation transfer function (MTF) of a radiographic screen-film system may exhibit erroneous oscillations resulting from the necessary truncation of the measured line spread function (LSF). The slit method was extended to permit evaluation of the LSF to approximately 0.001 of its maximum value. This reduces the MTF oscillations to a negligible level. The maximum MTF deviation was determined as a function of the truncation. Results show that for LSFs measured in the usual manner (0.01 truncation) the maximum MTF deviation is from 0.04 to 0.07, depending on the screen-film system. A theoretical analysis to estimate the error due to truncation is described, and a correction procedure is developed.

New device for accurate measurement of the x-ray intensity distribution of x-ray tube focal spots.

A new device has been developed with which the focal spot distribution can be measured accurately. The alignment and localization of the focal spot relative to the device are accomplished by adjustment of three micrometer screws in three orthogonal directions and by comparison of red reference light spots with green fluorescent pinhole images at five locations. The standard deviations for evaluating the reproducibility of the adjustments in the horizontal and vertical directions were 0.2 and 0.5 mm, respectively. Measurements were made of the pinhole images as well as of the line-spread functions (LSFs) and modulation transfer functions (MTFs) for an x-ray tube with focal spots of 1-mm and 50-mum nominal size. The standard deviations for the LSF and MTF of the 1-mm focal spot were 0.017 and 0.010, respectively.

Application of Longitudinal Magnification Effect to Magnification Stereoscopic Angiography: A New Method of Cerebral Angiography

A new method of stereoscopic cerebral angiography has been developed which employs 2X radiographic magnification. In order to obtain the same depth perception in the object as with conventional contact stereoscopic angiography, one can made the x-ray exposures at two focal spot positions which are separated by only 1 inch (2.5 cm), whereas the contact technique requires a separation of 4 inches (10 cm). The smaller distance is possible because, with 2X magnification, the transverse detail in the object is magnified by a factor of two, but the longitudinal detail, which is related to the stereo effect, is magnified by a factor of four, due to the longitudinal magnification effect. The small focal spot separation results in advantages such as improved stereoscopic image detail, better image quality, and low radiation exposure to the patient.

Evaluation Of Focal Spot Distribution By Rms Value And Its Effect On Blood Vessel Imaging In Angiography

It is well known that the x-ray intensity distribution of the focal spot can significantly affect radio-logic images, for example, blood vessel images in angiography (Refs.1, 2). For an accurate evaluation of the effect of this distribution on vessel images, it is essential to use the line spread function (LSF), the point spread function (PSF), or the optical transfer function (OTF) of the focal spot. However, for many years it has been common practice to describe the focal spot by its size, which is looked upon as a single figure of merit for the x-ray intensity distribution of the focal spot. Two methods are relatively well known; one is to measure the pinhole image size by a subjective judgment (Ref.3), and the other is to use the first-zero spatial frequency of the OTF for the determination of an equivalent uniform LSF (Ref.4).

The effect of radiographic magnification on blood vessel imaging with various screen-film systems.

From the computer simulation study of blood vessel imaging in cerebral angiography it is found that, with proper choice of the focal‐spot size and magnification ratio, radiographic magnification in conjunction with fast screens can give blood vessel images “equivalent” to or better than those from slower screens without magnification. However, this result is highly dependent on blood vessel diameter. If a sufficiently small focal spot is not available so that the proper choice of focal‐spot size and magnification cannot be made, then the vessel image from slower screens without magnification can be better than that from fast screens with magnification.