Enhancement and compression of digital chest radiographs.

The application of digital technologies to chest radiography holds the promise of routine application of image processing techniques to effect image enhancement. Because of their inherent spatial resolution, however, digital chest images impose severe constraints on data storage devices. Compression of these images will relax such constraints and facilitate image transmission on a digital network. We evaluated an algorithm for enhancing digital chest images that has allowed significant data compression while improving the diagnostic quality of the image. This algorithm is based on the photographic technique of unsharp masking. Image quality was measured with respect to the task of tumor detection and compression ratios as high as 2:1 were achieved. This compression can be supplemented by irreversible methods.

Evaluation of dry silver hard copies in digital radiography.

Diagnostic images produced by digital technologies are viewed on video terminals. Hard copies are subsequently made on film for use by radiologists and referring physicians. An alternative is to make prints on paper using "dry silver" technology. These prints are made by a camera that uses silver-coated paper. The paper is developed as it exits the camera and passes over a heating element. Thus dry silver paper is processed "dry," whereas standard film or print paper is processed in a chemical bath. The prints are viewed by reflected rather than transmitted light. We compared the diagnostic quality of the dry silver prints with that of conventional multiformat, transmitted-light images. Radiologists were asked to make diagnoses from both types of images. The results showed that the diagnoses made from the prints were identical to those made from the films. Furthermore, when the images were labeled with the diagnosis, there was no difference between the two formats in the information conveyed to the referring physician. We conclude that dry silver, reflection prints can provide an inexpensive, convenient method of making visual information available on patients' charts.

Evaluation of digital tomographic filters.

Recently, a technique has been proposed to filter conventional radiographs in order to obtain depth-dependent information by simulating standard tomography. This technique has been referred to as tomographic filtering or tomographic filtration process (TFP). It takes advantage of the finite size of the X-ray source, so that after processing the image of a particular layer is improved, while the others are degraded. This paper briefly reviews this technique and then concentrates on its analytical evaluation. A comparative assessment of tomographic filtering is produced taking as benchmarks two well established radiologic procedures: standard tomography and conventional radiology. The comparison is made on the basis of the following evaluation criteria: the exposure angle, the thickness of the cut, the rate of change of the transfer function, the signal-to-noise ratio, and the radiation dose. Practical evaluations are also shown.

Authors' reply.

This reply clarifies that it is possible to change the plane of cut by filtering a tomogram, if the presence of the out-of-focus images is tolerated, because the (projected) images of all the layers are superimposed on the film and any filtering will process all these images simultaneously.

Abandonment of endoprosthetic drainage technique in malignant biliary obstruction.

This report discusses a 3-year experience with the techniques of internal/external catheter and endoprosthetic stent drainage in 175 patients with obstructive jaundice. In 166 patients, the obstruction was bypassed satisfactorily. The complications encountered with these techniques are compared, and the radiation doses involved are emphasized. It is not necessary to put patients through the extra time and pain related to the placement of the endoprosthesis, because internal/external catheter placement produces the same desired result of biliary decompression without the higher radiation doses to the radiologist.

The use of high-speed film/screen combinations to improve diagnostic image quality.

Increased film speed, when used with the same intensifying screens, results in an increase in quantum "noise." Nevertheless, this increased noise is not the primary degrading factor of diagnostic quality during arteriography and cross-table myelography. The authors show that the use of high-speed film/screen combinations decreases the threshold diameter for blood vessel detectability on arteriograms. Also, these combinations can be used to increase the contrast between opacified spinal fluid and surrounding bone during cross-table myelography.

Digital tomographic filtering of radiographs.

Conventional radiographs do not provide information about the depths of details and structures because they are two-dimensional projections of three-dimensional bodies. Taking advantage of the finite size of the X-ray source and the divergent nature of the X-ray beam, a radiograph can be processed by two-dimensional digital filtering techniques, so that the image of a particular layer is improved, while the others are degraded. This technique is referred to as a tomographic filtration process (TFP). This paper develops the mathematical and physical foundations of the method. Based on a model of the radiologic process, which is described in the paper, the equations of image formation in standard tomography, conventional radiography, and tomographic filtering are derived and compared.

Design and implementation of digital tomographic filters.

A technique is proposed which allows the selective filtering of conventional radiographs in order to obtain depth-dependent information by utilizing the depth-dependent information contained therein. This technique, referred to as tomographic filtering or tomographic filtration process (TFP), takes advantage of the finite size of the X-ray source, so that after processing, the image of a particular layer is improved while the others are not. This paper starts with a brief review of technique and then concentrates on the design and implementation of digital tomographic filters. Examples are shown, including images of simulated radiographs processed with such filters. Evaluations of the performance of these filters show that the image quality cannot be as good as that of standard tomography or multiprojection reconstruction techniques; nevertheless they represent an improvement over conventional radiology, and highlight additional depth-dependent information contained in radiographs. This paper concludes with suggestions for further research in this area.

Radiation exposure reduction using carbon fiber.

The ability of carbon fiber cassette fronts to reduce x-ray exposure was evaluated by comparing a carbon fiber cassette with a Kodak X-Omatic cassette. Rare-earth screens were used. Although the transmission of the carbon fiber was considerably higher than that of the X-Omatic cassette, the radiation exposure reduction was significant only in the lower kVp ranges. Although some decrease in the primary-to-scatter ratio was measured, the overall image contrast with the carbon fiber cassette was only minimally reduced in the kVp range investigated.

Patient dose and image quality in computed tomography.

The relation between radiation dose in neurologic computed tomography scanning and image quality is described. Three different sets of images were obtained at varying exposures and were quantitatively evaluated by a panel of neuroradiologists. Anatomic information in the image varied with exposure. The relation of image quality and dose conforms to previous theoretical results and suggests that an optimal exposure level can be established.

A charge-compensated capacitor discharge generator.

A prototype capacitor discharge generator capable of fast termination and charge-compensated phototiming was used clinically for ten weeks. It was found to exceed all demands of ultra-fast rare-earth imaging systems.

Pinhole cameras: artifacts modifications, and recording of pinhole images of screen-film systems.

This paper will discuss three points concerning the x-ray pinhole camera: a) pinhole artifacts, which may erroneously be interpreted as off focus radiation, will be described along with a simple method of eliminating these artifacts, b) modifications to a commercially available pinhole camera which simplifies its use and allows for the proper leveling of the pinhole mounting plate will be presented, and c) the use of screen-film cassettes to reduce the required technique for recording the pinhole image will be discussed along with data indicating the expected error with this technique relative to dental film recording.

Comparison of dosimetry and image quality in computed and conventional tomography.

Comparison was made of tomograms of a phantom head and a normal adult cadaver head on GE CT/T, Delta 50, EMI 5005, and CGR Stratomatic units. Image quality was ranked as follows: First, GE CT/T 8800 system; second, EMI; third, GE CT/T 7800 and Delta. The GE CT/T 7800 imaged 3.7 line pairs at high contrast with an exposure to the head phantom of 2.5 R (6.5 X 10(-4) C kg-1). The EMI exposure level was 5 R (13 X 10(-4) C kg-1) in the fast mode and 12 R (31 X 10(-4) C kg-1) in the slow mode, while the Delta 50 exposure level was fixed at 2 R (5.2 X 10(-4) C kg-1). The GE CT/T 8800 could image 6.1 line pairs/cm in a high-contrast phantom with exposures as low as 100 mR (25.8 muC/kg) but a typical operating exposure would be about 1--2 R (2.58 -5.2 X 10 (-4) C kg-1). Dosimetry ranged from 1.1 to 5.5 R/scan section (2.8--14.2 x 10(-4) C kg-1) in the CT units but never increased by a factor of more than 2, irrespective of the number of sections scanned. In conventional tomography, however, exposure increased almost arithmetically with the number of contiguous sections scanned.

Measurements of the partial volume phenomenon.

The partial volume phenomenon can produce subtle changes in the reconstructed value of the linear attenuation coefficient which may in turn make the detection of an object more difficult. A model for studying the effects of the partial volume phenomenon is presented along with measurements made on the General Electric CT/T whole body scanner. The results of these measurements indicate that small differences in the linear attenuation coefficient may go unnoticed when the video display format is used exclusively to evaluate the CT image. It is suggested that this display format incorporate profiles of the CT numbers to assist the viewer in the evaluation of the CT scans.

The diagnostic quality of optically processed radiographs.

The diagnostic quality of optically processed radiographs was compared with that of unprocessed radiographs, using the number of lesions detected as a criterion. It was found that merely removing phase-shifted information with either a binary or focal spot filter alone did not improve image quality, however, use of the focal spot filter plus removal of all phase-shifted frequencies did lead to the detection of an increased number of lesions. The authors suggest that currently available optical filtration techniques should be evaluated with caution, as they may actually degrade diagnostic quality by increasing the visibility of phase-shifted information.

Phase effects in diagnostic radiological images.

The effect of phase on the quality of diagnostic radiological images is investigated on a theoretical basis and with the use of the computer simulations. Phase effects are divided into two categories: linear phase shift and asymmetric phase shift. Using a model of the x-ray focal spot distribution composed of two Gaussian functions, it is shown that the phase transfer function (PTF) is generally a continuous nonlinear function of spatial frequency. The linear phase shift serves only to shift the location of the image. The asymmetric phase shift in the model studied tends to accompany an improvement in the modulation transfer function (MTF) in the midfrequency region while introducing "wows" into the image. Contrast reversals are shown to be inherent in line spread functions (LSFs) which do not necessarily include linear or asymmetric phase effects, i.e., they are the result of another phenomenon.

The production of apparent image bifurcation.

It is well known that x-ray focal spots with split-intensity distributions can produce radiographs exhibiting image bifurcations. The authors observed such bifurcations in optical analogue images which were produced with both split- and uniform-intesity focal spots. The bifurcations observed with the split focal spot were real, while those observed with the uniform focal spot were not observed in photometer scans of the object. The authors explain these bifurcations by a consideration of the response function of the eye.