Roland Wong

Accurate characterization of image intensifier distortion

Image intensifier distortion due to photocathode curvature and electron optics is shown to be approximated by a simple two parameter odd-power polynomial. The accuracy of this fit was found to be far better than that of two other one parameter characterizations of distortion when applied to experimental data from four different model image intensifiers ranging in diameter from 9 to 14 in. The standard errors of the two parameters fits were less than 0.1 mm or 0.03% of the field of the IIs and were within the estimated measurement error.

Assessment of patient exposure for barium enema examinations.

Methods are described for the assessment of patient exposure during clinical fluoroscopic procedures. Values of the roentgen-area-product (RAP) and their distribution throughout the examination are presented for both single-contrast and double-contrast barium enema studies. The double-contrast procedure was measured to give 50% more radiation to the patient than the single-contrast procedure when the same size optical aperture is used between the intensifier and TV pick-up tube. However, it was possible to decrease the fluoroscopic RAP value by over a factor of two for the double-contrast procedure without an adverse clinical effect by increasing the area of the aperture diaphragm.

Reduction of fluoroscopic exposure for the air-contrast barium enema

In a fluoroscopic imaging system, image quality and patient dose are both affected by the optical system linking the image intensifier with the video camera. The effect on patient exposure of increasing the optical iris aperture size over that required for other procedures performed on the same imaging system was investigated for the air-contrast barium enema examination. Using a large-area transmission ionisation chamber to monitor the Roentgen-area-product of entrance exposure, a decrease in fluoroscopic radiation of greater than 50% was clinically documented for a fluoroscopic system utilising kVp and mA variable automatic brightness control. For this iris change, the video image was of acceptable quality for positioning and monitoring the patient, and no deleterious effect was detected in the conduct of the air-contrast exam. The availability of a variable-sized operator-selectable iris diaphragm would permit this dose-reduction approach to be extended to other fluoroscopic procedures.

Improving fluoroscopic image quality with continuously variable zoom magnification

Coning down is commonly used during fluoroscopy to increase image contrast by reducing scatter. However, the resulting image fills only part of a video display whose resolution is limited by line rate and bandwidth. Optical or electron-optical zooming can be used to magnify the collimated image so that it fills a larger fraction of the viewable area of the video frame to make more effective use of the available video-display capacity. Modulation-transfer functions (MTFs) were measured for various zoom factors achieved using a zoom lens and the image-intensifier (II) electronic magnification mode. Significant and continuing improvement in total system MTF was observed up to zoom magnifications of greater than 3.3. For larger zoom factors, the resolution limit becomes dominated by the intrinsic resolving power of the II and by geometric unsharpness rather than by the line rate of the video system. When the MTF at infinite zoom factor, obtained by extrapolation, was divided into the measured MTFs, the resultant MTFzs were shown to scale predictably with zoom factor. Only a slight improvement in MTF was obtained using the IIs electronic magnification mode compared to the same magnification using a zoom lens. It is concluded that, if improved image quality is the motivation for the use of coning down in fluoroscopy, then zooming to use fully the available video frame is warranted.

Design of rotating aperture cones for radiographic scatter reduction

A new multiple-scanning-slit or scanning-grid scatter-reduction geometry consisting of coaxial rotating aperture (RA) cones is described and compared with other RA assemblies such as the rotating aperture wheel (RAW) device. A unique feature of the new design is that the geometric and rotational axes of the conical RA surfaces coincide and are collinear with the x-ray focal spot. This arrangement of axes should provide the potential for greatly improved mechanical rigidity, higher rotational velocities, and the capability for static slit-pattern alignment. The common rotational axis of the cone assembly is angled obliquely to the central x-ray beam such that the intersections of the irradiated portion of the RA surfaces with the plane formed by the x-ray central beam and cone axes are straight lines parallel to the film plane. This geometry is compatible with the standard source-patient-image receptor radiographic relationship and allows for variable source-to-image-receptor distance.

Effect of a Multiple-Scanning Beam Device and Trough Filter on Scatter in Chest Radiography

The use of a trough compensation filter to reduce the large difference in x-ray transmission between the lung parenchyma and mediastinum results in beam hardening and increased scattered radiation which causes a reduction of subject contrast in the lung fields. When the Rotating Aperture Wheel (RAW) multiple-scanning-beam device is used with the filter, this image degradation in the lung fields can be prevented while obtaining improved visualization of the mediastinum. Quantitative comparisons of image contrast and scatter distributions over chest radiographs obtained using various com-binations of anti-scatter device and filter are examined.

Artifact patterns in multiple scanning beam radiography.

A limitation of the use of multiple scanning beams for scatter reduction in radiography is the possible creation of artifactual aperture patterns which can be caused: (1) by a synchronization between the pattern scanning frequency and the radiation waveform ripple frequency, or (2) by the fractional overlap of the scanning beams during the final beam pass. An analysis of the stages of artifact formation and the implications for various scanning beam approaches is presented.

Luminance range compression for video film digitizers

Video cameras are used in many film digitization and teleradiology systems. However, the density range of medical radiographs often exceeds the dynamic range of the camera, and all diagnostic information in the original image may not be captured. Information in both the high and low density areas of the film can be captured in a single video frame if the transmitted luminance range of the radiograph is reduced. This can be accomplished by spatially modulating the back illumination of the film such that areas of lesser density receive less illumination while areas of greater density receive greater illumination. In this work, the use of a video monitor is shown to be an effective means to provide spatially modulated light for compressing the transmitted luminance range and thereby expanding the apparent dynamic range of the video camera. A simple computer-interfaced video feedback system that determines the appropriate compression mask and a scheme for linearization of system response are described. This system provides an interactive means for control of the degree of range compression.

Methods Of Sensitometry For Low Energy Radiography

Inverse-square sensitometry for low kVp techniques is limited by air attenuation and beam hardening. It is thus difficult to obtain accurate H and D curves for mammographic screen-film combinations at technique factors used clinically. Methods are described for the determination of characteristic curves at low x-ray energies which provide composite intensity-scale curves using small changes in source-to-receptor distance and concomitant changes in either exposure time, beam attenuation, or kVp. H and D curves obtained at low kVps using the various sensitometric approaches are compared with those obtained using inverse-square sensitometry; the relative merits of the different methods are discussed.

Conical Rotating Aperture Geometries In Digital Radiography

Applications of conical rotating aperture (RA) geometries to digital radiography are described. Two kinds of conical RA imaging systems are the conical scanning beam and the conical scanning grid assemblies. These assemblies comprise coaxial conical surface(s) the axis of which is collinear with the x-ray focal spot. This geometry allows accurate alignment and continuous focusing of the slits or the grid lines. Image receptors which use solid state photodiode arrays are described for each type of conical RA system: multiple linear arrays for the conical scanning beam assembly and multiple area arrays for the conical scanning grid assembly. The digital rotating-aperture systems combine the wide dynamic range characteristics of solid state detectors with the superior scatter-rejection advantages of scanned beam approaches. The high scanning-beam velocities attainable by the use of rotating apertures should make it possible to obtain digital images for those procedures such as chest radiography which require large fields of view and short exposure times.