2 Scanned Projection Radiography

One of the outgrowths of modern computed tomographic (CT or CAT) scanning instrumentation is a method for performing digital radiography which has many of the features of an ordinary radiographic study but in which each data point (“voxel” or small rectangular portion of the painting) is recorded in a discrete, finite manner.5 This technique is often called the “scout view.” At present, it is available in any hospital radiology department with computed tomography equipment. The method consists of placing the x-ray tube and detector assembly in such a position that the painting may be moved linearly through the x-ray beam (Fig. 3). As it is moved, data are collected by the computer from the detector array, and the image of the painting is thus stored in computer memory for subsequent manipulation and display.

Fig. 3. Schematic of a scanned projection system. In the scanner, the fan-shaped x-ray beam is stationary, striking a linear array of electronic detectors positioned along an arc of a circle such that all are equidistant from the source. The painting is then passed linearly through the beam at a constant speed while the computer samples each detector at frequent, uniform intervals. Again, the analog signal is digitized and stored in the computer for future display.

The collected image, with or without digital processing, is displayed on a video monitor. In this image, the intensity of each individual point (“pixel”) in the image is proportional to the quantity of x-rays passing through the corresponding point in the painting. The spatial resolution of this system is acceptable but not as good as that obtained by digital fluoroscopy.

A major advantage of the scanned projection technique is that x-ray energies over a wide range may be used. This is possible because the electronics of the detector array can be optimized for the x-ray energy being employed. This calibration procedure is not available for either conventional radiography or digital fluoroscopy. Large fields-of-view are also possible (Figs. 4 and 5). The equipment is expensive ($800,000 to $1,200,000), but is widely available in major medical centers. We have had the opportunity to evaluate several of these systems and feel that they offer great promise in the evaluation of paintings.

Fig. 4. Self portrait, by Cornelius Hankins. Oil on canvas. Scanned projection radiographs windowed at different levels. See text for details. In A, the image is processed to display details of the canvas painting support (arrows). In B, processing is aimed to show the distribution of pigment. In C, the wood and metal supports of the painting are demonstrated.

Fig. 5. Mountain Lake, by Nell Choate Jones. Oil on canvas.

1. Photograph of the painting.
2. Film radiograph. Note lack of contrast and of correlation of the radiographic image with the painting. That is, the thickness of pigment application is independent of the visual display.
3. Scanned projection radiograph, windowed to show most painting details. See text. Pigment density pattern is seen in more detail than in B, but stll does not correlate with the painting.
4. Another scanned projection radiograph, windowed to show image details overlying the wood frame.

The scanned projection images in Figures 4 and 5 demonstrate a second common technique of digital image processing known as “windowing.” Image intensity, or “brightness,” using this equipment is measured on an arbitrary scale in units called Hounsfield units (HU). For the typical machine, the dynamic range is −1024 to 1024 (or more) HU, where −1024 is assigned to the attenuation of air, 0 to water, and 1024 (or the equipment maximum) to the densest material in the subject. If the entire dynamic range of image intensity is displayed simultaneously, the contrast resolution is low, as in film radiography. However, the windowing technique allows the viewer to select only a portion of the dynamic range for display at any one time. For example, in Figure 5C, the window center is −445 HU, and its height is 61. That is, in this display any pixel whose intensity is less than −506 HU (−445 −61) is displayed as black, and any whose intensity is greater than −384 HU (−445+61) is displayed as white. The intensity range between −506 and −384 is spread over the entire −1024 to 1024 dynamic range of the display from black to white, greatly increasing the contrast in the region of interest. To display the more intense portions of the image overlying the wooden frame, a second window whose center is at −376 and whose height is 100 was used (Fig. 5D). These static images can never demonstrate the information display capacity of this technique. It can be accomplished in “real time;” that is, the display can be altered as the window level and height are changed, providing for a continuously varying image that almost resembles a moving picture as it displays the various features of the painting.