3 Scanned Point Source System

A third digital radiographic method is the “flying spot” or scanned point source system.6 In this system, the x-ray image is formed by scanning a fine pencil beam of x-rays over the area being imaged (Figure 6). After passing through the object, the attenuated x-ray beam is detected by a large scintillation crystal whose light output is measured, digitized, and stored in computer memory. The formation of the pencil beam of x-rays is accomplished by a mechanical system of collimators which block the beam and move in such a manner that the beam is scanned over the imaged area in a rectilinear raster. The time required for imaging a painting of moderate size (16 � 24 or 20 � 24 inches) is of the order of five seconds.

Fig. 6. Scanned point-source system. The x-ray beam arising from the x-ray tube (a) is collimated to a fan shape by the primary collimator (b); these elements move in a linear direction (large arrow) as a subject is scanned. The secondary rotating collimator (c) further collimates the beam to a small pencil shape (d) which is detected by a crystal (e), digitized, and stored.

This technique has not been generally available because, until recently, the imaging time was of the order of fifteen seconds, and the spatial resolution was significantly less than with other methods. Although these time and resolution limitations are not so important in radiography of paintings, they are in clinical circumstances. Thus, these machines have not been widely employed in medical facilities. At present, images can be obtained with resolution of about two line pairs per millimeter. In addition, images can be stored and repeated images of the same area can be summed to provide higher quality pictures. Data can be manipulated to change contrast and other characteristics as for the other systems of digital radiography. This technique has not as yet received sufficient use to permit us to evaluate it and to compare it with the other two more widely accepted computer methods.