Chapter 34 Production of the digital radiographic image
Chapter contents
34.1 Aim
The aim of this chapter is to introduce the reader to the principles of digital imaging as practised in the radiological department. The assumption is made that the reader has read and understood Chapter 25 which dealt with the basic mechanisms of image production. The consequences of producing a digital image will be discussed in Chapter 36.
34.2 The digital image
The old method of producing a radiograph using film and intensifying screens is an example of an analogue image. The information (or data) it contains is represented by a range of continuously varying densities or shades of grey. If such an image is scanned as a series of horizontal lines and the densities plotted on a graph, we would see an appearance similar to that shown in Figure 34.1A (see page 252).
Figure 34.1 A horizontal line drawn across an image. (A) A conventional analogue image; (B) the same line as a digital image.
A digital image is divided into a series of small boxes called pixels, arranged in a series of rows and columns called a matrix (Fig. 34.2, see page 252). The density of each pixel has a numerical integer value. If we consider our initial radiograph, we could allocate the value 0 to the most dense value and 255 to the least dense value giving a digital scale of 256. Thus, any single pixel would have a discrete value between zero and 255 and our line would appear as a series of steps as shown in Figure. 34.1B. The smaller the image size and the larger the number of pixels in the image matrix, the better the spatial resolution of the image. If the pixels are too large, the individual pixels can be seen by the observer and distract from the image information.
Figure 34.2 An example of the matrix used for a digitized image. Each box of this matrix is a pixel.
Most modern digital imaging systems have a matrix of 1024×1024 pixels; the resultant image has 1 048 576 individual pixels. If such an image was displayed on a 20-cm square section of a monitor, each pixel will be a square of side just under 0.2 mm and so will be below the resolving power of the eye; thus we are aware of the overall image and do not see the individual pixels.
A digital image is any image in which the information is represented in discrete units, with integer values.
Digital imaging is used in all imaging modalities in the modern diagnostic imaging department. The process of converting the analogue radiation image from the patient into a digital image differs with each modality and application. However, the principle of changing this analogue signal into a digital one is common to all modalities and applications.
Digital imaging allows the construction of an image with a high spatial resolution, large dynamic range and good contrast resolution. In addition, the imaged data may also be processed by a computer to enhance the diagnostic value of the ‘raw’ unprocessed image. The data for these images, as already mentioned, can come from a variety of sources and will be received by some form of image receptor (imaging plate, digital array or transducer, for example). The signal then passes through several basic stages before a visible image is produced.
34.3 Digital image production
If the image is not already in the form of an electrical signal, the first stage of the conversion process is to convert the image to an electrical signal. (This may not be necessary with all imaging modalities.) The analogue electrical signal is converted into a digital one using a device known as an analogue-to-digital converter (ADC). Within the ADC, the signal undergoes three stages: scanning, quantization and coding.
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