Film-based and Digital Imaging Principles

Film-based imaging consists of X-ray interaction with electrons in the film emulsion, production of a latent image, and chemical processing that transforms the latent image into a visible one.  As such, radiographic film provides a medium for recording, displaying, and storing diagnostic information.³ Film-based images are described as analog images.  Analog images are characterized by continuous shades of gray from one area to the next between the extremes of black and white.  Each shade of gray has an optical density (darkness) related to the amount of light that can pass through the image at a specific site.⁴  Film displays higher resolution (Table 2) than digital receptors with a resolving power of about 16 lp/mm.⁵ However, film is a relatively inefficient radiation detector and, thus, requires relatively high radiation exposure.³  The use of rectangular collimation and the highest speed film are methods that reduce radiation exposure, but these techniques are not practiced commonly in private dental offices.⁶ Chemicals are needed to process the image and are often the source of errors and retakes.  The final result is a fixed image that is difficult to manipulate once captured.³

Figure 1

Digital imaging is the result of X-ray interaction with electrons in electronic sensor pixels (picture elements), conversion of analog data to digital data, computer processing, and display of the visible image on a computer screen.  Data acquired by the sensor is communicated to the computer in analog form.  Computers operate on the binary number system in which two digits (0 and 1) are used to represent data.  These two characters are called bits (binary digit), and they form words eight or more bits in length called bytes.  The total number of possible bytes for 8-bit language is 2⁸ = 256.  The analog-to-digital converter transforms analog data into numerical data based on the binary number system.  The voltage of the output signal is measured and assigned a number from 0 (black) to 255 (white) according to the intensity of the voltage.  These numerical assignments translate into 256 shades of gray.  The human eye is able to detect approximately 32 gray levels.⁷  Some digital systems sample the raw data at a resolution of more than 256 gray values such as 10 bit or 12 bit values.⁸  The large number of gray values is reduced to 256 shades of gray with the advantage of controlling under or overexposed images.⁸

Direct digital imaging systems produce a dynamic image that permits immediate display, image enhancement, storage, retrieval, and transmission.³  Digital sensors are more sensitive than film and require significantly lower radiation exposure.  Dynamic range or latitude is the range of exposures that will produce images within the useful density range.⁹  This corresponds to the straight line portion of the Hurter and Driffield (H & D) curve or the characteristic curve.  (Figure 1) This curve demonstrates the relationship between exposure (number of X-rays) and optical density (darkness) of an image receptor.  The scale of useful densities ranges from 0.6 (low density – light) to 3.0 (high density – dark).¹⁰  Beyond these parameters, the image is not diagnostic.  Typically, the H & D curve for film has a stretched letter S appearance with the top curve known as the shoulder and the bottom curve the toe.  Exposure changes in the shoulder (high exposure) and toe (low exposure) have little affect on density, but small changes in the straight-line portion between them significantly affect density.  The more vertical the straight-line portion of the curve is, the smaller the range and the narrower the film latitude.  In comparison, the dynamic range of charged-coupled devices (CCDs) is linear with no shoulder or toe and is much wider than film.⁵

Click Here To Navigate Within Article >> Abstract Introduction Film-based and Digital Imaging Principles Direct Digital Imaging Advantages and Disadvantages Diagnostic Utility of Digital Imaging Indirect or Scanned Digital Imaging Indirect Photostimuable Phosphor Plates Extraoral Imaging Image Enhancement Digital Subtraction Radiography Image Output DICOM Standard Checklist References About the Authors
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Citation Number: Vol. 3, No. 4, Page 025