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The Current Concept of Speed Should Not Be Used to Describe Digital Imaging Systems¹

¹ From the Department of Radiation Physics, SUNY Upstate Medical University, 750 E Adams St, Syracuse, NY 13210. Received April 27, 2004; accepted April 28. Address correspondence to the author (e-mail: hudaw@upstate.edu).

Speed has been used in screen-film radiography to quantify the amount of x-ray radiation incident on the receptor that is required to produce an image with a satisfactory film density. Historically, an exposure of 3.3 x 10^–7 C/kg (1.28 mR) has been used to correspond to a nominal speed of 100 (1). Doubling the screen-film speed to 200 then implies that the receptor dose is halved while a radiographic image with a satisfactory film density is still produced. In general, faster (higher) screen-film speeds are associated with lower radiation doses to the patient. A review of recent imaging journal articles reveals that this use of the concept of speed is also being applied to characterize digital imaging systems (2–5). In this editorial, I will consider whether it is appropriate to use the current concept of speed with digital imaging systems.

Digital radiography decouples the traditional relationship between radiation exposure and film density for screen-film systems. Digital imaging systems can operate over a wide range of radiation exposures and yet still yield satisfactory gray-scale rendering through a simple modification of the look-up table used to produce the printed and/or displayed images (6). The use of digital radiography systems thus eliminates the poor image quality and need to repeat examinations associated with over- or underexposure in conventional screen-film systems (7). Because different radiation exposures can produce an image with satisfactory gray scale rendering, ascribing a speed value to a digital imaging system is technically incorrect. However, changes in speed also affect the spatial resolution and noise characteristics of both screen-film and digital radiography systems, and these characteristics also require consideration.

For a given film, the speed of a screen-film combination is determined primarily by the screen composition and thickness, which also affect the spatial resolution performance. Fast screens are thick and have poorer spatial resolution performance, whereas slow screens are thin and have superior spatial resolution performance. Doubling the speed of a typical screen-film combination from medium to fast can reduce the dose to the patient by a factor of two, where the medium screen has a limiting spatial resolution of six line pairs per millimeter whereas the fast screen has 4.3 line pairs per millimeter (8). Digital imaging system resolution is generally determined by detector thickness and pixel size; the limiting spatial resolution for computed radiography and flat panel detectors is typically three line pairs per millimeter (9,10). It is important to note that for a given detector, the limiting high-contrast resolution of digital imaging systems is independent of the amount of radiation used to create a digital radiographic image.

Quantum mottle is the dominant source of noise in screen-film radiography and is determined by the number of x-ray photons used to produce the image. Increasing the speed of a given screen-film system is normally achieved by increasing the screen thickness, which means that fewer incident photons are required to generate an image. However, the total number of photons absorbed in the screen may be the same, which implies that the noise would be independent of screen thickness (8). This apparent paradox is best understood by comparing the amount of light required to blacken the film in a "thick" screen-film system with that required in a "thin" screen-film system, assuming the screen conversion gain remains constant. The light output of the thin and thick screens must be the same because the film requires the same amount of incident light; this implies that the total number of absorbed x-rays must remain constant. Screens with phosphors that have the same conversion gain will thus have similar total noise levels, irrespective of their actual thickness.

However, although the total amount of noise in screens of different thicknesses is the same, the observed noise pattern in thicker screens will appear more blurred (11). As with screen-film systems, most current digital imaging systems have noise characteristics that are dominated by quantum mottle. Image noise for current digital systems is determined by the radiation exposure incident on the digital receptor, and the exposure can be varied over a wide range. If the radiation exposure is increased by a factor of two, the image signal-to-noise ratio improves by a factor of 2^0.5.

The speed of a screen-film system is thus directly related to spatial resolution performance, whereas changes in dose will have no effect on the amount of blur in digital images. Specification of a speed for a given type of screen-film combination with a fixed conversion gain is independent of the noise level, whereas changing the dose in a digital system will affect the resultant signal-to-noise ratio. It is thus scientifically misleading, as well as technically meaningless, to use the current concept of speed to describe the performance of any digital imaging system.

To avoid the scientific and semantic difficulties of ascribing a speed to any digital imaging system, it is recommended that authors specify the amount of radiation incident on any digital receptor when characterizing the system performance. The amount of radiation can be expressed by using exposure in coulombs per kilogram or in roentgens (2.58 x 10^–4 C/kg = 1 R). The medical imaging community is currently in the process of substituting air kerma (which stands for "kinetic energy released in the medium") for exposure, since the latter involves the use of cumbersome units of measure (12). In conditions of electronic equilibrium, air kerma is equal to air dose (13). The mean energy (W) required to produce an ion pair in air is 33.97 eV (14), and an exposure of 2.58 x 10^–4 C/kg (1 R) therefore corresponds to an air kerma of 8.76 mGy. Specifying the amount of radiation used to create a digital radiograph by using either exposure and/or air kerma is relatively straightforward, involves easy measurements, and avoids the ambiguities associated with the term speed. Because digital detectors are generally quantum-noise limited, the amount of radiation used to create a digital image is directly related to the signal-to-noise ratio and patient dose. The appropriate amount of radiation in a diagnostic procedure performed with a digital system is one that strikes an appropriate balance between image quality (ie, signal-to-noise ratio) and patient dose (15).

FOOTNOTES

Author stated no financial relationship to disclose.

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