Читать книгу A Comprehensive Guide to Radiographic Sciences and Technology - Euclid Seeram - Страница 32

The overall system components of FSR are shown in Figure 2.1. These include the x‐ray generator, the x‐ray tube, a radiographic table, the image receptor, a chemical processing unit, and a light view‐box. A film‐based image is created as follows:

Figure 2.1 The overall system components of film screen radiography (FSR).These include the x‐ray generator, the x‐ray tube, the image receptor, a chemical processing unit, and a light view‐box, for viewing the film image.

1 X‐rays pass through the patient and fall upon the film to form a latent image.

2 The latent image is then rendered visible using chemical processing.

3 The visible image on the film is displayed on a light view‐box for viewing and interpretation by a radiologist. This image consists of varying degrees of blackening as a result of the amount of exposure transmitted by different parts of the anatomy. While the blackening is referred to as the film density, the difference in densities in the image is referred to as the film contrast. The film converts the radiation transmitted through the various types of tissues (tissue contrast) into film contrast.

4 The light from the view‐box is transmitted through the film and can be measured using a densitometer and is referred to as the optical density (OD), which is defined as the log of the ratio of the intensity of the view‐box (original intensity) to the intensity of the transmitted light. The OD is used to describe the degree of film blackening as a result of radiation exposure.

5 A plot of the OD as a function of the log of the relative radiation exposure is described by the well‐known characteristic curve (Hurter–Driffield Curve), which provides information about the film response to the exposure (Figure 2.2). There are three parts of the curve: the toe region, the slope, and the shoulder region. Exposures that fall in the toe and shoulder region of the curve will result in images that are light (underexposed) and images that are dark (overexposed), respectively. Acceptable image contrast is obtained when the exposure that falls within the slope of the curve. This slope defines the exposure latitude as well as the film contrast characteristics (the steeper the gradient, the higher the contrast).

6 The density of the image is hence used as an exposure indicator that provides immediate feedback to the technologist that the correct exposure technique factors have been used for the examination. This curve also shows that FSR has a fixed film speed (sensitivity) and a fixed‐dose requirement.

7 Furthermore, the characteristic curve can be used to describe the film speed, average gradient, the film gamma, and the film latitude. Only film speed and film latitude will be reviewed here. The interested reader should refer to any standard radiography physics text for a further description of the other terms. While film speed refers to the sensitivity of the film to radiation and it is inversely proportional to the exposure, film latitude describes the range of exposures that would produce useful densities (contrast).Figure 2.2 A plot of the OD as a function of the log of the relative radiation exposure is described by the well‐known characteristic curve (Hurter–Driffield Curve), which provides information about the film response to the exposure. See text for further explanation.

8 High‐speed films (fast films) require less exposure than films with low speeds (slow films). On the other hand, while wide‐exposure latitude films respond to a wide range of exposures, films with narrow exposure latitude can respond only to a small range of exposures. In the latter situation, the technologist has to be very precise in the selection of the exposure technique factors for examination.

A significant limitation of FSR that has been overcome by digital radiography (DR) relates to the narrow exposure latitude which means that exposure technique selection in FSR must be accurate to achieve an image with acceptable density and contrast. This poses a challenge as stated in point 8. For technologists, DR detectors “have wide exposure latitude, a variable speed class of operation, and image postprocessing capabilities that provide consistent image appearance even with underexposed or overexposed radiographs” [1]. This is illustrated in Figure 2.3. It is clear that DR images taken with low and high exposures appear visually the same on the viewing monitor (due to the image processing of DR systems). Note, however, while low exposures produce images with more noise (grainy image), high exposures produce images with less noise but at the expense of increased dose to the patient. The result is that technologists face a difficult task of recognizing underexposed and overexposed images. If overexposed images cannot be determined, the patient receives an unnecessary dose. Overexposures 5–10 times a normal exposure will appear acceptable to the technologist. Subsequently, this will lead to what has been popularly referred to as exposure creep or dose creep [1].

In FSR, the radiation exposure used for an examination is determined by the exposure technique factors selected on the radiographic control console by the technologist. These include the kilovoltage (kV), the milliamperage (mA), and the time of exposure in seconds (s). Consoles may allow the selection of mAs (mA × s). While the kV determines the x‐ray beam quality (beam penetration), the mA determines the quantity of photons falling on the patient per unit time. The length of time of the exposure is influenced by the exposure time. Today, automatic exposure control (AEC) is often used to ensure that correct exposure factors are used for examinations, and to reduce errors made by manual technique selection. The basic principle of an AEC timer is such when a preset quantity of radiation reaching the film detector (acceptable image density) is measured, the exposure is automatically terminated. The result is almost always perfect exposures.

Figure 2.3 DR detectors have wide‐exposure latitude and image postprocessing provides consistent image appearance even with underexposed or overexposed radiographs. See text for further explanation.

Another essential concept in FSR is image quality. The quality of a film‐based image can be described by several technical factors including resolution, contrast, noise, distortion, and artifacts. Only the first three will be reviewed in this chapter. Resolution includes two types, namely, spatial resolution and contrast resolution.

1 Spatial resolution refers to the detail or sharpness of the image, and is measured in line pairs/mm (lp/mm). The higher the number of lp/mm, the greater the sharpness of the image. FSR has the highest spatial resolution ranging from 5 to 15 lp/mm compared to all other imaging modalities [2]. As noted by Bushong [3], “Detail is affected by several factors such as the focal spot size, motion of the patient, and the image receptor design characteristics. Detail is optimum when small focal spots are used, when the patient does not move during the exposure, and when detail cassettes are used.”

2 Contrast resolution on the other hand describes the differences in tissue contrast that the film can show. As a radiation detector, film‐screen cannot show differences in tissue contrast less than 10%. This means that film‐based imaging is limited in its contrast resolution. For example, while the contrast resolution (mm at 0.5% difference) for FSR is 10, it is 20 for nuclear medicine, 10 for ultrasound, 4 for computed tomography (CT), and 1 for magnetic resonance imaging [2]. “The contrast of a radiographic film image, including the object, energy of the beam, scattered radiation, grids, and the film. The main controlling factor for image contrast, however, is kV. Optimum contrast is produced when low kV techniques are used. A grid improves radiographic contrast by absorbing scattered radiation before it gets to the film” [3].

3 Noise is seen on an image as having a grainy appearance. This occurs if few photons (quantity) are used to create the image. Noise can be reduced if more photons are used by using higher mAs settings; however, this will result in more dose to the patient. Less noise is produced when higher kV techniques are used for the same mA settings. The goal of the technologist is to use the lowest possible radiation dose and not compromise image quality. This is an important consideration in observing and working within the as low as reasonably achievable (ALARA) philosophy established by the ICRP.