Factors affecting detectability

The factors affecting performance detectability include detector properties, exposure parameters and image processing technologies.

2.8.7.1

Detector properties

A comparison study of four different types of detectors (i.e,. CR1, CR2, DR1 and DR2) conducted by Fernandez et al. [71] demonstrate the detectors’ performance in determining image quality and low-contrast detail (LCD) detectability. CR1 is a conventional AGFA CR compact plus system with AGFA MD40 plates and a scanning resolution of 10 pixels/mm, while the CR2 is a needle-based AGFA DXS system with AGFA CR HD 5.0 storage phosphor plates. The flat panel system, or DR1, is one-piece amorphous silicon panel with a caesium iodide scintillator (200 µm pixel size), and DR2 is the most recently developed DR detector, with a detector of amorphous silicon and caesium and a 143 µm pixel size [71]. By obtaining the IQFinv for all detectors (using a reference value of 0.3 mGy for the chest PA

examination), CR2 and DR2 provide better IQFinv than CR1 and DR1 (though DR2 shows

better trends with low doses). Moreover, the storage phosphor system promotes improved image quality, but the dose reduction is limited compared to the flat-panel detectors. Images acquired through the needle image plate/line scanner provide better low-contrast performance than images obtained using image plate/flying-spot scanners. As a result, a structured CR2 produces the best LCDs of the three detectors, as well as better image quality [71].

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In CR detectors, a capture element blur (as well as noise) occurs due to the movement of the laser beam during the scanning process of the image plate. Consequently, there is a delay in the photostimulable emission as laser beam quickly scans the phosphor screen [72]. However, the capture element blur is neglected for direct flat-panel detectors because the electric field application eliminates the charge dissipation. In the phosphor-based detectors, the capture element blur is the main source of blur. Decreasing the thickness of the sensitivity layer is a valuable method for reducing the blur [72]. Veldkamp et al. [50] conducted a study evaluating such digital radiography systems as CR, Direct-DR and Indirect-DR and comparing their performances. The high and low attenuation regions in the dual readout CR were better than the those in the single readout CR [73]. The LCD detectability for IDR was better than that for the CR system [7]. A comparison study of IDR and DDR demonstrated that IDR has better SNR values and, hence, better LCD detectability than DDR. Moreover, IDR has the ability to compromise between the radiation dose and image quality [74]. The differences among these several detector types demonstrate the variations in their performance in relation to detecting LCD. They also highlight the detectors’ limitations [50].

2.8.7.2

Exposure factors

The main exposure parameters in radiographic images are the tube potential (kVp) and the tube current in mA. The beam quality is altered by the radiographer using kVp and mA according to patient conditions and the particular radiological study. Modifications to the exposure factors influence patient’s dose and image quality simultaneously [30]. For example, adjusting the tube current (mA) controls the beam quantity, and the penetrating power of the beam is controlled by adjusting the tube potential (kV). Changing the exposure parameter could result in better penetration of the primary beam (by kV) and enhance the quality of X-ray production. Therefore, the scattered radiation is reduced due to good beam

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penetration, which lowers the dose absorbed by the patient. In general, a high tube potential decreases the patient dose without reducing the image contrast to an unacceptable level. Moreover, the image quality can be improved by lowering the exposure time; however, this may affect the effective dose and entrance skin dose (ESD). Thus, maintaining the same mAs is an alternative method that involves increasing the mA and decreasing the exposure time (s). This will improve the image quality by limiting the patient’s motion blurring through a shorter exposure time [30].

Enhanced subject contrast requires a low kVp (tube voltage) due to increased X-ray attenuation. This results in optimised LCD detectability. Moreover, this leads to increased digital system SNRs, as well as increased DQE values for the detectors. In contrast, the decreased kVp results in image blurring and great exposure doses due to increased mA [5, 48].

The mA plays an important role in decreasing the radiation dose and enhancing image quality. There is a significant correlation between low radiation doses and noise production. Reducing the radiation dose ultimately degrades the SNR level, thereby increasing the potential for noise and the loss of important details in the radiographic image. Moreover, an overexposed image shows a very black image area, which is not easily recognised by radiologists. Therefore, LCD detectability is increased with great radiation exposure, and it is essential to balance the detectability performance with the patient radiation dose [48].

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2.9.7.3 Image processing techniques

Image processing affects image outcomes, such that the benefits of different image processing techniques result in better image quality. Methods like multi-frequency processing algorithms and un-sharp mask filtering can improve image contrast [5]. Another method to improve image contrast is edge enhancement, which can reduce noise and change pixel values to improve contrast [75]. This method can cause misrepresentation to the structures; this can be considered its main drawback. Moreover, elimination of the image noise can be achieved by a smoothing processing technique. This uses a subtraction processing technique to provide a better anatomically structured image by removing the superimposed structures. However, this method may reduce spatial detail [75]. The application of image processing technology is difficult because improving one feature of image quality can create another image artefact. Therefore, the optimisation process must be utilised in parallel to the system characterisation in order to enhance the image quality without resulting in the loss of detail [75].

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Chapter 3 Low contrast detectability in