With its ability of capturing what’s going on inside the human body, X-rays has transformed the medical world and its impact on people’s life is phenomenal. Since the day X-rays was discovered, scientists have not stopped finding and developing the use of X-rays. Over the years, the medical imaging technology has been expanding and advancing in a rapid speed.
Digital imaging has now replaced the traditional film-screen system and become the most widely used technology. It is well known for making the work more efficient and allowing flexibility of staffing options. Moreover, it’s believed that digital imaging is capable of producing images of similar contrast to conventional radiography with giving less radiation dose to the patient.
Unlike conventional film-screen system, digital imaging technology records latent image on an electronically readable device rather than onto an actual film. It uses ‘discrete numbers, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values.’ (Johnston, 2010) Oakley (2003:11) defines a digital image as a string of binary numbers.
Digital imaging is used in general radiography as well as modalities such as Fluoroscopy, Angiography, CT etc. Digital imaging technology has lots of features that have made it superior to conventional imaging system and some of these features have contributed to the possibility of producing good images while delivering less radiation dose to patients.
In general radiography, there are currently two types of digital imaging systems that are used. One is Computed radiography (CR) and the other is Direct Digital radiography, also known as DR. The main difference between these two types of systems is that cassette is used in CR whereas DR is cassetteless. What’s used in DR is a large area, flat-panel detectors with integrated thin-film transistor (TFT). This type of detectors is well known for its high detective quantum efficiency (DQE) compare with other types of detectors. DQE is used to describe a detector’s efficiency in image capturing, so the higher the value of DQE, the more efficient the detectors are, thus, the less exposure is required to obtain a diagnostic image, which means less radiation dose is given to the patient. In clinical practice, especially with pediatric imaging, if both CR and DR systems are available, radiographers would choose DR over CR for pediatric populations because of the significant reduction of patient dose. In computed tomography (CT), the dose efficiency is always an important parameter of performance due to the fact that there’s a high dose of radiation involved in CT examination compare to other modalities. In Chesney’s equipment for student radiographers, it is suggested that detectors’ efficiency has a direct impact on dose efficiency. (Carter, 1994: 206) In other words, the higher the DQE, the better the dose efficiency.
Another very important feature of digital imaging is that the digital detectors have a wider and linear dynamic range, which is believed to have contributed to dose reduction as well as image quality. Dynamic range is defined as ‘ the range from minimum to maximum radiation intensity that can be displayed as differences in signal intensity (or as density differences in conventional radiography).’ (Prokop, 1997:3) A larger dynamic range enables the system to be more tolerant to the variation of exposures, thus it reduces the need for repeating studies that are due to minor errors in techniques. Also, with a wider dynamic range, good image contrast can be formed. A good example would be chest x-rays. Because there’s a wide range of exposure involved with that particular body region, therefore a wider dynamic range is desired for fitting all the exposures into the exposure range. (Sprawls, n.d.)
Image processing is one of the key features of digital imaging technology. The beauty of it is that it enables the radiographers to manipulate and improve the image quality of a less diagnostic image. This has greatly reduced the number of repeating images, thus, patient dose is reduced. There are mainly two ways of image processing that are carried out by computer. One is image enhancement and the other is image restoration. (Ball and Price, 1995: 322)
Digital image enhancement is designed to ‘alter the visual impact that the image has on the interpreter in a fashion that improves the information content’. (Freeman, n.d.) It can be achieved by manipulation of the image contrast by control of window width and window level. Window width controls the width selection of the band of luminance value in the digital signal and window level controls the level selection of the displayed band of values within the complete range. (Ball and Price, 1995: 322) In general radiography, windowing and gradation are pre-programmed into the system for each particular examination. Gradation processing controls the contrast, brightness and density of the final image. It identifies which part of an image requires high contrast, which then helps to enhance the region of interest and improve the image quality. (Fujifilm medical system, n.d.) After automatic gradation processing, image will then be displayed on a screen monitor. If the quality of resultant image is still not diagnostic enough, the radiographers are able to change the contrast and brightness of the displayed image manually until the image quality has reached to its optimal. Another good thing about image manipulation is that information such as anatomical marker can be added afterwards if the radiographer forgot to use it when taking the X-ray image. Adding anatomical markers afterwards isn’t a good practice, however, being able to do this has greatly reduced the number of retakes in clinical practice, which consequently reduces patient dose.
In CT, selecting the correct window width and window level plays an important part on image quality due to the density of our body parts varies from one and another. (Larson, n.d.) Therefore selecting the wrong window width or level can result in poor image quality. For instance, if a bone window is selected for looking at lungs, lung tissues will not be visualized at all in this setting; therefore it won’t help with diagnosis. There is also other data manipulation techniques involved in CT, such as image enlargement, edge enhancement, etc. Adding’ Bow-tie’ filters in CT also helps in reducing patient radiation dose while improve image quality. (Bushberg, 2002:114)
In digital subtraction angiography, other than brightness and contrast manipulation, radiographers can also perform ‘pixel shift’ to correct for mis-registration. Also, anatomy can be added to visualize landmarks. (Seibert, 2004) During the procedure, correct collimation manipulation can improve image quality as well as reducing patient dose.
Image restoration is ‘a method of reconstructing an image to correct the degradation that may have occurred as part of the image-forming process.’ (Balls and Price, 195:322). A good example of degradation of image would be ‘vignetting’ in fluoroscopy. Vignetting is defined as the loss of brightness towards the edges of an image and this is an inevitable cause when image intensifiers are used. Image intensifier helps to increase the resolution, contrast and brightness of an image; however, the amount of resolution, contrast and brightness is only intensified at its greatest in the center of an image and then gradually reduces towards the periphery. (Hendee and Ritenour, 2002:239). This type of image distortion can’t be avoided, however, the quality of the image can be restored afterwards by using specialized software.
Another feature of digital imaging technology is that all the images that are taken will be sent to PACs (patient archiving and communication system). PACS enables the images to be stored electronically and viewed on monitor screen. The viewers can optimize the image quality further by manipulation on the high -resolution monitor when viewing. In addition, PACS lowers radiation dose by reducing the need for repeat examinations through inaccurately exposed images or lost films. (Connecting for Health, n.d.)
As a radiographer, the main responsibility is to produce a diagnostic image while giving patient the dose that’s as low as reasonably practical. Digital imaging technology has certainly made this task more achievable. However, nothing is really perfect, there are still some imperfections with digital imaging. For example, the spatial resolution of digital radiography is not as good as conventional radiography. Also, radiographers can overexpose their patients without noticing due to the automatic processing in digital radiography. Therefore it’s important for the radiographer to be aware of the limitations of digital imaging technology in order to utilize and maximize its use. Moreover, it’s important to remember that digital imaging is still a fairly new technology in the medical imaging world and it is still developing, which means it’s crucial that radiographers are receiving continuous training and keeping their skills up to date with the latest technology in order to operate the equipments efficiently and correctly, which will then create a positive outcome of image quality and dose reduction.
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