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2. Imaging Methods Miroslav Heřman
This chapter discusses the basic principles of imaging methods, their most common indications and contraindications, and the general terms used in the examination report.
2.1 Radiography Radiography is an imaging technique using X-rays to view the internal structure of an object. Radiographs, usually shortened to X-rays, are results of the rays generated in the X-ray tube (Fig. 2.1) which pass through the area being examined, where they are partially absorbed and scattered, depending on the composition of the tissues being examined, and then they are captured by a detector. The original recording material was a photographic film. By developing the film we get the final image. Nowadays, radiographs are made digitally in most cases. The most significant benefits of digital radiography are the higher quality of the images acquired, the reduction of the dose, the possibility of subsequent modification of the image (e.g. highlighting the most important parts of the image, adjusting the brightness, contrast, enlargement, highlighting of the interfaces), and archiving images in digital form. There are two basic principles of digital imaging: computed radiography and digital radiography. In computed radiography (CR), the radiation passed through the object is captured on a plate containing a sensitive layer based on photostimulable phosphor. The image
a
is processed in a digitizer (reading device) by scanning the plate with a laser. The amount of light released is registered and the latent image of the phosphor layer is converted into digital form. The phosphor layer is then erased by the laser and is ready for further use. These plates are stored in similar cassettes to X-ray films and maintained in a similar way – after the image is captured, the cassette is inserted into the digitizer and the resulting digital image is displayed on a monitor, where it can be further edited. The entire process from exposure to the display of the image takes 0.5–2 minutes (Fig. 2.1). Digital radiography (DR) uses different technical principles, but in both systems the conversion of passed X-rays to electrical digital signals occurs directly in the detector of the device. This transfer is faster than in computed radiography and, moreover, there is no need to transport the cassette from the examining room to the digitizer, which significantly accelerates the whole process – the image is available a few seconds after its exposure (Fig. 2.2). A radiograph is a two-dimensional image of a threedimensional object. It is a summation image – it captures information about all the tissues through which radiation has passed, regardless of the order in which it occurred. Tissues that absorb more radiation produce opaque [shadowed, (hyper)dense] areas, while less absorbent tissues are presented as lucent (hypodense) areas. These terms are relative – they are always related to a normal condition. This terminology is based on traditional radiographs. Because the radiograph
b
Fig. 2.1 Computed radiography process. a) Radiation coming out from the X-ray tube ( ) passes through the object being examined (elbow) and interacts with the sensitive layer of the plate enclosed in the cassette ( ). The phosphor-coated imaging plate interacts with X-rays transmitted through the object to capture a latent image. b) The cassette is then marked with patient data ( ) and inserted into the digitizer ( ). In the digitizer, the film is removed from the cassette and scanned by a laser beam; the amount of energy released is registered at each point of the image and thus the latent image is “converted” to the resulting digital image, which is displayed on the monitor, checked or modified by a radiological technician, and sent to the digital archive.
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