Chapter 13 Digital radiography
As with the recent developments with general photography, where digital cameras are replacing film, this technology is also being used to replace X-ray film with digital imaging. Digital imaging has already been in use for many years in computed tomography (CT), magnetic resonance imaging (MRI), nuclear medicine (NM), fluoroscopy and ultrasound, but only recently has conventional X-ray equipment started to use digital technology.
Digital radiography has greater exposure latitude than X-ray film; therefore any over- or underexposure of an image can still be viewed. However, there is a trade off between radiation dose and image quality, as high exposures will give a very clear image but an unacceptable dose to the patient. Low exposures will result in a noisy (grainy) image and may miss diagnostically significant information. This is an advantage over the traditional film–screen combination where any exposure in the foot or shoulder areas of the characteristic curve will result in no usable image being obtained (Fig. 13.1).
Computed radiography (CR) has been used as a direct replacement in areas where previously film was used. It uses storage phosphor cassettes in standard X-ray rooms and has allowed radiology departments to make the transition from film to digital imaging without significant equipment changes.
The imaging plate is coated with a photostimulable phosphor (PSP), which captures X-rays once they have passed through the patient or object being imaged (Fig. 13.2). The PSP material has been ‘doped’ with small amounts of impurities which alter the physical properties of its crystalline structure.
A PSP material has a high absorbance at the energies used for diagnostic radiology and is able to store the irradiated energy and then release it as visible light in response to the stimulation of a beam of laser light within the reader. The family of europium doped barium fluorohalides fits these criteria – BaFX, where X can be chlorine (Cl), bromine (Br) or iodine (I).
As the X-rays pass through the PSP material, they interact with the electrons in the crystalline structure, giving them energy, which enables them to enter the conduction band. Some electrons return immediately to the valence band, but others remain ‘trapped’ in the forbidden zone between the two bands (Fig. 13.3). This trapped signal is proportional to the amount of radiation incident on the plate; these electrons form the latent image in the PSP.
To retrieve the latent image from the PSP it is placed in a CR reader, where a laser scans it (Fig. 13.4). The laser gives the electrons enough energy to return and leave the traps and to decay down to the ground or valence state. As these electrons move down in energy a blue light is emitted. This light is collected by a light guide and directed towards the photomultiplier tube. The light moves through the photomultiplier, is then amplified and then the signal is digitised using an analogue-to-digital converter, allowing the temporary storage of the image in digital format. This can then be sent to a monitor for viewing or to a printer.