One of the greatest advances in radiography over the thirty years is the advent of digital radiographic systems . These systems are generally divided into computed radiography ( CR ) and digital radiography ( DR )
DIAGNOSTIC IMAGING
Artefacts in Radiography
An Old Topic Digitally Revisited
Dr Christelle le Roux BVSc Hons . MMedVet ( Diag Im ) DipECVDI Senior Lecturer and Section Head , Section of Diagnostic Imaging Department of Companion Animal Clinical Studies Faculty of Veterinary Science University of Pretoria
One of the greatest advances in radiography over the thirty years is the advent of digital radiographic systems . These systems are generally divided into computed radiography ( CR ) and digital radiography ( DR )
The linear response to radiation exposure by digital radiography systems , and hence wide dynamic range , is one of its biggest advantages and therefore these systems are more lenient of exposure errors compared to film / screen systems . 1 The ability to use image processing is also one of digital radiography ’ s chief benefits , compared to traditional film-screen systems , in which the image could not be altered once acquired . 2
The term “ image processing ” includes all of the procedures or processes ( usually mathematical ) applied to the image detector and the raw data collected by the system in the course of image acquisition . 2
However , with the introduction of digital radiography , came the unavoidable introduction of its own set of artefacts . An artefact is an observed effect on the image or a structure that may spoil image quality , or may mimic or hide clinical pathology 1 , hence their recognition and avoidance are important . The artefacts that can occur with traditional film screen radiography , such as motion artefact , poor patient positioning , incorrect identification , and poor patient preparation ( such as debris on the skin ) 1 , can still occur in digital radiography and cannot be overcome .
This article will discuss the most common artefacts found in , and that are specific to , CR systems . Artefacts common to film / screen radiography will be mentioned briefly . Digital radiography ( DR ) will be discussed at another time .
Basic physics of CR
Computed radiography systems are cassette-based with a reading step after image acquisition . It uses a photostimulable phosphor imaging plate ( IP ) that is housed in the cassette for image acquisition . 3 , 4 When the IP is exposed to x-rays using standard radiographic equipment , electrons are excited to a higher energy state , which forms the latent image . 4
The cassette is then placed into a CR reader , which opens the cassette and extracts the IP from within . Rollers within the reader move the IP so that it can be scanned by a red laser beam . This laser light frees the trapped electrons and allows them to drop back to a lower energy state , with the emission of visible green light as a result . 4
The laser light and the released light photons have different wavelengths ( longer and shorter , respectively ), which allows the CR reader to distinguish the different light signals from each other . The shorter wavelength green light released from the IP is collected by a fibreoptic light guide and strikes the photomultiplier tubes ( PMT ). Here it produces an electronic signal , which is digitized and is stored in a display monitor , and can be sent to a picture archiving and communication system ( PACS ) and viewing stations . vet360
Issue 02 | MAY 2018 | 16