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Artefacts found in computed radiography

Department of Diagnostic Radiology, Mayo Clinic and Foundation, 200 First Street Southwest, Rochester, MN 55905, USA

View larger version (98K): [in a new window]   Figure 1. Imaging plate (IP) artefact. (a) Thumb radiograph showing cracks (white arrow) that usually first become visible on the IP edges. As deterioration progresses, cracks appear closer to the clinically used areas of the IP (black arrow). (b) In some instances, early cracking along the edge of the IP does not occur. This crack appears as a lucency near the radius, which could be confused with a foreign body. Artefact remedy: an IP must be replaced when cracks occur in clinically useful areas.

View larger version (129K): [in a new window]   Figure 2. Imaging plate (IP) artefact. (a) Residue from adhesive tape used to attach lead markers to the outside of the cassette has caused artefacts (arrow) when the tape came in contact with the IP. (b) Static caused a hair to cling to the IP on this skull image. Artefact remedy: IPs must be cleaned regularly or when artefacts such as these are noted. Cleaning frequency is dependent on the environment in which IPs are stored and used. Specific cleaning methods are available from the manufacturer. Lint-free cotton gauze and lens cleaner is recommended by one vendor [5].

View larger version (161K): [in a new window]   Figure 3. Imaging plate artefact. The dark line along the lateral portion of this upper abdomen is caused by backscatter transmitted through the back of the cassette. The line corresponds to the cassette hinge where the lead coating was weakened or cracked. Artefact remedy: to reduce backscatter, the radiographer should collimate when possible. Since backscatter cannot be eliminated in every case, knowledge of the radiographic appearance of cassette backs is useful.

View larger version (178K): [in a new window]   Figure 4. Plate reader artefact. The pattern of lines seen on this oblique hip view occurred intermittently. The artefact was traced to the plate reader's electronics. Artefact remedy: the electronic board that controlled the photomultiplier tube was replaced.

View larger version (159K): [in a new window]   Figure 5. Plate reader artefact. The horizontal white line (arrow) shown on this upright chest radiograph was caused by dirt on the light guide in the plate reader. The light guide collects light emitted from the imaging plate when it is scanned by the laser. Artefact remedy: the light guide of the photomultiplier tube was cleaned by service personnel.

View larger version (57K): [in a new window]   Figure 6. Plate reader artefact. This artifact occurred because the plate reader loaded two imaging plates (IPs) in a single cassette. After an exposure, the bottom IP was extracted, read and replaced as usual, leaving the top IP to be exposed numerous times. Artefact remedy: double-loaded cassettes will be discovered during routine IP cleaning. If a cassette containing two IPs is discovered, the IPs should be erased before being put back into use.

View larger version (116K): [in a new window]   Figure 7. Plate reader artefact. This bilateral knee image was spoiled when the incorrect erasure setting was used to eliminate a previous femur image. Evidence of this is the residual image of the lead marker in the top corner of the image, the tissue line from the previous image (upper arrow) and the additional line of collimation along the bottom of the image (lower arrow). Artefact remedy: radiographers must select the correct erasure setting according to the type of exposure that has occurred.

View larger version (107K): [in a new window]   Figure 8. Image processing artefact. (a) When too large a kernel size is selected for image enhancement, artefacts like the black halo surrounding the prosthesis can create the appearance that the prosthesis is loose. (b) The same image as (a) processed with a smaller kernel size.

View larger version (81K): [in a new window]   Figure 9. Image processing artefact. (a) Edge enhancement was increased from the default level for this paediatric chest image. Notice the marked increase in lung markings, which could indicate interstitial infiltrates. (b) The same image processed with normal edge enhancement. Artefact remedy: select a standard set of image processing parameters, especially for frequency and edge enhancement, with the help of applications specialists. Once a standard set of processing parameters are selected, post-processing changes should not be needed frequently.

View larger version (61K): [in a new window]   Figure 10. Image processing artefact. Owing to lack of primary beam collimation on this lateral lumbar spine, the amount of unattenuated radiation striking the imaging plate (IP) (anterior and posterior to the patient) altered the histogram so that it was outside the normal range for that body part selection. Artefact remedy: use the smallest IP practicable and collimate the beam to the body part. This is particularly important on small or slim patients.

View larger version (112K): [in a new window]   Figure 11. Image processing artefact. The prosthesis in this knee adds too many extreme pixel values to the image histogram. This results in an image where the difference between prosthesis and glue, or glue and bone is not well demonstrated. Artefact remedy: in computed radiography systems that provide different modes of operation, use of a different image processing mode will alleviate this problem. For example, a semi-automatic or fixed mode will cause the image histogram to be analysed differently from a fully automatic mode and will produce a more acceptable image in cases such as this.

View larger version (116K): [in a new window]   Figure 12. Laser printer artefact. This oblique foot image demonstrates two different artefacts. The fine white line (black arrow) running parallel with the long dimension of the film is an artefact from the laser printer. The line running perpendicular to the long dimension of the film (white arrow) is caused by dirt on the light guide of the photomultiplier tube in the plate reader, as in Figure 5. Artefact remedy: service personnel can use a camel hair lens brush to clean the mirror.

View larger version (152K): [in a new window]   Figure 13. Operator error. A wire mesh cart loaded with unexposed imaging plates (IPs) was placed too near a source of scatter radiation. Because of the high sensitivity of the phosphor to scatter, exposure that may have been minimally seen on a film/screen image was well demonstrated on this portable image. Artefact remedy: protect IPs from sources of scatter radiation. Erase IPs that have been unused for an unknown period of time. One vendor recommends that IPs be erased if left unused for more than 8 h [5]. We have had good results using 1 week as the time limit.

View larger version (98K): [in a new window]   Figure 14. Operator error. The moire' pattern seen in this knee image was caused by using a grid with a frequency of 33 lines cm-1, which was oriented with the grid lines parallel to the plate reader's scan lines. Artefact remedy: use grids with no less than 60 lines cm-1. In addition, grid lines should run perpendicular to the plate reader's laser scan lines.

View larger version (88K): [in a new window]   Figure 15. Operator error. This axillary shoulder was exposed through the back of a cassette. Artefact remedy: be sure radiographers are well educated about how to use the entire computed radiography system.