DR/CR artifacts Flashcards
What is this artifact?
Why does it occur? How do you fix it?
Quantum mottle (aka noise)
Occurs at low exposure (low signal to noise ratio) in both CR and DR
Minimzed by decreasing the display size
Corrected by repeating at a higher expusre (doubling exposure will reduce quantum mottle by 41%)
Saturation artifact (opposite of quantum mottle)
Exposure is too high –> Too many photons reach detector and the detector saturates; pixels are at maximum value and margins of structures are no longer visible. No amount of windowing can correct this.
Corrected by repeating the radiograph at a lower exposure
Planking
DR only!!! Calibration mask to correct x-ray field inhomogeneity
This artifact occurs due to over-exposure and appears as sharply demarcated rectangles in varying shadows of gray
Correct by decreasing exposure
Fading
CR only!! Post-exposure artifact
Occurs when plate sits for a period of time between acquiring image and reading image –> electrons become less excited and fall to lower energy state
Prevent by processing cassettes immediately after exposure
Light leak
CR only!!!
Cassette exposed to light –> partial erasure of information –> lighter image
Minimized with:
- proper maintenance of cassettes and machine
- reading plates immediately after exposure
Dirty light guide
CR only!!! Reading artifact
Dust on the light guide causes a white line that spans the length of the image
Corrected by cleaning the light guide
Faulty transfer
Workstation artifact - due to problem transferring data to workstation (loose cable or power fluctuation) –> pixels may be altered, misisng, ror replaced by electronic noise
Minimize artifact by using a stable power supply
Misplacement
Workstation artifact - also due to error in send date to the workstation (faulty cables or unstable power supply); appears inconsistently –> fragmented radiographs with fragments incorrectly located, multiplied, and/or superimposed.
Avoid by using stable power supply and reliable data transfer
Border detection
Workstation artifact - occurs when reader is trying to determine area to be analyzed. Borders may be incorrectly assigned –> cut off part of image
- if the imaging plate and FOC are not parallel (non-parallel collimator)
- Plate is divided for multiple exposures
- Object of interest is not centered
- Highly attenuating linear objects (Bone or metal) are present
Artifact can be reduced by properly centering object of interest and using semi-automatic methods for border detection
Dead pixels
DR only!! Exposure artifact
Occurs when detector elements fail –> appear as white dots or lines
Correct by recalibrating the detector to blur dead pixels or replacing detector
Moire (aka corduroy artifact)
Workstation artifact
Interference between grid and sampling frequency (grid and reader scan lines are parallel) –> curved, parallel, or wavy lines of increased density superimposed with entire image
Reduced by using an oscillating grid, orientating stationary grid perpendicular to the plate reader, or increasing frequency of grid lines
Uberschwinger/halo
Post-processing error
Dark halo around highly attenuating structures due to unsharp masking edge enhancement blurring (averaging) the pixel values
Can mimic implant loosening or pneumothorax; differentiated from pathology because even in thickness and conforms closely to the shape of the object
Avoid edge-enhancement when imaging orthopedic implants or algorithms are available to correct this artifact
Density threshold artifact (no image)
Workstation artifact.
It occurs when highly attenuating objects (such as metal) are present and are included in the histogram analysis. They are assigned the maximum pixel value; since the remainder of the objects on the radiograph are much less attenuating, they don’t contrast with each other well.
This can be avoided by setting a maximum value lower than that of metal.
