Post Processing And Artifacts

Question 1

Window and level

Answer 1

-most common image processing parameters (adjustments of brightness and contrast)
-window level: controls how bright/dark the image is
-window width: controls the contrast of the image
🔹wider= lower contrast
🔹narrower= higher contrast

Question 2

Shuttering/masking

Answer 2

• when viewing an image (film/digital), the unexposed edges around the image can be distractingly bright (causing veil glare: interferes with the image reception of the eye)
• with digital, automatic shuttering is used to black out the white collimation borders, eliminating veil glare
• can be manually removed and adjusted
• this is NOT MEANT to replace collimation

Question 3

Magnification

Answer 3

Two types
1. Magnifying glass function
🔹a box placed over a section of anatomy shows a magnified version of that section
🔹just like using a magnifying glass to enlarge JUST ONE AREA
2. zoom function
🔹enlarges ENTIRE IMAGE, so that it likely will not be able to fit on the screen- only a protion will be visible at a time
🔹need to use mouse to navigate the image

Question 4

Spatial frequency filtering

Answer 4

-after signal is obtained for each pixel, to shorten the processing time and minimize storage requirements, the signals are averaged
🔹one pixel will be averaged with the signals of the neighbouring pixels
-if more pixels are involved in averaging, the image will appear smoother
-EDGE ENHANCEMENT is when fewer pixels are used for the averaging
🔹the fewer pixels are included in the average, the greater the enhancement is

Question 5

Spatial frequency filtering

Answer 5

High pass filtering
-when frequencies are amplified
-sharpening effect
-increases contrast and edge enhancement
Low pass filtering
-when frequencies are suppressed 
-smoothing
-can lose small details, reduces noise and contrast

Question 6

Image stitching

Answer 6

• special cases where the anatomy/area of interest is too large to fit on one cassette
• multiple images are ‘stitched’ together to create one image (usually 3-4 CR cassettes are used) software program processes image to display as one single image

Question 7

Artifacts

Answer 7

-any undesirable density on the processed image (excluding scatter and fog)
-four main types
🔹imaging plate artifacts
🔹plate reader artifacts
🔹image processing artifcats
🔹printer artifcats

Question 8

Imaging plate artifacts

Answer 8

• usually due to aging or wear
• plate can become prone to cracks due to the constant removal and replacement within the reader (cracks will appear as areas of radiolucency on the image)
• backscatter: worn down or cracked areas of the lead coating on the cassette allow scatter to reach the IP (backscatter can be seen as dark line artifacts)
• can occur for different reasons: incorrect part selection, incorrect sampling
• very important to set appropriate facts and choose the correct body part so that the algorithm can produce the desired image
• poor technique and positioning can cause misrepresentation of the image

Question 9

Plate reader artifacts

Answer 9

-if there are extraneous line patterns that appear randomly (not regularly), this can be caused by issues with the electronics
-white lines parallel to the direction of the plate travel are caused by debris on the light guide
-requires cleaning or replacement by service personnel
-if multiple IPs are loaded into a single cassette
🔹very rare
🔹only one plate will be extracted, leaving the other one to be exposed multiple times
🔹similar effect as double exposing a film cassette
-insuffiecent erasure can result in residual information being on the IP before next exposure (can occur from over exposure or if the erasure lamp requires work
-moire artifact can occur with the orientation of a stationary grid (grid lines need to be perpendicular to the laser scan direction), not an issue with moving grids

Question 10

Operator error

Answer 10

• collimation: inappropriate collimation for the body part can result in an improper calculation of the exposure indicator, affects histogram
• cassette use: ex if image is taken on the back of a cassette hardware will be present as artifact on the image
• technical factors: under exposure produces quantum mottle, over exposure reduces contrast

Question 11

Flat panel artifacts (dead pixels)

Answer 11

• as detectors age, quantity of dead pixels increases, but may not need replacement if they are near edge
• software identifies dead pixels and uses an interpolation method to ‘fill in’ the dead pixels information from surrounding pixels
  
  • can be an issue if there is too many dead pixels

Question 12

Flat panel artifacts (incorrect gain calibration)

Answer 12

-used for the correction of flaws in the detector
-if there is an area with many dead pixels or poor connections between the conversion layer and the array-there will be a noticeable artifact on the screen
🔹gain calibration corrects this
🔹creates a mask of the defects, so when an image is taken, the software uses the mask to remove the unwanted densities so only the diagnostic information is displayed

Question 13

Image lag

Answer 13

-some flat panel systems allow for the user to take images faster than the detector can accommodate
-if there is an image taken before the detector has finished releasing all the signal for the previous image, there may be a faint image of the previous image still visible
-like a type of double exposure
-caused by:
🔹taking images in rapid succession
🔹over exposure
🔹areas with little beam attenuation (marker)

Question 14

Offset correction

Answer 14

-can correct image lag
-determines the amount of signal inherent in the detector
🔹creates a mask of the residual signal
-if this is done before residual signal has left the pixels, that information can be stored as inherent and cause a negative image of the signal- inverse image