UNIT 3: Digital Imaging Pre-Processing Flashcards
Pre-processing
performed by the computer on the raw image data to correct for flaws in the
image acquisition process (acquisition processing); largely automatic
Post-processing
all manipulation and adjustments made in order to refine the image
Pre-processing vs. Post-processing
-Pre-processing: image corrections
-Post-processing: image refinement
Digital Imaging Preprocessing Problems and Solutions
-Defective Pixel: Interpolate adjacent pixel signal
-Image Lag: Offset voltage correction
-Line Noise: Correct from dark reference zone
How are post-processing functions and operator adjustments related?
-Post-processing: all manipulation and adjustments made in order to refine the image (includes default and operator adjustments)
-Operator Adjustments: Post-processing functions carried out by the user after initial display
Post-Processing examples
(also image zoom)
5 steps in pre-processing
- Field uniformity
- Noise reduction for DEL dropout
- Exposure field recognition
- Histogram formation and Analysis
- Histogram rescaling
Post-Processing steps
- Gradation Processing (LUT)
- Detail Processing
- Formatting for Display
- Image Displayed
- Operator Adjustments
Field Uniformity potential sources of noise in image acquisition
-Faulty DELs/ DEL drop out
-Dark noise/Ambient Nose in CR (Background radiation)
-Dark current in DR and CR
-Phosphor layer thickness variations for indirect capture DR and CR systems
-Light guide variations in CR
-Anode heel effect especially at short SID’s
What does Field uniformity do?
correct for flaws in image acquisition
Field uniformity: Faulty DEL’s
Field uniformity: Dark Noise
-In CR
-Accumulation of background radiation on the CR plate prior to/or after the image is acquired
-Ensure the IR is erased prior to use and processed immediately after exposure
Field uniformity: Dark Current
-In DR and CR
-(noise) a small amount of current remaining in the electronics in the absence of x-ray exposure
Dark current in DR vs CR
-DR: caused from current moving in the electronics of the detector in the absence of x-ray exposure
-CR: typically is attributed to the CCD but may be caused by light guide variations in the reader as well.
(These elements create noise in the system that can degrade image quality and efficiency of the system)
Does DQE increase or decrease as Dark Current increases?
Decrease
What is a common QC test performed by radiographers on equipment to test for field uniformity issues?
-Flat fielding AKA Gain Calibration or Calibration tests
-A QC test performed on a digital detector by producing an image on the receptor without any attenuating material in the field of view
Flat field uniformity tests
-In image A, a flat field uniformity test demonstrates visual artifacts from the Bucky tray mechanism on a digital mammography machine.
-In image B, a DR image picked up the ionization chambers inside the Bucky tray. These artifacts must be detected and corrected before clinical use of digital equipment
Step 2: Noise reduction for DEL dropout. What is DEL dropout?
a DEL may be dead or faulty causing noise and/or signal loss. This can be a single DEL or an entire row.
Step 2 Noise Reduction for DEL dropout. What is a kernel? What is Interpolation?
-Kernel: (A matrix within a matrix) A computer processing code that is applied to an entire data set
-Interpolation: average of surrounding cells to fill in the gap
Dead DELS or Pixels: Kernels and Interpolation (malfunctioning pixels/detector elements)
To correct for a non-uniform field in which a small area of the detector is not picking up a signal (AKA dead DEL or pixel), a kernel is configured to average the signals surrounding the malfunctioning DEL and fill in the dead space with interpolation
Step 3: Exposure Field Recognition. What is another term for this?
Edge Detection Recognition
Step 3: Exposure Field Recognition
• System excludes areas outside the collimation from processing
-Requires parallel and symmetric collimation
Step 3: Exposure Field Recognition. What happens when it fails?
-This may lead to histogram errors resulting in issues with poor resolution, overly light or dark images and/or EI numbers that do not accurately represent the exposure to the IR
-Reasons for EFR/EDR failure:
• High exposure to the detector
• Extreme off centering of part on IR
• Improper collimation
• Area of high attenuation within the image (metal objects, barium, lead shield)
Step 3: Exposure Field Recognition. What is segmentation software?
-Creating multiple exposures on 1 IR
-Used in CR ONLY to identify and count the number of views per image receptor