MOD 3 - Pre and Post-processing

Pre-processing

• correction of "raw" image data done by the computer software

• minor faults: cleans any dust, scratches, reduces noise, adjusts for small defects in the receptor

Dead Pixels

• what

• correction

• pixels that don’t respond to radiation and appear as small dots prior to pre-processing

• fixed through pixel calibration: interpolation, assigning a digital value from the average of the surrounding 8 pixels

Row or Column Drop-Out

• what

• correction

• when an entire row or column of pixels don't work, appearing as vertical white lines

• interpolation, assigned the average value of neighbouring columns

Detector Seam

• what

• correction

• inherent flaw where two detector parts meet leading to a slight variation in IQ

• correction is interpolation, however wont work if too severe

Cluster Defect

• what

• correction

• when many pixels in a small area are defective, creating a uniform white cluster

• correction is interpolation

Flat-Field Uniformity

A component of the pre-processing stage where the electronic noise within the system is evened out (eg. anode heel effect)

Detector Calibration

• purpose

• types

• calibration allows our system to recognize where dead pixels are and allows for flat fielding levels to be determined

• types:

  • Offset calibration

  • Gain calibration

  • Pixel calibration

Offset Calibration

• to reduce the intrinsic noise caused by current running within the detector, as it interferes with the diagnostic quality of our images

• machine will do this automatically from once to several times a day

Gain Calibration

Flat Field Testing

• used to compensate for sensitivity variations that can occur across the detector

• looks for defective pixels, image wide gradient differences and variations in the signal

Pixel Calibration

performed to ensure that all DELs are displaying uniformly and fixed through interpolation

Windowing (Grayscale Processing)

• used to adjust the contrast and brightness of the image

• window level = controls brightness, the center of window range

• window width = controls contrast

wide window width = 

low contrast bc of more grey shades

increased window level =

darker image (window tint analogy)