Digital Imaging Considerations Computed Radiography

Transcription

1 Digital Imaging Considerations Digital Radiography Computed Radiography o Cassette based Direct or Indirect Digital Radiography o Cassetteless Computed Radiography 1

2 CR Image Acquisition Most like conventional radiography Cassette can be placed in Bucky or tabletop During exposure, the image plate stores trapped electrons in the plate s photostimulable phosphor The amount of energy trapped in each area reflects subject contrast of the part CR Image Sampling Once the image plate is exposed, the body part is selected Correct part must be selected so the correct lookup table (LUT) is applied when the image is rescaled The plate is sent to a reader 2

3 CR Image Sampling Infrared laser beam scans the plate and stored energy is released in the form of visible light The amount of light produced = amount of energy stored during the acquisition process Light is collected and converted to an electrical signal by the photomultiplier tube The electrical signal is sent to the analog-to-digital converter (ADC) to be digitized PM tube Analog ADC Computer Monitor CR Exposure Indicators Express amount of light given off by the IP Amount of exposure to the IP and patient Not a measure of dose, but an indication of what the patient received 3

4 CR Exposure Indicators System Exposure Acceptable Ideal Insufficient Excessive Change by Indicator Parameters Exposure Exposure Exposure Factor of 2 Kodak Exposure Index (EI) Below 1700 Above Fuji Sensitivity (S) number Below 100 Above Phillips Sensitivity (S) number Below 55 Above Agfa Log median value (LgM) Below 2.2 Above Direct & Indirect Digital Radiography 4

5 DR Image Processing Very similar to CR image processing o Proper body part must be selected o Ensures correct LUT is applied during rescaling o Electrical signals from flat panel detector sent to ADC to be digitized Thin film transistor (TFT) o Direct or Indirect system o Detector Shuttering Black surround o Blackening the brightness of the areas outside the exposure field o Provides a perceived enhancement of image contrast o Only shutter to exposed areas (match collimation) o Does not replace good collimation practices 5

6 Display Stations Vary with regard to resolution ability o Maximum # of pixels that the screen can demonstrate For full resolution display, monitor must display same # of pixels at which the system acquired the image Technologist vs. Radiologist Ambient Light Overall illumination of a room Reflects off the viewing monitor Levels should be low for best image analysis Image Analysis with Digital Systems 6

7 Demographics Must connect correct patient information with the CR cassette/dr computer Marker Placement Digital systems allow annotations o Do not cover original marker o Less reliable than markers added before exposure o Annotated images are not considered legal documents that will hold up in court 7

8 Adjusting Exposure Exposure indicator number estimates amount of automatic rescaling that occurred during image processing The more the value is outside of normal range, the more rescaling that occurred Lower than ideal numbers may necessitate repeating procedures due to mottled image appearance (quantum noise) Higher than ideal numbers may necessitate repeating procedures due to excessive gray-scale Next Quantum Noise Caused by insufficient number of photons reaching the IP Requires increase in kvp and/or mas EI = 1600 EI =

9 Dynamic Range Range of gray shades that the imaging system can display Higher dynamic range = better contrast resolution Screen film dynamic range = 1000 shades of gray Digital Radiography dynamic range = 16,384 shades of gray Windowing Postprocessing manipulation of the image s brightness and contrast to demonstrate an area of interest more accurately Adjust window level to increase or decrease brightness Adjust window width to increase or decrease the contrast Summary When compared to digital radiography, conventional radiography has better spatial resolution However, digital radiography has better contrast resolution than conventional radiography Digital radiography also has a much wider exposure latitude (more room for error) 9

10 Summary There is no substitute for proper kilovoltage peak and milliamperesecond settings. Insufficient penetration or overpenetration will result in loss of diagnostic information that cannot be manufactured by manipulation of the image parameters. 10