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Quality Assurance and Digital Radiography Information gathered for this lecture come from American College of Radiology Practice Guidelines for Digital Radiography 10/10/07 “Quality Management in The Imaging Sciences “ Jeffrey Pap Mosby 1998
The Digital Radiography Advantage • Digital Radiography offers many advantages over film based radiography. • Film response to radiation follows a sigmoidal response curve while digital response is linear. This results in a much wider dynamic response to exposure. The advantages include: – Reduced retakes due to exposure factors. – Improved contrast resolution. – Improved latitude.
The Digital Radiography Advantages • Much of the quality assurance for film based radiography is to assure proper photographic processing of the image. This is eliminated unless a film printer or multi-format camera is used. • Less space and perhaps less staff is required because of reduced storage space requirements. Images can be stored on a hard disk or CD.
The Digital Radiography Advantages • If the images are left in digital format, no darkroom or darkroom personnel is needed. • No film, processing chemistry and waste recovery costs. • Computed Radiography can be added to existing equipment. Direct Digital requires a total replacement of the image receptor.
Computed Radiography and Quality Control • Computed radiography still uses conventional radiographic equipment so basic radiography quality control testing is still needed. This would include: – – – Calibration of the generator Collimation Beam Alignment Collimation Linearity of exposure Spatial resolution (focal spot)
Computed Radiography and Quality Control • Calibration of the generator. – The linear dose response can be a double edged sword. It can mask calibration problem that can result in higher patient exposure. – Over exposed images really look good so this has resulted in dose creep. • Collimation & Beam Alignment – Collimation is critical for the computer to determine the proper contrast and density scale. • Spatial resolution (focal spot) – Film can record greater spatial resolution than digital radiography so resolution should be monitored more often than film based systems.
Computed Radiography and Quality Control • We have discussed much of the radiographic quality control earlier. • The end product in radiography is the image and accurate report of findings – Therefore for film, the processor function, screen contract and artifact avoidance was critical. – With digital radiography the computer monitor, image transfer and image receptor plate become the critical areas for quality assurance.
Monitor Testing • The acquisition workstation and secondary displays should be tested frequently using a standard image test pattern. • The SMPTE RP-133 (Society of Motion Picture and Television Engineers) or AAPM TG-18 QC test patterns are suitable.
Using the SMPTE Test Pattern • As a image fidelity test, the test pattern data files should be sized to occupy the full area used to display images. • The overall SMPTE image appearance should be inspected to assure that absence of gross artifacts.
Using the SMPTE Test Pattern • There should be no blurring or bleeding of bright display areas into dark areas or aliasing of the spatial imaging patterns. • All monitors used for interpretation should be tested at least monthly.
Using the SMPTE Test Pattern • For the dynamic range test, both the 5% and 95% contrast areas should be visible and distinctly different than the adjacent 0% and 100% areas. • Therefore:
Using the SMPTE Test Pattern • The brightness and contrast of the monitor is adequate if the 5% squares at both ends of the gray scale are visible. • The gray scale is shown as a series of squares in the center of the image that range from black (0%) to white (100%).
Using the SMPTE Test Pattern • It may be difficult to get the monitor to show both of these inner squares perfectly. • Most monitors do better showing the 95% square than the 5% square. • However, you might try adjusting the ambient light to improve the visibility of the 5% square.
Spatial Resolution & Aliasing • The spatial resolution (linearity) and aliasing (distortion) of the monitor are within acceptable limits if the high contrast bar patterns in the test image are distinct as simple black and white pairs.
Spatial Resolution & Aliasing • In each corner and the very center of the image, inspect the six squares filled with varying widths of alternating black/white horizontal and vertical lines.
Spatial Resolution & Aliasing • You should be able to differentiate all the lines from fat to narrow (6 pixels, 4 pixels and 2 pixels) and both horizontally and vertically.
BWH Pattern • The BWH pattern is a test of the continuous gray scale from the center to outside of the pattern. • It was developed at the Brigham Womens Hospital Radiology Department.
BWH Pattern • No concentric ring like features should be present in the image. • If they are present, the images displayed on your system will be of less than optimum quality.