Conversion to Digital Radiography from Film Radiography Steve Mango Worldwide Technical Manager Carestream NDT Rochester, NY
Overview: Overview of digital Basic computed radiography (CR) Basic digital radiography (DDA) Image quality measures ASTM standards Digital radiography system qualification Customer acceptance of digital Audit criteria
What is Digital Radiography? Sources, shielding, image: same as film Detector: different type for both CR & DR (film is a detector also) Everything after exposure to image formation is different from film Need computer and monitor Image is an array of rows and columns of pixels Each pixel has a value proportional to dose, which eventually produces an analog gray value (brightness) on a display monitor Digital radiography is a way for you to improve your throughput and reduce consumable expenditures while improving the end customer image experience.
What is a Digital Image? Digital Image Analog Image
Digital Conversion Other industries have converted (for example; photography) Medical and dental radiography have converted NDT is the last to go, still primarily film It s not a matter of if ; it is a matter of when Reduce consumables, reduce re- work, eliminate chemical disposal charges, eliminate film storage costs, improve image evalua?on, share and store files electronically.
What s so Great about Digital? The user can; adjust brightness adjust contrast apply magnification apply image processing apply measurements, do calculations, and obtain statistics apply annotations include stamps Saves inspector?me and consumable cost while improving produc?vity.
What do your people need to learn to be prepared for digital? Digital Imaging Fundamentals it not the same as film! ASTM Standards pertaining to Digital Digital System Qualification Digital System Performance Baselining & Monitoring Image Quality Measures Computer Literacy A Totally New User Interface!
Varied requirements for digital certification ASNT training requirements Level I: 24 total hours, 16 hours hands-on Level II: 24 total hours, 16 hours hands-on NAS410 training requirements NAS410 Level I: 16 total hours, 8 hours hands on NAS410 Level II: 40 total hours, 8 hours hands-on The Industry has standardized on training MAI training requirements curricula for the transition to digital radiography with common guidelines from FWGIDR, MAI Level I: 8 hours formal training plus 20 DWGNDT, and OJT MAI. hours MAI Level II: 40 hours formal training plus 120 hours OJT
Kinds of Radiography Radiographic Testing (RT) film Computed Radiography (CR) Digital Radiography (DR) Digital Detector Array (DDA) Linear Detector Array (LDA) Computed Tomography (CT)
Computed Radiography
Computed Radiography Invented in 1975 by George Lucky at Kodak First commercialized in the mid 1980 s Digital image from film like plates Work flow is more like film Imaging plates are flexible like film Dynamic range is four decades (i.e., 1X to 10,000X)
Components of Computed Radiography Imaging Plate (IP) photostimulable phosphor Laser scanner or reader Computer, Software, and Monitor (image viewing)
Computed Radiography Imaging Process Stimulating Laser Detector (PMT) Galvo-driven scanning mirror (Fast scan) Filter Translation stage "Slow scan" Clock/sync * Computer A/D convert Storage Display Hard-copy output device
Computed Radiography: Imaging Plate
Imaging Plates
Imaging plate selection
Imaging plate payback compared to film One 14 X 17 plate typically costs about the same as one box (100 sheets) of film Imaging plates wear out over time Film wears out too over time Proper care and handling of IPs is required to achieve favorable payback! Imaging plates are a reusable media compared to film which is only used once. With proper handling of the imaging plates they can easily be used hundreds (or thousands) of?mes. The longer they last, the faster the payback.
Comparison of CR to Film
Digital Radiography/DDAs
Digital Radiography Invented in the early 1960 s, the worlds first digital radiograph was via fluoroscopy Image is immediate The detector is an electronic device Workflow is less like film, more like a digital camera DDA is rigid, not flexible Highest sensitivity Has excellent image quality due to frame averaging
Digital Radiography System DR System DR Flat Panel Detector (DDA)
Components of Digital Radiography Radiation source Object manipulator (for different exposure positions) Digital Detector Array, DDA Analog to Digital Converter Computer, Software & Monitor
Digital Radiography Imaging Process
Digital Radiography Detectors - Indirect Contain a thin film transistor photodiode array pixel grid Converts radiation to light Scintillators typically utilize Gadolinium Oxysulfide Terbium doped particle phosphors Scintillators can utilize Cesium Iodide Thallium doped needle phosphors Light is converted to electronic signal by the panel amorphous silicon, α-si Analog to Digital conversion produces pixel value
Digital Radiography Detector Handling DR detector contains sensitive electronics Semi portable Drop shock adversely affects detector Temperature and moisture dependent Radiation damage increases with kv; generally above 200kV Periodic detector maintenance required DR panels contain very sensi?ve electronics. In a sta?c environment, where the panel is not moved, the risk is reduced. However, panels can damage easily. Replacement panel costs run about $75K.
Calibration and Corrections Raw images obtained from a DDA must be calibrated (corrected) to create an ideal image. These corrections consist of: Offset (dark image) the noise level in the absence of radiation is characterized and subtracted from raw images. Gain the individual pixel to pixel non uniformity (gain) is normalized (x-ray on) and applied to the raw images. Bad pixel correction underperforming pixels are characterized, mapped, and corrected in the raw image.
Calibration frequency Offset is the most frequently required calibration. This can be performed as often as every image. As such, these are not normally saved. Gain calibrations last much longer. They are required for different energy and geometry conditions. They may need to be done if the scintillator develops irregularities due to burn in. These calibrations are saved, and are recalled as required. Bad pixel mapping is provided by the manufacturer, and is required very infrequently. As the DDA develops bad pixels with age and/or use, an updated bad pixel map may be required.
DDA Calibration Example Uncalibrated Calibrated
Digital Radiography Bad Pixels
Comparison of DR to Film
Image Quality Measures
Digital Image quality measures With film, inspectors only had to understand film density With digital radiography inspectors need to understand terms such as; pixel value, digital detector response (PV, DDR) digital driving level (DDL) contrast to noise ratio (CNR) signal to noise ratio (SNR) interpolated basic spatial resolution (isrb) equivalent penetrameter sensitivity (EPS) An understanding of digital image quality measures is required to qualify a system, gain customer approval, and produce optimum images
Exposure Linearity Equal increments of Log Exposure (doubling mas) should yield a straight line response of pixel values in a 12-bit Log system
Dynamic Range or Latitude Digital Latitude Film Latitude Pixel value or density Digital Film Exposure CR has a huge dynamic range rela?ve to film. It s easy to get a good shot the first?me. Fewer reshoots, no double loading!
Pixel Value (Pixel Intensity, DDR) Proportional to Dose! Can not be treated the same as optical film density The numerical representation of the pixel intensity depends on the chosen scale (i.e., log vs linear) Bit depth determines total range of pixel values Example 12 bit log = 2^12 = 4096, values 0 to 4095 Example 16 bit linear = 2^16 = 65536, values 0 to 65535 Pixel values are converted to analog brightness for viewing
Pixel Value Adjustment Gray scale (display) value is the number assigned to pixel value Window & level change the display values, not the underlying pixel values Display value is the digital driving level DDL DDL is the shade of gray on the monitor Monitor bit depth determines # of possible gray levels (8-bit = 256 levels)
Contrast Pixel value difference between parts of image Radiographic contrast Subject contrast: atomic no. (Z), density, thickness KV, filter, scatter Detector sensitivity Contrast enhancement with processing Use step wedge to define contrast (image sensitivity) and latitude
Contrast Sensitivity Minimum percent change in an object which produces a perceptible change in the image Computed Radiography lower contrast sensitivity Digital Radiography higher contrast sensitivity
Contrast to Noise Ratio (CNR) Difference between pixel values of adjacent areas Divided by standard deviation of pixel value (N2 N1)/ σ N = pixel value σ = standard deviation Standards specify this contrast is measured inside the 4T hole, and adjacent to it
Signal to Noise Ratio (SNR) SNR = N/ σ N= pixel value; σ = standard deviation Signal desirable part of image: dose Noise undesirable part of image Scatter Geometric distortion Statistical variation or quantum noise
Spatial Resolution (SR) Amount of detail in the image Minimum resolvable separation between high contrast objects SR affected by many factors
Spatial Resolution (SR) Resolution is limited by effective pixel size!
ASTM Standards for Digital Radiography
ASTM Standards for both CR and DR (DDAs) Standard Guides (tutorials) Standard Practices for Performance Evaluation and Long Term Stability Standard Practices for Manufacturer Classification & Qualification Standard Practices for Examination Major revisions to CR standards in progress! DDA Standards due for update this year
ASTM E2445 CR Phantom
ASTM/USAF CR Phantom for low energy
HPX-1 Diagnostic Tool CR Phantom Valida?ng digital system performance at the start and finish of a project can be a sellable benefit to a customer (confirms work) as well as risk management for the inspec?on company.
SMPTE TV test pattern to evaluate display monitor performance
ASME code acceptance ASME Article 2, Appendix VIII Replacement of film by CR ASME Article 2, Appendix IX Replacement of film by DR ASME code, section V, Article 2 2T hole or essential wire must be displayed
Digital Radiography System Qualification
Example of a CR System Qualification
Minimum pixel value qualification Establishes an exposure range for optimum image quality Determines the minimum pixel value to achieve a predetermined image quality level Equivalent penetrameter sensitivity Signal to noise ratio Allows users to know that they have good image quality based upon the pixel value (pixel intensity) that they achieve Must be qualified for specific systems/parameters!
Customer Acceptance of Digital
Customer acceptance of digital What s in it for them? Why change? Is it as reliable as film? Demonstrate the same result as film. Productivity Will it make them more profitable? ROI (Note; may not recoup investment with one job) Operating expenses Image management and storage People can be reluctant to change. Digital imaging has a significant cost advantage rela?ve to film. It s worth the effort to convert.
RT inspection cost Source: Radiographic Inspection of Aircraft Components, Quality Digest, June 2006; Steven A. Mango.
Digital radiography audit criteria How to assess / prepare / comply? Have comprehensive written procedures and be prepared to demonstrate S/W features & image quality measures! Window/level/zoom/pan/1:1 pixel mapping Histogram, ROI stats, variable width line profile Display of DDRs and DDLs Neg/Pos image polarity Annotations DICONDE compliance, lossless image format Process controls (SNR, EPS tests, etc.) Written procedures include scanner settings, viewing cond.
Thank You!