Presentation on theme: "Grayscale Calibration Grayscale Standard Display Function"— Presentation transcript:
1 Grayscale Calibration Grayscale Standard Display Function accuGray™Grayscale CalibrationFor Image Display WorkstationsDICOMGrayscale Standard Display FunctionaccuGray
2 Imagine the following: Scenario 1Imagine the following:A trauma patient is admitted at an outpatient clinic or rural clinic at a remote location. Several X-rays are taken and sent to a hospital for consultation.
3 Scenario 1How can we make sure that what the physician sees at the clinic matches what is being displayed on the radiologist softcopy workstation at the hospital?????
4 Scenario 1The image is also sent to another radiologist for a second opinion. This radiologist has a diagnostic viewing station from another vendor - a different monitor and video display card. As a matter of fact, he has a new flat panel display instead of a conventional CRT.How do we make sure that the image again looks identical??????
5 Scenario 2Everyone misses something among the myriads of pixels that are tape archived for colleagues and lawyers to examine with 20/20 hindsight vision.Perhaps the missed findings were apparent in the image data, but a once-new PACS monitor had lost its ability to display those subtle differences.How can you know? What will you say when they ask what steps you took to assure that the display equipment you used was up to par?
6 Dicom StandardThese issues are addressed the Digital Imaging and Communications in Medicine (DICOM) standard (PS )Part 14 of this standard deals with grayscale consistency.Jointly developed by American College of Radiology (ACR) & National Electrical Manufacturers Association (NEMA)Standard method for transferring images and associated informationAllows images to be displayed consistently on different softcopy and hardcopy devicesdevices manufactured by various vendorsdevices having different technologies (CRT, LCD)devices with different characteristics (max. Brightness and/or Density)different ambient light environments.
7 output luminance = input drive2.5 Display GammaCRT monitors (and LCD monitors which emulate them) have a fixed power-law relationship between output luminance and input drive signal (gamma) of about 2.5.output luminance = input drive2.5Standard Windows signals or video signals applied to monitors require an inverse gamma offset to counteract the display’s gamma.An exact inverse correction produces a linear relationship between the driving signal and the resulting light output.
8 Our eye-brain detection system is not linear Display GammaOur eye-brain detection system is not linearmuch less sensitive in the blacks than in the whiteswhy map all black values into luminance changes to display on the monitor when we can’t notice the differences between small black value changes?DDL* values of darker areas mapped into large luminance incrementsDDL* values of whiter areas mapped to smaller luminance incrementsResulting curve is standardized as the DICOM Grayscale Standard Display Function (GSDF)*Digital Driving Level (DDL): digital value applied to a Display System to produce a luminance output
9 Perceptual Linearization DICOM Grayscale Standard Display Function (GSDF) generates a display function matching the perceptual characteristics of a human observer.Standard developed having human observer view monitor while luminance output of monitor changed in small steps from black to white and noting when they observed a Just Noticeable Difference (JND) in light output as the drive signal is changed.This standard display function creates a perceptually linearized display - equal changes in driving levels yield perceptually equivalent changes in luminance across entire luminance range.From the observer’s point of view, this means that equal steps in brightness sensation represent equal steps in the acquired image data.Smallest luminance change that average person can just perceive under ideal circumstances is approximately 256 shades of gray.
10 Grayscale Standard Display Function Perceptual LinearizationEqual steps in perceived brightness represent equal steps in the acquired image data.01.11101001000200400600800Grayscale Standard Display FunctionJND IndexDifferent changein absolute luminanceSame number of Just Noticeable Difference == same perceived contrast
11 Perceptual Linearization Most digital radiologic images are windowed to attempt to display 256 different shades of gray at any one time, (256 is approximate JND that humans can perceive)The perceptual linearization implemented with the DICOM Grayscale Standard Display Function (GSDF) ensures that pixel values that are supposed to increase brightness in a linear fashion, say 6, 7, and 8, for example, actually appear that way to the human eye, despite the nonlinearity of our perception.
12 Perceptual Linearization The DICOM GSDF provides a mechanism to standardize the appearance of images on monitors of different inherent brightness and with different response curves.Also recalibrate monitors whose luminance and response curves have changed over time.Now it is a matter of calibrating the monitor and printers according to this curve.Calibration tools for softcopy display devices include a luminance meter that can be placed on the monitor and allows the light output (luminance) to be measured at multiple digital driving levels (DDLs).This output can be compared with the actual Grayscale Standard Display Function (GSDF) and any differences can be calculated and saved. Once the differences are calculated, a corrective transfer function can be downloaded directly to the video display controller.
13 Color Lookup TableVideo Card Look Up TableDigitized images are made up of a matrix of pixels, each possessing at least three dimensions: two (or more) spatial and one intensity value. The quantized dimensions are stored on the computer as a file of binary numbers.In order to see the image on the computer monitor, the image pixel values must be mapped, one-to-one, to screen pixel values, via a Look Up Table (LUT). The LUT transfer function determines what screen values correspond to image pixel intensity values at all coordinates in the image.
14 Perceptually equalized Color Lookup TableIf each image intensity value is mapped to its corresponding screen value, the LUT is a linear function and can be graphed as a 45° straight line.By modifying the transfer function’s slope & screen mapping, LUT image intensity values may be selectively increased or decreased to perceptually equalize image luminance steps.LinearPerceptually equalized0.111010050150200250DDL or P-ValuesStandardCharacteristic Curve
15 Guidelines for Calibrating Grayscale Monitors Calibrating DisplaysGuidelines for Calibrating Grayscale MonitorsCalibrate each monitor regularly.Calibrate under regular viewing & lighting conditions.Adjust monitor contrast and brightness before/during calibration and never again.Calibrate with always the same background intensity (about 20%, or a value comparable to the average intensity emitted from the monitor).Calibrate to match the standard display function (DICOM).
17 accuGray™ Grayscale Calibration Four Primary uses of accuGray™Acceptance Testing of new displays.Calibration to Grayscale and Color standards.Visual Verification of proper operation.Conformance Tracking over time.
18 accuGray™ Grayscale Calibration Main Display screen
36 3. Visual Verification of proper operation. Display VeriLUM® Pattern – analyze 256 levels of gray, scan linearity, and focus.
37 3. Visual Verification of proper operation. Display User Patterns – analyze display’s capabilities with standard or custom medical test images.
38 4. Conformance Tracking over time. Insure and document that a display continues to provide a properly calibrated image over its lifetime.You’ll select from thesemenu items
39 4. Conformance Tracking over time. Measure Luminance Response for Tracking - Measure a color display’s grayscale tracking and save to measurement history
40 4. Conformance Tracking over time. Display Tracking History - displays documentation of a color display’s white background color measurement history.Click for graphical Tracking and JND plots that show the display’s performance history
41 4. Conformance Tracking over time. History Plot – shows the current color Tracking measurement and past performance of the display.
42 4. Conformance Tracking over time. JND Plot - shows the current JND measurement and past performance of the display.
43 4. Conformance Tracking over time. Plot Results - provides on-screen and printable documentation of the display’s past and present performance.
44 4. Conformance Tracking over time. Gamma Correction Table - shows the current gamma curve and gamma history of the display.
45 4. Conformance Tracking over time. Calibration Luminance Response - shows the current luminance calibration curve and prior calibration curves.
46 4. Conformance Tracking over time. Tracking Luminance Response - shows the current luminance response and prior response curves.
47 Benefits of a Properly Calibrated Video Display: Displays a sharper-focused, full resolution imageShows full detail in both the darkest & brightest parts of all scenesIs properly matched to the viewing environmentProduces a full range of accurate colors, including flesh tonesMinimizes picture artifacts (distortions)Ensures a proper perceptually linearized displaySatisfied user (Doctor)Properly diagnosed patientNo litigation