Achieving Medical Industry Display Standards Using Advanced Display Controller Boards and Empowered Calibration Application Packages for Medical Imaging
Abstract...3 Introduction...4 What is a DISPLAY calibration?...4 How is a DISPLAY calibrated with a BOARD LUT?...5 How is a DISPLAY calibrated with a DISPLAY LUT?...5 Why is luminance uniformity calibration necessary?...5 What is the difference between BOARD LUT and DISPLAY LUT?...5 What do commercial off-the-shelf boards offer?...7 What is the influence of a higher number of input addresses?...7 What is the influence of a higher number of output values?...8 DISPLAY versus Matrox Xenia BOARD Calibration Comparison Table...10 Absolute and relative errors in showing gray shade levels for each LUT type...11 Practical Experience...11 Calibration Result with ATI Radeon TM 9550 in 32 Bit Color Mode:...12 Calibration Result with Matrox XENIA in Extended Color Mode:...17 Conclusion...22 Acknowledgments & References...22
Abstract All video, graphic and display controller BOARDs with standard eight (8) bit output hardware Look Up Tables (LUTs) provide insufficient table entries to accurately display medical images. Products with ten (10) bit or greater output hardware LUTs achieve the desirable and excellent results.
Introduction It is a recurring issue - hardware calibration, display calibration, board calibration, LUT manipulation, and software calibration. Hardware sounds substantial and industry acceptance and marketing has made medical imaging professionals acknowledge this as a necessity that typically has resided within a display. Advances in board and software application level technologies which incorporate hardware calibration outside the display should be considered and embraced, if it provides significant benefits to the end user with respect to ease of use, conformance, universality, and cost. The goal of this white paper is to analyze the advantages and disadvantages of calibrating a display for medical imaging performed through either a board or within the display. A DISPLAY as referred to herein, is used in medical applications, mainly radiology and mammography, which has to conform to industry standards such as AAPM TG-18, JESRAX or DIN 6868-57. A LUT is used to make appropriate corrections between input and output levels, so that compensation is performed for the display to adequately render an image. Such LUTs may or may not be included in either a BOARD or a DISPLAY. What is a DISPLAY calibration? Display calibration is a process, with the help of a color or grayscale luminance measuring device, which adjusts a display s visual so that images conforms to various industry accepted standards. This standard can have target values for maximum white/black level, color temperature as well as gamma and/or luminance response. Calibration can also include color and/or luminance uniformity of the DISPLAY screen surface. In the case of a medical imaging, the following parameters are minimal: luminance response complying to DICOM GSDF for grayscale and CIE L* for color images stable white level of at least 170 cd/m 2, typically above 200 luminance uniformity of under 15% depending on locale as per DIN 6868-57
How is a DISPLAY calibrated with a BOARD LUT? A software calibration application draws gray and/or color squares on the DISPLAY. A measurement device placed over these squares on the DISPLAY measures color and luminance values for the software. The software analyzes the measured values and creates, from expected and measured data, a correction table. This correction table is generated in a format that can be stored in the BOARD LUT. How is a DISPLAY calibrated with a DISPLAY LUT? The same procedure is performed as for a BOARD LUT, except the correction table is generated in a format that can be stored in the DISPLAY LUT. Why is luminance uniformity calibration necessary? All displays have some form of light generation which emanates through from back to the front onto a DISPLAY surface. Due to the physical and architectural structure of the DISPLAY as a whole, the generated light is nonuniformly distributed, which causes light levels, for both gray and color images, to vary on different areas of the DISPLAY device. Such non-uniform areas would thus display different image data and interpretation is suspect. A DISPLAY which can be corrected, whether internally or by external devices, to reduce luminance uniformity errors will display medical images with greater precision and conformance to medical display standards as developed by AAPM, DIN and JESRA to name a few. Luminance uniformity for a DISPLAY allows image placement in different locations to be within acceptable visual limits such that image interpretation is accurate. DISPLAY luminance uniformity is of greatest concern in mammography, and worldwide, luminance uniformity is becoming adopted in more countries to enhance and accurately perform patient diagnosis. Since a DISPLAY is designed to be sold into many locales, a DISPLAY manufacturer designs for worldwide distribution, and would prefer one design for many versus many design for many. What is the difference between BOARD LUT and DISPLAY LUT? A BOARD LUT and a DISPLAY LUT is a hardware device or set of devices which can be used to store a correction or transformation table such that the image data is adapted before it is seen on the DISPLAY surface (the visible image to the user).
Each LUT can have both a varying number of entries and entry bit depth that can store the correction or transformation table. In general, we see 8 bit (256 entries or depth), 10 bit (1024 entries or depth), 12 bit (4096 entries or depth) and 13 bit (8192 entries or depth) for LUTs. Each entry represents one color and/or gray level. In the case of an 8 bit grayscale DISPLAY, the DISPLAY can only show 256 simultaneous levels of gray. Since a color display has three LUTs of 256 entries, each can store 256 possible bit combinations (8bits per entry) - so we can see that there are still only 256 levels in the Red, Green and Blue channel of each color and still only 256 levels of gray. Each entry stores one output data level. The output data can vary amongst any DISPLAY because in general they can offer 8 bit (256 levels), 10 bit (1024 levels), 12 bit (4096 levels) or 13 bit (8192 levels). On BOARDs, the output data can be as high as 16 bit or 65,535 levels!
What do commercial off-the-shelf boards offer? A Commercial off the shelf (COTS) BOARD has an 8 bit LUT. In our study with the Qubyx PerfectLUM calibration application, we use a Matrox Xenia BOARD which has programmable 8, 10 and 13 Bit onboard hardware LUTs. BOARD LUT Type Input addresses Output values COTS 8 bit 256 65535 Matrox MED 10 bit 1024 65535 Matrox XENIA - 13 Bit 8192 8192 A DISPLAY can have 8, 10, 11.5 and 13 bit LUTs. In general for a DISPLAY, the output value or range is smaller. DISPLAY LUT Type Input / Output Input addresses Output values 8 Bit / 8 Bit 256 256 10 Bit / 10 Bit 1024 1024 10 Bit / 12 Bit 1024 4096 12 Bit / 12 Bit 4096 4096 12 Bit / 13 Bit 4096 8192 What is the influence of a higher number of input addresses? A higher number of input addresses provide more shades in color or gray which can be displayed by the system. The number of levels is also often called "number of simultaneous shades/levels". A higher number represents a higher number of JND s that can also be displayed.
What is the influence of a higher number of output values? A higher number of output values provide more precision in luminance output. As a result, a higher number of output values also provide smaller, more detailed or refined steps. In the case of Matrox Xenia, one step up or one step down (out of 8192 steps) would generate less change in the luminance output of the BOARD than in a system with a BOARD which uses say 8 or 10 bit output. With a higher number of output values available, delicate corrections can be done without any loss of data while guaranteeing full dynamic range. As a result, the luminance output is more precise with the Matrox Xenia Series BOARD and closer to (and in many cases exactly that of) the DICOM GSDF or CIE L* target luminance reproduction. The higher precision improves the quality of the output DISPLAY system. But the relationship between input and output values should be correctly balanced. COTS BOARDS with 256 input values will, even with 65535 output values, not achieve the necessary results because the small number of input addresses already guarantees lost information.
DICOM Step Precision To Achieve Equivalent Target Luminance of 150 cd/m 2 150 140 130 256 1024 8192 8bpp 10bpp 120 Target Luminance. 110 100 90 80 70 60 50 40 30 20 Target Luminance using 13bit Precision Target Luminance using 10bit Precision Target Luminance using 8bit Precision 10 - Digital Driving Levels (DDL) from 0 to 8192 possibllities
DISPLAY versus Matrox Xenia BOARD Calibration Comparison Table Advantage highest quality correction table precision with least JND errors Disadvantage Features only available on Matrox Xenia Series independent from any type of DISPLAY BOARD luminance uniformity adjustment with Matrox Xenia provides highest precision calibration Ability to define various calibration curves for different color spaces for use with Color and/or grayscale displays DISPLAY may be calibrated even if connected to another BOARD under the condition that the BOARD output is the same Smaller LUT size and less output values reduce the correction table range giving less precision and resulting in greater JND errors DISPLAY A pre-calibrated display does not take into account the ambient light where it is installed, thus requiring another calibration Users may forget the need to recalibrate as they are not aware that the display characteristic change over time and location Calibration is specific to the display in the specific location Will drift over time 8 bit BOARD LUT DISPLAY LUT Matrox XENIA LUT Precision DICOM Precision. Number of JND s Always less than 256 256 JNDs with COTS BOARD Up to 1024 JNDs using 13-Bit LUT Memory LUT needs to be loaded after every boot LUT is saved in the display itself, but may need recalibration LUT is saved in the graphic board and no need to load after machine load Luminance/Chrominance Uniformity Adjustment Not available Not available in all DISPLAYs Always available with suitable calibration software like PerfectLUM
Absolute and relative errors in showing gray shade levels for each LUT type Graphic board: 256 -> 65536 Display LUT: 256 -> 1024 XENIA: 8192 -> 8192 Absolute error = 0.5 x (65536 / 256) = 128 = 0.5 x (1024 / 256) = 2 = 0.5 x (8192 / 8192) = 0.5 Relative error = absolute error * 100% / max_shades = 128 * 100% / 65536 = 0.1953125 % = 2 * 100% / 1024 = 0.1953125 % = 0.5 * 100% / 8192 = 0.006103515625 % Matrox XENIA board has only a relative error of 0.006103515625 % due to its high number of LUT input addresses and relationship to LUT output values. Practical Experience Performing a color and DICOM calibration comparison between a commercial off the shelf graphic board and a Matrox Xenia display controller board for medical imaging using a medical display Experiment Set Up: System: Graphicboard: Display: Colorimeter: Calibration Target: Windows XP Professional Matrox XENIA Pro ExteniColor Mode and 13bit LUTS ATI Radeon TM 9550 32 Bit Color and 8 bit Rein EDV Viewmedic MV C219 Datacolor Spyder3 Color temperature CIE D65 and Luminance Response DICOM GSDF The following figures demonstrate the weakness of commercial off the shelf graphic boards and the strengths of the Matrox Xenia display controller board when exploited appropriately.
Calibration Result with ATI Radeon TM 9550 in 32 Bit Color Mode: Figure 1: This result window from PerfectLum shows the Luminance response of the Display before calibration (Rose color), the target in Blue color and the Black line shows the result. The graphs are drawn in Log mode. The calibration result is very good as target and result graph nearly lie over each other.
Figure 2: This window from PerfectLum history database show the LUT correction that was applied on the Graphicboard or display. In this case the correction was applied in the ATI graphicboard with an 8 bit LUT. So 256 addresses and 65535 LUT values
Figure 3: This result window shows the number of JND s (Just Noticeable Differences) per Luminance interval with its mean value of 1.853 and a standard deviation of 0.061. The Software Verilum from Image-Smith is rating a calibration result with a JND deviation lower than 0.1 as Excellent
Figure 4: This result window in PerfectLum shows the so called AAPM graph with the target graph (Black line), the 10% deviation from this target line (Gray lines) and the results (Black points). The 10% deviation is the maximum deviation allowed by the AAPM TG18 standard for primary (Diagnostic) displays. This display has a maximum GSDF error of 8.46% and thus is within the APPM TG18 requirements for a diagnostics display.
Figure 5: This result window in PerfectLum show the Delta E between the target color and reached color after calibration. The target color was D65 with CIE values x=0, 312727 and y=0, 329023. After calibration different driving levels are measured and the CIE Delta E 2001 is calculated between target and result. Results are still excellent for driving levels down to RGB 90/90/90 but then the deviation does increase as the correction needs to be more precise and the LUT with only 256 addresses is too weak for this.
Calibration Result with Matrox XENIA in Extended Color Mode: Figure 6: The display calibration with XENIA gives a perfect result. Target and Result graphs do lie exactly over each other
Figure 7: The 13 Bit LUT with 8192 LUT addresses and 8192 LUT values gives bigger range for correction and a smother correction graph.
Figure 8: The JND standard deviation with XENIA is reduced to 0.032 only!
Figure 9: Maximum GSDF error only 5.77%. All within AAPM primary for diagnostic displays
Figure 10: Average delta E only 0.48! Only a delta E higher than 1 is noticeable by the human eye
Conclusion Advanced display controller boards such as the Matrox Xenia Series with higher precision capabilities that can correct for display luminance and chrominance uniformity errors, and can render the highest quality images, will offer displays a higher color and/or grayscale precision and reduce errors associated with the total number of simultaneously displayable shades/levels of color and/or gray, resulting in overall lower Just Noticeable Differences (JND)errors, higher total number of JND s and lower Delta E required for accurate patient image diagnosis. Acknowledgments & References All nationally and internationally recognized trademarks and trade names are hereby acknowledged. Windows is a registered trademark of Microsoft Corporation Verilum is a registered trademark of Image-Smith Inc. XENIA is a registered trademark of Matrox Inc. Radeon is a registered trademark of ATI Corporation JESRA X-0093-2005 Japan Industries Association of Radiological Systems Standards IEC 61223-3-6 Evaluation and routine testing in medical imaging departments-acceptance Tests-Imaging Display Devices NEMA Digital Imaging and Communications in Medicine (DICOM) Part 14: Grayscale Standard Display Function AAPM TG18 ASSESSMENT OF DISPLAY PERFORMANCE FOR MEDICAL IMAGING SYSTEMS Physique de la couleur, R. Sève (in French) Digitales Farbmanagement, Homann (in German) Optimizing the display function of display devices Authors: Tinglan Ji; Hans Roehrig; Hartwig R. Blume; Jose Guillen