Radiology Understanding Display Characteristics What You Should Know When Viewing Images from PACS

Radiology Disclosures Unfortunately no one is paying me to hawk their products Unfortunately no one is paying me to hawk their products Gratefully acknowledge the use of slides from OTech and Barco Gratefully acknowledge the use of slides from OTech and Barco

Radiology Terms AAPM TG-18 American Association of Physicists in Medicine Task Group 18 AAPM TG-18 American Association of Physicists in Medicine Task Group 18 COTS Common Off the Shelf COTS Common Off the Shelf CRT Cathode Ray Tube CRT Cathode Ray Tube LCD Liquid Crystal Display LCD Liquid Crystal Display DVI Digital Video Interface DVI Digital Video Interface DICOM Digital Imaging and Communications in Medicine DICOM Digital Imaging and Communications in Medicine GSDF Gray Scale Display Function GSDF Gray Scale Display Function

Radiology Preface We can see radiological images on any pc We can see radiological images on any pc So why don’t we just use our desktop pcs (is a display just a display?) So why don’t we just use our desktop pcs (is a display just a display?) We need to understand the characteristics that differentiate displays We need to understand the characteristics that differentiate displays We need to use that understanding to properly deploy image viewing workstations We need to use that understanding to properly deploy image viewing workstations

Radiology Image Display Images currently can be viewed on a CRT (cathode ray tube) or LCD (liquid crystal display) monitors, or film can be viewed on a lightbox Images currently can be viewed on a CRT (cathode ray tube) or LCD (liquid crystal display) monitors, or film can be viewed on a lightbox The various modalities impose specific requirements on workstations The various modalities impose specific requirements on workstations –Nuclear Medicine and Ultrasound require color displays, but lower resolution –CT/MRI depend more on contrast (bit depth) –CR/DR require high spatial resolution

Radiology CRT Display CRT CRT –Can display a true black –Industry moving to LCD because this is what customers want (not necessarily need) Sexy Sexy Take up less room Take up less room Produce less heat Produce less heat Ability to remote monitor Ability to remote monitor GE already sent out ‘end of service’ on CRTs GE already sent out ‘end of service’ on CRTs

Radiology CRT Display Our current CRTs require specialized video cards and cables which are different from standard digital pc video displays today Our current CRTs require specialized video cards and cables which are different from standard digital pc video displays today These cards are becoming very difficult to find These cards are becoming very difficult to find

Radiology

Radiology So Let’s Move on

Radiology LCD Display LCD LCD –High even luminance and no positional drift –Long lifetime –Small pixel sizes –Reduced reflection and sensitivity to ambient light –Off-angle viewing is an issue –No true black (look at LCD in dark room)

Radiology LCD Monitors Color monitors 1024x768 (15”), 1280x1024 (17,18,19”) or 1600x1200 (20,21”) Color monitors 1024x768 (15”), 1280x1024 (17,18,19”) or 1600x1200 (20,21”) Gray scale monitors typically 1280x1024 (1Megapixal), 1600x1200 (2Mp clinical) or 2k x 1.5k (3Mp diagnostic) [Mammo 4kx4k CRT] Gray scale monitors typically 1280x1024 (1Megapixal), 1600x1200 (2Mp clinical) or 2k x 1.5k (3Mp diagnostic) [Mammo 4kx4k CRT] When use portrait (up and down) display, can represent typical CR/DR at almost full resolution on 3 Megapixels When use portrait (up and down) display, can represent typical CR/DR at almost full resolution on 3 Megapixels On 2Mp monitors, only portions of the image can be displayed at maximal resolution, and have to ‘pan’ to view areas of the image On 2Mp monitors, only portions of the image can be displayed at maximal resolution, and have to ‘pan’ to view areas of the image

Radiology Video Cards Medical grade cards that can be properly calibrated and monitored Medical grade cards that can be properly calibrated and monitored COTS cards that for the most part do not provide an ability to calibrate them COTS cards that for the most part do not provide an ability to calibrate them Always use a DVI card when using digital (LCD) monitors. Analog signals do not translate accurately to a digital format. Always use a DVI card when using digital (LCD) monitors. Analog signals do not translate accurately to a digital format.

Radiology Major Concerns Given that we will be moving to LCD displays, there are two major concerns Given that we will be moving to LCD displays, there are two major concerns –Luminance and the ability to calibrate it –Stability of the display Consistent voltage to lamp Consistent voltage to lamp No changes over time as monitor warms up No changes over time as monitor warms up Ability to monitor changes and calibrate as needed Ability to monitor changes and calibrate as needed

Radiology Luminance is the quantity describing the brightness of a monitor (from black to white) Luminance is the quantity describing the brightness of a monitor (from black to white) ACR standards specify a luminance of at least 160 candela per square meter (cd/m 2 ), or 47 fL (foot Lamberts), for diagnostic reading ACR standards specify a luminance of at least 160 candela per square meter (cd/m 2 ), or 47 fL (foot Lamberts), for diagnostic reading Gray scale monitors provide higher luminance than color monitors Gray scale monitors provide higher luminance than color monitors Luminance affects both contrast resolution and spatial resolution (very important) Luminance affects both contrast resolution and spatial resolution (very important) High-performance monitors are still less bright than a view box by a factor of 5 to 10, and windowing and leveling are needed to compensate High-performance monitors are still less bright than a view box by a factor of 5 to 10, and windowing and leveling are needed to compensate

Radiology Luminance Lightbox/Alternator fL Lightbox/Alternator fL PC color monitor (CRT)20-40fL PC color monitor (CRT)20-40fL Medical imaging monitors Medical imaging monitors –ACR recommended minimum47fL –Low bright50-60fL –LSU Clinical Barco CRT65fL –NEC Color 15”, 18”, 20” LCD71fL –LSU Diagnostic Barco CRT85fL –High bright100fL or higher

Radiology LCD Display Examples

Radiology Color vs Monochrome Color monitors have decreased luminance, hence decreased contrast and spatial resolution Color monitors have decreased luminance, hence decreased contrast and spatial resolution Color becoming more useful (US, NucMed, 3D reconstructs etc.) Color becoming more useful (US, NucMed, 3D reconstructs etc.) Human eye has greater dynamic range (JND) color (500) vs grayscale ( ) Human eye has greater dynamic range (JND) color (500) vs grayscale ( ) But has decreased spatial resolution in the color spectrum But has decreased spatial resolution in the color spectrum Most displays will be in the grayscale spectrum of the color video/monitor system (bit depth) Most displays will be in the grayscale spectrum of the color video/monitor system (bit depth) Grayscale monitors do provide higher contrast capability Grayscale monitors do provide higher contrast capability

Radiology Stability The backlight is an important source of instabilities The backlight is an important source of instabilities –Temperature dependent –Time dependent COTS monitor luminance COTS monitor luminance –drops drastically over the first year of use (and contrast and spatial resolution drops with it) –Doesn’t stabilize for up to an hour or more when first turned on

Radiology

Radiology Temperature Dependance

Radiology Time Dependance

Radiology Monitor Calibration Brightness and contrast controls usually hidden, once set during calibration they should not be changed Brightness and contrast controls usually hidden, once set during calibration they should not be changed Manual or automated monitoring Manual or automated monitoring DICOM GSDF DICOM GSDF

Radiology Monitor Calibration DICOM Part 14 Grayscale Display Function (GSDF) DICOM Part 14 Grayscale Display Function (GSDF) –Use of LUT’s (look up tables) for monitors allows calibration to provide consistent image display on different monitors –Enables images to look the same regardless of the display or printer by calibrating JND points to specific luminance output

Radiology GSDF Graph

Radiology Calibrating to the GSDF Curve

Radiology Window Width and Level This is not the same as contrast and brightness (monitor controls for this) This is not the same as contrast and brightness (monitor controls for this) Actually displaying a subset of the available dynamic range of data Actually displaying a subset of the available dynamic range of data Take a CT image represented in 12 bits – this provides 4,096 different values Take a CT image represented in 12 bits – this provides 4,096 different values We can only discern around 200 values, or just noticable differences We can only discern around 200 values, or just noticable differences

Radiology

Radiology AAPM TG-18 Tests Document ‘Assessment of Display Performance for Medical Imaging Systems’ Document ‘Assessment of Display Performance for Medical Imaging Systems’ Not a standard but a documented system for monitoring and tracking displays Not a standard but a documented system for monitoring and tracking displays Defined test pattern images to enable assessment of display abnormalities Defined test pattern images to enable assessment of display abnormalities SMPTE test pattern (next slide) SMPTE test pattern (next slide) The 95/100% box is often not discernible on flat panel monitors due to lack of true black The 95/100% box is often not discernible on flat panel monitors due to lack of true black

Radiology SMPTE Test Pattern To properly display a test pattern on your monitor, go to /ami and click on ‘Test’ /ami

Radiology AAPM The test patterns used by the methodology in the TG-18 document are stored under the user ‘AAPM, TEST PATTERNS’ The test patterns used by the methodology in the TG-18 document are stored under the user ‘AAPM, TEST PATTERNS’ Document can be found at the AAPM web site, Document can be found at the AAPM web site,

Radiology Perfect World If you have a need to view an image If you have a need to view an image You have a need to view it on a workstation that is subject to quality control You have a need to view it on a workstation that is subject to quality control

Radiology Realities Need to calibrate and monitor the display systems that allow us to do that (diagnostic and clinical) Need to calibrate and monitor the display systems that allow us to do that (diagnostic and clinical) Impossible to provide this for every pc that someone may view an image on Impossible to provide this for every pc that someone may view an image on COTS pcs can be valid for viewing as long as there is a diagnostic report available COTS pcs can be valid for viewing as long as there is a diagnostic report available If there is no report, contact a Radiologist before making a clinical decision (a good thing to do in any case) If there is no report, contact a Radiologist before making a clinical decision (a good thing to do in any case)

Radiology Future Path Move to LCD displays Move to LCD displays Move manual ‘at the display’ calibration every 1-2 months to remote monitoring Move manual ‘at the display’ calibration every 1-2 months to remote monitoring Provide quality control at least at our diagnostic and clinical display stations Provide quality control at least at our diagnostic and clinical display stations Understand the limitations of COTS display stations we use at the institution Understand the limitations of COTS display stations we use at the institution