Guidance for Quality Assurance of PACS Diagnostic Display Devices

Transcription

1 Guidance for Quality Assurance of PACS Diagnostic Display Programme NPFIT DOCUMENT RECORD ID KEY Sub-Prog / PACS Project NPFIT-PAC-DES Prog. Director Max Jones Owner Jerry Norman Version 1.0 Author Peter Simpson Version Date 03/08/06 Status Approved Guidance for Quality Assurance of PACS Diagnostic Display Crown Copyright 2006 Page 1 of 15

2 Amendment History: Version Date Amendment History /08/05 First draft for comment /09/05 Authority and PACSnet comments. This version for approval /10/05 Redrafting of Purpose section to indicate how this document is expected to be utilised /03/06 Feedback from PACSnet and RCR /05/06 Update for pixel defects Forecast Changes: Anticipated Change None. When Reviewers: This document must be reviewed by the following. Indicate any delegation for sign off. Name Signature Title / Responsibility Date Version NHS CFH PACS Central Technical Team PACS National Clinical Reference Panel (NCRP) NHS CFH Cluster PACS CTA PACS National Implementatio n Reference Group (NIRG) Approvals: This document requires the following approvals: Name Signature Title / Responsibility Date Version Jerry Norman PACS Technical Team Lead Max Jones PACS Programme Crown Copyright 2006 Page 2 of 15

3 PACS National Clinical Reference Panel Director Distribution: As Reviewers and Approvals above plus:- PACS Cluster CTA s LSP PACS Crown Copyright 2006 Page 3 of 15

4 Document Status: This is a controlled document. This document version is only valid at the time it is retrieved from controlled filestore, after which a new approved version will replace it. On receipt of a new issue, please destroy all previous issues (unless a specified earlier issue is baselined for use throughout the programme). Related Documents: These documents will provide additional information. Ref no Doc Reference Number Title Version 1 NPFIT-SHR-QMS-PRP-0015 Glossary of Terms Consolidated.doc 11 See References, Section 5. Glossary of Terms: List any new terms created in this document. Mail the NPO Quality Manager to have these included in the master glossary above [1]. Term Acronym Definition Crown Copyright 2006 Page 4 of 15

5 Contents 1. Purpose Scope Introduction Working Environment Rooms Staffing requirements for display QA Room illumination and arrangement of displays Displays General display condition Image quality checks Contrast ratio Geometry Distance measurement Angle measurement Spatial resolution Contrast resolution Luminance uniformity Veiling Glare Reflectivity Maximum luminance DICOM GSDF conformance Noise Crosstalk Focus References ACR-NEMA DICOM Standard (PS 3) DIN V ISO 9241 and VESA Flat Panel Display Measurements (FPDM) Standard The American Association of Physicists in Medicine (AAPM) The Institute of Physics in Engineering (IPEM) The Royal College of Radiologists (RCR) Appendix A QA Specification and Testing Appendix B Additional Tests Crown Copyright 2006 Page 5 of 15

6 1. Purpose The purpose of this document is to provide outline guidance to NHS Connecting for Health (CfH) Cluster PACS Programmes (and their associated clinical reference groups) to enable the development and implementation of locally defined procedures for the Quality Assurance (QA) of PACS display devices. This document is not intended to be a statement of requirements for the LSP PACS to comply with. Each LSP PACS is responsible for delivering appropriate quality control measures as detailed in the Authority s Requirements, for example, requirement detailed below The Contractor shall ensure that all the following display devices shall have a facility for regular quality control and shall be able to be calibrated to DICOM grey scale display function: mammography reporting; orthopaedic reporting; cardiology reporting; radiology reporting; mammography review; orthopaedic review; cardiology review; and Accident & Emergency review Scope Displays Displays devices may be classified as Primary or Secondary. (They may also be referred to by the alternative names Diagnostic or Review respectively). Primary display devices are used for the clinical interpretation of medical images. Secondary display devices are used for review of medical images, usually in conjunction with the report. This document deals with the QA of primary diagnostic display devices used for clinical image interpretation. Where images are reviewed without a requirement for clinical interpretation, the image quality is considered to be of secondary importance. The quality of display in these circumstances should be considered locally, depending on the purpose of the review. 2. Introduction The QA of displays is essential to maintain both the performance of the display and the consistent presentation of images throughout an imaging department and beyond. Crown Copyright 2006 Page 6 of 15

7 Manufacturers of many primary class medical displays supply tools to support QA, principally for luminance and greyscale calibration. However there is no recommendation as to the frequency of these checks, only that they should be performed regularly. Many displays include luminance probes built in to the displays which continuously monitor and maintain the display s luminance and can be used to predict the lifetime of the backlight. Other performance parameters are less easily automated and rely on a degree of user intervention or on examination by appropriately qualified personnel. These tests are detailed below. The first section deals with the general working environment in which the displays are used; subsequent sections are concerned with the various checks that are to be performed on the displays themselves. Finally, there is a list of standards and other documentation that may be of assistance in running a QA programme. Recommended QA specifications and testing frequencies are listed in the appendix at the end of this document. 3. Working Environment 3.1. Rooms The lighting conditions of the areas used for the displays, and the locations of the displays within these areas, should be inspected Staffing requirements for display QA General daily QA checks that require viewing of the screen without use of specialist equipment are best done by those that routinely use the displays, i.e. radiographers, radiologists and other clinicians. The more in-depth checks that do require use of specialist equipment are best done by technical support staff, who should also be responsible for maintaining records of the QA checks Room illumination and arrangement of displays Check general room lighting conditions and arrangement of displays, including window blinds, room lighting, positioning of displays, adjustability of display positioning, desk height, chairs. Room illumination should be checked every 6 months Window Blinds Window blinds should allow for the complete blocking of external illumination (primarily daylight) Room Lighting There should be no direct lighting falling on the displays during their normal use. It may be necessary to have extra lighting available when the room is in use for purposes other than use of the displays (e.g. cleaning, meetings). Dimmer switches should be used to enable the overhead lighting to be lowered to an appropriate level. Supplementary task lighting, e.g. desk lamp, should be considered to allow reading of associated papers (e.g. request forms), though care must be taken with their positioning and luminance. Room lighting levels can be measured using an illuminance meter. Crown Copyright 2006 Page 7 of 15

8 Display Positioning Displays should be positioned such that any reflections are minimised, i.e. they should be side-on to any sources of illumination such as doors and should not be positioned directly opposite each other. Adjustability of display positioning, desk height and chairs: It should be possible for users to adjust the equipment so that it can be used comfortably and safely. 4. Displays Appendix A details a QA testing list together with the suggested testing frequency for diagnostic and review displays. These tests are further described in this section. The Appendix also notes whether tests should be performed as part of acceptance testing, and suggests action limits for the results of the tests General display condition Check the cleanliness and general condition of the display, keyboard and any associated wiring and base computer. Note: cleaning materials used for displays 4.2. Image quality checks Checks on image quality can be divided into several areas, listed below. Note that the listed equipment required is in addition to appropriate, contractor supplied, test images, which will be required for all tests. Note: In the interest of consistency, a standard set of images (e.g. SMPTE, AAPM, etc.) should be agreed across all Clusters Contrast ratio This is a measurement of the difference in luminance between the brightest areas of a display screen and the darkest areas. Equipment required: luminance meter Geometry A test to check that the geometry of the display is acceptable, i.e. to check that the straight lines are straight, any circles are circular and any square are square. Equipment required: ruler 4.5. Distance measurement A test to check that any distance measurements read from the screen are correct. This may also involve checking that measurements displayed by workstation software are correct Angle measurement A test to check that any distance measurements read from the screen are correct. This may also involve checking that measurements displayed by workstation software are correct. Crown Copyright 2006 Page 8 of 15

9 4.7. Spatial resolution Spatial resolution is a measure of the capability of the display to present information on two closely-spaced objects in the image such that the two objects can be discriminated. Spatial resolution is usually expressed as the number of black and white lines within one millimetre (lp/mm) that can be distinguished Contrast resolution Contrast resolution is a measure of the ability of the display to present areas of different image intensity such that there is a difference in luminance when displayed (i.e. it is the resolution of different grey levels). Equipment required: luminance meter 4.9. Luminance uniformity Displays should demonstrate equal luminance across the display screen and between all display screens attached to the same workstation. Equipment required: luminance meter Veiling Glare Veiling glare is a measure of how much a dark area in the image is brightened by surrounding bright areas. The effect of veiling glare is to reduce the contrast. In practice, this test is only meaningful for CRT displays. Equipment required: Luminance meter, screen mask (i.e. a mask used to cover the display screen apart from the area used to make the measurements required for this test. Alternatively, a funnel or cone may be used) Reflectivity Display screens should be manufactured so that there is a minimum of reflection from the screen. Reflections can prove distracting to the observer and can also serve to reduce the contrast resolution. Equipment required: luminance meter Maximum luminance It is important to measure the maximum luminance of a display regularly. A decline in maximum luminance leads to a commensurate decline in contrast resolution. Maximum luminance measurements can also act as a useful predictor for the lifetime of the backlight in liquid crystal displays. Equipment required: luminance meter DICOM GSDF conformance Part 14 of the DICOM standard describes a Grayscale Standard Display Function, which aims to ensure that images displayed on different devices have a similar appearance, provided that the devices are calibrated to the DICOM GSDF. Calibration to the GSDF should be checked regularly. Equipment required: luminance meter. Crown Copyright 2006 Page 9 of 15

10 4.14. Noise Noise in a displayed image is the random variation of luminance over the image. Noise in an image can potentially obscure diagnostic information and can also increase the fatigue of the user of the display. A visual impression of the noise in an image should be made Crosstalk Crosstalk is the unwanted leakage of electronic signal from one channel to an adjacent channel. In liquid crystal display systems this can lead to the intended luminance of a pixel (i.e. the intended voltage across the liquid crystal cell that forms the pixel) being modified away from the intended value. A visual impression of the presence of crosstalk in an image should be made. Note: Use of SMPTE test image is acceptable, as is the use of TG18-QC or indeed any image with appropriate bar patterns Focus A visual impression of the focus of CRT displays should be made and adjustments made when necessary. Crown Copyright 2006 Page 10 of 15

11 5. References The following references have informed the guidance contained in this document ACR-NEMA DICOM Standard (PS 3) In 1984, the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) formed a committee that produced and currently maintains the Digital Imaging and Communications in Medicine (DICOM) standard. The committee produced a document, Grayscale Standard Display Function (NEMA 2000), which specified a standardized display function known as the Grayscale Standard Display Function (GSDF) for grayscale images. The intent of the standard was to allow images transferred using the DICOM standard to be displayed on any DICOMcompatible display device with a consistent grayscale appearance DIN V The German standards institution, Deutsches Institut für Normung e.v. (DIN), standard 6868 part 57, Image Quality Assurance in X-Ray Diagnostics, Acceptance Testing for Image Display (DIN 2001) was developed as an acceptance testing standard addressing the requirements for display systems. The standard recognizes the use of DICOM GSDF ISO 9241 and The International Standards Organization (ISO) standard, ISO :1992, Ergonomic Requirements for Office Work with Visual Display Terminals (VDTs) Part 3: Visual Display Requirements (ISO 1992), gave test methods and performance requirements primarily for displays used for text based terminals. ISO :2001, Ergonomic Requirements for Work with Visual Displays Based on Flat Panels. Part 2: Ergonomic Requirements for Flat Panel Displays (ISO 2001) VESA Flat Panel Display Measurements (FPDM) Standard This standard was introduced to specify measurement methods and procedures for both CRT and flat panel display devices. However this does not recommend performance or compliance criteria for display applications The American Association of Physicists in Medicine (AAPM) The American Association of Physicists in Medicine (AAPM) has produced probably the most thorough document on the assessment of medical displays, Assessment of display performance for medical imaging systems. This describes methods and tools for the assessment of displays and suggests regimes for acceptance testing and for daily, monthly/quarterly and annual quality control tests. Crown Copyright 2006 Page 11 of 15

12 5.6. The Institute of Physics in Engineering (IPEM) In the UK the Institute of Physics in Engineering (IPEM) has recently published Report 91 Recommended Standards for the Routine Performance Testing of Diagnostic X-Ray Systems. Report 91 replaces IPEM report 77 and provides essential guidance for anyone responsible for diagnostic X-Ray equipment. This includes a section on quality assurance of diagnostic displays The Royal College of Radiologists (RCR) The UK Royal College of Radiologists (RCR) PACS & Teleradiology special interest group has produced guidelines on the specification and quality assurance of primary diagnostic display devices. This guidance includes a minimum and recommended screen display size and resolution for diagnostic image interpretation, in conjunction with best practice guidance on using diagnostic workstations. Crown Copyright 2006 Page 12 of 15

13 6. Appendix A QA Specification and Testing Table 1 Summary of Image Display Device Guidance from IPEM 2005 Recommended Standards for Routine Performance Testing of Diagnostic X-ray Imaging Systems, IPEM Report 91 Physical parameter Frequency Remedial level Image display monitor condition Greyscale contrast luminance ratio Distance and Angle Calibration Daily to weekly Image display devices should be clean, and the perceived contrast of the test pattern should be consistent between display devices connected to the same workstation. Verify that the 5% and 95% patches are visible. 3 monthly Ratio white to black (maximum to minimum luminance) < monthly ± 5 mm ± 3 Spatial Resolution 3 monthly Grade AAPM TG18-QC resolution patterns according to the reference score (CX > 4) Greyscale drift 6 to 12 monthly Black baseline ± 25% White baseline ± 20% DICOM greyscale calibration Luminance Uniformity Variation between monitors 6 to 12 monthly GSDF ± 10% 6 to 12 monthly U% > 30% 6 to 12 monthly Black baseline > 30% White baseline > 30% Room illumination 6 to 12 monthly > 15 lux Crown Copyright 2006 Page 13 of 15

14 Table 2 Summary of the RCR PACS & Teleradiology special interest group minimum and recommended specification and QA acceptance testing for primary diagnostic display devices used for clinical image interpretation. This guidance applies to all workstations where CR, DR, fluoroscopy, ultrasound, CT, MR, nuclear medicine and PET images are viewed (excluding mammography). Minimum [1] Recommended [1] Screen Resolution [2] (Native Pixel Array) Screen Size (Viewable Diagonal) [5] Maximum Luminance >= 1280 x 1024 [3] (~ 1.3 megapixels) >= 1500 x 2000 [4] (~ 3 megapixels) >= 42 cm (~17 ) >= 50 cm (~ 20 ) > 170 cd/m2 [6] >= 500 cd/m2 [7] Luminance Contrast Ratio >= 250:1 [6] [8] >= 500:1 (Maximum/Minimum) Grey-scale Calibration Within 10% GSDF [8] Calibrated to GSDF [5] >= 10 bit grey-scale Grey-scale bit depth 8 bit grey-scale (24 bit colour) [9] Video Display Interface Digital-analogue Digital video interface (DVI) Pixel Defects ISO classification [10] Class 2 (2 per million) Class 1 (0 defects) Notes [1] The minimum and recommended specifications for diagnostic display devices are only appropriate if clinical image viewing is performed according to image viewing guidelines. All diagnostic image interpretation should be performed on DICOM images making use of the application software zoom, pan, magnification, and windowing tools to optimise spatial and contrast resolution. [2] LCD devices should be run at their native resolution to ensure there is a 1:1 match between screen pixels and screen resolution, and therefore no loss of image quality due to screen interpolation. CRT displays can be run at a variety of resolutions with no loss of display quality; however care should be taken that the correct aspect ratio is maintained to avoid distortion of the image. [3] Where the majority of reporting performed on a diagnostic workstation is of cross-sectional imaging, lower resolution landscape style displays (>= 1.3 megapixels) are considered adequate, providing larger images are interpreted with the aid of systematic magnification. [4] High fidelity (>= 3 megapixels) portrait style displays are recommended in radiology and other areas where large numbers of plain radiographic images are reported to reduce requirements for systematic magnification, and thus reduce image interpretation and reporting times. [5] Display devices may be set initially to operate at a fraction of the maximum luminance in the manufacturer s specification. This can be adjusted to compensate for the decline in performance of the back-light over time whist maintaining grey-scale calibration. [6] AAPM TG18 recommendation [7] High luminance displays can increase the number of perceivable grey-scale levels (JND index steps) but may have a detrimental effect in user performance through fatigue and the Crown Copyright 2006 Page 14 of 15

15 human visual adaptation response. The optimum operating luminance level may vary between users. [8] IPEM 91 recommendation [9] 24-bit and 32-bit colour are equivalent to 8-bit monochrome grey-scale. Colour display devices are recommended for displaying colour images, but they generally perform less well than monochrome display devices in terms of maximum luminance and contrast ratio. [10] The number of permissible pixel defects per million is defined by the ISO standard. Class 1 panels should have no defects. Class 2 panels should be replaced if they have > 2 whole pixel defects per million. Appropriate use of application software zoom, pan and magnification tools can negate the effect of pixel defects in clinical practice. 7. Appendix B Additional Tests In addition to the tests listed in Appendix A, the following tests are recommended by PACSnet as being important parts of a QA programme. Quarterly tests: - Room inspection Check that window blinds, dimmer switches and lighting are in place and working appropriately. - Noise Check that the noise level in the displayed image is acceptable by using the test image and test method supplied in the AAPM Assessment of Display Performance for Medical Imaging Systems. (ref:1.25 AAPM) - Geometry of displayed image Check that straight lines in a test image are displayed correctly (ref: 1.25, AAPM) - Crosstalk and other video image artefacts Use AAPM TG18-QC, SMPTE or other similar test image to check that levels of crosstalk and other artefacts are acceptable (ref: 1.25, AAPM) Acceptance testing: - Veiling glare A new CRT display (or a single display from a set bought at the same time) should be tested for veiling glare (ref: 1.25, AAPM) - Reflectivity A new display (or a single display from a set bought at the same time) should be tested for reflectivity (ref: 1.25, AAPM) - Variation of luminance and contrast ratio with viewing angle A new display (or a single display from a set bought at the same time) should be tested for variation of luminance and contrast ratio with viewing angle (ref: 1.25, AAPM) Crown Copyright 2006 Page 15 of 15