Display device image quality

Background
When clinicians, such as radiologists, use a display to make a diagnosis, that display should be of appropriate image quality. Off-the-shelf products may not meet the image quality requirements.

There are many factors that determine the quality of a display. For example, displays in diagnostic radiology require high contrast resolution and, depending on the modality of the images being viewed, high spatial resolution. Displays designed for medical purposes should meet these requirements, but other display devices, although apparently similar, may not. However, some clinical applications can use displays intended for general retail or 'professional' use.

Manufacturers and/or vendors indicate the intended purpose of a product and will determine if it is suitable for its desired application.

Please refer to the Annex below for more information on pixel and display sizes

Types of display
Displays within the scope of this document fall into one of two categories: 'diagnostic' and 'review' (they may also be referred to by the alternative names 'primary' and 'secondary' respectively). Diagnostic displays are those used for making a primary diagnosis and/or for the purposes of displaying an image on which a report will be based. Review displays are used to show images for purposes other than making a primary diagnosis and so the requirements of these displays are less stringent than those for primary displays.

Diagnostic imaging
Radiology and radiotherapy imaging workstations - users should be aware of the different specifications of display matrix and video control circuits used in display devices. These devices can be either cathode ray tubes (CRT) displays or liquid crystal displays (LCD). Displays are usually described in terms of the number of pixels available, typically 1 MP (megapixel, or 1 million pixels), 2MP, 3MP or 5 MP. The 5 MP displays are usually used for only the most demanding applications, for example mammography.

Review displays
Displays may be used for image review purposes in a number of locations, including:

• Theatre displays
• Ward displays
• Outpatient clinics
• Video conferencing
• Projectors
• GP surgeries
• PC monitors/laptops

Display viewing environment
Diagnostic displays should be located in areas that have little or no natural daylight. Solid blinds should be fitted to windows if it is not possible to locate diagnostic displays in a room without windows. Dimmer switches should be employed to enable ambient lighting within rooms to be reduced to a low level. Lamps should be located to avoid direct illumination of the screen, and screens themselves should not be positioned such that they are directly facing each other. Consider using lampshades that diffuse and spread the light to prevent bright reflections on the display.

LCDs used for review purposes can operate adequately with some daylight present, but direct light from windows should be avoided. CRT displays offer better contrast resolution than LCD in low ambient light conditions but may be worse in bright rooms.

LCDs do not require such critical lighting requirements for general data and image review display, as long as they are not positioned in direct sunlight.

Users should be aware of the (external link) UK Health and Safety (Display Screen Equipment) Regulations 1992, Statutory Instrument No 2797.

Maintaining display quality
Display quality needs to remain at the specified settings, and should not degrade over time. This includes focus, luminance, symmetry, contrast and colour rendition. Display devices should have restricted user access to controls to prevent brightness and contrast being adjusted by inappropriate personnel.

Displays, whether monochrome or colour, may need colour matching when used in groups. Check colour balance when replacing one faulty display of a matched pair/array.

Displays classed as medical devices (for diagnostic imaging applications) need to be adjusted regularly and maintained by trained staff.

Users should be aware that image quality assurance programmes may require additional overheads, such as the cost of purchasing calibrated luminance meters (if these are required) and the additional maintenance and calibration costs.

Review-quality display devices do not require the same level of image quality assurance as diagnostic displays. However, they still need to be cleaned and the image quality checked; this could be achieved with in-house trained staff. Care should also be given to display monitors used, for example, at the homes of clinicians for remote reporting or review.

Note that LCD screens require the same checks as CRT displays. LCDs use a backlight that can change in brightness, or fail, depending on the amount of time it is switched on. Backlight technology is improving, but users should consider the life expectancy. LCD screens may suffer a unique type of fault: individual pixel failure may occur during the manufacturing process and users should check the LCD for such failures upon delivery.

The quality of all display systems will inevitably deteriorate over time to such a level where it will become necessary to replace them. These costs should be projected in appropriate budgets.

Technical help
For further technical support on this subject, please contact:

Cliff Double
Senior Medical Device Specialist
Tel: 020 7084 3039
E-mail: cliff.double@mhra.gsi.gov.uk

Richard Glover
Principal Medical Device Specialist
Tel: 020 7084 3245
E-mail: richard.glover@mhra.gsi.gov.uk

Jonathan Turner
Principal Evaluator, PACSnet
Tel: 020 8725 2619
E-mail: jonathan.turner@stgeorges.nhs.uk

Annex
Flat-panel LCD matrices

Notes
Windows will support two displays if the graphics card has two sockets.
DVI is the new pc standard for direct digital display connection (UXGA is still analogue).
Ensure that the operating system and the display driver can support the technology.