Universal calibration protocol heightens image quality in soft copy

A consistent approach to calibrating the many monitors used throughout a hospital for image viewing can lead to more efficient image review and help to weed out monitors that may need to be replaced, according to researchers in Canada.

"We use a systematic approach that works on practically any type or brand of monitor and computer," said Emily Seto, a biomedical engineer with the University Health Network in Ontario.

The study describes a method to provide image quality improvement by using an inexpensive calibration protocol (J Digit Imaging 2005; [Epub ahead of print]). The tool attempts to maximize the perceived image quality, while prolonging the useful lifespan of the monitors, Seto said. It also detects monitors that are no longer suitable for medical image review and should therefore be replaced.

The protocol involves five steps:

• cleaning the monitor

• setting the appropriate minimum luminance (dark level)

• setting the appropriate maximum luminance

• setting the display system to the DICOM 14 gray-scale standard display function (GSDF) to compensate for the nonlinearity of the human ocular motor response

• performing visual checks

To set the GSDF, bit values representing different gray-scale levels are mapped to specific luminance values. This mapping is stored in look-up tables (LUTs) on the graphics card. The graphics card should have the same number of LUTs as the number of monitors it is supporting, Seto said.

"The only required equipment is an affordable commercial monitor calibration toolkit, which is used to perform the measurements and to set the GSDF," she said.

Seto used VeriLUM from Image-Smiths of Germantown, MD.

The steps of the protocol are relatively straightforward and quick, according to Seto. It is not difficult to train IT staff to perform the calibration.

Monitor calibration becomes important because many soft-copy display systems, unlike traditional film, do not have well-established image quality assurance processes.

"Image quality assurance of soft-copy display systems is often neglected because it is perceived as a complicated and costly process," Seto said.

With PACS, and especially with Web-based review applications, standard desktop computers scattered around the hospital are now being used for image review, making image quality control even more difficult.

Currently, image quality assurance focuses mainly on expensive high-end display systems, which often have proprietary calibration methods.

"However, it is usually not feasible to standardize on a particular brand of expensive high-end hardware or to have multiple calibration techniques dependent on the brand," Seto said.

Standard desktops used for viewing medical images are rarely calibrated, she said.