The Advantages of Medical Diagnostic Displays
Aside from inherent component and manufacturing quality process differences displays for medical applications are markedly superior to those for commercial use. These hallmark requirements are:
- Luminance Stability, stability of brightness over life
Achieved by superior luminance stability circuitry
Assures long term stability for greater diagnostic utility
- Luminance Range, broadness of grayscale range
Higher maximum luminance for better image presentation
Higher contrast ratio
Better grayscale reproduction
- Safety Standards to which a unit is designed and made
There are many aspects to these statements of differentiation. Let’s explore each of them.
All displays age from the moment they are first used. Medical displays
are designed to have greater luminance stability through life. This
insures quality of image presentation and diagnostic yield to the
clinician as well as extended life for these costly devices.
This graph presents the characteristics of Aging vs. Time for medical and commercial displays.
A fundamental concern is that medical image displays must remain stable and constant in brightness and image presentation throughout their useful life. The reproducibility of images from day to day is paramount to the radiologist in clinically evaluating a patient’s condition. Leading to the importance of the ability to calibrate the device to a standard, which is constant throughout the displays life.
This graph presents the propensity of displays to drift as a function of time after powering on and as a result of component heat up as they rise to operating temperature.
To achieve the required performance constancy every component must operate at the top of the specification curve everyday the system is used. This holds for the LCD panel all the way down to the sensors in the unit that measure and adjust light output to present the very best image possible. There can be no weak links in this chain particularly in the realm of 5 MP units where FDA governance enters into standard setting. Only special LCD panels are used for medical applications. Often panels that fail the QA for medical qualification are used in higher end commercial products with specialized applications such as graphics or government mapping processes.
Design specifications in actual use translate in to JND, Just-noticeable difference, that is how well they perform in their use environment. A comparison of JND’s for commercial vs. medical displays is shown below. Note the superior performance difference between medical and commercial grades.
Aside from the total quantities produced, that cause commercial units to be lower in cost, performance superiority and features equate to market price in medical displays. The quality differences are measurable.
Medical displays must measure up to exacting safety standards set by governing bodies in the industry. Use environment, proximity to patients and FDA review and acceptance are a noted few of these standards. Note the various regulatory influences driving design and performance of medical displays.
From the moment an LCD display is manufactured it begins to degrade along with other performance aspects all contributing to accuracy in image presentation. Over a display’s life brightness wanes, stability can shift, image accuracy can drift all contributing to degraded image quality and clinical image rendering. The rigorous standards to which medical displays are designed and built slow the aging process and attendant unfavorable changes in performance.
Other specialized devices and features are available for medical monitors that record and correct changes from the aging process. These measures insure accurate presentation of image data and track the progression of change so that they are stored for observation over the term of life of the device. Calibration to DICOM standards is of the utmost importance to sustain image quality as images are presented, shipped to various PACS systems for review or to remote reading areas via teleradiology. A comprehensive review of DICOM and standards information is located at the DICOM official website http://dicom.nema.org
The data following presents the effect of a calibration system to render gamma correction to a DICOM standard. A marked result is affected by the use of the calibration system.
DICOM compliance is essential to accurate image presentation and clinical interpretation. As seen below in the image comparison a 5% difference in the DICOM compliant image and the non-DICOM grayscale image makes a significant diagnostic difference.
These highly specialized calibration system and software are required to insure performance throughout a usable life of 3 to 5 years. Many of these devices are built into the LCD glass itself, others are fixtures that assess the screen of the LCD and compute necessary adjustment through complex weighted mathematical processes deemed to support the requirement. Typically, commercial LCD displays do not have these features and commonly are adjusted for suitable performance by the user subjectively to satisfy the use pattern.
Calibration systems can function locally or remotely. They observe and control the following parameters.
- Alarm at Abnormal Circumstances
- Record Calibration History
- Present and Allow Adjustment of Desired Calibration Settings
In multi head medical LCD applications or applications with one monitor of the same type where a mobile radiologist reads scans at various locations from the same modality like performance is important to clinical quality and accurate patient diagnosis. Switching between LCD displays, at a single reading station or from room to room or site to site making on the fly visual adjustments can be challenging and compromise interpretation unless all of these units reliably present information uniformly. Therefore, there is a need for performance matching in all aspects of measurement. Only the tightest specification standards can deliver this characteristic. Commercial displays are well outside of this category.
Dead pixels can contribute to misdiagnosis though blocked pathology, anomalies and guised disease processes. Commercial LCD displays because of their intended use commonly pass QA tests and are approved for shipment with an acceptable number of bad or dead pixels. Medical displays have a much higher standard for acceptance for this measurement parameter and often have internal algorithms, which measure the performance of each and every pixel. When questionable or dead pixels are discovered sophisticated interpolation adjustments are affected to assure uniform performance and maximal quality of information displayed.
A common form of in use pixel damage is the result of taping the LCD displays injuring the array of cells receiving the blow. While commercial displays rarely have any protection over the face of the LCD panel allowing the blow to be fully absorbed and resultant damage incurred, medical LCD panels have robust cover materials that diffuse the blow across a larger surface area absorbing the shock. As an attendant benefit the cover is also made of materials that enhance image visualization and reduce eyestrain. The later feature directly impacts the quality of the image being displayed and the reader’s ability to integrate the field of view of the image.
Since the mid 1960’s when stringent standards came into being for electrical devices used in hospitals protection of user and patient regulations has progressed at a constant, progress rate. When first set forth these rules focused on devices that present the opportunity to bring harm to a patient to which they were attached or with which they may come in contact. Today these well-defined, highly regulated and monitored standards are present in all areas of healthcare. LCD displays and their components, internal and external, are affected by these rules requiring adherence to additional performance standards, ongoing biomedical and performance monitoring and associated cost.
In every regard, LCD’s are a marked improvement over CRT’s, in their performance, their associate risk and extended longevity. LCD displays present new options unafforded by conventional technology such as multi format, multi modality image/administrative display, lower power consumption, heat produced, greater resolution, risk of exposure to higher operating voltages and current. Just as it is un fare to compare today’s high performance LCD’s to conventional CRT’s it is also un fare to compare LCD displays manufactured for medical applications over those intended for the commercial environment. Medical LCD’s remain in a special performance class all their own meeting a very unique and specialized purpose which calls for a remarkably different device that that used in general commercial applications.
In Summary medical displays are:
- Compliant to highly rigorous medical standards
- Incorporate very sophisticated QA control systems and software
- DICOM Compliant and calibrated
These features make medical displays ideally suited for the demands of their use surrounding and superior to those displays designed and manufactured for commercial use.
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