Selecting the best monitor for you
“The physician performing the official interpretation must be responsible for the quality of the images being reviewed and understand the elements of quality control of digital image management systems” [ACR 2007)
In medical imaging, it is important that there be a visual consistency in how a given digital image appears. In the early days of softcopy imaging, images would look differently, depending on the particular display system or workstation where the rendering was done. One important reason for this problem is that electronic displays – irrespective of the technology used - show a variable behavior in converting electric input values to luminance. Clearly, this is unwanted in medical applications where the image and consequently, the diagnosis, needs to be invariable and consistent, independent when, where and on what hardware you choose to render the electronic image.
For these reasons, DICOM Part 14 Grayscale Standard Display Function was developed, according to NEMA procedures. The main objective of the DICOM Grayscale Standard Display Function (or GSDF) is to guarantee device independent representation of images.
Commercial off-the-shelf displays have some of the characteristics that are appropriate for review and diagnosis of medical images, but they lack others, which should be of great concern to the physician providing the interpretation. Standard computer displays offer limited resolution with a form-fit factor (landscape) that is not optimized for diagnostic imaging. DICOM Distribution can provide a variety of Medical Grade Displays offering resolution up to 2048 x 2560 (5 megapixel) in portrait or landscape that corresponds better with the image format of the medical images. Higher resolution allows the physician to see much more detail without panning or zooming the image. As a result, image quality is higher and productivity is increased.
Luminance Range of a monitor is also of great importance. Consumer grade displays typically offer a maximum luminance of 250 – 300 cd/m2. State-of-the-art medical displays by contrast achieve luminance levels of more than 1000 cd/m2, much closer to conventional film. According to DICOM Part 14, a larger luminance range results in a broader spectrum of grayscale that can be discerned by the human eye (also known as Just Noticeable Differences or JND’s). As a result, subtle lesions will be easier to detect on a medical display and physicians can reach a diagnosis faster.
Whether used individually or as part of a multi-monitor viewing system, these Diagnostic Monitors must:
- Maximize the number of grey tones that can be perceived on the display and provide optimal perception of single contrast steps
- Have the ability to be recalibrated to ensure on-going life time conformance
- Provide consistency of image display across all displays
- Meet the requirements for the modality (such as FDA clearance for mammography)
The DICOM GSDF does not specify functions for display of color images, as the specified function is limited to the display of grayscale images. Color Display Systems may be calibrated to the Grayscale Standard Display Function for the purpose of displaying grayscale images.
ACR Technical Standard for Electronic Practice of Medical Imaging
Display device guidelines are currently divided into two basic categories of digital image data set size when used for rendering the official interpretation: small matrix size (e.g., computed tomography [CT], MRI, ultrasound, nuclear medicine, digital fluoroscopy, and digital angiography), and large matrix size (e.g., digital radiography, computed radiography, digitized radiographic films, and digital mammography).
For both small-matrix and large-matrix digital image data, the initial data setshould provide full resolution data for processing, manipulation, and subsequent display. Ideally, the image display’s resolution abilities should be at least as good as those of the acquisition matrix:
Small-matrix: the initial digital image data set should provide a minimum of 256 x 256 matrix size at a minimum 10-bit pixel depth for processing, or manipulation with no loss of matrix size or bit depth at display.
Large-matrix: the digital image data set should be acquired with a minimum of 2.5 lp/mm spatial resolution at a minimum 10-bit pixel depth. An increased spatial resolution of 5 lp/mm at a minimum 10-bit pixel depth should be considered for pediatric imaging or the imaging of small body parts.
The Small-matrix monitors used for primary diagnosis should be capable of providing the following elements of display:
a. Matrix size should be as close to the “for processing” image data as possible, or attainable with magnification.
b. Bit depth should be at least 8 bits (higher dynamic range is advisable with higher luminance values).
c. The total number of images acquired in the study needs to be accessible during interpretation.
d. Clinically relevant technical parameters should be accessible.
The Large Matrix monitors used for primary diagnosis should provide the following elements of display capability:
- A 5 MP (2,048 x 2,560) monitor exceeds the ACR standard of a displayed resolution of at least 2.5 lp/mm when viewing a 14" x 17" image and thus is sufficient for viewing all types of CR/DR images. The FDA recommends that only monitors that have been cleared for digital mammography use by FDA’s Office of Device Evaluation be used for interpreting digital mammograms
- A 2 MP (1,200 x 1,600) or 3 MP (1,535 x 2,048) monitor needs a 2X magnification when viewing 14" x 17" images but no magnification when viewing 8" x 10" image).
- Similarly, a 1K x 1.2K (1,024 x 1,280) will not permit a 10" x 12", 12" x 14", or a 14" x 17" image with at least 2.5 lp/mm resolution without zooming or magnifying the image. The choice of monitor will be determined by many factors. The display requirements of the monitor will largely be determined by the needs of the clinical task. Of primary importance is the ability of the monitor and video card to display the acquired image at full resolution and adequate bit depth. For monitors that cannot display the full field of view of the acquired image, this requires that the entire image data set be stored by panning a region of interest. Note that this process may not be convenient and may impact the productivity of clinicians.
- Conform to the current DICOM grayscale display function standard.
NOTE: Care should be taken to control the lighting in the reading room to eliminate reflections in the monitor and to lower the ambient lighting level as much as is feasible.
Diagnostic Workstations used for official interpretation and for small-matrix and large-matrix systems should be capable of the following:
- Maximum luminance of the gray-scale monitors of at least 50 foot lamberts.
- Selection of image sequence.
- Accurately associating the patient and study demographic characterizations with the images of the study performed.
- Window and level adjustment.
- Pan functions and zoom (magnification) functions capable of meeting guidelines for display of all acquired data.
- Rotating or flipping the images, provided labeling of patient orientation is preserved.
- Calculating and displaying accurate linear measurements and pixel value determinations in values appropriate for the modality (e.g., Hounsfield units for CT images), if those data are available.
- Displaying prior application of irreversible compression ratio, processing, or cropping.
- Displaying the total number of images acquired in the study.
Grayscale Standard Display Function
The American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) formed a joint committee to develop a standard for Digital Imaging and Communications in Medicine (DICOM). While other parts of the DICOM Standard specify how digital image data can be moved from system to system, it does not specify how the pixel values should be interpreted or displayed. DICOM Part 14 Grayscale Standard Display Function (or GSDF) specifies a function that relates pixel values to displayed Luminance levels.
An application that knows this relationship between digital values and display Luminance can produce better visual consistency in how that image appears on diverse display devices. The relationship that the GSDF defines between digital image values and displayed Luminance is based upon measurements and models of human perception over a wide range of Luminance, not upon the characteristics of any one image presentation device or of any one imaging modality. It is also not dependent upon user preferences, which can be more properly handled by other constructs such as the DICOM Presentation Lookup Table (please see our PACS section).
A digital signal from an image can be measured, characterized, transmitted, and reproduced objectively and accurately. However, the visual interpretation of that signal is dependent on the varied characteristics of the systems displaying that image. Currently, images produced by the same signal may have completely different visual appearance, information, and characteristics on different display devices.
DICOM Distribution understands the importance of these requirements. We offer an extensive line of color and monochrome Medical Grade Display monitors from a wide variety of manufacturers, designed for use individually or in a multi-monitor viewing system. DICOM Distribution can provide a system designed specifically for your requirements, within your budget.
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