Essentials of Digital Imaging

Transcription

1 Essentials of Digital Imaging Module 3 Transcript 2016 ASRT. All rights reserved.

2 Essentials of Digital Imaging Module 3 Display 1. ASRT Animation 2. Welcome Welcome to the Essentials of Digital Imaging Module 3 Display. 3. License Agreement 4. Objectives After completing this module, you will be able to: Differentiate between a cathode ray tube and a liquid crystal display monitor. Define the characteristics of a monitor that affect image display. Explain the features of a modality workstation monitor. Describe how a modality workstation functions. 5. CRT Monitor Animation CRT technology has been around much longer than LCD equipment, and is still used to provide excellent images. An electron gun, or cathode, creates images by beaming electrons towards an anode. Instead of striking the anode, however, these electrons are deflected toward the monitor s fluorescent screen to construct the image. There are 2 types of image acquisition: interlaced and progressive. There is an obvious difference between how the image is produced based on how they acquired data is transferred to the monitor display. The progressive process fills each line from top to bottom, whereas interlaced acquisition fills every other line top to bottom and then returns to the top to fill in the missing lines of data. 6. LCD and LED-LCD Monitors The LCD and LED-LCD monitors are constructed of 2 sections of polarized glass with liquid crystals sandwiched between the glass layers. The crystals allow different light waveforms to pass through the monitor and be viewed on the screen, diffusing the light as appropriate. The difference between the two monitor types is the light source for the liquid crystals. A traditional LCD monitor uses a cold cathode fluorescent light source and the LED-LCD uses a light emitting diode as the light source. 7. Display Characteristics There are several factors that affect image display. Although technologists can t modify the image display characteristics of a digital imaging system, it s important to understand the role a display monitor has in viewing digital images. The monitors used with acquisition workstations can be cathode ray tube (CRT) or liquid crystal display (LCD) types. Their dimensional shape is one way to distinguish between them. A CRT monitor is much deeper from the screen to the back of the device. An LCD monitor is thinner from front to back. In general, a CRT monitor provides more luminance, especially in the center of the image. They have lower measurable black levels and better color reproduction. The phosphor granularity of a CRT adds to the spatial noise recognition of the image, the viewable area is actually smaller than the stated size, and the aspect ratio is 4:3. Not only are CRT monitors more responsive on redraw they are more durable than LCD monitors. On the other hand, an LCD monitor produces less screen glare, or veiling glare, offers better spatial resolution, and is easier to move and reposition. The LCD monitor consumes less energy and provides a larger viewing area that is closer to the described size. Additionally, it leaves a smaller footprint than the CRT counterpart while providing a wider screen aspect ratio of 16:9. A limitation of the LCD is that it can only display the designed resolution. There are several characteristics of the monitor that can affect its display which include spatial resolution, vertical and horizontal resolution, the size of the display, aspect and matrix ratio, pixel size and pixel pitch.

3 8. CRT and LCD Resolution For a CRT monitor, spatial resolution is determined by the bandwidth, line spacing, and electron beam size. The vertical resolution depends on line spacing and spot size which is the ability of the tube to focus the electron beam. Horizontal resolution depends on bandwidth and spot size. An LCD monitor's resolution is determined by the pixel size and pitch. The smaller the pixel size the greater the spatial resolution. The pixel pitch is the distance between the cells of the same color. The vertical and horizontal resolution is determined by the pixel pitch in that given direction. 9. Display Size The size of the display screen also plays a role in resolution. The screen is measured diagonally from corner to corner. For a CRT monitor, the measurement is taken from the outside edges of the display casing. For an LCD monitor, the screen is measured from the inside of the beveled edge. There are several display characteristics, uses, and sizes for image viewing monitors such as XGA, an extended graphics array, up through WUXGA, a wide ultra XGA display. Each display type has its own unique resolution. These resolution values are similar to a laptop computer or your home PC, with resolutions such as 1024 x 768 or 1600 x These values represent the relationship of pixels on a vertical and horizontal axis across the monitor. An important consideration is the difference in resolution between the image acquisition work station and the diagnostic workstation used by the radiologist. The increased resolution of the diagnostic workstation can display suboptimal characteristics that are not apparent on the acquisition workstation. For example, a common issue is lost spatial resolution from motion seen on the diagnostic workstation. 10. Aspect Ratio Aspect ratio is another important display characteristic. Aspect ratio refers to the proportional relationship of the width and height of the display screen. Most CRT monitors have an aspect ratio of 4:3, whereas LCD monitors have an aspect ratio of 16:9 when viewed in a landscape format. 11. Matrix Ratio Matrix size also affects how an image appears. The larger the matrix, the more pixels on the display monitors, which results in smaller pixels and greater resolution. A 1 x 1 matrix doesn't really demonstrate what is contained in the image. The 100 x 100 display matrix provides a better image of the letter R in terms of spatial resolution compared with the smaller matrix sizes. 12. Pixel Size and Pixel Pitch Pixel size and pixel pitch values vary depending on the display monitor being used. The number of pixels is an important display characteristic. As the number of pixels per square centimeter increases, the monitor is able to display radiographs in a higher-resolution. Pixel pitch, also known as dot pitch, determines the sharpness of an image. In a CRT monitor, the pitch is the distance between phosphors. Gaps between pixels represent dead space meaning no image information can be displayed in that area. In an LCD monitor, the pitch measures from the center of 1 pixel to the center of an adjacent pixel and includes any gaps between them. The closer the pixels are together, the sharper the image because there are more pixels in the area. 13. Knowledge Check 14. Knowledge Check 15. Gray Scale To accurately display the gray scale in a radiograph, the display monitor needs to be able to demonstrate at least 256 shades of gray which is equivalent to 8 bits. In addition to the minimum gray-scale display, the American College of Radiology, or ACR, has determined the minimum requirement for spatial

4 resolution on diagnostic monitors must be 2.5 line pairs per millimeter when viewing a 14 x 17-inch, or 35.6 x 43.2 cm, image. 16. Display Noise Display noise, also referred to as monitor noise, is not the sound that you hear but rather what is visible on a finished radiograph. Monitors have 2 types of display noise. Spatial noise is caused by phosphor granularity. This type of image noise is the result of random luminance variations from construction characteristics and electrical signal variability inherent in LCD monitors. Temporal noise occurs when the electron beam intensity fluctuates as the beam scans back and forth across the back of the phosphor in the monitor. 17. Contrast Resolution The contrast resolution of a monitor is the difference between the maximum and minimum luminance of the display. Color depth determines the colors that can be displayed and depends on the graphics adapter and color capabilities of the monitor. Color bit depth refers to the number of bits used to create the color of a single pixel. 18. Maximal Luminance Maximal luminance, when referring to a video display, refers to monitor brightness. Current ACR standards specify that gray-scale monitors used for interpreting radiographs provide a minimum of 171 candela per square meter. The minimum level of luminance is called the black level, and CRTs have a measurably lower black level than LCD displays. Analog display brightness differs significantly compared with digital image display brightness. This is one reason that viewing angle and ambient lighting surrounding a workstation has such a tremendous effect on a technologist's ability to evaluate a digital image. Analog images on view boxes are much brighter. A mammography view box has a luminance between 3,000 and 7,500 cd/m2 where a standard view box has a luminance between 1,500 and 3,500 cd/m2. An LCD gray-scale monitor at a technologist s workstation has a luminance of 120 to 1000 cd/m2. If the monitor is a CRT then the luminance is 120 to 450 cd/m2. As a comparison, a standard PC color monitor displays at a luminance of 65 to 130 cd/m Refresh Rate The refresh rate of CRT monitors refers to the number of times an image is drawn on the screen each second. Refresh rates range from 60 to 120 images per second. Large CRT monitors flicker more visibly. Refresh rates vary. The refresh rate for a 1280 x 1024 matrix might be 85 hertz, or 85 cycles per second, whereas a 1600 x 1200 matrix has a refresh rate of 75 Hz, or 75 cycles per second. 20. Persistence Persistence is the delayed light emission from the phosphor after a CRT screen refreshes. Persistence helps when viewing static images, but it produces image lag in a cine-type display. Persistence with LCD based monitors is the result of the liquid crystals staying in one position following the termination of the applied electrical signal. In the case of LCD based monitors, image persistence is also known as image retention. 21. Technologist Workstation The technologist workstation has modality specific software that enables the workstation to be multifunctional. Technologists often can choose the patient name and exam from a modality work list, select the type of examination performed and the processing parameters for the image data, and determine how to display the resulting image. Technologist workstation monitors are used in bright, or well lit, areas. Monitoring luminance, or the brightness of a monitor display, is an important consideration when evaluating image quality. The monitor must allow a technologist to discern motion and to ensure anatomy is demonstrated properly. The ability of the monitor to display contrast directly affects the visibility of structures in an image. The display monitor is only as functional as its video card.

5 22. Tiling Depending on the manufacturer's software, the examination list screen may display thumbnail-sized images of the examinations. Most often the display includes images recently received by the workstation as the images are acquired during a series of radiographic exposures. Thumbnails vary based on manufacturer specific designs and are selectable. Selecting a thumbnail allows technologists to view a larger image to determine whether it can be accepted or rejected according to the diagnostic principles of the examination. 23. Activity Tracking Modality workstations can track any activity associated with an imaging exam that displays on the workstation comprehensive list, such as date and time. The list of displayed items is configurable but not comprehensive. More examination data is available than is displayed on the exam list screen. For example, the times associated with an examination might include when the imaging order was received by the work list and when the patient's individual images were performed which is based on specific plate processing times, the image capture, and when the image was sent to PACS. 24. Image Data To view an individual study, the user activates a command on the screen showing all the examinations stored on the workstation. The screen then displays information about the images selected that includes the date and time the image is processed, the anatomical menu used to process the image, exposure indicator, which is identified by exposure to the digital receptor, exposure factors, in the case of cassetteless digital imaging systems, brightness level, and contrast level. The data displayed on the image frame and the examination screen can be configured by the user. However, the addition or rejection of certain data fields displayed might be restricted based on an individual technologist s permissions. An image list can offer configurable displays for patient demographics, examination data, image data, and the plate exposure number. Certain users are able to access specific data fields in addition to being able to make general modifications. 25. Knowledge Check 26. Knowledge Check 27. Super Users The terms advanced user and "super user" often apply to technologists with higher levels of access than the rest of the team. Super users can have permission to turn certain data displays on and off and add data to free fields. The configurable areas that advanced users might access include patient demographics, examination data, image data, and plate exposure number. Changes made in these areas often are more complex than simple orders to change the display or add a data column. Super users may control additional data sets or end user access to those data sets. 28. Retrieve To Display Manufacturers provide different types of user controls to manage images, even though the end product is very similar. A touch screen, mouse, or keyboard is used to activate the modality workstation for retrieveto-display commands. As with most computer-based retrieval functions, it s possible to indicate the quantity and sequence of images, or studies, the user wants to retrieve. The display screen provides a retrieval method or methods to step through the images or studies. 29. Editing Image Information The designated privilege levels restrict who can edit and modify images and demographic data. Manufacturers' systems allow varying amounts of change during and after installation. In most cases, the institution and job responsibilities of users drive who can make these changes. It s important to understand that modifying demographic and image data alters the examination and permanent patient record. Therefore, all changes must comply with institutional guidelines. If an order is

6 modified after a work list entry is generated, this change must be described in the patient's chart with an explanatory note. Again, the ability to make these changes depends on the privilege level of the user. 30. Manipulating Image Appearance Editing image data can include altering image orientation, image and display annotation, stitching, applying shuttering or a black border, adjusting image brightness and modifying image contrast. Technologists must follow departmental policy and procedure and standard practice for any and all modifications. Imaging protocols require placing the digital capture device appropriately so that the resulting image is displayed in the proper orientation. On occasion, when a patient's condition or exam parameters make this impossible, modality workstation software allows the image to be rotated or flipped. The image on the left is the original image and the image on the right has been flipped. The ability to flip images makes it critical for the technologist to use lead, left or right side, anatomical markers at the time of image acquisition. Doing so would help to minimize the potential for incorrect placement of electronic left or right side anatomical markers that might be used in addition to lead markers for image acquisition. 31. Image Flip The main processing page on Fuji s cassette-based system allows an image to be flipped. The image is flipped horizontally when the technologist selects the button on the left. This action moves the image much like how you turn the page of a book. When the button immediately to the right of the horizontal flip button is pressed, the image is flipped vertically, or upside down. These buttons can be pressed as many times as necessary until reaching the desired orientation. Konica s cassette-based system also allows an image to be flipped from the main processing page, although the user interface is very different. In this case, a menu of different images is brought up when the technologist selects image flip. These images are thumbnail size and display all the possible variations for flipping the image in horizontal and vertical directions. The technologist can look through the thumbnail images and select the one that presents the desired orientation. The image automatically is flipped once the thumbnail is selected. If the technologist does not approve of this orientation, the entire process can be quickly repeated. 32. Annotating Images Imaging departments have established protocols for the type and placement of the data displayed on a digital image. The actual display may be different on a modality workstation and a PACS workstation. There are a variety of annotations available that can be added to an image keeping in mind that left and right side anatomical markers should always be used during image acquisition for legal reasons. Technologists must include certain basic information on every digital image, just as they do for analog images. Generally, the technologist performing the examination should alter the data, and most systems provide a way to add text. Additional text may include predetermined labels such as, "erect," "10 min" and so forth. Another method adds text by using a free-form text box. When using these display annotations, technologists must follow standard practice and departmental policies and procedure. Fuji s cassette-based system allows image annotation from the processing menu. After the user selects image annotation, a menu of preset annotation markers is displayed. Fuji s system has 7 pages of markers to choose from. The technologist selects the desired marker and then clicks on the image in the general area where the marker should be displayed. The marker is automatically placed on the image where it then can be moved to an exact location and enlarged if necessary. Multiple markers can be placed on the same image. The Fuji cassette-based system also allows technologists to type free text in a marker and place it on the image using a dialog box. A technologist can type a note by clicking on the box in the image annotation menu of the processing main menu. In this case the technologist typed upright. Once the technologist has typed the necessary information, the marker can be placed on the image in the same way that other

7 annotation markers are placed. This marker also can be enlarged and moved to any desired location. Technologists should avoid placing the annotation directly over the anatomy displayed in the image. 33. Stitching For examinations such as a scoliosis series or leg length study in which the anatomical parts are longer than 43 cm, or 17 inches, multiple imaging plates or multiple exposures can be used to acquire the image. When these images are obtained, the equipment provides an indexing mark to ensure that the entire anatomical region is displayed. The technologist can open the processed images on the workstation and use a software function to join the separate images into a single-image format. This process is commonly referred to as stitching. After the cassettes are processed in the correct order, the software uses the identifying markers in the cassettes to join the anatomy, creating a continuous image. 34. Overlapping PSP An alternative to making separate exposures for a scoliosis examination or a long-leg, hip-to-ankle, examination is to use a specially designed cassette. The exam is performed as it would be with a filmscreen image receptor and only 1 exposure is needed. The cassette is designed so that the photostimulable phosphor plates, or PSPs, inside the cassette overlap slightly to cover all the relevant anatomy. Markers inside the cassette assist the stitching software to reconstruct the full-length image. After the exposure is complete, the single cassette on the top is detached from the longer cassette and placed into the plate reader to extract the image. The longer cassette on the bottom contains 2 photostimulable phosphor plates. The cassette is fed from 1 side first to unload 1 PSP. Once the image extraction process is complete for that plate, the cassette is turned around and the final PSP is extracted from the other end of the long cassette. In all, 3 cassettes are processed for a hip-to-ankle examination and 2 plates are processed for a scoliosis exam. It s important to identify the exam to the plate reader before beginning the image extraction process, or the stitching software won t automatically stitch all the images together. A beam-attenuating marker also should be placed on each PSP plate in case an image is misidentified. 35. Image Shuttering A workstation software function allows a black mask to be placed over bright areas which is a practice known as shuttering. Other terms for shuttering include black border, black mask and collimation, although this type of collimation shouldn t be confused with beam restriction. The mask can be configured to apply automatically or manually, depending on the manufacturer. The ability to adjust the mask size depends on the vendor as does the option to select a rectangle, circle, or polygon from the menu and use the computer s mouse to resize the shape to the desired shutter size over the image. Once the technologist has shuttered the image or images to the desired size, the changes are accepted. The area outside of the shuttered area is darkened, although a faint image of the anatomy can still be seen outside the shuttered area. Keep in mind that proper beam restriction, or collimation, to the area of interest is the accepted clinical practice when working with digital equipment. However, when the image is displayed, it is typically surrounded by an unexposed area that appears clear and extremely bright compared with the exposed area. This bright area can result in viewer fatigue when displayed on PACS. The black mask, border or shuttering should not be used in place of effective beam restriction to crop off anatomy. It is critical to apply the black mask or shuttering to an image outside the actual collimated border of the exposure field. This practice is important to show that the technologist exercised prudent judgment in using beam restriction. The Carestream system allows a technologist to shutter an image manually using a function called a mask. The technologist uses a touch-sensitive screen to position the boundaries of the mask in the

8 desired location. Once the lines of the mask have been properly set and accepted, the area outside the shuttered area darkens, making it easier for the radiologist to view the pertinent anatomy. 36. Image Brightness and Contrast Windowing is the term used when changing the clarity and variances of an image. By changing the window levels, the technologist is adjusting what is called the brightness and contrast of that image. Brightness refers to the overall lightness or darkness and contrast is the degree of differences in densities of adjacent structures on an image. The differences in clarity and anatomical structures are manipulated using varying degrees of technical factor selection. 37. Institutional Protocol Although the capabilities of all digital imaging displays are similar, different terminology is used and different processes are followed. It is essential to become familiar with the protocols and equipment used at each facility. 38. Knowledge Check 39. Knowledge Check 40. Conclusion This concludes Essentials of Digital Imaging: Module 3 -- Display. You should now be able to: Differentiate between a cathode ray tube and a liquid crystal display monitor. Define the characteristics of a monitor that affect image display. Explain the features of a modality workstation monitor. Describe how a modality workstation functions. 41. References AAPM Report No An exposure indicator for digital radiography. July American Association of Physicists in Medicine website. Accessed February 14, Bushong SC. Radiologic Science for Technologists: Physics, Biology, and Protection. 10th ed. St Louis, MO: Mosby Elsevier; Carlton RR, Adler AM. Principles of Radiographic Imaging: An Art and A Science. 5th ed. Clifton Park, NY: Thomson Delmar Learning; Carroll QB. Radiography in the Digital Age: Physics, Exposure, Radiation Biology. Springfield, IL: Charles C Thomas; Carter CE, Vealé BL. Digital Radiography and PACS. St Louis, MO; Mosby Elsevier; Practice Standards for Medical Imaging and Radiation Therapy. Radiography practice standards. June 19, American Society of Radiologic Technologists website. Accessed February 14, Seeram E. Digital Radiography: An Introduction for Technologists. Clifton Park, NY: Delmar Cengage Learning; Strategic Document. Version , April 11, Digital Imaging and Communications in Medicine website. Accessed February 14, 2013.