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ARRT & Other DIGITAL Terms Defined. April 2009 Wk 9/10 RT 255. Display Workstations. Conventional film/screen radiography uses large multiviewer lightboxes. With early PACS, radiologists thought that they needed 4-6 monitors. Now, the number of monitors has dropped to an average of 2.
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ARRT & Other DIGITAL Terms Defined April 2009 Wk 9/10 RT 255
Display Workstations • Conventional film/screen radiography uses large multiviewer lightboxes. • With early PACS, radiologists thought that they needed 4-6 monitors. • Now, the number of monitors has dropped to an average of 2. • Development of viewing software and better hardware.
Name the 3 types of monitors • Two major types of monitors with a third type gaining acceptance: • CRT (?) • LCD (?) • Plasma screen
3 types of monitors • CRT (cathode ray tube) • LCD (liquid crystal display) • Plasma screen
Medical Use of Monitors • Most medical monitors used to display radiographic images are monochrome high-resolution monitors. • Early displays used were primarily CRTs, but as the LCD technology has gotten better, more LCDs are taking the place of the older CRTs.
ARRT DEFINITIONS Image Display= MONITORS • viewing conditions • (i.e.,luminance,ambient lighting) • spatial resolution • contrast resolution/dynamic range • DICOM gray scale function • window level and width function
How does this affect viewing images ? Different monitors viewing conditions luminance,ambient lighting ARRT DEFINITIONS
How does this affect viewing images? Surrounding light impacted what was seen on image – now : With different monitors: LCD gives more light. LCD can be used in areas with a high amount of ambient light. “dark rooms” not necessary viewing conditions luminance,ambient lighting
Technologist workstation monitors are used in brightly lighted areas. So monitor luminance, the brightness of a monitor display, is an important consideration. Also, the monitor must allow a technologist to visualize enough detail to discern motion and that the recorded lines are sharp and visible.
Monitor Advantages and Disadvantages • Most consumers want a monitor that can provide the highest resolution for the best price. • Most radiology applications have traditionally used the CRT because of its superior resolution, but as the LCD technology has progressed, more and more departments are buying the more slim and light weight LCDs to replace the bulky CRTs.
MONITORS:Display Workstations • Early PACS reading rooms required supplemental air-conditioning to offset the heat from multiple CRTs. • Resolution and orientation of the monitor is also a factor in determining which type of monitor is to be used. • Most cross-sectional imaging is read on a 1K square monitor. • Most computed radiography (CR) and digital radiography (DR) images are read on at least a 2K portrait monitor
1.3 megapixels to 5 megapixels. mammography imaging = at least 5 megapixel resolution is required. LCD
The CRT is the most popular monitor on the market. It consists of a cathode and anode within a vacuum tube. Cathode boils off a cloud of electrons, and then a potential difference is placed on the tube. A stream of electrons is sent across to the anode, which in the case of the monitor is a sheet of glass coated with a phosphor layer. CRT
CRT • Electrons strike the phosphor on the glass. • Impact causes the glass to emit a color based on the intensity of impact and area that the electrons strike. • The electrons interact with a red, green, or blue dot to form the color and image that is being sent from the video card signal.
CRT • Electron beam starts in the upper left hand corner and scans across the glass from side to side and top to bottom. • Once beam reaches the bottom, it starts back over at the top left. • Most monitors have 350 lines to be scanned.
LCD • LCD produces images by shining or reflecting light through a layer of liquid crystal and a series of color filters. • Monitor is two pieces of polarized glass with a liquid crystal material between the two. • Light is allowed through the first layer of glass.
LCD • When a current is applied to the liquid crystal, it aligns and allows light in varying intensities through to the next layer of glass through color filters. • Light forms the colors and images seen on the display.
LCD: Display Workstations • LCD has dropped in price and has risen in quality. • LCD will soon take over PACS display market because of its size, resolution, and lack of heat production. • LCD requires less maintenance.
CRT Luminance higher in the center Lower measurable black levels Phosphor granularity adds to spatial noise Viewable area smaller than stated size Better color reproduction More responsive on redraw More rugged Aspect ratio 4:3 LCD Less veiling glare Consumes less energy Increased spatial resolution Larger viewing area by described size Display limited to designed resolution Can position screen Smaller footprint and lighter Widescreen aspect ratio 16:9 CRT vs LCD
Aspect ratio Ratio is the width of the monitor to the height of the monitor. Most CRT monitors have an aspect ratio of 4:3, and LCD monitors have a ratio of 16:9. Viewable area The viewable area is measured from one corner of the display to the opposite corner diagonally. Monitors – more terms…
Plasma Display • Plasma monitors are new to the consumer market. • They have been used in government and military applications since the late 1960s. • They are made up a many small fluorescent lights that are illuminated to form the color of the image. • Monitor varies the intensities of the various light combinations to produce a full range of color.
Image Display= MONITORS • viewing conditions • (i.e.,luminance,ambient lighting) • spatial resolution • contrast resolution/dynamic range • DICOM gray scale function • window level and width function
ARRT DEFINITIONS MONITORS: Spatial Resolution • Spatial resolution refers to the amount of detail present in any image. • Phosphor layer thickness and pixel size determines resolution in CR. • The thinner the phosphor layer is, the higher resolution. • Film/screen radiography resolution at its best is limited to 10 line pairs per millimeter (lp/mm). • CR resolution is 2.55 lp/mm to 5 lp/mm, resulting in less detail.
MODULATION TRANSFER FUNCTION - MTF • A measure of the ability of the imaging system to preserve signal contrast as a function of the spatial resolution. • Every image can be described in terms of the amount of energy for each of its spatial frequency components. • MTF often is regarded as the ideal expression of the image quality provided by a detector.
ARRT definitions digital image characteristics • spatial resolution • sampling frequency • DEL (detector element size) • receptor size and matrix size • image signal (exposure related) • quantum mottle • SNR (signal to noise ratio) or • CNR (contrast to noise ratio)
Spatial Resolution • A radiograph typically does not show soft tissue structures • Digital image shows not only the soft tissue but also the edge of the skin. • Giving the appearance of more detail.
MONITORScontrast resolution/dynamic range • Appearance of more detail is due to the wider dynamic recording range / contrast resolution • and does not mean that there is additional detail • Because so many more densities are recorded in CR (wide dynamic range), images appear more detailed.
contrast resolution • The contrast resolution of a monitor is the difference between the maximum and minimum luminance of the display.
CR and DR Contain a detector that can respond in a linear manner as compared to S shape of H&D Curve Exposure latitude is wide, allowing the single detector to be sensitive to a wide range of exposures. Exposure Latitudeor Dynamic Range
Why do digital systems havesignificantly greater latitude? • Linear response give the imaging plates greater latitude • Area recieving little radiation can be enhanced by the computer • Higher densities can be separated and brought down to the visibile density ranges • (Brightness in DR replaces density)
Monitors - RESOLUTION • Pixel is a basic picture element on a display. • A pixel is “any of the small discrete elements that together constitute an image.” • Resolution-# of pixels contained on a display • Relationship: • More pixels in an image, the higher the resolution & more information that can be displayed. • Resolution also is defined as the process or capability of distinguishing between individual parts of an image that are adjacent.
Nyquist frequency • The highest spatial frequency that can be recorded by a digital detector. • is determined by the pixel pitch. • The Nyquist frequency is half the number of pixels/mm. • A digital system with a pixel density of 10 pixels/mm would have a Nyquist frequency of 5 line pair/mm.
10 PIXEL DENSITY WHAT IS THE NYQUIST FREQUENCY= ?
ARRT definitions sampling frequency • The frequency that a data sample is acquired from the exposed detector. • It is expressed in pixel pitch and pixels per mm. • Sampling frequency may be determined by receptor size depending on the vendor. • KODAK 8x10 better detail than 14x17 • Use of the smallest imaging plate possible for each exam results in the highest sampling rate. • When the smallest possible imaging plate is selected, a corresponding matrix is used by the computer algorithm to process the image.
Pixel “picture element,” • the smallest area represented in a digital image. • A digital radiography image consists of a matrix of pixels which is typically several thousand pixels in each direction. • Pixel density – A term that describes the number of pixels/mm in an image. Pixel density is determined by the pixel pitch.
DEL (detector element size)receptor size and matrix size • a pixel or picture element. • The typical number of pixels in a matrix range from about 512 × 512 to 1024 × 1024 and can be as large as 2500 × 2500. • The more pixels there are, the greater the image resolution. • The image is digitized by position (spatial location) and by intensity (gray level).
DELs collect electrons that are extracted from the detector assembly and converted into a digital value by an ADC. That process creates the image that displays on our monitor. DEL size controls the recorded detail, or spatial resolution, for the flat-panel device. The technologist can’t change the size of the DEL, which is fixed for that piece of equipment. . DELs – detector elements
Detective Quantum Efficiency • Known as the fill factor, the larger the area of the TFT photodiodes, the more radiation can be detected and the greater amount of signal generated. • Consequently, the greater the area of the TFT array, the higher the DQE. • Over 1 million pixels are read & converted
FILL FACTOR • A field-effect transistor (FET) or silicon TFT • Isolates each pixel element • Reacts like a switch to send the electrical charges to the image processor
Detective Quantum Efficiency • How efficiently a system converts the x-ray input signal into a useful output image is known as detective quantum efficiency, or DQE. • DQE is a measurement of the percentage of x-rays that are absorbed when they hit the detector.
Detective Quantum Efficiency • In other words, CR records all of the phosphor output. Systems with higher quantum efficiency can produce higher-quality images at a lower dose. • Indirect and direct DR capture technology has increased DQE over CR. • However, DR direct capture technology, because it does not have the light conversion step and consequently no light spread, increases DQE the most.
SNR (signal to noise ratio) orCNR (contrast to noise ratio) ARRT definitions • SNR (signal to noise ratio): there is always a very small electric current flowing in any circuit - is called background electronic noise. • It is similar to the fog on a radiograph in that it conveys no information and serves only to obscure the electronic signal. • CNR (contrast to noise ratio): measure for assessing the ability of imaging an procedure to generate clinically useful image contrast. • gives an objective measure of useful contrast.
Image Display • spatial resolutioncontrast resolution/dynamic range
What is a 3-D array of Pixels ? A voxel (a volumetric pixel) is a volume element, representing a 3-D value space. A pixel which represents 2D image data.
The space from the center of a pixel to the center of the adjacent pixel. It is measured in microns (μm). Pixel pitch is determined by sampling frequency for cassette-based PSP systems and by DEL spacing for TFT flat panel. Pixel Pitch
The Nyquist Theorem • Theorem states that when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal so that the reconstruction of the original image will be nearly perfect. • At least twice the number of pixels needed to form the image must be sampled. • If too few pixels are sampled, the result is a lack of resolution.