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Review CR & DIGITAL IMAGING (1) 2012 – RT 244 wk 15 References: Bushong Vol 9 www.sprawls.org/resources/DIGRAD/classroom.htm 1 Objectives Digital imaging review Review CR fundamentals 2 Digital Imaging Image acquisition that produces an electronic image that can be viewed and manipulated on a computer. Examples? 3 Methods to Digitize an Image • • • • 1. Film Digitizer 2. Video Camera (vidicon or plumbicon) 3. Computed Radiography 4. Direct Radiography – PACS – DICOM 4 Digital DDR CR Radiography Direct Capture Indirect Capture Direct-to-Digital Radiography (DDR)-Selenium Computed Radiography (CR) - PSL Direct-to-Digital Radiography Silicon Scint. Laser Scanning Digitizers 5 6 Computer Language • Computers operate on the Binary Number System • It has only two digits, 0 and 1 • Computers function by converting all data into binary values. 7 8 Byte Represents one character, digit, or value. A bit describes the smallest unit of measure 0 or 1 – computers ultimately understand only 0 or 1 Byte are 8 bits A kilobyte represents 1024 bytes, megabyte is 1 million bytes, gigabyte is approximately 1 billion bytes 9 Alphabet in Binary 10 What is a Pixel? 11 Basics of Digital Images • digital images are a (matrix) of pixel (picture element) values 12 Pixel 13 14 Computed Radiography Fundamentals of Computerized Radiography 15 CR SYSTEM COMPONENTS What are the CR system components? 16 CR SYSTEM COMPONENTS CASSETTES (phosphor plates) ID STATION IMAGE PREVIEW (QC) STATION DIGITIZER VIEWING STATION 17 Imaging Plate (IP) Contained in a cassette Handled the same as S/F cassettes Processed more like daylight processor with no chemicals IP has lead backing to reduce scatter 18 CR – PSP plate • photostimulable phosphor (PSP) plate • Exit photons energizes the PSP plate • The energy is stored in traps on plate (latent image) • PLATE scanned in CR READER 19 Imaging Plate Construction A thin sheet of plastic IP’s have several layers ◦ A protective layer. This is a very thin, tough, clear plastic that protects the phosphor layer ◦ A phosphor or active layer. This is a layer of photostimulable phosphor that “traps” electrons during exposure 20 Active Layer - Crystals • The materials that make up the PSP plate are from the barium fluorohalide family. • Barium fluorohalide, chlorohalide, or bromohalide crystals. The most common crystal uses is barium fluorohalide with europium 21 Acquiring the Image What is the correct order? • Violet light is captured by PMT – is amplified and converted into a digital signal • cassette is put into the reader, the imaging plate is extracted • light is sent to the analog to digital converter (ADC). To convert light to binary. • e- return to ground state, visible light is emitted • remnant beam interacts with electrons in the barium fluorohalide crystals • imaging plate is scanned with a helium laser beam or solid-state laser diodes 22 Acquiring the Image • The remnant beam interacts with electrons in the barium fluorohalide crystals. This interaction stimulates, or gives energy to, electrons in the crystals, allowing them to enter the conductive layer, where they are trapped in an area of the crystal known as the color or phosphor center. • This trapped signal will remain for hours, even days, although deterioration begins almost immediately. IR should be processed as soon as possible. • The trapped signal is never completely lost. 23 Imaging Plate Construction A reflective layer. This is a layer that sends light in a forward direction when released in the cassette reader. This layer may be black to reduce the spread of stimulating light and the escape of emitted light. Some detail is lost in this process. 24 IP Construction 25 Cross section of a PSP screen 26 Needle PSP increase the absorption of xrays and limit the spread of light emission 27 IP Design Designed to optimize the intensity of light release. (CE) Enhance the absorption of x-rays (DQE) Limit the spread of light emission for more detail. 28 Photostimulable Luminescence • When the cassette is put into the reader, the imaging plate is extracted and scanned with a helium laser beam or, in more recent systems, solid-state laser diodes. This beam, about 100μm wide with a wavelength of 633 nm (or 670 to 690 nm for solid state), scans the plate with red light in a raster pattern and gives energy to the trapped electrons. 29 X-ray interaction with a PSP screen 1 X-ray interactions with the screen phosphors causes an e- to excited 2 When e- return to ground state visible light is emitted 30 CR Phosphor Plates ABSORPTION EMISSION LASER STIMULATION X-RAY ELECTRON TRAP ELECTRON TRAP LIGHT 31 CR Reader – PSP plate Stimulates the matrix of trapped E- by a RED OR ULTRAVIOLET laser light Trapped E- energy is released in a form of VIOLET/BLUE light Violet light is captured by PMT – is amplified and converted into a digital signal 32 Producing a PSL signal 50% of the excited e- return to ground state immediately, resulting in light (VIOLET/BLUE) emission. Slow scan = plate Fast scan = laser 33 How CR works Released light is captured by a PMT (photo multiplier tube). An ultrasensitive photomultiplier tube or CCD (charged couple device) PSP light is amplified by the PMT or CCD This light is sent to the analog to digital converter (ADC). To convert light to binary. 34 Sequence of CR imaging 35 Processing of digital images can be used to change most image characteristics. • Three possibilities include processing methods to: • Adjust and optimize the image contrast characteristics • LUT & Processing Algorithms • Reduce image noise • Increase visibility of detail • Some type of digital image processing is used with most of the medical imaging modalities. 36 Brightness & Contrast • Optimum kVp & mAs has changed for digital • kVp changes for SUBJECT contrast – not image contrast • mAs does not influence DENSITY the same as it did with F/S • The image is POSTPROCESSED – with changing PROCESSING ALGORITHMS 37 38 39 • digital processing methods that can be used to adjust the contrast characteristics of an image. •Look Up Table (LUT) processing •Windowing •Are used in digital radiography as well as with many of the other imaging modalities. 40 41 DIGITAL “DENSITY” 42 Dynamic Range • • • • • The range of exposure values to which the image receptor will respond. The greater the range of values that a receptor will respond to the greater the dynamic range. 43 Characteristic curve of radiographic film 44 45 46 47 Widow level & width Same photons at the image receptor Image is post processed – changing Brightness and contrast of image appearance 48 • The ability to window is a valuable feature of all digital images. • Windowing is the process of selecting some segment of the total pixel value range • and then displaying the pixel values within that segment over the full brightness (shades of gray) range from white to black. 49 windowing • Important point...Contrast will be visible only for the pixel values that are within the selected window. • All pixel values that are either below or above the window will be all white or all black and display no contrast. • The person controlling the display can adjust both the center and the width of the window. The combination of these two parameters determine the range of pixel values that will be displayed with contrast in the image. 50 advantages of windowing? 51 52 53 54 55 Detective Quantum Efficiency DQE • An indicator of the potential “speed class” or dose level required to acquire an optimal image. • The DQE performance is obtained by comparing the image noise of a detector with that expected for an “ideal” detector having the same signal-response characteristics. 56 57 58 59 60 Exposure Latitude • • • • It is the optimal exposure range relative to the “ideal” exposure that produces a quality image at an appropriate patient dose. 61 Why do digital systems have significantly greater latitude? • Linear response give the imaging plates greater latitude • Area receiving little radiation can be enhanced by the computer • Higher densities can be separated and brought down to the visible density ranges 62 Exposure Latitude • The analog receptor exposure latitude ranges from approximately • 30% underexposed • to 50% overexposed relative to • the “ideal” exposure level. 63 Exposure Latitude The digital image receptor • exposure latitude ranges from • approximately • 50% underexposed • to 100% over exposure • relative to the “ideal” exposure level. 64 65 Exposure Indicators • Imaging plates get a signal from the exposure they receive • The value of the signal is calculated from the region identified as the anatomy of interest • The signal for the plate is an average of all signals given to the plate Note It is important to note that just because a • digital imaging system has the capacity to • produce an image from gross underexposure • or gross overexposure it does not equate to • greater exposure latitude. • The reason the system is capable of producing an image when significant exposure errors occur is through a process called automatic rescaling. 67 • In a digital system, underexposure of • 50% or greater will result in a mottled • image. • In a digital system, overexposure • greater than 200% of the ideal will result • in loss of image contrast. 68 Darker Lighter Histogram showing pixel values in an image. The pixel values in gray are on the horizontal with the total number for each on the vertical. Screen / Film Imaging = self regulating 2 mAs Under exposed 6 mAs Correct exposure 24 mAs Over exposed 70 CR imaging results in 10,000 shades of gray Fixed kVp exposures mAs = 0.5 S = 357 mAs = 1.0 S = 175 mAs = 2.0 S = 86 mAs = 5.0 S = 35 71 LUT • • • • Look Up Table (LUT) Each anatomic area has a LUT Used to adjust contrast and density Other terms that may be used for this – Contrast rescaling – Contrast processing – Gradation processing – Tone scaling LUT • The image data from the histogram is rescaled for application of the LUT • The LUT maps the adjusted data through a “S” curve that is similar to an H & D curve • The result is an image that has the correct contrast and brightness (density) Characteristic curve & histogram Underexposed Overexposed Just right! LOOK UP TABLE (LUT) Linear LUT Black Saturation White Saturation Black Shirt Facial Tones * No Detail in Black Areas * High Contrast * Only Detail in White Areas can be seen * No Detail in White Areas * Low Contrast * Only Detail in Black Areas can be seen Digital Images – Bit Depth • Pixel values can be any bit depth (values from 0 to 1023) • Bit depth = # or gray shades available for image display • Image contrast can be manipulated to stretched or contracted to alter the displayed contrast. • Typically use “window width” and “window level” to alter displayed contrast and brightness 79 Display Bit Depth 1 bit 6 bit 8 bit 2 shades shades 64 shades 256 80 BIT Depth • The number of gray shades available for image display. Number of gray shades is 2n . • Where n is the number of bits available for each pixel 81 Digital - Grayscale Bit depth. Number of gray shades available for display • 8 bit 256 • 10 bit 1024 • 12 bit 4096 • 14 bit 16384 82 83 84 85 CR Image Quality Pixels, Field of View, Image receptor Sampling frequency, Quantization, Nyquest frequency Noise Magnification Image compression (lossless vs lossee) Questions? 86