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172 Cards in this Set
- Front
- Back
What are intensifying screens?
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Radiolucent plastic, coated w/ light-emitting phosphors that produce light when exposed to radiation
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Who invented intensifying screens?
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Thomas Edison
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What are intensifying screens used for?
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- Used to amplify the effects of incoming x-ray beam
- Requires less radiation (=less dose) to produce same film density |
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What are the layers of the intensifying screen?
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Base, reflective layer, phosphor layer, protective coating
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Intensifying Screen - Base
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- must be flexible but rigid while being chemically inert
- radiolucent polyester plastic |
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Intensifying Screen - Reflective Layer
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- forces light through base towards film
- reflects light toward the film |
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Intensifying Screen - Phosphor Layer
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- active layer of screen
- absorbs incident x-ray photons & emits light photons |
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Intensifying Screen - Protective Coating
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- a layer of radiolucent plastic that protects the phosphor layer
- right next to film |
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What must phosphors have?
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1) High Atomic Number
2) High Conversion Efficiency 3) Appropriate Spectral Emission 4) Minimal Phosphorescence |
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Phosphors - High Atomic Number
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- increases the probability of an incident x-ray photon interaction
- permits PE & Compton interactions |
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Phosphors - High Conversion Efficiency
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- ability to accept the x-ray photon & then emit energy as light photons
- conversion efficiency should be 1 x 10^3 light photons per incident x-ray photon (w/ energy of 50 keV) |
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The higher the conversion efficiency, the _______ (less/more) radiation required to produce an equal density.
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Less
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Phosphors - Appropriate Spectral Emission
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- indication of the precise wavelength of light emitted by the phosphor
- need to match the wavelength sensitivity of screen with same wavelength sensitivity of film |
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Phosphors - Minimal Phosphorescence
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Luminescence -- ability of a material to emit light in response to excitation
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Fluorescence
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- INSTANTANEOUS emission
- occurs within the time it take an electron to complete one orbit of the affected shell (1 ns) |
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Phosphorescence
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- DELAYED emission
- occurs when the light is emitted for a period longer than necessary for 1 orbit of the affected shell electron - associated w/ "afterglow" or "screen lag" |
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Ideal intensifying screens should....
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i. Possess fluorescence
ii. Have minimal phosphorescence iii. Be replaced periodically (5-7 years) iv. Be replaced simultaneously |
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Roughly _________ photons per mm^2 must exit an object being examined in order to produce a radiographic image.
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50,000
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What was used in the past for screen phosphors?
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Calcium tungstate (5% conversion efficiency)
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What's used now for screen phosphors?
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Rare-earth
- conversion efficiency of 15-20% - made of gadolinium & lanthanum (high atomic #s) |
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What's an advantage of the rare-earth screen phosphors?
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Increased speed while maintaining resolution
Disadvanage: increased cost |
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Resolution
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- the ability to accurately image an object
- controlled by size of phosphor crystals, the thickness of the phosphor layer, & concentration of the crystals |
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(Direct/Inverse) ____________ relationship between crystal size & resolution.
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Inverse
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Increasing crystal density ______________ (increases/decreases) resolution & screen speed
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Increases
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Resolution is measured in....
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line pairs per millimeter lp/mm
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An increase in lp/mm would __________ (increase/decrease) spatial resolution
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Increase
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Naked eye can resolve _______ lp/mm
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10-20
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Film-Screen Contact
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Poor contact between radiographic film & the intensifying screen will reduce the resolution of the images
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Quantum Mottle
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- caused by insufficient quantity of photons striking the intensifying screen
- has grainy appearance |
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Speed (sensitivity) of the intensifying screen is determine by...
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The same factors that control resolution
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Increasing crystal size ______________ (increases/decreases) speed
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Increases
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Intensification Factor
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- Most accurate factor that measures the speed or sensitivity of an intensifying screen
- measurement of how well can it take a photon & convert it to a light photon |
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How is the intensification factor calculated?
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IF = Dn/Ds
Dn = nonscreen mR measurement (large0 Ds = screen mR measurement |
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Descriptive Ratings
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High Speed = 200-1200
Par Speed (Universal/Medium Speed) = 100 Slow Speed (fine detail, high resolution) = 20-80 |
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K-Shell Absorption Edge
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Deals w/ the binding energy of the "K" shell of the phosphor of the intensifying screen
Tungsten = 70 keV K-shell binding energy Gadolinium = 50 keV Lanthanum = 39 keV Causes erratic density production in response to minor adjustments in kVp settings |
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Asymmetrical Screens
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- used in highly efficient intensifying screens
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The back of a film cassette may contain a sheet of lead foil that serves what purpose?
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Reduce the residual beam & absorb backscatter
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The appropriate film-screen combo for a specific clinical situation must be selected based on...
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The combined qualities of the film & the screen
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Blue-sensitive film _________ (will/will not) respond to most of the wavelengths emitted by the rare-earth phosphors.
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Will not
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Green-sensitive film is sensitive to...
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Entire range of phosphor emissions, including yellow-green wavelengths
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Speed is ___________ (directly/inversely) related to patient dose
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Directly
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Fine detail screen, _____ (slow/fast) speed, _______ (lots/little) of exposure
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Slow / Lots of
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The more latitude something has, the ________ (more/less) different amounts of exposure will get us an appropriate image.
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More
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Relative Speed
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- Combo of both film speed & screen speed
- changing either film or screen speed result in relative speed change |
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Relative Speed Equation
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(mAs1/mAs2) = (RS2/RS1)
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Narrow Latitude (steep slope) = ______ [high/low] contrast
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High
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3 Parts to a Computer System
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Hardware, Software, Someone to Input Info
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Mainframe Computers
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- very large
- capable of multiple actions at once - ex. the type of computer used to run a large hospital |
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Minicomputers
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- not large server-based systems
- performs multiple, complex operations w/ high levels of input & output - must be very fast - ex. computers part of CT machines that construct a 3D image |
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Microcomputers
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- personal computers
- tied back to either a minicomputer or a mainframe computer for communication w/ servers |
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Input Hardware
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Keyboard, mouse, scanner, microphone, etc.
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Processing Hardware
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CPU, memory for computing each command, BUSes (ex. USB)
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Output Hardware
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Hard copy items (printers, voice synthesizers), computer screen, video display
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Storage Hardware
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Magnetic tapes/discs, optical discs, portable hard drive, flash memory
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Array Processors
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Bigger part of CPU that function w/ part of memory storage
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LAN
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Local Area Network
- based on functionality of the internet - only serves to connect a finite number of computers to one another |
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HIS
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Hospital Information Network
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RIS
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Radiology Information Network
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PACS
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Picture Archiving and Communication Systems
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DICOM
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Digital Imaging and Communications in Medicine
- how digital imaging is transferred - includes both a file format & a standard network communications protocol |
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Computed Radiography (CR)
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- use photostimulable storage phosphor IP known as a PSP/IP; held inside cassette or IR
- 2 step process: expose the plate, process the plate to acquire the image |
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Direct Digital Radiography (DR)
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- without a scintillator
- x-rays are converted directly to an electrical signal - selenium flat panal imaging system |
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Indirect Digital Radiography
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- x-rays converted to light, then to an electrical signal
- amorphous silicon is usually paired w/ a thin film transistor (TFT) array |
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Charged-Coupled Devices (CCD)
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Take incoming photons & emit light photons, which are then converted to an x-ray signal
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Binary Number System
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- base 2 system; only 0s and 1s (binary digits)
- bits are each a discrete unit |
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What does an 8 bit "word" form?
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A "byte"
Represents each of the 26 letters of our alphabet |
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Computer memory is often rated in terms of total ________
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Bytes
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Matrix
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Series of boxes in a grid laid out in rows & columns
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Pixel
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- Individual boxes in the matrix
- Location determined by its "address" using X-Y coordinate system |
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Field of View (FOV)
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-Overall dimension of image matrix
- determined by the size of the detector - if FOV remains the same, then as matrix size increases, the pixels get smaller, increasing spatial resolution |
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Spatial Location
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- domain used in digital imaging (the before & after)
- representative x. y coordinates |
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Spatial Frequency
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- used in MRI
- frequency refers to the number of times the signal changes per unit length - small anatomical structures produce high frequencies, vis versa |
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Fourier Transform (FT)
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Math algorithm that's applied to change an image from the spatial location domain to the spatial frequency domain (inverse FT can return it back)
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Analog
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A mechanism in which data is represented by continuously variable physical quantities (e.g. info entered into computers)
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What 2 processes must happen to see digital image?
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Analog-to-Digital Conversion (ADC) & Digital-to-Analog Conversion (DAC)
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Analog-to-Digital Conversion (ADC)
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- requires sampling, quantization, & coding
- represented as a sine wave illustrating continuous function (shows smooth transition) - any time we see an image on the screen, we're seeing an analog image |
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Scanning
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Image is divided into individual pixels within a matrix; light is transmitted through individual pixels
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Sampling
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Measures the brightness of each individual pixel (areas of high density have less light coming through them)
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Quantifying
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Assigning on an integer to represent the brightness of light transmitted through the image; grayscale
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How can you calculate image dimensions?
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MxNxk bits (M = matrix columns, N = rows)
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Nyquist Theorm
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-Each sine wave of an analog signal must be sampled at least twice per cycle
- more samples that are taken, more accurate the model based on the original (ADC accuracy) |
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Accuracy and speed are __________ (directly/inversely) proportional
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Inversely
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Digital Imaging Processes (from most simple to most complex)
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Point Operations
Local Processing Operations Global Operations Geometric Operations |
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Point Operations
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- least complicated, most frequently used
- takes grayscale data to output matrix pixels w/ little manipulation (affects appearance of pixel) |
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Histogram
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- scale to determine nature of the output image based upon the number of shades of gray used
- if modified, brightness/contrast will change |
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Wide histogram implies...
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More contrast (vis versa is true)
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If the values of the histogram are on the lower end, how will the image appear?
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Dark (vis versa is true)
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Look-Up Tables (LUT)
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Process that takes a structure that's low contrast & converts it so that the difference btw. contrasting tissues is increased
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Local Processing Operations)
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- manipulation of data from an individual pixel base on comparison of value of the pixels immediately surrounding it
- if a pixel is very different than the others, it may be manipulated to fit in better |
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Spatial Location Filtering - Convolution
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Done by taking a weighted avg. of the surrounding pixels; can be used to help smooth areas or help define lines or enhance edges
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High spatial frequency will show ________ (small/large) changes in gray in either the horizontal or vertical directoins
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Large
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High-Pass Filtering
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- edge enhancement or sharpness in a spatial domain that's blurred
- used in high contrast regions (skeleton) - problem --> increased noise |
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Low-Pass Filtering
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- output image is smoothed, intended to reduce the noise
- image detail is compromised (blurred edges) |
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Global Operations
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- all pixels from input image used to change value of a pixel in output image
- popular method is to use FT in filtering images in the frequency domain rather than the spatial domain location - can process images for edge enhancement, image sharpness, & image restoration |
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Geometric Operations
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Involve changes to the image in relation to size, orientation, or position (e.g. rotating or resizing the image)
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PSL
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Photostimulable Luminescence Radiography
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SPR
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Storage Phosphor Radiograph
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DLR
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Digital Luminescence Radiography
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DSP
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Digital Storage Phosphor Radiography
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PSP
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Photostimulable Phosphor IP
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History of CR/DR - 1600s
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Discovery of the Bolognese stone of Italy
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History of CR/DR - 1800s
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Becquerel worked on the notion of de-excitation of atoms by optical means
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History of CR/DR - 1983
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Fuji produced first commercial CR imaging system for use in diagnostic radiology (FCR-101)
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Physics of CR Image Formation
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IP contains PSP layered on base to provide support; PSP must be scanned by laser of specific wavelength in order to render the latent image visible
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Time for luminescence must be shorter than...
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1 µsec
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Latent Image Formation
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- x-ray photons hit the PSP IP
- europium atoms are ionized by radiation & electrons move from valence band to conduction band (higher energy state) - electrons in conduction band are free to travel to "F center" |
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The number of trapped electrons is ______________ (proportional/not proportional) to the absorbed radiation
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Proportional
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Photostimualted Excitation
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Process in which a laser beam scans a PSP IP inside a CR reader to render the latent image visible
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Why must the laser lights be capable of being absorbed by F centers?
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- causes trapped electrons to move up to the conduction band, where they are free to return to valence band
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Photostimulable Luminescence (PSL)
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The transition of the electrons from a higher energy to a lower one, which results in an emission of bluish-purple light
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Layers of CR IP
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Protective Layer
CR Phosphor Layer Electroconductive Layer Support Layer Light Shield Layer Protective layer |
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Standard Resolution CR IP
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- thicker phosphor layers
- may absorb more radiation - faster speed |
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High Resolution CR IP
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- thinner phosphor layer
- provide sharper images - slower speed - similar to emulsion layers of film/intensifying screen phosphor layers |
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Cassettes
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- made of aluminum or aluminum honeycomb panel
- front of cassette is radiolucent - lead backing on back prevents backscatter radiation |
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CR Image Processing Imaging Cycle
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"Ready-to-use" IP is exposed; energy stored on IP; exposed IP processed in CR reader to render latent image visible; laser light erase it
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Cassette-Based CR Reader Types
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- IP has to physically be taken into CR reader for scanning & erasing
- it all involves transport of IP, which can result in damage |
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Cassette-Less CR Reader Types
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- incorporate single fixed IP encased in special housing that's part of the stationary unit
- no contact w/ the IP in the unit when it's read |
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Components of a Scanner
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Laser source, IP transport system, light channeling guide, photodetector, & ADC
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Laser Source
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- stimulates trapped electrons
- stimulation of IP causes it to emit light at a much different wavelength than the laser |
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Point Scanner
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Scan in a method called the raster pattern (like reading a book, from left to right)
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Line Scanner
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- much faster than point scanner b/c they cover more area in the same amount of time by scanning line by line
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Pixel Pitch
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Physical distance between pixels
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IP Transport System
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Removes IP from cassette & transports for exposure to laser, erasure, & re-insertion into cassette
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Light Channeling Guide
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Emitted light is optically filtered & collected by light channeling guide (light collection optics)
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Photodetector
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After light is captured by channeling guide, it's sent to photodetector which converts PSL into an electrical analog signal
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ADC
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PSL is converted to electrical analog signal, is amplified, & is digitized by the ADC ("signal conditioning")
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"Slow Scan" Direction
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Direction in which the IP travels
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"Fast Scan" Direction
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Laser beam movement across the IP
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Dual-Side Reading
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- newer technology where 2 sets of photodetectors are used to capture PSL from front & back side of IP
- more signal obtained, improved signal to noise ratio, improved image quality |
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What are pre-processing operations?
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Used to identify, correct, & scale raw image data when IP is scanned in reader before image is displayed for viewing
"acquisition processing" |
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Exposure Field Recognition
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Purpose is to identify the appropriate raw data values to be used for image grayscale rendition; provide an indication of average radiation exposure to IP
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What are the steps of exposure field recognition?
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1) Collimated edges detected, anatomical structures are detected by specific algorithms
2) Histogram of info on IP is created ("measured/scanned") 3) Grayscale rendition of image occurs |
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Incorrect exposure values result in...
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Under/Over exposure, can be used as a quality control tool to facilitate optimization of radiation protection
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What does Fuji use to represent the quality control?
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"S" Number (sensitivity number)
S - 200 / exposure to IP (mR) Sweet Spot = 200 |
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S number is __________ (directly/inversely) proportional to exposure
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Inversely
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What does AGFA use to represent the quality control?
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LGM or log of median value
LGM = 2.2 + log (exposure in mR) |
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Each increment of 0.3 LGM corresponds to...
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Doubling or halving of the exposure level
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What does Kodak use to represent the quality control?
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Exposure Index (EI)
Sweet spot = 2000 |
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Exposure Index is __________ (directly/inversely) proportional to exposure
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Directly
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Acceptable Range for the 3 Types
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Fuji = 150-300
LGM = 2.05-2.35 Kodak = 1850-2150 Incident Exposure = 0.67-1.33 |
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Algorithms
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Contrast enhancement (optimize image contrast), spatial frequency (edge enhancement [decreasing grayscale]), multi-scale, multi-frequency enhancement, dual-energy, & disease-specific processing
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Image Quality Detectors - Spatial Resolution
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- related to size of pixels in image matrix
- smaller the pixel size, greater the spatial resolution Pixel Size = FOV / Matrix Size |
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Image Quality Detectors - Density Resolution
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- linked to bit depth
- greater the bit depth, the better the density resolution |
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Image Quality Detectors - Noise
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- electronic noise (system noise)
- quantum noise (determined by # of x-ray photons interacting w/ the detector to create an image) |
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Detective Quantum Efficiency (DQE)
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- measure of the efficiency & fidelity with which the detector can perform this task
- takes into consideration SNR, system noise |
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What allows imaging plates to capture more data during acquisition?
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The linear exposure range, which is largely dynamic
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What do over- & under- exposure describe?
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Actual exposure (mR) to the IR
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Overexposure implies _______ (dark/light) images
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Dark
Low contrast b/c of extra scatter (vis versa also true) |
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Dynamic range of screen/film is....
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From the straight-line portion of H&D curve
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Dynamic range of digital is...
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A linear response (has more than 50x in dynamic range)
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What does the speed of the receptor determine w/ screen-film?
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The appropriate level of receptor exposure required to achieve the optimal image density
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What does the speed of the receptor determine w/ digital systems?
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The speed class at which the system operated
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If digital exposure is <50% of the optimal signal level...
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Quantum Mottle may be objectionable to the viewer (too few photons)
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If digital exposure is >100% of the optimal signal level...
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ALARA is violated; extra scatter generated will decrease contrast (200% more radiation)
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What 3 things are required for high image clarity?
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1) Selection of correct technical factors
2) Correctly positioned anatomical part 3) Precise alignment of the beam, the part, the IR |
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How can one reduce patient exposure by 12-15%?
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Performing routine radiography at 48" rather than typical 40"
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How does grid cutoff show up on digital images?
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Flattening of the luminance from plate & loss of contrast
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What 2 factors have a significant role in determining amount of scatter radiation generated?
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Beam size & Water content of patient
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Lack of collimation generates ________ (more/less) scatter, reducing contrast
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More
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When is there an increased risk of grid cutoff?
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On portables, due to poor central ray & grid/IR alignment
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Moiré Artifacts (aliasing)
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- lines seen as artifacts on CR images
- reduce risk by selecting appropriate grid frequency that isn't near scanning frequency of IP |
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What determines image contrast on digital image?
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Processing algorithm (instead of kVp)
Can use 10-20 more kVp for digital systems to limit patient dose |
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What happens when the acquired data width is reduced in digital imaging?
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More of the anatomy may be visualized for a single image display setting
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kVp for a non-grid exam should never exceed...
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80
Because of increased sensitivity to scattered radiation |
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The actual exposure latitude of the typical digital system is about _________ (double/half) that of the typical screen/film system
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Double
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Exposure levels < 50% below optimal level will...
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Produce excessive mottle
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Exposure levels >100% above optimal level will...
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Produce excessive scatter that degrades the image quality
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Most common reason for histogram analysis errors is....
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Exposure field recognition failure
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What's a way to avoid exposure field recognition error?
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Acquire 1 image on the smallest plate available (some companies don't utilize highest detail until smallest plate is employed)
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What's undercutting?
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Occurs when intense raw radiation degrades the structure visibility along the margin of the structure
To reduce this, collimate |
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What increases geometric blur?
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Short SID, long OID, large focal spot
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Max spatial resolution is equal to...
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Nyquist frequency (1/2x pixel pitch, mm)
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