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Carter Veale Ch 4-7

Digital Rad and PACS Ch. 4-7

QuestionAnswer
What are the layers (in order) for a CR imaging plate? Protective / phosphor (active)/ reflective / conductive / color (newer plates) / support / backing
How is the latent image created in CR? Remnant beam interacts with electrons (e-) in the photostimulable phosphor in the IP which stimulates the PSP crystal e- to enter and become trapped in the phosphor center
How is the latent image created in conventional radiography? Remnant beam interacts with an intensifying screen which emits light that exposes the film
Where is the latent image stored for CR? The phosphor center in the PSP
What is the photomultiplier in CR? Detects the light intensity information from a PSP plate as the laser scans it, amplifies the light and sends it to a digitizer
How is the light signal from the photomultiplier digitized (in CR)? Each light photon is assigned a numerical value
How is the latent image made visible in CR? IP is put in a reader that scans the e- (that have been trapped in the storage phosphor center) with a helium-neon laser beam or solid-state laser diodes, which releases the e- and a blue light is emitted as they relax to a lower energy level
How does the ADC digitize the analog signal in CR? Assigns each picture element or pixel a value that corresponds to a level of brightness
What is spatial resolution? Amount of detail in an image
What determines spatial resolution in CR? Phosphor layer thickness and pixel size
CR images have more detail than conventional, however it has less spatial resolution. Why is this? Dynamic range in CR is much higher so the loss in spatial resolution is made up for by the increase in contrast resolution
What is speed class in CR? A reflection of the amount of light (PSL) emitted by the plate during the reading process, related to the level of exposure received by the IP
Why should the lowest possible speed class be used in CR? It is determined by the radiation exposure to the IP
How often should CR IR's be put through an erasure mode? 48 hours or at least once a week
What is meant by "dynamic range"? The number of recorded densities in a digital radiograph
What is the recommended range of kVp for CR? 45 to 120 kVp / technique outside this range are inconsistent and produce too much or too little excitation of the phosphors
What is the recommended range of kVp for conventional? Minimum value of 70kVp
What happens to an image when mAs chosen is not sufficient? Number of photons produced will be insufficient causing insufficient phosphor stimulation and results in quantum mottle
What is the main advantage and main disadvantage of using a grid with a high ratio? More scatter radiation is absorbed but positioning is more critical
What is used to calculate the exposure indicator number? The light given off by the IP (which is determined by radiation exposure)
What is the relationship of an 'S' number to the amount of exposure? Inverse
What is the relationship of an 'EI' number to the amount of exposure? Direct
What is automatic data recognition? Processing mode in which the computer analyzes data according to set parameters, by recognizing collimation and performing histogram analysis
What does automatic data recognition automatically adjust? Latitude and sensitivity
What is a histogram? Graphic representation of the numerical tone values of an x-ray exposure; each system has a region of interest(on the IP)where the part to be examined is recognized for analysis
What happens when a body part is positioned in the wrong part of an IP? Histogram analysis will be incorrect; will have poor contrast
With automatic data recognition, poor image quality can occur due to what technologist errors? Over or under collimation and overlapping exposures (when using multiple on one IP)
What is an artifact? Any undesirable density on the processed image other than those caused by scatter radiation or fog
What are the common types of artifacts? Imaging plate, plate reader, image processing, and printer
kVp value is selected for what? Penetration and the type and amount of contrast desired
mAs value is selected for what? The number of photons needed for a particular body part
When an IP is read, what captures the light released? PMT- photomultiplier tube
How does direct conversion DR work? Photoconductor in the DR plate absorbs x-rays and converts them to e- and are stored in TFT dectectors
What is a TFT (thin-film transistor)? Array of small pixels, each containing a photodiode that absorbs e-, generating electrical charges and sends the signal to the image processor
How does indirect conversion DR work? Two step process- X-rays strike a scintillator that produces light, the light then strikes the amorphous silicon in a TFT allowing the conduction of the e- and the signal is sent to the processor
What is DQE (detective quantum efficiency)? How efficiently a system converts the x-ray signal into a useful output image; AKA the effects of noise and contrast on the imaging system
What is DQE used to measure? The quality of a digital image
In DR, why is detector size is critical? It must be large enough to cover entire area to be imaged and small enough to be practical
The effect of noise is usually expressed as what? SNR (signal-to-noise ratio); signal is the useful image information, noise is the not so useful information
A high SNR is good because? It represents low system noise and results in the more capture of useful image information and thus a higher quality image
What is the Nyquist Theorem? The sampling frequency must be greater than twice the bandwidth of the input signal, so reconstruction of the original image will be nearly perfect
What is the Nyquist Theorem in reference to? Signal sampling, such as conversion from an analog to digital image
In CR, at what rate does latent image e- lose its energy if not read right away? 25% of its energy in 8 hours
What is the benefit of automatic rescaling? Images are produced w/ uniform brightness and contrast regardless of amount of exposure
What is a LUT (look-up table)? Luminance values derived during image acquisition (histogram); contains standard contrast, speed, and latitude for a given exam
What can be increased or decreased by changing the slope of a LUT graph? Contrast
What can be increased or decreased by moving the line of a LUT graph up or down the y-axis? Brightness (density)
What is exposure latitude? Range of exposures that can be used while still resulting in a diagnostic quality image
In CR, what results if the exposure is more than 50% above ideal? Quantum mottle
In Cr, what results if the exposure is more than 200% above the ideal exposure? Contrast is lost
Created by: Emily Gilligan Emily Gilligan on 2010-11-14



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