RAD141 - Chap 2b - Digital Radiography Image Quality
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| What are digital images? | numeric representation of the x-ray intensities that are transmitted through the patient, viewd on a computer monitor, referred to as soft-copy images
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| What do matrix and pixels describe in digital images? | a pixel is a picture element; a matrix is a collection of pixels (a pixel is similar to a single box on graph paper; the matrix is the graph paper)
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| What does wide latitude mean? | wide range of acceptance of exposure factors to produce an acceptable image; digital imaging requires less precise settings than film imaging
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| What factors are used to evaluate digital image quality? | brightness, contrast, resolution, distortion, exposure index, noise
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| What is brightness? | the intensity of light that represents the individual pixels in the image; replaces film-based term density
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| How is brightness controlled? | by the processing software through the application of predetermined digital processing algorithms; user can adjust the brightness of the digital image after exposure
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| What is contrast? | the difference in brightness between light and dark areas of an image; contrast resolution refers to an imaging system's ability to distinguish between similar tissues
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| What are the controlling actors for contrast? | through software; the user can manipulate the contrast of the digital image
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| What is bit depth? | the range of possible shades of a pixel; determined by the manufacturer; the greater the bit depth of a system, the greater the contrast resolution
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| What are the most common bit depths used? | 10, 12, and 16; a bit depth of 10 is 2^10 -> 1024 intensities
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| What are the different pixel sizes used? | acquisition pixel size -> minimum size that is inherent to the acquisition system; and display pixel size -> the minimum pixel size that can be displayed by a monitor
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| What would an example of a high pixel size monitor? | a 17 x 17 monitor with 9 megapixels (3000 * 3000)
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| What must be controlled in digital imagin to obtain appropriate image contrast? | scatter radiation -? digital detectors are more sensitive to low-energy radiation; accomplished thru use of grids, close collimation, & selection of optimal kV
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| What is resolution? | the recorded sharpness or detail of structures on the image
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| What factors affect resolution? | same as film-screen imaging (focal spot size, geometric factors, motion) and acquisition pixel size; also, display matrix of the monitor affects perceived resolution
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| How is resolution size measured? | in microns; typically 100-200 microns -> 5 to 2.5 line pairs per mm
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| What is distortion? | the misrepresentation of object size or shape as projected onto radiographic recording media -> same as film-based imaging; same factors
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| What is exposure index? | a numeric value that is representative of the exposure the image receptor received; may also be called the sensitivity (s) number
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| What is the exposure index dependent on? | intensity of radiation striking the IR; calculated from mAs, kV, total detector area irradiated, objects exposed (e.g. air, metal implants, patient anatomy); dependent on manufacturer/technique, directly or indirectly proportional to radiation striking IR
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| How is "S" number related to radiation? | "S" number (used by some manufacturers) is inversely proportional to the radiation striking the detector; ex: "S" number for certain examss is 150-250; an "S" value over 250 would indicate underexposure; lower than 150 would indicate overexposure
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| How is an exposure index related to radiation? | when manufacturers use exposure index (instead of "S" number), it is directly proportional to the radiation striking the IR; if an acceptable exposure index is 2.0 to 2.4, value lower than 2.0 -> underexposure; number higher than 2.4 -> overexposure
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| Why is it important to check the exposure index? | it verifies that the ditital radiographic images were obtained with the least possible dose to the patient
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| How do the technologist's and radiologist's workstations differ? | typically the radiologist's workstation provides far superior spatial and contrast resolution due to increased display matrix with smaller pixels and superior brightness characteristics
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| What is noise? | a random disturbance that obscures or reduces clarity; translates into a grainy or mottled appearance of the image
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| How can noise be described? | SNR -> signal-to-noise ratio; # of x-ray photons that strike the detector (mAs) is the "signal"; other factors are classified as "noise"; a high SNR is desirable (signal > noise); low SNR is undesirable
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| What should not be sacrificed to obtain high SNR? | exposure factors should not exceed what is required so that patient dose is not unnecessarily increased; exposure index must be checked
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| What other factors contribute to noise? | scatter radiatio, which can be mitigated through use of grids and correct collimation; electronic noise from inherent noise in the electronic system, nonuniformity of the IR, and power fluctuations can't be easily controlled
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| What is post-processing? | changing or enhancing the electronic image in order to improve its diagnostic quality; algorithms are applied to the image to modify pixel values
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| Why is exposure index important in post-processing? | an exposure index below the acceptable exposure index range can't be overcome by post-processing; more "signal" can't be created with post-processing
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| What are the different post-processing options available? | windowing, smoothing, magnification, edge enhancement, subtraction, image reversal, and annotation
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| What is windowing? | the adjustment of image contrast and brightness on the monitor; window width controls the contrast of the image; window level controls the brightness
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| What is smoothing? | brightness values of adjacent pixels can be brought closer together
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| What is subsraction? | the removal of background anatomy to allow visualization of contrast media-filled vessels (used in angiography)
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