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RAD141-Chap 2b

RAD141 - Chap 2b - Digital Radiography Image Quality

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
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)
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
What factors are used to evaluate digital image quality? brightness, contrast, resolution, distortion, exposure index, noise
What is brightness? the intensity of light that represents the individual pixels in the image; replaces film-based term density
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
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
What are the controlling actors for contrast? through software; the user can manipulate the contrast of the digital image
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
What are the most common bit depths used? 10, 12, and 16; a bit depth of 10 is 2^10 -> 1024 intensities
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
What would an example of a high pixel size monitor? a 17 x 17 monitor with 9 megapixels (3000 * 3000)
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
What is resolution? the recorded sharpness or detail of structures on the image
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
How is resolution size measured? in microns; typically 100-200 microns -> 5 to 2.5 line pairs per mm
What is distortion? the misrepresentation of object size or shape as projected onto radiographic recording media -> same as film-based imaging; same factors
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
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
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
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
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
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
What is noise? a random disturbance that obscures or reduces clarity; translates into a grainy or mottled appearance of the image
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
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
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
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
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
What are the different post-processing options available? windowing, smoothing, magnification, edge enhancement, subtraction, image reversal, and annotation
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
What is smoothing? brightness values of adjacent pixels can be brought closer together
What is subsraction? the removal of background anatomy to allow visualization of contrast media-filled vessels (used in angiography)
Created by: debmurph