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Digital Image Defs

Appendix for Digital Image Acquisition and Display

QuestionAnswer
Amorphous silicon (a-Si) the devel. of a-si materials,have the same structure as single crystals over short distances but are less ordered over larger distances, has enabled fabrication of flat-panel TFT arrays large enough to be used as the basis for all flat panel XR detec
Amorphous selenium (a-Se) Amorphous selenium layers have the same structure as single crystals over short distance, but are less ordered over larger distances. As a result, amorphous selenium layers provide uniform x-ray detection over the large areas needed by flat-panel x-ray de
Automatic rescaling (Auto ranging, rescaling, scaling, normalization) software function maps the gray-scale to the values of interest (VOI) in the histogram. This feature provides image brightness that is at a prescribed level over a large range of exposure. With some digital systems the image brightness will be consistent
Bit depth the available grayscale for image acquisition and display. Bit depth is equal to 2n, where 'n' is the number of bits. Bit depth cannot be changed after equipment is purchased and is a vendor-specific system characteristic (i.e., 8 bits = 256 shades of gra
Complimentary metal-oxide semi-conductor (CMOS) a photgraphic detector. None are in use except for intraoral dental imaging.
contrast resolution the smallest exposure change (signal difference) that can be captured by a detector. Ultimately, contrast resolution is limited by the dynamic range and the quantization (number of bits per pixel) of the detector. Increased contrast resolution is consider
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" detecor having the same signal-response characteri
detector size or field of view (FOV) the detector size and FOV describe the useful image acquisition area of an imaging device. Cassette-less digital systems have a fixed OV which makes some projections difficult, while cassette-based CR systems have flexible FOVs like screen/film.
Detector element (DEL) the detector element is the smallest resolvable area in a TFT- or CCD-based digital imaging device
Dynamic range the range of exposure over which a detector can acquire image data. Digital image acquisition systems are capable of capturing an image across a much larger range of exposures than film-screen. The increased dynamic range allows more anatomical structures
exposure latitude the range of receptor exposures that provides a quality, low-noise imageat an appropriate patient exposure consistent with ALARA. Exposure latitude is not the exposure range which will be rescaled to consistent image brightness.
histogram a data set, in a graphical form, of the pixel digital values vs. the prevalence number of those values inthe image. The horizontal axis represents pixel exposure, the vertical axis represents incidence of those values. The software has histogram models fo
image noise all images have unwanted fluctuations in brightness that are unrelated to the object being imaged. These are collectively described as image noise.
look up table (LUT) the default gradient curve applied to the data set of your image determining the intial display contrast. The LUT can be adjusted after the initial image processing has been applied.
matrix size the matrix size is the number of pixels that make up the image; this is normally expressed in terms of the number of pixels in two orthogonal directions (length and width of the image). The matrix size is dependent on VOV and pixel size.
modulation transfer function (MTF) a measure of the ability of the imaging system to preserve signal contrast as a function of the spatial resolution. every image can be described in terms of the amount of energy for each of its spatia frequency components.
Nyquist frequency the highest spatial frequency that can be recorded by a digital detector. Determined by the pixel pitch. The pixel pitch is determined by sampling frequency for cassette-based PSP systems and by DEL spacing for TFT flat panel. half the # of pixels/mm
photodiode an electronic element that converts light into charge. With indirect TFT detectrs this is accomplished by a light-sensitive amorphous silicon photodiode on top of the TFT array.
photoconductor amorphous selenium TFT detectors, the a-Se layer forms a continuous x-ray-sensitive photoconductor that converts x-ray energy directly to charge. This charge can be directly "read out" by the TFT array. A photodiode is not necessary with a-Se detectors.
pixel a "picture element", or pixel, the smallest area represented in a digital image. A digital radiography image consists of a matrix of pixels, which is typically several thousand pixels in each direction
pixel density a term that describes the number of pixels/mm in an image. pixel density is determined by the pixel pitch.
pixel pitch the space from the center of a pixel to the center of the adjacent pixel. It is measured in microns (um). Pixel pitch is determined by the DEL size or the sampling frequency.
processing algorithm the mathematical codes used by the software to generate the image appearance desired by the viewer. The processing algorithm includes gradient processing (brightness and contrast), frequency processing (edge enhancement and smoothing), and others
quantization all xray digital receptors respond smoothly and continuously to the incident exposure. Digital images require each pixel to be assigned a unique value (quantized), so that a unique gray shade is assigned to that pixel.
sampling frequency the frequency that a data sample is acquired from the exposed detector. Sampling frequency is expressed in pixel pitch and pixels per mm. may be determined by receptor size, depending on the vendor. As receptor size dec., samp freq ^, spat res ^
scintillator a material that absorbs x-ray energy and re-emits part of that energy as visible light. Indirect TFT flat panel detectors use a scintillator. 2 modern high-eff x-ray scintillators are cesium iodide & gadolinim oxysulfide.
signal to noise ratio (SNR) noise, especially quantum noise, ultimately limits our ability to see an object's edge (signal difference); SNR can be used to describe the edge conspicuity of a particular obj. under well defined exposure conditions.
spatial resolution a characteristic of the imaging system. Max spat. res. (Nyquist freq-line pairs per mm)is equal to half the number of pixel/mm. Spa res depends on the sampling freq for cassette-based systems & the detector element size for cassette less
structured (needle) phosphor a phosphor layer with columnar phosphor crystals within the active layer. Resembles needles lined up on end and packed together.
thin film transitor (TFT) an electronic switch on flat panel detectors commonly made of amorphous silicon. The TFT allows the charge collected at each pixel to be independently transferred to external electronics, where it is amplified and quantized.
tiling a prcess whereby several flat panel detectors are joined to obtain one larger detector. Tiling results in segments tha have unequal response requiring flat-field correction for flat panel detectors.
turbid phosphor a phosphor layer with a random distribution of phosphor crystals within the active layer. this layer can be used in both cassette-based and cassette-less systems and is similar to a standard intensifying screen used with film.
Created by: radtech10