n. short for ‘picture element’, the smallest individual component of an electronically produced image. Numerical values are ascribed to each pixel, which describe its position and relative intensity and/or colour. A two-dimensional *matrix of pixels produces the final image.
an image made up of *pixels. Each pixel has numbers (digits) to describe its position and shade on the *grey scale. The more shades available, described by the number of computer bits required to store the shade of grey, the more accurately the image represents the original tissue contrast. An 8-bit computer image shows 28 (256) possible shades of grey, close to the maximum the human eye can differentiate. 12-bit (4096 levels of grey) images are of much higher quality and take up more memory. They can be manipulated more easily by computer using image enhancement techniques. Compare analogue image.... digital image
n. (in radiology) the conversion of an *analogue image to a *digital image. The image is broken down to pixels and numerical values assigned to each pixel for its position and to describe its shade on the *grey scale. This allows storage, electronic manipulation, and transfer via computer links of any images, including radiographs or CT, MRI, or ultrasound scans.... digitization
(in radiology) a scale representing the possible gradient of densities from black to white for each *pixel in an image. In an *analogue image this gradient is smooth. A *digital image has many discrete steps. The more steps allowed, the closer to representing the true analogue image it comes, although more steps require more computer memory. Images can be manipulated by *windowing. This concept is particularly valuable in *computerized tomography. See Hounsfield unit; digitization.... grey scale
the numerical unit assigned electronically to each *pixel in a computerized tomography (CT) image, according to its X-ray density. The fixed points on the scale are arbitrarily assigned as ?1000 for air and 0 for water. The CT image is viewed in a ‘window’. The range of Hounsfield units displayed (window width) and the centre point of the range of interest (window level) can be varied by the radiologist in order to observe specific tissues (see windowing). The unit was named after Sir Godfrey Hounsfield (1919–2004), who developed CT scanning in the 1950s. Symbol: HU.... hounsfield unit
adj. (in computerized tomography or magnetic resonance imaging) denoting the acquisition of data when the slice thickness is similar in size to that of an individual *pixel in all planes, i.e. the *voxel is a cube. Computerized reconstruction in any plane will not suffer any loss of detail. The concept is particularly applicable for *multidetector computerized tomography, in which slice thickness of less than 1 mm is used.... isotropic
n. 1. (in histology) the substance of a tissue or organ in which more specialized structures are embedded; for example, the ground substance (extracellular matrix) of connective tissue. 2. (in radiology) the division of an image into rows and columns with equally sized elements (*pixels). The final image is completed by assigning a density to each of these elements. Increasing the number of pixels in the matrix improves the resolution of the final image. A typical value could be 256 rows × 256 columns.... matrix
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