iaea human health series publications - SEDIM

esolution pattern. A tool for determining the limiting spatial resolution of an imaging system. It is composed ofgroups of several highly X ray attenuating strips spaced by an equal width of non-attenuating material, eachstrip and adjacent spacer being referred to as a ‘line pair’. The reciprocal of the total width (in millimetres)occupied by the line and the space determines the number of line pairs per millimetre (lp/mm) for that group.Each group of strips is smaller than the previous group; that is, it consists of more line pairs per millimetre.The last group in which each bar is visible as a distinct line indicates the limiting spatial resolution (in lp/mm)of the system.screening mammography. X ray breast examination of asymptomatic women in an attempt to detect breast cancerwhen it is small, nonpalpable and confined to the breast.source to image distance (SID). The shortest distance between the point of X ray production (focal spot) and theimage receptor (detector).spatial frequency. A spatial pattern such as an image can be represented as the summation of a set of spatialsinusoidal functions of appropriate amplitudes, each sinusoid covering a specific distance (e.g. millimetres)per cycle. The spatial frequency is the reciprocal of that distance and is specified in cycles/millimetre.spatial resolution. The fineness of spatial detail that an imaging system can demonstrate. This can be measuredfrom the image of a resolution pattern in terms of the number of line pairs per millimetre (see resolutionpattern). Spatial resolution can be affected by the size of the X ray source, geometric magnification, blurringof signal in the X ray detector, the del size and pitch, and the pixel size. The limit imposed by data samplingis that the maximum spatial resolution (Nyquist frequency — line pairs per millimetre or lp/mm) is equal toone half the number of pixels/mm (i.e. if the sampling frequency is 5 pixels/mm, the maximum spatialresolution is 2.5 lp/mm). Spatial resolution depends on the sampling frequency for cassette based systems andthe detector element size for cassette-less systems. With TFT based detectors, the actual spatial resolution isnear the Nyquist frequency. With PSP based CR systems, the spatial resolution is less than the Nyquistfrequency to the light spread from the PSP plate during image extraction. Unlike screen film systems, there islittle correlation between exposure level and spatial resolution.standard breast. A 53 mm thick compressed breast consisting of 29% fibroglandular and 71% adipose tissue usedas the ‘average’ breast for dosimetry calculations. This may be represented by 45 mm of PMMA, a thicknessthat attenuates approximately the same amount as the standard breast.structured noise. A background pattern in a radiograph that often adds visual clutter and degrades or masks thedetection of a lesion. In a clinical image, normal anatomy can provide structured noise. In phantom images,structured noise is typically the result of systematic artefacts such as roller marks in screen film images, gridlines or poor flat fielding of digital images.system spatial resolution. Resolution of the whole system and not just one component, such as the focal spot orgeometric resolution or the detector resolution.tolerance values. Values that express the range over which the parameter is allowed to vary before the item is nolonger considered to be operating within limits. These ranges are classified into two categories: ‘achievable’and ‘acceptable’.viewbox. A device providing a relatively uniform surface luminance for viewing mammographic films.Mammographic viewboxes should have a luminance level of at least 3000 cd/m 2 (nit).176