Table 7.
Examples of measures to assess image quality/technical efficacy of imaging methods and description of the measures
| Measure | Description |
|---|---|
| Objective image quality | |
| Contrast resolution | Degree of density difference between two areas of an image, especially between image of an object and background |
| Line Spread Function (LSF) andPoint Spread Function (PSF). | Spatial density distribution on an X-ray image of a narrow slit or a small point |
| Modulation Transfer Function (MTF) | A description of unsharpness over a range of spatial frequencies |
| Noise | Fluctuations of signal over an image, as a result of a uniform exposure. For a digital detector pixels may on average receive a specific number of X-ray photons, but in practise some will have fewer and some will have more, resulting in a grainy pattern or mottle. As number of x-rays is increased, by increasing exposure factors, amount of noise increases, but proportion of signal that is noise decreases |
| Noise Power Spectrum (NPS) | Noise variance analysed in terms of its spatial frequency content |
| Signal-to-Noise Ratio (SNR) | Ratio of strength of a signal to uncertainty with which it is measured as function of spatial frequencySignal-to-noise ratio (SNR) is a generic term which, in radiology, is a measure of true signal (i.e., reflecting actual anatomy) to noise (e.g., random quantum mottle)A lower SNR generally results in a grainy appearance to images |
| Contrast- to-Noise Ratio (CNR) | Amount of lesion contrast relative to amount of noise (mottle) is key determinant of visibility of a given lesion. Ratio of lesion contrast to image mottle is known as contrast-to-noise ratio (CNR). CNR is mainly used for optimisation purposes in combination with radiation doses and can be defined as ratio between lesion or structure contrast and image noise |
| Detective Quantum Efficiency (DQE) | A measure, which combines effects on modulation, spatial frequency and noise of an image receptor |
| Semi-subjective image quality | |
| Spatial resolution | Ability to reproduce small objects or to separate images of two objects close to each otherA single useful measurement is limiting resolution, being the highest resolvable bar in a resolution (line-pair) test grating. Involves raters making assessment of images of a line-pair test grating |
| Subjective image quality | |
| Contrast-detail resolution | Evaluation of image quality by grading clarity of reproduction of anatomical or pathological structures |
| Visibility of defined structures | |
| (Visual grading analyses) | |
| Artefacts | Evaluation of image artefacts from various sources, e.g. metal and motion |