(LCD) The low contrast detectability represents the ability of a CT scanner to distinguish between objects that have similar x-ray attenuation coefficients. In cases of other diagnostic imaging modalities it means the ability of imaging equipment to differentiate between objects that have similar properties.

Image quality is an important value of all radiographic imaging procedures. Accurate measures of both image quality and patient radiation risk are needed for effective optimization of diagnostic imaging. Images are acquired for specific purposes, and the result depends on how well this task is performed. The imaging performance is mainly influenced by the imaging procedure, examined object, contrast agents, imaging system, electronic data processing, display, maintenance and the operator. Spatial resolution (sharpness), contrast resolution and sensitivity, artifacts and noise are indicators of image quality.
A high image contrast provides the discrimination between tissues of different densities.
The image resolution states the distinct visibility of linear structures, masses and calcifications.
Noise and artifacts degrade the image quality. In computed tomography (CT), high spatial resolution improves the visibility of small details, but results in increased noise. Increased noise reduces the low contrast detectability. Noise can be reduced by the use of large voxels, increased radiation dose, or an additional smoothing filter, but this type of filter increases blurring.
An image acquisition technique taking these facts into account maximizes the received information content and minimizes the radiation risk or keeps it at a low level.

See also As Low As Reasonably Achievable.

(LCR) The low contrast resolution describes the ability to discriminate between tissues with slightly differences in attenuation properties. The LCR depends on the stochastic noise.
The low contrast resolution is usually expressed as the minimum detectable size of an image structure, for a fixed percentage difference in contrast relative to the adjacent background.
A strength of computed tomography (CT) is its ability to visualize structures of low contrast in an object, a task that is limited by noise and is closely associated with the radiation dose. For example, a reduction of the dose at constant spatial resolution affects the visibility of structures with low contrast (e.g. vessels in the liver), due to increased noise. The visibility of these low contrast structures can partly be improved by decreasing the spatial resolution, while keeping the dose constant.

See also CT Number, Image Quality and Low Contrast Detectability.