<unk>  is the ability to distinguish between differences in intensity in an image. The measure is used in medical imaging to quantify the quality of acquired images. It is a difficult quantity to define because it depends on the human observer as much as the quality of the actual image. For example, the size of a feature affects how easily it is detected by the observer. One definition of image<unk>  is: : where SA and SB are signal intensities for signal-producing structures A and B in the region of interest. A disadvantage of this definition is that the<unk>  C can be negative. An alternative definition is: : where Sref is a reference signal intensity, which is independent of the type of signal-producing structure under investigation. This series of pictures shows that the number of row of holes that are perceived decreases as the image<unk>  decreases from 5.1% to 3.7% to 2.2% to 1% (left to right) In MRI, determining<unk>  is of high importance for calibration because the operator has a high degree of control of how the signal intensities of various structures vary in the images by using different MRI methods and imaging parameters. Unlike most other imaging modalities, such as x-ray CT in which the Hounsfield units value for water is set to zero, there is no standard reference signal for MRI. Thus the<unk> -to-noise ratio is often employed as an index for<unk>  because this metric does not require a reference signal.<unk>  or<unk> -detail is an approach to describing the image quality in terms of both the image<unk>  and<unk> .<unk>  is usually measured by generating a pattern from a test object that depicts how image<unk>  changes as the structures being imaged get smaller and closer together. The picture below shows one such set of images produced using the low<unk>  detectability inserts of the phantom employed in the MRI accreditation program of the American College of Radiology.
Contrast resolution
Closing capacity
Core stability
Neuronal tuning
Diastolic function
Partial volume 
Concept creep
Tissue selectivity