09-04 Definition of Contrast
There is only one step from picture elements to image contrast.
Contrast itself is a quite controversial term in medical imaging. It describes the relative difference of intensities of two adjacent regions within an examined object on a gray or color scale. Several definitions of contrast have been proposed during the years.
It is quite difficult to give an exact definition of contrast on a conventional x- ray image. Here, definition of contrast is merely qualitative, except when using a special measuring device – or digitizing the analogue image. Digitalization of images in nuclear medicine and x-ray CT opened the door to more straightforward quantitative approaches to contrast. Now, picture elements are available. Their gray-scale intensity can be expressed in numbers. The numerical difference between two intensities allows quantitative definition of contrast.
If there is no difference between two neighboring pixels, they cannot be distinguished and thus no contrast exists. The bigger the difference in the intensity of two pixels, the better will be the contrast (Figure 09-06).
A quantitative definition of contrast is given by the following equation:
C = (Ia - Ib) / (Ia + Ib)
where C is contrast and Ia and Ib are the signal intensities of two adjacent pixels or voxels.
It is important to understand that image intensity in magnetic resonance imaging is not standardized. MR imaging does not possess any correlation to Hounsfield units in x-ray CT. The signal intensity of an MR image can represent a mixture between T1-, T2-, and ρ-values, flow, diffusion, perfusion, and other factors influencing the signal emitted by structures within a volume element.
Only normalization of images, e.g., with a water-filled vial outside the patient's body, allows an approximation to be made and can be used to calculate relative signal intensities, which then can be compared. However, these values are only semiquantitative. They vary between different MR equipment and have no diagnostic value.