A contrast medium (or contrast agent) is a chemical substance introduced to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue, by altering the relaxation times.
The chemical composition of the contrast media determines the specific usage. Similar to nuclear imaging is the intention in development of MR contrast media a high affinity to different organs or even tumors (e.g. necrosis avid contrast agent).
In 'contrast' to nuclear imaging contrast agents MR contrast media do not contain radiopharmaceuticals and the concentrations are about 100 times higher. Nuclear imaging contrast agents are direct contrast agents;; they are directly visible caused by their radioactivity. MR contrast agents affect the targeted tissue; they are indirect contrast agents.
See also Contrast Agents, the info sheet gives an overview and more in-depth information about different types of MRI contrast medium.

Low-contrast resolution: The ability to image objects with similar subjects contrast; contrast detectability.
High-contrast resolution: The ability to image small objects having high subject contrast and spatial resolution.

Contrast reversal is an MR image phenomenon where the dark parts become bright, and the bright parts become dark. This is usually most prevalent in sequences utilizing an extended TR.

(CNR) In Magnetic Resonance Imaging MRI, Contrast to noise ratio is the relationship of signal intensity differences between two regions, scaled to image noise. Improving CNR increases perception of the distinct differences between two clinical areas of interest. A contrast to noise ratio is a summary of SNR and contrast. It is the difference in SNR between two relevant tissue types.
(A and B): CNR = SNRA - SNRB

See also Signal Intensity, Signal to Noise Ratio and Medical Imaging.

Convolution is a mathematical way of combining two signals to form a third signal. It is the single most important technique in digital signal processing. This operation is mostly used together with Fourier transformations for MRI signal / image processing.