Contrast in MR Imaging

The observer's ability to differentiate between different structures in images depends on image contrast. MRI has become an important diagnostic tool in medical imaging because it provides the necessary contrast between various soft tissues required to identify pathologic processes. Image contrast, C(A,B), between two different structures, A and B, within the object may be defined as follows:

J-ref where 1(A) and 1(B) are the image intensities of A and B, respectively; Iref is an arbitrary reference intensity [1], In the case of MRI, there is no standard lref, therefore, the term "image contrast" is often used to characterize the difference between the intensities 1(A) and 1(B).

Variations in local relaxation times 7\ and T2, and the amount of hydrogen nuclei per unit volume (known as proton density or N(H)) are the predominant sources of contrast in proton MRI. The use of local relaxation times for identification of pathology by MRI was inspired by the observation that malignant tissue can have longer T} and T2 than normal tissue [2]. Although the relaxation properties of 1H nuclei in tissue are not completely understood, there is evidence that the pathological states of different tissues are characterized by specific biochemical processes that can alter relaxation times and proton density [3,4],

In this chapter we consider different mechanisms of contrast in 1H MRI as well as techniques used to improve contrast in MR images. The emphasis of the following discussion is on the relationship between image contrast and NMR relevant tissue parameters, such as inherent relaxation times and proton density.

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