Chapter 4 - Contrast in MR Imaging

Abstract

Magnetic resonance imaging (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. The observer's ability to differentiate between different structures in images depends on an image contrast. This chapter considers different mechanisms of contrast in MRI as well as techniques used to improve contrast in Magnetic resonance (MR) images. The emphasis is on the relationship between image contrast and NMR relevant tissue parameters, such as inherent relaxation times and proton density. The dependence of MRI contrast on intrinsic relaxation times and proton density can be studied by using the equation describing steady-state transverse magnetization in a given pulse sequence. Certain materials, known as contrast agents, can enhance MR image contrast by altering relaxation times. Contrast agents are frequently used in diagnostic MRI in order to achieve better assessment of local physiologic and anatomic conditions, or to improve detection of malignancy. Because contrast agents are usually administered internally, they must possess low toxicity and be easily excreted from the body. Unlike contrast agents used in nuclear medicine and clinical radiography, MRI contrast agents affect the signal indirectly via interaction with the hydrogen nuclei. Contrast agents that significantly alter the local magnetic field in the specimen are frequently referred to as susceptibility agents.