Contrast mechanisms in MR imaging.
This paper is a brief introduction to tissue-specific parameters and the utilization of various MR imaging sequences to display these parameters in order to differentiate normal from pathologic tissue and function. The three dominant tissue-specific parameters discussed are proton density, longitudinal relaxation time T1, and transverse relaxation time T2. For the utilization of gradient-echo sequences, transverse relaxation time T2(*) is introduced, more dependent on the environment or tissue interfaces than on the tissue itself. Another tissue-specific parameter is the concentration of macromolecules and their hydration layers as targeted with magnetization transfer imaging. Still another tissue-specific parameter is the chemical environment. Functional parameters that influence the contrast are diffusion, perfusion, flow, or motion. The sequence-related utilization of these tissue-specific parameters start with magnetization preparation as in spectral suppression of fat signal, relaxation-dependent elimination of fat or cerebrospinal fluid (CSF) signal, simple inversion pulse, magnetization transfer saturation, or diffusion weighting. Possible contrast mechanisms for the tissue-specific parameters are discussed for each of the commonly used sequences, whether of spin-echo type or of gradient-echo type, with or without magnetization preparation, conventional single-echo acquisition, or contemporary multiecho acquisition.
Institut für Röntgendiagnostik des Klinikums der Universität Regensburg, Franz-Josef-Strauss-Allee 11, D-93042 Regensburg, Germany.
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Nitz
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Reimer
D003287:Contrast Media; D006801:Humans; D007089:Image Enhancement; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*
D003287:Contrast Media