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The main strength of MR derives from the relatively large dynamic range of gray scale or intensity values between different types of tissues that the contrast mechanisms of this imaging modality generate. The microscopic properties of tissues that produce these differences in intensity values in MR images include proton density (p), spin-lattice relaxation time (T1), spin-spin relaxation time (T2), flow, susceptibility, magnetization transfer, and diffusion, among others. Of these, the first three mechanisms were the first to be utilized for clinical MR imaging. All three arise from microscopic magnetic properties of the nuclei, i.e., the protons of the water molecules that abound in human tissue. The MR imaging experiment consists of translating the differences in the value of these properties into differences in intensity values and mapping their spatial dependence, i.e., establishing a one-to-one correspondence between different points in the anatomy with different pixels in the image. Since there are a number of textbooks (1–3) devoted entirely to the physics and/or clinical aspects of MR imaging and spectroscopy, we will not consider the details in this chapter.Instead, we will describe aspects of MR imaging and spectroscopy in sufficient detail so as to enable the reader to both understand the techniques and tailor their use to the needs of the specific clinical questions pertaining to CNS infections. |
