Читать книгу Essential Concepts in MRI - Yang Xia - Страница 11

This book explores the physics phenomenon that provides the foundation for and the engineering architectures that facilitate the widespread applications of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI). NMR is the physics phenomenon at the basis of every MRI experiment. The first word, “nuclear,” refers to the core player of this phenomenon – stable atomic nuclei. The protons in a common water molecule are the most useful nuclei because of their high sensitivity and simplicity. Please make a note that when we say proton in NMR and MRI literature and in this book, we mean hydrogen atom, not the nucleon. Since these nuclei are stable, there is never any radioactivity in NMR. The second word, “magnetic,” refers to the environment that these nuclei must have – the nuclei need to be immersed in a magnetic field, which can be generated in several ways including the use of a permanent magnet. The third word, “resonance,” refers to a concept in physics where a system has the tendency to oscillate at the maximum amplitude at a certain frequency f (Figure 1.1). This resonance system can be mechanical (e.g., the pendulum studied by Galileo Galilei in 1602, and the collapse of several suspension bridges in Europe in the 1800s by marching soldiers), acoustic (e.g., many musical instruments), and electromagnetic (e.g., an electronic receiver in your radio and television). To receive the signal from a particular channel or station among the tens or hundreds of channels and stations available, the resonant frequency of a receiver in a radio or television set is adjusted either manually by turning a knob/dial in an analog circuit of a classical (i.e., pre-digital) radio or TV, or by scanning automatically over a range of frequencies in digital receivers. When the right frequency is met, the signal can reach the maximum.

Figure 1.1 The resonance phenomenon, where the signal amplitude reaches a maximum at a particular frequency f0.

Let us clarify the terminology of the NMR phenomenon, since it has several acronyms as well as sub-fields. NMR is the original and full name of the phenomenon, which now commonly refers to its physical principles. NMR spectroscopy is the spectroscopic application of NMR, which seeks the chemical information in the process; this term is used commonly in basic science and in particular in physics and chemistry. NMR imaging is the imaging application of NMR, which mainly seeks the spatial information in the process; this term is used mainly by the non-medical imaging community. MRI is identical in content to NMR imaging, which is the term that is commonly used in the medical community (and by everyone else who is not in basic science). Microscopic MRI (µMRI) and NMR microscopy are the high-resolution versions of MRI.