Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) can be used to observe signals from various nuclei, but this book focuses only on the proton (1H) of water molecules (H2O) as the nucleus of interest. This is because more than 90% of protons in the body are located in water molecules, and theMRI signal is therefore dominated by water. When we perform NMR spectroscopy, we put our sample (in this case, water) in a tube and place it inside a magnet.
We then input energy into the sample (a process called excitation) and observe the signal emitting from the sample. This is similar to ringing a bell and listening to the sound. There are only three types of information in the signal: frequency (i.e., high or low pitch), intensity (i.e., loudness), and phase. To visualize this information, we use a waveform diagram
These waves are often called time-domain data because the horizontal axis is the time elapsed while we are listening to the signal. After Fourier transform, the time axis transforms to the so-called frequency domain, in which the horizontal axis is frequency
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