Basic principles of magnetic resonance imaging

We have come full circle from spinning quarks to three-dimensional (3D) medical images. The bulk of MRI is now performed using slice-selective gradients, where radio-frequency energy is applied to excite the hydrogen nuclei. By stepping a phase-encoding gradient during each repetition time and using a frequency-encoding gradient as the data are sampled, the 3D human object can be reduced to many individual points or voxels. By acquiring multiple slices at once, the time efficiency of imaging can be vastly improved. Many newer strategies use variations of this technique to acquire multiple lines of data during a single echo, enshrining spin warp imaging as the most important method of signal acquisition for MRI.