Signal Generation

Abstract

This chapter provides some basic information regarding the origin of the MRI signal. The MRI signal is generated by the interaction of applied magnetic fields with the nuclei of hydrogen atoms in the body. Hydrogen nuclei (protons, or “spins”) tend to align themselves with the large static magnetic field generated by the MRI system, and rotate or precess about the direction of that field at a characteristic frequency called the Larmor frequency. The application of additional radiofrequency (RF) energy at the same frequency excites the magnetized protons causing them to tip into the plane perpendicular to the main field. The magnetized protons which have been perturbed in this fashion undergo a process of relaxation that returns them back into alignment with the main field. During the course of relaxation, a signal is emitted which is detected using receiver coils and digitally sampled. The relaxation of spins is governed by time constants known as T₁ and T₂, and these time constants play a major role in determining the contrast between tissues in an image. The encoding of spatial information is accomplished using magnetic field gradients that alter the precession frequency of spins based on their position in the scanner. The Fourier transform is then used to reconstruct an image from the encoded data. Many of the basic concepts introduced in this chapter are covered in greater detail in later chapters.