Generating a Signal: RF Pulses and Echoes

Abstract

In order to generate an MR signal from the net magnetisation, the RF oscillating magnetic field is used to deliver energy into the population of protons at the Larmor Frequency. The Larmor frequency (resonant frequency) is proportional to the strength of the magnetic field and is typically in the Megahertz range. The RF field is normally applied as a short pulse, known as an rf pulse. When it is applied, the proton magnetic moments start to rotate together about the main magnetic field, causing the net magnetisation to move away from its alignment with the main magnetic field and to rotate around it. The greater the amount of energy applied by the RF pulse, the greater the angle that the net magnetisation makes with the B_o field. This angle that the net magnetisation is rotated through by the RF pulse is known as the flip angle. Commonly used RF pulses for imaging are a low flip angle RF pulse, a 90° RF pulse, 180° RF refocusing pulse and a 180° RF inversion pulse. The low flip angle and 90 RF pulses are used as excitation pulses and generate a transverse component of magnetisation which rotates about the main magnetic field direction, generating an oscillating magnetic field that is detected by the RF receiver coil as an MR signal. The MR signal generated by a single RF pulse gradually decays and is known as a free induction decay or (FID).