ABSTRACT
In order to generate an image, the protons in the region of the fetus must be caused to emit a signal. Once the pregnant patient is placed into the main magnetic field, protons align with the field, resulting in a net magnetization pointing in the same direction as the main magnetic field. This net magnetization is known as the longitudinal magnetization. However, the protons generating the longitudinal magnetization do not generate a signal until they are “excited” with a radio-firequency (RF) pulse tuned to the resonant frequency of the protons at the static field strength that is being used for imaging. This excitation causes some fraction of the longitudinal magnetization to be transferred into a plane perpendicular to the longitudinal axis. This plane is often referred to as the transverse plane, and magnetization in this plane is referred to as transverse magnetization. The amount of magnetization transferred from the longitudinal axis to the transverse plane can be visualized as tipping the longitudinal magnetization by an angle. Thus RF pulses are often characterized by the angle of the tip they produce; for example, a 90 or 180 pulse.
