MR Physics & Techniques for Clinicians: Spin Echo Imaging

Spin echo imaging is often called the workhorse of MRI as it is frequently a large part of many clinical protocols. By definition, the “spin echo” is the occurrence of transverse magnetization that has been re-phased by a refocusing RF pulse. Note that transverse magnetization exists following an excitation RF pulse that tips the magnetization from the equilibrium, longitudinal direction into the transverse plane. The spin echo effect was explained by Erwin Hahn in 1950 [1], and further developed by Carr and Purcell who pointed out the advantages of using a 180° refocusing RF pulse for the second pulse [2]. Another improvement was made by Meiboom and Gill in orientating the axes of the excitation and refocusing RF pulses perpendicular to each other [3]. These two improvements give rise to the term “CPMG condition”. The figure below shows the spin echo pulse sequence, which illustrates a couple of important parameters when describing the pulse sequence: TE: time to echo from the excitation RF pulse TR: repetition time between adjacent excitation RF pulses