kehräysilmiö

The kehäräysilmiö, also known as the spin echo phenomenon, is a fundamental concept in magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy. It describes a specific sequence of radiofrequency pulses used to generate a detectable signal from a sample.

The process begins with a 90-degree pulse, which tips the net magnetization of the sample's nuclei into the transverse plane. In this plane, the magnetization starts to dephase due to variations in the local magnetic field. This dephasing causes the signal to decay. To counteract this, a 180-degree pulse is applied after a time delay. This second pulse effectively flips the magnetization vector, causing the dephased spins to rephase.

The rephasing occurs at a time equal to the initial time delay following the 90-degree pulse. At this point, the magnetization is refocused, creating a strong, transient signal called the spin echo. The amplitude of the spin echo is influenced by factors such as T2 relaxation, which represents the irreversible loss of transverse magnetization due to spin-spin interactions.

The spin echo technique is crucial in MRI for several reasons. It allows for the acquisition of images free from the effects of static magnetic field inhomogeneities, which would otherwise cause rapid dephasing and signal loss. By measuring the amplitude of spin echoes at different echo times, information about tissue relaxation times can be obtained, contributing to contrast in MRI images. The kehäräysilmiö is a cornerstone of many advanced MRI pulse sequences.