Basic Principles of Nuclear Magnetic Resonance for Medical Imaging
In 1946, the phenomenon of nuclear magnetic resonance (NMR) was discovered independently by F. Bloch and E.M. Purcell. They exploited the characteristic resonance signals of different structures as a physicochemical analytical tool. However, it took anoth
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Basic Principles of Nuclear Magnetic Resonance for Medical Imaging GEORG M. BONGARTZ and HILDE BOSMANS
characteristic frequency and under two possible orientations, parallel or anti-parallel to the axis of the 3.1 Introduction 47 magnetic field. The orientation can be destroyed by 3.2 Nuclear Magnetism and external (electro-) magnetic energy at the same resoNuclear Magnetic Resonance 48 nance frequency. Thereafter, the spins return to a state 3.2.1 Isolated Spin 48 of equilibrium and release the excitation energy as 3.2.2 A System of Spins: electromagnetic radiation at their characteristic freThe Macroscopic Nuclear Magnetic Moment 49 3.2.3 Excitation of the Spin System 49 quency. The return to equilibrium, also called nuclear The Free Precession Signal 50 3.2.4 spin relaxation, is characterized by relaxation times 3.2.5 Spin Echo 50 that are specific for each biological tissue. The differ3.3 Relaxation 52 ence between relaxation times of various tissues is the Sequences 53 3.4 source of contrast in NMR imaging. Due to the fact 3.4.1 Spin Echo Sequences 53 Gradient Echo Sequences 54 3.4.2 that the frequency is a characteristic of the particular nuclei, and due to both the bioavailability of protons and their large magnetic moment, the resultant magnetic signal of protons is detected in MR imaging. The signal produced by a global excitation of all the hydrogen spins of the human body does not pro3.1 vide any "spatial" information. P. Lauterbur was the Introduction first to apply additional linear magnetic field gradiIn 1946, the phenomenon of nuclear magnetic res- ents for spatial encoding of the signal. Still today, onance (NMR) was discovered independently by F. in order to obtain an image of the hydrogen spins, Bloch and E.M. Purcell. They exploited the charac- the electromagnetic excitation is combined with teristic resonance signals of different structures as the application of small additional magnetic fields a physicochemical analytical tool. However, it took whose amplitudes change as a function of position another 30 years to introduce the technique into med- (magnetic field gradient). These gradients modulate (encode) the resonance frequency of the nuclear spins ical imaging. NMR is based on the fact that some nuclei possess as a function of their position. Spectral analysis of the an intrinsic angular momentum or a spin that can be nuclear spin signal after the excitation provides the expressed in the presence of a magnetic field. This spatial distribution of the signal, i.e., the NMR image, angular momentum provides the nucleus with mag- the intensity of which depends on the density and netic properties similar to those of a compass needle. relaxation of the hydrogen spins in the particular tisHydrogen nuclei have such a nonzero magnetic sues. The basic instrumentation for the acquisition of moment. When placed in an external magnetic field, these hydrogen nuclei (or protons) begin a preces- NMR images follows from the above description: sional motion around the direction of the field with a 1. A magnet
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