The NMR Signal

To begin an MRI session, the subject is placed horizontally into the bore of a high-field magnet. In a typical clinical MRI system this magnet has a field strength of 1.5 T. A small fraction of the hydrogen nuclei (single protons) of the water molecules in the body of the subject become aligned with the field of this magnet. (Many other atoms and nuclei with magnetic moments are also aligned, but for the purposes of this article and almost all fMRI applications, it is the hydrogen nuclei of water molecules that are the source of the signal.) The hydrogen nuclei are oriented in a random collection of directions, relative to the main magnetic field, but there is a small statistical preference to have the longitudinal component of their orientations (i.e., the component in the direction of the main magnet) to be aligned with the main field. To the extent that a proton is not perfectly aligned with the main magnet, it will precess (spin) around the orientation of the main magnet. However, because the orientation of each proton is random (except for the component along the direction of the main field), and because it is only the randomly oriented transverse components (i.e., the components perpendicular to the main field) that generate a signal, the collection of spinning protons do not yield a net, detectable magnetic field.

Application of a radio frequency (RF) pulse of magnetic energy, presented at the frequency of the precession (i.e., the resonant frequency), causes all the hydrogen nuclei to change orientation (nutate). By controlling the power and duration of the RF pulse, the nuclei can be rotated to any desired angle relative to the main magnetic field. Typically, the parameters of the device are set so that the protons are rotated 90°. When the protons continue to precess in this new orientation, the net magnetic field that was originally induced in the body by the main magnet, and that was previously aligned with the main magnet, is now oriented 90° away and thus generates a detectable, changing magnetic field as it spins (precesses). A coil of wire around the subject will have a current generated within because of the changing magnetic field (Faraday's law). This current is the raw signal detected in a MRI scanner (Fig. 1).

fact that each individual proton experiences a slightly different local magnetic field due to interactions with nearby water molecules and other biological tissues and thus precesses at a slightly different frequency from its neighbors. With time (typically on a scale of tenths of seconds for most brain tissue) these protons get out of phase so that their respective magnetic fields are no longer lined up and therefore do not generate a detectable, macroscopic signal in the surrounding coil of wire. This is sometimes called the spin-spin component of transverse relaxation because it is based on the interaction of the spins (which imply magnetic fields) of nearby nuclei. If the magnetic field were perfectly uniform, then the net decay rate of the signal would be given by the exponential decay rate T2, which is driven by the combination of spin-spin transverse relaxation and the T1 longitudinal component. In the brain tissue of interest, T2 is almost entirely determined by the spin-spin relaxation.

In reality, there are other sources of magnetic field nonuniformity. Imperfections in the main magnet, variable magnetic susceptibility of the differing parts of the human body that has been inserted into the magnet, and changes in blood chemistry caused by externally injected "contrast agents'' all contribute to nonuniformities in the magnetic field experienced by the precessing protons. Most important for fMRI, some chemicals that occur naturally in the body also distort the magnetic field. Deoxyhemoglobin is such a molecule, and as its local concentration is varied, the amount of distortion also varies. The rate of exponential decay of the NMR signal is influenced by all of these factors (Fig. 1).

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