Figure 1 The basics of the NMR signal. The discussion of Section II.A is represented here in a simplified, cartoon form, with the story proceeding from left to right. Protons can be thought of conceptually as positively charged spheres that are always spinning, and this spin about an axis gives the proton an inherent orientation as well as a net magnetic moment along the axis of the spin. Before entering the magnet, the protons (with their magnetic field and spin direction indicated by arrows) are randomly oriented. There is no main field surrounding the body (Bo=0) and there is no induced field within the body (M0=0). The body is placed in the main magnet (B0 b 0). All the protons immediately start processing in a direction around B0, but because the orientation and phases are random there is no net signal. After a few seconds, a small fraction of the protons change orientation to line up with B0, which results in the creation of a net magnetic field in the body (M0 > 0) oriented in the same direction as B0. The individual protons are still precessing, now with a common net orientation, but the components of that rotation perpendicular to B0 are still random so there is no detectable signal. A radio frequency (RF) pulse is applied for a brief period of time, causing all the protons to change their orientation by 90° so that the net induced magnetic field M0 is now perpendicular to B0. Now the individual precessing protons are aligned in such a way that the common component of orientation (M0) goes around B0 in a perpendicular direction and generates a macroscopically detectable current—the signal. The strength of that signal decreases exponentially with time for a variety of reasons described in Section II.B.

from different points in the three-dimensional volume. (The application of these nonuniform magnetic fields causes the raw NMR signal to decay even more rapidly than the FID, but the signal can still be measured, and various procedures can be used to create "echos" that enable the recovery of more information.)

The technology of MRI is based on the flexible (but complicated) application of multiple RF pulses and multiple gradients, synchronized precisely (and typically described in a pulse sequence diagram). The pulse sequence diagram indicates how a given slice of the brain is selected for imaging, how individual volume elements (voxels) are detected within each slice, and how the resulting signals are preferentially selected to obtain information about arterial blood flow or about the concentration of deoxyhemoglobin in venous blood flow. Some imaging pulse sequences use multiple RF-pulses in the generation of NMR signals that will yield a single plane of imaging data. Some imaging pulse sequences [such as echo-planar imaging (EPI)] generate data for an entire plane from a single RF pulse. EPI is rapid (with an entire plane collected in less than 50 msec), but it is associated with more expensive hardware and various limitations in spatial resolution or susceptibility to imaging artifacts and distortions.

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