By Dr. Ir. Marinus T. Vlaardingerbroek, Dr. Ir. Jacques A. den Boer (auth.)
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Extra resources for Magnetic Resonance Imaging: Theory and Practice
Example text
A An x-gradient coil. b The actual fie ld in the (x , z) plane. 1o v Gxx dxdydz, ( 1. 17) where V is the volume. The integration in the x d irection yields a factor proportional to r3. The y integration yields a fa ctor proportional to rand the z dimensions of a gradient coil always scale with r, so the z integration also yields a factor proportional to r. 17) is therefore proportional to r 5. 38 1. MRI and its Hardware energy decreases by almost a factor of 2. We return to this point in the next section.
The total vector sum of the magnetic moments within a volume element considered (several cubic mm) decreases and so the total 16 1. MRI and its Hardware magnetization and the induced signal decrease. The total decay caused by both effects is called "T:; decay" and is two to three times as fast as pure T2 decay. The decay mechanisms are further disc Lissed in more detail ill Sect. 3. Actually the FID is not measured in imaging applications (as opposed to spectroscopy applications), since the measurements can start only some time after the effective excitation time (which is the center of the excitation pulse at the maximum of the main RE lobe), so that the T:; process already has partly destroyed the signal.
Their temperature must be very constant, since the field changes with temperature at about 1000 ppm per K. 3 System Architecture ---f--+'---jL---~ 31 H(Alm ) Fig. 17. Magnetization curve for ferroelectric material Permanent magneti c mate rial IRQNYOKE... Fig. 18. Integration path l for a permanent magnet with iron yoke Another type of magnets based on permanent magnetic material is the cylindrical magnet. Examples are shown in Fig. 19. Zijlstra [15] has shown for the magnet in Fig. 19a, that when the angle of the magnetization in a certain point is twice the angle between the position vector and the vertical, we have an ideal homogeneous vertical field (note that the field direction is transverse and differs from that in a superconducting magnet; this has consequences for the RF coil design).