DK1212_C005 - 5 Magnetic Resonance Imaging Magnetic...

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177 5 Magnetic Resonance Imaging Magnetic resonance imaging (MRI) is based on a rather complex physical phenomenon of nuclear magnetic resonance (Bloch 1946 [50]; Purcell et al., 1946 [51]), which is basically the exchange of energy between elementary particles placed in a strong magnetic field and the irradiating electromagnetic field of a particular fre- quency. It is, precisely taken, governed by laws of quantum mechan- ics and should be described in corresponding terms. Nevertheless, from the viewpoint of medical image data acquisition, processing, and interpretation, the description at the level of individual atoms or their nuclei is, in most respects, not necessary. We will thus present a macroscopic approximation, introduced originally by Bloch (1946), where large sets of nuclei are the subject of observation or measure- ment. These sets (sc., spin isochromats * ) can often be considered to encompass a volume of a voxel— a three-dimensional space element of the discrete image data. In some cases, when some of the param- eters influencing the behavior of the group are not homogeneous * The name is derived from the property that the nuclei in the group are precessing at the same frequency — see below. © 2006 by Taylor & Francis Group, LLC
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178 Jan throughout the voxel volume, a subvoxel view is necessary; never- theless, even such a smaller subvolume still contains a huge number of particles, allowing the acceptance of the macroscopic view when analyzing the external magnetic behavior of the subgroup. MRI is entirely dependent on digital processing of the mea- sured data that have to be converted to the image form by means of algorithms based on the theory underlying this book. While it is impossible here to go into details of many branches of the highly developed MRI field, the purpose of this chapter is to give a consis- tent and comprehensible explanation of the principles of MRI data acquisition and processing, which are generally considered difficult (and often described in an obscure or vague way). The explanation in Sections 5.1 to 5.5 is thus intended to clarify the image-forming procedures in the manner enabling the user to understand and well interpret the image properties. Definitely, it should not be considered an in-depth explanation of MRI phenomena or imaging practices. Particularly, the advanced quantum physics of nuclear magnetic resonance is completely omit- ted, as well as the construction details and technical solutions of MRI systems, and particularities of MRI in specialized diagnostic applications. For a more detailed study or for further reference, see [9–11], [15], [26], [30], [34], [37], [38], [47]. 5.1 MAGNETIC RESONANCE PHENOMENA 5.1.1 Magnetization of Nuclei A single proton (the nucleus of a hydrogen atom or a particle of another atomic nuclei), besides being positively charged, has a prop- erty called spin , which can be attributed to rotational movement of the proton, to enable easier understanding. The charged rotating particle has a magnetic moment m , like a magnetic dipole. When
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This note was uploaded on 02/27/2008 for the course BME 525 taught by Professor Singh during the Fall '07 term at USC.

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DK1212_C005 - 5 Magnetic Resonance Imaging Magnetic...

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