ABSTRACT
Magnetic resonance (MR) is a well-known diagnostic technique today widely used especially for cardiac and neurological applications. MR has an excellent spatial resolution and a good temporal resolution, and allows obtaining high-quality clinical images often essential to the diagnosis or monitoring of several diseases. Moreover, MR is considered a safe technology because it has just the ability to change the position of atoms, but not to alter their structure, composition, and properties, as the ionizing radiations do. In fact, the electromagnetic radiations involved in an MR procedure (see Figure 10.1) have no enough energy to detach electrons from atoms
or molecules, such as other higher energetic radiations can do (X-rays, radiations used in nuclear medicine, etc.). However, as in any sanitary intervention, even in an MR diagnostic procedure there are intrinsic hazards that must be understood, acknowledged, and taken into consideration; for this reason, the analysis of the interaction between the magnetic fields and the biological tissues that undergo an MR procedure is essential. These interactions are caused from different physical phenomena which, on the one hand, are responsible for the signal generation that contribute to the final image but, on the other hand, can cause dangerous biological effects for the patient or signal artifacts. The study of these interactions has become more important in the past years due to growing interest for high-static magnetic
Valentina Hartwig
CONTENTS
10.1 Introduction ............................................................................................................................................................... 191 10.2 Effects of Static MF ................................................................................................................................................... 193
10.2.1 Magnetic Induction .................................................................................................................................. 193 10.2.2 Magneto-Mechanical Effects ................................................................................................................... 193 10.2.3 Electron Spin Interaction ......................................................................................................................... 194 10.2.4 Other Effects .............................................................................................................................................. 194
10.3 Effects of Gradient MF ............................................................................................................................................. 194 10.4 Effects of RF MF ........................................................................................................................................................ 195 10.5 Effects of Combination of Static, Gradient, and RF MFs ..................................................................................... 196 10.6 Other MR-Related Safety Issues ............................................................................................................................. 196
10.6.1 Quench Hazards in Superconducting Magnets ................................................................................... 196 10.6.2 Contrast Agents ......................................................................................................................................... 196 10.6.3 Claustrophobia .......................................................................................................................................... 197 10.6.4 Pregnancy .................................................................................................................................................. 197
10.7 Risks for MR Occupational Workers ...................................................................................................................... 197 10.8 Biomedical Implants ................................................................................................................................................. 197 10.9 MR Safety Guidelines and Regulations ................................................................................................................ 198 10.10 Methods to Quantify Interaction between MR Field and Biological Tissue: Dosimetry ............................... 199
10.10.1 Static MF ..................................................................................................................................................... 200 10.10.2 Gradient MF ............................................................................................................................................... 200 10.10.3 RF Magnetic Field ..................................................................................................................................... 201
10.10.3.1 Analytical Dosimetry ............................................................................................................ 202 10.10.3.2 Numerical Dosimetry ........................................................................................................... 202 10.10.3.3 Experimental Dosimetry ...................................................................................................... 205
10.11 Conclusion ................................................................................................................................................................. 206 References ................................................................................................................................................................................ 207
field (MF) MR scanners, which assure a higher signalto-noise ratio (SNR), and hence a better quality for the final images, but imply heavier risks for patients and occupational workers. Although the radiation used is not ionizing, there are several effects to be considered for safety assurance and engineering aspects relative to MR signal and image generation. The knowledge of these phenomena is important not only for the design of transmission/reception coils and acquisition sequences but also for the choice of acquisition parameters for each diagnostic exam.
