interactions of these fields with non‐biological matter. For those wishing to become MR Safety Experts, Chapter 2 should be read in conjunction with Appendix I supplemented by reading further reading of standard electromagnetism texts.
Revision Questions
1 The Larmor frequency of a 1.5 T MRI scanner is approximately:10 MHz42.58 MHz64 MHz85 MHz128 MHz
2 In a 1.5 T MRI scanner, if the B1+ amplitude required to produce a 90° flip angle is 10 μT, what B1+ amplitude is required in a 3T scanner if the pulse shape is unchanged?5 μT10 μT20 μT40 μT0.01 mT
3 Which of the following is true for B1?It is applied along the z‐direction along the magnet boreIt is a single sinusoidIt is a radio waveIt is generated by the x and y gradient coilsIt rotates with the magnetization precession.
4 Which of the following is untrue for the static field spatial gradient in a superconducting MRI system?It is measured in tesla per meterIt is required for image acquisitionIt is always presentIt is responsible for the translational magnetic forceIt is reduced in extent in a self‐shielded magnet.
5 If an imaging gradient system has a peak amplitude of 50 mT m−1 and a slew rate of 200 T m−1 s−1 what is the minimum achievable rise time for a full amplitude pulse?10 μs0.1 ms0.2 ms0.25 ms0.4 ms
6 Which of the following is not acceptable terminology for MR safety according to ASTM‐F2503?MR safeMR unsafeMR compatibleMR conditionalMR acceptable.
References
1 1. Organisation for Economic Co‐operation and Development (2017). Health care resources: medical technology. https://stats.oecd.org/ (accessed 12 January 2019).
2 2. Kanal, E., Borgstede, J.P., Barkovich, A.J. et al. (2002). American College of Radiology white paper on MR safety. American Journal of Roentenology 178:1335–1347.
3 3. Delfino, J.G., Krainak D.M., Flesher S.A. et al. (2019). MRI‐related FDA adverse event reports: a 10‐year review. Medical Physics doi: 10.1002/mp. 13768.
4 4. International Electrotechnical Commission (2015). Medical Electrical Equipment – Part 2‐33: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis. IEC 60601‐2‐33 3.3 edn. Geneva: IEC.
5 5. Lui, Y., Shen, J., Kainz, W. et al. (2013). Numerical investigations of MRI RF field induced heating for external fixation devices. BioMedical Engineering OnLine 12:12 doi.org/10.1186/1475‐925X‐12‐12.
6 6. ASTM F2503‐13 (2015). Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment. West Conshohocken, PA: ASTM International.
Further reading and resources
1 McRobbie, D.W., Moore, E.A., Graves, M.J., et al. (2017). MRI from Picture to Proton 3rd edn. Chapters 3, 4, 8, 9, and 10. Cambridge, UK: Cambridge University Press.
Notes
1 1 In type 1 superconductors the magnetic field inside the material is zero due to the Meissner Effect.
2 2 This is not true in tissue. See Chapter 2, page 54.
3 3 Using complex notation where, the operator eiωt signifies circular motion.
2 Let’s get physical: fields and forces
BASIC LAWS OF MAGNETISM
The fundamental laws of magnetism were summarized by Scottish physicist James Clerk Maxwell in four equations. These equations are not for the faint‐hearted nor for the mathematically challenged, but if you aspire to be an expert in MRI safety, then you should have a good understanding of their consequences. By comparison, if you did not understand Newton’s laws of gravitation or Einstein’s theory of relativity you would not become a rocket scientist. Maxwell’s equations underpin everything in electromagnetism: the biological effects of EM fields, interactions with implants, electromagnetic modeling of field exposures and specific absorption rate (SAR), projectiles and magnet safety, magnetic shielding, fringe field gradients, and acoustic noise. A full understanding requires some knowledge of vector calculus and differential equations (see Appendix 2) but for now we will not need this. Those aspiring to be MR Safety Experts should read this chapter in conjunction with Appendix 1.
Understanding Maxwell’s Equations
Maxwell’s equations are given in Appendix 1. Here we describe their main consequences for MRI safety.
Electrical charge and electric fields
Gauss’s Law (Maxwell’s first equation) describes how electrical charges produce static electric fields E. Electric fields start at a positive charge and are directed towards their conclusion at negative charges (Figure 2.1). We are not going to use Gauss’s Law much, although it has relevance in minimizing unwanted electric fields in coil design, and at some tissue boundaries where charge may accumulate.
Figure 2.1 Electric field lines begin at a source of positive charge and terminate at a negative charge: (a) single point positive charge; (b) positive and negative point charges; (c) capacitor with a potential difference V between the plates.
Magnetic fields
Maxwell’s second equation states that the “divergence of B is zero.” This means that there is no magnetic equivalent of electrical charge – no “magnetic monopoles”. Magnetic sources are not like electrostatic ones, but exist as dipoles with a north and south pole (just like the Earth). Magnetic field lines have no beginning or end, but form complete loops from north pole to south (Figures 2.2, 1.23). The nature of the B0 fringe field depends upon this.
