In this case
(1.10)
Consequently B1+rms depends upon the
flip angle
number of RF pulses (echoes, slices, etc.)
RF pulse shape
TR.
B1 is also a time‐varying magnetic field. We can calculate the magnitude of dB/dt for a circulating field B1+e−iωt as3
(1.11)
The rate of change is proportional both to the frequency and the amplitude. As B1+ is typically μT and f in MHz, RF dB/dt is of the order of a few tesla per second.
Scanning modes
The IEC standard 60601‐2‐33 [4] defines three modes for scanning:
Normal mode: mode of operation of the MR equipment in which none of the outputs has a value that may cause physiological stress to patients.
First‐level controlled mode: mode of operation of the MR equipment in which one or more outputs reach a value that may cause physiological stress to patients which needs to be controlled by medical supervision.Software allowing access to this mode must require specific acknowledgement by the operator that the first‐level controlled mode has been entered.
Second‐level controlled mode: mode of operation of the MR equipment in which one or more outputs reach a value that may produce significant risk for patients, for which explicit ethical approval is required (i.e. a Human Studies protocol approved to local requirements).
“Outputs” refers to the magnitude of the magnetic fields. Clinical scanners are usually restricted to the Normal and First Level Modes.
OTHER MEDICAL DEVICES
Along with an understanding of MRI hardware and fields it is important to understand how these interact with other medical devices. A system to categorize the MRI safety (we used to say “compatibility”) of other devices: implants, accessories, medical equipment, tools, fire extinguishers, gas tanks, etc. uses three labels [6]:
MR Safe means that the device poses no risk to the patient in the MR environment. Image quality may be affected.
MR Conditional means that the device poses no additional risk to the patient when introduced to the MR environment under specified conditions.
MR Unsafe means that the device may not be introduced into the MR environment as it poses significant risk to the patient and/or staff.
The approved signs are shown in Figure 1.29. The “MR environment” generally means the MR examination room, or areas with a fringe field exceeding 0.5 mT, rather than just the scanner itself. The safety of implants is considered in Chapters 9–11.
Figure 1.29 MR device labeling according to ASTM‐F2503 (IEC‐62570) [6]. The two MR Safe symbols are equivalent; either may be used.
CONCLUSIONS
MRI incidents can lead to injury or death. The most frequent incidents are thermal, followed by mechanical, projectile, and hearing loss. We have considered the basic elements of MRI acquisitions, the components of the scanner and the magnetic fields encountered. There is a symmetry about the magnitude and time‐variance of the fields: B0 is of the order of tesla; the imaging gradient fields a thousand times lower, typically milli‐tesla; B1 is typically one thousand times less again, in micro‐tesla. At the same time the temporal variations range from zero to one hertz for movement in the static field, to kHz for the gradients, and MHz for B1.
In the next chapter we consider the physical 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,