target="_blank" rel="nofollow" href="#ulink_4a360d7f-ba6e-53c3-9a6d-e3293ff4aae6">Figure 1.27 RF pulse consisting of the carrier (Larmor) frequency multiplied by a shape function or pulse envelope. The example shown is a truncated sinc (sinx/x) function.
This is equal to the area under the curve of the pulse envelope. Three important points arise:
1 for the same pulse shape and duration, the B1 amplitude is proportional to the flip angle;
2 for the same pulse shape and duration, the B1amplitude required to produce a given flip angle is independent of B0;
3 the peak amplitude of B1 alone is not sufficient to characterize the RF exposure.
MYTHBUSTER:
The amplitude of the B1 RF excitation pulse does not depend upon the static field strength B0.
B1+ and B1+rms
The parameter B1+rms is used to characterize the average B1 exposure. The “+” refers to the rotating component of B1 responsible for excitation of the magnetization. An efficient coil should not generate a B1−. RMS stands for root‐mean‐square and is a type of averaging used for time‐varying waveforms. For example, the RMS value for a sinusoidal waveform is 1/√2 or approximately 0.71 of the peak amplitude (Figure 1.28a). B1+rms is defined as
(1.9)
Figure 1.28 (a) the RMS value of a sinusoid is the peak amplitude divided by √2; (b) B1+ RMS for a train of N RF pulses of amplitude B1+, duration τ within time T.
calculated over 10 second intervals (T=10 s). The easiest way to visualize this is to consider a regular train of N rectangular RF pulses (Figure 1.28b), each of amplitude B1+ and duration tp. 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
