The coils operate in a resonant mode as tuned circuits, resulting in current amplification to achieve greater B1 at the Larmor frequency. They are driven by powerful RF amplifiers, rated at tens of kilowatts (kW). An important aspect of RF generation is impedance matching, usually to 50 Ω (ohms), to ensure the maximum power transfer from the amplifier to the coil. B1 is of the order of micro‐tesla (μT) peak amplitude.
In 3 T systems, operating at 128 MHz, the B1‐field in tissue is often quite non‐uniform. In this instance parallel transmit systems can help. These utilize multi‐element Tx coils powered by independent amplifiers capable of changing both the amplitude and phase (relative direction) of the RF pulses (Figure 1.19).
Figure 1.19 Parallel transmit: two (or more) independent RF power amplifiers drive elements of the transmit coil.
RF reception
The purpose of the RF receiver coils is to detect the tiny (micro‐volt) MR signals. A parallel tuned circuit is used (Figure 1.20) to magnify the voltage prior to pre‐amplification and further processing. The receive coil requires protection circuitry to prevent the large transmit pulses from coupling into the coil. A simple means of achieving this is to use crossed diodes and a detuning capacitor. During the large Tx pulses the diodes conduct and so the total capacitance becomes the sum of both capacitors and the circuit is off‐resonance. During signal detection the diodes do not conduct, and Cd is “invisible.” A fault in this circuitry can lead to large induced currents in the coil and potential heating or burns for the patient.
Figure 1.20 Receive coil and pre‐amplifier: (a) the coil has inductance L and resistance R; Cd is a detuning capacitor; (b) response of the coil during signal reception (red line) and RF transmission (blue line).
ELECTROMAGNETIC FIELDS
The extent and magnitude of the fields involved in MRI are summarized in table 1.1 and Figure 1.21.
Table 1.1 Magnetic fields in MRI.
| Field | Amplitude | Frequency / Slew rate | Pulse duration |
| Static field B0 | 0.2‐7 T | 0 Hz | Always present |
| Static fringe field spatial gradient dB/dz | 0‐25 T m–1 | 0 Hz | Always present |
| Imaging gradients Gx, Gy, Gz | 0‐80 mT m–1 | 0‐10 kHz 0‐200 T m–1 s–1 | 0‐10 ms |
| RF transmit field B1 | 0‐50 μT | 8‐300 MHz | 0‐10 ms |
Figure 1.21 Relative magnitude of magnetic fields used in MRI.
Static field
Definition of magnetic flux density and the tesla
Whilst MR practitioners commonly refer to their magnets in terms of “magnetic field strength”, this nomenclature is scientifically incorrect. The proper term is magnetic flux density, denoted as B. B is a vector field with components in each direction Bx, By and Bz. MRI is only sensitive to Bz and that is what we refer to colloquially as the “field.” Magnetic flux density has the SI (International System) unit of the tesla (T). An older unit is the gauss (G). One tesla equals 10 000 G.
The scientific definition of the tesla is in terms of force. Referring to Figure 1.22, one tesla is the amount of magnetic flux density which exerts a force of one newton (N) on a charged particle of charge one coulomb (C) moving at right angles to the field direction with a velocity of one meter per second (m s−1). It’s not an easy definition, but the fact that it is defined in terms of force is highly apt for MR safety!
Figure 1.22 Definition of the SI unit tesla.
MYTHBUSTER:
The unit of “magnetic field strength” is not the tesla, but is amperes per meter. B is the magnetic flux density.
So, what is magnetic field strength in actuality? It is given the symbol H and has units of amperes per meter (A m−1). It is defined in terms of a cylindrical electromagnet, just like our scanner – the current in the windings generates an H‐field. In free space
(1.5)
μ0 is the magnetic permeability in a vacuum, equal to 4π x10−7 henrys per meter (H m−1).
One way of visualizing magnetic fields is through magnetic field lines. If you have ever done the experiment of introducing iron filings to the proximity of a simple bar magnet you may have observed the pattern shown in Figure 1.23a. These illustrate the magnetic “lines of force”. A small compass needle positioned anywhere will align with these. We can think of the magnetic flux density as being the intensity of grouping of these lines: the more closely grouped together, the stronger the B‐field.
