Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 30

In most instances a body RF transmit coil is used (Figure 1.17). This typically has a “birdcage” design, operating in quadrature to produce a circularly‐polarized magnetic field B₁. Some coils may operate in transmit and receive mode (T/R) denoted symbolically as in Figure 1.18. Examples are dedicated T/R head and knee coils. Tx coils usually have a cylindrical geometry, entirely encompassing the anatomical region to produce a uniform B₁ so that everything in the FOV experiences the same flip angle.

Figure 1.17 Transmit 8‐rung ‘birdcage’ coil to produce a circularly polarised (rotating) B₁₊ field orthogonal to B₀.

Figure 1.18 IEC 60601‐2‐33 [4] compliant coil labelling: left‐ transmit only; middle‐ transmit‐receive; right‐ receive only.

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.