Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 103

In order to better understand the RF interactions, we can consider radio antenna theory. In radio transmission for broadcasting and telecommunications, the RF field is commonly divided into different regions illustrated in Figure 2.29. Closest to the transmitter is the reactive or inductive near field. This is the mode of operation of the MR transmit coil, producing primarily a magnetic B₁‐field. This region is said to extend to λ/2π or 0.159×λ. In a 1.5 T scanner this would extend in air to 0.75 m from the iso‐centre and half that distance for a 3 T scanner, encompassing the entire coil volume.

Figure 2.29 Field regions in air and within the patient’s tissues. In the inductive near field zone the B₁ magnetic field dominates. In the radiative near field B₁ and E₁ are highly non‐uniform. For comparison the wavelengths in air and muscle are shown. Top view of the scanner bore.

Figure 2.30 Measurements of B₁ and E₁ in a gel phantom on axis and off‐axis up to 60 cm from the end of a transmit head coil in tissue equivalent material.

Source [8]. Licenced under CC BY 4.0. Licensee Frontiers.

The radiative near field or Fresnel region extends between one and two wavelengths. At 3 T with a patient in situ, this region will extend from 0.25 to 0.5 m and double those distances in a 1.5 T scanner. In the Fresnel zone the field behavior can become very complex with local maxima and minima. This results in the B and E fields extending well beyond the end of the coil within tissue. Figure 2.23 shows measurements of B and E in a phantom where significant amounts of both fields exist up to 60 cm beyond the end of a head only transmit coil, i.e. well beyond where we might otherwise consider the RF field to end [7] . This behavior has implications for patients with implants.