Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 102

Figure 1.26 showed the uniformity of the B₁‐field in air produced by a typical birdcage transmit coil. However, the presence of the patient’s tissues affects the nature and amplitude of the field (Figure 2.28). In a dielectric medium the EM wavelength λ has a different value than in air or vacuum and the familiar equation for wavelength (λ = c/f) becomes

(2.33)

Figure 2.28 B₁ uniformity map for a head phantom at 7 T showing the effect of (left) non‐optimized and (right) optimized parallel transmission. Figure courtesy of Kawin Setsompop, Massachusetts General Hospital, A.A. Martinos Center for Biomedical Imaging.

where c is the speed of light (3×10⁸ ms⁻¹).

The “wavelength” in air is around 4.7 m at 64 MHz (1.5 T) and 2.3 m at 128 MHz (3 T) and B₁ in air is a magnetic field in the near field region with minimal E‐field components, except close to the coil windings and tuning capacitors. However, the dielectric constant, ε_r of tissues changes the wavelength within the patient. Often the relative permittivity ε_r of water, with a value stated to be around 80, is used to estimate wavelength in tissue. This reduces the wavelength at 64 and 128 MHz to 0.51 m and 0.26 m. These dimensions are comparable to the patient’s dimensions and also to the body transmit coil dimensions.