Radiofrequency (RF) coils are components of every scanner and are used for two essential purposes — transmitting and receiving signals at the resonant frequency of the protons within the patient [Schenck, 1993]. The precession occurs at the Larmor frequency of the protons, which is proportional to the static magnetic field. At 1T this frequency is 42.58 MHz. Thus in the range of field strengths currently used in whole-body scanners, 0.02 to 4 T, the operating frequency ranges from 0.85 to 170.3 MHz. For the commonly used 1.5-T scanners, the operating frequency is 63.86 MHz. The frequency of MRI scanners overlaps the spectral region used for radio and television broadcasting. As an example, the frequency of a 1.5-T scanner is within the frequency hand 60 to 66 MHz, which is allocated to television channel 3. Therefore, it is not surprising that the electronic components in MRI transmitter and receiver chains closely resemble corresponding components in radio and television circuitry. An important difference between MRI scanners and broadcasting systems is that the transmitting and receiving antennas of broadcast systems operate in the far field of the electromagnetic wave. These antennas are separated by many wavelengths. On the other hand, MRI systems operate in the near field, and the spatial separation of the sources and receivers is much less than a wavelength. In far-field systems, the electromagnetic energy is shared equally between the electric and magnetic components of the wave. However, in the near field of magnetic dipole sources, the field energy is almost entirely in the magnetic component of the electromagnetic wave. This difference accounts for the differing geometries that are most cost effective for broadcast and MRI antenna structures.
Ideally, the RF field is perpendicular to the static field, which is in the z direction. Therefore, the RF field can be linearly polarized in either the x or y direction. However, the most efficient RF field results from quadrature excitation, which requires a coil that is capable of producing simultaneous x and y fields with a 90-degree phase shift between them. Three classes of RF coils — body coils, head coils, and surface coils — are commonly used in MRI scanners. These coils are located in the space between the patient and the gradient coils. Conducting shields just inside the gradient coils are used to prevent electromagnetic coupling between the RF coils and the rest of the scanner. Head and body coils are large enough to surround the legion being imaged and are designed to produce an RF magnetic field that is uniform across the region to be imaged. Body coils are usually constructed on cylindrical coil forms and have a large enough diameter (50 to 60 cm) to entirely surround the patient’s body. Coils are designed only for head imaging (Figure 12.12) have a smaller diameter (typically 28 cm). Surface coils are smaller coils designed to image a restricted region of the patient’s anatomy. They come in a wide variety of shapes and sizes. Because they can be closely applied to the region of interest, surface coils can provide SNR advantages over head and body coils for localized regions, but because of their asymmetric design, they do not have uniform sensitivity.
A common practice is to use separate coils for the transmitter and receiver functions. This permits the use of a large coil — such as the body coil — with a uniform excitation pattern as the transmitter and a small surface coil optimized to the anatomic region — such as the spine — being imaged. When this two-coil approach is used, it is important to provide for electronically decoupling of the two coils because they are tuned at the same frequency and will tend to have harmful mutual interactions.
FIGURE 12.12 Birdcage resonator. This is a head coil designed to operate in a 4-T scanner at 170 MHz. Quadrature excitation and receiver performance are achieved by using two adjacent ports with a 90-degree phase shift between them. (Reprinted with permission from Schenck and Leue, 1991.)
