In magnetic resonance imaging (MRI) scanners the magnetic field B strongly varies based on the coil resistance. Coil resistance may be affected by patient load, and reactance, which is affected by off-resonance. In conventional MRI systems, time consuming power adjustments and tuning are required to correct for changes in the impedance of the transmission coil. If not properly adjusted, the off-resonance caused therefrom may yield negative effects on multi-slice imaging, on off-center imaging, and may cause other image quality degradation issues.
MRI radio frequency (RF) coils create the B1 field that rotates the net magnetization in a pulse sequence. RF coils may also detect precessing transverse magnetization. The difference in energy between the two orientations for the nuclei subjected to the B0 and B1 fields depends on the type of atom and the strength of the B0 field. Higher strength B0 fields may produce an increased signal to noise ratio (SNR) but may also cause other issues. For instance, increasing the B0 field strength increases the energy differential between the two orientations but requires more RF energy to induce the transition between the orientations, which in turn increases the frequency of the RF signal required to produce a B1 field. For example, RF energy applied at a frequency of approximately 64 MHz is used to bring a hydrogen nucleus into resonance in a 1.5 T B0 field while RF energy applied at a frequency of approximately 300 MHz is used to bring the same hydrogen nucleus into resonance in a 7 T B0 field.
Coils may be used for transmitting RF energy that is intended to cause nuclear magnetic resonance (NMR) in a sample. The frequency at which NMR will be created depends on the magnetic field present in the sample. Both the main magnetic field B0 produced by the MRI apparatus and the additional magnetic field B1 produced by a coil contribute to the magnetic field present in the sample. For a circular loop coil, the transmit B1 field equals the coil sensitivity. A circular loop of radius a carrying a current I produces on axis the field: B=μ0 I a2/[2(a2+z2)3/2].
An imaging coil needs to be able to resonate at a selected Larmor frequency. Imaging coils include inductive elements and capacitive elements. The resonant frequency, v, of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit (e.g. LC circuit) according to:
  v  =      1          2      ⁢      Π      ⁢              LC            
Positioning coils in a transmit array may produce a more uniform B1 field. However, transmit arrays produce additional problems. For example, to produce a uniform B1 field it may be necessary to control the current flowing through each coil of the array. However, an RF pulse is defined by a voltage level input to an amplifier and thus unique loading of different coils in the transmit array may lead to different currents on the different coils. Different coils may experience unique loading due, for example, to different properties in different tissues being imaged and the proximity of the different coils to those different tissues
Additionally, RF coils for MRI may need to be tuned and matched. Tuning involves establishing or manipulating the capacitance in a coil so that a desired frequency is produced. Matching involves establishing or manipulating the resistance in a coil so that a desired resistance is achieved. When tuning, the impedance z may be described by Z=R+jX=1/(1/(r+jLω)+jCω). Tuning may be performed to achieve a desired tuning frequency for a coil. ω0 identifies the desired tuning frequency. ω0, may be, for example, 63.87 MHz at 1.5 T. The size of a conventional coil facilitates estimating inductance L. With an estimate of L in hand, values for capacitors can be computed to produce a desired resonant peak in an appropriate location with respect to ω0. Once capacitors are selected, the resonant peak can be observed and a more accurate L can be computed. The matching can then be adjusted to produce the desired resistance. Once the desired resistance is achieved, then the coil and its driver are power matched.
A conventional loop coil has elements that produce a resistance (e.g., coil copper trace and the coil loading) and that produce an inductance (e.g., copper trace). A conventional loop coil may include a matching capacitor and a tuning capacitor. Conventionally, the resistor, inductor, and capacitor may all have been two terminal passive elements that were soldered to copper wire or copper foil that was attached to a printed circuit board.