A modified perturbation method for tuning and testing volume head coils was developed. The common perturbation method, utilizing the change in resonance frequency of a resonator in response to the presence of a small dielectric or magnetic probe, was modified to modulate the frequency shift due to rotation of a probe. This modification enabled the RF magnetic and electric fields as well as the angular distribution of current in the longitudinal elements of the coil to be mapped. The latter serves as a quick test of magnetic field homogeneity by comparing the measured distribution with the sinusoidal function required for the field to be homogenous. High frequency (Ͼ100 MHz) MRI and magnetic resonance spectroscopy (MRS) provide valuable biomedical information due to their intrinsically higher spatial and spectral resolution and signal-to-noise ratio (SNR) (1-3). However, at these higher frequencies the use of standard humansized "bird-cage" (BC) coils (4) becomes difficult due to self-resonance and radiation losses (5). To overcome these limitations transverse electromagnetic (TEM) based head and body coils have been described (3,6 -8). This design decreases radiation losses, improves current distributions (no end ring current), and provides a better loaded quality factor (Q). However, TEM coils require more careful tuning and adjustment than a standard BC coil. Specifically, the splitting between resonance modes in the TEM coil are substantially smaller than that of the BC coil, making the magnetic field B 1 distribution of the TEM coil more susceptible to a sample distortion (7,9 -11). Adjustment of the B 1 field distribution on the bench can be time-consuming, requiring multiple measurements for each alteration of the coil. To aid in the bench optimization of RF coils, we developed a modified perturbation method to provide measurements of the magnetic and electric field distributions along with a rapid visualization of the current flow in each coil element. This method utilizes the change in resonance frequency of the TEM coil in response to the presence of a small dielectric, magnetic, or conductive probe. Previously, this effect has been applied to map fields in microwave cavity resonators operating in the GHz range and to test properties of dielectric and magnetic materials (12)(13)(14). Also, Doty et al. (15) used this method to measure the magnetic field filling factor for MRI coils.
THEORYThe resonance frequency shift due to the placement of a small dielectric or conductive body into a cavity resonator based on the perturbation theory (14) is given by:where E 1 , D 1 , H 1 , B 1 are unperturbed fields, E 2 , D 2 , H 2 , B 2 are field changes, V S and V C are small sample and cavity volumes, respectively. Substituting D 2 and B 2 with:and the numerator of Eq.[1] to four times the energy of the entire cavity W t we obtain:where E 1 and H 1 are perturbed electric and magnetic fields, respectively. Assuming fields H 1 , H 1 , and E 1 , E 1 to be homogeneous over the small sample volume V S (in our case V S ...