Design and calibration of electric field probes in the range 10-120 MHz.
In view of potential thermal hazards, there is a need to determine the specific absorption rate (SAR) distributions associated with radiofrequency coils used in magnetic resonance imaging (MRI) (typically 10-120 MHz). Electric field (E-field) distributions in tissue-equivalent phantoms may be determined using a probe comprising a dipole antenna and a detector. The geometry of the dipole dictates the sensitivity of the device, thus two designs are discussed in this paper. Both probes are compact, have a spatial resolution of 2.5 cm3, operate at MR frequencies and have a response independent of the dielectric characteristics of the phantom material. Calibration of these probes requires a system capable of producing a known E-field both in air and in a tissue-like medium at frequencies between 10 and 120 MHz. Transverse electromagnetic wave (TEM) cells answering these specifications are described and the calibration procedure outlined. Accurately calibrated E-field probes can make field measurements in phantoms which can be used to verify predictions from numerical models. These numerical techniques may then be used to predict E-fields, and hence SAR, in patients.
Taylor
H C
HC
Radiological Sciences Unit, Hammersmith Hospital, London, UK.
Burl
M
M
Hand
J W
JW
D002138:Calibration; D004867:Equipment Design; D008279:Magnetic Resonance Imaging / Q000009:adverse effects / Q000295:instrumentation* / Q000592:standards; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D011827:Radiation; D012680:Sensitivity and Specificity