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MRI system measures its own highfrequency field

Especially interesting for
  • physicians and patients
  • manufacturers of MR equipment

A transversal electromagnetic mode (TEM) cell developed at PTB allows electro-optical E-field probes to be calibrated directly in a clinical magnetic resonance imaging (MRI) system. With a first test arrangement, the uncertainty of the Efield reached at 123 MHz and 300 MHz, respectively, was better than 0.4 dB. This procedure is suited for all whole-body MRI scanners, since the TEM cell itself is broadband.

The rapid development of novel procedures of magnetic resonance imaging (MRI) requires, among other things, radio frequency (RF) electromagnetic fields within or in the immediate vicinity of the MR system to be measured with great precision. These measurements are used, on the one hand, to evaluate the efficiency of RF components of the MRI system and, on the other hand, to further develop concepts for the limitation of the specific absorption rate (SAR) in the human body, in conformity with the relevant standards. Another metrological challenge is the EMC testing of medical devices, e. g. injectors for contrast agents or implants such as, e. g., pacemakers, that are operated inside or on the body and are therefore exposed to the strong pulsed RF fields of the MRI system. The measurement of pulsed RF electromagnetic fields inside an MRI system is considerably complicated by the simultaneous presence of the static magnetic field (from 1.5 T to 7 T) and of strong magnetic field transients of > 50 T/s. Electro-optical RF field sensors are actually suited for such measurements, their calibration does, however, not exhibit a good long-term stability, as the optical components used (laser, fibre connectors) drift strongly.

For the on-site calibration of electrooptical RF field probes, a non-magnetic open TEM cell was, thus, developed as a computable field source and optimized for use in strong magnetic field transients. It is positioned in the MRI system, together with the sensor to be calibrated. The calibration procedure consists in measuring the flip angle of the nuclear magnetization of an aqueous sample in the TEM cell; the flip angle is directly related to the E-field of the TEM cell via fundamental constants. The particularity of the new procedure resides in the fact that the MRI scanner itself is used as a measuring instrument by exploiting the excited nuclear spin of the water protons in the system as “traceable” field sensors for a RF magnetic field component. This measuring concept was implemented directly in the hardware and the software of PTB’s 3-tesla MRI system and of the 7-tesla MRI system in Berlin-Buch, respectively, and used to calibrate an electro-optical E-field sensor. The measurement yielded an uncertainty of the E-field of < 0.4 dB at a frequency of 123 MHz (in the 3-tesla MRI scanner) and 300 MHz (in the 7-tesla MRI scanner), respectively. After having been calibrated in this manner, the sensor can then be immediately used for E-field measurements inside the magnet and its surroundings. The new procedure allows numerical simulations on the spreading of RF electromagnetic fields inside the human body to be validated and will, thus, facilitate compliance with requirements laid down in standards.


Frank Seifert
Department 8.1 Medical Metrology
phone: +49 (0) 30 3481-7377
e-mail: frank.seifert@ptb.defrank.hohls(at)ptb.de

Scientific publication

Seifert, F. et al.: TEM cell for calibration of an electro-optic E-field sensor in a clinical scanner. Proc. Intl. Soc. Mag. Reson. Med. 19 (2011) 3777