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MR technology

Research group 8.11

MR coil development: Simulation, metrological characterization and patient safety

The objective of these activities is to develop and metrologically characterize radio-frequency coils for high-field and ultra-highfield MRI, in particular of multi-channel (pTx) coils for parallel transmission. The use of these RF coil arrays allows a better control of the distribution of the radio-frequency magnetic B1 fields, for example in order to homogenize the distribution of the excitation field B1+ in the human body or to maximize it at specific positions in the body.

For this purpose, the coils are controlled individually and coherently with different HF amplitudes and phases. Due to this type of control, both the spatial distribution of the specific absorption rate (SAR) in the body and also the local increase in the tissue temperature associated with it depend on a great number of parameters. Therefore, ensuring patient safety, for example in accordance with Norm IEC 60601-2-33 , represents a great metrological challenge. Another subject of the working group's current research with impacts on the further development of the relevant standards IEC 60601-2-33 and ISO TC 10974 is the safety assessment of metallic implants in the pTx-capable MRI.

For a detailed characterization of pTx coils, the local distribution of the specific absorption rate in the body and the maximum temperature increase associated must, therefore, be calculated for different steering conditions of the pTx arrays to be characterized with sophisticated simulation procedures, and the parameters which are important for patient safety must be determined.

The simulation calculations are validated by in-situ measurements in the MR scanner or by bench measurements in the RF laboratory. For this purpose, a novel metrological infrastructure has been developed, especially within the scope of the EMRP-Project HLT06 'MR Safety' in which both MR-based measurement procedures and sensor-based measurement procedures are used. The in-situ calibration of time domain E and H field probes in the MR scanner by means of an MR-TEM cell is, thereby, decisive for the metrological quality of the model validation. 

 

 

Fig.1: Comparison of the measured (a) and calculated (b) |B1+| distribution for a 4-channel CSA array, in the case of which only the top element was driven with 1 kW of RF power. Next to it (c), the counter-rotating component |B1-|. After a global scaling of the simulation data with a factor of 1.08, these components agree quantitatively with the experiment. Bottom: horizontal (d) and vertical (e) profiles of the |B1+| distributions.

 

Fig.2: Setups for the metrological characterization of MR coils in an MR scanner. These comprise diverse phantoms, data acquisition systems for MR-compatible fiber-optical E and H field probes as well as MR-compatible calibration facilities.

 

Fig.3: MR-TEM cell for in-situ calibration of fiber optic E and H field probes in the MR scanner. In the lower compartment, a small sphere filled with water is located in a polystyrene block. A flip angle measurement allows the transversal E and H field components in the upper compartment of the TEM cell to be traced back to the gyromagnetic ratio of water protons. The MR-TEM cell is a broadband device (up to 500MHz) and, thus, suited for all MR field strengths which are currently relevant.

 

Left:

Fig.4: Waveguide (60 cm in diameter) for measurement of the far-field radiation of MR coils for 7-tesla MRI. At frequencies of approx. 300 MHz, a significant part of the HF power applied to an MR coil is irradiated. A reliable safety assessment requires the determination of the complete power balance of the coil.

Right:

Fig.5: MR-compatible RF current probe with fiber optic signal transducer for in-situ measurements in the MR scanner of RF currents in metallic implants. The design of the RF current probe is based on a Rogowski coil with slotted RF screen. Calibration in the scanner is performed with a special RF coil which is controlled via the MR-TEM cell with a calibrated RF signal.

 

Left:

Fig.6: 2-channel pTx head coil for 7-tesla MR spectroscopy. The coil is approved for use in human beings and allows |B1+| fields of up to 45µT to be generated in the back of the head.

Right:

Fig.7: 4-channel pTx heart coil for 7-tesa MRI.

 

Fig.8: 8-channel pTx head coil for 7-tesal MRI. The coil allows |B1+| fields of up to 50µT to be generated in the brain of humans and is, thus, particularly suited for MR spectroscopy.

 

Fig.9: Measuring setup with 8-channel pTx head coil for 7 tesla to determine the B1+ fields (amplitude and phase) of all coil channels with a fiber optic time-domain H field sensor.

 

Fig. 10: Correlation of peak spatial SAR (psSAR10g - averaged over 10g) with peak brain tissue temperature for an 8-channel 7-tesla pTx-head coil. 500 different steering vectors with a total forward power of 8W (red squares) or a maximum power of 1.5W per channel (black squares) were examined. The control vector with the highest psSAR10g leads to an only moderate heating of the brain tissue during the measurement.

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Selected references

G. Weidemann, F. Seifert, W. Hoffmann, H. Pfeiffer, R. Seemann, B. Ittermann
Measurements of RF power reflected and radiated by multichannel transmit MR coils at 7T
Opens external link in new windowMagn Reson Mater Phy DOI 10.1007/s10334-016-0551-6.

F. Seifert, G. Wübbeler, S. Junge, B. Ittermann, H. Rinneberg
Patient safety concept for multichannel transmit coils
J Magn Reson Imaging 26, 1315-21 (2007).

E. Kirilina, A. Kühne, T. Lindel, W. Hoffmann, K. H. Rhein, T. Riemer & F. Seifert
Current CONtrolled Transmit And Receive Coil Elements (C2ONTAR) for Parallel Acquisition and Parallel Excitation Techniques at High-Field MRI
Opens external link in new windowAppl Magn Reson 41, 507-23 (2011).

F. Seifert, E. Kirilina, T. Riemer
Transmitter/receiver antenna for MR with improved decoupling between antenna elements
United States Patent US 7, 916, 920 B2.

L. Winter, E. Oberacker, C.Ozerdem, Y. Ji, F. von Knobelsdorff-Brenkenhoff, G. Weidemann, B. Ittermann, F. Seifert, T. Niendorf
On the RF Heating of Coronary Stents at 7.0 Tesla MRI
Opens external link in new windowMagnetic Resonance in Medicine (2014).

A. Kühne, S. Goluch, P. Waxmann, F. Seifert, B. Ittermann, E. Moser, E. Laistler
Power balance and loss mechanism analysis in RF transmit coil arrays
Opens external link in new windowMagnetic Resonance in Medicine (2014).

T. Klepsch, T. D. Lindel, W. Hoffmann, H. Botterweck, B. Ittermann, F. Seifert
Calibration of fiber optic RF E/H-field probes using a magnetic resonance (MR) compatible TEM cell and dedicated MR measurement techniques
Opens external link in new windowBiomed Tech 57, 119-22 (2012).

A. Graess, W. Renz, F. Hezel, M. A. Dieringer, L. Winter, C. Oezerdem, J. Rieger, P. Kellman, D. Santoro, T. D. Lindel, T. Frauenrath, H. Pfeiffer, Th. Niendorf
Modular 32-Channel Transceiver Coil Array for Cardiac MRI at 7.0T
Opens external link in new windowMagnetic Resonance in Medicine (2014).

O. Bottauscio, A. M. Cassarà, J. W. Hand, D. Giordano, L. Zilberti, M. Borsero, M. Chiampi, G. Weidemann
Assessment of computational tools for MRI RF dosimetry by comparison with measurements on a laboratory phantom
Opens external link in new windowPhysics in Medicine & Biology 60, 5655-5680 (2015).

A. Kuehne, P. Waxmann, W. Hoffmann, H. Pfeiffer, R. Seemann, F. Seifert, and B. Ittermann
Parallel transmission experiments using an extensible RF pulse generator
Proc Intl Soc Mag Reson Med 21, 4404 (2013).

F. Seifert, G. Weidemann, B. Ittermann
Correlation of psSAR and tissue specific temperature for 7T pTx head coils - a large scale simulation study
Proc Intl Soc Mag Reson Med 23, 380 (2015).

F. Seifert, G. Weidemann, B. Ittermann
Q matrix approach to control implant heating by transmit array coils
Proc Intl Soc Mag Reson Med 23, 3212 (2015).

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