Conductive Head Mannequin Anthropomorphic (CHEMA)

 Leonardo M. Angelone, Christos Vasios, Graham Wiggins, Patrick Purdon, Giorgio Bonmassar [1]

 

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OBJECTIVE:  Design and build an anatomically accurate conductive head phantom.  The phantom can be used for temperature measurements in EEG-MRI settings with matching configuration between numerical models and experimental settings [1].

BACKGROUND. Temperature measurements on phantoms have been performed using MRI - compatible temperature probes (Luxtron Co., Santa Clara, CA; FISO Technologies Inc, Quebec, Canada) to estimate the RF-induced heating during MRI [25,29-31] and in particular when using metallic implants [32-34].  Temperature measurements on a sphere and an animal model have been used to validate results of FDTD numerical simulations [35].  In real measurements involving EEG electrodes, the conductivity of the phantom must be similar to that of the human head in order to realistically approximate the interaction between electrodes and head tissues.  Moreover, the resistance between the electrode and the phantom must be similar to that encountered in real experiments or clinical settings.  The phantom used for the measurements must provide a solid and conductive surface for an appropriate contact with the electrodes.

METHODS.A custom-made silicone mold was created from MRI data as described [1] (Figure 3B).  A transparent Plexiglas frame was built to hold the halves of the mold together (Medical Modeling LLC, Golden, CO).  The mold was composed of two equivalent parts (sagittal cut) that could be perfectly sealed together.  A hole was present at the bottom of the mold for pouring the following mix: a) 4.5 l ± 5% distilled H20, b) 135 gr Agarose Type 1A - A0169 (Sigma Aldrich), and c) 40.55 gr NaCl (purity: 98% Catalog No. 31,016-6 Sigma Aldrich) (Figure 3C).  The NaCl concentration was selected in order to have a conductivity of σ=0.6-1 S/m [39], in the range of human head tissues conductivity  at 300MHz (approximate RF frequency of our 7 T magnet) [20].  The relatively high percentage of Agarose (3%) in the final mix created a solid gel that allowed placing the EEG electrodes/leads directly on the phantom surface.  This ensured electrical connection between leads and the phantom surface during the measurements (Figure 1C).

RESULTS.  The CHEMA phantom was used in a study evaluating the effect of EEG electrodes/leads during MRI recording [1]. The study used numerical simulations and temperature measurements and showed that EEG electrodes/leads changed the electromagnetic field of the RF coil.  The effect of EEG electrodes/leads depended on the lead resistivity and confirmed the worst case when using copper leads [17].  Higher resistivity leads (e.g., carbon fiber) performed better than copper EEG leads for both SAR simulations and temperature measurements.  Overall SAR simulations did not exhibit a substantial improvement when using a discrete resistor of any value between each EEG electrode and lead.  These results were confirmed by the temperature measurements in CHEMA using the common value of 10KΩ. 

CONCLUSIONS.  The potential presence of “hot spots” and increased temperature suggest that caution should be taken when selecting EEG leads for simultaneous EEG-fMRI recordings.  Subject safety issues rise more prominently when the subject cannot communicate with the operator (e.g., patients under anesthesia) or when using high-power sequences.  The use of distributed high-resistive leads and solutions such as the RTS technology [37] are recommended.

ACKNOWLEDGEMENTS. This work was supported by NIH grants R01 EB002459and P41 RR014075 and the MIND institute.

REFERENCES

1.   Angelone LM, Vasios CE, Wiggins G, Purdon PL., Bonmassar G.  On the effect of resistive EEG electrodes and leads during 7 Tesla MRI: simulation and temperature measurement studies.  Magnetic Resonance Imaging 2006, 24: 801–812

17. Angelone LM, Potthast A, Segonne F, Iwaki S, Belliveau JW, Bonmassar G. Metallic electrodes and leads in simultaneous EEG-MRI: specific absorption rate (SAR) simulation studies. Bioelectromagnetics 2004;25(4):285-95.

20. Gabriel C, Gabriel S, Corthout E. The dielectric properties of biological tissues: I. Literature survey. Phys. Med. Biol. 1996;41:2231–2249.

25. Kangarlu A, Shellock FG, Chakeres DW. 8.0-Tesla human MR system: temperature changes associated with radiofrequency-induced heating of a head phantom. J Magn Reson Imaging 2003;17(2):220-6.

29. Shellock FG. Magnetic resonance procedures : health effects and safety. Salt Lake City, Utah: Amirsys; 2003. 456 p. p.

30. Nguyen UD, Brown JS, Chang IA, Krycia J, Mirotznik MS. Numerical evaluation of heating of the human head due to magnetic resonance imaging. IEEE Trans Biomed Eng 2004;51(8):1301-9.

31. Yang QX, Wang J, Collins CM, Smith MB, Zhang X, Ugurbil K, Chen W. Phantom design method for high-field MRI human systems. Magn Reson Med 2004;52(5):1016-20.

32. Park SM, Nyenhuis JA, Smith CD, Lim EJ, Foster KS, Baker KB, Hrdlicka G, Rezai AR, Ruggieri P, Sharan A and others. Gelled versus nongelled phantom material for measurement of MRI-induced temperature increases with bioimplants. Magnetics, IEEE Transactions on 2003;39(5):3367-3371.

33. Chou CK, McDougall JA, Chan KW. RF heating of implanted spinal fusion stimulator during magnetic resonance imaging. IEEE Trans Biomed Eng 1997;44(5):367-73.

34. Baker KB, Tkach JA, Nyenhuis JA, Phillips M, Shellock FG, Gonzalez-Martinez J, Rezai AR. Evaluation of specific absorption rate as a dosimeter of MRI-related implant heating. J Magn Reson Imaging 2004;20(2):315-20.

35. Gajsek P, Walters TJ, Hurt WD, Ziriax JM, Nelson DA, Mason PA. Empirical validation of SAR values predicted by FDTD modeling. Bioelectromagnetics 2002;23(1):37-48.

37. Bonmassar G. Resistive Tapered Stripline (RTS) in Electroencephalogram Recordings During MRI. IEEE Trans on Microw Theory and Tech 2004;52(8):1992-1998.

39. Armenean C, Perrin E, Armenean M, Beuf O, Pilleul F, Saint-Jalmes H. RF-induced temperature elevation along metallic wires in clinical magnetic resonance imaging: influence of diameter and length. Magn Reson Med 2004;52(5):1200-6.

 

Figure 1 [1]

Fig1-1

(A) Thirty two  electrodes co-registered on the anatomically accurate homogeneous head model; (B) Head model with electrodes/leads inside the end-cap TEM coil used for the simulations;  (C) Anatomically accurate phantom (CHEMA) with 32 electrodes/leads in place; (D) CHEMA with EEG electrodes/leads inside the end-cap TEM coil used for the measurements.  The similarity of dimensions, size and shape of the models allowed a more accurate comparison between simulations (top) and measurements (bottom).

Figure 3 [1] 

Fig3-1

Different construction phases of the Conductive Head Mannequin Anthropomorphic (CHEMA): 
(A) Anatomically accurate numerical head model; (B) silicone-made mold based on the head model; (C) CHEMA inside mold.

 Figure 7 [1] 

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Temperature changes observed at different probe locations during Experiment #1 for EEG electrodes/leads with 10 kΩ and without.  The increase of temperature between the two models was different outside CHEMA, on the EEG paste of Cz electrode.  Inside CHEMA, however, the increase of temperature was similar for both models, with maximum increases underneath Cz electrode.  See also Table 2.

Figure 8 [1]

Fig8-1

Left: Temperature distribution on the head model at the peak-temperature time-step (30 min after onset).  Right: Temperature changes in time during simulations at the center of the head and at electrodes Cz and Fp1.  The spatiotemporal distribution of temperature is similar to that observed during the real measurements.