| Zugriffsnummer | 26470 |
| Dokumenttyp | Konferenzartikel |
| Sprache | Englisch |
| Titel | Assessment of RF induced heating of coronary stents in 7T MRI [poster] |
| Autor(in); Institution |
Santoro, D.; Berlin Ultrahigh Field Facility, Max-Delbrück-Centrum für Molekulare Medizin, Berlin, GERMANY
Vogt, J.M.; Department of Physics, Humboldt University Berlin, GERMANY
Renz, W.; Siemens Healthcare, Erlangen, GERMANY
Gellermann, J.; Experimental and Clinical Research Center (ECRC), Charité Campus Berlin-Buch, GERMANY
Seifert, Frank; 8.1, Medizinische Messtechnik, PTB-Berlin
Tkachenko, V.; Experimental and Clinical Research Center (ECRC), Charité Campus Berlin-Buch, GERMANY
Schulz-Menger, j; Experimental and Clinical Research Center (ECRC), Charité Campus Berlin-Buch, GERMANY
Niendorf, T.; Berlin Ultrahigh Field Facility, Max-Delbrück-Centrum für Molekulare Medizin, Berlin, GERMANY
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| Quelle/Jahr | Proceedings of the International Society for Magnetic Resonance in Medicine ; ISMRM 19th annual meeting & exhibition: 19 (2011), 3770 |
| Availability | [CD-ROM] ; file name: 3770.pdf |
| ISSN | 1545-4428 |
| Verlag | Berkeley, Calif.: ISMRM |
| Konferenzangaben | ISMRM-ESMRMB Joint Annual Meeting ; ISMRM 19th Scientific Meeting, Montreal, 07-13, May, 2011, Kanada |
| Zusammenfassung | Introduction: The baseline signal-to-noise ratio (SNR) advantage of ultrahigh field MRI holds the promise to enhance, spatial and/or temporal resolution in MRI. Such improvements would benefit an ever growing set of indications for cardiovascular MR (CMR), including the characterization of ischemic disorders on themyocardial tissue level through mapping microstructures, and parametric imaging. However, intracoronary stents used for treatment of coronary artery disease(CAD) are currently considered to be contra-indications for CMR at 7.0 T. The presence of a metallic implants in combination with RF wave lengths and RF powerdeposition used at 7.0 T may induce local heating which might cause myocardial tissue damage, influence coagulation or endothelial function. For all these reasons it isessential to carefully assess RF induced heating in coronary stents commonly used in percutaneous coronary intervention. To meet this goal this work examines RFinduced heating of coronary stents in agarose phantoms using electromagnetic field (EMF) simulations, fiber optic temperature measurements and MR thermometry at 7.0 T. Methods: An eight rung highpass circular polarized birdcage RF coil with an inner diameter of 20 cm was build to provide RF power which is beyond that of clinicalstandards. Cylindrical phantoms (V=600ml, d=15cm diameter, 4% agar gel, NaCl-concentration = 3g/l) were setup. A cobalt chromium alloy coronary stent with l=4 cmand d=4 mm (Biotronik, Bülach, CH) was placed into one phantom while the other agarose phantom was used as a control. EMF simulations were performed using thedesign and geometry of the birdcage coil and of the phantoms (conductivity of 0.77 S/m, the relative permittivity εr= 58.24). A FDTD method (CST software, Darmstadt, Germany) was used for EMF and temperature simulation. The proton resonance frequency method (PRF) was used to monitor temperature changes in the phantom experiments. This method derives the temperature changes (ΔT) from the phase difference (Δφ) of two MR images: ΔT = Δφ /(α·TE·γ B0) where α is the temperature-dependent chemical shift coefficient (-0.01 ppm/K). A variation of this method has been used, which makes use of the difference of two sets of phase images acquired with different TE to cancel residual phase contributions. For validation, temperature was also measured in four different locations of the phantom using a fiber optics system (Luxtron). RF heating was achieved by repeating the single pulse experiment, with FA=360° (transmit voltage =231V) , TR=1.2ms, Averages=240 (5minutes). After every 5 minutes of RF heating, two 3D-GRE images were acquired (TR=20ms, TEs=4/10ms, FA=20°, FOV=128x128x20cm3, totalacquisition time= 72s). Results: Figs. 1, 2 demonstrate qualitative agreement between simulated and experimental temperature change maps. The simulated map was derived for the stationarysolution. Hence it shows much higher ΔT. Fig. 3 shows temperature changes in the presence of a metallic stent in an agarose phantom. For increased accuracy RF irradiation with an absorbed power of 11W resulting in a SAR value of 18.3W/kg was used, which is far beyond the local SAR limits given by the IEC guidelines. Dueto the MRI artifact induced by the stent itself, data obtained right at the stent are not considered. Consequently only areas in close proximity to the stent, which showedenough signal in the magnitude image were included. Fig 4 shows the temperature raise in time in the experiment of Fig.3, measured with fiber optics and MR thermometry. Discussion: Our results show an overall agreement between the simulated temperature maps in a virtual phantom and the temperature maps derived from phantom experiments using MR thermometry. For the chromium alloy coronary stent no extra hot spots were evident in the 3D temperature maps in our setup. Further careful investigations are required before intracoronary stents can be declared to be MR safe in a 7.0 T environment since the presence of metallic stents disturbs the image quality in the close vicinity of the stents. |
Zitierung
Santoro, D., Vogt, J., Renz, W., Gellermann, J., Seifert, F., Tkachenko, V., Schulz-Menger, J., & Niendorf, T. (2011). Assessment of RF induced heating of coronary stents in 7T MRI [poster]. ISMRM-ESMRMB Joint Annual Meeting ; ISMRM 19th Scientific Meeting, Montreal, 07-13, May, 2011, Kanada.