This lecture will cover basic safety issues related to MRI, focusing on power deposition and radio-frequency heating in the patients. Specific absorption rate (SAR) and its relation to temperature will be discussed. Various methods to simulate, predict,control and mitigate SAR and temperature will be introduced. Finally, the effects of RF coil geometry, field strength/frequency will be explained.
The objective is to give a brief introduction to the following:
- Specific Absorption Rate (SAR), temperature and it's relation to patient safety in MRI
- Current/future methods for quantifying, limiting, and mitigating SAR & temperature
- RF safety issues related to imaging patients with implants
-The effects of field strength and RF coil design
MRI utilizes radio-frequency (RF) magnetic-field pulses that excite the spins in the body to generate images. Although undesirable, an RF electric field, is often generated in the body as a result of this process. Power absorbed by the body under this electric field is determined by the specific absorption rate (SAR) and needs to be kept at a level that is safe to the patients(1). In addition to the SAR limits, safety limits are present for absolute temperature in the body,head and the extremities(1).
The bio-heat equation (2) establishes the relation between SAR and the temperature distribution in the body. The equation uses tissue dependent thermal parameters such as specific heat capacity,thermal conductivity and perfusion.These parameters can be assumed constant or temperature dependent(3,4).
Electromagnetic and thermal simulations can be performed in order to predict SAR and temperature. However as it has been shown in previous studies, accurate modeling of the coils and the subject is crucial, especially when the field strength increases (5-7). The electric field of the coil strictly depends on the tissue distribution, therefore using realistic human models is preferred( 8,9). Working with subject specific models is also an option, and is shown to enable more accurate modeling of the SAR and the temperature.
In addition to computational efforts, imaging methods are developed to predict SAR from subject-specific B1 ( a component of transverse magnetic field of the RF coil) measurements(10). Similar approach is also governed to quantify conductivity variation in the body which is crucial for SAR calculation. These methods have their limitations because only some and not all components of the magnetic field can be measured with MRI.
Optimizing RF coils has also a huge impact on SAR and temperature. Depending on the application and the field strength, different coil designs may have advantages over the others in terms of patient safety, power efficiency and image homogeneity (11,12).
Another approach to mitigate SAR and manage RF power deposition is to optimize RF excitation pulses. It has been shown that RF pulses can be re-designed for multi-channel excitation in order to control SAR (13) and temperature(14) while preserving/improving image homogeneity.
RF field interactions may also pose risk to patients with metallic implants(15). Implants with long conductive structures interact with RF fields and may heat up excessively. Various methods have been proposed to tackle with this problem, involving modification of the implant (16) and the RF coil/excitation (17-19).
1)IEC. International Electrotechnical Commission. International standard,. Medical equipment: particular requirements for thesafety of Magnetic resonance equipment, 3rd edition.Geneva 2010
2)Pennes HH. Analysis of tissue and arterial blood temperatures in the resting humanforearm (Reprinted from Journal of Applied Physiology, vol 1, pg 93-122, 1948). J ApplPhysiol 1998;85(1):5-34.
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6)Jin J, Liu F, Weber E, Crozier S. Improving SAR estimations in MRI using subject-specific models. Phys Med Biol 2012;57(24):8153.
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8) Ackerman MJ. The visible human project. Proceedings of the IEEE 1998;86(3):504-511.
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10) Katscher U, Voigt T, Findeklee C, Vernickel P, Nehrke K, Doessel O. Determination of electric conductivity and local SAR via B1 mapping. IEEE transactions on medical imaging. 2009 Sep;28(9):1365-74.
11)Guérin B, Gebhardt M, Serano P, Adalsteinsson E, Hamm M, Pfeuffer J, Nistler J, Wald LL. Comparison of simulated parallel transmit body arrays at 3 T using excitation uniformity, global SAR, local SAR, and power efficiency metrics. Magnetic resonance in medicine. 2015 Mar 1;73(3):1137-50.
12) Erturk MA, Wu X, Eryaman Y, Van de Moortele PF, Auerbach EJ, Lagore RL,DelaBarre L, Vaughan JT, Ugurbil K, Adriany G, Metzger GJ. Toward imaging the body at 10.5 tesla. Magn Reson Med 2016.
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14)Boulant N, Wu X, Adriany G, Schmitter S, Ugurbil K, Van de Moortele PF. Directcontrol of the temperature rise in parallel transmission by means of temperature virtualobservation points: Simulations at 10.5 Tesla. Magn Reson Med 2016;75(1):249-256
15)Henderson, J. M., Tkach, J., Phillips, M., Baker, K., Shellock, F. G., & Rezai, A. R. (2005). Permanent neurological deficit related to magnetic resonance imaging in a patient with implanted deep brain stimulation electrodes for Parkinson’s disease: case report. Neurosurgery, 57(5), E1063.
16)Bottomley, P. A., Kumar, A., Edelstein, W. A., Allen, J. M., & Karmarkar, P. V. (2010). Designing passive MRI-safe implantable conducting leads with electrodes. Medical physics, 37(7), 3828-3843.
17)Eryaman Y, Akin B, Atalar E. Reduction of implant RF heating through modification of electric field. Magn Reson Med 2011;65:1305–1313.
18)Gudino N, Sonmez M, Yao Z, Baig T, Nielles-Vallespin S, Faranesh AZ, Lederman RJ, Martens M, Balaban RS, Hansen MS, Griswold MA. Parallel transmit excitation at 1.5 T based on the minimization of a driving function for device heating. Medical physics. 2015 Jan 1;42(1):359-71.
19)Etezadi-Amoli M, Stang P, Kerr A, Pauly J, Scott G. Controlling radiofrequency-induced currents in guidewires using parallel transmit. Magnetic resonance in medicine. 2015 Dec 1;74(6):1790-802.