MRIsafety.com

251
Bioeffects of Radiofrequency Fields

Bioeffects of Radiofrequency Fields

The majority of the radiofrequency (RF) power transmitted for MR imaging or spectroscopy procedures is transformed into heat within the patient’s tissue as a result of resistive losses. Not surprisingly, the bioeffects associated with exposure to RF radiation are related to the thermogenic aspects of this electromagnetic field.

Prior to 1985, there were no reports concerning the thermophysiologic responses of human subjects exposed to RF radiation during MR procedures. Since then, many investigations have characterized the thermal effects of MR-related heating.

SPECIFIC ABSORPTION RATE

Thermoregulatory and other physiologic changes that a human subject exhibits in response to exposure to RF radiation are dependent on the amount of energy that is absorbed. The dosimetric term used to describe the absorption of RF radiation is the specific absorption rate (SAR). SAR is the mass normalized rate at which RF power is coupled to biological tissue and is typically indicated in units of watts per kilogram (W/kg). The relative amount of RF radiation that an individual encounters during an MR procedure is designated as the whole-body-averaged SAR. Other SAR levels relative to the body part exposed or peak SAR level (i.e. the amount in one gram of tissue) may also be reported by the MR system.

Measurements or estimates of SAR are not trivial, particularly in human subjects. There are several methods of determining this parameter for the purpose of RF energy dosimetry. The SAR that is produced during an MR procedure is a complex function of numerous variables including the frequency (i.e. determined by the strength of the static magnetic field of the MR system), the type of RF pulse used (e.g., 90° vs. 180° pulse), the repetition time, the type of transmit RF coil used, the volume of tissue contained within the transmit RF coil, the shape of the anatomical region exposed, as well as other factors.

With regard to RF energy, the U.S. Food and Drug Administration currently indicates that MR procedures that exceed certain SAR values may pose significant risks.

THERMOPHYSIOLOGIC RESPONSES TO MR PROCEDURE-RELATED HEATING

Thermophysiologic responses to MR procedure-related heating depend on multiple physiological, physical, and environmental factors. These include the duration of exposure, the rate at which energy is deposited, the response of the patient’s thermoregulatory system, the presence of an underlying health condition, and the ambient conditions within the MR system.

In regards to temperature regulation in human subjects, when exposed to a thermal challenge, the human body loses heat by means of convection, conduction, radiation, and evaporation. Each mechanism is responsible to a varying degree for heat dissipation, as the body attempts to maintain thermal homeostasis. If the thermoregulatory effectors are not capable of dissipating the heat load, an accumulation or storage of heat occurs along with an elevation in local and/or overall tissue temperatures.

Various health conditions may affect an individual’s ability to tolerate a thermal challenge including cardiovascular disease, hypertension, diabetes, fever, old age, and obesity. In addition, medications including diuretics, beta-blockers, calcium blockers, amphetamines, and sedatives can alter thermoregulatory responses to a heat load. Certain medications have a synergistic effect with RF radiation with respect to tissue heating. The environmental conditions (i.e. ambient temperature, relative humidity, and airflow) that exist in the MR system will also affect tissue temperature changes associated with RF energy-induced heating.

The first study of human thermal responses to RF radiation-induced heating during an MR procedure was conducted by Schaefer, et al. Temperature changes and other physiologic parameters were assessed in volunteer subjects exposed to a relatively high, whole-body-averaged SAR (approximately 4.0-W/kg). The data indicated that there were no excessive temperature elevations or other deleterious physiologic consequences related to exposure to RF energy.

Several studies were subsequently conducted involving volunteer subjects and patients undergoing MR procedures, with the intent of obtaining information that would be applicable to patients typically encountered in the MR setting. These investigations demonstrated that changes in body temperatures were relatively minor (i.e. less than 0.6 degrees C). While there was a tendency for statistically significant increases in skin temperatures to occur, these were of no serious physiological consequences.

Interestingly, various studies reported a poor correlation between body temperature and skin temperature changes versus whole-body-averaged SARs associated with clinical MR procedures. These findings are not surprising considering the range of thermophysiologic responses that are possible in human subjects relative to a given SAR level. For example, as previously indicated, an individual’s thermoregulatory system can be greatly impacted by the presence of an underlying condition or medication that can impair the ability to dissipate heat.

An extensive investigation by Shellock, et al. (1994) was conducted in volunteer subjects exposed to MR examinations performed at a whole-body-averaged SAR of 6.0-W/kg. To date, this is the highest level of RF energy that human subjects have been exposed to in association with MR procedures. Tympanic membrane temperature, six different skin temperatures, heart rate, blood pressure, oxygen saturation, and skin blood flow were monitored. The findings indicated that an MR procedure performed at a whole body averaged SAR of 6.0-W/kg can be physiologically tolerated by an individual with normal thermoregulatory function.

VERY-HIGH-FIELD MR SYSTEMS

Clinical MR systems now operate at a static magnetic field strength of 3-Tesla, many research scanners operate at 4-Tesla and 7-Tesla, and one at 9.4-Tesla. These very-high-field MR systems are capable of depositing RF power that exceed those associated with a 1.5-Tesla MR system. Therefore, investigations are needed to characterize thermal responses in human subjects to determine potential thermogenic hazards associated with the use of these powerful MR systems, especially since frequency related differences likely exist. To date, several reports have studied MR procedure-related heating associated with very-high-field MR systems utilizing modeling techniques as well as other experimental methods.

REFERENCES

Alon L, et al. Method for in situ characterization of radiofrequency heating in parallel transmit MRI. Magn Reson Med 2013;69:1457-65.

Atalar E. Radiofrequency safety for interventional MRI procedures. Acad Radiol 2005;12:1149-1157.

Bermingham JF, et al. A measurement and modeling study of temperature in living and fixed tissue during and after radiofrequency exposure. Bioelectromagnetics 2014;35:181-91.

Boss A, et al. Tissue warming and regulatory responses induced by radio frequency energy deposition on a whole-body 3-Tesla magnetic resonance imager. J Magn Reson Imag 2007;26:1334-9.

Bottomley PA. Turning up the heat on MRI. J Am Coll Radiol 2008;5:853.

Bottomley PA, Edelstein WA. Power deposition in whole body NMR imaging. Med Phys 1981;8: 510-512.

Budinger TF. Nuclear magnetic resonance (NMR) in vitro studies: Known thresholds for health effects. J Comput Assisted Tomog 1981;5:800-811.

Collins CM. Numerical field calculations considering the human subject for engineering and safety assurance in MRI. NMR Biomed 2009;22:919-26.

Collins CM, Wang Z. Calculation of radiofrequency electromagnetic fields and their effects in MRI of human subjects. Magn Reson Med 2011;65:1470-82.

de Greef M, et al. Specific absorption rate inter-subject variability in 7-T parallel transmit MRI of the head. Magn Reson Med 2013;69:1476-85.

Drinkwater BL, Horvath SM: Heat tolerance and aging. Med Sci Sport Exer 1979;11:49-55.

Gorny KR, et al. Calorimetric calibration of head coil SAR estimates displayed on a clinical MR scanner. Phys Med Biol 2008;53:2565-76.

Gowland PA, De Wilde J. Temperature increase in the fetus due to radio frequency exposure during magnetic resonance scanning. Phys Med Biol 2008;53:L15-8.

Graesslin I, et al. A specific absorption rate prediction concept for parallel transmission MR. Magn Reson Med 2012;68:1664-74.

Graesslin I, et al. Comprehensive RF safety concept for parallel transmission MR. Magn Reson Med 2015;74:589-998.

Hand JW, et al. Prediction of specific absorption rate in mother and fetus associated with MRI examinations during pregnancy. Magn Reson Med 2006;55:883-893.

Hand JW, et al. Numerical study of RF exposure and the resulting temperature rise in the foetus during a magnetic resonance procedure. Phys Med Biol 2010;55:913-30.

Hartwig V, et al. Biological effects and safety in magnetic resonance imaging: A review. Int J Environ Res Public Health 2009;6:1778-98.

International Commission on Non-Ionizing Radiation Protection (ICNIRP) statement, medical magnetic resonance procedures: Protection of patients. Health Physics 2004;87:197-216.

International Electrotechnical Commission (IEC), Medical Electrical Equipment, Particular requirements for the safety of magnetic resonance equipment for medical diagnosis, International Standard IEC 60601-2-33, 2002.

Israel M, et al. Electromagnetic field occupational exposure: Non-thermal vs. thermal effects. Electromagn Biol Med 2013;32:145-54.

Kangarlu A, Shellock FG, Chakeres D. 8.0-Tesla MR system: Temperature changes associated with radiofrequency-induced heating of a head phantom. J Magn Reson Imag 2003;17:220-226.

Kangarlu A, Ibrahim TS, Shellock FG. Effects of coil dimensions and field polarization on RF heating inside a head phantom. Magnetic Resonance Imaging 2005;23:53-60.

Kenny WL. Physiological correlates of heat intolerance. Sports Med 1985;2:279-286.

Kikuchi S, et al. Temperature elevation in the fetus from electromagnetic exposure during magnetic resonance imaging. Phys Med Biol 2010;55:2411-26.

Jauchem JR. Effects of drugs on thermal responses to microwaves. Gen Pharmacol 1985;16:307-310.

Murbach M, et al. Whole-body and local RF absorption in human models as a function of anatomy and position within 1.5T MR body coil. Magn Reson Med 2014;71:839-845.

Murbach M, et al. Thermal tissue damage model analyzed for different whole-body SAR and scan durations for standard MR body coils. Magn Reson Med 2014;71:421-31.

Neufeld E, et al. Analysis of the local worst-case SAR exposure caused by an MRI multi-transmit body coil in anatomical models of the human body. Phys Med Biol 2011;56:4649-59.

Oh S, et al. Experimental and numerical assessment of MRI-induced temperature change and SAR distributions in phantoms and in vivo. Magn Reson Med Magn Reson Med 2010;63:218-23.

Rowell LB. Cardiovascular aspects of human thermoregulation. Circ Res 1983;52:367-379.

Schaefer DJ. Health effects and safety of radiofrequency power deposition associated with magnetic resonance procedures. In: Shellock FG, Editor. Magnetic Resonance Procedures: Health Effects and Safety. Boca Raton, FL: CRC Press, 2001;55-74.

Shellock FG. Radiofrequency energy-induced heating during MR procedures: A review. J Magn Reson Imag 2000;12:30-36.

Shellock FG, Crues JV. MR procedures: Biologic effects, safety, and patient care. Radiology 2004;232:635-652.

Shellock FG, Crues JV. Temperature, heart rate, and blood pressure changes associated with clinical MR imaging at 1.5-T. Radiology 1987;163:259-262.

Shellock FG, Crues JV. Corneal temperature changes associated with high-field MR imaging using a head coil. Radiology 1988;167:809-811.

Shellock FG, Crues JV. Temperature changes caused by clinical MR imaging of the brain at 1.5 Tesla using a head coil. Am J Neuroradiol 1988;9:287-291.

Shellock FG, Rothman B, Sarti D. Heating of the scrotum by high-field-strength MR imaging. Am J Roentgenol 1990;154:1229-1232.

Shellock FG, Schatz CJ. Increases in corneal temperature caused by MR imaging of the eye with a dedicated local coil. Radiology 192;185:697-699.

Shellock FG, Schaefer DJ. Radiofrequency energy-induced heating during magnetic resonance procedures: Laboratory and clinical experiences In: Shellock FG, Editor. Magnetic Resonance Procedures: Health Effects and Safety. Boca Raton, FL: CRC Press, 2001;75-96.

Shellock FG, Schaefer DJ, Crues JV. Alterations in body and skin temperatures caused by MR imaging: Is the recommended exposure for radiofrequency radiation too conservative? Br J Radiol 1989;62:904-909.

Shellock FG, Schaefer DJ, Kanal E. Physiologic responses to MR imaging performed at an SAR level of 6.0 W/kg. Radiology 1994;192:865-868.

Shrivastava D, et al. Radiofrequency heating at 9.4T: In vivo temperature measurement results in swine. Magn Reson Med 2008;59:73-8.

Shrivastava D, et al. Radio frequency heating at 9.4T (400.2 MHz): In vivo thermoregulatory temperature response in swine. Magn Reson Med 2009;62:888 95.

Shrivastava D, et al. Radiofrequency heating in porcine models with a "large" 32 cm internal diameter, 7T (296 MHz) head coil. Magn Reson Med 2011;66:255-63.

Shrivastava D, et al. In vivo radiofrequency heating in swine in a 3T (123.2-MHz) birdcage whole body coil. Magn Reson Med 2014;72:1141-50.

Shuman WP, et al. Superficial and deep-tissue increases in anesthetized dogs during exposure to high specific absorption rates in a 1.5-T MR imager. Radiology 1988;167:551-554.

Stuchly MA, Abrishamkar H, Strydom ML. Numerical evaluation of radio frequency power deposition in human models during MRI. Conf Proc IEEE Eng Med Biol Soc. 2006;1:272-275.

U.S. Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health, Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices, Guidance for Industry and Food and Drug Administration Staff. June 20, 2014.

van Lier AL, et al. Radiofrequency heating induced by 7-T head MRI: Thermal assessment using discrete vasculature or Pennes' bioheat equation. J Magn Reson Imag 2012;35:795-803.

van Rhoon GC, et al. CEM43?C thermal dose thresholds: A potential guide for magnetic resonance radiofrequency exposure levels? Eur Radiol 2013;23:2215-27.

Voigt T, et al. Patient-individual local SAR determination: In vivo measurements and numerical validation. Magn Reson Med 2012;68:1117-26.

Wang Z, Lin JC, Vaughan JT, Collins CM. Consideration of physiological response in numerical models of temperature during MRI of the human head.J Magnetic Resonance Imaging 2008;28:1303-1308.

Wang J. Issues with radiofrequency heating in MRI. J Appl Clin Med Phys 2014;15:5064.

Weintraub MI, Khoury A, Cole SP. Biologic effects of 3 Tesla (T) MR imaging comparing traditional 1.5-T and 0.6-T in 1,023 consecutive outpatients. J Neuroimaging 2007;17:241-5.

Wolf S, et al. SAR simulations for high-field MRI: How much detail, effort, and accuracy is needed? Magn Reson Med 2013;69:1157-68.

Zhang X, et al. Quantitative prediction of radio frequency induced local heating derived from measured magnetic field maps in magnetic resonance imaging: A phantom validation at 7?T. Appl Phys Lett 2014;105:244101.

  Shellock R & D Services, Inc. email: Frank.ShellockREMOVE@MRIsafety.com.
  Copyright © 2024 by Shellock R & D Services, Inc. and Frank G. Shellock, Ph.D. All rights reserved.