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.
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