Thermal effects of MR imaging: worst-case studies on sheep

Temperatures in Pigs During 3 T MRI Temperatures, Heart Rates, and Breathing Rates of Pigs During RF Power Deposition in a 3 T (128 MHz) Body Coil

Exposure to radiofrequency (RF) power deposition during magnetic resonance imaging (MRI) induces elevated body-tissue temperatures and may cause changes in heart and breathing rates, disturbing thermoregulation. Eleven temperature sensors were placed in muscle tissue and one sensor in the rectum (measured in 10 cm depth) of 20 free-breathing anesthetized pigs to verify temperature curves during RF exposure. Tissue temperatures and heart and breathing rates were measured before, during, and after RF exposure. Pigs were placed into a 60-cm diameter whole-body resonator of a 3 T MRI system. Nineteen anesthetized pigs were divided into four RF exposure groups: sham (0 W/kg), low-exposure (2.7 W/kg, mean exposure time 56 min), moderate-exposure (4.8 W/kg, mean exposure time 31 min), and high-exposure (4.4 W/kg, mean exposure time 61 min). One pig was exposed to a whole-body specific absorption rate (wbSAR) of 11.4 W/kg (extreme-exposure). Hotspot temperatures, measured by sensor 2, increased by mean 5.0 ± 0.9°C, min 3.9; max 6.3 (low), 7.0 ± 2.3°C, min 4.6; max 9.9 (moderate), and 9.2 ± 4.4°C, min 6.1, max 17.9 (high) compared with 0.3 ± 0.3°C in the sham-exposure group (min 0.1, max 0.6). Four time-temperature curves were identified: sinusoidal, parabolic, plateau, and linear. These curve shapes did not correlate with RF intensity, rectal temperature, breathing rate, or heart rate. In all pigs, rectal temperatures increased (2.1 ± 0.9°C) during and even after RF exposure, while hotspot temperatures decreased after exposure. When rectal temperature increased by 1°C, hotspot temperature increased up to 42.8°C within 37 min (low-exposure) or up to 43.8°C within 24 min (high-exposure). Global wbSAR did not correlate with maximum hotspot. Bioelectromagnetics. 2021;42:37-50. © 2020 The Authors. Bioelectromagnetics published by Wiley Periodicals LLC on behalf of Bioelectromagnetics Society.

Parallel transmission RF pulse design with strict temperature constraints

RF safety in parallel transmission (pTx) is generally ensured by imposing specific absorption rate (SAR) limits during pTx RF pulse design. There is increasing interest in using temperature to ensure safety in MRI. In this work, we present a local temperature correlation matrix formalism and apply it to impose strict constraints on maximum absolute temperature in pTx RF pulse design for head and hip regions. Electromagnetic field simulations were performed on the head and hip of virtual body models. Temperature correlation matrices were calculated for four different exposure durations ranging between 6 and 24 min using simulated fields and body-specific constants. Parallel transmission RF pulses were designed using either SAR or temperature constraints, and compared with each other and unconstrained RF pulse design in terms of excitation fidelity and safety. The use of temperature correlation matrices resulted in better excitation fidelity compared with the use of SAR in parallel transmission RF pulse design (for the 6 min exposure period, 8.8% versus 21.0% for the head and 28.0% versus 32.2% for the hip region). As RF exposure duration increases (from 6 min to 24 min), the benefit of using temperature correlation matrices on RF pulse design diminishes. However, the safety of the subject is always guaranteed (the maximum temperature was equal to 39°C). This trend was observed in both head and hip regions, where the perfusion rates are very different.

Validating subject-specific RF and thermal simulations in the calf muscle using MR-based temperature measurements

PURPOSE

Ongoing discussions occur to translate the safety restrictions on MR scanners from specific absorption rate (SAR) to thermal dose. Therefore, this research focuses on the accuracy of thermal simulations in human subjects during an MR exam, which is fundamental information in that debate.

METHODS

Radiofrequency (RF) heating experiments were performed on the calves of 13 healthy subjects using a dedicated transmit-receive coil while monitoring the temperature with proton resonance frequency shift (PRFS) thermometry. Subject-specific models and one generic model were used for electromagnetic and thermal simulations using Pennes' bioheat equation, with the blood equilibration constant equaling zero. The simulations were subsequently compared with the experimental results.

RESULTS

The mean B1+ equaled 15 µT in the center slice of all volunteers, and 95% of the voxels had errors smaller than 2.8 µT between the simulation and measurement. The intersubject variation in RF power to achieve the required B1+ was 11%. The resulting intersubject variation in median temperature rise was 14%. Thermal simulations underestimated the median temperature increase on average, with 34% in subject-specific models and 28% in the generic model.

CONCLUSIONS

Although thermal measures are directly coupled to tissue damage and therefore suitable for RF safety assessment, insecurities in the applied thermal modeling limit their estimation accuracy. Magn Reson Med 77:1691-1700, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Feasibility of measuring thermoregulation during RF heating of the human calf muscle using MR based methods

PURPOSE

One of the main safety concerns in MR is heating of the subject due to radiofrequency (RF) exposure. Recently was shown that local peak temperatures can reach dangerous values and the most prominent parameter for accurate temperature estimations is thermoregulation. Therefore, the goal of this research is testing the feasibility of measuring thermoregulation in vivo using MR methods.

THEORY AND METHODS

The calves of 13 volunteers were scanned at 3 tesla. A Proton Resonance Frequency Shift method was used for temperature measurement. Arterial Spin Labeling and phase contrast scans were used for perfusion and flow measurements respectively. The calves were monitored during extreme RF exposure (20 W/kg, 16 min) and after physical exercise.

RESULTS

Temperature increases due to RF absorption (range of the 90th percentile of all volunteers: 1.1-2.5°C) matched with the reference skin temperature changes. Increases in perfusion and flow were defined on the whole leg and normalized to baseline. Perfusion showed a significant increase due to RF heating (ratio compared with baseline: 1.28 ± 0.37; P < 0.05), the influence of exercise was much greater, however (2.97 ± 2.45, P < 0.01).

CONCLUSION

This study represents a first exploration of measuring thermoregulation, which will become essential when new safety guidelines are based on thermal dose.

In vivo radiofrequency heating in swine in a 3T (123.2-MHz) birdcage whole body coil

PURPOSE

To study in vivo radiofrequency (RF) heating produced due to power deposition from a 3T (Larmour frequency = 123.2 MHz), birdcage, whole body coil.

METHODS

The RF heating was simulated in a digital swine by solving the mechanistic generic bioheat transfer model (GBHTM) and the conventional, empirical Pennes bioheat transfer equation for two cases: 1) when the swine head was in the isocenter and 2) when the swine trunk was in the isocenter. The simulation results were validated by making direct fluoroptic temperature measurements in the skin, brain, simulated hot regions, and rectum of 10 swine (case 1: n = 5, mean animal weight = 84.03 ± 6.85 kg, whole body average SAR = 2.65 ± 0.22 W/kg; case 2: n = 5, mean animal weight = 81.59 ± 6.23 kg, whole body average SAR = 2.77 ± 0.26 W/kg) during 1 h of exposure to a turbo spin echo sequence.

RESULTS

The GBHTM simulated the RF heating more accurately compared with the Pennes equation. In vivo temperatures exceeded safe temperature thresholds with allowable SAR exposures. Hot regions may be produced deep inside the body, away from the skin.

CONCLUSION

SAR exposures that produce safe temperature thresholds need reinvestigation.

Thermal simulations in the human head for high field MRI using parallel transmission

PURPOSE

To investigate, via numerical simulations, the compliance of the specific absorption rate (SAR) versus temperature guidelines for the human head in magnetic resonance imaging procedures utilizing parallel transmission at high field.

MATERIALS AND METHODS

A combination of finite element and finite-difference time-domain methods was used to calculate the evolution of the temperature distribution in the human head for a large number of parallel transmission scenarios. The computations were performed on a new model containing 20 anatomical structures.

RESULTS

Among all the radiofrequency field exposure schemes simulated, the recommended 39°C maximum local temperature was never exceeded when the local 10-g average SAR threshold was reached. On the other hand, the maximum temperature barely complied with its guideline when the global SAR reached 3.2 W/kg. The maximal temperature in the eye could very well rise by more than 1°C in both cases.

CONCLUSION

Considering parallel transmission, the recommended values of local 10-g SAR may remain a relevant metric to ensure that the local temperature inside the human head never exceeds 39°C, although it can lead to rises larger than 1°C in the eye. Monitoring temperature instead of SAR can provide increased flexibility in pulse design for parallel transmission.

Radiofrequency heating in porcine models with a "large" 32 cm internal diameter, 7 T (296 MHz) head coil

Temperatures were measured in vivo in four pigs (mean animal weight = 110.75 kg and standard deviation = 6.13 kg) due to a continuous wave radiofrequency (RF) power irradiation with a 31.75 cm internal diameter and a 15.24 cm long, 7 T (296 MHz), eight channel, transverse electromagnetic head coil. The temperatures were measured in the subcutaneous layer of the scalp, 5, 10, 15, and 20 mm deep in the brain, and rectum using fluoroptic temperature probes. The RF power was delivered to the pig's head for ∼3 h (mean deposition time = 3.14 h and standard deviation = 0.06 h) at the whole head average specific absorption rate of ∼3 W kg(-1) (mean average specific absorption rate = 3.08 W kg(-1) and standard deviation = 0.09 W kg(-1)). Next, simple bioheat transfer models were used to simulate the RF power induced temperature changes. Results show that the RF power produced uniform temperature changes in the pigs' heads (mean temperature change = 1.68°C and standard deviation = 0.13°C) with no plateau achieved during the heating. No thermoregulatory alterations were detected due to the heating because the temperature responses of the pre-RF and post-RF epochs were not statistically significantly different. Simple, validated bioheat models may provide accurate temperature changes.

(19)F MRI for quantitative in vivo cell tracking

Cellular therapy, including stem cell transplants and dendritic cell vaccines, is typically monitored for dosage optimization, accurate delivery, and localization using noninvasive imaging, of which magnetic resonance imaging (MRI) is a key modality. (19)F MRI retains the advantages of MRI as an imaging modality, and also allows direct detection of labeled cells for unambiguous identification and quantification, unlike typical metal-based contrast agents. Recent developments in (19)F MRI-based in vivo cell quantification, the existing clinical use of (19)F compounds and current explosive interest in cellular therapeutics have brought (19)F imaging technology closer to clinical application. We review the application of (19)F MRI to cell tracking, discussing intracellular (19)F labels, cell labeling and in vivo quantification, as well as the potential clinical uses of (19)F MRI.

Radio frequency heating at 9.4T (400.2 MHz): in vivo thermoregulatory temperature response in swine

In vivo thermoregulatory temperature response to radio frequency (RF) heating at 9.4T was studied by measuring temperatures in nine anesthetized swine. Temperatures were measured in the scalp, brain, and rectum. The RF energy was deposited using a four-loop head coil tuned to 400.2 MHz. Sham RF was delivered to three swine to understand the thermal effects of anesthesia (animal weight = 54.16 kg, SD = 3.08 kg). Continuous wave (CW) RF energy was delivered to the other six animals for 2.5-3.4 h (animal weight = 74.01 +/- 26.0 kg, heating duration = 3.05 +/- 0.29 h). The whole-head specific absorption rate (SAR) varied between 2.71 W/kg and 3.20 W/kg (SAR = 2.93 +/- 0.18 W/kg). Anesthesia caused the brain and rectal temperatures to drop linearly. Altered thermoregulatory response was detected by comparing the difference in the temperature slopes before and after the RF delivery from zero. RF heating statistically significantly altered the rate of cooling down of the animal. The temperature slope changes correlated well with the RF energy per unit head weight and heating duration, and the maximum rectal temperature change during heating in heated animals. The temperature slope changes did not correlate well to the whole-head average SARs.

Radiofrequency heating at 9.4T: in vivo temperature measurement results in swine

In vivo temperatures were correlated to the whole head average specific absorption rate (SAR(avg)) at 9.4T using 12 anesthetized swine (mean animal weight = 52 kg, standard deviation = 6.7 kg). Correlating the temperatures and SAR(avg) is necessary to ensure safe levels of human heating during ultra-high field MR exams. The temperatures were measured at three depths inside the brain, in the rectum, and at the head-skin of swine. A 400 MHz, continuous wave RF power was deposited to the head using a volume coil. The SAR(avg) values were varied between 2.7-5.8 W/kg. The RF power exposure durations were varied between 1.4-3.7 hr. To differentiate the temperature response caused by the RF from that of the anesthesia, the temperatures were recorded in four unheated swine. To study the effect of the spatial distribution of the RF and tissue properties, the temperature probes were placed at two brain locations (n = 4 swine for each location). Results showed that the in vivo brain temperatures correlated to the SAR(avg) in a geometry-dependent manner. Additionally, 1) the skin temperature change was not the maximum temperature change; 2) the RF heating caused an inhomogeneous brain temperature distribution; and 3) the maximum temperature occurred inside the brain.

Computation of temperature elevation in rabbit eye irradiated by 2.45-GHz microwaves with different field configurations

This study calculated the temperature elevation in the rabbit eye caused by 2.45-GHz near-field exposure systems. First, we calculated specific absorption rate distributions in the eye for different antennas and then compared them with those observed in previous studies. Next, we re-examined the temperature elevation in the rabbit eye due to a horizontally-polarized dipole antenna with a C-shaped director, which was used in a previous study. For our computational results, we found that decisive factors of the SAR distribution in the rabbit eye were the polarization of the electromagnetic wave and antenna aperture. Next, we quantified the eye average specific absorption rate as 67 W kg(-1) for the dipole antenna with an input power density at the eye surface of 150 mW cm(-2), which was specified in the previous work as the minimum cataractogenic power density. The effect of administrating anesthesia on the temperature elevation was 30% or so in the above case. Additionally, the position where maximum temperature in the lens appears is discussed due to different 2.45-GHz microwave systems. That position was found to appear around the posterior of the lens regardless of the exposure condition, which indicates that the original temperature distribution in the eye was the dominant factor.

Ocular effects of radiofrequency energy

Radiofrequency (RF) energy has been reported to cause a variety of ocular effects, primarily cataracts but also effects on the retina, cornea, and other ocular systems. Cataracts have been observed in experimental animals when one eye was exposed to a localized, very high RF field and the other eye was the unexposed control. The results show that 2450 MHz exposures for >or=30 min at power densities causing extremely high dose rates (>or=150 W/kg) and temperatures (>or=41 degrees C) in or near the lens caused cataracts in the rabbit eye. However, cataracts were not observed in the monkey eye exposed to similar exposure conditions, reflecting the different patterns of energy absorption (SAR, specific absorption rate) distribution, due to their different facial structure. Since the monkey head is similar in structure to the human head, the nonhuman primate study showed that the incident power density levels causing cataracts in rabbits and other laboratory animals cannot be directly extrapolated to primates, including human beings. It is reasonable to assume that an SAR that would induce temperatures >or=41 degrees C in or near the lens in the human eye would produce cataracts by the same mechanism (heating) that caused cataracts in the rabbit lens; however, such an exposure would greatly exceed the currently allowable limits for human exposure and would be expected to cause unacceptable effects in other parts of the eye and face. Other ocular effects including corneal lesions, retinal effects, and changes in vascular permeability, have been observed after localized exposure of the eye of laboratory animals to both continuous wave (CW) and pulsed wave (PW) exposures, but the inconsistencies in these results, the failure to independently confirm corneal lesions after CW exposure, the failure to independently confirm retinal effects after PW exposure, and the absence of functional changes in vision are reasons why these ocular effects are not useful in defining an adverse effect level for RF exposure. While cataracts develop after localized exposure of the eye at SARs >or= 150 W/kg, whole body exposure at much lower levels (14-42 W/kg) is lethal to rabbits. Two studies reported cataracts in this animal after 30 daily exposures at SARs at the upper end of the lethal range, e.g., 38-42 W/kg; however, long term exposure of rabbits (23 h/day, 6 months) at 1.5 W/kg (17 W/kg in the rabbit head) did not cause cataracts or other ocular effects. A long term (1-4 years) investigation of monkeys exposed at high SARs (20 and 40 W/kg to the monkey face) found no cataracts or other ocular effects or change in visual capability. The results of these long term studies support the conclusion that clinically significant ocular effects, including cataracts, have not been confirmed in human populations exposed for long periods of time to low level RF energy. The results of four recent human studies show that there is no clear evidence of an association between RF exposure and ocular cancer.

Thermoregulatory responses to RF energy absorption

This white paper combines a tutorial on the fundamentals of thermoregulation with a review of the current literature concerned with physiological thermoregulatory responses of humans and laboratory animals in the presence of radio frequency (RF) and microwave fields. The ultimate goal of research involving whole body RF exposure of intact organisms is the prediction of effects of such exposure on human beings. Most of the published research on physiological thermoregulation has been conducted on laboratory animals, with a heavy emphasis on laboratory rodents. Because their physiological heat loss mechanisms are limited, these small animals are very poor models for human beings. Basic information about the thermoregulatory capabilities of animal models relative to human capability is essential for the appropriate evaluation and extrapolation of animal data to humans. In general, reliance on data collected on humans and nonhuman primates, however fragmentary, yields a more accurate understanding of how RF fields interact with humans. Such data are featured in this review, including data from both clinic and laboratory. Featured topics include thermal sensation, human RF overexposures, exposures attending magnetic resonance imaging (MRI), predictions based on simulation models, and laboratory studies of human volunteers. Supporting data from animal studies include the thermoregulatory profile, response thresholds, physiological responses of heat production and heat loss, intense or prolonged exposure, RF effects on early development, circadian variation, and additive drug-microwave interactions. The conclusion is inescapable that humans demonstrate far superior thermoregulatory ability over other tested organisms during RF exposure at, or even above current human exposure guidelines.

The effects of 1.5T magnetic resonance imaging on early murine in-vitro embryo development

Although no ionizing radiation is involved, patients undergoing magnetic resonance imaging (MRI) are exposed to powerful static magnetic fields, magnetic field gradients, and radio-frequency fields that may be potentially damaging. Our study aims to document the effect of MRI imaging sequences on early murine embryo development (two-cell to blastocyst stage) in vitro. Two-cell murine embryos were exposed to various lengths of MRI using pulse sequences employed in present day clinical imaging. Early murine embryo development was documented in vitro, and blastocyst development rates were computed for both the control and exposed groups. There were no significant differences detected in the rate of blastocyst formation between the control groups and the embryos exposed to MRI.

Skin temperature increase during local exposure to high-power RF levels in humans

The local temperature response of the skin on heating due to prolonged exposure to RF radiation by a surface coil was investigated in five healthy volunteers. Temperature changes induced by RF radiation were measured at the skin of the calf muscle by a fluoroptic probe. Exposure to superficial specific absorption rate (SAR) levels of 6.5, 12 and 22 W/kg resulted in skin temperature increases, the highest temperature recorded was 38.3 degrees C. Although the maximum values of each temperature curve correlated with the applied superficial SAR levels, these values did not exceed the recommended temperature limit for the extremities such as given by the Food and Drug Administration (FDA).

Electromagnetic and thermal modeling of SAR and temperature fields in tissue due to an RF decoupling coil

The finite difference time domain method is used to calculate the specific absorption rate (SAR) due to a butterfly surface coil in a realistic tissue model of the leg. The resulting temperature distribution and temperature changes are found using a finite difference solution to the bioheat transfer equation. Reasonable agreement is found between predicted temperature changes and those measured in vivo provided that the resulting hyperthermia does not induce noticeable changes in perfusion. The method is applicable to radiofrequency dosimetry problems associated with high Bo field magnetic resonance systems and where knowledge of spatial variation in SAR is important in assessing the safety of new magnetic resonance procedures.

Magnetic resonance imaging can cause focal heating in a nonuniform phantom

To test if the radiofrequency fields of a magnetic resonance imager could cause focal heating, two cylindrical phantoms were made from a mixture of agar and saline. The first phantom was uniform; the second was nonuniform in that a narrow bridge of agar was produced. Both phantoms were exposed to high levels of radiofrequency power (140 W) at 63 MHz and the temperature rises were measured. In the uniform phantom, the temperature increased as the radius increased. In the bridge phantom, the narrow bridge heated three times greater than at the opposite uniform periphery, and over five times the average of the uniform phantom. This experiment demonstrates that the radiofrequency fields of magnetic resonance imagers can cause focal heating if the exposed object is nonuniform. Since nonuniformity is present in the human body, as the radiofrequency power of magnetic resonance imaging techniques increases, focal heating in patients is a concern.