Superficial- and deep-tissue temperature increases in anesthetized dogs during exposure to high specific absorption rates in a 1.5-T MR imager

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.

L'uomo elettromagnetico e la tomografia a RM

In questi ultimi anni gli effetti biologici indotti dai tre campi energetici dell'imaging a RM sono passati in seconda linea rispetto agli effetti esercitati sui materali ferromagnetici e sui dispositivi elettronici sensibili presenti nel corpo del soggetto esaminato. Tuttavia, l'aumento dei valori di campo previsto in un prossimo futuro rinnova l'interesse sull'interazione magnetobiologica vera e propria.

Come base di partenza per comprendere meglio gli effetti biologici correlati con gli esami a RM, l'autore prende in esame le componenti elettriche, magnetiche ed elettromagnetiche dell' uomo elettromagnetico. Nella review di aggiornamento, vengono considerati gli effetti sul sistema nervoso centrale e periferico, sull'apparato cardiocircolatorio, sullo sviluppo embrionale, gli effetti da esposizione cronica e professionale ad alti campi, gli effetti su cellule in coltura e su sistemi enzimatici.

Un particolare sviluppo è riservato al significato biologico degli impulsi di gradiente temporale (dB/dt) e degli impulsi a radiofrequenza e viene anche proposta una ipotesi dell'autore relativa alla genesi di infrasuoni e ultrasuoni nei tessuti del paziente per effetto dei gradienti e, rispettivamente, degli impulsi a radiofrequenza.

I fatti esaminati portano a concludere che l'odierna, apparente innocuità dell'imaging a RM potrebbe non essere più tale con l'aumento del campo magnetico statico oltre i 2–4 T e con l'aumento conseguente dell'onda a RF, del SAR e dei gradienti.

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.

Magnetic resonance safety

Magnetic resonance imaging (MRI) has a superior soft-tissue contrast compared to other radiological imaging modalities and its physiological and functional applications have led to a significant increase in MRI scans worldwide. A comprehensive MRI safety training to protect patients and other healthcare workers from potential bio-effects and risks of the magnetic fields in an MRI suite is therefore essential. The knowledge of the purpose of safety zones in an MRI suite as well as MRI appropriateness criteria is important for all healthcare professionals who will work in the MRI environment or refer patients for MRI scans. The purpose of this article is to give an overview of current magnetic resonance safety guidelines and discuss the safety risks of magnetic fields in an MRI suite including forces and torque of ferromagnetic objects, tissue heating, peripheral nerve stimulation, and hearing damages. MRI safety and compatibility of implanted devices, MRI scans during pregnancy, and the potential risks of MRI contrast agents will also be discussed, and a comprehensive MRI safety training to avoid fatal accidents in an MRI suite will be presented.

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.

Patient safety issues in magnetic resonance imaging: state of the art

The presence of a static magnetic field (Bo), a radiofrequency field (RF), a dynamic gradient which varies in time and loud noises during an MR examination could increase patient risk. Specifically, a magnetic field could interfere with ferromagnetic material leading to one of the following five dangerous interactions: 1) projectile effect, 2) twisting, 3) burning, 4) artefacts and 5) device malfunction. The projectile effect is when an object is attracted by the magnet with the risk, as reported in literature, of hitting the patient, operators and/or the instrument. Objects which typically can undergo this effect are oxygen and helium cylinders, IV stands, cleaning trolleys, chairs, lamp holders, scissors, forceps, clampers, traction weights, monitoring instruments, and especially metallic splinters within the patient. Twisting (torsion) typically occurs with cerebral vascular clamps and cochlear implants. If parts of implants are involved a malfunction may result. Burns can be caused when electrically conductive material is introduced within the magnet, for example, ECG electrodes, monitoring cables and coils which are in contact with the patient's skin, as well as tattoos and eye-liners that contain iron-oxides. Artefacts can be induced by RF emission of implanted devices which can be mistaken for noise of the receiving coil. Implanted devices can induce signal voids which mask or simulate pathologies. Electrical or mechanical malfunction of implanted devices includes pacemakers which can stimulate inappropriately or at an elevated frequency yielding a distorted ECG with altered T-waves. The risk for patients can be reduced by specific educational programs within individual radiology departments which include other specializations and external referring physicians with the aim of developing a standardized safety protocol.

MR procedures: biologic effects, safety, and patient care

The technology used for magnetic resonance (MR) procedures has evolved continuously during the past 20 years, yielding MR systems with stronger static magnetic fields, faster and stronger gradient magnetic fields, and more powerful radiofrequency transmission coils. Most reported cases of MR-related injuries and the few fatalities that have occurred have apparently been the result of failure to follow safety guidelines or of use of inappropriate or outdated information related to the safety aspects of biomedical implants and devices. To prevent accidents in the MR environment, therefore, it is necessary to revise information on biologic effects and safety according to changes that have occurred in MR technology and with regard to current guidelines for biomedical implants and devices. This review provides an overview of and update on MR biologic effects, discusses new or controversial MR safety topics and issues, presents evidence-based guidelines to ensure safety for patients and staff, and describes safety information for various implants and devices that have recently undergone evaluation.

Three-dimensional T1rho-weighted MRI at 1.5 Tesla

PURPOSE

To design and implement a magnetic resonance imaging (MRI) pulse sequence capable of performing three-dimensional T(1rho)-weighted MRI on a 1.5-T clinical scanner, and determine the optimal sequence parameters, both theoretically and experimentally, so that the energy deposition by the radiofrequency pulses in the sequence, measured as the specific absorption rate (SAR), does not exceed safety guidelines for imaging human subjects.

MATERIALS AND METHODS

A three-pulse cluster was pre-encoded to a three-dimensional gradient-echo imaging sequence to create a three-dimensional, T(1rho)-weighted MRI pulse sequence. Imaging experiments were performed on a GE clinical scanner with a custom-built knee-coil. We validated the performance of this sequence by imaging articular cartilage of a bovine patella and comparing T(1rho) values measured by this sequence to those obtained with a previously tested two-dimensional imaging sequence. Using a previously developed model for SAR calculation, the imaging parameters were adjusted such that the energy deposition by the radiofrequency pulses in the sequence did not exceed safety guidelines for imaging human subjects. The actual temperature increase due to the sequence was measured in a phantom by a MRI-based temperature mapping technique. Following these experiments, the performance of this sequence was demonstrated in vivo by obtaining T(1rho)-weighted images of the knee joint of a healthy individual.

RESULTS

Calculated T(1rho) of articular cartilage in the specimen was similar for both and three-dimensional and two-dimensional methods (84 +/- 2 msec and 80 +/- 3 msec, respectively). The temperature increase in the phantom resulting from the sequence was 0.015 degrees C, which is well below the established safety guidelines. Images of the human knee joint in vivo demonstrate a clear delineation of cartilage from surrounding tissues.

CONCLUSION

We developed and implemented a three-dimensional T(1rho)-weighted pulse sequence on a 1.5-T clinical scanner.

Fast MRI of RF heating via phase difference mapping

A method is presented for the rapid acquisition of temperature maps derived from phase difference maps. The temperature-dependent chemical shift coefficients (TDCSCs) of various concentrations of aqueous cobalt and dysprosium-based compounds were measured. The largest TDCSC calculated was for 100 mM DyEDTA, which had a TDCSC of -0.09 PPM/K; 160 mM CoCl2 had a TDCSC of -0.04 PPM/K. These temperature-dependent chemical shifts (TDCSs) result in phase changes in the MR signal with changing temperature. Agarose phantoms were constructed with each paramagnetic metal. A fast gradient-echo (FGRE) MR image was acquired to serve as the baseline image. A "test" MRI procedure was then performed on the phantom. Immediately afterwards, a second FGRE MR image was acquired, serving as the probing image. Proper image processing as a phase difference map between the probing image and the baseline image resulted in an image which quantitatively described the temperature increase of the phantom in response to a particular "test" imaging experiment. Applications of this technique in assessing the safety of pulse sequences and MR coils are discussed.

Potential heating effect in the gravid uterus during MR HASTE imaging

Our purpose was to evaluate if temperature changes occur in maternal or fetal tissues during HASTE imaging.

METHODS

Pregnant pigs were scanned with the HASTE technique, and temperatures were measured with phase maps and temperature probes inserted into the amniotic fluid and fetal brain.

RESULTS

Fiberoptic probes showed that no heating occurred in fetal tissues or amniotic fluid during HASTE imaging.

CONCLUSION

Our current HASTE protocols do not deposit a significant amount of heat in the gravid uterus. J. Magn. Reson. Imaging 2001;13:856-861.

Infusion port dislodgement of bilateral breast tissue expanders after MRI

Tissue expanders are placed routinely for breast reconstruction, and magnetic resonance imaging (MRI) is a common diagnostic procedure. Many studies have reported on the safety of MRI in patients with nonferromagnetic implants; however, many tissue expanders contain ferromagnetic components. The authors present a case of bilateral tissue expander infusion port dislodgment after MRI. A 56-year-old woman underwent bilateral mastectomy and immediate reconstruction with McGhan BIOSPAN tissue expanders. These implants contain integral nonferromagnetic infusion ports, as well as small, powerful Magna-Site magnets. Several weeks postoperatively the patient underwent MRI of her spine, which was ordered by her primary physician for back pain. Subsequently, the infusion ports could not be located with the finder magnet. A chest radiograph was obtained, which demonstrated bilateral dislodgment of the infusion ports. Surgical removal and replacement of the tissue expanders were required. Safety considerations of MRI have been discussed extensively in the literature, and data on MRI with various implanted devices have been obtained. The potential risks of performing MRI on patients with metallic implants include conduction of electrical currents, heating of the implant, misinterpretation resulting from artifact, and the possibility of movement or dislodgment of the implant. The small magnet integral to many tissue expanders may be overlooked by patients and physicians during pre-MRI screening. All patients undergoing tissue expansion with implants that contain integral ports should be thoroughly warned about the potential hazards of MRI.

Radiofrequency energy-induced heating during MR procedures: a review

During an MR procedure, most of the transmitted RF power is transformed into heat within the patient's tissue as a result of resistive losses. Not surprisingly, the primary bioeffects associated with the RF radiation used for MR procedures are directly related to the thermogenic qualities of this electromagnetic field. This review article discusses the characteristics of RF energy-induced heating associated with MR procedures, with an emphasis on thermal and other physiologic responses observed in human subjects.

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.

Effects of magnetic resonance imaging on polymorphonuclear neutrophil functions

RATIONALE AND OBJECTIVES

Limited research has been performed on the effects of magnetic resonance (MR) imaging on the immune system. To our knowledge, there are no reported studies of MR imaging effects on the polymorphonuclear neutrophil (PMN) system. Therefore, we evaluated the influence of MR imaging exposure on PMNs.

METHODS

In vivo and in vitro studies were performed on 36 patients undergoing MR imaging. The following were estimated in blood samples: leukocyte and PMN count, PMN phagocytosis and bactericidal capacity, percentage of cells with expression of surface receptor for the Fc fragment of immunoglobulin G (IgG), PMN superoxide, hydrogen peroxide production, and plasma lysozyme activity. Another sample of patients was used to eliminate temperature as an influence on changes in PMN functions.

RESULTS

Both in vitro and in vivo MR imaging led to a decrease in PMNs and an increase in PMN phagocytosis, bactericidal capacity, hydrogen peroxide production, and percentage of cells with expression of surface receptor for Fc IgG. Superoxide anion production did not change significantly. Elevated temperature, stress, and anxiety were excluded as influences on our results.

CONCLUSION

The PMN system is affected seriously by MR imaging.

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.