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.

Magnetic resonance fingerprinting based thermometry (MRFT): application toex vivoimaging near DBS leads.

The purpose of this study is to demonstrate the first work ofT1-based magnetic resonance thermometry using magnetic resonance fingerprinting (dubbed MRFT). We compared temperature estimation of MRFT with proton resonance frequency shift (PRFS) thermometry onex vivobovine muscle. We demonstrated MRFT's feasibility in predicting temperature onex vivobovine muscles with deep brain stimulation (DBS) lead.B0maps generated from MRFT were compared with gold standardB0maps near the DBS lead. MRFT and PRFS estimated temperatures were compared in the presence of motion. All experiments were performed on a 3 Tesla whole-body GE Premier system with a 21-channel receive head coil (GE Healthcare, Milwaukee, WI). Four fluoroptic probes were used to measure the temperature at the center of a cold muscle (probe 1), the room temperature water bottle (probe 2), and the center and periphery of the heated muscle (probes 3 and 4). We selected regions of interest (ROIs) around the location of the probes and used simple linear regression to generate the temperature sensitivity calibration equations that convertT1maps and Δsmaps to temperature maps. We then repeated the same setup and compared MRFT and PRFS thermometry temperature estimation with gold standard probe measurements. For the MRFT experiment on DBS lead, we taped the probe to the tip of the DBS lead and used a turbo spin echo sequence to induce heating near the lead. We selected ROIs around the tip of the lead to compare MRFT temperature estimation with probe measurements and compared with PRFS temperature estimation. Vendor-suppliedB0mapping sequence was acquired to compare with MRFT-generatedB0maps. We found strong linear relationships (R2> 0.958) betweenT1and temperature and Δsand temperatures in our temperature sensitivity calibration experiment. MRFT and PRFS thermometry both accurately predict temperature (RMSE < 1.55 °C) compared to probe measurements. MRFT estimated temperature near DBS lead has a similar trend as the probe temperature. BothB0maps show inhomogeneities around the lead. MRFT estimated temperature is less sensitive to motion.

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.

MRI-related heating near deep brain stimulation electrodes: more data are needed.

Magnetic resonance imaging (MRI) of patients with implanted deep brain stimulation (DBS) devices poses a challenge for healthcare providers. As a consequence of safety concerns about magnetic field interactions with the device, induced electrical currents and thermal damage due to radiofrequency heating, a number of stringent guidelines have been proposed by the device manufacturer. Very few detailed investigations of these safety issues have been published to date, and the stringent manufacturer guidelines have gone unchallenged, leading some hospitals and imaging centers around the world to ban or restrict the use of MRI in DBS patients. The purpose of this review is to stimulate research towards defining appropriate guidelines for the use of MRI in patients with DBS. Additionally, this review is intended to help healthcare providers and researchers make sound clinical judgments about the use of MRI in the setting of implanted DBS devices.

Depth of thermal dispersion of monopolar radiofrequency heating in the vaginal wall.

The use of energy-based devices to treat genitourinary syndrome of menopause, termed vaginal thermotherapy (VTT), has gained significant interest in recent years. Among the primary safety concerns of this relatively new procedure is the possibility of unintentionally heating tissues adjacent to the vaginal wall, i.e., heating too deeply. Herein we use numerical simulations to evaluate monopolar radiofrequency-based (RF) VTT specifically focusing on the resultant depth of heating through a range of input parameters. Varying RF power, exposure time, and the simulated rate of blood perfusion, we map the parameter space identifying which combinations of input parameters are likely to heat past the depth of the vaginal wall and affect adjacent tissue. We found that the device parameters commonly used in the literature are likely to heat past the vaginal wall and merit further investigation. In addition, we found that the parameter typically used to describe VTT devices, total energy delivered, does not reliably indicate the resultant depth of heat dispersion.

Evaluation of the RF-induced lead-tip heating of AIMDs using a Volume-Weighed Tissue-Cluster Model for 1.5T MRI.

The RF-induced lead-tip heating of AIMDs is related to the tangential electric field distribution along the AIMD lead paths in patients and the electromagnetic behavior (represented by the transfer function model) of the AIMDs. To evaluate the in-vivo RF-induced lead-tip heating of AIMDs using in-vitro methods, the electric field distribution is critical. In this paper, we proposed a Volume-Weighed Tissue-Cluster Model, a feasible bench method, to simplify the evaluation of the in-vivo RF-induced lead-tip heating of AIMDs. The incident electric field distribution inside this simplified model is highly correlated to that of the original inhomogeneous human body model. Compared to the RF-induced lead-tip heating results in the original model, the maximum error of the lead-tip heating in this Volume-Weighed Tissue-Cluster Model is less than 1 °C. The correlation coefficients of the temperature rise between the two models are higher than 0.997.Clinical Relevance- Simplified and accurate anatomical models can be used to emulate the in-vivo heating assessment for MRI safety.

Performance of external and internal coil configurations for prostate investigations at 7 T.

Three different coil configurations were evaluated through simulation and experimentally to determine safe operating limits and evaluate subject size-dependent performance for prostate imaging at 7 T. The coils included a transceiver endorectal coil (trERC), a 16-channel transceiver external surface array (trESA) and a trESA combined with a receive-only ERC (trESA+roERC). Although the transmit B(1) (B 1+) homogeneity was far superior for the trESA, the maximum achievable B 1+ is subject size dependent and limited by transmit chain losses and amplifier performance. For the trERC, limitations in transmit homogeneity greatly compromised image quality and limited coverage of the prostate. Despite these challenges, the high peak B 1+ close to the trERC and subject size-independent performance provides potential advantages especially for spectroscopic localization where high-bandwidth radiofrequency pulses are required. On the receive side, the combined trESA+roERC provided the highest signal-to-noise ratio and improved homogeneity over the trERC resulting in better visualization of the prostate and surrounding anatomy. In addition, the parallel imaging performance of the trESA+roERC holds strong promise for diffusion-weighted imaging and dynamic contrast-enhanced MRI.

Effect of the extracranial deep brain stimulation lead on radiofrequency heating at 9.4 Tesla (400.2 MHz).

PURPOSE: To study the effect of the extracranial portion of a deep brain stimulation (DBS) lead on radiofrequency (RF) heating with a transmit and receive 9.4 Tesla head coil. MATERIALS AND METHODS: The RF heating was studied in four excised porcine heads (mean animal head weight = 5.46 +/- 0.14 kg) for each of the following two extracranial DBS lead orientations: one, parallel to the coil axial direction; two, perpendicular to the coil axial direction (i.e., azimuthal). Temperatures were measured using fluoroptic probes at four locations: one, scalp; two, near the second DBS lead electrode-brain contact; three, near the distal tip of the DBS lead; and four, air surrounding the head. A continuous wave RF power was delivered to each head for 15 min using the coil. Net, delivered RF power was measured at the coil (mean whole head average specific absorption rate = 2.94 +/- 0.08 W/kg). RESULTS: RF heating was significantly reduced when the extracranial DBS lead was placed in the axial direction (temperature change = 0-5 degrees C) compared with the azimuthal direction (temperature change = 1-27 degrees C). CONCLUSION: Development of protocols seems feasible to keep RF heating near DBS electrodes clinically safe during ultra-high field head imaging.

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.

An analytical study of 'Poisson conduction shape factors' for two thermally significant vessels in a finite, heated tissue.

To conveniently and properly account for the vessel to vessel and vessel to tissue heat transfer rates to predict in vivo tissue temperature distributions, this paper analyses two different types of Poisson conduction shape factors (PCSFs) for unheated and/or uniformly heated, non-insulated, finite tissue domains. One is related to the heat transfer rate from one vessel to another (vessel-vessel PCSF (VVPCSF)) and the other is related to the vessel to tissue heat transfer rates (vessel-tissue PCSF (VTPCSF)). Two alternative formulations for the VTPCSFs are studied; one is based on the difference between the vessel wall and tissue boundary temperatures, and the other on the difference between the vessel wall and the average tissue temperatures. The effects of a uniform source term and of the diameters and locations of the two vessels on the PCSFs are studied for two different cases: one, when the vessel wall temperatures are lower than the tissue boundary temperature, i.e., the vessels cool the tissue, and vice versa. Results show that, first, the VVPCSFs are only geometry dependent and they do not depend on the applied source term and the vessel wall and tissue boundary temperatures. Conversely, the VTPCSFs are strong functions of the source term and of the temperatures of the vessel walls and tissue boundary. These results suggest that to account for the vessel to vessel heat transfer rates, the VVPCSFs can be evaluated solely based on the vessel network geometry. However, to account for the vessel to tissue heat transfer rates, the VTPCSFs should be used iteratively while solving for the tissue temperature distributions. Second, unlike the tissue boundary temperature-based VTPCSFs which may become singular only in heated tissues, the average tissue temperature-based VTPCSFs have the potential to become singular in both unheated and heated tissues. These results suggest that caution should be exercised in the use of the VTPCSFs since they may approach singularity by virtue of their definition and thus may introduce large errors in the evaluation of tissue temperature distribution. Presented results are new and complementary to the previous shape factor results since these include the effect of (1) source term and (2) unequal vessel-tissue heat transfer rates from the two vessels to the tissue.

Heating induced near deep brain stimulation lead electrodes during magnetic resonance imaging with a 3 T transceive volume head coil.

Heating induced near deep brain stimulation (DBS) lead electrodes during magnetic resonance imaging with a 3 T transceive head coil was measured, modeled, and imaged in three cadaveric porcine heads (mean body weight = 85.47 ± 3.19 kg, mean head weight = 5.78 ± 0.32 kg). The effect of the placement of the extra-cranial portion of the DBS lead on the heating was investigated by looping the extra-cranial lead on the top, side, and back of the head, and placing it parallel to the coil's longitudinal axial direction. The heating was induced using a 641 s long turbo spin echo sequence with the mean whole head average specific absorption rate of 3.16 W kg(-1). Temperatures were measured using fluoroptic probes at the scalp, first and second electrodes from the distal lead tip, and 6 mm distal from electrode 1 (T(6 mm)). The heating was modeled using the maximum T(6 mm) and imaged using a proton resonance frequency shift-based MR thermometry method. Results showed that the heating was significantly reduced when the extra-cranial lead was placed in the longitudinal direction compared to the other placements (peak temperature change = 1.5-3.2 °C versus 5.1-24.7 °C). Thermal modeling and MR thermometry may be used together to determine the heating and improve patient safety online.

A generic bioheat transfer thermal model for a perfused tissue.

A thermal model was needed to predict temperatures in a perfused tissue, which satisfied the following three criteria. One, the model satisfied conservation of energy. Two, the heat transfer rate from blood vessels to tissue was modeled without following a vessel path. Three, the model applied to any unheated and heated tissue. To meet these criteria, a generic bioheat transfer model (BHTM) was derived here by conserving thermal energy in a heated vascularized finite tissue and by making a few simplifying assumptions. Two linear coupled differential equations were obtained with the following two variables: tissue volume averaged temperature and blood volume averaged temperature. The generic model was compared with the widely employed empirical Pennes' BHTM. The comparison showed that the Pennes' perfusion term wC(p)(1-epsilon) should be interpreted as a local vasculature dependent heat transfer coefficient term. Suggestions are presented for further adaptations of the general BHTM for specific tissues using imaging techniques and numerical simulations.

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.

Readdressing the issue of thermally significant blood vessels using a countercurrent vessel network.

A physiologically realistic arterio-venous countercurrent vessel network model consisting of ten branching vessel generations, where the diameter of each generation of vessels is smaller than the previous ones, has been created and used to determine the thermal significance of different vessel generations by investigating their ability to exchange thermal energy with the tissue. The temperature distribution in the 3D network (8178 vessels; diameters from 10 to 1000 microm) is obtained by solving the conduction equation in the tissue and the convective energy equation with a specified Nusselt number in the vessels. The sensitivity of the exchange of energy between the vessels and the tissue to changes in the network parameters is studied for two cases; a high temperature thermal therapy case when tissue is heated by a uniformly distributed source term and the network cools the tissue, and a hypothermia related case, when tissue is cooled from the surface and the blood heats the tissue. Results show that first, the relative roles of vessels of different diameters are strongly determined by the inlet temperatures to those vessels (e.g., as affected by changing mass flow rates), and the surrounding tissue temperature, but not by their diameter. Second, changes in the following do not significantly affect the heat transfer rates between tissue and vessels; (a) the ratio of arterial to venous vessel diameter, (b) the diameter reduction coefficient (the ratio of diameters of successive vessel generations), and (c) the Nusselt number. Third, both arteries and veins play significant roles in the exchange of energy between tissue and vessels, with arteries playing a more significant role. These results suggest that the determination of which diameter vessels are thermally important should be performed on a case-by-case, problem dependent basis. And, that in the development of site-specific vessel network models, reasonable predictions of the relative roles of different vessel diameters can be obtained by using any physiologically realistic values of Nusselt number and the diameter reduction coefficient.

9.4T human MRI: preliminary results.

This work reports the preliminary results of the first human images at the new high-field benchmark of 9.4T. A 65-cm-diameter bore magnet was used together with an asymmetric 40-cm-diameter head gradient and shim set. A multichannel transmission line (transverse electromagnetic (TEM)) head coil was driven by a programmable parallel transceiver to control the relative phase and magnitude of each channel independently. These new RF field control methods facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.4T head images or localize anatomic targets. Prior to FDA investigational device exemptions (IDEs) and internal review board (IRB)-approved human studies, preliminary RF safety studies were performed on porcine models. These data are reported together with exit interview results from the first 44 human volunteers. Although several points for improvement are discussed, the preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T.