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Purpose To investigate how the simultaneous exposure to gradient and RF fields affects the temperature rise in patients with a metallic hip prosthesis during an MRI session. Methods In silico analysis was performed with an anatomically realistic human model with CoCrMo hip implant in 12 imaging positions. The analysis was performed at 1.5 T and 3 T, considering four clinical sequences: turbo spin-echo, EPI, gradient-echo, and true fast imaging sequence with steady precession. The exposure to gradient and RF fields was evaluated separately and superposed, by adopting an ad hoc computational algorithm. Temperature increase within the body, rather than specific absorption rate, was used as a safety metric. Results With the exception of gradient-echo, all investigated sequences produced temperature increases higher than 1 K after 360 seconds, at least for one body position. In general, RF-induced heating dominates the turbo spin-echo sequence, whereas gradient-induced heating prevails with EPI; the situation with fast imaging sequence with steady precession is more diversified. The RF effects are enhanced when the implant is within the RF coil, whereas the effects of gradient fields are maximized if the prosthesis is outside the imaging region. Cases for which temperature-increase thresholds were exceeded were identified, together with the corresponding amount of tissue mass involved and the exposure time needed to reach these limits. Conclusion The analysis confirms that risky situations may occur when a patient carrying a hip implant undergoes an MRI exam and that, in some cases, the gradient field heating may be significant. In general, exclusion criteria only based on whole-body specific absorption rate may not be sufficient to ensure patients' safety.

To evaluate specific absorption rate (SAR) and temperature distributions resulting from pediatric exposure to a 7T head coil.Exposure from a 297-MHz birdcage head transmit coil (CP mode single-channel transmission) was simulated in several child models (ages 3-14, mass 13.9-50.4 kg) and one adult, using time-domain electromagnetic and thermal solvers. Position variability, age-related changes in dielectric properties, and differences in thermoregulation were also considered.Age-adjusted dielectric properties had little effect in this population. Head average SAR (hdSAR) was the limiting factor for all models centered in the coil. The value of hdSAR (normalized to net power) was found to decrease linearly with increasing mass (RChildren experienced higher SAR than adults for the 297-MHz head transmit coil examined in this work. Thermal simulations suggest that core temperature changes could occur in smaller subjects, although experimental data are needed for validation.

Purpose To investigate safe limits for neonatal imaging using a 7T head coil, including both specific absorption rate (SAR) and temperature predictions. Methods Head-centered neonate models were simulated using finite-difference time domain-based electromagnetic and thermal solvers. The effects of higher water content of neonatal tissues compared with adults, position shifts, and thermal insulation were also considered. An adult model was simulated for comparison. Results Maximum and average SAR are both elevated in the neonate when compared with an adult model. When normalized to B 1 + , the SAR experienced by a neonate is greater than an adult by approximately a factor of 2; when normalized to net forward power (forward-reflected), this increases to a factor of 2.5-3.0; and when normalized to absorbed power, approximately a factor of 4. Use of age-adjusted dielectric properties significantly increases the predicted SAR, compared with using adult tissue properties for the neonates. Thermal simulations predict that change in core temperature/maximum temperature remain compliant with International Electrotechnical Commission limits when a thermally insulated neonate is exposed at the SAR limit for up to an hour. Conclusion This study of two neonate models cannot quantify the variability expected within a larger population. Likewise, the use of age-adjusted dielectric properties have a significant effect, but while their use is well motivated by literature, there is uncertainty in the true dielectric properties of neonatal tissue. Nevertheless, the main finding is that unlike at lower field strengths, operational limits for 7T neonatal MRI using an adult head coil should be more conservative than limits for use on adults.

This paper presents an extended comparison between numerical simulations using the different computational tools employed nowadays in electromagnetic dosimetry and measurements of radiofrequency (RF) electromagnetic field distributions in phantoms with tissue-simulating liquids at 64 MHz, 128 MHz and 300 MHz, adopting a customized experimental setup. The aim is to quantify the overall reliability and accuracy of RF dosimetry approaches at frequencies in use in magnetic resonance imaging transmit coils. Measurements are compared against four common techniques used for electromagnetic simulations, i.e. the finite difference time domain (FDTD), the finite integration technique (FIT), the boundary element method (BEM) and the hybrid finite element method-boundary element method (FEM-BEM) approaches. It is shown that FDTD and FIT produce similar results, which generally are also in good agreement with those of FEM-BEM. On the contrary, BEM seems to perform less well than the other methods and shows numerical convergence problems in presence of metallic objects. Maximum uncertainties of about 30% (coverage factor k = 2) can be attributed to measurements regarding electric and magnetic field amplitudes. Discrepancies between simulations and experiments are found to be in the range from 10% to 30%. These values confirm other previously published results of experimental validations performed on a limited set of data and define the accuracy of our measurement setup.

Purpose The paper presents a computational study for the estimation of the temperature elevation occurring in a human subject carrying metallic hip prostheses when exposed to the magnetic field produced by gradient coils. Methods The simulations are performed through validated numerical codes, which solve the electromagnetic and thermal equations applied to a high-resolution anatomical human model. Three different sets of gradient coils (traditional, split and uniplanar) are considered to evaluate the maximum steady-state temperature elevation in the human body. This result is then rescaled to take into account the waveform of the signal, the duty-cycle and the duration of the scan. Results Several exposure situations obtained by changing the patient's position are analyzed, finding temperature elevations on the order of some degrees. Conclusion The results are of possible concern and provide evidence of the need for further specific investigations aimed at assuring the safety of potential patients carrying metallic hip implants. Magn Reson Med 74:272-279, 2015

A hybrid finite element–boundary element method, developed to solve eddy-current problems in the frequency domain, is applied to the electromagnetic analysis of voxel-based human models. A specific procedure employs a massively parallelized algorithm implemented on a multiple Graphic Processing Units (GPU) code to speed up the solution of large systems whose matrix exceeds the RAM capability. The database structure used for the electromagnetic problem is also suitable for a successive thermal analysis to evaluate the distribution of the temperature elevation due to the energy deposited by the waves in the tissues. Finally, some examples of application are presented.

This paper aims at evaluating the thermal effects produced by the gradient coils on patients carrying metallic hip prostheses when undergoing check-up in combined magnetic resonance imaging with a linear accelerator systems. The computations are performed by two noncommercial codes specifically developed by the authors for voxel based human models. The electromagnetic field problem is solved by a hybrid finite element-boundary element technique implemented in a massively parallelized GPU system. The temperature elevation due to the electromagnetic exposure is then evaluated through a finite element code. The computations are performed for a large number of situations, considering both radial and axial arrangements of the patient.

Numerical simulations of specific absorption rate (SAR) and temperature changes in a 26-week pregnant woman model within typical birdcage body coils as used in 1.5 T and 3 T MRI scanners are described. Spatial distributions of SAR and the resulting spatial and temporal changes in temperature are determined using a finite difference time domain method and a finite difference bio-heat transfer solver that accounts for discrete vessels. Heat transfer from foetus to placenta via the umbilical vein and arteries as well as that across the foetal skin/amniotic fluid/uterine wall boundaries is modelled. Results suggest that for procedures compliant with IEC normal mode conditions (maternal whole-body averaged SAR(MWB)or = 2 W kg(-1) (continuous or time-averaged over 6 min)), whole foetal SAR, local foetal SAR(10 g) and average foetal temperature are within international safety limits. For continuous RF exposure at SAR(MWB) = 2 W kg(-1) over periods of 7.5 min or longer, a maximum local foetal temperature38 degrees C may occur. However, assessment of the risk posed by such maximum temperatures predicted in a static model is difficult because of frequent foetal movement. Results also confirm that when SAR(MWB) = 2 W kg(-1), some local SAR(10g) values in the mother's trunk and extremities exceed recommended limits.

Miniature microwave helical antennas for use in thermal therapy applications are usually investigated using muscle-equivalent phantoms. In this paper, an alternative method using an electromagnetic solver based on the finite integration technique is used to simulate a range of 915 MHz helical antennas within a medium with the dielectric properties of muscle. By avoiding the stair-casing effect associated with many solvers, this method enables accurate simulations of non-orthogonal geometric objects such as helical antennas to be achieved. The effects of coil-spacing and insertion depth on the SAR distribution produced by the antennas were characterized and showed good agreement with previously published results obtained using a muscle phantom and a thermographic camera. The simulations confirm that the performance of helical antennas depends on insertion depth. Modification of the coil density demonstrated improvement of the return loss characteristics and changes to the resulting SAR profile.

The design and performance of a silicon-based RF detector coil for use in magnetic resonance (MR) spectroscopy applications is described. The coil is fabricated using microelectromechanical systems (MEMS) technology by deep reactive ion etching (DRIE) of an oxidized silicon substrate carrying electroplated conductors. The DRIE step simultaneously forms a sample trough and creates a trepan cut around the coil so that it may be detached from the substrate by cleaving short sections of sprue. A single-turn coil with Q-factor ~16 at 63.6 MHz is demonstrated and MR spectroscopy experiments are described. An accurate numerical model based on the time-domain finite integration technique is used to investigate the effect on the Q-factor of deliberately structuring the substrate to limit the volume of silicon exposed to RF energy. The model is compared with known analytic approximations, and good agreement is obtained.

The use of the Allan deviation for the analysis of signal noise and drift components is considered in the context of microwave radiometry. The noise behaviour of two types of microwave radiometer is modelled and compared with measurements of the performance of these radiometers analysed using the Allan deviation method.

Design and development of a prototype endocavitary probe for high-intensity focused ultrasound delivery with integrated magnetic resonance imaging Purpose: To integrate a high intensity focused ultrasound (HIFU) transducer with an MR receiver coil for endocavitary MR-guided thermal ablation of localized pelvic lesions. Materials and Methods: A hollow semicylindrical probe (diameter 3.2 ern) with a rectangular upper surface (7.2 cm X 3.2 cm) was designed to house a HIFU transducer and enable acoustic contact with an intraluminal wall. The probe was distally rounded to ease endocavitary insertion and was proximally tapered to a 1.5-cm diameter cylindrical handle through which the irrigation tubes (for transducer cooling) and electrical connections were passed. MR compatibility of piezoeeramic and piezocomposite transducers was assessed using gradient-echo (GRE) sequences. The radiofrequency (RF) tuning of identical 6.5 cm X 2.5 cm rectangular receiver coils on the upper surface of the probe was adjusted to compensate for the presence of the conductive components of the HIFU transducers. A T1-weighted (T1-W) sliding window dual-echo GRE sequence monitored phase changes in the focal zone of each transducer. High-intensity (2400 W/cm(-2)), short duration (< 1.5 seconds) exposures produced subtherapeutic temperature rises. Results: For T1-W images, signal-to-noise ratio (SNR) improved by 40% as a result of quartering the conductive surface of the piezoceramic transducer. A piezocomposite transducer showed a further 28% improvement. SNRs for an endocavitary coil in the focal plane of the HIFU transducer (4 cm from its face) were three times greater than from a phased body array coil. Local shimming improved uniformity of phase images. Phase changes were detected at subtherapeutic exposures. Conclusion: We combined a HIFU transducer with an MR receiver coil in an endocavitary probe. SNRs were improved by quartering the conductive surface of the piezoceramic. Further improvement was achieved with a piezoeomposite transducer. A phase change was seen on MR images during both subtherapeutic and therapeutic HIFU exposures.

Magnetic Resonance Imaging (MRI) has become an indispensable medical imaging modality with about 30 million patient exams in the EU every year and an excellent history of safe use. Nevertheless, it is continuously evolving and recent technological developments such as ultrahigh magnetic fields, parallel transmission, or MRI guided radiotherapy promise to significantly enhance the quality and the range of applicability of MRI. A major reason why these technological developments are not yet used in the clinical practice are unresolved safety issues. If the patient risk cannot be quantified reliably, a 'safety first' attitude naturally prevails preventing the routine use of new technologies or the scanning of subjects at high risk, e.g. carriers of metallic medical implants. The EMRP joint research project HLT 06 "Metrology for MRI Safety" aimed at providing such risk assessments for certain new developments or applications in MRI. The project was concluded in 2015 and some key results will be presented here.

Miniature coils for magnetic resonance imaging and spectroscopy are demonstrated. A rectangular Helmholtz arrangement based on a pair of substrates separated by spherical alignment spacers is investigated. An optimal geometry is developed using simple theory and verified by numerical analysis. Prototypes are fabricated using silicon substrates shaped by anisotropic etching to form a trough to hold an internal sample and pits for use as alignment features, and carrying Cu/Au conductors fabricated by electroplating. Q values of 11 are obtained, and devices containing an internal proton source are developed for use as fiducial markers in 1 H magnetic resonance imaging at 1.5 T. Magnetic resonance spectroscopy is also performed on the internal source, with a SNR of ≈85.

Purpose: To assess the magnetic field interactions on 60 heart valve prostheses subjected to a 4.7 T MR system. It addresses the question of whether heart valves deemed safe at 1.5 T may pose safety hazards as patients are exposed to increased static magnetic fields.Materials and Methods: Ex vivo testing was performed to evaluate translational and rotational forces on 60 heart valves using previously described techniques.Results: Translational forces were detected on 58 heart valves ranging from 0.5degrees to 7.5degrees. Seven valves exhibited paramagnetic/weakly ferromagnetic behavior, and 51 valves exhibited diamagnetic behavior. Rotational forces were observed for 46 valves.Conclusions: Criteria previously used for safety assessment of heart valve prostheses and expressed in terms of magnetic forces suggest the forces observed in this study arc compatible with the safe use of these valves in magnetic resonance (MR) systems with static fields up to 4.7 T.

High-intensity focused ultrasonography is the only completely non-invasive thermal therapy. To date its applications have been limited but clinical indications are expanding with enhanced technological advances that have increased the accuracy of targeting and decreased the duration of treatment times. We report its first use for rectal cancer.

The paper presents a computational study for the estimation of the temperature elevation occurring in a human subject carrying metallic hip prostheses when exposed to the magnetic field produced by gradient coils.The simulations are performed through validated numerical codes, which solve the electromagnetic and thermal equations applied to a high-resolution anatomical human model. Three different sets of gradient coils (traditional, split and uniplanar) are considered to evaluate the maximum steady-state temperature elevation in the human body. This result is then rescaled to take into account the waveform of the signal, the duty-cycle and the duration of the scan.Several exposure situations obtained by changing the patient's position are analyzed, finding temperature elevations on the order of some degrees.The results are of possible concern and provide evidence of the need for further specific investigations aimed at assuring the safety of potential patients carrying metallic hip implants. Magn Reson Med 74:272-279, 2015. © 2014 Wiley Periodicals, Inc.

In this study we present a design for a multi-frequency microwave radiometer aimed at prolonged monitoring of deep brain temperature in newborn infants and suitable for use during hypothermic neural rescue therapy. We identify appropriate hardware to measure brightness temperature and evaluate the accuracy of the measurements. We describe a method to estimate the tissue temperature distribution from measured brightness temperatures which uses the results of numerical simulations of the tissue temperature as well as the propagation of the microwaves in a realistic detailed three-dimensional infant head model. The temperature retrieval method is then used to evaluate how the statistical fluctuations in the measured brightness temperatures limit the confidence interval for the estimated temperature: for an 18 degrees C temperature differential between cooled surface and deep brain we found a standard error in the estimated central brain temperature of 0.75 degrees C. Evaluation of the systematic errors arising from inaccuracies in model parameters showed that realistic deviations in tissue parameters have little impact compared to uncertainty in the thickness of the bolus between the receiving antenna and the infant's head or in the skull thickness. This highlights the need to pay particular attention to these latter parameters in future practical implementation of the technique.

Twenty years ago concerns were raised that exposure to power frequency (or extremely low frequency (ELF)) electromagnetic fields (EMFs) may be associated with an increased risk of cancer or other health hazards. Subsequently no associations have been shown between laboratory magnetic field exposures and carcinogenesis in either animal or cellular models. Indeed, studies have demonstrated that magnetic fields are not associated with cancer. However, the puzzle remains that the results of some epidemiological studies may be interpreted as suggesting that living close to high-voltage transmission (HVT) lines appears to increase slightly the risk of childhood leukaemia. Alternatively, these results could result from small biases and errors in individual studies, which might not necessarily be the same in each study. The nature of the epidemiological studies (power-line, wire code, magnetic field or appliance based) appears to determine whether and how the EMFs associated with HVT lines might be a risk factor. It is possible that a simple association with either magnetic or electric field exposure may not be the whole answer, and an alternative mechanism is always a possibility. Although the interpretation of the available evidence by most expert bodies has led them to conclude that exposure to power frequency electric and magnetic fields is not a human health hazard, a working group under the auspices of the US National Institute of Environmental Health Sciences (NIEHS) concluded that there was a possible low risk associated with certain exposures to ELF magnetic fields. NIEHS itself interpreted the finding as insufficient to warrant aggressive regulatory concern but stated that, because virtually everyone is routinely exposed to ELF EMFs, passive regulatory action is warranted, such as a continued emphasis on educating both the public and the regulated community on means aimed at reducing exposures. These analyses, conclusions and advice are not contradicted by subsequent studies, and therefore the conclusion of the World Health Organisation that further research is needed seems valid.

Numerical simulation and comparative analysis of acoustic fields generated by two-dimensional phased arrays designed for ultrasonic surgery is conducted. The case of movement of a single focus by an array with the surface shaped as a part of a spherical shell with the curvature radius 120 mm is considered. The influence of the number of elements (varying from 64 to 1024), their diameter (from 2.5 to 10 mm), frequency (from 1 to 2 MHz), and the degree of sparseness of the elements at the array surface on the field characteristics is studied. The calculations are performed for arrays with elements positioned at the surface both regularly (in square, annular, or hexagonal patterns) and randomly. Criteria for the evaluation of the “quality” of intensity distributions in the field generated by an array in the case of movement of a single focus are suggested. Of all arrays studied, the best quality of distributions is obtained for an array containing 256 elements of diameter 5 mm randomly positioned at the array surface. The quality of the intensity distributions for arrays consisting of 255, 256, and 1024 elements positioned regularly (in square, annular, and hexagonal patterns) is inferior to the corresponding quality for arrays with randomly positioned elements. The irregularity in elements’ positioning considerably improves the distribution quality by suppressing the secondary intensity peaks in the field generated by the array; or, alternatively, it provides an opportunity to obtain the same distribution quality with a fraction of the number of elements in the array. The effects of the number and shape of elements, errors in phase setting, frequency modulation of signals, and non-uniform distribution of amplitudes over the array surface on the distribution quality are analyzed.

Clinical studies of hypothermal neural rescue therapy for newborn infants who have suffered hypoxia-ischaemia are currently hindered by the difficulty in measuring deep brain temperature. This paper addresses: the specific requirements for this measurement problem, the design of a proposed radiometer system, a method for retrieving the temperature profile within the cooled head, and an estimation of the precision of the measurement of deep brain temperature using the technique. A five-frequency-band radiometer with a contact-type antenna operating within the range 1-4 GHz is proposed to obtain brightness temperatures corresponding to temperature profiles predicted by a realistic thermal model of the cooled baby head. The problems of retrieving the temperature profile from this set of brightness temperatures, and the estimation of its precision, are solved using a combination of model fitting and Monte Carlo techniques. The results of this paper show that the proposed technique is feasible, that it is expected to provide a good estimate of the temperature profile within the cooled baby-head, and that the estimated precision (2/spl sigma/) of the temperature measured in the deep brain structures is better than 0.8 K, depending upon the estimation procedure used.

Introduction of hypothermia therapy as a neuroprotection therapy after hypoxia-ischemia in newborn infants requires appraisal of cooling methods. In this numerical study thermal simulations were performed to test the hypothesis that cooling of the surface of the cranium by the application of a cooling bonnet significantly reduces deep brain temperature and produces a temperature differential between the deep brain and the body core. A realistic three-dimensional (3-D) computer model of infant head anatomy was used, derived from magnetic resonance data from a newborn infant. Temperature distributions were calculated using the Pennes heatsink model. The cooling bonnet was at a constant temperature of 10 degrees C. When modeling head cooling only, a constant body core temperature of 37 degrees C was imposed. The computed result showed no significant cooling of the deep brain regions, only the very superficial regions of the brain are cooled to temperatures of 33-34 degrees C. Poor efficacy of head cooling was still found after a considerable increase in the modeled thermal conductivities of the skin and skull, or after a decrease in perfusion. The results for the heatsink thermal model of the infant head were confirmed by comparison of results computed for a scaled down adult head, using both the heatsink description and a discrete vessel thermal model with both anatomy and vasculature obtained from MR data. The results indicate that significant reduction in brain temperature will only be achieved if the infant's core temperature is lowered.

Specific absorption rate (SAR) due to a butterfly surface coil in a realistic model of the leg was calculated for frequencies 64 or = 149 MHz. The corresponding maximum deltaT at 64, 128, and 213 MHz were 0.3, 1.3, and 5.0 degrees C, respectively.

Computer modeling of spherical-section phased arrays for ultrasound surgery (tissue ablation) is described. The influence on performance of the number of circular elements (68 to 1024), their diameter (2.5 to 10 mm), frequency (1 to 2 MHz), and degree of sparseness in the array is investigated for elements distributed randomly or in square, annular, and hexagonal patterns on a spherical shell (radius of curvature, 120 mm). Criteria for evaluating the quality of the intensity distributions obtained when focusing the arrays both on and away from their center of curvature, and in both single focus and simultaneous multiple foci modes, are proposed. Of the arrays studied, the most favorable performance, for both modes, is predicted for 256 5-mm diameter, randomly distributed elements. For the single focus mode, this performed better than regular arrays of 255 to 1024 elements and, for the case of nine simultaneous foci produced on a coplanar 3/spl times/3 grid with 4-mm spacing, better than square, hexagonal, or annular distributed arrays with a comparable number of elements. Randomization improved performance by suppressing grating lobes significantly. For single focus mode, a several-fold decrease in the number of elements could be made without degrading the quality of the intensity distribution.

We demonstrate the feasibility of using in-fibre Bragg gratings to measure MHz acoustic fields and temperature simultaneously. We achieved a noise-limited pressure resolution of ˜4.5×10-4 Atm/vHz and a temperature resolution of 0.2°C.

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 B o field magnetic resonance systems and where knowledge of spatial variation in SAR is important in assessing the safety of new magnetic resonance procedures.

We investigate the feasibility of using in-fibre Bragg gratings to measure ultrasonic fields for medical applications. Two signal processing schemes for interrogating the gratings are described. Preliminary results for each scheme (one a homodyne approach, the other a heterodyne one) give noise-limited pressure resolutions of and atm respectively, each within a 1 Hz bandwidth. The second scheme, however, gives a more stable response.

Some practical aspects of planar linear ultrasound phased arrays for transrectal thermotherapy of prostate diseases are discussed. Several regimens for driving the array are investigated and spatial distributions of ultrasound intensities are measured in water and compared with computer simulations. Practical recommendations for suppressing grating lobes based on the use of subsets of elements and de-activation of several elements in the array are given. Treatment safety could be increased by adopting these measures since the relative intensities and power in grating lobes and other secondary intensity peaks are decreased, as is the overall ultrasound energy introduced into the body without significant reduction in the maximum power at the focus.

PURPOSE The objective of this study was to determine whether the thermal dose delivered during hyperthermia treatments and other thermal factors correlate with outcome after combined radiation and hyperthermia of breast carcinoma recurrences. Data were from the combined hyperthermia and radiation treatment arms of four Phase III trials, which when pooled together, demonstrated a positive effect of hyperthermia. METHODS AND MATERIALS Four Phase III trials addressing the question of whether hyperthermia could improve the local response of superficial recurrent breast cancer to radiation therapy were combined into a single analysis. Thermal dosimetry data were collected from 120 of the 148 breast cancer recurrence patients who received hyperthermia. The data were analyzed for correlations between thermal parameters as well as important clinical parameters and outcome (complete response rate, local disease free survival, time to local failure, and overall survival). RESULTS Five thermal parameters were tested, all associated with the low regions of the measured temperature distributions. Max(TDmin) and Sum(TDmin) were associated with complete response where TDmin is the minimum thermal dose measured by any of the tumor temperature sensors during a treatment: Max(TDmin) is the maximum of TDmin over a series of treatments. Using a categorical relationship with a cutoff of 10 min for Sum(TDmin), the complete response rate was 77% for Sum(TDmin) > 10 min and 43% for Sum(TDmin) < or = 10 min (p = 0.022, adjusted for study center and significant clinical factors). The overall complete response rate for hyperthermia and radiation was 61% compared to 41% for radiation alone. Either Max(TDmin) or Sum(TDmin) were also associated with local disease free survival, time to local failure and overall survival. CONCLUSIONS An earlier report of this trial demonstrated a significant benefit when hyperthermia was added to radiation in the treatment of breast cancer recurrences. The analysis of thermal factors demonstrates that parameters representative of the low end of the measured temperature distributions are associated with initial complete response rate, local disease-free survival, time to local failure and overall survival.

High-intensity focused ultrasound (HIFU) is a rapidly maturing technology with diverse clinical applications. In the field of oncology, the use of HIFU to non-invasively cause tissue necrosis in a defined target, a technique known as focused ultrasound surgery (FUS), has considerable potential for tumour ablation. In this article, we outline the development and underlying principles of HIFU, overview the limitations and commercially available equipment for FUS, then summarise some of the recent technological advances and experimental clinical trials that we predict will have a positive impact on extending the role of FUS in cancer therapy.

This European Federation of Organisations for Medical Physics (EFOMP) Policy Statement outlines the way in which a Safety Management System can be developed for MRI units. The Policy Statement can help eliminate or at least minimize accidents or incidents in the magnetic resonance environment and is recommended as a step towards harmonisation of safety of workers, patients, and the general public regarding the use of magnetic resonance imaging systems in diagnostic and interventional procedures., peer-reviewed

Online ISSN: 1521-186X; International audience; We estimate that there are about 100,000 workers from different disciplines, such as radiographers, nurses, anesthetists, technicians, engineers, etc., who can be exposed to substantial electromagnetic fields (compared to normal background levels) around magnetic resonance imaging (MRI) scanners. There is a need for well-designed epidemiological studies of MRI workers but since the exposure from MRI equipment is a very complex mixture of static magnetic fields, switched gradient magnetic fields, and radiofrequency electromagnetic fields (RF EMF), it is necessary to discuss how to assess the exposure in epidemiological studies. As an alternative to the use of job title as a proxy of exposure, we propose an exposure categorization for the different professions working with MRI equipment. Specifically, we propose defining exposure in three categories, depending on whether people are exposed to only the static field, to the static plus switched gradient fields or to the static plus switched gradient plus RF fields, as a basis for exposure assessment in epidemiological studies.

*MRC Cyclotron Unit, Hammersmith Hospital, London, United Kingdom; +MRC Cancer Trials Office, Cambridge, United Kingdom; *Departments of Hyperthermia and Statistics, Daniel den Hoed Cancer Center, Rotterdam,The Netherlands; “Department of Radiotherapy, Academic Medical Center, Amsterdam,The Netherlands; apartments of Radiation Oncology, Biostatistics, and Clinical Physics, Princess Margaret Hospital/Ontario Cancer Institute, University of Toronto, Canada purpose: Claims for the value of hyperthermia as an adjunct to radiotherapy in the treatment of cancer have mostly been based on small Phase I or II trials. To test the benefit of thii form of treatment, randomized phase III trials were needed. Methods and Materials: Five randomized trials addressing this question were started between 1988 and 1991. In these trials, patients were eligible if they had advanced prhnary or recurrent breast cancer, and local radiotherapy was indicated in preference to surgery. In addition, heating of the lesions and treatment with a prescribed (re)irradiation schedule had to be feasible and informed consent was obtained. The primary endpoint of ail trials was local complete response. Slow recruitment led to a decision to collaborate and combii the trial rest&s in one analysis, and report them simuhaneously in one publication. Interim analyses were carried out and the trials were closed to recruitment when a previously agreed statistically sign&ant difference in complete response rate was observed in the two larger trials. Results: We report on pretreatment characteristics, the treatments received, the local response observed, duration of response, time to local failure, distant progression and survival, and treatment toxicity of tbe 306 patients randomized. The overali CR rate for RT alone was 41% and for the combined treatment arm was 59%, giving, after stratification by trial, an odds ratio of 2.3. Not alI trials demonstrated an advantage for the combined treatment, although tbe 95% confidence intervals of the different trials all contain the pooled odds ratio. The greatest effect was observed in patients with recurrent lesions in previously irradiated areas, where further irradiation was limited to low doses. Conclusion: The combined result of the five trials has demonstrated the efficacy of hyperthemia as an adjunct to radiotherapy for treatment of recurrent breast cancer. The implication of these encouraging results is that hyperthermia appears to have an important role in the clinical management of this disease, and there should be no doubt that further studies of the use of hypertbermia are warranted. Breast cancer, Hyperthermia, Radiotherapy, Randomized trial. Reprint requests to: Jill B. Whaley, MRC Cancer Trials Office, 5 Shaftesbury Road, Cambridge CB2 2BW UK Accepted for publication 6 March 1996.

We present a method for the calculation of electrical potentials at the surface of realistic head models from a point dipole generator based on a 3D finite-difference algorithm. The model was validated by comparing calculated values with those obtained algebraically for a three-shell spherical model. For a 1.25 mm cubic grid size, the mean error was 4.9% for a superficial dipole (3.75 mm from the inner surface of the skull) pointing in the radial direction. The effect of generator discretization and node spacing on the accuracy of the model was studied. Three values of the node spacing were considered: 1, 1.25 and 1.5 mm. The mean relative errors were 4.2, 6.3 and 9.3%, respectively. The quality of the approximation of a point dipole by an array of nodes in a spherical neighbourhood did not depend significantly on the number of nodes used. The application of the method to a conduction model derived from MRI data is demonstrated.

There are a number of potential advantages to be gained by using an array of applicators in hyperthermia treatments compared with single applicator systems, These advantages include the possibility of greater spatial control of power deposition and conformability to nonplanar sites. Arrays of applicators can be driven either coherently or incoherently. In the case of coherent operation, an added advantage is the ability to steer power deposition by varying the phases of the antennas. In this study, the authors investigated the relative merits of the two modes of operation when a 2/spl times/2 planar array of current sheet applicators is used. The effective field size (EFS) of the array was calculated using a Gaussian beam representation of the applicators on a layered model in which the fat layer had its thickness varied. Good agreement was obtained between the square of the electric field distribution (E/sup 2/) and quantitative experimental results. It is shown that when the planar array is used with a fat layer greater than about 2 mm present, it should be driven incoherently as this results in a significantly larger EFS than that obtained when the array is driven coherently. >

The interaction between the RF field and metallic prosthetic devices may lead to an increase in specific absorption rate (SAR) with consequent tissue heating in areas surrounding the prostheses and to distortions in the B1 field, thus changing the local image contrast. Numerical simulations of the electric field, B1 field, and local SAR averaged over 10 g of tissue (SAR10g) around bilateral CoCrMo alloy hip prostheses in situ in anatomically realistic voxel models of an adult male and female due to RF fields from a birdcage coil driven at 64 or 128 MHz are described. Results suggest that significant perturbation of the B1 field occurs close to a prosthesis and that the electric field is enhanced in the vicinity of the ends of a prosthesis leading to enhancement of local SAR10g in these regions. Caution is required when the whole-body-averaged SAR is 2 W kg−1 since the SAR10g close to an extremity of a prosthesis is likely to exceed standard/safety guideline limits under these conditions. Compliance could conflict with metal artifact reduction sequences using shorter B1 RF pulses with increased amplitude and consequently higher SAR. Keywords: RF safety; hip prostheses; local SAR; numerical modelling; metallic implants; MRI safety

Models have been developed for the signal changes occurring in human peripheral muscle after thermal stress for some methods of measuring temperature in vivo. These include the dependency of T1 on temperature, observed by both direct, and magnetization transfer, experiments, and the temperature sensitivity of the diffusion coefficient and the chemical shift of the water proton line. Results show the relative complexity of the T1-dependent models when there are significant variations in, possibly amongst other things, tissue perfusion, and how the predictions made using them match the behavior of the practical values quite well. Using changes in the diffusion coefficient appears less problematic, although there can still be significant errors in temperature calibrations. The problems with this approach are likely to be due to the consequences of anisotropy in tissue.

Previous results for the measurement of T1 as a means of monitoring temperature have demonstrated a linear calibration with a coefficient of the order of 1.5%/degrees C. We have studied the changes in T1 further, and observe that changes in tissue perfusion are substantial contributors to the effects observed, and that the model of what is happening is complex and may not permit a simple temperature calibration.

[1] Clinical trials of hypothermic brain treatment for newborn babies are currently hindered by the difficulty in measuring deep brain temperatures. As one of the possible methods for noninvasive and continuous temperature monitoring that is completely passive and inherently safe is passive microwave radiometry (MWR). We have developed a five-band microwave radiometer system with a single dual-polarized, rectangular waveguide antenna operating within the 1–4 GHz range and a method for retrieving the temperature profile from five radiometric brightness temperatures. This paper addresses (1) the temperature calibration for five microwave receivers, (2) the measurement experiment using a phantom model that mimics the temperature profile in a newborn baby, and (3) the feasibility for noninvasive monitoring of deep brain temperatures. Temperature resolutions were 0.103, 0.129, 0.138, 0.105 and 0.111 K for 1.2, 1.65, 2.3, 3.0 and 3.6 GHz receivers, respectively. The precision of temperature estimation (2σ confidence interval) was about 0.7°C at a 5-cm depth from the phantom surface. Accuracy, which is the difference between the estimated temperature using this system and the measured temperature by a thermocouple at a depth of 5 cm, was about 2°C. The current result is not satisfactory for clinical application because the clinical requirement for accuracy must be better than 1°C for both precision and accuracy at a depth of 5 cm. Since a couple of possible causes for this inaccuracy have been identified, we believe that the system can take a step closer to the clinical application of MWR for hypothermic rescue treatment.

Publisher Summary The measurement of the temperature of the human body has been of clinical importance for the p200 years. However, during the past 50 years, technology and physical science have so advanced that the noncontact measurement of body temperature is now a reality. Radiometry of the ear, perhaps the most recent, has become a common technique, replacing the mercury-in-glass thermometer. Microwave imaging research continues, with its potential for less superficial temperature detection than in IR thermography. In this last area, the detectors and imaging systems have developed far beyond the pioneer devices of the 1950s and 1960s. A wide demand for thermal imaging in industry, military, and civil engineering has ensured a substantial development in technology, assisted by the now commonplace computer image processing. This chapter brings together the research and applications of thermal radiometry to medicine. The application of an IR ear thermometer is presented in the chapter. To measure thermal radiation emitted by the ear, specific detectors and optical designs must be used. In principle, two types of IR detectors can be applied: quantum and thermal detectors. Usually thermopile and pyro-electric detectors are used in IR ear thermometers. Heat transfer is principally affected by three main modes. The first is conduction, requiring contact between the object and the sensor. The second mode of heat transfer is convection, and the third is radiation.

Randomized phased arrays can offer electronic steering of a single focus and simultaneous multiple foci concomitant with low levels of secondary maxima and are potentially useful as sources of high intensity focused ultrasound (HIFU). This work describes laboratory testing of a 1 MHz random phased array consisting of 254 elements on a spherical shell of radius of curvature 130 mm and diameter 170 mm. Acoustic output power and efficiency are measured for a range of input electrical powers, and field distributions for various single- and multiple-focus conditions are evaluated by a novel technique using an infrared camera to provide rapid imaging of temperature changes on the surface of an absorbing target. Experimental results show that the array can steer a single focus laterally to at least +/-15 mm off axis and axially to more than +/-15 mm from the centre of curvature of the array and patterns of four and five simultaneous foci +/-10 mm laterally and axially whilst maintaining low intensity levels in secondary maxima away from the targeted area in good agreement with linear theoretical predictions. Experiments in which pork meat was thermally ablated indicate that contiguous lesions several cm(3) in volume can be produced using the patterns of multiple foci.

A method for focusing high-intensity ultrasound (HIFU) through a rib cage that aims to minimize heating of the ribs while maintaining high intensities at the focus (or foci) was proposed and tested theoretically and experimentally. Two approaches, one based on geometric acoustics and the other accounting for diffraction effects associated with propagation through the rib cage, were investigated theoretically for idealized source conditions. It is shown that for an idealized radiator, the diffraction approach provides a 23% gain in peak intensity and results in significantly less power losses on the ribs (1% vs. 7.5% of the irradiated power) compared with the geometric one. A 2-D 1-MHz phased array with 254 randomly distributed elements, tissue-mimicking phantoms and samples of porcine rib cages are used in experiments; the geometric approach is used to configure how the array is driven. Intensity distributions are measured in the plane of the ribs and in the focal plane using an infrared camera. Theoretical and experimental results show that it is possible to provide adequate focusing through the ribs without overheating them for a single focus and several foci, including steering at ± 10–15 mm off and ± 20 mm along the array axis. Focus splitting caused by the periodic spatial structure of ribs is demonstrated both in simulations and experiments; the parameters of splitting are quantified. The ability to produce thermal lesions with a split focal pattern in ex vivo porcine tissue placed beyond the rib phantom is also demonstrated. The results suggest that the method is potentially useful for clinical applications of HIFU, for which the rib cage lies between the transducer(s) and the targeted tissue. (E-mail: vera@apl.washington.edu)

A new technique is described for the treatment of menorrhagia by heating the whole of the endometrial cavity of the uterus. A capacitively coupled probe at 27.12 MHz is inserted into the uterine cavity, which causes the basilis layer to be raised to approximately 50-55 degrees C whilst the rest of the pelvic contents remain at approximately normal (body) temperature. A major advantage of the method is that no special hysteroscopic skills are required, unlike the two other techniques currently used for endometrial ablation: the Nd-Yag laser or the hysteroresectoscopic loop. Also no toxic flushing/distension fluids are necessary, as are required for all hysteroscopic surgery. The method, however, does require the application of a large amount of RF power to the probe and so care must be taken to position the probe correctly in order to prevent any serious complications. Of 32 patients given a single treatment at a power level of 550 W for 20 min, the 'success rate' was 84% with 31% becoming amenhorrhoiec and 53% showing significant reduction in menstrual bleeding. However, retreatment is possible and by this means, combined with improved treatment techniques, an even higher success rate could be achieved. In two of the earlier patients treated at 550 W a fistula was produced at the anterior vaginal wall which had to be surgically repaired. The probe was subsequently modified, since when this problem has not recurred. This new treatment approach offers an alternative to hysterectomy in the treatment of menorrhagia and may offer a number of significant advantages over methods currently used for endometrial ablation.

Numerical modelling of the interaction between electromagnetic fields (EMFs) and the dielectrically inhomogeneous human body provides a unique way of assessing the resulting spatial distributions of internal electric fields, currents and rate of energy deposition. Knowledge of these parameters is of importance in understanding such interactions and is a prerequisite when assessing EMF exposure or when assessing or optimizing therapeutic or diagnostic medical applications that employ EMFs. In this review, computational methods that provide this information through full time-dependent solutions of Maxwell's equations are summarized briefly. This is followed by an overview of safety- and medical-related applications where modelling has contributed significantly to development and understanding of the techniques involved. In particular, applications in the areas of mobile communications, magnetic resonance imaging, hyperthermal therapy and microwave radiometry are highlighted. Finally, examples of modelling the potentially new medical applications of recent technologies such as ultra-wideband microwaves are discussed.

A novel monolithic implementation of a Wu-King profiled antenna intended for use in critical time-domain/UWB/imaging applications is presented. The simulated device was fabricated on a high-resistivity Si substrate on top of which a layer of GaAs is grown, segmented and patterned to serve as the radiating element. The device's measured return loss indicates a -10 dB BW > 50 GHz and time-domain measurements using a 500 pS input pulse prove that the device is practically non-reverberant in air. © 2008 IOP Publishing Ltd.

Purpose To simulate exposure (e.g., during interventional procedures) of a worker close to an operating MR scanner by calculating electric fields and current density within an anatomically realistic body model due to a z-gradient coil and to compare results with safety guidelines and European Directive 2004/40/EC. Materials and Methods Electric field and current density in an adult male model located at three positions within the range 0.19–0.44 m from the end of a generic z-gradient coil were calculated using the time-domain finite integration technique (FIT). Frequency scaling was used in which quasistatic conditions were assumed and results obtained at 1 MHz (assuming tissue conductivity values at 1 kHz) were scaled to 1 kHz. Results Current density (averaged over 1 cm2) in central nervous system (CNS) tissues up to 20.6 mA m−2 and electric fields (averaged over 5 mm) up to 4.1 V m−1 were predicted for a gradient of 10 mT m−1 and slew rate of 10 T m−1 second−1. Conclusion Compliance with 2004/40/EC, and with basic restriction values of Institute of Electrical and Electronics Engineers (IEEE) C95.6-2002, was predicted only at impracticably low gradients/slew rates in the ranges 4.9–9.1 mT m−1/4.9–9.1 T m−1 second−1 and 5–21 mT m−1/5–21 T m−1 second−1, respectively. J. Magn. Reson. Imaging 2007;26:1286–1295. © 2007 Wiley-Liss, Inc.

The sections in this article are 1 Introduction 2 Interactions Between rf Fields and Tissues 3 Biological Effects of rf Fields 4 MR Procedures and rf Concerns 5 Dosimetry and Modeling 6 International and National rf Safety Guidelines 7 Future Developments In rf Safety 8 Biographical Sketch Related Articles

Many medical applications of High Intensity Focused Ultrasound (HIFU) would benefit if a volume of predetermined dimensions and geometry, within which the effect (thermal, cavitation, etc.) of HIFU will occur, could be synthesized and steered electronically. This goal could be achieved by synthesizing multiple foci using random two‐dimensional phased arrays. The spatial resolution of 2‐D phased arrays (i.e. the minimum spacing between individual foci compatible with distinguishing two or more neighbouring foci) and control of their amplitudes is discussed. Each array element contributes to the synthesis of many foci simultaneously. It is shown that the spatial resolution is slightly larger than the ultrasound wavelength (λ), and that it is possible to generate a set of tightly packed foci with inter‐foci spacing of 0.7 λ and 1.3 λ using sub‐patterns with a smaller number of foci spaced ⩾1.5–2 λ apart. Significant advantages of random arrays compared to traditional therapeutic arrays in which elements are distributed in a regular manner are shown. It is demonstrated that use of a 1MHz random array to steer a pattern of 25 foci at +/− 10 mm off the array’s central axis results in both a decrease in the maximum intensity of grating lobes (6–9 dB lower than that associated with a 1 MHz regular array) and a spatial distribution of these maxima over a much greater volume compared to the case of the regular array.

The first and second sessions of the Workshop focussed on the basics of ultrasound and infrasound, their applications in both industry and medicine, and metrology and protection standards for ultrasound applications.