Polynomial chaos expansion of SAR and temperature increase variability in 3 T MRI due to stochastic input data.

Objective.Numerical simulations are largely adopted to estimate dosimetric quantities, e.g. specific absorption rate (SAR) and temperature increase, in tissues to assess the patient exposure to the radiofrequency (RF) field generated during magnetic resonance imaging (MRI). Simulations rely on reference anatomical human models and tabulated data of electromagnetic and thermal properties of biological tissues. However, concerns may arise about the applicability of the computed results to any phenotype, introducing a significant degree of freedom in the simulation input data. In addition, simulation input data can be affected by uncertainty in relative positioning of the anatomical model with respect to the RF coil. The objective of this work is the to estimate the variability of SAR and temperature increase at 3 T head MRI due to different sources of variability in input data, with the final aim to associate a global uncertainty to the dosimetric outcomes.Approach.A stochastic approach based on arbitrary Polynomial Chaos Expansion is used to evaluate the effects of several input variability's (anatomy, tissue properties, body position) on dosimetric outputs, referring to head imaging with a 3 T MRI scanner.Main results.It is found that head anatomy is the prevailing source of variability for the considered dosimetric quantities, rather than the variability due to tissue properties and head positioning. From knowledge of the variability of the dosimetric quantities, an uncertainty can be attributed to the results obtained using a generic anatomical head model when SAR and temperature increase values are compared with safety exposure limits.Significance.This work associates a global uncertainty to SAR and temperature increase predictions, to be considered when comparing the numerically evaluated dosimetric quantities with reference exposure limits. The adopted methodology can be extended to other exposure scenarios for MRI safety purposes.

Efficient prediction of MRI gradient-induced heating for guiding safety testing of conductive implants.

PURPOSE: To propose an efficient numerical method to predict the temperature increase of an implantable medical device induced by any linearly polarized homogeneous magnetic field, according to the ISO 10974 methodology for testing of gradient-induced device heating. THEORY AND METHODS: The concepts of device-specific power and temperature tensors are introduced to mathematically describe the electromagnetic and thermal anisotropic behavior of the device, from which the device heating for an arbitrary exposure direction can be predicted. The proposed method is compared to a brute-force approach based on simulations, and validated by applying it to four reference orthopedic implants with a commercial simulation software. RESULTS: The proposed method requires about 5 % $$ \% $$ of the time required by the brute-force approach, and 30 % $$ \% $$ of the memory occupancy. The temperature increase predicted by the proposed method over a range of incident magnetic field exposures deviated from brute-force direct simulations by less than ± $$ \pm $$ 0.3 % $$ \% $$ . CONCLUSION: The proposed method allows efficient prediction of the heating of an implantable medical device induced by any linearly polarized homogeneous magnetic field using a small fraction of the simulations required by the brute-force approach. The results can be used to predict the worst-case orientation of the gradient field, for subsequent experimental characterization according to the ISO 10974 standard.

Computational dosimetry in MRI in presence of hip, knee or shoulder implants: do we need accurate surgery models?

Objective.To quantify the effects of different levels of realism in the description of the anatomy around hip, knee or shoulder implants when simulating, numerically, radiofrequency and gradient-induced heating in magnetic resonance imaging. This quantification is needed to define how precise the digital human model modified with the implant should be to get realistic dosimetric assessments.Approach. The analysis is based on a large number of numerical simulations where four 'levels of realism' have been adopted in modelling human bodies carrying orthopaedic implants.Main results. Results show that the quantification of the heating due to switched gradient fields does not strictly require a detailed local anatomical description when preparing the digital human model carrying an implant. In this case, a simple overlapping of the implant CAD with the body anatomy is sufficient to provide a quite good and conservative estimation of the heating. On the contrary, the evaluation of the electromagnetic field distribution and heating caused by the radiofrequency field requires an accurate description of the tissues around the prosthesis.Significance. The results of this paper provide hints for selecting the 'level of realism' in the definition of the anatomical models with embedded passive implants when performing simulations that should reproduce, as closely as possible, thein vivoscenarios of patients carrying orthopaedic implants.

Classification Scheme of Heating Risk during MRI Scans on Patients with Orthopaedic Prostheses.

Due to the large variety of possible clinical scenarios, a reliable heating-risk assessment is not straightforward when patients with arthroplasty undergo MRI scans. This paper proposes a simple procedure to estimate the thermal effects induced in patients with hip, knee, or shoulder arthroplasty during MRI exams. The most representative clinical scenarios were identified by a preliminary frequency analysis, based on clinical service databases, collecting MRI exams of 11,658 implant carrier patients. The thermal effects produced by radiofrequency and switching gradient fields were investigated through 588 numerical simulations performed on an ASTM-like phantom, considering four prostheses, two static field values, seven MR sequences, and seven regions of imaging. The risk assessment was inspired by standards for radiofrequency fields and by scientific studies for gradient fields. Three risk tiers were defined for the radiofrequency, in terms of whole-body and local SAR averages, and for GC fields, in terms of temperature elevation. Only 50 out of 588 scenarios require some caution to be managed. Results showed that the whole-body SAR is not a self-reliant safety parameter for patients with metallic implants. The proposed numerical procedure can be easily extended to any other scenario, including the use of detailed anatomical models.

A contribution to MRI safety testing related to gradient-induced heating of medical devices.

PURPOSE: To theoretically investigate the feasibility of a novel procedure for testing the MRI gradient-induced heating of medical devices and translating the results into clinical practice. METHODS: The concept of index of stress is introduced by decoupling the time waveform characteristics of the gradient field signals from the field spatial distribution within an MRI scanner. This index is also extended to consider the anisotropy of complex bulky metallic implants. Merits and drawbacks of the proposed index of stress are investigated through virtual experiments. In particular, the values of the index of stress evaluated for realistic orthopedic implants placed within an ASTM phantom are compared with accurate heating simulations performed with 2 anatomic body models (a man and a woman) implanted through a virtual surgery procedure. RESULTS: The manipulation of the proposed index of stress allows to identify regions within the MRI bore where the implant could affect the safety of the examinations. Furthermore, the conducted analysis shows that the power dissipated into the implant by the induced eddy currents is a dosimetric quantity that estimates well the maximum temperature increase in the tissues surrounding the implant. CONCLUSION: The results support the adoption of an anisotropic index of stress to regulate the gradient-induced heating of geometrically complex implants. They also pave the way for a laboratory characterization of the implants based on electrical measurements, rather than on thermal measurements. The next step will be to set up a standardized experimental procedure to evaluate the index of stress associated with an implant.

CoSimPy: An open-source python library for MRI radiofrequency Coil EM/Circuit Cosimulation.

BACKGROUND AND OBJECTIVES: The Electromagnetic/Circuit cosimulation method represents a valuable and effective strategy to address those problems where a radiative structure has to interact with external supporting circuitries. This is of particular concern for Magnetic Resonance Imaging (MRI), radiofrequency (RF) coils design, where the supporting circuitry optimisation represents, generally, a crucial aspect. This article presents CoSimPy, an open-source Python circuit simulation library for Electromagnetic/Circuit cosimulations and specifically optimised for MRI, RF coils design. METHODS: CoSimPy is designed following an Object-orientated programming. In addition to the essential methods aimed to performed the Electromagnetic/Circuit cosimulations, many others are implemented both to simplify the standard workflow and to evaluate the RF coils performance. In this article, the theory which underlies the fundamental methods of CoSimPy is shown together with the basic framework of the library. RESULTS: In the paper, the reliability of CoSimPy is successfully tested against a full-wave electromagnetic simulations involving a reference setup. The library is made available under https://github.com/umbertozanovello/CoSimPy together with a detailed documentation providing guidelines and examples. CoSimPy is distributed under the Massachusetts Institute of Technology (MIT) license. CONCLUSIONS: CoSimPy demonstrated to be an agile tool employable for Electromagnetic/Circuit cosimulations. Its distribution is meant to fulfil the needs of several researchers also avoiding duplication of effort in writing custom implementations. CoSimPy is under constant development and aims to represent a coworking environment where scientists can implement additional methods whose sharing can represent an advantage for the community. Finally, even if CoSimPy is designed with special focus on MRI, it represents an efficient and practical tool potentially employable wherever electronic devices made of radiative and circuitry components are involved.

Heating of hip joint implants in MRI: The combined effect of RF and switched-gradient fields.

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.