Draft standard on a test method for measurement of gradient magnetic field induced heating on or near non-active implants during magnetic resonance imaging submitted to ASTM F04.15 MR.

Confidentiality Status: PU - Public, fully open (remember to deposit public deliverables in a trusted repository) Deliverable Cover Sheet Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or EURAMET. Neither the European Union nor the granting authority can be held responsible for them. The project has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States. 1 of 2 21NRM05 STASIS D7: Draft standard on a test method for measurement of gradient magnetic field induced heating on or near non-active implants during magnetic resonance imaging submitted to ASTM F04.15 MR. Organisation name of the lead participant for the deliverable: MR:comp (MRC) Due date of the deliverable: 30 September 2025 Actual submission date of the deliverable: 30 September 2025 2 of 2 Summary As a results of the STASIS project, the attached document entitled “Standard Test Method for Testing gradient-induced heating on passive medical implants” was submitted as a work item (WK95470) to the ASTM subcommittee F04.15 on 26th of June 2025. The kickoff meeting for the ASTM subgroup to evaluate the draft is scheduled for Q4/2025. In summary, the aims of the STASIS project deliverable have been successfully achieved, fulfilling Objective 4 of the project. Designation: X XXXX-XX Work Item Number: 0 Date: 0 1 This document is not an ASTM standard; it is under consideration within an ASTM technical committee but has not received all approvals required to become an ASTM standard. You agree not to reproduce or circulate or quote, in whole or in part, this document outside of ASTM Committee/Society activities, or submit it to any other organization or standards bodies (whether national, international, or other) except with the approval of the Chairman of the Committee having jurisdiction and the written authorization of the President of the Society. If you do not agree with these conditions please immediately destroy all copies of the document. Copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. All Rights Reserved. Technical Contact: Include Ballot Rationale Here (Required for all Ballots): Draft Number (if applicable): Standard Test Method for Testing gradient-induced heating on passive medical implants StandardTitle1 This standard is issued under the fixed designation X XXXX; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (ε) indicates an editorial change since the last revision or reapproval. 1. Scope 1.1 This test method covers measurement of gradient-induced heating on a passive medical implant and its surroundings during magnetic resonance imaging (MRI). 1.2 This test method is one required to determine if the presence of a passive implant may cause injury to the patient with the implant during an MR procedure. Other safety issues that should be addressed include magnetically induced displacement force and torque, as well as radiofrequency induced heating and proper device function while in various configurations in the MR environment. 1.3 The amount of gradient induced heating will depend on the |dB/dt| rms (root mean square value of the time rate of change of the magnetic field) and on the gradient field orientation. 1.4 This test method assumes that testing is done on devices that will be entirely inside the body. For other implantation conditions (for example, external fixation devices, percutaneous needles, catheters or tethered devices such as ablation probes), modifications of this test method are necessary. 1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. 1 This test method is under the jurisdiction of ASTM Committee committeeTitle and is the direct responsibility of Subcommittee subcommitteeTitle. Current edition approved XXX. XX, XXXX. Published XX XXXX. DOI:10.1520/XXXXX-XX 1 2 3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 4 5 6 7 8 9 10 11 12 13 X XXXX 2 1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 1.7 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of international Standards, Guidesand Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee. 2. Referenced Documents 2.1 ASTM Standards: F2052 Test Method for Measurement of Magnetically Induced Displacement Force on Medical Devices in the Magnetic Resonance Environment F2119 Test Method for Evaluation of MR Image Artifacts from Passive Implants F2213 Test Method for Measurement of Magnetically Induced Torque on Medical Devices in the Magnetic Resonance Environment F2503 Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment F2182 Standard Test Method for Measurement of Radio Frequency Induced Heating On or Near Passive Implants During Magnetic Resonance Imaging 2.2 ISO Standards: ISO_TS_10974_2018(E) - Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device ISO/IEC Guide 99:2007 - International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM) 3rd Edition 14 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 15 X XXXX 3 IEC 60601-2-33:2022 - Medical electrical equipment - Part 2-33: Particular requirements for the basic safety and essential performance of magnetic resonance equipment for medical diagnosis 3. Terminology 3.1 Definitions: 3.1.1 gelled saline—phantom medium consisting of sodium chloride and polyacrylic acid in water as specified in this test method. 3.1.2 implant, n—in medicine, an object, structure, or device intended to reside within the body for diagnostic, prosthetic, or other therapeutic purposes. 3.1.3 isocenter—geometric center of the gradient coil system, which generally is the geometric center of a scanner with a cylindrical bore. 3.1.4 magnetic resonance imaging (MRI)—imaging technique that uses static and time varying magnetic fields to provide images of tissue by the magnetic resonance of nuclei. 3.1.5 magnetic resonance system (MR system)—combination, as specified by the MR manufacturer , of items of equipment, at least one of which is MR equipment , to be interconnected by functional connection or by use of a multiple socket-outlet 60601-2-33, Ed. 4.0 3.1.6 MR Conditional—an item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use. Field conditions that define the specified MR environment include field strength, spatial gradient, dB/dt, radio frequency (RF) 16 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 17 X XXXX 4 fields, and specific absorption rate (SAR). Additional conditions, including specific configurations of the item, may be required. 3.1.7 MR Safe—an item that poses no known hazards in all MR environments NOTE 1—MR Safe items include nonconducting, nonmagnetic items such as a plastic petri dish. An item may be determined to be MR Safe by providing a scientifically based rationale rather than test data. 3.1.8 MR test system—MR system or an apparatus that reproduces the time-varying gradient magnetic field of this type of system. 3.1.9 MR Unsafe—an item that is known to pose hazards in all MR environments. NOTE 2—MR Unsafe items include magnetic items such as a pair of ferromagnetic scissors. 3.1.10 passive implant—an implant that serves its function without supply of electrical power. 3.1.11 pulsed gradient magnetic field—the magnetic field in MRI that is used to spatially encode the MR signal. 3.1.12 peak temperature increase—the spatial maximum value of the temperature increase over the implant surface. 3.1.13 worst orientation—relative orientation between dB/dt vector and implant which causes the highest peak temperature increase. 3.1.14 implant CAD file—file containing the description of the 3D implant geometrical model suitable to be open with CAD (Computer Aided Design) software. 3.1.15 measured temperature increase value—quantity value representing the temperature increase measurement result International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM) 3rd Edition 18 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 19 X XXXX 5 4. Summary of Test Method 4.1 The test method relies on a computational model preliminary validated against laboratory experiments under standardized exposure conditions. 4.2 The passive implant to be tested is placed completely within a phantom filled with an appropriate medium with physical properties (i.e., thermal conductivity, thermal capacity, mass density) similar to the averaged properties of the human body. The phantom material is a gelled saline (consisting of a saline solution and a gelling agent). The implant is exposed to a spatially uniform linearly polarized harmonic magnetic field. 4.3 The test procedure is divided into four steps: (1) Preliminary simulations are carried out to determine the orientation relative to the applied field that produces the largest heating and the locations where the maximum local temperature increase is expected. (2) The implant is placed in the center of the phantom container and oriented according to the simulation outcomes. Temperature increase is measured by placing probes on the implant surface. (3) The computational model is refined until the maximum relative deviation between the measured temperature increase value and the simulation results obtained in the same reference point is lower than 20 % is obtained. When this condition is met, the computational model is considered validated and will represent the sole means for the testing. (4) In case the computational model has been subjected to refinement, the simulations to determine the largest heating are repeated and the result provides the testing output. 5. Significance and Use 20 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 21 X XXXX 6 5.1 This test method describes a test procedure for evaluating the peak temperature increase, consequent to the worst orientation, induced by gradient coil power deposition during an MR procedure, involving the exposure to a specific dB/dt vector. The method allows the characterization of the propensity of an implant to heat, rather than the prediction of heating during a specific MR procedure in a patient. The results may be used to identify the implant positions within the scanner which guarantee the safety of a patient with the implant during an MR examination. 6. Apparatus 6.1 Test Apparatus—The test apparatus consists of a suitable phantom, an MR test system for production of the dB/dt field and a modeling software. The phantom MR test system and modeling software are utilized to approximate the physical environment that the patient and device experience during an MR procedure. MR test system and modeling software are utilized to establish the heating behavior of a device in a known dB/dt field in a standardized phantom. 6.2 MR test system—The MR test system should be able to generate a harmonic linearly polarized dB/dt vector spatially uniform (less than 10 %) within the volume where the implant will be located. The device should be able to generate a |dB/dt| rms value equal to 42 T/s at a frequency not exceeding 1750 Hz. The experimental device should be able to guarantee the stability of the applied dB/dt vector (maximum acceptable variation less than 1 %) during the duration of the experiment. 6.3 Temperature Sensor—Temperature on the implant surface should be measured by using temperature probes immune to electromagnetic interference (e.g., optical probes). The sensitive 22 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 23 X XXXX 7 elements should have a side lower than 1 mm. The probe accuracy should be lower than ±0.1 °C close to the calibration point (23 °C). 6.4 Modeling software—A state-of-art electromagnetic and thermal modeling software should be used for the simulations. The software should be able to import the implant CAD file to be tested and discretize it on the basis of the adopted mathematical methodology. 7. Test Specimens 7.1 While this test method may be used on prototype or predicate devices, for purposes of implant qualification and to ensure patient safety relative to the use of MRI technology, the implant evaluated according to this test method should be representative of a finished device according to its intended use or in-situ condition. 7.2 Other than described as in 7.1, for purposes of implant qualification, the implant shall not be altered in any manner prior to testing other than positioning or otherwise configuring the implant in the orientation that generates the greatest heating. A justification for such orientation shall be provided through simulations. 8. Reagents and Materials 8.1 When using a phantom within an MR test system, there are often practical limitations to the phantom container size, and the boundary conditions of the phantom can affect temperature 24 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 25 X XXXX 14 11. Keywords 11.1 exposure; gelled-saline; implant; MRI (magnetic resonance imaging); MR safety; phantom; gradient-induced heating; dB/dt. ANNEX (Mandatory Information) A1. PHANTOM THERMAL PROPERTY MEASUREMENT A1.1 General considerations For a given recipe set, the parameters should be assessed and demonstrated at least once. Reliable temperature measurements can only be conducted if the Grashof number is sufficiently large, i.e. the viscosity of the medium or gel should be much larger than 1 Pa∙s. An appropriate method for measuring the viscosity should be selected, e.g. rheometers. The viscosity should be determined for the entire temperature range, of which the lowest number should be used. This subclause describes two methods to determine the heat capacity of tissue simulating media used for implant testing. Furthermore, it is necessary to demonstrate either by simulation or by experiment that the effect of heat conduction and convection are negligible in the assessment of GC heating for the maximum spatial gradient, temperature difference and duration of heating. The simulation approach can be used when the thermal conductivity and the Grashof number are known. It can also be demonstrated by experiment for the conditions of maximum spatial temperature gradient T, temperature difference (ΔT) and duration of assessment (Δt). The demonstration of negligible effects related to conduction and convection is not required for formulations that contain greater than 95 % water and the properties of water can be used. 38 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 39 X XXXX 15 A1.2 Method to determine heat capacity The most common way of determining the specific heat capacity is differential scanning calorimetry. It can be used to determine specific heat capacity as a function of temperature as well as the location of phase transition temperatures (relevant for gels). The temperature of the unknown sample and of a reference material is changed in the same way according to a predefined curve with constant cooling and heating rates. At a constant rate the temperature difference observed between the reference material and its surroundings is the result of the different heat capacities of the reference material and the surrounding material. The time dependence of the temperature difference is measured. The differential scanning calorimeter is calibrated with the known reference material, any temperature difference can be directly converted into heat flow. In such a way the heat capacity is determined as a function of the temperature. A1.3 Method to determine thermal conductivity The hot wire method is appropriate for determining thermal conductivity. A thin wire is embedded into the sample to be investigated, simultaneously serving as a heating element and a temperature sensor. During the experiment, the wire is heated with a constant electrical power source. The temporal development of the increase in temperature at the hot wire is calculated from the resistance of the wire. This temperature increase is primarily dependent on the thermal conductivity of the sample. The thermal conductivity is determined by considering both the thermal contact resistance between the sample and the wire and the axial heat losses to the temporal temperature development. 40 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 41 X XXXX 16 42 347 348 43 X XXXX 17 A2. THERMAL MATRICES A2.1 Introduction The following steps should be followed to compute the Thermal Matrices . The Thermal Matrices allow to compute the temperature increase in the positions of the implant to be tested, , according to: where is the time-derivative of the linearly polarized, spatially uniform, harmonic magnetic field vector causing the temperature increase. The Thermal Matrices also allow to compute the peak temperature increase obtained when the implant to be tested is exposed to the worst dB/dt vector and the direction of the the worst dB/dt vector. The following nomenclature is adopted: : the thermal matrix associated with point of the implant to be tested : temperature increase in when the implant to be tested is exposed for 30 minutes to a specific dB/dt vector : the unitary amplitude dB/dt vector responsible for the peak temperature increase on the implant to be tested 44 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 45 X XXXX 18 : the peak temperature increase when the implant to be tested is exposed for 30 minutes to : maximum temperature increase in considering a 30 minutes exposure to all possible dB/dt vectors with unitary amplitude : temperature increase in considering a 30 minutes exposure to a unitary amplitude dB/dt vector oriented along the j-th Cartesian direction. : temperature increase in considering a 30 minutes exposure to a dB/dt vector with unitary amplitude along both the j-th and k-th Cartesian directions with . A2.1 Thermal Matrices Computation Six electromagnetic simulations followed by transient thermal simulations are performed to compute the Thermal Matrices . Whereas it is possible to compute such matrices also for points in the phantom or inside the implant, for the purpose of the present testing procedure it suffices to limit their computation to the positions on the implant surface. 1. Compute the temperature increase, , in each position when the implant to be tested is radiated by a unitary amplitude dB/dt vector oriented along each j-th Cartesian direction. 2. Compute the quantities as 46 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 47 X XXXX 19 represent the diagonal elements of the Thermal Matrix 3. Compute the temperature increase , in each in each position when the implant is radiated by a dB/dt vector with unitary amplitude along both the j-th and k-th Cartesian directions with . 4. Compute the quantities as represent the off-diagonal element of Thermal Matrix 5. Construct the Thermal Matrices The procedure allows to compute a number of 3 x 3 Thermal Matrices equal to the number of positions considered. A2.2 Peak Temperature Increase and Worst dB/dt Vector Computation The following steps allow to compute the peak temperature increase and the dB/dt vector responsible for the peak temperature increase on the implant to be tested. 1. Compute the maximum eigenvalue of all the Thermal Matrices . This corresponds to the quantity 48 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 49 X XXXX 20 2. Compute the maximum among all the . This correspond to the quantity and it is reached in point : 3. Compute the unitary amplitude eigenvector associated with the eigenvalue of the Thermal Matrix . This corresponds to 4. Compute the peak temperature increase due to an exposure to a 42 T/s dB/dt vector oriented as : 50 402 403 404 405 406 407 408 409 51 X XXXX 21 APPENDIX (Nonmandatory Information) X1. X1.1 Introduction The proposed test allows retrieving the peak temperature increase on the implant to be tested when exposed to the dB/dt vector that maximizes the heating. Whereas this result guarantees a conservative safety approach, it can result in important overestimation of the actual peak temperature increase for strongly anisotropic implants. For these implants the worst dB/dt vector orientation could lead to much higher temperature increases with respect to those due to the actual dB/dt vectors generated by an MRI scanner. Since these depend on the scanner gradient coils and the MR pulse sequence, any information related to the implant geometry has to be interfaced to the MRI scanner in order to be effective. For this reason the proposed test is restricted to provide the peak temperature increase when the implant is exposed to the worst dB/dt vector. Anyway, whenever required, it is possible to compute so-called Anisotropy Coefficients which introduce additional information about the geometry of the implant to be tested. The Anisotropy Coefficients, combined with the information of the magnetic field generated by the gradient coils and the MR pulse sequence waveforms, allows to increase the maximum |dB/dt| rms at which the implant to be tested can be safely scanned. This potentially avoids the exclusion of safe scanning scenarios. In particular, the generated by a specific MR gradient coil set and MR sequence in the position inside the scanner bore, can be expressed as: 52 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 53 X XXXX 22 where:  is the the magnetic field produced by the i-th gradient coil when the gradient at the isocenter is 1 T/m along the coil reference direction  and is the gradient in T/m generated by the i-th gradient coil during the MR sequence execution TR is the repetition time of the MR sequence In absence of the skin effect, the temperature increase scales with the square of the . With the expression above it is therefore possible to limit the positions of an implant within those where the peak temperature increase over the implant surface will be lower than a specific value. As such, the represents a safety index, , whose value should not be exceeded in order to guarantee safety. However, the above expression brings no information about the anisotropy of the implant and the is limited independently of the direction of the vector. To add such an information, the Anisotropy Coefficients are introduced. These coefficients range from 0 to 1 and are used to reduce the value of for a specific vector according to: 54 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 55 X XXXX 23 This appendix describes how the Anisotropy Coefficients, , can be computed. The Anisotropy Coefficients encode the information about the shape of the implant to be tested and can be used, together with the maximum peak temperature increase at a of 42 T/s, to identify gradient-safe operating areas of the implant to be tested inside an MR scanner running an MR sequence. The appendix assumes that the Thermal Matrices are available for all the positions considered in the simulations. Furthermore, it assumes the implant under test is oriented into the phantom according to the expected orientation inside the scanner with the z-axis corresponding to the inferior (I) to superior (S) direction, the x-axis corresponding to the left (L) to right (R) direction and the y-axis to the anterior (A) to posterior (P) direction. X1.2 Anisotropy Coefficients Computation The following steps describe how the Anisotropy Coefficients can be obtained. 1. Define a number of directions across the hemisphere with a longitude between −180° and 180° and a latitude between 0° and 90°. The angular step must be at least 22.5° in order to map satisfactorily the hemisphere. 2. For each unitary amplitude vector, compute: 56 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 57