- Open Access
Radio Frequency MRI coils and safety: how infrared thermography can support quality assurance
Egyptian Journal of Radiology and Nuclear Medicine volume 52, Article number: 277 (2021)
The safety controls in Resonance Magnetic Imaging (MRI) diagnostic site are numerous and complex. Some of these are contained in international directives and regularly conducted by medical physics expert after acceptance tests, consisting of a series of checks, measurements, evaluations called quality controls (QCs) and made to guarantee the image quality of the equipment. In this context, ensuring that the coils are in proper operating conditions is important to prevent and reduce errors in use and to preserve patient safety.
A study by thermography was conducted to evaluate temperature changes of MRI coils during Quality Control (QC), in order to prevent any problems for the patient due to Radio Frequency waves. This experiment involves use of a thermal camera to detect temperature variations during MRI scans using head and body coils of two different tomography 1.5 T and 3.0 T static magnetic field. Thermal camera was positioned inside the MRI room to acquire images every 15 s for all the scansions duration. The observations have shown a temperature increase only for body coil of 1.5 MRI tomography, whereas no significative temperature variation has occurred for the other coils under observation. This temperature increase was later related to a fault of such coil.
The authors believe this simple method useful as first approach, during routinely QCs, to verify coils functioning and so to avoid patient hazards and are preparing a methodological study about functioning of the coils with respect to their temperature variation.
A medical imaging system produces images that allow accurate and timely diagnoses and improves evaluation of such images using protocols of increasing quality and standardizing . In particular, Magnetic Resonance Imaging (MRI) is a highly sophisticated imaging modality commonly used in clinical diagnoses . It is a non-invasive technique which provides images of internal tissues without applying ionizing radiations [3, 4]. MRI scanners combine three different electromagnetic fields, i.e., static magnetic field (typically symbolized by B0), radiofrequency (RF) field (B1), generated by coils, that can operate in transmit and receive mode with high signal-to-noise ratio and wide field homogeneity , and magnetic field gradients in the three spatial directions, to select the region of interest for spatial encoding of image. The possibility that some hazards for patient can be associated with performing MRI diagnostic images concerns, above all, B0, RF (B1) and magnetic field gradient.
It is necessary to establish regular and adequate Quality Assurance (QA) procedures to guarantee the maintenance of consistent image quality over the imaging equipment lifetime and to ensure safe and accurate operation of the whole process with respect to patients, workers and population.
Every QA program should include periodic tests to identify any degradation in image quality  reducing the ability to detect and correctly interpret abnormal findings that could imply a decrease in diagnostic accuracy.
MRI QA programs for images evaluation are well established [9,10,11,12,13], whereas evaluation of proper operating of hardware components of scanners, such as coils, bed, is more difficult, because no specific standardized procedures and guidelines are available. In this context, every responsible person, expert in charge, shall set up his own program.
About the coils, the RF pulses generated for image production are transmitted through free space from coils to the patient.
Such pulses can induce electrical currents in conducting materials and in human body can heat tissues quite dramatically, resulting in superficial skin burning if there is a malfunction or malposition .
There are unquestionable evidence of MRI-related reports of patients’ burns (thermal injuries or incidents) that strongly indicate the need for increased awareness, education and understanding concerning this rare, but real, MRI-related hazard .
Only for specific absorption rate (SAR) evaluation, the safety of RF exposure to clinical MRI is regulated by the US Food and Drug Administration and the International Electrotechnical Commission’s guidelines for RF exposure adopted in Europe [16, 17]. The SAR, which describes potential heating of the patient’s tissue due to RF, is automatically calculated by the system when is set a sequence to acquisition.
In all cases, coils should be periodically checked before use on patient to ensure the absence of frayed insulation, exposed wires and other hazards .
Some of these use methods that produce images directly whereas probe techniques generate B1 map from different points in the space. The perturbing sphere method has recently been applied to map the RF fields from MRI coils but it can provide accurate efficiency measurements only when the electric and magnetic field components are well separated in space [21, 22].
In this paper, a simple method that use non-contact Infra-Red (IR) thermography is presented in order to evaluate the proper operating of coils.
This method involves the use of an IR Camera to detect if a temperature variation occurs during MRI scans.
The wavelength range of this radiation is (0.7–350) µm ; this range can be subdivided in three bands: near infrared (NIR), medium infrared (MIR) and far infrared (FIR). According to thermal radiation theory, blackbody is considered as a hypothetical object that absorbs all incident radiations and radiates a continuous spectrum according to Plank’s law . The total emissive power of a blackbody is described by Stefan–Boltzmann’s law (Eq. 1):
where E is the total emissive power, σ is the Stefan–Boltzmann’s constant and T represents the absolute temperature in kelvin.
where ε represents the emissivity of the emitting surface.
In this experiment, thermography is used during MRI phantom tests, to evaluate the heat delivered by coils during scans, to inspect, without interference non-destructive manner, eventual surface crack in materials and to individuate superficial abnormal behavior. The local temperature increase, besides, can be correlated to electric field around the coil .
The thermal camera detected the infrared (IR) energy emitted by the investigated coils and created electronic images based on information about the temperature differences . In fact, due to thermal conductivity and specific heat, each object or its region has its own temperature; a thermal camera can detect all the field of view, and the objects appear as distinct in a thermal image, although less detailed [29,30,31].
The thermography measurements were conducted on two different MRI scanners of the same Institute, 3.0 T Philips Achieva and 1.5 T Philips Intera. In both cases, the in use body and head coils were tested. All the investigated RF coils were loaded with the phantom used during image QC procedures. Two different probes were used for temperature measurements:
(a) The FLUKE 62 mini IR Thermometer.
(b) The FLIR T440 Camera.
The Fluke 62 mini IR thermometer works in the temperature range − 20 °C to + 500 °C with an accuracy of ± 1.5 °C and an emissivity from 0.10 to 1. It allows us to measure single spot temperature in a contactless manner. The FLIR T440 camera allows us to display images in both the IR range (7.5–13) µm and in the visible (0.4–0.7) µm one. Thermal images are normally grayscale in nature: black corresponds to cold objects surface and white to hot; gray shades indicate intermediate temperatures. The FLIR T440 camera, however, processes images in false colors so that users better identify objects at different temperatures . The FLIR T440 camera was positioned inside the MRI room, because it is a MRI conditional device, and the distance between the RF coil and the thermal camera was (1.00 ± 0.01) m.
IR radiations emitted by a surface strictly depends on the micro-climatic conditions such as humidity, airflow, and surrounding temperature. For this reason, it is necessary to conduct measurements in controlled environments.
The temperature and humidity conditions inside the MRI room were constantly monitored using the thermohygrometric probes installed by manufacturer inside the MRI room, while the ventilation was measured using an anemometer probe. In Table 1, the measured micro-climatic values are reported.
To measure temperatures accurately, the emissivity represents the most important parameter to correctly set.
The emissivity is a measure of how much radiation is emitted from the object, compared to that from a perfect blackbody of the same temperature. According to Watmough , the emissivity of a surface of a wavelength λ and a view angle β is given by Eq. 3:
where β is defined as:
where nλ = refractive index. The emissivity value was set by using the database reported in . The acquisition of thermal images was performed during MRI scans. The camera acquired one image every 15 s. Total acquisition time was about 20 and 27 min for head and body scans, respectively. The FLIR T440 resolution was equal to ± 0.1 °C and the accuracy ± 2% of the reading . All the acquired images were stored in.TIFF format for later post-processing and the analysis of the raw data was performed by using the FLIR tools available on the dedicated software. The post-processing analysis was conducted to evaluate temperature variations. For each acquired image, three regions of interest (ROI) were selected and tracked over time. The segmentation of ROI intends to separate the main part from the rest of the image . Using Area Tool of the software, the three circular ROIs were located on different coils positions in the images; in particular ROIs 1, 2 and 3 were located on the left, medium and right region, respectively, of each coil image. The average temperature profile of each ROI versus time was then evaluated.
To assess the compatibility of the two different probes, 3.0 T Philips Achieva MRI head coil temperature was measured, during a routinely scan, by placing both probes simultaneously in the same position, consistently with their physical dimensions. The results of the measurements are presented in Fig. 1. The Wilcoxon test was used to evaluate the consistency of the experimental data presented in Fig. 1 and the resulting confidence level of p = 0.59 made it possible to conclude that there were no significant differences between the measurements obtained from the two different instruments. Having assessed the consistence of the probes, for practical reasons, the remaining measurements were conducted using only the FLIR T440 Camera in order to obtain data on a large image and not only from a single spot. Thermal images were acquired throughout MRI routinely scans, carried out during periodic QCs.
Visible and IR images of both head and body coils of the 3.0 T Philips Achieva MRI are depicted in Fig. 2. In the same figure, the ROIs used for evaluating temperature profile are also highlighted.
The average temperature profile versus time of each ROI pointed out on the 3.0 T MRI head coil is shown in Fig. 3a, whereas Fig. 3b reports the average temperature versus time trend of the ROIs. In particular, in Fig. 3a and b black, red and blue lines are referred to ROIs enumerate 1, 2 and 3, respectively, depicted in Figs. 2a and c.
Figure 4 shows visible and infrared images of the 1.5 T MRI body coil. In this case, a hot spot was noticed during QCs in the left region of the body coil. Figure 5a reports the average temperature versus time trend of the ROIs located in different positions of the 1.5 T MRI body coil.
In particular, the black line indicates the average temperature over time trend of the ROI located on the hot spot. The hot spot trend was also compared with the ROI 2 (red line) and the ROI 3 (blue line). In this case, the ROI 2 was positioned on the coil amplifier (following the manufacturer technicians’ indications) and the ROI 3 on the right side of the body coil.
In Fig. 5b, the curve trend of the temperature over time of the ROIs depicted on the head coil, always used on the same scanner, is reported; black, blue and red lines are referred to ROI 1, 2 and 3, respectively.
The obtained results highlighted that no differences in temperature were noticed when both the head and body coils available on the 3.0 T Philips Achieva scanner were monitored. In particular, the maximum temperature rise was equal to 1.5 °C when the head coil was monitored and 1.0 °C when the body coil was tested. The scenario changed when the body coil in use on the 1.5 T Philips Intera scanner was analyzed using the thermal camera. The IR camera revealed an unusual behavior and, during QCs, a high-temperature spot appeared in the IR images, far from coil amplifier. The temperature rise of the hot spot is equal to 12 °C, whereas the ROIs located on the coil amplifier and on the right side of the coil showed a temperature increase of 1.5 °C. The high-temperature spot appeared only on the body coil, which subsequently inspected, showed a failure. In fact, a maximum temperature increase of only 1.5 °C was observed, when the head coil has been mounted on the MRI scanner and analyzed by using the thermal camera.
Always using the images acquired by using the thermal camera, it was possible to observe that on the surface of the phantom, there were no variations in temperature and therefore it was possible to conclude that no thermal variation was due to the SAR.
In addition, in order to evaluate the performance of the MR system, the SNR was evaluated on the images acquired by using MR scanner and the obtained results were compared with those found during previous QCs measurements. It was possible to observe a slight decrease in the SNR, such as not to invalidate the diagnosis, and not attributable to a malfunction of the coil; in fact, it can suffer from systematic errors due to scanner instabilities .
The aim of this experiment is to propose a non-contact thermography modality in order to evaluate the MRI coils proper functioning as a simple approach could be added to a QA program. To the knowledge of the authors, this is the first study where a thermal camera is used to conduct QCs on MR coils. Such procedure does not affect image quality, but it examines coils functionality and, consequently, patient wellness during MR scans. Besides it is possible to identify the source of eventual equipment malfunction, pointing to preventive or immediate maintenance requirements in order to manage QCs and the identification of image degradation. IR non-destructive tests are used to reveal surface defects of manufactured products, and thermal camera is used by coil manufacturers during the final testing phase to identify hot spots. This study implies the use of a thermal camera inside a MRI room during routinely QCs, when no patient is inside the gantry. Thermal camera represents a non-contact and non-invasive technique and the interpretation of pseudo-colors thermal images is fast and easy without further elaborations. This is a real-time technique which enables monitoring of dynamical variations of surface temperature and it is suitable for repeated use. In this experiment, the feasibility and the relevance of thermography, during routinely QCs, have been demonstrated. A coil malfunctioning was in fact found and also the manufacturer’s tests revealed a non-proper operating of the RF body coil under investigation. In this context, the temperature increase could be a significant risk to produce severe discomfort in patients.
The MRI is considered by the research and medical community to be safe for workers when compared to other more invasive image modalities . Various hazards nevertheless can be associated with the use of MRI diagnostic for patients that are exposed to a combination of static, gradient and RF fields and their consequence . Besides, if magnet is superconductive, presence of cryogenic gas is also dangerous.
The authors believe that any initiative for patient safety is useful and can be included in a program of MRI Quality Assurance.
Availability of data and materials
Magnetic resonance Imaging
Specific absorption rate
Region of interest
Paul JM, Stewart SL (2020) Peer review in MRI: A quality improvement programme and pilot study. Radiography. https://doi.org/10.1016/j.radi.2020.09.021
Capstick M, McRobbie M, Hand J, Christ A, Kuhn S, Hanson Mild K, Cabot E, Li Y, Melzer A, Papadaki A, Prüssmann K, Quest R, Rea M, Ryf S, Oberle M, Kuster N (2008) An investigation into occupational exposure to electromagnetic fields for personnel working with and around medical Magnetic Resonance Imaging equipment. Project VT/2007/017, European Commission, Brussels
Lauterbur PC (1973) Image formation by induced local interactions: examples employing nuclear magnetic resonance. Nature 242(5394):190–191. https://doi.org/10.1038/242190a0
Mansfield P (1977) Multi-planar image formation using NMR spin echoes. J Phys C Solid State Phys 10:55–58
Ahmad SF, Kim YC, Choi IC, Kim HD (2020) Recent progress in birdcage RF coil technology for MRI system. Diagnostics 10(12):1017. https://doi.org/10.3390/diagnostics10121017
Mutic S, Palta JR, Butker EK, Das IJ, Huq MS, Loo L-ND, Van Dyk J (2003) Quality assurance for computed-tomography simulators and the computed-tomography-simulation process: report of the AAPM Radiation Therapy Committee Task Group No. 66. Med Phys 30(10):2762–2792. https://doi.org/10.1118/1.1609271
Bonanno L, Marino S, Morabito R, Barbalace G, Sestito A, Testagrossa B, Acri G (2019) Evaluation of US and MRI techniques for carotid stenosis: a novel phantom approach. Radiol Med (Torino) 124(5):368–374. https://doi.org/10.1007/s11547-018-0971-7
Chen C-C, Wan Y-L, Wai Y-Y, Liu H-L (2004) Quality assurance of clinical MRI scanners using ACR MRI phantom: preliminary results. J Digit Imaging 17(4):279–284. https://doi.org/10.1007/s10278-004-1023-5
Price RR, Axel L, Morgan T et al (1990) Quality assurance methods and phantoms for magnetic resonance imaging - Report of AAPM Nuclear Magnetic Resonance Task Group No-1. Med Phys 17:287–295
Acri G, Testagrossa B, Sestito A, Bonanno L, Vermiglio G (2018) CT and MRI slice separation evaluation by LabView developed software. Z Med Phys 28(1):6–13. https://doi.org/10.1016/j.zemedi.2017.09.009
Acri G, Tripepi MG, Causa F, Testagrossa B, Novario R, Vermiglio G (2012) Slice-thickness evaluation in CT and MRI: an alternative computerised procedure. Radiologia Medica. https://doi.org/10.1007/s11547-011-0775-5
Firbank MJ, Harrison RM, Williams ED, Coulthard A (2000) Quality assurance for MRI: practical experience. Br J Radiol 73(868):376–383. https://doi.org/10.1259/bjr.73.868.10844863
Safety Guidelines for Magnetic Resonance Imaging Equipment in Clinical Use - MHRA (UK) - February 2021
Durbridge G (2011) Magnetic resonance imaging: fundamental safety issues. J Orthop Sports Phys Ther 41(11):820–828. https://doi.org/10.2519/jospt.2011.3906
Dempsey MF, Condon B (2001) Thermal injuries associated with MRI. Clin Radiol 56(6):457–465. https://doi.org/10.1053/crad.2000.0688
Bottomley PA (2008) Turning up the heat on MRI. J Am Coll Radiol 5(7):853–855. https://doi.org/10.1016/j.jacr.2008.04.003
European Committee for Electrotechnical Standardization. Particular requirements for the safety of magnetic resonance equipment for medical diagnosis. European Committee for Electrotechnical Standardization. IEC 60601-2-33, 2002 Brussels, Belgium
Shellock FG, Kanal E (1996) Burns associated with the use of monitoring equipment during MR procedures. J Magn Reson Imaging 6:271–272
Giovannetti G, Hartwig V, Positano V, Vanello N (2014) Radiofrequency coils for magnetic resonance applications: theory, design, and evaluation. Crit Rev Biomed Eng 42(2):109–135. https://doi.org/10.1615/CritRevBiomedEng.2014011482
Labbé A, Authelet G, Baudouy B, van der Beek CJ, Briatico J, Darrasse L, Poirier-Quinot M (2021) Recent advances and challenges in the development of radiofrequency HTS coil for MRI. Front Phys. https://doi.org/10.3389/fphy.2021.705438
Giovannetti G, Francesconi R, Landini L et al (2004) Conductors geometry and capacitors quality for performance optimization of low frequency birdcage coils. Concepts Magn Resonance B (Magn Reson Eng) 20B:9–16
Giovannetti G, Frijia F, Menichetti L et al (2012) Coil sensitivity estimation with perturbing sphere method: application to 13C birdcages. Appl Magn Reson 42:511–518. https://doi.org/10.1007/s00723-012-0323-z
Jones BF (1998) A reappraisal of the use of infrared thermal image analysis in medicine. IEEE Trans Med Imaging 17(6):1019–1027. https://doi.org/10.1109/42.746635
Maldague X (2001) Theory and practice of infrared technology for non destructive testing -, 1st edn. Wiley, New York
Modest MF (2003) Radiative heat transfer, 2nd edn. Academic Press, California
Stuart B (2015) Infrared spectroscopy. In: Kirk-Othmer Encyclopedia of Chemical Technology, pp 1–18.https://doi.org/10.1002/0471238961.0914061810151405.a01.pub3
Ibrahim TS, Lee R (2006) Evaluation of MRI RF probes utilizing infrared sensors. IEEE Trans Biomed Eng 53(5):963–967. https://doi.org/10.1109/TBME.2006.871892
Lüdemann L, Wlodarczyk W, Nadobny J, Weihrauch M, Gellermann J, Wust P (2010) Non-invasive magnetic resonance thermography during regional hyperthermia. Int J Hyperth 26(3):273–282. https://doi.org/10.3109/02656731003596242
Oswald-Tranta B, Wally G (2006) Thermo-inductive surface crack detection in metallic materials. In: 9th European Conference on NDT, Berlin, Germany
Vrana J, Goldammer M, Baumann J, Rothenfusser M, Arnold W, Thompson DO, Chimenti DE (2008) Mechanisms and models for crack detection with induction thermography. AIP Conf Proc 975:475–482. https://doi.org/10.1063/1.2902698
Wilson J, Tian GY, Abidin IZ, Yang S, Almond D (2010) Pulsed eddy current thermography: system development and evaluation. Insight - Non-Destructive Test Condit Monit 52(2):87–90. https://doi.org/10.1784/insi.2010.52.2.87
Watmough DJ, Fowler PW, Oliver R (1970) The thermal scanning of a curved isothermal surface: implications for clinical thermography. Phys Med Biol 15(1):301. https://doi.org/10.1088/0031-9155/15/1/301
Flir user’s manual - Flir T4xx series (available online https://assets.tequipment.net/assets/1/26/FLIR_T4xx_User_Manual.pdf)
Acri G, Tripepi MG, Vermiglio V, Sansotta C, Testagrossa B, Causa F, Vermiglio G (2010) Telethemographic evidence of tattoos dermatological relevance. Acta Medica Mediterranea 26:17–23
Montin E, Lattanzi R (2021) Seeking a widely adoptable practical standard to estimate signal-to-noise ratio in magnetic resonance imaging for multiple-coil reconstructions. J Magn Resonance Imaging. https://doi.org/10.1002/jmri.27816
Acri G, Inferrera P, Denaro L, Sansotta C, Ruello E, Anfuso C, Testagrossa B (2018) dB/dt evaluation in MRI sites: Is ICNIRP threshold limit (for workers) exceeded? Int J Environ Res Public Health 15(7):1298. https://doi.org/10.3390/ijerph15071298
Hartwig V, Giovannetti G, Vanello N, Lombardi M, Landini L, Simi S (2009) Biological effects and safety in magnetic resonance imaging: a review. Int J Environ Res Public Health 6(6):1778–1798. https://doi.org/10.3390/ijerph6061778
This research received no external funding.
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Testagrossa, B., Ruello, E., Gurgone, S. et al. Radio Frequency MRI coils and safety: how infrared thermography can support quality assurance. Egypt J Radiol Nucl Med 52, 277 (2021). https://doi.org/10.1186/s43055-021-00659-y
- Quality assurance