WebNIR

Premise

In 2008, the then ISPESL promoted a research project (in which, in addition to ISPESL, operational units of IFAC and ENEA participated) to further investigate the assessment of exposure to EMF of operators working in magnetic resonance imaging (MRI) departments. The project had mainly dosimetric objectives, to achieve which it was necessary to develop a method for acquiring the waveform of the magnetic field perceived as variable over time by an operator moving in the magnetostatic field. This waveform is also fundamental for the determination of the radiometric quantities foreseen by the specific ICNIRP guideline of 2014, relating to the prevention of the effects resulting from such movement, i.e. the maximum variation ΔB over 3 seconds and the weighted peak indices referred to the limits for the stimulation effects.

This method subsequently evolved into the proposal of a possible standard approach to the problem of evaluating occupational exposure due to the movement of an operator in any non-homogeneous magnetostatic field. This approach is based on a Narda-Metrolab THM-1176 probe, worn by the operator himself or by a volunteer who simulates his behaviour. The probe must be fixed to a point on the subject's body, as shown in the figures. As a fixing point, the head is often chosen, due to the importance of this organ in relation to the effects of the exposure studied: in this case, the internal frame of a cycling helmet can be used, as shown in the figure. The probe is then connected to a personal computer using a sufficiently long USB extension cable. The probe management software supplied as standard by the manufacturer is run on the personal computer.

The subject is then asked to perform the typical movements of the various activities carried out in relation to the task studied, in areas where an intense static magnetic field is present; during these movements, the instantaneous values of the magnetic induction vector detected by the probe are stored in a file on the personal computer.

The web tool to which this information sheet refers was created to allow the processing of these data files, using them directly in the native format generated by the measurement probe mentioned and its standard management software.

Purpose

This tool allows the processing of the magnetic field that varies very slowly over time measurements, to obtain the radiation protection parameters provided for by the ICNIRP-2014 Guidelines Guidelines for limiting exposure to electric fields induced by movement of the human body in a static magnetic field and by time-varying magnetic fields below 1 Hz: in particular, the maximum variation ΔB in any interval of 3 seconds and the index weighted peak for stimulation effects.

The instrument is particularly useful for assessing the exposure suffered by an individual who moves in an environment where there is an intense non-homogeneous static magnetic field (such as in magnetic resonance departments): the movements of the exposed subject give rise to a magnetic field perceived by the subject as slowly varying over time. For this purpose, the measurement sensor must be fixed on a part of the subject's body (for example the head), so that it participates in a supportive manner in his movements.

Important note

The limits proposed by the cited ICNIRP-2014 Guidelines do not have binding value, since these Guidelines have not been integrated into any legally relevant national or community document. However, compliance with these limits - also recommended by the Non-binding guide to good practice for the implementation of Directive 2013/35/EU on electromagnetic fields - has a recognized scientific basis.

General approach

The instrument is designed to process the data files produced by the Narda-Metrolab THM1176 Hall effect magnetometer (connected to a personal computer through its native USB interface) and is able to automatically recognize and manage three different formats of these files, adopted by the manufacturer in three different phases of the probe's life: the two historical ones and the current EZMag3D software export (see at the bottom of this sheet).
The data files in question are made up of vector samples of magnetic induction acquired at sufficiently close successive instants of time.

The data file generated by the THM1176 magnetometer is sent to the WebNir server, where it is decoded and processed to calculate the relevant radiation protection parameters and build the graphs shown among the results.

Limitations

• The THM1176 magnetometer, at least the tested specimens, was able to save on PC reliably and continuously a flow of measurements with a maximum speed of 10 vector samples per second. This limits the maximum frequency of the perceived magnetic field spectrum reconstructed by the instrument to 5 Hz.
• To avoid unacceptably long processing times, the calculation of the maximum ΔB variation over 3 seconds is performed only if the product of the sampling frequency and the number of samples is less than one million; otherwise the value N/A is shown.

Input data file format

The input data format is the one provided by the Narda-Metrolab magnetometer THM1176.
This, depending on the installed firmware, can save the measured data in the following formats.

First version

12,317243 10,265700 6,797100 -0,359600 MT 20634 0x0000000000060315
12,320505 10,301500 6,753200 -0,261200 MT 20614 0x000000000006031f
12,358934 10,318400 6,794500 -0,329600 MT 20620 0x0000000000060329
12,319015 10,285600 6,771700 -0,329600 MT 20659 0x0000000000060333
12,322512 10,305300 6,745800 -0,373200 MT 20635 0x000000000006033d
12,339373 10,312900 6,765800 -0,358000 MT 20656 0x0000000000060347
12,308422 10,318700 6,695000 -0,445700 MT 20646 0x0000000000060351
12,299327 10,297000 6,713600 -0,415700 MT 20624 0x000000000006035b
12,295990 10,286300 6,724000 -0,413800 MT 20625 0x0000000000060364
12,293187 10,297800 6,702000 -0,401200 MT 20681 0x000000000006036e
...

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Second version

Block B Bx By Bz Units Temperature Timestamp Serial No. Comment
0 0.000644 -0.000290 -0.000567 -0.000101 T 20989 2013.06.19 22:02:20.604394039 0000105
0 0.001074 -0.000590 -0.000800 -0.000406 T 20989 2013.06.19 22:02:20.704394039 0000105
0 0.000783 -0.000187 -0.000565 -0.000510 T 20989 2013.06.19 22:02:20.804394039 0000105
0 0.000745 -0.000294 -0.000457 -0.000510 T 20989 2013.06.19 22:02:20.904394039 0000105
0 0.000851 -0.000499 -0.000464 -0.000510 T 20989 2013.06.19 22:02:21.004394039 0000105
0 0.000723 -0.000301 -0.000236 -0.000613 T 20989 2013.06.19 22:02:21.104394039 0000105
0 0.000731 -0.000605 -0.000356 -0.000204 T 20989 2013.06.19 22:02:21.204394039 0000105
0 0.000706 -0.000290 -0.000567 -0.000305 T 20989 2013.06.19 22:02:21.304394039 0000105
0 0.001073 -0.000484 -0.000908 -0.000303 T 20989 2013.06.19 22:02:21.404394039 0000105
0 0.000869 -0.000605 -0.000357 -0.000511 T 20989 2013.06.19 22:02:21.504394039 0000105
1 0.000511 -0.000404 -0.000238 -0.000204 T 21005 2013.06.19 22:02:21.604394039 0000105
1 0.000615 -0.000397 -0.000459 -0.000101 T 21005 2013.06.19 22:02:21.704394039 0000105
...

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Third version (EZMag3D software)

This is the manufacturer's current software export: a header with the Source;EZMag3D signature and the measurement unit declared in the Parameters line (Units=mT); the data, semicolon-separated, start after the column-header line.

Source;EZMag3D v1.2.3
Timestamp;2025.08.02T11:22:13
Description;Magnetic Field Measurement File, Metrolab Technology SA

Type;tMXR_BODY_EZMAG3D
Version;1.0
Comment;
Instrument;THM1176-MF 0002598

Parameters;Units=mT;Range=0.008;AutoRange=1;TriggerMode=Continuous;AcqRate=10;DispRate=1;Avg=100;FreqMin=0;FreqMax=5;Bins=5;BlockSize=10

Timestamp;B;B.B';Bx;By;Bz;Temp;dB
2025.08.02T11:21:14.049454055;0.0744983;-0.0525771;0.069817;0.0196624;-0.0169992;21171;0.05
2025.08.02T11:21:14.149454055;0.0710563;-0.0623716;0.0576843;0.0240541;-0.0338072;21171;0.05
...