LEVELS OF EXPOSURE TO MAGNETIC FIELDS GENERATED BY MAGNETOTHERAPY EQUIPMENTS

C. GOICEANU*, R. DĂNULESCU*, EUGENIA DĂNULESCU*, F. M. TUFESCU**, DORINA-Emilia CREANGĂ**

* Institute of Public Health, Department of Occupational Health, 14 Victor Babes St, 700465 Iasi, Romania; E-mail: cristian.goiceanu@samun.ro , cristig3@yahoo.co.uk

** "Al. I. Cuza" University of Iasi, Faculty of Physics, 11 Carol Boulevard, 700506 Iasi, Romania

Abstract. Our study investigated the magnetic field levels generated by magnetotherapy equipments during their operation in physiotherapy departments. Field measurements were carried out in various points that were considered relevant for the exposure of the operator of magnetotherapy equipment and, in some cases, for the exposure of the patient. Several models of magnetotherapy equipments were included in this study, both new technology and some older type equipments. Our main focus consisted in occupational exposure of medical personnel, but the recorded data also allowed us to emphasize some characteristics of patient exposure. Especially in the case of the new technology equipments, the compliance with occupational exposure guidelines is met, provided that the medical personnel conform to the good practice recommendations.

Key words: magnetotherapy, human exposure, magnetic fields.

introduction

Therapy using magnetic field action on human body is famous in Europe by several centuries, but some sources mention the use of magnets even several millenniums ago. The first form of magnetic field therapy consisted in the application of natural magnets on specific points located on body surface. Latter, artificial magnets were manufactured and employed for magnetic field therapy. In the 19th century, solenoid coils were built with the purpose of immersing human body in a magnetic field supposedly to accomplish some healing action [1].

After the end of the World War II, a variety of devices generating time-varying magnetic fields were built with the aim to be used for therapeutic purposes. Production of magnetotherapy devices started in Japan and quickly moved to Europe, first in Romania and the former Soviet Union [2]. Mid 1970s, in the USA biphasic low frequency signal started to be used for treatment of fractures. During last decades, low-frequency pulse magnetotherapy, also called pulsed magnetic field (PMF) therapy, has been extensively used by employing various waveshapes for the treatment of many diseases and disorders [3], [4], [5], [6].

Magnetotherapy has become one of the main physiotherapy procedures that are largely employed worldwide. Its extended use has led to a concern regarding possible consequences on human health related to exposure to magnetic fields generated by magnetotherapy equipment. Patients are intentionally exposed to high levels of magnetic fields, but their daily as well as total exposure duration is limited. On the other hand, medical personnel operating the magnetotherapy equipments is exposed to lower levels comparing to the patients, but exposure last for many years. This paper focuses on occupational exposure during the use of therapy equipments employing time-varying magnetic fields.

materials and methods

In our study, we investigated the emissions of some types of magnetotherapy equipments in use in physiotherapy departments.

Equipments under test. Two generation of devices generating time-varying magnetic fields were included in our tests:

-          an older generation that employs sinusoidal pulses,

-          new generation devices that employs pulses with programmable characteristics regarding frequency and waveshape.

Applicators. Magnetic field measurement was accomplished for all available applicators:

-          ring solenoids,

-          cylinder solenoids,

-          local flat applicators.

Various working regimes of devices regarding waveshape and magnitude were taken into account.

Measurement equipment. The equipment we used for all magnetic field measurements consists in a low-frequency field analyzer from Narda model EFA-300 together with Narda magnetic field probes.

Methodology. We measured magnetic field levels in relevant points for the exposure of magnetotherapy equipment operators. Although our study focused on the exposure of medical personnel, some measurements were also carried out in some points related to patient location during the therapy - e.g. very close to applicators. Given that in the case of occupational exposure from magnetotherapy equipment there is no specific standard for field level measurement, our measurement protocols are based on general measurement methodology [7], [8] which was adapted to the specific needs in the domain of magnetotherapy:

o   Determination of magnetic field levels was mainly accomplished by spot measurements of magnetic flux density,

o   Two kinds of measurement were carried out: total field measurement and frequency-weighted measurement were carried out,

o   Additionally, spectral analysis was employed.

 

Frequency-weighted measurements and spectral analysis were carried out to allow checking compliance with exposure standards when harmonics are present -since the exposure limit expressed as magnetic field reference level, is frequency dependent. Moreover, spectral analysis was employed to better emphasize the characteristics of magnetic fields generated by magnetotherapy equipments.

results

Magnetic field levels inside or in the proximity of applicators depend on the working regimes of devices, i.e. the parameters set by the operator of magnetotherapy equipment: magnitude of the field, frequency, waveshape. We measured field levels for various working regimes of magnetotherapy equipments and the result of our measurements is presented as intervals of measured values as seen in Table 1. These intervals of values do not represent dynamic range of the device emission parameters, but minimum and maximum rounded values of measured field levels in the proximity of magnetotherapy equipments during usual therapy types. We focused on therapy types that correspond to frequent diagnostics of patients undergoing magnetotherapy and mainly on those emission parameters that are common to both older and newer generations of equipment.

Table 1

Measured magnetic field levels for older and newer generations of magnetotherapy equipments.

Measurement points	Magnetic flux density B ( μT )
	Old generation equipments	New generation equipments
Inside solenoids or very close to applicators.	50 - 2000	50 - 3000
Close to the therapy bed, in front of applicators.	3 - 100	2 - 30
Close to the magnetotherapy base unit.	2 - 30	1 - 10

Recorded magnetic field levels at distances of 30 - 50 cm from magnetotherapy base unit or from applicators ranged between about 2 - 100 μT for old generation equipments under test and between about 1 - 30 μT for new generation equipments. Reference levels set by occupational exposure standards for magnetic low-frequency fields are of 500 μT at 50 Hz and decrease as frequency increases down to 30.7 μT around 1 kHz. Consequently, magnetic field levels measured in points were operator is supposed to be present during magnetotherapy (sitting or standing in front of base unit, or else standing or passing near the bed where the patient lies) are, generally, below the reference levels set by occupational exposure standards.

Occasional high local exposure can occur when operator do not fully comply with procedures, rules and good practice recommendations. When the operator gets too close or even touches or holds the applicators, he reaches point with high levels of magnetic field. Our measurement showed maximum values of 2000 - 3000 μT.

Given that the limits for human exposure to varying magnetic fields depend on frequency, when exposure field is non-sinusoidal, checking compliance with exposure limits requires frequency-weighted measurements or spectral analysis. The spectral composition of the generated magnetic fields is related to the waveshape of the applied current. Some waveshapes much used in many generations of magnetotherapy devices are sinusoidal pulses and rectangular pulses and their spectra are presented in (Fig. 1 and Fig. 2). The weight of various components is much dependent on the chosen waveshape.

Fig. 1.  Spectrum of sinusoidal pulses generated by magnetotherapy equipment.

Fig. 2.  Spectrum of rectangular pulses generated by magnetotherapy equipment.

DISCUSSION

Generally, magnetic field levels measured in points where medical personnel might be present do not exceed the exposure levels. On the other hand, especially when medical personnel do not fully comply with good practice procedures and gets closer to the applicators, occasional high local exposure can occur.

One of the most frequent cases when the operator places himself to an area with high level of magnetic field is when he comes very close to the applicator leading to a high momentary exposure of the upper part of the leg or of the trunk. Moreover, high local exposure of hands occurs sometimes when the operator touches the applicator at patient request to diminish the discomfort by finding applicator optimum position.

Situations requiring operator intervention during device operation are often related to the lack of patient comfort as a consequence of the failure of initial applicator positioning in an optimum manner. Another category of situations requiring operator intervention during device operation is related to inadequate functioning of parts of the equipment. In a specific case we observed that inadequate functioning of the applicator led to a local temperature increase in a spot area located on the circumference of the ring solenoid. Another case consisted in troubles with the electric wire connecting the applicator to the main unit.

Spectral analysis shows a wide low-frequency spectrum for non-sinusoidal waveshapes. Comparing to older generation, new generation magnetotherapy equipments can generate more spectral components with significant spectral weight. This feature of modern equipments is due to generation of more complex pulses with programmable characteristics of waveshape and of time evolution of sequences. Consequently, when the level of dominant frequency represents an important percentage of the magnetic field reference level at that frequency, an analysis of the next highest components of the spectrum is to be achieved.

Analysing the magnetic field levels generated by the two generation of equipments, one can note that for similar levels applied to the patient, the levels in points where operator is often present are lower for newer generation equipments. Modern equipments use new type magnetic applicators that focus the field on patient side (e.g. internal side in the case of solenoids) and significantly reduce the field on the opposite side. Therefore, the spurious magnetic field to which operator may be exposed is diminished.

cONCLUSIONs

Measured magnetic field levels generated by magnetotherapy equipments measured in points were operator is supposed to seat, stand or pass during patient therapy do not exceed the reference levels set by occupational exposure standards. Occupational exposure of medical personnel is increased for short time duration when they disregard good practice procedures or when their intervention is required by unexpected situations related to patient or to malfunction of parts of equipment. High momentary exposure of hands, upper part of legs or parts of the trunk can occur in situations requiring operator to optimize position of applicators during device operation.

Comparing the magnetic field levels generated by the two generation of equipments, for similar levels applied to the patient, a better protection of the operator is provided by newer generation equipments by means of better focusing the field on patient side. Therefore, the level of magnetic field to which operator may be exposed is decreased in the case of devices employing new technologies. On the other hand, magnetic fields emitted by new generation equipments have a more complex spectrum and, when the level of dominant frequency represents an important percentage of the reference level, spectral analysis is needed.

references

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