Exposure to MRI-Related Magnetic Fields and Vertigo in MRI Workers

Transcription

1 Utrecht University Institute for Risk Assessment Sciences (IRAS) Exposure to MRI-Related Magnetic Fields and Vertigo in MRI Workers Kristel Schaap, Yvette Christopher-de Vries, Hans Kromhout This project was funded by:

2 Magnetic Resonance Imaging (MRI) Potential exposure to 3 types of electromagnetic fields (EMF): During the imaging process: Radiofrequency pulses (MHz frequencies) Switched gradient fields (khz frequencies) Always present: Static magnetic field (SMF, 0 Hz) Strenght of magnet (actually flux density) is expressed in Tesla (T) Stray field around a scanner Main source of exposure for MRI staff 2

3 The static magnetic stray field Presence within the static magnetic stray field results in two types of exposure

4 B = 200 mt B = 0 mt t = 2 s ΔB = 200 mt Δt = 2 sec ΔB/Δt = 100 mt/s

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6 Acute effects of exposure to a static magnetic field Safety risks attraction of metal objects, deregulation of electronic devices, swipe cards, and electronic implants 6

7 Acute effects of exposure to a static magnetic field Safety risks attraction of metal objects, deregulation of electronic devices, swipe cards, and electronic implants Neurocognitive effects E.g. Visuo-spatial orientation (van Nierop et al.; de Vocht et al.) Effects on vestibular functions and balance (van Nierop et al.; Mian et al.) Transient acute symptoms reported by patients and staff e.g. Vertigo/dizziness, metallic taste, nausea 7

8 Study context Increasing scanner strengths increasing exposure levels Increasing numbers of exposed in 2010 ~ 150 MRI departments in NL with (potentially) exposed Relatively new research field Various health effects of SMF exposure still relatively unknown MRI staff may experience acute short-lasting symptoms while working close to an MRI scanner Literature suggests a positive association with magnet strength of MRI scanner (de Vocht et al. 2006; Wilen & de Vocht 2011; Heinrich et al 2013) No clear indication of the prevalence of these symptoms among MRI staff 8

9 Study objectives Get more insight into symptoms reported during occupational exposure to MRI-related static magnetic fields (SMFs): How often do these occur among clinical and research MRI staff? (incidence) How are these symptoms associated with exposure to static magnetic fields and/or motion-induced timevarying magnetic fields? 9

10 Study setup: Workplace visits of 1 or 2 weeks 14 MRI facilities - Scanning human subjects or animals - Scanners of Tesla Population (n=331 subjects) MRI staff (SMF exposed and unexposed work shifts) Additional SMF unexposed control group of CT radiographers Work shift diaries with information on exposure and symptoms: worked in MRI scanner room (yes/no) type & strength of MRI scanner experienced any of 19 listed symptoms during the work shift? duration of symptoms Repeated measurements: 1-6 diaries per person 10

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12 Common closed bore scanner Upright scanner Extremity scanner Small bore animal scanner 4 mei

13 Study setup: Workplace visits of 1-2 weeks per MRI facility A. Personal shift-based exposure measurements, using SMF dosimeters: SMF (B) and TVMF (db/dt) Repeats where possible Measurement rate: 10Hz B. Work-day diary Scanner type and field strength Performed scan procedures and other tasks Types of patients scanned C. General questionnaire Job title Body height Years of MRI experience

14 Symptoms Target symptoms A priori unrelated symptoms Vertigo Black spots Nausea Irritated eyes Head ringing Irritated skin Magnetophosphenes Hot flashes Metallic taste Earache Headache Palpitation Tiredness Concentration problems Vomiting Instability Light-headedness Blurred vision IRAS Occupational exposure to MRI-related EMF 14 Strange smell

15 Symptoms Target symptoms A priori unrelated symptoms Vertigo Nausea Head ringing Magnetophosphenes Metallic taste Headache Tiredness Concentration problems Vomiting Instability Light-headedness Blurred vision IRAS Occupational exposure to MRI-related EMF 15 Strange smell Black spots Irritated eyes Irritated skin Hot flashes Earache Palpitation Core symptoms

16 Adjusted odds ratios (95%CI) for scanner category in the symptom-reporting class Scanner category Target symptoms Core symptoms A priori unrelated symptoms Unexposed (reference) T closed bore 1.88 ( ) 2.41 ( ) 0.75 ( ) 3T closed bore 2.14 ( ) 4.92 ( ) 0.28 ( ) 7T closed bore 4.17 ( ) ( ) n.a. <1.5T various types 1.47 ( ) 0.84 ( ) 1.53 ( ) >4.7T small bore 0.72 ( ) n.a ( ) Adjusted for gender, age, workload, use of solvents, alcohol ingestion 16

17 Shifts reporting at least one symptom within the specified category Scanner category N obs Target symptoms Core symptoms A priori unrelated symptoms Unexposed 134 (19%) (1%) (3%) 1.5T closed bore 259 (28%) (5%) (7%) 3T closed bore 131 (35%) (9%) (2%) 7T closed bore 31 (39%) (29%) (0%) <1.5T various types 49 (27%) (4%) (14%) >4.7T small bore 57 (16%) (0%) (7%) Overall 619 (26%) (6%) (5%) Individual target symptoms were reported in 0 12% of shifts 17

18 Individual symptoms Not statistically tested. However, similar trends observed for core symptoms vertigo, metallic taste: Unexposed 1.5T closed bore 3T closed bore 7T closed bore <1.5T various types >4.7T small bore Vertigo 0.0% 3.5% 7.6% 22.6% 2.0% 0.0% Metallic taste 0.0% 0.4% 0.8% 19.4% 2.0% 0.0% not reported during non-mri shifts transient (duration <1 minute to <15 minutes) 18

19 Statistical method: analysis at shift level Strong clustering of symptoms within subjects Finite Mixture Model (FlexMix package, R): Random effects assumed to be from a discrete distribution: Each subject belongs to one of several (latent) classes. 2 steps: 1. A model to estimate number of classes and predict classmembership 2. A class-specific model A 2-class model best fitted the data. Subjects were assigned to one of two classes: 1. non-reporting class 2. symptom-reporting class 19

20 Exposure-response curve for vertigo for fast and slow moving individuals 20

21 Threshold levels for reaching 5% of symptomatic group reporting vertigo Exposure Type Metric Peak exposure (B) SMF TWA SMF Exposed TWA full-shift Peak exposure (db/dt) TVMF TWA SMF Exposed (db/dt) TWA full-shift (db/dt) 5% level 409 mt 48 mt 3 mt 477 mt/s 6 mt/s 0.6 mt/s 21

22 Summary of main results Largest study of MRI-related symptoms among MRI workers Increased incidence of experiencing specific transient symptoms with closed-bore MRI scanners of 1.5 to 7 Tesla Positive association with scanner strength. This is in agreement with other studies among people working with MRI scanners in similar and other work environments Wilen & De Vocht, 2011; De Vocht et al., 2006; Schenck et al., 1992 Effects appear to be driven mainly by vertigo and metallic taste reported during 6% and 2% of the MRI shifts, respectively not reported during non-mri shifts 22

23 Summary of main results Exposure response steepest for full-shift TWA TVMF exposure. Based on the exposure response curve for this metric it is estimated that above a full-shift TWA of 0.6 mt/s sensitive individuals (at least 9% of our study population) will report vertigo with a probability higher than 5%. With MRI system magnets getting increasingly stronger, number of workers experiencing MRI-related symptoms expected to increase 23

24 Thanks to: Lutzen Portengen (IRAS) Frank de Vocht (University of Bristol) Catherine Mason Evelyne Cambron-Goulet Study participants and participating institutions Publications: Schaap et al Occupational exposure of healthcare and research staff to static magnetic stray fields from Tesla MRI scanners is associated with reporting of transient symptoms. Occup Environ Med 71: (open access) Schaap et al Exposure to static and time-varying magnetic fields from working in the static magnetic stray fields of MRI scanners: a comprehensive survey in the Netherlands. Ann Occup Hyg 58: Schaap et al Exposure to MRI-related magnetic fields and vertigo in MRI workers. Occup Environ Med 73: This work was supported by the Netherlands Organization for Health Research and Development (ZonMw) within the program Electromagnetic Fields and 24 Health Research, under grant numbers and