3.1 Introduction
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Standard operation with the TMS coil held at distances of approximately 1–2 ft from the belly
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Accidental TMS coil discharge when the coil is in direct contact with the belly or in its immediate vicinity
3.2 Methods and Materials
3.2.1 Existing Computational Models of a Pregnant Woman
3.2.2 Construction of FEM (CAD) Full-Body Pregnant Woman Model and Model Topology
3.2.3 Tissue Properties
Tissue | σ (S/m)/εr |
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Amniotic fluid | Cerebrospinal fluid |
Fetus | Mean of muscle, uterus, and blood |
Fetal brain |
\( \left[\left(\frac{\sigma \_\mathrm{fetalbrain}\left(64\ \mathrm{MHz}\right)}{\sigma \_\mathrm{fetus}\left(64\ \mathrm{MHz}\right)}\right)+\left(\frac{\sigma \_\mathrm{fetalbrain}\left(127\ \mathrm{MHz}\right)}{\sigma \_\mathrm{fetus}\left(127\ \mathrm{MHz}\right)}\right)/2\right]\times \sigma \_\mathrm{fetus} \)
|
\( \left[\left(\frac{\varepsilon \_\mathrm{fetalbrain}\left(64\ \mathrm{MHz}\right)}{\varepsilon \_\mathrm{fetus}\left(64\ \mathrm{MHz}\right)}\right)+\left(\frac{\varepsilon \_\mathrm{fetalbrain}\left(127\ \mathrm{MHz}\right)}{\varepsilon \_\mathrm{fetus}\left(127\ \mathrm{MHz}\right)}\right)/2\right]\times \varepsilon \_\mathrm{fetus} \)
| |
Placenta | Average muscle |
3.3 Study Design
3.3.1 TMS Coil
3.3.2 Pulse Form and Duration
3.3.3 Coil Current
3.3.4 Coil Positions
3.3.5 Accidental Coil Discharge
3.3.6 Frequency-Domain Computations
3.3.7 Time-Domain Computations
3.3.8 Finding Maximum Peak Current Density/Electric Field Strength in Individual Tissues
3.4 Results: Pregnant Patient
3.4.1 Qualitative Behavior of Induced Currents in the Body of a Pregnant Patient at Different Frequencies (Pulse Durations)
3.4.2 Quantitative Results for Maximum Peak Electric Field at One SMT Unit
3.4.3 Comparison with the Recommended Safe Value of Electric Field
3.4.4 Observations from the Quantitative Solution
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Values of the peak electric field obtained using the condition of one STM unit weakly depend on the pulse duration. This is in contrast to the results shown in Fig. 3.7, where the dependence on frequency is paramount. The reason is the normalization condition of one SMT unit, which means, for example, that the amplitude of the coil current is significantly increased for the 1.0 ms pulse.
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The largest fields are observed in the placenta/uterus.
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The smallest fields are observed in the fetal brain.
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Peak values for two different coil orientations are quite similar.
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The third trimester is characterized by somewhat larger values of the maximum peak electrical field as compared to the first and second trimesters.
3.4.5 Comparison with Upper Analytical Estimate for Electric Fields/Eddy Currents
3.4.6 Using the Analytical Estimate for Predicting Maximum Fields for Different Patients
Nearest distance from coil center to uterus | Rounded estimate for the maximum peak electric field in the entire fetal area |
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50 cm | <170 mV/m |
60 cm | <100 mV/m |
70 cm | <60 mV/m |
80 cm | <40 mV/m |
90 cm | <30 mV/m |
3.5 Results: Pregnant Operator and Accidental Coil Discharge
3.5.1 Quantitative Results for Maximum Peak Electric Field at One SMT Unit
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For coil positions in close proximity to the belly (A-1, A-2, C-1), the peak electric field in the fetal volume may exceed the safe limit of 800 mV.
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When the distance from the coil center to the nearest point of the uterus is less than 60 cm, the maximum peak values in excess of 100 mV/m may be observed (this number is adopted from Table 3.4).
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When the distance from the coil center to the nearest point of the uterus is greater than 60 cm, the upper estimate from Table 3.4 can be applied.