TMS positioning in MRI using NMR probes
Yi-Cheng Hsu1, Ying-Hua Chu1, Pu-Yeh Wu1, Shang-Yueh Tsai2, and Fa-Hsuan Lin1

1Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan, 2Institute of Applied Physic, National Chengchi University, Taipei, Taiwan

Synopsis

We propose a method and a system to precisely place the TMS coil inside the MRI using NMR probes.The positioning can be completed in 0.1 s with high translation (0.015 mm) and rotation precision (0.0047°) as well as low bias (~0.8 mm in 50 mm FOV).

PURPOSE

Transcranial magnetic stimulation (TMS) uses transient magnetic field to induce local neuronal depolarization noninvasively1. Allowing both observation and manipulation, integrating TMS and fMRI in the same experimental setup can be a valuable tool to study brain connectivity. Compared to TMS EEG, TMS-fMRI has higher and more homogeneous spatial resolution. More importantly, TMS-fMRI holds the promise of detecting deep brain responses to exogenous magnetic field stimulation. The BOLD activation at remote but inter-connected regions has been successfully detected using simultaneous TMS-fMRI2. However, positioning a TMS coil is a tedious but critical job. The position and the orientation of a TMS coil determine the anatomical site and the effectiveness of stimulation. Outside the MRI scanner, it is possible to use optical devices to co-register between a TMS coil and head/brain3. Optical tracers can also be used inside a MRI scanner to place the TMS coil if they are not obscured. Alternatively, functional indices (e.g. thumb movement) can be used to first locate a functional brain area and then to adjust TMS coil position/orientation with respect to this reference site4. However, this approach has rather limited spatial resolution and needs a precise functional area. NMR-active droplets have been used to estimate the head position in real time to provide information for correcting motion related artifacts5. These droplets can be immersed in a miniature RF coil to increase SNR. Such field probes can be quickly localized inside MRI and used to monitor field fluctuations during data acquisition6. Here, we propose a method and a system to precisely place the TMS coil inside the MRI. Specifically, multiple NMR probes are mounted on a TMS coil. Their positions were estimated from spectra of NMR signals measured with the presence of three orthogonal gradient fields. Our approach has high translation (0.015 mm) and rotation precision (0.0047°) as well as low bias (~0.8 mm). MRI gradients caused 0.76% (0.38 mm in 50 mm FOV) measurement error.

METHODS

Our system consisted of four home-made NMR probes7. Probes were integrated to our 8-channel TMS-MRI RF coil array8 . Before TMS coil positioning, brain anatomy was measured by a 3D T1-weighted (MPRAGE) sequence. Probe positions were measured using three gradient echo pulse sequences (Figure 1) with 10 mT/m strength in x, y, and z direction separately (flip angle = 10°, TE = 11 ms, and TR = 30 ms). A 10-ms readout after gradient rewinding was used to measure the off-resonance when no gradient field was presented. After we measured probes positions, $$$\overrightarrow{(r_{meas}^i ) }$$$, $$$i = 1,…,4$$$, the orientation and the position of the TMS coil was estimated by minimizing the following squared error. $$$∑_{i=1}^4||((\overrightarrow{r_{meas}^i }-\overrightarrow{r } )-\overrightarrow{R(r_0^i )} ||_2^2 $$$, where $$$\overrightarrow{r_0^i }$$$ denoted the probe position when the TMS coil was placed at the iso-center at the beginning of the experiment. $$$\overrightarrow{r_0^i }$$$ denoted the unknown instantaneous translation of the center of TMS coil. R was an unknown rotation matrix to be estimated. To investigate the stability of TMS coil positioning, we repeated the measurement 100 times. We calculated the standard deviation of TMS coil position $$$(x,y,z)$$$ and angle $$$(\overrightarrow{u},\overrightarrow{v})$$$. To investigate the positioning error caused by the MRI gradient, two probes separated by 50 mm were repetitively placed 570 times evenly over a 3D region of 25 cm x 25 cm x 14 cm. This error was calculated as the standard deviation of the estimated distance between two probes over measurements.

RESULT

Figure 2 shows the picture of our positioning apparatus. Figure 3 depicts the definition of position and orientation of a TMS coil. Standard deviations for $$$(x,y,z)$$$ and $$$(\overrightarrow{u},\overrightarrow{v})$$$ were (9 μm, 15 μm, 6 μm) and (0.005°, 0.004°), respectively. The average and the standard deviation of the 50-mm probe separation was 50.8 mm and 0.38 mm (0.76% of the separated distance), respectively.

DISCUSSION

Preliminary results suggest that probes can be used to achieve high precision positioning of a TMS coil. The bias of the positioning can be only confirmed to the accuracy of probe separation (50 mm in the current experiment). For each positioning estimation, the measurement took only 0.1 s. This can be further accelerated to allow real-timeTMS coil positioning. The error caused by MRI gradient is expected to be corrected by accurate mapping of the magnetic field generated by gradients. Ultimately, our approach can be an accurate and convenient solution of TMS coil positioning inside MRI.

Acknowledgements

This study was supported by Ministry of Science and Technology, Taiwan (MOST 104-2314-B-002-238, MOST 103-2628-B-002-002-MY3), National Health Research Institute, Taiwan (NHRI-EX104-10247EI), and Ministry of Economic Affairs, Taiwan (100-EC-17-A-19-S1-175).

References

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3. Ettinger GJ, Leventon ME, Grimson WEL, Kikinis R, Gugino V, Cote W, Sprung L, Aglio L, Shenton M, Potts G, Alexander E. Experimentation with a transcranial magnetic stimulation system for functional brain mapping. Lect Notes Comput Sc 1997;1205:477-486.

4. Herwig U, Schonfeldt-Lecuona C, Wunderlich AP, von Tiesenhausen C, Thielscher A, Walter H, Spitzer M. The navigation of transcranial magnetic stimulation. Psychiat Res-Neuroim 2001;108(2):123-131.

5. Sengupta S, Tadanki S, Gore JC, Welch EB. Prospective real-time head motion correction using inductively coupled wireless NMR probes. Magnetic Resonance in Medicine 2014;72(4):971-985.

6. De Zanche N, Barmet C, Nordmeyer-Massner JA, Pruessmann KP. NMR probes for measuring magnetic fields and field dynamics in MR systems. Magnetic Resonance in Medicine 2008;60(1):176-186.

7. Ying-Hua Chu, Yi-Cheng Hsu, Fa-Hsuan Lin, Spiral Imaging Trajectory Mapping Using High Density 25-Channel Field Probe Arra, Intl. Soc. Mag. Reson. Med. (2015); 1014

8. Pu-Yeh Wu, Ying-Hua Chu, Aapo Nummenmaa, Thomas Witzel, Shang-Yueh Tsai, Wen-Jui Kuo, Fa-Hsuan Lin, 10-Channel TMS-Compatible Planar RF Coil Array for Human Brain MRI at 3T”, Intl. Soc. Mag. Reson. Med. (2015); 625

Figures

Figure 1. The gradient echo pulse sequences for measuring probe position

Figure 2. Four probes were integrated to 8-channel TMS-MRI RF coil array.

Figure 3. The definition of position and orientation of a TMS coil.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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