J Korean Soc Magn Reson Med.  2011 Apr;15(1):11-21. 10.13104/jksmrm.2011.15.1.11.

Accurate Localization of Metal Electrodes Using Magnetic Resonance Imaging

• Joe E
• Ghim MO
• Ha Y
• Kim DH

• KMID: 2041223
• DOI: http://doi.org/10.13104/jksmrm.2011.15.1.11

Abstract

PURPOSE
Localization using MRI is difficult due to susceptibility induced artifacts caused by metal electrodes. Here we took an advantage of the B0 pattern induced by the metal electrodes by using an oblique-view imaging method.
MATERIALS AND METHODS
Metal electrode models with various diameters and susceptibilities were simulated to understand the aspect of field distortion. We set localization criteria for a turbo spin-echo (TSE) sequence usingconventional (90degrees view) and 45degrees oblique-view imaging method through simulation of images with various resolutions and validated the criteria usingphantom images acquired by a 3.0T clinical MRI system. For a gradient-refocused echo (GRE) sequence, which is relatively more sensitive to field inhomogeneity, we used phase images to find the center of electrode.
RESULTS
There was least field inhomogeneity along the 45degrees line that penetrated the center of the electrode. Therefore, our criteria for the TSE sequence with 45degrees oblique-view was coincided regardless of susceptibility. And with 45degrees oblique-view angle images, pixel shifts were bidirectional so we can detect the location of electrodes even in low resolution. For the GRE sequence, the 45degrees oblique-view anglemethod madethe lines where field polarity changes become coincident to the Cartesian grid so the localization of the center coordinates was more facilitated.
CONCLUSION
We suggested the method for accurate localization of electrode using 45degrees oblique-view angle imaging. It is expected to be a novelmethodto monitoring an electrophysiological brain study and brain neurosurgery.

Keyword

Microelectrode localization; Field inhomogeneity; Susceptibility artifact; Metallic artifact

MeSH Terms

Artifacts
Brain
Electrodes
Imidazoles
Magnetic Resonance Imaging
Magnetic Resonance Spectroscopy
Magnetics
Magnets
Neurosurgery
Nitro Compounds
Imidazoles
Nitro Compounds

Figure

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  Fig. 1 (a) Simulation results of field inhomogeneity pattern due to an electrode with d=125 µm, χ=77.2 ppm (tungsten). (b) Field inhomogeneity along the solid line (90°) and dashed line (45°) across the center of the electrode shown in (a). The blue line and red line in (b) indicate the 90° and 45 °line profiles, respectively. Note that there is a null-field line where least field inhomogeneity occurs in 45° line.

• 

  Fig. 2 Simulation results of signal pile-up patterns due to an electrode with d=250 µm, χ=77.2 ppm (tungsten) in (a) 90° and (b) 45° view angle and their RO/PE line-plots ((c) 90°, (d) 45°). The lines indicate the estimated coordinate center by using localization criteria.

• 

  Fig. 3 Simulation results of various susceptibilities with electrodes of R=0.5 mm, χ=77.2 ppm (a, d), 182 ppm (b, e), and 279 ppm (c, f) in 90° (a-c) and 45° (d-f). Red lines and green lines indicate the real center and incorrectly estimated coordinate center respectively.

• 

  Fig. 4 Simulation results of various resolutions with electrodes of R=0.25 mm and (a) res=0.15 mm2, (b) res=0.25 mm2, (c) res=0.5 mm2, (d) res=1.0 mm2. First rows are simulated TSE images and second rows show the relative location of electrodes in pixels of respective resolution. Third rows are the RO/PE line-plot results of images. Red lines indicate the coordinate center of the electrode. Each column shows how the relative location of an electrode affects the pixel pile-up pattern in image and localization of electrode in line-plots.

• 

  Fig. 5 2DGRE results in (a) 90° and (b) 45°. First and second rows are GRE magnitude and phase images of an electrode, respectively. Third rows show the grid which marks the real location of electrodes. Resolution of each column is shown at the top of the first image. Red lines indicate the estimated center of the electrode.

• 

  Fig. 6 2DTSE results in (a) 90° and (b) 45°. Each subfigure consists of four sets of images with respective resolution that are shown at the top left of the image sets. In each image set, there are TSE images of an electrode and the reference grid on the left side and respective RO/PE line-plots on the right side. Red lines indicate the estimated center of the electrode.

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  Fig. 7 (a) 3DTSE results for the rat brain model. The image on the top left of each view angle is the axial image showing the electrode position and image below it is series of 5 coronal images showing inhomogeneity due to the electrode. The regions near electrode center (red box) are magnified in the image on the right. (b) and (c) are RO/PE line-plots at the position of the yellow lines in (a) for 90° and 45°, respectively. Red lines indicate the estimated center and the yellow box shows the expected region where the electrode is located. Note that there is small amount of image distortion at the center slice of coronal image in 45° view angle. It implies that the RO line of that slice has almost crossed the electrode center, where the null-field exists.

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