Reduction of dielectric artifacts within an InSightec ExAblate 4000 head transducer
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KeywordsRadio Frequency Standing Wave Essential Tremor Electrical Equipment High Intensity Focus Ultrasound
High intensity focused ultrasound (HIFU) ablation of the brain has been increasingly used as a non-invasive treatment for essential tremor and neuropathic pain. However, artifacts arise in magnetic resonance (MR) images as a result of dielectric properties of the water bath coupler and of the tissues being investigated. As radio frequency (RF) waves enter a dielectric medium, their wavelengths decrease by a factor equal to the square root of the medium’s relative permittivity (Webb 2011). The shortened wavelength can cause a standing wave pattern within the sample, with regions of constructive and destructive interference. This RF inhomogeneity results in dielectric artifacts that appear as shaded regions in the intensity image. Modifying the RF coil can be used to bypass this issue, but is non-trivial. We present a simpler solution by decreasing the permittivity of the water bath, thereby reducing the standing wave effect that creates dielectric artifacts.
To decrease the permittivity of the water bath, sodium chloride (NaCl) was dissolved at concentrations of 12.5 to 62.5 mM at 12.5 mM increments. Higher concentrations correlated with lower permittivity. A fast gradient echo (FGRE) T1W sequence (TR 250, TE 13.2, FOV 40 cm x 40 cm, slice thickness 5 mm, 256 x 128 matrix) was performed on a 3T GE scanner. A region of interest (ROI) was drawn at the center of the head transducer along the superior/inferior direction. Signal to noise ratio (SNR) was calculated, in which signal was the mean intensity across the ROI and noise was the standard deviation of background.
Results and conclusions
Stanford Bio-X Bioengineering Fellowship, NIH P01 CA159992, Grant, InSightec, General Electric
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