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Use of Dielectric Pads to Improve Clinical 7T MRI Knee Image Uniformity

A Fagan1*, K Amrami1 , M Frick1 , P Kollasch2 , J Felmlee1 , (1) Mayo Clinic, Rochester, MN, (2) Siemens Healthineers, Rochester, MN


(Sunday, 7/29/2018) 4:30 PM - 5:00 PM

Room: Exhibit Hall | Forum 8

Purpose: To investigate the use for routine clinical applications of custom-made dielectric pads to mitigate an radiofrequency (RF) coil receiver-sensitivity deficit problem and also the inherent B�+ inhomogeneity present at 7T MRI for musculoskeletal imaging in the knee. At 7T (300 MHz for ¹H), large displacement currents are generated in high permittivity materials which effect a B�+ field focusing in their vicinity, increasing the flip angles achievable in nearby tissue.

Methods: Dielectric pads were manufactured from high electric permittivity materials to maximize the magnitude of the displacement currents. Care was taken to avoid any additives which might introduce susceptibility artifacts or excessively load the RF coil. A 40% vol/vol ratio of CaTiO₃ / D₂O was prepared, with a 0.5% mass fraction of a gelling agent (hydroxyethylcellulose) to reduce separation of the CaTiO₃ from the deuterated water over time. The small amount of gelling agent ensured that the commitment increase in conductivity was minimized, as this would lead to reduced B�+ efficiency. Pads of varying dimensions were manufactured and tested using phantom and volunteer scanning.

Results: A relative permittivity value of 104 was obtained in the manufactured pads, which were not visible in the images due to the use of D₂O. Experiments with varying thickness of dielectric pads achieved a 27% increase in measured signal in T₂-weighted TSE images in the vicinity of a 10 mm thick pad – thicker pads were impracticable for routine patient scanning due to coil size restrictions. Significant improvements in overall image uniformity in vivo was achieved with pads placed on either side of the knee, in a central coronal plane.

Conclusion: By increasing Bâ‚?+ and hence flip angles in their vicinity, dielectric pads made from high permittivity materials increase the measured signal in areas of deficit and improve knee image uniformity at 7T.

Funding Support, Disclosures, and Conflict of Interest: P. Kollasch is an employee of Siemens Healthineers.


Magnetic Fields, MRI, Image Artifacts


IM- MRI : Development (New technology and techniques)

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