An anatomically realistic temperature phantom for radiofrequency heating measurements

Abstract

Purpose: An anthropomorphic phantom with realistic electrical properties allows for a more accurate reproduction of tissue current patterns during excitation. A temperature map can then probe the worst-case heating expected in the unperfused case. We describe an anatomically realistic human head phantom that allows rapid three-dimensional (3D) temperature mapping at 7T.

Methods: The phantom was based on hand-labeled anatomical imaging data and consists of four compartments matching the corresponding human tissues in geometry and electrical properties. The increases in temperature resulting from radiofrequency excitation were measured with MR thermometry using a temperature-sensitive contrast agent (TmDOTMA(-)) validated by direct fiber optic temperature measurements.

Results: Acquisition of 3D temperature maps of the full phantom with a temperature accuracy better than 0.1°C was achieved with an isotropic resolution of 5 mm and acquisition times of 2-4 minutes.

Conclusion: Our results demonstrate the feasibility of constructing anatomically realistic phantoms with complex geometries incorporating the ability to measure accurate temperature maps in the phantom. The anthropomorphic temperature phantom is expected to provide a useful tool for the evaluation of the heating effects of both conventional and parallel transmit pulses and help validate electromagnetic and temperature simulations.

Keywords: MR safety; local SAR; parallel transmit; phantom; temperature estimation; thermometry.

FIG. 1. Phantom design

(a) The 37 tissue type model generated from 1 mm isotropic MR image volumes of a human head. (b) Sagittal cut through CAD model, shown in lateral and oblique views. White corresponds to outside surfaces or surfaces facing the cavities, whereas yellow surfaces are facing regions printed in plastic. (c) From left to right: surface meshes for the brain, muscle, the eyes and the plastic compartment.

FIG. 2. TmDOTMA⁻ imaging

(a) Water and TmDOTMA⁻ images separated by 31.5 kHz in frequency encoding direction. (b) Four echoes acquired with multi-echo GRE, TE₁ = 2.5 ms, ΔTE = 1.67 ms. The measured signal decay is consistent with a T₂* of approximately 4 ms.

FIG. 3. Constructed phantom

(a) Photographs of the completed phantom. From left to right: Front view, side view and details of the mechanical sealing: o-ring indentations surrounding filling channels and small holes for screws (top) and screwed on sealing cap (bottom) (b) CT scan of the empty phantom. (c) MRI of completed phantom filled with gel. (d) B₁⁺ map (magnitude) of the gel-filled phantom at 7 T.

FIG. 4. Temperature imaging calibration

(a) Water image showing test tube and fiber optic probe inserted into the tube. The red circle shows approximate position of ROI around the probe tip. (b) Phase image acquired at the TmDOTMA⁻ resonance frequency. (c) Cooling curve measured simultaneously with fiber optic probe (red asterisks) and temperature imaging (blue circles) demonstrating accurate calibration of temperature imaging. Insert: Example temperature map at t = 810 s after the beginning of the experiment.

FIG. 5. Birdcage coil heating experiment

(a) Temperature change over time for two locations in the phantom experiencing different amounts of heating. The temperature curves measured by the fiber optic probes and the temperature calculated from the temperature imaging using an ROI of 4 voxels (0.5 cm³) around the probe tip (blue = fiber optic probe 1, red = fiber optic probe 2) are displayed. (b) Example sagittal slice of temperature map for t = 30 min (after onset of imaging/heating), approximate locations of ROIs at probe tips marked with circles. Note the red circle marks the location of the one probe tip located within this slice—the blue circle marks the point within the slice nearest to the second probe tip. (c) Water image used to locate probe tip, with the approximate location of one fiber optic probe marked with red circle.

FIG. 6. Local RF coil heating experiment

(a) Photograph of the phantom with the RF loop coil used for the local heating experiment. (b) Relative temperature maps shown in sagittal, axial and coronal views at t = 10 min after RF heating. The white bar indicates the approximate position of the RF loop coil