Measuring Cranial Soft Tissue Thickness with MRI or Force-Compensated Tracked Ultrasound

Aims: A new approach to patient tracking in cranial stereotactic radiosurgery relies on contact-free localisation of the cranial bone. It requires accurate information about the soft tissue thickness on the patient's forehead, which in this work is measured using two independent modalities: magnetic resonance imaging (MRI) and force-compensated tracked ultrasound.
Methodology: High resolution MRI scans and ultrasound data of the forehead were recorded and the soft tissue thickness was extracted. The datasets were registered using the iterative closest point algorithm with high accuracy (RMS error < 0.5 mm after artefacts from data acquisition were removed). Tissue deformation was analysed using a robotic setup with force control where the ultrasound transducer was pressed against the skin.
Results: The force compensation setup showed that a tissue compression factor of 0.75 can be assumed for typically applied forces of 7-10N. This factor was confirmed by comparing histograms of soft tissue thickness. Comparing soft tissue thickness as measured by MRI and ultrasound showed a mean error of 0.14mm and a standard deviation of 0.87mm.
Conclusion: We could show that, using MRI as a ground truth, data from 2D ultrasound can be compensated for pressure and can also be used to generate realistic values of soft tissue thickness.