Measuring cranial soft tissue thickness with MRI or pressure-compensated tracked ultrasound

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