Thyroid Volumetry using freehand 3D ultrasound (tomographic Ultrasound): exemplary comparison with 2D model and MRI
Markus Krönke, Resident Physician, Nuclear Medicine, Klinikums rechts der Isar, Technische Universität München, Munich, Germany
Dr. Thomas Wendler, Senior Scientist, Computer Aided Medical Procedures and Augmented Reality, Technische Universität München, Munich, Germany
Thyroid volumetry plays an essential role in the calculation of the applied dose of radioiodine in patients with Morbus Basedow and local autonomies. Currently, three ultrasound (US) measurements taken by hand in two different planes are used to calculate volume using different mathematical models. Literature shows that these measurements do not only show high intra-observer and inter-observer variation, but may also underestimate the overall volume of the thyroid by up to 30%. Tomographic ultrasound (tUS) might be a user-independent alternative that yields smaller errors. In order to validate this, we compared tUS volumetry with the conventional 2D US approach and calculations based on magnetic resonance images (MRI) as the gold standard.
A patient undergoing MRI of the neck was recruited to receive 2D US and tUS examination. MRI (T1 vibe sequence with 0.625×0.625x1mm3 resolution, 1.5 T) and tUS were segmented using an interactive segmentation software that is based on Graph Cuts (Figure 1 & Figure 2). The conventional Brunn formula was used to calculate the thyroid volume with 2D US. The calculated volumes of the thyroid were 13.0, 13.1 and 12.8 cm3 for MRI, tUS and the 2D US respectively. The intra-observer variation in this patient was 5% for 2D US, based on three consecutive measurements.
Thyroid volumetry with tUS and MR showed an excellent match with a difference smaller than 1%. In this particular case, also 2D US showed a small difference to MRI with an average difference of <2% and a standard deviation of 5%. If these results are confirmed on a larger patient collective, tUS could have a major impact on dosimetry and possibly on the effectiveness of radioisotope therapy.