Magnitude of speed of sound aberration corrections for ultrasound image guided radiotherapy for prostate and other anatomical sites.

PURPOSE The purpose of this work is to assess the magnitude of speed of sound (SOS) aberrations in three-dimensional ultrasound (US) imaging systems in image guided radiotherapy. The discrepancy between the fixed SOS value of 1540 m∕s assumed by US systems in human soft tissues and its actual nonhomogeneous distribution in patients produces small but systematic errors of up to a few millimeters in the positions of scanned structures. METHODS A correction, provided by a previously published density-based algorithm, was applied to a set of five prostate, five liver, and five breast cancer patients. The shifts of the centroids of target structures and the change in shape were evaluated. RESULTS After the correction the prostate cases showed shifts up to 3.6 mm toward the US probe, which may explain largely the reported positioning discrepancies in the literature on US systems versus other imaging modalities. Liver cases showed the largest changes in volume of the organ, up to almost 9%, and shifts of the centroids up to more than 6 mm either away or toward the US probe. Breast images showed systematic small shifts of the centroids toward the US probe with a maximum magnitude of 1.3 mm. CONCLUSIONS The applied correction in prostate and liver cancer patients shows positioning errors of several mm due to SOS aberration; the errors are smaller in breast cancer cases, but possibly becoming more important when breast tissue thickness increases.