A CT based correction method for speed of sound aberration for ultrasound based image guided radiotherapy

Purpose: To introduce a correction for speed of sound (SOS) aberrations in three dimensional (3D) ultrasound (US) imaging systems for small but systematic positioning errors in image guided radiotherapy (IGRT) applications. US waves travel at different speeds in different human tissues. Conventional US-based imaging systems assume that SOS is constant in all tissues at 1540 m/s which is an accepted average value for soft tissues. This assumption leads to errors of up to a few millimeters when converting echo times into distances and is a source of systematic errors and image distortion in quantitative US imaging. Methods: At simulation, US applications for IGRT provide a computed tomography (CT) image coregistered to a US volume. The CT scan provides the physical density which can be used in an empirical relationship with SOS. This can be used to correct for different SOS in different tissues within the patient. For each US scan line each voxel's axial dimension is rescaled according to the SOS associated to it. This SOS correction method was applied to US scans of a PMMA container filled with either water, a 20% saline water solution or sunflower oil, and the results were compared to the CT. The correction was also applied to an US quality assurance (QA) phantom containing rods with high ultrasound contrast. This phantom was scanned with US through a container filled with the same three liquids. Finally, the algorithm was applied to two clinical cases: a prostate cancer patient and a breast cancer patient. Results: After the correction was applied to the phantom images, spatial registration between the bottom of the phantom in the US scan and in the CT scan was improved; the difference was reduced from a few millimeters to less than one millimeter for all three different liquids. Reference structures in the QA phantom appeared at more closely corresponding depths in the three cases after the correction, within 0.5 mm. Both clinical cases showed small shifts, up to 3 mm, in the positions of anatomical structures after correction. Conclusions: The SOS correction presented increases quantitative accuracy in US imaging which may lead to small but systematic improvements in patient positioning.