Purpose: One central aspect in ultrasound (US)-guided radiotherapy is the registration between planning US and CT images: for optimal anatomical correspondence, US_ref and CT_ref should be acquired simultaneously. However, especially for abdominal targets, large US probe dimensions and metal artefacts pose challenges for simultaneous acquisition inside the CT bore. In this study, five matrix array probes were compared regarding size and artefact magnitude.
Methods: Five diagnostic probes for real-time volumetric imaging were used: 3V-D, 4V-D, 4Vc-D (GE); X6-1, X5-1 (Philips). Each probe was fixed on a torso phantom (BluePhantom, CAE) in a medial position perpendicular to its surface and subsequently received a CT scan (Somatom DefinitionAS, Siemens) using a standard radiotherapy protocol (voxel size 0.9x0.9x1.0mmÂ³, no artefact reduction). Artefacts were assessed over the entire volume and in different regions of interest (ROIs) relative to a reference scan without probe by calculating total variation (TV) and the percentage of affected voxels with mean absolute differences > 25HU.
Results: Probe lengths were 13.4cm (3V-D), 11.2cm (4V-D), 11.6cm (4Vc-D), 10.0cm (X6-1) and 9.0cm (X5-1). Artefacts were observed for all probes in the axial slices intersecting the probe. TV increase was 4.6% (X6-1), 4.4% (3V-D), 3.7% (4V-D), 2.4% (X5-1) and 1.9% (4Vc-D) over the entire scan area but most pronounced directly below the probe with increases from 166% (4Vc-D) up to 390% (X6-1) . At 15cm depth, this reduced to between 10% (4Vc-D) and 41% (3V-D). For ROIs 6.5cm inferior of the probes, TV increase ranged from 0.1% (4Vc-D) to 3.6% (3V-D). The percentage of affected voxels was 22.1% (3V-D), 19.1% (4V-D), 18.9% (X6-1), 15.0% (X5-1) and 13.9% (4Vc-D).
Conclusion: Especially state-of-the-art probes with smaller dimensions could fit inside the bore and reduce CT artefacts by up to 44%. Combined with artefact-reducing reconstruction, simultaneous CT-US acquisition could potentially increase registration accuracy in US-guided radiotherapy.
Funding Support, Disclosures, and Conflict of Interest: Parts of this work were supported by the German Federal Ministry of Education and Research (grant no. 13GW0228B), the German Research Foundation (grant no. ER 817/1-1), the Ministry of Economic Affairs, Employment, Transport and Technology of Schleswig-Holstein and GE healthcare.