Electromagnetically tracked ultrasound for small animal imaging

The fusion of computed tomography (CT) and ultrasound (US) imaging has recently been examined for guiding interventional procedures such as biopsy, local drug delivery and radiofrequency ablation. During combined CT/US procedures, US facilitates real time visualization of blood flow and tissue structure, while CT provides a three-dimensional view of an extended region and is capable of differentiating tissue types based on Hounsfield units (HU). Our laboratory is working to extend this fusion to include therapeutic US co-registered with CT. Here, we demonstrate the ability to differentiate fat, protein-rich regions, and bone with CT by comparing segmented CT images to histology. Then we present methods for semi-automated registration between tracked US and CT with the objective of imaging and guiding acoustic therapy in small animals. A calibration phantom and fiducial markers were used to generate accurate transformation between the real-time US and pre-acquired CT image stack. The mean error in transducer calibration was 0.7±0.5mm with a maximum error of 1.75mm. Target registration error (TRE) of 1.98+/-1.12mm was observed between US and CT images of a dual-modality phantom. The combined CT/US system provides CT-based guidance with enough accuracy to plan and guide US-induced hyperthermia.