This work describes the design and implementation of a system for liver tumor ablation guided by ultrasound. Features of the system include spatially registered ultrasound visualization, ultrasound volume reconstruction, and interactive targeting. Early results with phantom experiments indicate a targeting accuracy of 5-10mm. The system serves as a foundation for further clinical studies and applications of image-guided therapy to liver procedures.
Endorectal ultrasound is currently the gold standard for the staging of rectal cancer; however, the accurate staging of the disease requires extensive training and is difficult, especially for those clinicians who do not see a large number of patients per year. Therefore, there is a need for a semi-automatic staging system to assist the clinicians in the accurate staging of rectal cancer. We believe that the unwrapping of the circular ERUS images captured by a spatially tracked ERUS system is a step in this direction. The steps by which a 2D image can be unwrapped are described thereby allowing the circular layers of the rectal wall to be displayed as flat layers stacked on top of each other. We test the unwrapping process using images from a cylindrical rectal phantom and a human rectum. The process of unwrapping endorectal ultrasound images qualitatively provides good visualization of the layers of the rectal wall and rectal tumors and supports the continual study of this novel staging system.
We calibrate a tracked laparoscopic ultrasound probe for application in image-guided surgery and 3-D volume reconstruction. With a plane-mapping technique, the spatial relationship between the ultrasound beam emitted from the tip of the probe to the local coordinate system of the probe was determined by mapping it with an optically tracked pointer. A cross-wire calibration technique was also performed for comparison. The accuracy and precision of the calibrated probe was evaluated by measuring its ability to localize targets in a water bath. Target registration error depended upon probe position, varying from an average 0.88 mm for the fixed probes to 6.09 mm for a moving probe. This error can be reduced to 4.54 mm by accounting for target localization error which is the error determining the position of the probe itself. These results validate the plane-mapping calibration technique for this type of ultrasound probe, and better probe tracking is expected to reduce the overall registration error.
Endorectal Ultrasound (ERUS) is essential for the accurate staging of rectal cancer. Staging is important to the treatment of patients with rectal cancer because it will determine whether the patient receives preoperative radiotherapy for the purpose of tumor downstaging. ERUS images are intrinsically different from images taken by Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) in that ultrasound provides 2D images while CT and MRI provide 3D data sets that can be rendered into volumes and then re-sliced and viewed as 2D images in any desired orientation. This fundamental difference between ultrasound and tomographic imaging modalities creates a problem when a direct comparison between ultrasound and CT or MRI is desired. To accomplish the goal of following tumor volume over time, an accurate ultrasound volume must be constructed. By optically tracking the ERUS probe as data is collected, the intensity value for each pixel is saved and then inserted into the nearest voxel in the ERUS volume matrix. We validate the accuracy of volume reconstruction by finding the 3D coordinates of targets that are inside of the ERUS volume and comparing them to their known physical locations.