IMAGE REGISTRATION ACCURACY WITH LOW-DOSE CT: HOW LOW CAN WE GO? Omkar Dandekar 1,2 , Khan Siddiqui 3 , Vivek Walimbe 4,5 , Raj Shekhar 2 1 Department of Electrical and Computer Engineering, University of Maryland, College Park, MD 20740 2 Department of Diagnostic Radiology, University of Maryland, Baltimore, MD 21201 3 VA Maryland Health Care System, Baltimore, MD 21201 4 Biomedical Engineering Center, The Ohio State University, Columbus, OH 43210 5 Dept of Biomedical Engineering, The Cleveland Clinic Foundation, Cleveland, OH, 44195 ABSTRACT Image-guided interventions are known to lead to improved outcomes and significantly faster patient recovery as compared with conventional open, invasive procedures. Common intraoperative imaging techniques such as endoscopy and fluoroscopy are two-dimensional (2D), and provide a 2D representation of the 3D anatomy. Use of recently emerged multislice computed tomography (CT) can facilitate 3D visualization of anatomy during an intervention. The speed and dimensionality of these CT scanners make their use in image-guided interventions technically feasible. For clinical acceptance, however, the net radiation dose to the patient and the interventionist must be minimized. This article suggests a strategy to reduce radiation dose and describes an evaluation scheme to identify the optimal dose which does not sacrifice the specificity of the image-guided procedure. Our work indicates at least a tenfold reduction in radiation dose. 1. INTRODUCTION Minimal invasiveness is the defining feature and the primary motivator for the adoption of image-guided interventions. Other equally important advantages of image-guided interventions are improved outcomes, shorter procedures and a quick recovery [1]. The current use of image guidance can be found in neurosurgery, focal cancer ablative therapies, and radiosurgeries and radiotherapies. In many clinical applications, image- guided intervention has become the standard of care (e.g., laparoscopic cholecystectomy). Image-guided procedures are usually planned on preoperative CT/magnetic resonance (MR) images. However, due to normal tissue motion, the anatomy at the time of the surgical procedure generally differs from the anatomy at the time of planning. Such misregistration between pre- and intraoperative anatomy renders the planned treatment and navigation, based solely on preoperative data, inaccurate and unsafe. To compensate for this misregistration, the anatomy needs re-imaging immediately prior to and during the intervention. Registering preoperative images then with the intraoperatively collected images provides valuable information for updating operative/surgical navigation plan. An interesting aspect of this approach is a possibility to augment the current anatomy, with supplementary information (such as contrast) from the preoperative scan, which will tremendously aid navigation and localization. With the advent of multislice CT (up to 64), intra- operative imaging is now possible with computed tomography. Comparatively, magnetic resonance imaging (MRI) continues to lag in speed, while use of ultrasound is limited due to poor image quality and, in image-guided surgeries specifically, the difficulty of scanning across the pneumoperitoneum with CO 2 insufflation. Radiation exposure to the patient and the interventionist, however, continues to be a concern with using CT, and must be minimized. Our primary radiation dose reduction strategy is to acquire a standard-dose CT image preoperatively and scan the dynamic operative field subsequently using low- dose CT. Using high-speed nonrigid 3D image registration techniques [2] our group has developed, the preoperative CT image can be registered to low-dose intraoperative CT images. Registered preoperative CT will show the dynamic intraoperative anatomy and will substitute the low-dose CT images. These diagnostic quality images can be 3D rendered and used for intraoperative guidance and navigation. Capability of viewing hidden structures using CT together with the additional capability of virtually inserting tracked tools in the 3D renderings will add a new dimension to image- guided interventions. Our proposed dose reduction strategy necessitates the determination of the lowest acceptable dose for intraoperative CT, which permits accurate image registration. This article describes an evaluation scheme to judge the registration accuracy with low-dose CT. The following sections describe this method and the results in detail. 502 0-7803-9577-8/06/$20.00 ©2006 IEEE ISBI 2006