title: Real-time tracking of liver motion and deformation using a flexible needle.
abstract: OBJECTIVE
  A real-time 3D image guidance system is needed to facilitate treatment of liver masses using radiofrequency ablation, for example. This study investigates the feasibility and accuracy of using an electromagnetically tracked flexible needle inserted into the liver to track liver motion and deformation.
  
  
  METHODS
  This proof-of-principle study was conducted both ex vivo and in vivo with a CT scanner taking the place of an electromagnetic tracking system as the spatial tracker. Deformations of excised livers were artificially created by altering the shape of the stage on which the excised livers rested. Free breathing or controlled ventilation created deformations of live swine livers. The positions of the needle and test targets were determined through CT scans. The shape of the needle was reconstructed using data simulating multiple embedded electromagnetic sensors. Displacement of liver tissues in the vicinity of the needle was derived from the change in the reconstructed shape of the needle.
  
  
  RESULTS
  The needle shape was successfully reconstructed with tracking information of two on-needle points. Within 30 mm of the needle, the registration error of implanted test targets was 2.4 ± 1.0 mm ex vivo and 2.8 ± 1.5 mm in vivo.
  
  
  CONCLUSIONS
  A practical approach was developed to measure the motion and deformation of the liver in real time within a region of interest. The approach relies on redesigning the often-used seeker needle to include embedded electromagnetic tracking sensors. With the nonrigid motion and deformation information of the tracked needle, a single- or multimodality 3D image of the intraprocedural liver, now clinically obtained with some delay, can be updated continuously to monitor intraprocedural changes in hepatic anatomy. This capability may be useful in radiofrequency ablation and other percutaneous ablative procedures.
authors.affiliation: Department of Diagnostic Radiology and Nuclear Medicine, University of Maryland School of Medicine, Baltimore, MD, USA.
authors.forename: Peng
authors.initials: P
authors.lastname: Lei
authors.forename: Fred
authors.initials: F
authors.lastname: Moeslein
authors.forename: Bradford J
authors.initials: BJ
authors.lastname: Wood
authors.forename: Raj
authors.initials: R
authors.lastname: Shekhar
mesh_terms: D000818:Animals; D017115:Catheter Ablation; D055590:Electromagnetic Phenomena; D005240:Feasibility Studies; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional; D008107:Liver Diseases / Q000601:surgery*; D023421:Models, Animal; D009038:Motion; D009339:Needles; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D025321:Surgery, Computer-Assisted / Q000379:methods; D013552:Swine; D014057:Tomography, X-Ray Computed