title: A new method for tracking organ motion on diagnostic ultrasound images.
abstract: OBJECTIVE
  Respiratory-gated irradiation is effective in reducing the margins of a target in the case of abdominal organs, such as the liver, that change their position as a result of respiratory motion. However, existing technologies are incapable of directly measuring organ motion in real-time during radiation beam delivery. Hence, the authors proposed a novel quantitative organ motion tracking method involving the use of diagnostic ultrasound images; it is noninvasive and does not entail radiation exposure. In the present study, the authors have prospectively evaluated this proposed method.
  
  
  METHODS
  The method involved real-time processing of clinical ultrasound imaging data rather than organ monitoring; it comprised a three-dimensional ultrasound device, a respiratory sensing system, and two PCs for data storage and analysis. The study was designed to evaluate the effectiveness of the proposed method by tracking the gallbladder in one subject and a liver vein in another subject. To track a moving target organ, the method involved the control of a region of interest (ROI) that delineated the target. A tracking algorithm was used to control the ROI, and a large number of feature points and an error correction algorithm were used to achieve long-term tracking of the target. Tracking accuracy was assessed in terms of how well the ROI matched the center of the target.
  
  
  RESULTS
  The effectiveness of using a large number of feature points and the error correction algorithm in the proposed method was verified by comparing it with two simple tracking methods. The ROI could capture the center of the target for about 5 min in a cross-sectional image with changing position. Indeed, using the proposed method, it was possible to accurately track a target with a center deviation of 1.54±0.9 mm. The computing time for one frame image using our proposed method was 8 ms. It is expected that it would be possible to track any soft-tissue organ or tumor with large deformations and changing cross-sectional position using this method.
  
  
  CONCLUSIONS
  The proposed method achieved real-time processing and continuous tracking of the target organ for about 5 min. It is expected that our method will enable more accurate radiation treatment than is the case using indirect observational methods, such as the respiratory sensor method, because of direct visualization of the tumor. Results show that this tracking system facilitates safe treatment in clinical practice.
authors.affiliation: Heavy Ion Medical Center, Gunma University, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan.
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Kubota
authors.affiliation: Heavy Ion Medical Center, Gunma University, 3-39-22 Showa-machi, Maebashi, Gunma 371-8511, Japan.
authors.forename: Akihiko
authors.initials: A
authors.lastname: Matsumura
authors.affiliation: Research Center of Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
authors.forename: Mai
authors.initials: M
authors.lastname: Fukahori
authors.affiliation: Medical Physics Section, Kanagawa Cancer Center, 1-1-2 Nakao, Asahi-ku, Yokohama 241-8515, Japan.
authors.forename: Shin-ichi
authors.initials: S
authors.lastname: Minohara
authors.affiliation: Research Center Hospital of Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan.
authors.forename: Shigeo
authors.initials: S
authors.lastname: Yasuda
authors.affiliation: Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama 226-8503, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Nagahashi
mesh_terms: D000465:Algorithms; D005704:Gallbladder / Q000000981:diagnostic imaging; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation / Q000379:methods; D008099:Liver / Q000098:blood supply / Q000000981:diagnostic imaging; D009038:Motion; D011446:Prospective Studies; D012119:Respiration; D013995:Time; D014463:Ultrasonography / Q000295:instrumentation / Q000379:methods*; D014680:Veins / Q000000981:diagnostic imaging