title: Assessment of pixel shift in ultrasound images due to local temperature changes during the laser interstitial thermotherapy of liver: in vitro study.
abstract: Laser interstitial thermotherapy (LITT) is an internal ablation therapy consisting of percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with maximum diameter of 5 cm. It is very important to control the temperature increase, because tissue carbonization occurs with high temperatures, which can damage normal tissues. In this research, pixel shift changes on ultrasound B-mode images with temperature changes were measured. LITT in vitro was performed on 12 freshly excised sheep liver tissues using a Nd:YAG laser with a bare-tip optical fiber. The 1 W power setting was used for 700 s exposure time (2477 J/mm2). Invasive temperature monitoring was performed during the heating and cooling by attaching microthermocouples to the tissue. At the same time, ultrasound B-mode images were saved on the computer for each 5 degrees C temperature increase from 25 degrees C to 100 degrees C, for noninvasive temperature monitoring. These pixel shifts were measured by an echo-tracking algorithm. Linear and nonlinear regression analyses between the independent variable (temperature change) and the dependent variable (pixel shift on images) were performed. Regression functions and correlation coefficients were determined. It was shown that with a correlation coefficient of 0.998, the cubic function was suitable. Pixel shift increased for each 5 degrees C temperature increase and the maximum shift was observed during 60 to 70 degrees C. Beyond these temperatures, the pixel shift decreased. In this method, because of evaporation of tissue water and bubble formation and tissue carbonization, monitoring greater than 100 degrees C was difficult. It is possible to monitor temperature changes on the ultrasound B-mode images in interstitial laser thermotherapy of liver. Also, with the improvement of image processing, this method could be used for noninvasive temperature monitoring for a large number of patients during LITT.
authors.affiliation: Medical Physics Department, Tarbiat Modares University, Tehran, Iran. mokhtarm@modares.ac.ir
authors.forename: Manijhe
authors.initials: M
authors.lastname: Mokhtari-Dizaji
authors.forename: Tina
authors.initials: T
authors.lastname: Gorjiara
authors.forename: Hossein
authors.initials: H
authors.lastname: Ghanaati
mesh_terms: D000465:Algorithms; D000818:Animals; D001831:Body Temperature / Q000502:physiology; D006979:Hyperthermia, Induced / Q000379:methods*; D007091:Image Processing, Computer-Assisted / Q000379:methods; D053685:Laser Therapy; D008099:Liver / Q000000981:diagnostic imaging* / Q000503:physiopathology; D008113:Liver Neoplasms / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000628:therapy*; D008991:Monitoring, Physiologic / Q000379:methods; D012044:Regression Analysis; D012756:Sheep; D014463:Ultrasonography