title: Feasibility of non-invasive temperature estimation by the assessment of the average gray-level content of B-mode images.
abstract: This paper assesses the potential of the average gray-level (AVGL) from ultrasonographic (B-mode) images to estimate temperature changes in time and space in a non-invasive way. Experiments were conducted involving a homogeneous bovine muscle sample, and temperature variations were induced by an automatic temperature regulated water bath, and by therapeutic ultrasound. B-mode images and temperatures were recorded simultaneously. After data collection, regions of interest (ROIs) were defined, and the average gray-level variation computed. For the selected ROIs, the AVGL-Temperature relation were determined and studied. Based on uniformly distributed image partitions, two-dimensional temperature maps were developed for homogeneous regions. The color-coded temperature estimates were first obtained from an AVGL-Temperature relation extracted from a specific partition (where temperature was independently measured by a thermocouple), and then extended to the other partitions. This procedure aimed to analyze the AVGL sensitivity to changes not only in time but also in space. Linear and quadratic relations were obtained depending on the heating modality. We found that the AVGL-Temperature relation is reproducible over successive heating and cooling cycles. One important result was that the AVGL-Temperature relations extracted from one region might be used to estimate temperature in other regions (errors inferior to 0.5 °C) when therapeutic ultrasound was applied as a heating source. Based on this result, two-dimensional temperature maps were developed when the samples were heated in the water bath and also by therapeutic ultrasound. The maps were obtained based on a linear relation for the water bath heating, and based on a quadratic model for the therapeutic ultrasound heating. The maps for the water bath experiment reproduce an acceptable heating/cooling pattern, and for the therapeutic ultrasound heating experiment, the maps seem to reproduce temperature profiles consistent with the pressure field of the transducer, and in agreement with temperature maps developed by COMSOL®MultiPhysics simulations.
authors.affiliation: Centro de Informática e Sistemas, Polo II, Departamento de Engenharia Informática, Pinhal de Marrocos, Universidade de Coimbra, 3030-290 Coimbra, Portugal. Electronic address: cteixei@dei.uc.pt.
authors.forename: C A
authors.initials: CA
authors.lastname: Teixeira
authors.affiliation: Laboratory of Ultrasound/National Institute of Metrology, Standardization and Industrial Quality (Inmetro), Duque de Caxias, Brazil.
authors.forename: A V
authors.initials: AV
authors.lastname: Alvarenga
authors.affiliation: Laboratorio de Acústica Ultrasonora, Facultad de Ciencias, Universidad de la República, Iguá 4225, Montevideo 11400, Uruguay.
authors.forename: G
authors.initials: G
authors.lastname: Cortela
authors.affiliation: Biomedical Eng. Program/COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil.
authors.forename: M A
authors.initials: MA
authors.lastname: von Krüger
authors.affiliation: Biomedical Eng. Program/COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil.
authors.forename: W C A
authors.initials: WC
authors.lastname: Pereira
mesh_terms: D000818:Animals; D002417:Cattle; D004867:Equipment Design; D005240:Feasibility Studies; D006358:Hot Temperature; D007091:Image Processing, Computer-Assisted; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging*; D015203:Reproducibility of Results; D062125:Thermometry / Q000295:instrumentation / Q000379:methods*; D014159:Transducers; D014464:Ultrasonic Therapy / Q000295:instrumentation / Q000379:methods*; D014463:Ultrasonography; D014867:Water
chemical_list: D014867:Water
keywords: Average gray-level variation; B-mode image processing; Non-invasive temperature estimation