Ultrasonic thermometry by a localized measurement of the longitudinal wave velocity using sound field information

Temperature monitoring during medical interventions such as hyperthermia or radio frequency ablation is a demanding task. An alternative to expensive MRI measurements are ultrasound measurements exploiting the dependence of sound velocity on temperature. In this paper, a new measurement technique is presented that determines the sound velocity spatially resolved in tissue phantoms with the aim of temperature monitoring. The measurements are performed with annular arrays operating element-wise in pulse-echo mode to allow synthetic focusing. Unlike speckle techniques, which also evaluate echoes from scatterers, this method allows to determine the true position of scatterers and absolute values of local sound velocity, rather than evaluating only relative changes. This is achieved by varying the set of delay times used for focusing and determining the set that gives the maximum amplitude of the focused echo. The measurement technique is described in detail and measurement results are shown. The achievable resolution (1mm axial and 0.4mm lateral) and accuracy of 98% are determined. Sound field simulations are used to model the measurement process and vary measurement parameters. Thus, the effect of parameter variations on the achievable accuracy can be evaluated very precisely.