An apparatus for deriving tissue temperature from thermal strain includes a thermal strain measuring module. The module uses ultrasound (156 158) to measure thermal strain in a region within a subject that surrounds a location (166a 166f) where a temperature sensor is disposed. Also included is a temperature measurement module configured for via the sensor measuring a temperature at the sensor while the sensor is inside the subject. Further included is a patient specific thermal strain to temperature change proportionality calibration module. The calibration module is configured for calibrating (S238) a coefficient and for doing so based on a measurement of a temperature parameter at that location derived from output of the temperature measurement module and on a measurement of thermal strain at that location obtained via the strain measuring module. The coefficient is usable in conjunction with a thermal strain measurement derived from another location within the region in evaluating(S242) for that other location another temperature parameter.
