A heat sink parameter determination apparatus for determining a parameter of a heat sink within an object, the heat sink parameter determination apparatus (23, 71) comprising: - a heat source parameter providing unit for providing a parameter of a heat source, which defines a heating of the object, the heat source being an ablation element for ablating a tumor within the object, and the heat sink being a blood vessel within the object which defines a cooling of the object, - a model providing unit for providing a model describing a temperature distribution in the object, while the object is heated by the heat source, wherein the model depends on the heat sink parameter and the heat source parameter, - a heat sink location providing unit for providing a heat sink location and an initial heat sink parameter for initializing the provided model, - a temperature distribution measuring unit (13, 71) for measuring a temperature distribution in the object close to the provided heat sink location, the temperature distribution measuring unit (13, 71) comprising an ultrasound probe configured to acquire ultrasound data in one or more scan planes traversing the heat sink, the one or several scan planes each comprising an intersection area where the heat sink intersects the respective scan plane, and - a parameter determination unit for determining the heat sink parameter by starting the determination of the heat sink parameter with the initialized model, and minimizing a deviation between only parts of the temperature distribution, which have been measured within the one or several scan planes and having a shortest distance to a border of the intersection area that are smaller than a predefined distance from the provided heat sink location, and a modeled temperature distribution, wherein the modeled temperature distribution is modeled based on the provided heat source parameter and the heat sink parameter to be determined by using the provided model. - a heat source parameter providing unit for providing a parameter of a heat source, which defines a heating of the object, the heat source being an ablation element for ablating a tumor within the object, and the heat sink being a blood vessel within the object which defines a cooling of the object, - a model providing unit for providing a model describing a temperature distribution in the object, while the object is heated by the heat source, wherein the model depends on the heat sink parameter and the heat source parameter, - a heat sink location providing unit for providing a heat sink location and an initial heat sink parameter for initializing the provided model, - a temperature distribution measuring unit (13, 71) for measuring a temperature distribution in the object close to the provided heat sink location, the temperature distribution measuring unit (13, 71) comprising an ultrasound probe configured to acquire ultrasound data in one or more scan planes traversing the heat sink, the one or several scan planes each comprising an intersection area where the heat sink intersects the respective scan plane, and - a parameter determination unit for determining the heat sink parameter by starting the determination of the heat sink parameter with the initialized model, and minimizing a deviation between only parts of the temperature distribution, which have been measured within the one or several scan planes and having a shortest distance to a border of the intersection area that are smaller than a predefined distance from the provided heat sink location, and a modeled temperature distribution, wherein the modeled temperature distribution is modeled based on the provided heat source parameter and the heat sink parameter to be determined by using the provided model.