High intensity focused ultrasound (HIFU) is a non-invasive thermal ablation technique. Ablating malignant tissue entirely while preserving as much healthy tissue as possible is challenging, requiring real-time monitoring for optimal results. Magnetic Resonance Imaging (MRI) is the current gold standard for HIFU monitoring, providing reliable temperature information in real-time. However, MRI has several disadvantages including its high cost and its lack of portability. Accordingly, there is motivation to guide thermal ablation procedures with real-time ultrasound (US) monitoring techniques exploiting changes in acoustic properties with rising temperature and ablation progression. As the speed of sound (SOS) changes with the increase in temperature, we propose a new method using an active external US element detecting SOS changes to generate temperature images during HIFU ablation.