The invention provides for a medical apparatus (100) comprising a high intensity focused ultrasound system (122) and a magnetic resonance imaging system (102). A memory (152) stores machine executable instructions(280, 282, 284, 286) and pulse sequence commands (260) for an acoustic radiation force imaging protocol. The memory further stores first sonication commands(262) and second sonication commands (264) for controlling the high intensity focused ultrasound system to sonicate the sonication region according to the acoustic radiation force imaging protocol. The pulse sequence commands specifies the acquisition of the magnetic resonance data for multiple pulse sequence repetitions. The pulse sequence commands specifies for each of the multiple sequence repetitions a first group of motion encoding gradients (406) and a second group (408) of motion encoding gradients. The execution of the machine executable instructions causes a processor controlling the medical apparatus to: acquire (300) first magnetic resonance data (266) by controlling the magnetic resonance imaging system with the pulse sequence commands and by controlling the high intensity focused ultrasound system with the first sonication commands； acquire (302) second magnetic resonance data (268) by controlling the magnetic resonance imaging system with the pulse sequence commands and by controlling the high intensity focused ultrasound system with the second sonication commands； reconstruct (304) a first motion encoded image (270) from the first magnetic resonance data； reconstruct (306) a second motion encoded image (272) from the second magnetic resonance data； and construct (308) a displacement map (274) from the difference of the first motion encoded image and the second motion encoded image.L'invention concerne un appareil médical (100) comprenant un système d'ultrasons focalisés à haute intensité (122) et un système d'imagerie par résonance magnétique (102). Une mémoire (152) stocke des instruction
