A medical instrument (300, 400) comprising: - a magnetic resonance imaging system for acquiring magnetic resonance data (342, 354) from an imaging zone and; - a high intensity focused ultrasound system with an adjustable focus, - a processor for controlling the medical instrument; and - a memory containing machine readable instructions (360, 362, 364, 450, 452, 454, 456) for execution by the processor; wherein execution of the instructions causes the processor to - acquire (100, 202) first magnetic resonance data with the magnetic resonance imaging system, - reconstruct (102, 204) a first magnetic resonance image (344, 502) using the first magnetic resonance data, wherein from the first magnetic resonance image a treatment plan is formed for controlling the high intensity focused ultrasound system and the formation of the treatment plan includes identification of one or more graphical objects in the first magnetic resonance image, wherein execution of the instructions further causes the processor to - receive (104, 206) a registration of the one or more graphical objects (346, 510, 512) to the first magnetic resonance image, wherein the registration defines spatial positions of the one or more graphical objects with respect to the first magnetic resonance image, and wherein execution of the instructions further causes the processor to repeatedly: - acquire (106, 210) second magnetic resonance data using the magnetic resonance imaging system; - reconstruct (108, 212) a second magnetic resonance image (356, 502') using the second magnetic resonance data; - receive (110, 214) repositioning coordinates (358, 700) in the second magnetic resonance image for a first group (348, 510) selected from the one or more graphical objects, wherein the repositioning coordinates describe a repositioning of the first group in the second magnetic resonance image with respect to the first magnetic resonance image; and - determine (112, 216) a coordinate transformation (359, 702) of a second group (350, 512) selected from the one or more graphical objects by applying a coordinate transformation model to the repositioning coordinates and wherein execution of the instructions further causes the processor to repeatedly modify the treatment plan using the repositioning coordinates and the coordinate transformation and - control the high intensity focused ultrasound system in accordance with the modified treatment plan. - a magnetic resonance imaging system for acquiring magnetic resonance data (342, 354) from an imaging zone and; - a high intensity focused ultrasound system with an adjustable focus, - a processor for controlling the medical instrument; and - a memory containing machine readable instructions (360, 362, 364, 450, 452, 454, 456) for execution by the processor; wherein execution of the instructions causes the processor to - acquire (100, 202) first magnetic resonance data with the magnetic resonance imaging system, - reconstruct (102, 204) a first magnetic resonance image (344, 502) using the first magnetic resonance data, wherein from the first magnetic resonance image a treatment plan is formed for controlling the high intensity focused ultrasound system and the formation of the treatment plan includes identification of one or more graphical objects in the first magnetic resonance image, - acquire (100, 202) first magnetic resonance data with the magnetic resonance imaging system, - reconstruct (102, 204) a first magnetic resonance image (344, 502) using the first magnetic resonance data, wherein from the first magnetic resonance image a treatment plan is formed for controlling the high intensity focused ultrasound system and the formation of the treatment plan includes identification of one or more graphical objects in the first magnetic resonance image, - receive (104, 206) a registration of the one or more graphical objects (346, 510, 512) to the first magnetic resonance image, wherein the registration defines spatial positions of the one or more graphical objects with respect to the first magnetic resonance image, and - acquire (106, 210) second magnetic resonance data using the magnetic resonance imaging system; - reconstruct (108, 212) a second magnetic resonance image (356, 502') using the second magnetic resonance data; - receive (110, 214) repositioning coordinates (358, 700) in the second magnetic resonance image for a first group (348, 510) selected from the one or more graphical objects, wherein the repositioning coordinates describe a repositioning of the first group in the second magnetic resonance image with respect to the first magnetic resonance image; and - determine (112, 216) a coordinate transformation (359, 702) of a second group (350, 512) selected from the one or more graphical objects by applying a coordinate transformation model to the repositioning coordinates and - control the high intensity focused ultrasound system in accordance with the modified treatment plan.