A medical apparatus comprising: - an ultrasound transmitter (444, 602) and receiver (446, 604) system for acquiring ultrasound data, wherein the ultrasound data is descriptive of the speed of ultrasound (304, 306) along at least two paths (606, 1114) in a subject (430, 1100); - a medical imaging system for acquiring medical image data descriptive of the at least two paths in the subject; - a processor for controlling the medical apparatus; - a memory containing machine executable instructions (490, 492, 494, 496, 498, 500, 502, 504), wherein execution of the instructions causes the processor to: - acquire (100, 200) the medical image data using the medical imaging system; - acquire (102, 202) the ultrasound data using the ultrasound transmitter and receiver system; - segment (104, 204) the medical image data identifying at least two tissue types (434, 436, 610, 612, 708, 710); - determine (106, 206) at least two distances in the subject for each of the at least two paths in the subject, wherein the at least two distances are equal to the distance traveled in the at least two tissue types by ultrasound generated by the ultrasound transmitter and receiver system; - calculate (108, 208) the speed of ultrasound in each of the at least two tissue types in accordance with the ultrasound data and with the at least two distances for each of the at least two paths, wherein the medical apparatus further comprises a high intensity focused ultrasound system for sonicating a target zone within the subject, wherein the high intensity focused ultrasound system comprises an ultrasound transducer for generating ultrasonic energy, wherein the ultrasound transducer comprises multiple ultrasound transducer elements (432, 802, 1102), wherein the phase of each of the multiple ultrasound transducer elements is controllable; wherein execution of the instructions further cause the processor to calculate a set of transducer element phases in accordance with the segmented medical image data and the speed of ultrasound in each of the at least two tissue types. - an ultrasound transmitter (444, 602) and receiver (446, 604) system for acquiring ultrasound data, wherein the ultrasound data is descriptive of the speed of ultrasound (304, 306) along at least two paths (606, 1114) in a subject (430, 1100); - a medical imaging system for acquiring medical image data descriptive of the at least two paths in the subject; - a processor for controlling the medical apparatus; - a memory containing machine executable instructions (490, 492, 494, 496, 498, 500, 502, 504), wherein execution of the instructions causes the processor to: - acquire (100, 200) the medical image data using the medical imaging system; - acquire (102, 202) the ultrasound data using the ultrasound transmitter and receiver system; - segment (104, 204) the medical image data identifying at least two tissue types (434, 436, 610, 612, 708, 710); - determine (106, 206) at least two distances in the subject for each of the at least two paths in the subject, wherein the at least two distances are equal to the distance traveled in the at least two tissue types by ultrasound generated by the ultrasound transmitter and receiver system; - calculate (108, 208) the speed of ultrasound in each of the at least two tissue types in accordance with the ultrasound data and with the at least two distances for each of the at least two paths, wherein the medical apparatus further comprises a high intensity focused ultrasound system for sonicating a target zone within the subject, wherein the high intensity focused ultrasound system comprises an ultrasound transducer for generating ultrasonic energy, wherein the ultrasound transducer comprises multiple ultrasound transducer elements (432, 802, 1102), wherein the phase of each of the multiple ultrasound transducer elements is controllable; wherein execution of the instructions further cause the processor to calculate a set of transducer element phases in accordance with the segmented medical image data and the speed of ultrasound in each of the at least two tissue types. - acquire (100, 200) the medical image data using the medical imaging system; - acquire (102, 202) the ultrasound data using the ultrasound transmitter and receiver system; - segment (104, 204) the medical image data identifying at least two tissue types (434, 436, 610, 612, 708, 710); - determine (106, 206) at least two distances in the subject for each of the at least two paths in the subject, wherein the at least two distances are equal to the distance traveled in the at least two tissue types by ultrasound generated by the ultrasound transmitter and receiver system; - calculate (108, 208) the speed of ultrasound in each of the at least two tissue types in accordance with the ultrasound data and with the at least two distances for each of the at least two paths, wherein the medical apparatus further comprises a high intensity focused ultrasound system for sonicating a target zone within the subject, wherein the high intensity focused ultrasound system comprises an ultrasound transducer for generating ultrasonic energy, wherein the ultrasound transducer comprises multiple ultrasound transducer elements (432, 802, 1102), wherein the phase of each of the multiple ultrasound transducer elements is controllable; wherein execution of the instructions further cause the processor to calculate a set of transducer element phases in accordance with the segmented medical image data and the speed of ultrasound in each of the at least two tissue types.