A magnet assembly for an apparatus that combines MR imaging with radiation therapy comprising a cryostat with an innner chamber, that includes : first and second annular sections separated and spaced apart from each other along a first direction, the first and second annular sections having disposed therein corresponding first and second annular volumes of a cryogenic fluid , and a third annular section extending in the first direction between the first and second annular sections and connecting the first and second sections to each other allowing for thermal communication between the first and second annular sections of the inner chamber by an exchange of cryogenic fluid between the first and second annular sections , the third annular section having a third annular volume of the cryogenic fluid disposed therein and superconducting coils for maintaining a uniform magnetic field for proper MR imaging disposed in the first and second annular sections, wherein the depth of the third annular volume of cryogenic fluid perpendicular to the first direction is less than the depth of the first annular volume of cryogenic fluid and the depth of the second annular volume of cryogenic fluid, and wherein an internal width of the first annular section in a plane perpendicular to the first direction and an internal width of the second annular section in a plane perpendicular to the first direction are each more than ten times an internal width of the third annular section in a plane perpendicular to the first direction, so that a radiotherapy beam can be configured to pass through the third annular section of the cryostat and where the internal width (W1, W2, W3) being the cross-sectional dimension of the interior of the inner chamber in the respective first, second, and third annular section , respectively. first and second annular sections separated and spaced apart from each other along a first direction, the first and second annular sections having disposed therein corresponding first and second annular volumes of a cryogenic fluid , and a third annular section extending in the first direction between the first and second annular sections and connecting the first and second sections to each other allowing for thermal communication between the first and second annular sections of the inner chamber by an exchange of cryogenic fluid between the first and second annular sections , the third annular section having a third annular volume of the cryogenic fluid disposed therein and superconducting coils for maintaining a uniform magnetic field for proper MR imaging disposed in the first and second annular sections, wherein the depth of the third annular volume of cryogenic fluid perpendicular to the first direction is less than the depth of the first annular volume of cryogenic fluid and the depth of the second annular volume of cryogenic fluid, and wherein an internal width of the first annular section in a plane perpendicular to the first direction and an internal width of the second annular section in a plane perpendicular to the first direction are each more than ten times an internal width of the third annular section in a plane perpendicular to the first direction, so that a radiotherapy beam can be configured to pass through the third annular section of the cryostat and where the internal width (W1, W2, W3) being the cross-sectional dimension of the interior of the inner chamber in the respective first, second, and third annular section , respectively. <EOS>