A method for determining respiratory induced blood mass change from a four-dimensional computed tomography, the method comprising: receiving, at a data processing hardware, a four-dimensional computed tomography image set including a first three-dimensional computed tomography image of a thoracic volume and a second three-dimensional computed tomography image of the thoracic volume; determining, at the data processing hardware, a spatial transformation from the first three-dimensional computed tomography image to the second three-dimensional computed tomography image, comprising: executing, at the data processing hardware, a deformable image registration, DIR, function on the first three-dimensional computed tomography image and the second three-dimensional computed tomography image; delineating, by the data processing hardware, a first plurality of subvolumes within the first three-dimensional computed tomography image; warping, by the data processing hardware, each of the first plurality of subvolumes onto the second three-dimensional computed tomography image, wherein the step of warping each of the first plurality of subvolumes includes estimating a blood mass change for each of the subvolumes; determining, at the data processing hardware, a spatial distribution of blood mass change within the thoracic volume based on the spatial transformation; and outputting, from the data processing hardware, a respiratory-induced blood mass change image generated based on the determined spatial distribution of blood mass change. receiving, at a data processing hardware, a four-dimensional computed tomography image set including a first three-dimensional computed tomography image of a thoracic volume and a second three-dimensional computed tomography image of the thoracic volume; determining, at the data processing hardware, a spatial transformation from the first three-dimensional computed tomography image to the second three-dimensional computed tomography image, comprising: executing, at the data processing hardware, a deformable image registration, DIR, function on the first three-dimensional computed tomography image and the second three-dimensional computed tomography image; delineating, by the data processing hardware, a first plurality of subvolumes within the first three-dimensional computed tomography image; warping, by the data processing hardware, each of the first plurality of subvolumes onto the second three-dimensional computed tomography image, wherein the step of warping each of the first plurality of subvolumes includes estimating a blood mass change for each of the subvolumes; executing, at the data processing hardware, a deformable image registration, DIR, function on the first three-dimensional computed tomography image and the second three-dimensional computed tomography image; delineating, by the data processing hardware, a first plurality of subvolumes within the first three-dimensional computed tomography image; warping, by the data processing hardware, each of the first plurality of subvolumes onto the second three-dimensional computed tomography image, wherein the step of warping each of the first plurality of subvolumes includes estimating a blood mass change for each of the subvolumes; determining, at the data processing hardware, a spatial distribution of blood mass change within the thoracic volume based on the spatial transformation; and outputting, from the data processing hardware, a respiratory-induced blood mass change image generated based on the determined spatial distribution of blood mass change. <EOS>