A medical instrument (100, 600) comprising, - a medical imaging system (102, 102') for acquiring medical imaging data (400, 400') from a subject within an imaging zone; - a subject support with a support surface, wherein the support surface is configured for receiving the subject, wherein the subject support is configured for supporting the subject in an initial position wherein in the initial position the subject is outside of the imaging zone; - a camera system configured for imaging the support surface when the subject support is in the initial position; - a memory comprising machine executable instructions; and - a processor for controlling the medical instrument, wherein execution of the machine executable instructions causes the processor to: - place the subject support in the initial position; - control the camera system to repeatably acquire a series of repeated images; - detect the placement of one or more background objects that at least partially obscure the support surface within the series of repeated images; - detect one or more foreground objects that obscure at least a portion of the one or more background objects in the series of repeated images; - construct a background object surface image at least partially by stitching together the series of repeated images to replace image regions containing background objects obscured by the one or more foreground objects; - determine a three-dimensional object surface using the background object surface image; - detect the subject in one of the series of repeated images; - calculate a subject segmentation of the subject in the one of the series of repeated images; - determine a visible subject surface using the subject segmentation and the one of the series of repeated images; and - calculate a three-dimensional subject model by estimating a volume defined by the three-dimensional object surface and the visible subject surface. - a medical imaging system (102, 102') for acquiring medical imaging data (400, 400') from a subject within an imaging zone; - a subject support with a support surface, wherein the support surface is configured for receiving the subject, wherein the subject support is configured for supporting the subject in an initial position wherein in the initial position the subject is outside of the imaging zone; - a camera system configured for imaging the support surface when the subject support is in the initial position; - a memory comprising machine executable instructions; and - a processor for controlling the medical instrument, wherein execution of the machine executable instructions causes the processor to: - place the subject support in the initial position; - control the camera system to repeatably acquire a series of repeated images; - detect the placement of one or more background objects that at least partially obscure the support surface within the series of repeated images; - detect one or more foreground objects that obscure at least a portion of the one or more background objects in the series of repeated images; - construct a background object surface image at least partially by stitching together the series of repeated images to replace image regions containing background objects obscured by the one or more foreground objects; - determine a three-dimensional object surface using the background object surface image; - detect the subject in one of the series of repeated images; - calculate a subject segmentation of the subject in the one of the series of repeated images; - determine a visible subject surface using the subject segmentation and the one of the series of repeated images; and - calculate a three-dimensional subject model by estimating a volume defined by the three-dimensional object surface and the visible subject surface. - place the subject support in the initial position; - control the camera system to repeatably acquire a series of repeated images; - detect the placement of one or more background objects that at least partially obscure the support surface within the series of repeated images; - detect one or more foreground objects that obscure at least a portion of the one or more background objects in the series of repeated images; - construct a background object surface image at least partially by stitching together the series of repeated images to replace image regions containing background objects obscured by the one or more foreground objects; - determine a three-dimensional object surface using the background object surface image; - detect the subject in one of the series of repeated images; - calculate a subject segmentation of the subject in the one of the series of repeated images; - determine a visible subject surface using the subject segmentation and the one of the series of repeated images; and - calculate a three-dimensional subject model by estimating a volume defined by the three-dimensional object surface and the visible subject surface.