The present disclosure provides a system comprising a SPECT or PET device; a CT device; and a computer comprising memory and a processor in communication with the memory, the memory comprising a computer application program for a method of performing dosimetric analysis of an organ. The computer application program is executable by the processor to perform the method. The method comprising receiving single photon emission computed tomography (SPECT) or positron emission tomography (PET) images at time instances, the SPECT or PET images relating to the organ. The method then receives a computed tomography (CT) image at one of the time instances, the CT image relating to the organ. Virtual CT images are then generated at the other time instances based on the received SPECT or PET images and the CT image. An absorbed dose of ionising radiation on the organ can then be measured based on the received SPECT or PET images, the received CT image, and the generated virtual CT images. The method generates the virtual CT images using any one of: SPECT to SPECT (or PET to PET) registration, CT to SPECT (PET) registration, and SPECT (PET) to CT registration.