A method or computer program for forming a digital model of a patient's teeth. The method includes the steps of: obtaining a plurality of projection images acquired by scanning a negative impression of the patient's teeth using a computed tomographic imaging apparatus; generating, from the plurality of projection images, an air volume model of the air that surrounds the negative impression; defining a transition surface along edges of the air volume model that touch the negative impression; forming the digital model of the patient's teeth according to the defined transition surface; and displaying, storing, or transmitting at least a portion of the digital model of the teeth.