This question was open ended in the sense that no specific imaging modality or anatomical information was requested. Internal volume of the heart would probably be referring to the volume of the ventricles, which, when calculated during different phases of the cardiac cycle can give information such as ejection fraction, which is useful in assessing cardiac function. Typically a modality that gives soft tissue data (such as MRI) would be used so that each cross section would show the difference between different tissues (ie, myocardium, blood, etc). These slices can be segmented manually or automatically to detect the borders of these different tissues. The myocardium surrounds the 'internal volume' of the heart, so it can be segmented in each slice. The segmented areas can be integrated along whatever axis the cross sections occur to determine the internal volume.