The major hurdles currently preventing advance and innovation in thoracic insufficiency syndrome (TIS) assessment and treatment are the lack of standardizable objective diagnostic measurement techniques that describe the 3D thoracoabdominal structures and the dynamics of respiration. Our goal is to develop, test, and evaluate a quantitative dynamic magnetic resonance imaging (QdMRI) methodology and a biomechanical understanding for deriving key quantitative parameters from free-tidal-breathing dMRI image data for describing the 3D structure and dynamics of the thoracoabdominal organs of TIS patients. In this paper, we propose an idea of a shape sketch to codify and then quantify the overall thoracic architecture, which involves the selection of 3D landmark points and computation of 3D dynamic distances over a respiratory cycle. We perform two statistical analyses of distance sketches on 25 different TIS patients to try to understand the pathophysiological mechanisms in relation to spine deformity and to quantitatively evaluate improvements from pre-operative to post-operative states. This QdMRI methodology involves developing: (1) a 4D image construction method; (2) an algorithm for the 4D segmentation of thoraco-abdominal structures; and (3) a set of key quantitative parameters. We illustrate that the TIS dynamic distance analysis method produces results previously unknown and precisely describes the morphologic and dynamic alterations of the thorax in TIS. A set of 3D thoracoabdominal distances and/or distance differences enables the precise estimation of key measures such as left & right differences, differences over tidal breathing, and differences from pre- to post-operative condition.