Dynamic magnetic resonance imaging in assessing lung function in adolescent idiopathic scoliosis: a pilot study of comparison before and after posterior spinal fusion

BACKGROUND Restrictive impairment is the commonest reported pulmonary deficit in AIS, which improves following surgical operation. However, exact mechanism of how improvement is brought about is unknown. Dynamic fast breath-hold (BH)-MR imaging is a recent advance which provides direct quantitative visual assessment of pulmonary function. By using above technique, change in lung volume, chest wall and diaphragmatic motion in AIS patients before and six months after posterior spinal fusion surgery were measured. METHODS 16 patients with severe right-sided predominant thoracic scoliosis (standing Cobb's angle 50 degrees -82 degrees , mean 60 degrees ) received posterior spinal fusion without thoracoplasty were recruited into this study. BH-MR sequences were used to obtain coronal images of the whole chest during full inspiration and expiration. The following measurements were assessed: (1) inspiratory, expiratory and change in lung volume; (2) change in anteroposterior (AP) and transverse (TS) diameter of the chest wall at two levels: carina and apex (3) change in diaphragmatic heights. The changes in parameters before and after operation were compared using Wilcoxon signed ranks test. Patients were also asked to score their breathing effort before and after operation using a scale of 1-9 with ascending order of effort. The degree of spinal surgical correction at three planes was also assessed by reformatted MR images and correction rate of Cobb's angle was calculated. RESULTS The individual or total inspiratory and expiratory volume showed slight but insignificant increase after operation. There was significantly increase in bilateral TS chest wall movement at carina level and increase in bilateral diaphragmatic movements between inspiration and expiration. The AP chest wall movements, however, did not significantly change.The median breathing effort after operation was lower than that before operation (p < 0.05).There was significant reduction in coronal Cobb's angle after operation but the change in sagittal and axial angle at scoliosis apex was not significant. CONCLUSION There is improvement of lateral chest wall and diaphragmatic motions in AIS patients six months after posterior spinal fusion, associated with subjective symptomatic improvement. Lung volumes however, do not significantly change after operation. BH-MR is novel non-invasive method for long term post operative assessment of pulmonary function in AIS patients.