A method for morphometric study of the intercostal muscles by high-resolution ultrasound

Besides their main function of assisting in breathing, the intercostal muscles also play an important role in maintaining the balance of forces acting on the thoracic cage including the thoracic spine. Since it is virtually impossible to conduct a morphometric evaluation of these muscles, a study was undertaken to standardize an ultrasound method for accurate determination of the area of the intercostal muscles and hence, indirectly, their function. In a pilot study, the area of the intercostal muscles was determined on the torso of a fresh specimen of a grown-up lamb, using high-resolution ultrasound and CT, and by direct measurements of the intercostal space at two points equidistant from the midline on the left and the right sides of the back of the specimen. The size of the intercostal muscles was determined either by tracing or from the perpendiculars of the area of the muscles both on sonographs and on CT scans. The results showed that measurements derived from the perpendiculars of the muscle area on the sonographs give better estimates than those derived from CT scans, and were in good accordance with the direct measurements of the corresponding intercostal space on the specimen. To evaluate the applicability of the method in vivo the area of the intercostal muscles at maximal inhalation and exhalation was determined in one adult person. It was found that measurements at maximal inhalation were more accurate than those taken at maximal exhalation. It is concluded that ultrasonography is a reliable, safe, easy to apply and high-resolution method for measurements of the area and, indirectly, of the activity of the intercostal muscles in humans, and that the measurements are more accurate at maximal inhalation.