The Haller index, created in 1987 by J. Alex Haller, S. S. Kramer, and S. A. Lietman, [1] is a mathematical relationship that exists in a human chest section observed with a CT scan. It is defined as the ratio of the transverse diameter (the horizontal distance of the inside of the ribcage) and the anteroposterior diameter (the shortest distance between the vertebrae and sternum). [2]
where:
More recent studies show that simple chest x-rays are just as effective as CT scans for calculating the Haller index and recommend replacing CT scans with CXR to reduce radiation exposure in all but gross deformities. [3] [4] [5]
A normal Haller index should be about 2.5. Chest wall deformities such as pectus excavatum can cause the sternum to invert, thus increasing the index. [6] [7] In severe asymmetric cases, where the sternum dips below the level of the vertebra, the index can be a negative value. [8]