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]
<math>\ HI = \frac {\text{distance 1}}{\text{distance 2}}</math>
where:
HI is the Haller Index
distance 1 is the distance of the inside ribcage (at the level of maximum deformity or at the lower third of the sternum)
distance 2 is the distance between the sternal notch and vertebrae.
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]
