Hypoperfusion Asymmetry Index
The Hypoperfusion Asymmetry Index (HAI) measures the volume of significant hypoperfusion voxels based on p-values and extent thresholds in each hemisphere of the brain. This is a powerful tool for lateralizing seizure onset to the correct hemisphere for post-ictal injections. The HAI is calculated everytime you run the Hypoperfusion Statistics from the SPECT tool. This number can be found on the DOS-like Command Line prompt that opens with BioImage Suite. The formula for the HAI is
HAI = ((kleft - kright) /(kleft + kright))where kleft is the number of significant voxels on the left hemisphere and kright is the number of significant voxels on the right hemisphere.
A positive HAI indicates more hypoperfusion in the left hemisphere,
while a negative HAI indicates more hypoperfusion in the right hemisphere.
For purposes of reviewing the calculations, let us pretend that the sample patient data were from a post-ictal injection. The HAI would then be 0.71. Since the HAI is positive, this would lateralize seizure onset to the left hemisphere. Based on McNally et al., 2005, this lateralization is expected to be correct in approximately 80% of patients if the injection occurred in the post-ictal period (which it did not for this patient).