The basic idea of ISAS is to compute the difference between an ictal and interictal SPECT scan for a single patient.
Significant increases and decreases in CBF (cerebral blood flow) between the interictal and ictal SPECT can then be detected.
However, the side (L or R) of seizure onset can be reliably determined based on which hemisphere has greater overall SPECT decreases (hypoperfusion asymmetry index).
All that is needed is a computer running MATLAB and an operator with sufficient imaging experience to download and implement the SPM (statistical parametric mapping) analysis.
McNally KA, Paige AL, Varghese G, Zhang H, Novotny EJ, Spencer SS, Zubal IG, Blumenfeld H. (2005).
Chang DJ, Zubal IG, Gottschalk C, Necochea A, Stokking R, Studholme C, Corsi M, Slawski J, Spencer SS, Blumenfeld H (2002).
Evaluation of ictal brain SPET using statistical parametric mapping in temporal lobe epilepsy.
O'Brien TJ, So EL, Mullan BP, Hauser MF, Brinkmann BH, Bohnen NI, Hanson D, Cascino GD, Jack CR, Jr., Sharbrough FW (1998).
This paper describes SISCOM (subtraction ictal SPECT coregistered with MRI), a widely used method of ictal-interictal difference imaging (see also below).