During data acquisition in a tomographic scan of a subject unscattered events and scatter events are collected. Once an achievable spatial diversity gain is determined for the group, it is compared to a required spatial diversity gain, and if the condition is met at, the validated group can begin spectrum sensing to identify a vacantsuitable channel for operation. The adjustment in ABD delay restores the time alignment of the ECAN internal echo estimate and offsets variations in echo tail delay introduced by network induced AJB delay changes.