Personally I'd use the sides of the lower torso. This allows the current more tissue to pass through, lowering the chances of issues where current will conduct directly to the bone increasing chances of skin burns or unpleasant feeling when current is concentrated into a small area.
I'd also use the same place for a scan to start with.
The only time I'd use either side of the spine is if I'm trying to isolate the scan to the lower spine and have a good working experience on pad placement, to avoid the issue mentioned above.
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