The DI in the HELIX 7 SI is a computer generated blended image from the data received by the SI piezoes (because there is no DI piezo the XHS 9 HDSI 180 T or the XNT 9 SI 180 T) ...
The longer SI piezoes in the XHS 9 HDSI 180 T create a thinner SI beam ...which equates to better SI detail ...
So the DI image will also have better detail because the DI image is basically the SI data (manipulated to look like a DI image) ...
Rickie