{HEAD ~ The line source image} is maintained in {TAIL ~ focus} during scanning by synchronous refocussing of the imaging system 
is maintained in
{ HEAD ~ The line source image } is maintained in { TAIL ~ focus } during scanning by synchronous refocussing of the imaging system
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH