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