%BFT_SUB_IMAGE Subtract one low-res image from  high-res one.%%USAGE :  [hi_res] = bft_sub_image(hi_res, lo_res, element, start_time)%%INPUT  : hi_res  - High resolution RF image. One column per scan line%         lo_res  - Low resolution RF image. One column per scan line%         element - Number of element, used to acquire the low resolution%                   image%         time    - Arrival time of the first sample of the RF lines.%%OUTPUT : hi_res - The high resolution image%%VERSION 1.0 Feb.29 2000, Svetoslav Nikolovfunction hi_res1 = add_image(hi_res, lo_res, element, start_t)hi_res1 = bft(15, hi_res, lo_res, element, start_t);