<prompt> Trans rectal ultrasonogram in evaluation of carcinoma prostate most useful for:Taking guided biopsyIdentifying seminal vesicle invasionNodal samplingMeasuring the extent of invasion  <response>: TRUS is the imaging technique most frequently used to assess the primary tumor, but its chief use is directing prostate biopsies, not staging - Harrison Thus primary role of TRUS is guiding prostate biopsies Other uses are assessing extent of invasion if cancer is detected. measurement of prostate volume which is needed in the calculation of the PSA density. also used in performance of cryosurgery brachytherapy. Ref :Smiths Urology 17e p361 