when the IWS probe occupied 60% of lumen XSA sites were judged to be severely narrowed if the IWS probe occupied >90% of the lumen XSA. At all sited* mildly, severely narrowed, pze- or post-II determination of ASS %SA WBS consistently possible. In contzadt( determination of PiEL XSA narrowing was not possible in

severely narrowed well as 14/26 (54%) owed sites. HOE@ def tive determination of

R$L XSA was infrequently achieved due consistently distinguish individual la wall. Not only wab distinction between c0nsiatentl.y achieved, but id@~tifi~~t~o~ of media/adventitial boundary was equally difficult. Conclusion: although quantification of luminal XSA is usually feasible, accurate in vivo assessment of the intimallmedisl and medial/adventitisl borders is more often not possible with cuzrantly available IWS instrumentation.: this limitation has important implications for many of proposed applications of IWS.